Note: Descriptions are shown in the official language in which they were submitted.
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WO 96/15719 PCTIUS95/14265
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BONE DENSITOMETER WIl'H FII~l CaSSETTE
Field of the Invention
This is a continuation-in-part of application serial
number 08/241,270 filed May 10, 1994 based on a PCT filing of
September 10, 1993 which is a continuation-in-part of serial
number 08/067,651 filed May 26, 1993 which is a divisional of
U.S. Patent 5,228,068 filed September 14, 1992; and a
continuation-in-part of application 08/073,264 filed June 7,
1992, which is a continuation of application number 07/862,096
filed April 2, 1992 which is a continuation of application
07/655,011 filed February 13, 1991.
The present invention relates to the general field of bone
densitometry and vertebral morphology and relates, in
particular, to an automated technique for the determinAtion and
analysis of vertebral morphology utilizing techniques of bone
densitometry and to an apparatus for use with that technique
and having the ability to simultaneously record an image
digitally and on film.
Back~round of the Invention
Densitometry
Digital bone densitometry devices such as the DPX machines
manufactured by LUNAR Corporation of Madison, Wisconsin or the
QDR machines manufactured by Hologic, Inc. of Waltham,
Massachusetts, are used to generate broadly based values of
bone character, such as bone mineral content ("BMC") or bone
mineral density ("BMD"). These machines analyze bone in vivo
by the use of dual energy measurements which permit the
attenuating effects of surrounding soft tissue to be largely
eliminAted. Such information about bone character, and in
particular, about bone character in the spine is often relied
on to diagnose and treat bone depletive disorders such as
osteoporosis.
Morphometry
In the case of osteoporosis, bone density measurements
alone are not definitive for diagnosis. The clinician must
also look for evidence of spinal fracture. J. A. Kanis, et al.
2~096~
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Osteoporosis Int. 1:182-188 (1991). Det~rmin;ng whether a
fracture is present is important both on clinical grounds and
for research purposes. In the clinical setting, a patient may
display a reduced BMD but the clinician is hesitant or
unwilling to begin a particular treatment regimen without a
diagnosis of fracture or deformity. In the research setting,
diagnosis of fracture is important in studying the incidence
and prevalence of osteoporosis in a population, or as an entry
criterion to a clinical study, or as a measure of efficacy with
regard to a particular treatment. In this regard, the European
Foundation for Osteoporosis has published guidelines for
clinical trials in osteoporosis which recomm~n~s a definition
of osteoporosis as a "disorder where one or more fractures has
arisen due to an increase in the fragility of bone," and an
endpoint of fracture reduction in studies of efficacy of new
drugs for the treatment of osteoporosis. J.A. Kanis, et al.
While the presence or absence of vertebral fracture is
critical in the diagnosis of osteoporosis, diagnosis of
vertebral fracture is often difficult. Over one-half of such
fractures are asymptomatic, and in cases of min;~l symptoms
obvious fracture or deformity will often not be observed
particularly if there is no previous radiological record for
comparison.
Vertebral morphometry techniques promise to make the
det~rm;n~tion of vertebral fracture or deformation more
objective. These approaches rely on certain indexes or
normative values of vertebral body ~;me~sions. See e.g. Minne
et al., "A Newly Developed Spine Deformity Index (SDI) to
Quantitate Vertebral Crush Factors in Patients with
Osteoporosis," Bone and Mineral, 3:335-349 (1988); J. C.
Gallagher et al., "Vertebral Morphometry: Normative Data,"
Bone and Mineral, 4:189-196 (1988); Hedlund et al., "Vertebral
Morphometry in Diagnosis of Spinal Fractures," Bone and
Mineral, 5:59-67 (1988); and Hedlund et al., ~'Change in
Vertebral Shape in Spinal Osteoporosis," Calcified Tissue
International, 44:168-172 (1989).
In using vertebral morphometry to diagnose fractures, the
clinician cG._I~ollly employs analog radiological imaging
techniques. In essence, an analog x-ray image of the patient's
:
2 ~ 0 0 9 6 9
W096/15719 PCT~S95/14265
vertebrae is taken, and printed onto a fixed media, such as an~
x-ray radiographic film print. The ~rint is made to a specific
scale relative to the original human, i.e., one-to-one, or a
specificaliy reduced or expanded scale. Then the clinician
manually measures the size of a vertebra by using a ruler and a
straight edge and actually draws on the film to outline the
vertebral body, and then measures with the ruler between
criteria lines drawn onto the film itself.
A combined evaluation of bone density and morphometry, at
a m;nimllm, requires the clinician diagnosing or treating
osteoporosis to use two relatively expensive medical devices: a
bone densitometer and an x-ray imaging device. And yet
conventional x-ray imaging devices are not well suited for
morphometric measurements. The conical shape of the x-ray beam
in conventional x-ray machines causes the magnification of the
image produced to be variable depending on the location of the
object relative to the plane of the radiograph. In particular,
the front edge of the object, away from the radiographic plate
is more magnified than the back edge toward the radiographic
plate. The result is that bone edges perpendicular to the
plane of the plate, which for morphological measurement should
produce a sharp visual demarcation, produce a blurred boundary
on the cone beam radiograph.
Distortions of the spine are particularly acute for cone
beam exposures at the edges of the cone beam where the beam is
most angled. For vertebral morphology, the angulation obscures
and distorts intervertebral spacing at the top and bottom of a
field rendering morphological measurements, for example of body
height, imprecise.
SummarY of the Invention
The present invention incorporates a radiographic film
cassette into a sc~nning x-ray densitometer to simultaneously
produce a high resolution, broad spectrum x-ray radiograph and
a digitized bone density image without significantly increasing
35 the sc~nning time or x-ray dose to the patient.
Specifically, the invention is a densitometer having an x-
ray source producing a poly-energetic beam of x-ray radiation
directed to a patient, the latter positioned by a support
device in the x-ray beam. A film cassette is fixed to the
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patient support opposite the x-ray source with respect to the ,
patient and holds a standard broad spectrum radiographic film
within the x-ray beam after the x-ray has passed through the
patient. A dual energy detector is positioned to receive the
poly-energetic beam after it has passed through both the
patient and the film cassette and generates electrical signals
indicating the attenuation of the poly-energetic beam within a
distinct first and second energy range. An electronic computer
receives and combines these electrical signals to produce a
digital bone density image.
Thus, it is one object of the invention to provide both an
analog film image and a digital bone density image
simultaneously during a single bone density scan. The digital
bone density image may be used to evaluate bone density while
the analog film image may be used to analyze or confirm
morphometric data. The analog film image may also be used to
confirm the proper operation of the densitometer, or as an
archival copy of the scan. The analog film image may provide
improved spatial resolution and as a radiograph is familiar to
and readily interpreted by experienced radiographers.
It is another object of the invention to provide a highly
efficient use of the x-ray exposure of a bone density scan.
The radiation source necessarily emits x-rays outside of the
first and energy range of the detector or otherwise
undetectable by the dual energy detector. These x-rays may be
detected and recorded by the x-ray film.
The radiation source may be collimated to a fan beam and
scanned across the film together with a linear detector array.
Thus, it is another object of the invention to take
advantage of the improved collimation of scAnning densitometry
systems to provide a film image with good edge contrast
resulting from the eliminAtion of parallax or variable
magnification incident in normal radiographs. High edge
contrast improves the value of the radiograph for morphometric
measurements.
The invention also provides a self-calibrating method of
det~rmining whether a given vertebra has been crushed, such
crushing as would increase the vertebra's density possibly into
the range that would suggest healthy bone. The invention
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WO96/15719 PcT~sssll426s
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detects such crushing by comparing the height of each vertebra~
to a stAn~rd uniquely generated for each patient by
statistically analyzing that patient's vertebrae under the
assumption that most vertebrae will not have significant
crushing. Thus, the stAn~rd adapts readily to different
patients and is potentially more sensitive than a database
st~n~rd based on a number of different patients.
Specifically, the method of detecting fracture includes
the step of reading into the memory of a digital computer an
array of pixels acquired on a densitometer and analyzing those
pixels first to identify pixels attenuated by bone rather than
soft tissue alone, and second, to relate those pixels to
distinct vertebrae. The height of the vertebrae is then
measured by determin;ng the locations of separated pixels
within a single vertebra. Heights for multiple vertebrae are
analyzed to develop a statistically normal height for that
patient. A deviation between the height of individual vertebra
and the normal height is then determined and that deviation is
indicated to the operator to identify the crushed vertebra.
Thus, it is another object of the invention to provide an
indication to the operator that fracture may have occurred and
thus the bone density reading obtained by the densitometer may
not be an accurate indication of bone health.
Other objects, advantages, and features of the present
invention will become apparent from the following specification
when taken in conjunction with the accompanying drawings.
Brief Description of the Drawinqs
Fig. 1 is a schematic illustration of an instrument for
use in the present invention showing a first embodiment
employing a pencil beam and a raster scan and a second
embodiment employing a fan beam and a linear scan;
Fig. 2 is an illustration of a lateral view of a vertebra
- illustrating measurements used in det~rmining indicia used in
the present invention;
Fig. 3 is a graph plotting bone density versus position in
a horizontal scan line across a vertebra such as that
illustrated in Fig. 2;
Fig. 4 is a graph plotting bone density versus position in
a vertical line across a vertebra;
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WO96/1~719 PCT~S95/14265
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Fig. 5 is a flow chart illustrating the method of the
present invention in analyzing vertebral morphology;
Fig. 6 is an illustration of a vertebra and its vertical
and horizontal graphs similar to Figs. 2, 3 and 4 showing a
first method of det~rm;n;ng an analysis axis;
Fig. 7 is an illustration of a vertebra showing
corresponding graphs taken along two different axes showing a
second method of determining an analysis axis;
Fig. 8 is a lateral view of a vertebra aligned with the
analysis axis showing the generation of measurement zones over
which vertebral height is averaged in preparation to analyzing
the morphometry of the vertebra;
Fig. 9 is a schematic diagram of the bone mineral density
values over a portion of a vertebra illustrating one method of
determining the borders of a vertebra wherein for clarity the
density values within a range associated with tissue are shown
by the letter "T" and the density values in a range indicating
bone are shown by the letter "B";
Fig. 10 is an elevational view of the instrument of Fig. 1
viewed along the scanning direction showing ~llov~ e~lt of the
source and detector between a lateral position and an anterior-
posterior position in one embodiment of the present invention;
Fig. 11 is a schematic illustration of an instrument for
use in the present invention showing a third embodiment in
which the patient is scanned in a st~n~ing position;
Fig. 12 is an illustration of an anterior-posterior view
of a femur showing the determin~tion of the femur's axis and
the identification of fiducial points at the proximal and
distal ends;
Fig. 13 is an anterior-posterior view of the interface
between the femur head and the acetabulum showing the placement
of cut lines to determine joint spacing as aligned with a graph
showing rate of change in x-ray attenuation along one cut line
used for the calculation of joint spacing;
Fig. 14 is a flow chart similar to that of Fig. 5 showing
the method of the present invention in measuring limb length
and joint spacing;
Fig. lS is a planar view of a metacarpal bone of a human
hand showing the determination of a reference axis with respect
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to measurements of cortical to trabecular bone and joint
spacing within the hand;
Fig. 16 is a flow chart similar to that of Fig. 14 showing
the steps of obt~;ning the measurements shown in Fig. 15;
Fig. 17 is a figure similar to Fig. 1 showing the
placement of a film cassette beneath the patient table for
simultaneously acquiring a digital dual energy density image
and an analog radiographic image by means of a scanning fan
beam;
Fig. 18(a) is a simplified elevational view along line 18-
-18 of Fig. 17 showing the position of the radiographic film
cassette between the dual energy detector and the patient
during scanning;
Fig. 18(b) is a detail of Fig. 18(a) showing an
alternative configuration of the dual energy detector;
Fig. 19 is a schematic representation of an anterior-
posterior scan of a spine showing regions of pixels measuring
bone and a graph aligned with the scan having a vertical axis
corresponding to vertical location in the scan and a horizontal
axis corresponding to the sum of pixel values for a row of scan
data permitting the identification of the vertebra by minimAs
or rows of low total bone value; and
Fig. 20 is a graph of height of each vertebra, as
determined from Fig. 19, by order of vertebra in the spine
showing the determin~tion of a typical vertebral height by two
methods which provide a standard against which to detect crush
fraciures of ihe veriebrae.
DescriPtion of the Preferred Embo~;ment
DensitometrY Hardware
Shown in Fig. l is a simplified schematic of an x-ray
based digital x-rsy device 10 of the type described in the
- preferred embodiment of the present invention. The digital x-
ray device 10 includes a dual energy x-ray radiation source 12
and a detector 13, both of which are mounted on a rotatable C-
arm 14, which extends on either side of a supine patient 16 so
as to direct and receive radiation along a radiation axis 24
through the patient 16. The C-arm 14 is designed to be rotated
in a vertical plane as indicated by arrows 9 as supported by a
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WO96/1~719 PCT~S95tl4265
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collar 15 so as to allow both an anterior-posterior ("AP") view
of the spine or other bones or a lateral view of the same. The
C-arm 14 may also be moved longit~ n~lly along the patient's
body in a scanning direction 19 and may be positioned under the
control of servo motors as is understood in the art.
The digital x-ray device 10 of the preferred embodiment
has the capability of switching from a dual energy x-ray to a
single energy x-ray mode. ~Single-energy x-ray" refers to
ionizing radiation at a narrow band of energies of a few keV in
the diagnostic imaging range (20-100 keV). "Dual energy x-ray"
or ~polychromatic x-ray" refers to radiation at two or more
bands of energy, emitted simultaneously or in rapid succession,
or a single broad band energy of more that a few keV over the
diagnostic imaging range.
Switching from dual energy to single energy may be done
either by affecting the source, e.g~, removing or adding a K-
edge filter, or by controlling the switching of energies, i.e.,
switching between high and low x-ray tube voltage, or by
affecting the detector e.g. selecting only one energy level
during a particular study, or a combination of source and
detector.
In the preferred embodiment, a dual energy x-ray beam is
used for the measurements of bone character (i.e. BMC and BMD)
whereas a single energy x-ray beam is used for automated
morphometric measurements. It has been determined that the
single energy beam provides greater precision (i.e. higher data
density per pixel) in the resulting scan than a dual energy
system. However, the novel features of the invention can also
be combined with the features of strictly dual energy x-ray
densitometers to permit measurement of the morphometry as well
as the bone density of the subject. Alternatively, a single
energy beam may be used alone for morphometry measurements
without densitometry measurements.
For purposes of illustrating the present invention, a
study of the morphology of a human vertebra and other bones
will be described. It should be understood, however, that the
invention is not limited to studies of humans but can be
applied to animals as well as humans.
The digital x-ray device 10 of the preferred embodiment
WO96/15719 6 9 PCT~Sg5/14265
also has the capability of selecting between a fan beam 23 of
x-rays which is collimated and oriented toward the vertebra
such that the plane of the fan beam 23 is perpendicular to the
longitudinal axis of the spine; or a pencil beam. being
substantially the centermost ray only of the fan beam 23 along
the radiation axis 24. When the fan beam configuration is
selected, the detector 13 is a linear array of detector
elements subtending the fan beam 23 for providing simultaneous
measurements along a number of rays of the fan beam 23
associated with each such detector element. When the pencil
beam configuration is adopted, only a limited number of
detector elements 13' are employed and measurement is made only
along the single ray of the pencil beam. A cone beam (not
shown in Fig. 1) may also be used, in which case the detector
13 is a matrix of rows and columns of detector elements
covering the area of the fan beam 23 opposite the patient 16.
The fan beam 23, when used, is scanned along the
longitudinal axis of the spine or scAnning direction 19. The
use of a narrow fan beam 23 perpendicular to the spine allows
imaging of the spine, or other long bones generally aligned
with the spine such as the femur, with minimAl distortion along
the longit~l~; n~ 1 axis resulting in the ability to measure
vertebral ~im~n~ions in this axis with greater accuracy than
possible with a cone beam. For greater accuracy in the
horizontal axis, the fan beam 23 may also be oriented so that
the vertebral body or other bone is irradiated by the center
portion of the beam rather than the edges which are subject to
distortion. Since the center of a fan beam 23 has little
angulation, the resulting data is comparable to that obtained
with a pencil beam and yet a scan can be obtained much faster.
Alternatively, when the pencil beam is used, a raster scan
17 of the lateral view of the vertebral body is done. The
raster scan moves the radiation axis back and forth in the
anterior-posterior direction along successive scan lines
separated in the longitudinal direction so that the radiation
axis moves generally along the scan direction 19. The raster
scan 17 results in the slower acquisition of data but provides
the least distortion from parallax.
If a cone beam is used, the digital output must be
WO96/lS719 ~ C ~ ~ ~ 6 ~ PCT~S95/14265 ~
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reformatted to compensate for ray alignment in order to allow
more accurate measurement of ~;m~nsion. The cone beam
acquisition may be performed at discrete stationary locations
or may be acquired continuously as the radiation axis 24 is
scanned along the scanning direction 19.
The rotatable C-arm 14 carrying the radiation source 12
and the detector 13 is connected to, and operates under the
control of, a general-purpose digital computer 18, which is
specifically programmed for use in operating the digital x-ray
device 10 and analyzing the data and includes specialized
algorithms for carrying out the calculations required by the
present invention. In addition, the present invention includes
a data acquisition system ("DAS") and a data storage device
(both of which are not shown) and may be included in the
computer 18 and a display 22 for outputting the data analysis.
Referring now to Fig. 11, in a second embodiment of the
digital x-ray device 10' preferable for use in studies where
the patient's spine and other bones should be under the natural
load imposed by the weight of the patient's body, the patient
16 rPmAins in a stAn~ing position with the hands above the head
resting on a horizontal grip bar 21 positioned above the
patient's head. The grip bar 21 serves to position and
stabilize the patient between the source 12 and detector 13.
In this embodiment, the source 12 and detector 13 rotate about
a vertical axis and the C-arm 14 on which they are mounted
rotates in a horizontal plane as indicated by arrow 9'.
The C-arm 14 may be moved vertically along the patient's
body as indicated by direction arrow 19' and may be translated
in a horizontal plane as indicated by arrow 33 to provide
complete flexibility in allowing overlapping scans of the
patient 16 for studies that involve a wider path than is
subtended by the detector 13. In other respects, the
vertically oriented digital x-ray device 10' operates
analogously to its horizontal counterpart shown in Fig. 1.
A single digital x-ray device 10 may be advantageously
employed for both stA~ing and supine studies of the patient 16
by incorporating a pivot (not shown~ in the supporting
structure of the digital x-ray device 10 so that it may swing
from the vertical position of Fig. 11 to the horizontal
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position of Fig. 1 for the different types of studies. It will
be understood to those of ordinary skill in the art that the
other components of the devices of Fig. 1 and Fig. 11 are
common to both machines and thus that this pivoting design may
provide a flexible, cost effective single machine.
In most general terms, the radiation source 12 emits
radiation of a certain energy level or levels along the
radiation axis 24 at defined locations along the scan. The
radiation passes through the vertebra 20 being scanned and is
then received by the detector 13. The analog output of the
detector 13 is sampled and digitized so as to produce a signal
consisting of discrete data elements by a data acquisition
system ("DAS") which may then transmit the digitized signal to
the computer 18 which stores the data in computer memory (not
shown) or on a mass storage device.
Film Cassette
Referring to Figs. 17 and 18(a) and 18(b), table 26 is
constructed of an epoxy impregnated carbon fiber laminated over
a foam plastic core or any similar material which provides an
extremely light structure that is generally radiolucent and
stiff. Importantly, the table 26 provides an extremely uniform
attenuation so as to prevent the introduction of artifacts into
a radiographic image taken through the table, especially in the
vertical or anterior-posterior direction.
A film cassette 25 is mounted on the underside of the
table 26, to hold a radiographic film 27 in a generally
horizontal plane. The cassette 25 is attached to the table 26
by radiolucent ret~;~;ng tabs 29 which permit the cassette 25
to be both easily attached to the table 26 and ~e~ ed from the
table 26 to be taken to a darkroom to remove the film 27 so
that the film 27 may be developed. In order to conform to the
bottom surface of the table 26 which is downwardly convex, the
cassette 25 is box-like with a cylindrically concave upper
surface that may fit closely to the table 26. By reducing the
cassette~s displacement from the table 26, more clearance is
provided between the cassette 25 and the detector 13.
The film 27, when positioned within the cassette 25, is
enclosed in a radiation perme~hle but light opaque outer casing
that permits the film 27 to be handled in normal room light.
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WO96/15719 PCT~S95/14265
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Preferably, the walls of the cassette 25 are constructed of
thin aluminum to provide an opaque and durable enclosure that
minim; zes the attenuation of the x-rays passing though the
cassette 25. Importantly, the entire cassette 25 is
constructed to provide both minimAl and uniform attenuation to
the x-ray beam passing through the patient and the table 26 so
that the beam exiting the cassette 25 may be further detected
by means of the detector 13. Thus, the cassette 25 includes no
beam stopping structure on its lower wall such as may be
present in some commercial x-ray cassettes.
A conventional x-ray grid 31 may be positioned above the
film 27 to improve contrast by reducing scatter such as is well
known in the art. Such a grid may have a lamellae (not shown)
that are canted to follow the general angle of the rays of the
fan beam 23 from the source 12.
After the fan beam 23 passes through the cassette 25, it
is received by the detector 13 which includes an x-ray
absorbing stop plate 35 attached to the C-arm 14. The stop
plate 35 presents a generally horizontal surface when the C-arm
14 is in the anterior-posterior orientation. On the top of the
stop plate 35, toward the patient, are high and low energy
detector elements 37(a) and 37(b) forming parts of the linear
array of the detector 13 and together subtending the thickness
of the fan beam 23.
When the fan beam 23 is poly-energetic, discrimination
between high and low energy attenuation of x-rays by the
patient can be done by the detector 13 which illustrates side
by side linear array detectors. The detector elements 37(a)
are selectively sensitive to high energies and detector
elements 37(b) are selectively sensitive to low energies. In
this case, during the scan along scAnning direction 19, each
array of detector elements 37(a) and 37(b) forms either a high
or low energy image, and these two images are aligned and
mathematically combined to produce the necessary bone density
information according to mathematical algorithms known in the
art.
An alternative design shown in Fig. 18(b) can be a stacked
array detector as illustrated in detector 13'. In this
arrangement, elements 37(a)' and 37(b)' are selecti~ely
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WO96/15719 PcT~s95/14265
-13-
sensitive to low and high energy spectrums, respectively. A
particular advantage of the stacked array detector is that it
can easily accommodate a multilinear array or area detector
design. Such stacked detectors are described and clAim~ in
Barnes, U.S. patents 4,626,688 and 5,l38,l67, incorporated
herein by reference.
During the sc~nn i ~g of the poly-energetic x-ray fan beam
23 across the patient, the x-ray film 27 is also exposed
progressively across its surface by the uniform and highly
collimated fan beam 23. The collimation of the fan beam 23
reduces scatter and provides accurate edge definition of the
vertebra 20.
Generally the low energy x-rays striking the high energy
radiation detec~or 37(a) and the low energy x-rays striki~g the
high energy detector 37(b) are rejected and do not form a
useful part of the bone density image. Nevertheless these rays
all expose the film 27 and thus are fully utilized in the film
image. Importantly, the present invention recognized that,
given the quantum efficiencies of the detector 13 and film 27,
the interposition of the film 27 and cassette 25 does not
significantly increase the exposure time required to acquire
the bone density image and thus the film increases the imaging
information that may be obtained for a given dose to the
patient during a bone density scan.
Once the scan is complete, the signals provided by the
detector 13 are reconstructed into an image on the computer 18
and the cassette 25 is lelliuved so that the film 27 may be
developed. Because the film 27 is exposed simultaneously with
the collection of data by detector 13, the film image may be
used to confirm the operation of the scanner and the
positioning of the patient. Unlike the bone density image
formed from the data from detector 13, the film image is broad
spectrum and will closely match the characteristics of a
~onventional radiograph in terms of contrast and resolution.
For this reason, the film image may be preferred by trained
radiographers for certain diagnoses. In particular, it should
be noted that because the detector 13 necessarily detects a
narrow energy range, even a simulated broad spectrum image
produced from the data from the detector 13 will not be
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W O 96/15719 PC~rnUS95/14265
- 14 -
identical to the image on the film 27.
The film may also be used for archival purposes,
eliminating the need for an expensive film printer for
producing archival images. Also, the film image may be used
for morphometric measurements, with the physician measuring
relevant ~imen~ions directly on the film, typically as
illuminated on a light table.
The above described invention of combining a film cassette
with a solid state dual energy discriminating detector is
subject to many modifications and variations which will become
apparent to those of ordinary skill in the art. Not the least
of which is the substitution of a computed radiography or
stimulable phosphor plate such as that developed by Fuji for
the film. Accordingly, it is intended that the present
embodiment illustrated herein embraces all such modified forms
as might come within the scope of the claims.
Automated MorPhometr~
Vertebral Studies
Manual measurement of bone morphometry is subject to
errors both in the determin~tion of the edges of the bone and
the orientation of the measurement between edges. Preferably,
therefore, the morphometric measurements are made automatically
by computer analysis of the data obtained by the detector 13.
Such analyses can apply statistical techniques to multiple data
points to provide a more robust and repeatable detprm;n~tion of
bone edges and orientations.
Referring now to Fig. 5, upon completion of the scanning
of the patient 16 by the source 12 and detector 13, as
indicated by process block 60, the computer 18 of the present
invention arranges the data elements obtained in the scan in a
matrix per process block 62. Each data element of the matrix
is associated with a spatial location defined by the position
of the C-arm 14 when the data element is acquired during the
scan. The spatial locations among the data elements differ by
the distance the instrument, e.g., the source 12 and detector
13, moves between taking each data point, both laterally and
(in the case of the pencil beam) vertically between scans.
For a digital x-ray device 10 such as that in Fig. 1 which
uses a fan beam 23, data elements are taken in a series of
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WO96/15719 PCT~S95/14265
-15-
scans by moving the x-ray source 12 and detector 13 in short
longitudinal steps. If a pencil beam is used, data elements
are taken in short vertical scans in the anterior-posterior
direction. In either case, the matrix of data elements is
assembled from a series of these scans to collect data over an
area defined by the length of the scan along the longitudinal
scAnning direction l9 or the vertical lines of the raster scan
17.
In studying the morphology of the human vertebra, it is
preferred that the scan be taken from a lateral direction
through the subject's spine and a single energy mode is
selected. Each data element has a relative value proportional
to the amount of radiation absorbed by the tissue at the
corresponding location. In turn, the absorption of radiation
by a tissue correlates to certain physical properties of that
tissue. For example, bone absorbs a greater amount of
radiation than does soft tissue. The data elements thus
obtained, are referred to PBM for pseudo bone mineral content.
The numbers are pseudo values because they are non-calibrated
and therefore ~imensionless. At this point in the analysis,
therefore only the relative differences between the data
elements are significant, not their absolute values. While the
calibration for each data element could be done at this point,
it is consumptive of computer resources, and thus is deferred
at this point and the PBM values are used. The matrix of
values thus obtained is a representation of the relative
density of the patient's vertebra viewed laterally.
Once the matrix is assembled, the computer 18
automatically conducts a local comparison of data elements to
determine the juncture of data elements attributable to bone
and data elements attributable to soft tissue. In order that
the purpose and the results obtAine~ from such a scan may be
- readily understood, reference is had to Fig. 2 which
illustrates an idealized set of vertebrae 20. Each of the
vertebrae 20 has characteristic boundary regions indicated by
the reference numbers in Fig. 2 with respect to a single
vertebra. Each of the vertebrae has an anterior border 30, a
posterior border 32, a superior border 34, and an inferior
border 36. Additional elements of the vertebrae 20 located
2 ~ 0 Q ~ 6 ~
WO96/15719 PCT~S95/14265
-16-
rearward of the posterior border are referred to as the
posterior elements 38. The region between adjacent vertebrae
20 is referred to as the intervertebral zone and is indicated
at 40.
Superimposed on the lowest of the illustrated vertebrae 20
of Fig. l is a series of horizontal lines, representing the
raster scans 17 of the digital x-ray device lO as employed when
the digital x-ray device lO is operating with a pencil beam.
The results of that raster scan is the matrix of digital
values, with each point value in a single scan being displaced
one unit distance from the previous measured digital value.
Referring to Figs. l and 2, the patient 16 is supported in
the supine position on a table 26 so that the vertebrae 20 of
the spine are generally aligned with the sc~nn i ng direction l9.
Nevertheless, it will be recognized that because of the
curvature of the spine, the angle of the vertebrae 20, i.e.,
the angle of the anterior border 30, a posterior border 32, a
superior border 34, and an inferior border 36 with respect to
the scAnning direction l9 will vary among vertebrae 20.
Whereas this variation may be accommodated by the trained eye
of a physician in estimating the distances that describe the
morphology of the vertebrae 20 for the automation of such
measures, the orientation of the vertebra 20 with respect to
the raster scan 17 or the sc~nn i ng direction lg must be
established to provide repeatable and accurate morphology
measurements.
The first step in evaluating the relative placement of a
vertebra 20, indicated by process block 64 of Fig. 5, is a
det~rmin~tion of the approximate location of each vertebrae 20
as identified by its approximate center 28. Preferably, the
centers 28 are located by evaluating the horizontal and
vertical graphs of Figs. 3 and 4.
Shown in Fig. 4 is a superior-inferior graph of the spinal
column of vertebrae 20. The vertical axis of the graph
represents the units of body density measured at the detector
13 while the horizontal axis represents the spatial locations
of data elements along a line such as indicated at 46 in Fig.
2. Ideally, line 46 is centered along the spine as positioned
by several arbitrary horizontal graphs of adjacent vertebrae 20
WO96/15719 ~~ ~ 2 ~ ~ ~ 6 9 PCT~S95/14265
-17-
as will be described below. Alternatively, the graph of Fig. 4
may represent not the data elements along a single line 46 but
an average of data elements along anterior-posterior lines.
Generally, when a pencil beam is used, the data elements
in the vertical graph are not derived from a single line of
raster scan 17 o digital x-ray device 10 but rather are
reassembled using suitable digital techniques from the entire
matrix of data elements collected by the digital x-ray device
10. As reassembled, the values of the graph of Fig. 4
represent a sequential series of data elements taken in a
superior-inferior direction. This set of data elements is
equivalent to the result which would be obtained from a single
superior-inferior scan.
Note that the graph of Fig. 4 includes local min;m~ 50 and
local m~;m~ 51. These mini~ 50 represent areas of low
density and the m~xim~ 51 represent areas of high density. The
location of the intervertebral zones 40 are readily
ascertAin~hle as the local minim~ 50 and the approximate
inferior border 52 and superior border 54 of t:he vertebrae 20
are recognizable as the portions of the graph on either side of
the local min;m~ 50. The superior-inferior centers of the
vertebrae may be identified as points halfway between the local
m;n;m~ 50.
Referring now to Fig. 3, a horiæontal graph is constructed
along each anterior-posterior line of the scan pattern. The
horizontal graph of Fig. 3 like the vertical graph of Fig. 4
has, as its vertical axis, bone mineral densit:y. The
horizontal axis of the graph of Fig. 3 is the number of an
anterior-posterior scan line. Also like the vertical graph of
Fig. 4, the horizontal graph has local min;m~ 44. The local
m;n;mum 44 represents the approximate posterior border 32 of
the vertebra 20 located between the m~ x; mA created by the
posterior elements 38 and the m~x;m~ created by the major
portion (i.e. the body) of the vertebra 20 itself. M; ~ m 45
indicates the approximate anterior border of the vertebra 20.
Thus, the center 28 of each vertebrae 20 may be approximated as
illustrated in Fig. 6 as the intersection of the scan line in
the anterior-posterior direction which is halfway between
points 44 and 45 and the superior-inferior center 53 of the
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WO96/15719 PCT~S95/14265
-18-
vertebra 20 which is halfway between the local m; n;mnm 50 of
the vertical graph.
Alternatively, in a semi-automatic mode, the centers 28
may be identified interactively by having an operator of the
S densitometer lO observe an image produced by representing the
matrix of data elements as a series of pixels having gray
values in the image proportional to their density values on the
display term;nAl 22. The operator may move a cursor with a
track-ball or mouse-type cursor control device (not shown) to
position the cursor at the center of the vertebral images so
displayed thereby selecting the centers 28. The screen
position of the cursor at the time of selection is recorded and
related a spatial coordinate of the discrete data values
forming the image, thus relating the selected points on the
image to the centers 28 of the vertebra as recorded in the
matrix of discrete data.
These centers 28 may be refined by the above described
center determi n; ng process or used directly for the
morphometric measurements to be described below, the latter
which intrinsically correct for small error in the center
determi~tion.
Referring now to Fig. 5, once the center 28 of each
vertebrae 20 has been detected as indicated by process block
64, a coordinate system is established aligned to each
vertebrae 20. Referring now to Fig. 6, a rectangular area lO0
may be established about the center 28 of each vertebrae 20
having an anterior-posterior width equal to the distance
between min;m~ 44 and 45 and having a superior-inferior height
equal to the distance between ,mi n i m~ S0. If necessary, the
exact ~imen~ions of the rectangle may be adjusted by the
operator to conform with the image of the vertebrae as
described above. As defined, the rectangular area lO0 will be
aligned with its sides parallel or perpendicular with the scans
of raster scan 17 or the sc~nning direction l9.
The PBM data elements, within the rectangular area lO0,
are then summed to produce an alignment value. This alignment
value indicates roughly the total bone mass of the vertebra 20
within rectangular area lO0 and is thus a general measure of
the "fit" of the rectangular area lO0 to the vertebra 20.
9 6 9
~ WO96/15719 PcT~s95/14265
--19--
A new rectangle 102 is then created also centered about
point 28 but perturbed by angle ~ and a new alignment value is
calculated. If the perturbed rectangle 102 produces a lower
alignment value, a new perturbed rectangle 102 is generated
with a rotation in the opposite direction. If, however, a
higher alignment value is obtained with perturbed rectangle
102, a further rectangle 102 is generated with additional angle
and a new alignment value calculated. This process is
repeated until there is detected a decrease (after an increase)
in alignment value within the pertu~bed rectangle 102.
Thus, rectangle 102 is gradually rotated in one way or the
other until the alignment value is m~imi zed. It is found that
the orientation of rectangle 102 which gives the highest
alignment value is also the orientation which m~ximi zes the
amount of vertebra 20 contained within borders of rectangle
102. Thus, upon completion of the rotation process, the
rectangle 102 is a best fit of a rectangle 102 to the vertebra
20 and establishes a coordinate system for analyzing the
vertebra 20 morphology. Specifically, all measurements of the
vertebra 20 are taken along parallels to the vertical or
horizontal edges of the rectangle 102. A column axis 108
parallel to the vertical edge of rectangle 102 is identified to
indicate this axis of measurement as distinguished from the
srAnn i ng direction 19.
Alternatively, and in a second embodiment shown in Fig. 7,
the coordinate system for the measurement of vertebral
morphology may be established by creating a column averaged
graph 104 taken along line 46 in the scAnning direction 19. The
vertical axis of the column averaged graph is a line number of
a row of data elements, and the horizontal axis of the column
averaged graph is the total density of the data elements of
that row, that is, the sum of the data element in that row.
For a vertebrae 20 tipped with respect to the sc~ning
direction 19, the column averaged graph 104 will exhibit a
relatively low rate of change as a result of the obliquely
advancing rows of data elements extending arrow 106 which
crossing the inferior border 36 and superior border 34 at an
angle. In a manner similar to that described with respect to
Fig. 6, a new column axis 108 is iteratively generated and
WO96/15719 2 ~ Q ~ PCT~S95114265
-20-
canted with respect to the scAnn;ng direction 19 by an angle
A new column averaged graph 104' is generated with respect to
this column axis 108. If the row orientation 106' of the new
column axis 108 better aligns with the inferior border 36 and
the superior border 34 of the vertebra 20, the column averaged
graph 104' will exhibit a more rapid rate of change in total
row bone density with respect to row number. The derivative of
the column averaged graph 104' may be taken and the peak value
of the derivative compared between column averaged graphs 104
with other column axes 108 (at different angles ~) to select a
column axis 108 that produces the greatest such derivative
value within a limited angular range. This column axis 108 is
selected as the reference axis for future morphological
measurements.
Thus, although the scan direction 19 may not be aligned
with the vertebra 20 so that the anterior and posterior borders
30 and 32 are substantially parallel to the scAnning direction
19 and the superior border 34 and inferior border 36
substantially perpendicular to the scAnn;ng direction 19, a new
column axis 108 may be determined and measurements of the
vertebrae morphology taken with respect to that column axis 108
to provide improved accuracy and repeatability in the
measurement of the vertebrae morphology. This alignment of a
coordinate system represented by column axis 108 to each
vertebra is indicated at process block 68 of Fig. 5.
Referring now to Fig. 8, in general, the column axis 108
will differ from the scAnning direction 19. Once the column
axis 108 has been determined for a given vertebra 20, the data
elements are effectively "rebinned" to comport with that new
coordinate system. The rebinning may be accomplished by
generating a new series of locations within the vertebra 20
corresponding to evenly spaced lines and columns aligned with
the new column axis 108. Interpolated data elements lying on
these locations at the new columns and rows are obtained by a
bi-linear interpolation of the nearest neighbor actual data
elements weighted according to the actual locations of those
data elements. New vertical and horizontal graphs are then
constructed from these interpolated data elements much in the
manner as described with respect to Fig. 6 with the horizontal
WO96/lS719 ~ 2 ~ 0 9 6 9 PCT~Sg5/14265
-2l-
graph displaying average density for a vertical column of
interpolated data elements and the vertical graph displaying
the average density for a row of the interpolated data
elements.
Referring to Fig. 8, the m;nim~ of the horizontal and
vertical graphs are used to derive an analysis rectangle llO in
the same manner as that described with respect to the
rectangular area lO0 of Fig. 6, the analysis rectangle llO
being aligned with the column axis 108 and encompassing
principally the vertebra 20 and not the posterior elements 38.
This analysis rectangle llO is then divided by the computer
into zones as indicated by process block 68 of Fig. 5. In the
preferred embodiment, three zones are selected, a posterior
zone 112, a medial zone 114 and an anterior zone 116. In the
preferred embodiment, the zones are rectangular having an
anterior to posterior width of l/4 that of the analysis
rectangle llO and extending the full height of the analysis
rectangle llO in the superior to inferior direction and evenly
spaced therein. The relative width and number of zones is
arbitrary and can be varied according to the needs of the user
and the ~imen~ions of the analysis rectangle llO.
Alternatively, in a semi-automatic mode, the measurement zones
may be determined interactively by an operator who indicates
general regions on an image of the data, as described above
,where data within those regions is to be analyzed.
In the example of Fig. 8, the poster1or, medial and
anterior zones define a set of data elements that will be
employed to produce ~iducial measurements with respect to the
morphology of the vertebra 20. The first set of such
measurements is to determine the average height of that portion
of the vertebral body in the superior to inferior direction of
each of the three zones of the analysis rectangle llO.
In one embodiment, this is done by starting with the
posterior zone 112; the data elements within the zone are
automatically summed across rows by the computer to produce a
zone graph 118. The vertical axis of the zone graph 118 is a
row number of data elements corresponding to an anterior-
posterior row and the horizontal axis is the total bone mass of
the data elements of that row and within the posterior zone
2~0~9
WO96/15719 PCT~S95/1426S
-22- .
112. This zone graph 118 differs from the other graphs
described thus far in that it is effectively focused on the
posterior zone 112 and thus sensitive to the morphology of that
region alone.
A first row 120 is automatically identified by the
computer on that graph, and thus with respect to the vertebra
20, at the rising edge of the graph 118 associated with the
superior border 34 in the posterior zone 112. A number of
criteria may be employed in selecting this first row 120, such
as, the first row having a bone mass value to exceed a fixed
predetermined threshold. In the preferred embodiment, the row
is selected as that which has a PBM value first exceeding 30%
of the peak graph value of the zone graph 118. Ideally, the
computer selects as a first row 120 the row which best
lS approximates the position of the superior border 34 if the
position of its contour in the posterior zone (as weighted by
PBM value) were averaged out. In actual practice, a deviation
of the selected first row 120 from the true average of the
superior border 34 is not significant as long as the same
criteria is used each time in selecting the first row 120 and
hence constancy of measurement is obtained.
Likewise, a second row 122 is selected from the graph 118
in its inferior border 36 in the posterior zone 112. Here, the
falling edge of the graph 118 is P~mi ned and the second row
122 selected as that row which first falls below 30% of the
m~xi mllm PBM value for the graph 118. The distance between
these rows 120 and 122 is automatically determined and termed
the posterior height and represented by P and is assumed to be
the average height of the vertebral body in the region of the
posterior zone.
Each of the other zones, the medial zone 114 and the
anterior zone 116, are likewise analyzed by generating a zone
graph 118 and identifying two rows, one at the superior border
34 and one at the inferior border 36. The distance between
these columns for the medial zone becomes the medial height M
and for the anterior zone becomes the anterior height A. The
extractions of these morphological height values is indicated
in Fig. 5 as process block 70.
In the preferred embodiment, the height values A, M, and P
~ ~ 0 9 ~ 9
WO96/15719 PcT~S95/14265
-23-
are calculated by multiplying the number of rows of data
elements between the first row 120 and the second row 122 for
each zone by the distance between each row of data elements in
the analysis rectangle 110. That distance between rows of data
5 points is a characteristic of the digital imaging technique
r used, and will be known. The computer will automatically
perform the above described analysis on each zone of the
analysis rectangle.
In an alternative embodiment, the average height of each
10 zone is determined by the computer by automatically identifying
in each zone, data pairs; one data element of each pair
corresponding to a location on the superior border and the
other data element corresponding to a location on the opposite,
inferior border. The two data elements of each pair have a
15 relationship to one another such that an imaginary line
transecting each data element of a data pair would be
reasonably parallel to the column axis 108. Each data element
of a data pair is selected by the computer by performing a
local comparison of the PBM values of data elements. For
20 example in selecting data elements lying on the superior
border, the computer would ~Amine the PBM values of adjacent
elements. As illustrated in Fig. 9, a data element 111 having
neighboring data elements of similar value in the anterior,
posterior and inferior orientations but of markedly lesser
25 value in the superior orientation, would be assumed to lie at
or near the superior border. In like manner, the computer will
automatically ~Amine all the data elements in the top 1/3
position of zone 112 to determine those data elements lying on
the superior border. Once data elements on the superior border
30 had been selected, a similar analysis would be conducted on the
data elements in the lower 1/3 portion of zone 112 to select
data elements lying on or near the inferior border portion of
zone 112. With data elements selected on each border, the
computer would then pair data elements on the superior border
35 with data elements on the inferior border by assigning the data
elements to columns reasonably parallel to column axis 108.
Having organized the data elements into pairs and assigned the
pairs to columns, the computer employs an algorithm to
automatically determine the distance between each data element
2~0~9~9
WO96tl5719 PCT~S95114265
-24-
in a pair by multiplying the number of data elements found
between each data element of a data pair by the distance
between each data element. As stated above, the distance
between data elements is a characteristic of the digital
imaging technique used. The distance between data elements of
a pair is taken as the inferior to superior height of the
vertebra 20 at the particular location of the column associated
with that pair.
Having determined the height of all the columns in zone
112, the heights are summed and an average height is obtained
for zone 112. In similar fashion, average heights are obtained
for the medial zone 114 and the anterior zone 116.
By a similar process, the analyses rectangle llO may be
divided into several (preferably three equal) horizontally
extending fiducial zones (not shown) and columns identified at
the anterior and posterior borders 30 and 32 of the verteb,ra 20
to determine the average widths of such horizontally extending
fiducial zones. In the preferred embodiment, three zones, a
superior zone S, a central zone C and an inferior zone I are
selected and automatically measured.
In each zone S, C and I, an anterior column and a
posterior column is identified in a process analogous to that
described above with respect to the selection of the first and
second rows 120 and 122 of a posterior zone 112, a medial zone
114 and an anterior zone 116. A center column is also
determ;ned for the zones S, C and ~ being exactly half way
between the anterior and posterior identified columns. The
intersection of the identified columns of the zones S and I and
the rows posterior zone 112, a medial zone 114 and an anterior
zone 116 define a set of fiducial points. For example, the
intersection of the first row 120 for the posterior zone 112
and the first column for zone C defines one such fiducial
point.
The identified columns of the zones S, C and I also serve
to create measures S, C, and I corresponding to measures A, M,
and P, as described above, but exten~ing in an anterior-
posterior direction.
Thus it will be understood that fiducial points at the
"corners~' and center superior and inferior borders 34 and 36 of
WO96/15719 ~ ~ Q 0 9 6 ~ PCT~S95/14265
-25-
the vertebra 20 may be established and that the separation of
these fiducial points with respect to one another measured
automatically. Although each of these fiducial points has a
specific location, they represent an average of the BMD values
of the surrounding data points and hence are robust against
small errors in the BMD measurements at any given data point.
Once the computer has identified these fiducial points,
the computer automatically uses this data to accurately define
the shape and size of the vertebra being studied which, at the
option of the operator can be displayed visually such as on a
CRT device or a printing device. More importantly, however,
the computer is proyldl~u--ed to use the data regarding shape and
size to form~ te indicia of vertebral condition having
clinical or diagnostic value and then to visually display the
indicia for use by the operator either in diagnosing a clinical
condition or increasing the accuracy of bone density
measurements if such measurements are being made.
Using the invention described herein, measurements can be
automatically obtained for a single vertebra, or can be
obtained for several vertebrae. It is possible to use the
analysis performed by the algorithm for several purposes. This
analysis is most effectively directed to the vertebral body,
that is the portion of the vertebra excluding the posterior
elements. Various measurements of a vertebral body which are
obtained by the invention herein described can be used to
provide indicia of disease or deformity as described below.
Additionally, the measurements obtained of a single vertebral
body can be compared to those of adjacent vertebral bodies, as
determi~ed from a single scan, to determine if one or more
vertebra has been subjected to a trauma or other incident which
produces an abnormality in that vertebra. Alternatively, the
indicia of a vertebral body can be compared to those obtained
~ from a normal reference population, to de~rmine aberrant or
abnormal vertebrae, either singly or collectively, in a given
patient. Such normative results may be adjusted for body
height, sex, and weight of the individual patient, as well as
for maturity of the individual, with the various normals being
held in a database. Another alternative is that the indicia
for the vertebral bodies can be compared from time to time in
~ ~ o ~
WO 96115719 PCT/US95/14265
--26--
the same individual, to show changing vertebral morphology ove~
time, which can be indicative of the progress of clinically
significant conditions.
E2~A~LE 1~ ERIOR HEIGH~
A particular indicia of interest for vertebral morphology
is anterior height of the vertebra. It has already been
described above, in connection with the description of the
fiducial points, how the algorithm automatically calculates the
distance from inferior border to superior border for each
fiducial zone. The anterior height of the vertebral body is
the distance between the two end plates, or the distance
between the superior border and the inferior border at or
adjacent to the anterior border. In the prior art, the point at
which the anterior height of the vertebra was preferably
measured was within the first 5 to 10 mm from the extreme
anterior border of the vertebral body. In the preferred
embodiment of the present invention, an anterior zone 116 is
selected which occupies the anterior 1/4 portion of the
analysis rectangle 110 and within this zone the computer
determines an average height of the anterior portion of the
vertebra 20. If compared with the prior art technique of
selecting a particular point for measurement of anterior
height, the techni~ue of the present invention, which
automatically determines an average height within a preselected
fiducial zone, is found to be superior in terms of
reproducibility. This anterior height A, as determined by the
present invention, may be displayed to an operator in absolute
units of measurement, such as millimeters, or the computer can
provide height relative to the average height either of normal
values of vertebrae in the general population or the other
vertebrae of that patient.
EXAMPLE 2: POSTERIOR HEIGHT
Another indicia of interest is the measurement of
posterior height P of the vertebra. Like anterior height,
posterior height measurements made in the prior art are taken
at a single location within 5 to 10 mm of the posterior border
of the vertebra. The present invention provides an automatic
measurement of the average height of the posterior region of
the vertebra 20 lying within the posterior fiducial zone 112
~ ~ 0 0 9 6 9
o96/15719 PcT~ss5/14265
-27-
which occupies the posterior l/4 of the analysis rectangle llO.`
Like anterior height, posterior height may be displayed to an
operator in absolute units of measurement, such as millimeters,
or the computer can provide height relative to the average
5 height either of normal values of vertebrae in the general
population or the other vertebrae of that patient.
E~AMPLE 3: ANTERIOR/POST~RIOR HEIGHT CONPARISON
An important indicia which the computer can be programmed
to automatically obtain is a comparison of anterior height A to
posterior height P. Typically a 15% decrease in anterior
vertebral height, either relative to a norm, relative to the
same individual in previous measurements, or relative to the
posterior height of the same vertebra, is taken as an index of
anterior vertebral fracture, a clinically significant
indication.
EXANPLE 4: W~ ANGLE
Another indicia of vertebral morphology which can be
automatically obtained with the present invention is the wedge
angle. The wedge angle of the vertebra is defined as the
degree of departure from a parallel relationship that linear
extensions of the planes of averages of the inferior and
superior borders would create. In the prior art, this is
calculated based on the overall anterior and posterior height
of the vertebra. From these values of anterior and posterior
height and from the distance between the locations at which the
measurements were taken, it becomes possible to calculate the
angle between hypothetical straight lines ext~ing through the
superior and inferior borders of the vertebra. In the present
invention, the distance between A and P for purposes of
plotting the wedge angle is C. Since A, P and C are average
values, variability in the wedge angle due to variation in the
selection of the location for height and width measurement is
~ avoided. Typically a 15 wedge angle of a vertebra is taken to
indicate that wedge fracture has occurred in the vertebra. A
wedge fracture is a recurrent clinical condition, observed as a
modality of vertebral fracture recognized in clinical
literature.
EXAMPLE 5: BICONCAVITY INDEX
Another indicia of vertebral morphology which the computer
, 2201~6~
WO96/15719 PCT~S9~/14265
-28-
of the present invention can be progr~mm~ to automatically
measure is referred to as the biconcavity index. The index of
biconcavity of the vertebral body is calculated by comparing
the degree to which the height of the central portion of the
vertebral body deviates from the average height of the
posterior and anterior borders of the body. In other words,
this is measuring the deformation of the vertebral body as it
tends to become a concave object. This biconcavity indicates a
degree of deformation of the vertical body associated with
relatively poor vertebral condition. This quantity can be
calculated automatically by the computer which uses an
algorithm to compare M to the average of A and P. M can also
be compared to adjacent vertebrae, or to average values from a
normal reference population previously obtained. A 15%
preferential decrease in central height of the vertebral body
as compared to the anterior and posterior borders, is often
taken to represent a central fracture or a condition of
biconcavity.
EXAMPLE 6: ~y~h ~OPH~
Another indicia of vertebral morphology which the computer
of the present invention can be proyl~lu.,ed to measure is
hypertrophy of the end plates of the vertebrae or hypertrophy
of nodes located within the vertebra. Hypertrophy refers to a
condition where portions of the vertebral body have a relative
density which is abnormally greater than that typically seen
with other vertebrae. In conventional densitometry, localized
areas of high or low density are ignored and only larger area
averages are obtained. This can lead to misleading
interpretations of bone mineral levels when discontinuities are
present as caused by hypertrophy. The ability of the present
invention to define and reproducibly locate a variety of zones:
the posterior zone 112, the m~ l zone 114, the anterior zone
116 and zones S, C, and I, allow bone density to be evaluated
separately at a variety of places within the vertebra. By
comparing the bone density in S or I zones to the C zone, for
example, end plate hypertrophy may be detected. Alternatively,
a hypertrophied node, may be detected by evaluating the bone
density over the entire region within the fiducial points at
the "corners" of the vertebra to identify any data points, or a
- - -
~ WOs6/15719 2 2 0 ~ 9 6 ~ PCT~S95,l4265
-29-
small set of data points, within that defined area that have
values which differ by more than a predefined amount from the
statistical norm of all the values within that area.
While this indicia is not generated specifically for
clinical value, in and of itself, it is useful insofar as it
represents information about a region of vertebra having unique
characteristics which must be excluded from otherwise valid
measurements of bone density or mineral content. For this
reason, it may be used to provide a warning to the operator
indicating that the bone mineral data for a particular vertebra
may need close review.
EXAMPLE 7: lr.l~hV~hl~KAL SPACING
Intervertebral spacing may be readily determined from the
evaluation of the fiducial points at the superior border of one
vertebra and the inferior border of the next superior vertebra.
Essentially, the intervertebral spacing is the distance between
the corresponding fiducial points for these two vertebrae.
As a result of the possible difference in column axes 108
for the two vertebrae, the intervertebral distance may
preferably be evaluated by considering the average distance
between two edges, one defined by line segments joining the
fiducial points at the superior border of the inferior
vertebra, and the other defined by line segments joining the
fiducial points at the inferior border of the superior
vertebrae.
2200~69 ~
WO 96/15719 PCT/US95/14265
--30--
E~A~PLE 8: WARNING OF D~ L1V~ V~;KI~:~A
It is specifically intended that the indicia of
morphological character of the vertebral body, whether height,
compression, wedge, or biconcavity, are utilized by the
instrument for two discreet purposes. One purpose is to create
a warning to the operator that the bone mineral density data
for a particular vertebra should not be utilized since it may
be inappropriate. It can readily be understood that a porous
bone if crushed will have a higher measured density than a
porous bone which has not been subject to crushing. In that
instance, the higher density of the crushed bone is not an
indication of the health of that bone, quite to the contrary.
Accordingly, it is appropriate for the instrument of the
prèsent invention to create an indicational warning to the
operator as indicated by process block 76 in Fig. 5 when one or
more of the indicia of significant vertebral body fracture have
been detected. In any event, the result is that a heightened
accuracy of bone mineral density calculation is obtained by the
bone densitometer, as well as providing the benefit of
potential diagnosis of clinically significant conditions of
vertebral deterioration.
The calculation of these various indicia from the
measurements of A, M, and P, and S, I and C and their
separation is indicated in process block 72 of Fig. 5. At
subsequent process block 76, an indication may be presented to
the operator of abnormal indications which have been detected
by the method, if any. The abnormal conditions can be indicia
described above which are outside the normal values to be
expected for patients of the category of the patient who has
been scanned. If such an indication has occurred, the operator
will then know that the average value for bone density for the
particular vertebrae having the discontinuity should not be
utilized for clinical purposes.
EXANPLE 9: PREDICTING V~l ~KAL FRACTURE
A decrease in bone mass, or the presence of one or more
vertebral fractures, is associated with an increase in the
li~elihood of future vertebra fractures. A decrease in bone
mass of two stAn~Ard deviations is associated with an increase
of four to six times in the likelihood of future vertebral
~ WOg6/15719 2 ~ O O ~ ~ 9 PCT~S95/14265
-31- .
fractures whereas the existence of two fractures, as determined
by a morphologic measurement of anterior height or A is
associated with an increase of twelve times in the likelihood
of future vertebral fractures. See, Ross, et al., "Pre-
Existing Fractures and Bone Mass Predict Vertebral FractureIncidence in Women", Annals of Internal Medicine, v.114-11:919
(1991) .
A combination of bone mass measurement and morphometric
evaluation of fracture is associated with an increase of
seventy-five times in the likelihood of future vertebral
fractures and provides correspondingly improved predictive
power. The present invention, which allows a densitometer to
be used in making morphometric measurements, should prove
valuable in conveniently providing both bone mass and frac~ure
data for such combined measurements.
EXaMPLE 10: D~AL ANGLE MORPHOLOGY
AND BND MEasoRl~M~NT
Referring to Fig. 10, prior to the lateral scan of the
patient 16, an anterior-posterior dual energy scan may be
performed with the x-ray source 12 in position 101 as rotated
about the patient 16 on C-arm 14. As is understood in the art,
dual energy scA~ning provides an improved ability to
distinguish between x-ray attenuation caused by tissue as
opposed to bone allowing more accurate BMD det~rmin~tions~ but
also provides less accuracy for morphology measurements. The
anterior-posterior positioning of the C-arm 14 also improves
the bone density measurement to the extent that the amount of
intervening tissue is reduced at that angle.
Referring also to Fig. 5, the BMD values obtained from the
anterior to posterior scan may be employed to calculate BMD as
indicated by process block 78 and to calculate bone area as
indicated by process block 80 according to techniques well
known in the art. Rather than directly displaying the BMD
values however, a lateral scan is then performed with the x-ray
source 12 at position 101 so that the radiation axis 24 is
horizontal. The calculated values of BMD for various points in
the anterior-posterior scan of the patient may be matched
approximately to those corresponding points in the lateral scan
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WO96/15719
-32-
and the indicia of the morphometric measurements o~ a given
vertebra 20 may be matched to the calculated BMD values of
process block 80 and bone area values of process block 82. If
the indicia for a given vertebra are abnormal, then at process
block 84, where BMD and area calculations may be displayed, a
suitable warning may be given to the operator that the BMD
values and area values are suspect as indicated by process
block 76.
The correlation of anterior to posterior scanned points to
lateral scanned points simply matches the longitll~in~l
coordinates of each such point and makes the assumption that
the patient has not shifted on the table appreciably between
scans. Alternatively, the intervertebral zones 40 may be
derived for each of the anterior-posterior and lateral scans
and the data from each scan shifted to correlate the graphs of
each scan so that the intervertebral zones 40 match.
The ability to use a densitometer to make morphologic
measurements is critical to this augmentation of BMD and bone
area calculations by morphometric indicia as it allows both
measurements to be made without shifting the patient.
The indicia calculated at any one time for a patient may
be compared to subsequent or previous determ;n~tions on the
same patient. By such comparison, changes in vertebral
morphology over time can be tracked. In addition, the digital
2~ image obtained at subsequent det~rmin~tions following an
initial det~r~;nAtion may be subtracted from the stored initial
image to produce a differential image. The boundary conditions
used for det~rm; n ing morphological indicia may be used to
precisely overlap such se~uential images.
Alternati~ely, the indicia calculated for a patient may be
compared to values ContA i ne~ in a data base of reference values
categorized by sex, age or other criteria.
The following emho~ nts illustrate the use of the
in~ention to make morphological measurements of the femur, hip
joint, and metacarpal bones of the hand. It is to be
understood that these examples are illustrative only and do not
limit the invention in any way. In particular, it should be
appreciated that the measurements described for the femur and
metacarpal may be applied to other bones and joint spaces, such
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THE INIERNAlIONAL APPLICATION
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of time.
Det~rm;ning the location of the axis 146 of the femur 134
re~uires that the matrix of data acquired be truncated by the
operator, or according to automatic methods, to that data 151
contained within a rectangle roughly circumscribing the entire
femur with a limited inclusion of other adjacent bones.
Further, the femur axis 146 must be approximately oriented
along the scanning direction 19. This selection and
orientation is typically accomplished setting the limits and
orientation of the AP scan per process block 130.
Once the relevant data is selected, a top portion 138 is
identified including the superior one-sixth of the selected
data 151 as measured along the scAnning direction. Similarly,
a bottom portion 140 including an inferior one-sixth of the
selected data 151 is identified. The r~m~ining central portion
142, including approximately two-thirds of selected data 151
generally excludes the ends (epiphysis) of the femur and
include only the shaft (diaphysis).
In the central portion 142, each row 144 of data is
analyzed and the center data value 143 of the bone values for
that row 144 determined. The orientation of each row 144
(exaggerated in Fig. 12 for clarity) is perpendicular to the
sc~nn;ng direction 19 but generally not perpendicular to the
long axis of the femur 134.
The process of identifying the centermost bone value for
each data element in the rows 144 is repeated until a set of
centers 143 are established along the long shaft of the femur
134. A line fit to these centers 143 establishes the femur
axis 146. As noted above, the femur axis 146 provides a more
reproducible reference for subsequent measurements than the
sc~nn; ng direction 19.
Once the femur axis 146 has been established, the data
values of the matrix are rebinned as has been previously
described, so that the data values follow rows 144' and columns
perpendicular and parallel to the axis of the femur 134.
Referring still to Figs. 12 and 14 once the femur axis 146
has been located, two measurement zones 150 and 152 are
established, per process block 158, in order to identify
fiducial points of the proximal limit 154 and the medial
F - = -
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WO96/15719 PCT~s95/14265
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epicondyle 156. The former measurement zone 150 extends in a
superior direction approximately from the beginning of the
superior portion 138 (as determined from the rebinned data
values) along the femur axis 146.
Within this measurement zone, per process block 160, each
rebinned row 144' is ~x~m;ned to find the superior most row
144' still ha~ing bone values and aligned with the femur axis
146. This row is considered to be the height of the femur 134
and contains the proximal limit 154 and forms one endpoint for
the measurement of the length of the femur 134.
The second measurement zone 152 extends in an inferior
direction from the beginning of the inferior portion 140 and
proximally from the femur axis 146. The position of the medial
epicondyle 156 is considered to be the break in the smooth
curve of the femur surface joining the diaphysis to the ysis.
This break point may be determined by considering the first
derivative of row value of the medial-most data element in each
row 144 of the measurement region 152 identified to bone, as
one progresses in an inferior direction. The first row 144'
where the derivative goes to zero is considered the location of
the m~iAl epicondyle 156. This row 144~ is taken as the
second endpoint in the det~rm;n~tion of the length of the femur
134.
Per process block 162, the femur length L may be
calculated by subtracting the row coordinates for the two row
endpoints embracing the proximal limit 154 and the medial
epicondyle 156.
Once the femur length L has been calculated, the results
may be displayed per process block 164 or printed out per
process block 166 via the computer 18 and display 22. The
length L may be displayed as a function of time, through the
compilation of a number of measurements over a period of time
- or may be compared to a database of ~normals~ as previously
described with respect to the vertebral measurements.
- 35 The length of the femur may be used to provide an
indication of bone growth. Thus, it is of primary importance
that measurements of femur length taken years apart may be
accurately compared. The use of the fiducial points centered
around the medial epicondyle and the proximal limit 154 is
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WO96/15719 PCT~S95/14265
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intended to provide such reproducibility. The referencing of
the measurements to the femur axis 146 which is located by the
mathematical combination of a large amount of data within
region 142 helps ensure this reproducibility.
Referring to Figs. 13 and 14, a second measurement of the
joint spacing between the femur head 170 and the acetabulum
168, may be performed on the femur 134. The measurement of
joint spacing may permit the evaluation of joint function and
the tracking of degenerative joint disease such as arthritis.
Again, when measurements must be made over a period of time it
is imperative that variability caused by changes in the
measurement technique, rather than the joint spacing, be
reduced to a mi n; mnm .
The joint spacing begins again with the detprm;n~tion of
the femur axis 146, as has been previously described with
respect to process blocks 130, 132, and 137, to provide a
reproducible reference. Once the femur axis 146 has been
identified, the data in the superior zone 138 and the m~A; ~1
side of the femur axis 146 is analyzed on a row by row basis,
starting at the inferior edge of the superior zone 138 to
identify an inferior and superior inflection point 163 and 165,
respectively. The inferior inflection point 163 is the center
of the last tissue data elements in a row 144' having bone
values flanking them. Thus, inferior inflection point 163 is
the highest point of the downward concavity of the femur neck.
If two such points occur in a given row, the point closest to
the femur axis 146 is selected.
The superior point of inflection 165 is the center of the
first detected tissue elements having bone values flanking them
after the inferior inflection point 163 as progressively
superior rows of data 144' are P~m;neA. The superior point of
inflection 165, then, is the lower most point of the upward
concavity of the femur neck.
These points 163 and 165 are determined at process block
3S 172 in preparation for detPrmi n; ~g a femur neck axis 184 of the
femur neck and a center 174 of the femur head.
Once points 163 and 165 are detPrminPA, a measurement
rectangle 167 having a width of one centimeter and a length of
four centimeters is aligned with an inflection axis 169 passing
220Q9~9
96/15719 PCT~S95tl4265
-37-
through the points 165 and 163 such that the inflection axis
169 is coincident with the long axis of the measurement
rectangle 167 and so that points 165 and 163 are equidistant
from the center of the measurement rectangle 167.
Data elements within the measurement rectangle 167 are
organized into rectangle rows along the rectangle's width and
rectangle columns along the rectangle's length in a manner
analogous to the rebinning described with respect to process
block 137 and rows 144. The data elements of the measurement
rectangle 167 are then analyzed to determine a centerline (not
s.hown) across the width of the measurement rectangle 167 and
symmetrically bi-secting the bone elements contained within the
measurement rectangle 167. Specifically, each rectangle column
of data in the measurement rectangle 167 is analyzed to find
the centermost data element within that rectangle column and a
line is fit to those centerpoints. This centerline
approximates the femur neck axis 184, that is, the line of
symmetry of the extension of the femur's neck.
Once this centerline is determined, the measurement
rectangle 167 is rotated so that its axis of symmetry along its
width is aligned with this centerline. This realignment
typically involves a translation and rotation of the
measurement rectangle 167.
. As rotated, the new data within the measurement rectangle
167 is arranged in new rows and columns with respect to the
measurement rectangle and that data is analyzed to det~rmine
the rectangle column having the min;m-lm length of contiguous
bone elements among all rectangle columns within the
measurement rectangle 167. This rectangle column corresponds
roughly to the narrowest portion of the femur neck within the
measurement rectangle 167. The measurement rectangle 167 is
then moved along its short axis, and thus generally along the
axis 184 of the femur neck, so as to position this determined
mi n i mllm width of the neck approximately at the center recta~gle
~ 35 column.
The det~rminAtion of the centerline of the bone data
within the rectangle columns of data is then repeated and the
measurement rectangle shifted again to align its short axis
with this centerline and the measurement rectangle is again
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WO96/15719 PCT~S95/14265
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moved to position the shortest column of bone data at its
centermost column in an iterative fashion. These two steps of
repositioning the measurement rectangle 167 are repeated until
the incremental adjustments drop be~ow a predetermined amount
or for a predetermined number of times so that the measurement
rectangle's short axis is coincident with the axis 184 of the
femur neck and the measurement rectangle is positioned
straddling the narrowest portion of the femur neck.
The position of the femur neck axis 184 is set equal to
the short axis of the measurement rectangle 167, thus
positioned.
For the purpose of measuring joint spacing and certain
other ~im~nsionsr an average width of the femur neck is
determined from the length of the bone elements in the
centermost column of data of the measurement rectangle 167.
Next, a centerpoint 174 corresponding approximately to the
center of the femur head is identified along the femur neck
axis 184 displaced from the center of the measurement rectangle
167 by the average neck width. This calculation reflects an
approximation that the radius of the femur head is equal to the
average width of the femur neck.
Minor variations in this location of the center point 174
may be tolerated because of the primary reliance on the
location of the femoral axis 146.
At process block 178, cut lines 180 and 182 radiating from
the center 174 are established based on a neck axis 184 and
femur axis proceeding from the center 174. Cut line 182 is
parallel to femur axis 146 proceeding in a proximal direction
and cut line 180 is spaced from cut line 182 by 60 in a
clockwise direction.
Between cut lines 180 and 182, five more cut lines are
defined (not shown) proceeding from center point 174 and spaced
at every 10 about that ~el~Lel point 174.
The values of the attenuation "A" of x-rays indicated by
the data values of the matrix may be determ;ned along each cut
line by bilateral interpolation as is well understood in the
art. The derivative 183 of these attenuation values (dA/dx) as
a function of a distance x along the individual cut lines is
then determined starting at the center point 174 and proceeding
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WO96/15719 PCT~S95/14265
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outward from the head 170 of the femur, at process block 190.
The distance between the first two m; n i m~ 186 of the
derivative 183 along each cut line is taken as the joint
spacing for that cut line and the joint spacings for all cut
lines are averaged, at process block 1~2, to produce an average
joint spacing. The joint spacing may be displayed and printed
out per process blocks 164 and 166 or may be compared to
normals contained in a data base of such values.
The ability to fix this measurement of joint spacing with
respect to a robust reference of the femur axis 146 and to
average a number of values of joint spacing improves the
reproducibility of this measure.
Certain of the measurements obtained by these studies may
be used for the purposes of evaluating the strength of the hip
joint. For example, the angle between the femur axis I46 and
the femur neck axis 184, together with the distance between the
intersection of these two axes and the center 174 of the femur
head, and the width of the femur neck provide a measure of the
mechanical strength of the neck under the weight of the patient
16.
These measurements may also be performed on a patient 16
having an artificial hip joint so as to provide an indication
of any possible shifting of the prosthetic joint with respect
to the femur 136. This shifting may be characterized by
changes in the distances between the center 174 and the
intersection of the femur axis 146 and the femur neck axis 184,
or the distance between this latter point and the intersection
of the femur neck axis 184 with the lateral most portion of the
femur 134.
Metacarpal Studies
Referring now to Figs. 15 and 16, similar studies to those
described with respect to the femur 134 may be advantageously
made of the human hand and in particular of the third
- metacarpal bone 200 (center finger) of the human hand.
Like the femur, it is preferred that the scan of the hand
be taken in anterior/posterior direction with the hand in the
anatomic position, that is, from the back of the hand through
the palm of the hand, per process block 202. Again, the scan
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WO96/15719 PCT~S95/14265
-40-
data is assembled into a matrix of data values within computer
18 as generally indicated by process block 204.
The third metacarpal bone 200 to be measured is generally
identified by the operator by selecting a point on the
metacarpal bone 200 and selecting that data related to the
third metacarpal by connectivity algorithms known in the art.
Alternatively, this selection process may be performed
automatically.
Generally, the hand of the patient 16 is oriented so that
the metacarpal bone 200 extends along the columns of scan data
indicated by arrow 19. However, it will be understood, as
described above, that the columns of scan data will not
necessarily align with the axis of the metacarpal bone 200.
For this reason,. in a manner analogous to those described
above, coordinates are first established with respect to the
metacarpal bone 200.
In particular, once the data elements have been isolated
into bone or soft tissue and according to those associated with
the metacarpal bone 200, a center portion 206 of the data of
the metacarpal bone 200 covering the diaphysis is selected and
the centermost bone value 207 of each row 208 within this
center portion 206 is identified and aligned fit to those
centers 207 indicating the metacarpal axis 212. As with the
femur, the center portion of the data used for the
det~rmin~tion of the metacarpal axis 212 may be the center two
thirds of the rows of the scan matrix.
Once this metacarpal axis 212 is determined, the data
values are rebinned, as has been described, so that subsequent
measurements may be made with respect to this axis 212. This
reb;nning is indicated at process block 214.
The metacarpal length is readily det~rmined, per process
block 216, by reviewing the rebinned data along the column
aligned with metacarpal axis 212. A distal point 220 is
det~rmin~ by moving distally from the center region of the
metacarpal bone 200 along the metacarpal axis 212 until the
first non-bone value is detected, that non-bone value
corresr~n~ i ng to cartilage between the metacarpal and the
proximal phalanx 213. Likewise, a proximal point 218 is
det~rmined by moving proximally from the center region of the
WO96/15719 ~ 9 ~ 9 PCT~S95/14265
-41-
metacarpal bone 200 along the metacarpal axis 212 until the
first non-bone value is detected.
Joint spacing between the distal epiphysis of the
metacarpal bone 200 and the opposing face of the proximal
phalanx 213 may next be determined, as indicated by process
block 222, by evaluating the rebinned data symmetrically
located on either side of the metacarpal axis 212 within a
predetermined range indicated by lines 224 and proceeding
distally until the first bone values are detected in each
column of rebinned data after the end of bone values of the
metacarpal bone 200. The total area of non-bone data contained
between the bones of the metacarpal bone 200 and the proximal
phalanx 213 is then divided by the number of columns within the
range of lines 224 to provide a joint spacing having the
robustness of a statistical average.
Alternatively, each column of data 221 near the joint and
within the range indicated by lines 224 may be differentiated
to produce a derivative graph 223. The positive and negative
peaks 225 about the nonbone values of the joint cartilage are
taken as the opposed ends of the distal epiphysis and the
metacarpal bone 200 and their separation measured. The average
separation for each column of data 221 within lines 224 is then
averaged to produce a value of joint spacing.
Yet another measurement that may be advantageously made
once the data has been rebinned is that of cortical thickness
as indicated by process block 226. Within the data of the
metacarpal bone 200, as isolaied from soft tissue by the graph
process previously described, is a denser cortical layer 228
and a less dense trabecular center 230. The relative
proportions of these two portions 223 and 230 may provide a
more sensitive measure of the change in bone structure than,
for example, the thickness of the metacarpal bone 200. This
measurement involves differentiating between these two
different bone types within the data acquired.
The first step in such measurements is to position the
measurement rectangle 232 at the center of the metacarpal bone
200. This may be done automatically as guided by the previous
measurements of axis 217 and points 220 and 218. The
measurement rectangle 232 has a width, oriented along the axis
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of the metacarpal bone 200, of 0.5 cm and a length of 2.0 cm.
The exact size of the measurement rectangle 232 may be
adjusted, depending on the patient's size as will be apparent
to those of ordinary skill in the art. Each row of data values
233 within the measurement rectangle 232, aligned across the
metacarpal axis 212, is differentiated to produce a derivative
graph 234. The positive and negative peaks 236 of this graph
234 are taken as the locations of the interfaces between soft
tissue and cortical bone and between cortical bone and
trabecular bone. These locations are averaged with others of
its kind for each row of the data of the measurement rectangle
232 to provide average measurements of cortical and trabecular
thickness for the indicated 0.5 cm. of length.
The results of process blocks 216, 222, and 226 may be
displayed and printed out for review per process blocks 240 and
242. The values may be reviewed directly or compared to
statistical norms cont~i ne~ within a data base of stAn~rd
values.
Detection of Crush Fractures
Referring now to Fig. 19, a matrix of data 201 having
horizontal rows and vertical columns of pixels 210 may be
obtained by an anterior-posterior scan of the patient as
described previously with respect to Fig. 5, process blocks 60
and 62. Each pixel 210 may be identified to either bone or
soft tissue as also previously described and as is generally
known in the art. Preferably, the matrix 201 covers a number
of vertebrae 20(a) through 20(f).
From the scanned matrix 201, the computer 18 may determine
values for a graph 215 much like that of Fig. 4, plotting the
total bone content of the pixels 210 in each row of the matrix
201 against row number, to produce a total bone graph line 219.
This graph line 219 will have periodic min;m~ 50 for rows of
pixels 210 that extend through the intervertebral spaces 246 of
the spine. These min;m~ 50 may be used to identify each of the
pixels 210 to particular vertebrae 20(a) through 20(f). That
is, pixels 210 in a row above the row of a minim~ 50 and below
the row of the next higher mi nim~ 50 are identified to the same
vertebra 20.
For each such vertebrae 20(a) through 20(f), a height H1-
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571~ PCT~s95/14265
-43-
H6 may now be determined directly from graph 219 either by
measuring the separation between the mi~im~ 50, already
calculated, or preferably by taking the first row of graph line
219 having a total bone value exceeding a predet~rmined
threshold, for example 30% of the peak value of the graph 219
between the m i n i mA S 50, for a particular vertebra 20.
Generally, there will be some variation between the values
of H1-H6 for a given healthy individual but substantially more
variation between particular values of Hl-H6 between healthy
individuals.
Referring now to Fig. 20, the height values Hl-H6 may be
effectively plotted by the computer 18 against the order of
their respective vertebrae 20(a) through 20(f) or according to
the vertical center of the vertebra 20(a) through 20(f) as
lS computed from the values H1-H6, but in either case preserving
the sequence of those vertebrae 20 to produce bar chart 248. A
simple arithmetic average 241 of the values H1-H6 may be
computed and this average compared to each value Hl-H6 to
produce a deviation value ~ as follows:
~ = Hi ~ N ~ Hk- (1)
3c=1
where N is the total number of vertebra measured. Here,
large negative deviations ~ indicate a possible crush fracture
of the corresponding vertebra 20 which may suggest discounting
the validity of the density measurement obtained from that
vertebra 20.
In the example of Figs. 19 and 20, both the vertebra 20(f)
(H6) and 20(d) (H4) are somewhat smaller than the average 241.
However, vertebra 20(d) is significantly smaller leading to an
indication of possible crush fracture. The amount of deviation
that indicates a crush fracture is a clinical det~rminAtion
that must be developed empirically.
Alternatively, the normal variation in heights Hl-H6 of
vertebra 20 as one moves through the spine of an individual may
be accommodated by fitting a line or curve of low order 243 to
the height values Hl-H6. In the example of Figs. 19 and 20,
the vertebral height H decreases somewhat from top to bottom.
Here, a straight line fit, using well known techniques such as
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WO96/lS719 PCT~S95/14265
-44-
least squares, produces a statistical measure exceeded by all
the vertebra 20 except vertebrae 20(d) which as previously
described may have a crush fracture.
Alternatively, the height values Hl-H6 may be compared to
a database of normal values, where again the normals are
adjusted for body height, sex, and weight of the individual
patient, as well as for maturity of the individual. The
normals may be further adjusted based on the statistical
measure of the vertebrae of the individual, again making the
assumption that the majority of the vertebrae are healthy.
Thus, the technique provides a threshold for det~rmining
that a vertebra is fractured that is automatically tailored to
the individual, and that can be tailored even to variations of
vertebrae within the individual. This technique potentially
elimin~tes the need for a routine lateral scan of the patient,
but nevertheless provides an indication that lateral scans may
be desirable to detect other indicia of fractures such as
concavity and wedge angles. Generally, the invention makes a
robust bone density measurement possible with a single anterior
posterior scan.
It is thus envisioned that the present invention is
subject to many modifications which will become apparent to
those of ordinary skill in the art. Accordingly, it is
intended that the present invention not be limited to the
particular embodiment illustrated herein, but embraces all such
modified forms thereof as come within the scope of the
following claims.
