Note: Descriptions are shown in the official language in which they were submitted.
CA 02571466 2006-12-22
METHOD AND APPARATUS FOR COLLECTING DATA FOR DETECTING AND
LOCATING DISTURBANCES
Field of the Invention
The present invention relates generally to a method and apparatus for
collecting data,
which may be used for detecting and locating disturbances, such as objects or
radiations. More
particularly, the invention relates to use of a hand-held sensing device
capable of sensing its own
position in space as well as a magnitude of the disturbance at each of several
positions for
collecting and evaluating such data. Still more particularly, the invention
relates to detecting and
locating disturbances providing weak signals or that are weakly interacting.
In this latter
connection, the invention has a very particular relation to detecting and
locating medical objects
unintentionally left behind inside the body cavity of a medical patient.
Background
There are many contexts in which it is desired to ascertain the presence of a
concealed
object. In medical practice particularly, and even more particularly in
surgical practice, there is a
critical need to track the locations of medical tools, devices, aids,
materials, and other objects.
And the most critical need-of all is to ensure that no such medical objects
are unintentionally left
behind inside a patient's body cavity after a surgical procedure has been
completed.
For providing best estimates of the position or location of an object,
detectors adapted for
sensing the object are preferably fixed in space at known positions in the
space. To most closely
realize this goal when using a movable detector, the detector is preferably
moved automatically
CA 02571466 2006-12-22
and repeatably by robotic devices to ensure that measurements correspond to
specific points in
space. However, in the medical theater, there are typically a number of lines
and wires
connecting the patient to various devices and implements that are present in
the vicinity, there
may be a number of surgeons and other personnel standing in the vicinity, and
there is an
S abundance of equipment, trays and carts for holding tools in proximity to
this personnel, so that .
the area around the patient's body is cramped and difficult to negotiate. For
these reasans as well
as others, it has been found to be impractical to use fixed sensing devices,
or a sensing device
that is moved on, e.g., a track, or with a pre-programmed robotic ann, for
sensing medical
objects inside apatient.
Position sensing using hand-held devices is known in medical imaging, and
position
tracking has been used to track the movements of surgical instruments. For
example, Sliwa 3r.,
et al., U.S. Patent No. 6,122,538 ("the '538 Patent"), refers to a movable
ultrasound transducer
probe utilizing a set of translational accelerometers for measuring both
translational and angular
accelerations. Position and angle changes are deduced by double integration.
However, the ' 538
Patent acknowledges that the system is subject to substantial inaccuracies,
making it difficult or
impossible to return to a previous scan position. Hence, the '538 Patent
proposes a hybrid
system that also incorporates a gyroscopic sensor array having three
orthogonal gyroscopes. This
type of device is commonly referred to in the art as an "inertial guidance
system," although the
system need not be used for guidance and can be used, as suggested in the
above reference,
merely for sensing.
Ferre et al., U.S. Patent No. 6,687,531 ("the '531 Patent"), provides an
example of
tracking the movements of a surgical instrument. In one embodiment, the
location of the
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instrument may be determined with respect to fixed transmitters (or receivers)
attached to the
patient's head by triangulation. In another embodiment, a field is generated
using three
orthogonally disposed magnetic dipoles, and the lcrlown near-field
characteristics are used for
position detection.
Neither of the aforementioned systems is suited to searching for, or detecting
the location
of, concealed and unknown objects. The ' 538 Patent merely detects the
location of a probe that
15 111 the possession of a user of the system. The probe is not used to
acquire data about another
object having unknown characteristics. The ' 531 Patent merely tracks the
location of a probe as
it moves in the body. The standard prior art methodologies for tracking the
location of surgical
instruments or other medical objects left in a patient's body are either to X-
ray the patient, which
is undesirable for obvious reasons, or simply to count the objects used during
the surgery before
and after the surgery.
Some more sophisticated approaches for detecting medical objects in a
patient's body
cavity have been proposed. These fall into two basic categories, depending on
the constitution of
the medical object. On the one hand, metal medical objects can be detected
using, essentially, a
metal detector. Avrin et al., U.S. Patent No. 5,842,986 ("the '986 Patent"),
is exemplary. On the
other hand, some medical objects, such as sponges, do not interact with
electromagnetic fields.
However, such objects can be tagged with RF1D (acronym for "radio frequency
identification")
tags and the tags can be stimulated to produce an output that can be detected.
Blair et al., U.S.
Patent No. 6,026,818 ("the ' 818 Patent"), is exemplary of this approach.
Hobbyists use metal detectors for discerning the presence of buried coins and
other
artifacts, and metal detectors are also used, now ubiquitously, in security
systems for discerning
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the presence of concealed weapons. Metal detectors may sense radiation
reflected or otherwise
transmitted from the object, or may sense a change in the inductance of a coil
that is induced by
the presence of a nearby metal object. The '986 Patent in particular proposes
applying a low
strength, time varying magnetic field to a screening region of a body and
sensing a responsive
magnetic field from a retained ferrous body within the screening region. As in
standard metal
detection practice, only the component of the responsive field that oscillates
at the frequency of
excitation is used, to minimize the effect of background noise. However,
unlike the standard
practice, the location of the source of the responsive field is inferred from
the response at a single
location in space by measuring field gradients. Further, to increase detector
sensitivity over
standard metal detection techniques, the '986 Patent proposes the use of an
improved
magnetorestrictive sensor.
For detecting non-metallic medical objects, the ' 818 Patent proposes
detecting the output
of an RFID tag that has been attached to the medical object. Such tags are
attached to medical
objects by the manufacturer. The tags produce a coded output when stimulated
by a time-varying
electromagnetic field at the proper frequency. The output is a W eak, narrow
band signal that is
modulated to identify a particular product/manufacturer combination.
As the '818 Patent explains, there is a problem obtaining sufficient signal
strength from a
small tag. However, increasing the size of the tag to provide greater signal
strength is
problematic for surgical sponges, which are asserted to be the most common
medical object for
which detection is important, because deformation of the sponge will often
deform the tag, and
using a large, non-deformable tag would defeat the purpose of the sponge.
The proposed solution to this problem is to increase the sensitivity of the
detector so that
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it is capable of detecting the weak signals produced by small, inexpensive
tags by stimulating the
tag with a pulsed signal which covers a wide frequency range that includes the
frequency of the
tag. The pulsed signal triggers a continuing response signal from the tag, at
its single frequency,
which increases to a point where it becomes differentiated' from background
noise.
Both the '986 and the '818 Patents propose useful and desirable improvements
to
increasing detector sensitivity, for detecting the presence of and, to some
extent, for locating the
respective classes of medical objects within a body cavity in the operating
room environment.
However, the '986 patent does not recognize that the increased sensitivity of
its detector will be
accompanied by an increased sensitivity to noise, and that the standard
practice of measuring
only components of the responsive field that are synchronized to the frequency
of excitation will
not provide any additional noise reduction than has always been available in
metal detectors, and
which is known to be insufficient for this medical purpose.
Moreover, neither the ' 986 nor the ' 818 Patents recognize that the operating
room is filled
with other, similar medical objects, as well as many sources of general, broad-
band
electromagnetic radiation (such as CRT's), that are not located inside the
patient but that are
nevertheless nearby and will be detected. Many of the medical obj ects left
behind inside a
patient, such as needles and tagged sponges, will produce small signals or
will only weakly
interact with an interrogating field. Many of the external objects and sources
will produce a
stronger signal or other indication of presence, and neither patent provides
any guidance for
ensuring that what is detected is in fact of interest. Moreover, the
improvements proposed in
these patents are specific to sensing particular classes or types of objects;
neither patent provides
any guidance for improving the accuracy, precision, or specificity of
detection and location in
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general, independent of what is being detected and the sensor technology being
used.
Therefore, as the present inventor has recognized, there is a need for a
method and
apparatus for collecting data for detecting and locating disturbances that has
particular
applicability in the context of searching for medical objects, such method and
apparatus also
having wider applicability as discussed hereinbelow.
S ummary
A method and apparatus for collecting data for detecting and locating
disturbances in
space is disclosed. A plurality of distinct regions of the space are
identified, and a device
adapted for sensing positions of the device and respective representations of
a field parameter at
the positions is also disclosed. Preferably, the device comprises an inertial
guidance system for
determining the positions so that it can be hand-held and yet provide high
accuracy and precision
without requiring an articulated arm with position encoders.
According to a first, data collection aspect of the invention, the device is
moved through
the space to obtain a plurality of sets of the representations and
corresponding positions.
Positions falling within the regions are quantized, and, for each of the
regions, the representations
associated with the quantized positions therein are combined.
According to a second, presence testing aspect of the invention, the device is
moved
through the space in a first pass, to obtain first sets of the representations
and the corresponding
positions and thereby define baseline scan results. In addition, in a second
pass distinct from the
first pass, the device is moved through the space to obtain second sets of the
representations and
the corresponding positions and thereby define test scan results. The
positions falling within the
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regions are quantized. For each of the regions having associated baseline scan
results, the
baseline scan results associated with the quantized positions therein are
combined, defining
combined, quantized baseline scan results, and for each of the regions having
associated test scan
results, the test scan results associated with the quantized positions therein
are combined,
defining combined, quantized test scan results. Respective representations of
the difference
between the combined, quantized test scan results and the combined, quantized
baseline scan
results are formed, defining ultimate data associated with said regions.
Whether the ultimate data
corresponding to at least one of the regions exceeds a threshold is evaluated.
According to a third, location determining aspect of the invention, the device
is moved
through the space to obtain a plurality of sets of the representations and
corresponding positions.
The positions falling within the regions are quantized. For each of the
regions having associated
scan results, the scan results associated with the quantized positions therein
are combined,
defiling combined, quantized scan results. A first set of the combined,
quantized scan results
corresponding to at least two of the regions is identified. One or more
possible locations of the
disturbance in the space are triangulated using the first set.
Preferably further, the possible locations falling within the regions are
quantized, defining
quantized possible locations associated with the regions. For each of the
regions having
associated possible locations, the possible locations associated therewith are
counted. One or
more of the regions in which the location of the disturbance is most likely
are identified by
determining for which of the regions the counting produces a higher value than
is produced for
other regions.
It is to be understood that this summary is provided as a means of generally
determining
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CA 02571466 2006-12-22
what follows in the drawings and detailed description and is not intended to
limit the scope of the
invention. Objects, features and advantages of the invention will be readily
understood upon
consideration of the following detailed description taken in conjunction with
the accompanying
drawings.
Brief Description of the Drawings
Figure 1 is a schematic perspective illustration of a hand-held sensing device
for use
according to the present invention.
Figure 2 is a cross-sectional view of a surgical patient showing movement of
the hand-
held device of Figure 1 in a space around the surgical patient according to
the present invention.
Figure 3 is a block diagram of a loop sensor and a portion of a control
circuit for
controlling the loop sensor for sensing changes in permeability to very high
stability and
sensitivity according to the present invention.
Figure 4 is a pictorial view of a space partitioned into cellular regions
according to the
present invention and also showing a slice of the space lying in the plane of
Figure 2.
Figure 5 is a pictorial view of the slice of Figure 4 partitioned into
cellular areas that
correspond to a sub-set of the cellular regions of Figure 4 according to the
present invention.
Figure 6 is a pictorial illustration of a cell region, illustrating data
corresponding to a
baseline scan according to the present invention.
Figure 7 is a pictorial illustration of the cell region of Figure 6,
illustrating data
con-esponding to a detection scan according to the present invention.
Figure 8 is a pictorial illustration of the cell region of Figure 6,
illustrating data
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corresponding to difference results according to the present invention.
Figure 9 is a graphical illustration of the construction of a locus of points
according to the
present invention.
Figure 10 is a graphical illustration of a three-dimensional surface according
to the
present invention.
Figure 11 is a pictorial view of the slice of Figure 4 partitioned into
cellular areas
according to the present invention and showing two paths of the hand-held
device of Figure 1
that lie on the slice and that pass through the cellular areas.
Figure 12 is the same view of the slice as shown in Figure 11 with positions
on the paths
quantized within the cellular areas according to the present invention, and
showing an example
of movements of the quantized positions of the two paths according to the
present invention.
Figure 13 is the same view of the slice as shown in Figure 11 showing
quantized
positions for a single path resulting from the movements of Figure 12, for use
in developing the
surface of Figure 10 according to the present invention.
Detailed Description of Preferred Embodiments
Introduction
As mentioned above, the invention relates to a method and apparatus for
collecting data,
which may be used for detecting and locating disturbances. The term
"disturbance" is used
generally herein to refer to any matter or energy that is capable of being
detected remotely. A
source may create a disturbance in space by emitting into the space either
energy , e.g., photons,
or matter, e.g., gases. An object may disturb space by having material
properties that interact
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CA 02571466 2006-12-22
with an applied field; for example, a ferrous metal object fixedly disposed
within the magnetic
field of the earth disturbs that magnetic field in the vicinity of the object.
Particular
embodiments disclosed herein pertain more particularly to such data collection
for detecting and
locating medical objects within the body cavity of a surgical patient;
however, it should be
S understood that the invention may be used in any other desired data
collection context without
departing from its principles.
Sensing the presence of a medical object, concealed within a patient's body
cavity, in the
electromagnetically harsh environment of the operating room, presents unique
and difficult
challenges. Medical objects often provide only a weak signal, or otherwise
only weakly interact
with ambient or applied fields, so that their presence cannot easily be
discerned over the
electromagnetic noise that exists in the operating room. Further, as the size
of medical patients
varies considerably, the closest proximity that can be obtained by an external
sensing device may
be as high as 36 inches. Moreover, the body is primarily a saline solution, so
that already weak
electromagnetic signals emanating from either metal objects or RF>D tags
present within the
body will be severely attenuated with distance. While the magnetic
susceptibility of ferrous
objects is useful for identifying ferrous objects, because the measurement is
generally insensitive
to the presence of salt water, many medical objects are non-ferrous, and some
medical objects,
such as needles, are quite small and will produce an exceedingly weak
response. RFm tags also
produce very weak signals, and are problematic for use on metal objects which
axe very common
obj ects of interest.
It is also important in the medical application to discern the location of an
object for
which presence has been detected. While it is not so important to a hobbyist
looking for buried
CA 02571466 2006-12-22
artifacts, or an airport security screening device, to signal the precise
location of an object that
has been detected, the lack of having a precise location for a medical object
in a patient's body
cavity places a burden on the surgeon of searching the body cavity, which
delays the completion
of the surgery and adds to the risk of damage to the patient.
Accordingly, the present invention addresses a number of needs. Generally, the
invention
provides for improved (a) disturbance data collection, (b) disturbance
presence detection, (c)
disturbance location determination, and (d) disturbance location
visualization. In addition, a
novel and improved sensing device is disclosed for use in conjunction with
disturbance data
collection.
The various features of the invention are preferably used in combination with
each other,
but may be used in any desired combination and may be used separately.
Moreover, the features
of the present invention may advantageously be combined with additional
features, such as those
proposed in the '986 and '818 Patents discussed above. It should also be
understood that sensing
technologies depend on the disturbance being sensed. For example, for sensing
chemical objects,
an appropriate sensing device would be adapted for detecting the presence of a
particular
chemical by reacting with liquid or gas evolving from the object. It is
generally intended that the
methodologies described herein may be utilized in conjunction with any type of
sensor or sensing
technology appropriate to the disturbance being sensed.
Reference will now be made in detail to specific preferred embodiments of the
invention,
examples of which are illustrated in the accompanying drawings. Wherever
possible, the same
reference numbers are used in the drawings and the description to refer to the
same or like parts.
Device Description
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Figure 1 shows a hand-held device 10 for use according to the present
invention. As will
be explained in more detail below, the device measures a field parameter, such
as magnetic
permeability, at different points in a space as the device is moved through
the space by hand. For
example, as shown in Figure 2, the device 10 may be moved about in a space 11
by hand,
represented by paths "A" and "B." While the paths "A" and "B" are shown in two-
dimensions, it
will be understood that the space may be, and typically is, three-dimensional,
so that the paths
may represent a course of travel anywhere within a three-dimensional space.
Also shown in Figure 2 is an idealized cross-section of a patient 13 lying on
a table 15.
The patient may be a human being or any other animal. In a preferred method
according to the
invention, the device 10 is moved in space surrounding the patient to obtain
measurements of a
field parameter useful for determining (a) whether a medical object is present
inside the patient,
and (b) if so, what the location of the medical object is. The medical object
is typically either a
metal object, such as a clamp or a needle, or a non-metallic object, such as a
sponge to which is
attached an RFID tag. However, it will be understood that the principles
described herein may be
applied to numerous other situations and circumstances in which it is desired
to determine the
presence of disturbances, or to discern the location of any such disturbances
that are present.
Returning to Figure 1, the device 10 includes a position sensing portion 14
adapted to
measure the position of the device in space. It is desired to provide as much
precision and
accuracy in the sensed position as is possible or practical, but there is no
requirement that the
precision and accuracy of the disturbance sensing portion 12 exceed
any,particular threshold.
As has been provided in the art of medical imaging before, the position
sensing portion
14 preferably includes three accelerometers 16, namely 16a, 16b, and 16c, all
arranged to respond
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CA 02571466 2006-12-22
to accelerations directed along different axes, preferably orthogonal axes
such as the x, y, and z
Cartesian axes shown. As is well known, the x, y, and z position of the device
at any instant can
be obtained by double integration of the outputs of the respective
accelerometers, assuming that
the orientation of the device is maintained as it is moved about. However,
since the device 10 is
hand-held and is moved through space by hand, the orientation of the device
will generally not be '
maintained fixedly in space. Therefore, the outputs of the accelerometers must
be corrected for
changes in orientation, requiring measurements of orientation corresponding to
the measured
positions.
To provide measurements of orientation, three gyroscopes 18, namely,l8a, 18b,
and 18c,
are provided to respond to changes in angle of the device 10 along different
axes, preferably the
same axes used for measuring acceleration. For example, with reference to the
coordinate system
defined.by the axes x, y, and z, the gyroscope 18a is sensitive to changes in
angle within the x-y
plane (commonly referred to as "yaw"), the gyroscope 18b is sensitive to
changes in angle within
the x-z plane (commonly referred to as "roll"), and the gyroscope 18c is
sensitive to changes in
angle within the y-z plane (commonly referred to as "pitch").
The gyroscopes 18 provide as outputs changes in angle that can be used to
correct the
outputs of the accelerometers by the use of trigonometry. For example,
consider that an
accelerometer, e.g., 16a, sensitive to accelerations along the "x" axis,
measures an acceleration.
Further consider that no acceleration is measured by an accelerometer, e.g.,
16b, sensitive to
accelerations along the "y" axis. However, consider that a gyroscope 18a,
sensitive to changes in
angle within the "x-y" plane, measures that the angle of the device 10 within
this plane has
deviated 45 degrees away from the "x" axis toward the "y" axis. In that case,
the acceleration
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CA 02571466 2006-12-22
measured on the "x" axis is actually the acceleration along a line oriented at
45 degrees with
respect to the "x" axis. The acceleration along the "x" axis corrected for
this change in angle is
obtained by multiplying the measured acceleration by the cosine of 45 degrees.
Similarly, the
acceleration along the ''y" axis corrected for this change in angle is
obtained by multiplying the
measured acceleration by the sine of 45 degrees.
The principles of operation of such position sensing mechanisms are well
known, so that
further discussion is omitted as not being particularly pertinent to the
invention. However, it may
be noted in passing that such mechanisms, including accelerometers and
gyroscopes, are now
being included in standard integrated circuits, and such integrated circuits
are preferable for use
in a hand-held device because it reduces weight, power consumption and cost.
For purposes of convenience, but not of limitation, a position sensing
mechanism that
determines position by use of accelerometers and gyroscopes will be referred
to herein as an
"IGS" system (acronym for inertial guidance system). The IGS provides the
particular advantage
that positions can be determined in a hand-held device and yet provide high
accuracy and
precision without requiring an articulated arm. By contrast, position sensing
using a GPS
(acronym for global positioning system) does not provide the required
precision, and articulated
arms providing fox more precise and accurate position sensing using position
encoders at the
joints will interfere with the lines and wires commonly surrounding a surgical
patient.
Referring again to Figure l, the device 10 further includes a disturbance
sensing portion
12 for sensing a field parameter indicative of the presence and magnitude of a
disturbance within
the measurement space.
Exemplary field parameters for use in sensing the presence and properties
(e.g., size) of
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metal objects are magnetic permeability and electrical conductivity.
Particularly, metal obj ects
perturb both magnetic and electric fields. Metal objects perturb electric
fields because their
conductivity attracts and therefore increases the density of electric field
lines existing in an
otherwise non-conductive space, e.g., a space filled with air. Similarly,
magnetically permeable
metal objects perturb magnetic fields because their magnetic permeability
attracts and therefore
increases the density of magnetic field lines existing in an otherwise non-
magnetically permeable
space. It is a recognition of the present inventor that magnetic fields,
unlike electric fields and
electromagnetic waves or signals, are unaffected by saline cavities such as
are present in animals.
Thus, the magnetic field parameter is a preferred field parameter according to
the present
, invention for collecting data regarding metal objects.
It is desired to provide as much precision and accuracy in sensing the
presence and
magnitude of a disturbance as is possible or practical. However, as will be
explained hereafter,
methods according to the invention improve on the raw precision and accuracy
provided by the
disturbance sensing portion 12, so it is not necessarily critical that the
precision and accuracy of
the disturbance sensing portion 12 exceed any particular threshold.
Figure 1 shows a loop sensor 20 of the disturbance sensing portion 12 for
sensing
magnetic permeability. The sensing is based on the principle that the
inductance of the loop
sensor is responsive to the presence of nearby metal objects. For general
purposes, the
inductance of the loop sensor may be monitored by a control circuit 23 through
use of the known
relationship between the voltage across an inductor and the current therein,
V= L di/dt.
However, unusually high stability and sensitivity in sensor measurements are
needed to
discern reliably the presence of, and locate, medical objects in the bodies of
medical patients in
CA 02571466 2006-12-22
the operating room. Figure 3 shows a block diagram of the loop sensor 20 along
with a pertinent
portion of a novel control circuit 23 that is particularly adapted for meeting
these needs. Through
these means, the device 10 provides for tracking extremely small changes in
the inductance of a
coil of wire by utilizing matched delay paths and a high-speed time sampling
circuit to compare
the phase shift between voltage and current while the coil is being driven at
its resonant
frequency.
The device 10 senses changes in permeability as a result of changes in the
effective
inductance of the loop sensor 20, due to a variable proximity of the loop
sensor to metal obj ects.
To provide for high sensitivity, a resonant circuit 50 is formed from the loop
sensor 20, a
capacitor C1, and a resistor R. The resonance circuit is maintained at a fixed
resonant frequency
Fo that is the actual resonance frequency of the circuit 50. Changes in
inductance of the loop
sensor 20 are sensed as a phase differential. At resonance, the phase
differential is zero, but on
either side of resonance, the phase differential changes dramatically between
being positive (at
+90 degrees) and being negative (at - 90 degrees).
The actual resonance frequency Fo is generally unknown because it can vary
within about
10 - 15% of an ideal, calculated value based on circuit parameters. The
circuit is controlled to
operate at resonance, however, by use of a "feedback control cycle" initiated
by the CPU wherein
the CPU monitors the phase differential to determine when the phase
differential passes through
resonance, and tunes a programmable oscillator 52 via a signal "M" to drive
the circuit 50 at the
frequency Fo. Once a convergence on the actual resonant frequency Fo is
determined, the
feedback control cycle is discontinued and the driving frequency remains fixed
until a new
feedback control cycle is initiated as indicated below.
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The phase differential is determined by comparing the phase of the voltage
output of the
circuit 50 and the phase of the voltage output of the programmable oscillator
52. These phases
are input to a phase shift monitoring circuit 54 at inputs INL and INR
thereof, respectively.
A low noise, low output-impedance buffer B1 is used to interface the
oscillator 52 to the
resonant circuit 50. The frequency of the oscillator 52 may be programmed by
adjusting a
programmable clock divider CD operating on the output of a crystal oscillator
C(~S.
The phase shift monitoring circuit 54 receives the signals input at INL and
INR in
parallel, and passes these parallel inputs through twin, matched paths
comprising matched filters
F and matched zero crossing detectors Z, each matched pair being provided on
the same chip
substrate to cancel out phase differential between the paths due to inequality
of device parameters
due to manufacturing variations. Matched delay paths are provided by utilizing
matched,
common substrate semiconductor devices for both analog and digital circuit
elements in the
circuit 23.
A phase gate PG receives the outputs of the zero crossing detectors and
outputs a signal
T~lag which specifies the differential in phase between the two paths. T lag
is used to start and
stop a counter that counts at very high frequency compared to the frequency
Fo, as controlled by a
clock multiplier 57 that is phase locked to the oscillator 52, to obtain a
digital signal, "Interval,"
representing an interval of phase differential which is a high precision
fraction of Fo,, preferably
on the order of 1:100,000 or better.
The production of the signal "Interval" is repeated at times defined by the
base frequency
of the oscillator 52 to produce samples of "Interval" at a frequency on the
order of 1000
samples/second. These samples are provided to a functional block 56 that forms
a short term
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running average of the samples. This averaging is performed over a short
period of time,
providing a response time on the order of 100 times a second, to essentially
low-pass filter or
smooth the samples to produce measurement data, "Tave."
A functional block 60 produces long term running average signals, "Tbase,"
based on
averages of the short term running average taken over long periods of time
defined by the base
oscillator 52 as modified by another clock divider 59, extending on the order
of one second
As mentioned above, the circuit 50 is controlled to operate at resonance by
use of a CPU
that monitors the phase differential to determine when the phase differential
passes through
resonance, and tunes a programmable oscillator 52 to drive the circuit 50 at
the frequency Fo. To
monitor the phase differential for implementing this control, the CPU uses
Tbase to ensure that
discerned resonant -frequency is not affected by variations due to changing
proximities to metal
objects that are the subject of search, as these variations will average out
over long times..
The signals Tbase are preferably accumulated by the CPU, which resets the
black 60, and
are averaged over even longer times, e.g., an the order of hours or days, to
provide a reference
signal Tref (not shown) that averages out variations, such as those due to
changes in temperature,
that occur over very long times. Tref is subtracted from all of the
measurement data Tave to
provide ultimate measurement data for which all reasonable environmental and
circuit parameter
variations have been compensated for. Tref is also used to provide a starting
or seed value of the
programmable clock divider CD used in a feedback control cycle to facilitate
fast convergence to
the resonant frequency Fo when the device 10 is first turned on.
The frequency of the resonant circuit is preferably chosen to be high enough
to prevent
interference with ECG measurements and to facilitate blocking 60Hz raise, but
still low enough
18
CA 02571466 2006-12-22
that the high frequency counter CNT can be.implemented using standard
integrated circuit
technology. As this technology is always improving, it is expected that the
upper bound of the
resonant frequency can be increased over time.
Magnetic field effects due to local 60 Hz noise produce a bimodal distribution
of phase
differential due to the sampling frequency being much higher than 60 Hz. An
algorithm within
the CPU cancels out the bimodal distribution and extracts a valid center value
of the measured
delay. In order to enhance the accuracy of this extraction process, the CPU is
supplied with an
ambient 60 Hz zero-crossing disturbance signal "S" produced by a functional
block 63.
An exemplary field parameter for collecting data regarding RFID tags is
electric field
strength, which in particular is discerned as an electric signal emanating
from the RF)D tag. For
this purpose, an antenna optimized for the frequency of the signal transmitted
from the RFID tag
is used in place of or in addition to the loop sensor 20.
The device 10 still further includes a sampling portion 22 having a clock 24.
The
sampling portion 22 outputs a signal in response to the clock 24 that
instructs the sensing portion
1 S 12 to make a measurement and store the measurement in a memory 26. The
clock 24 is also used
to instruct the position sensing portion 14 to register the sensor's
coordinates at the sample time.
It is not essential that the clock 24 output a regular clock signal at a
single frequency; however, at
least a real-time clock base is preferably provided to ensure that samples are
in fact taken
throughout the entire space in which the hand-held device 10 is moved.
Data Collection Methodolouy
The device 10 is enabled by the aforedescribed adaptations to collect data in
space. The
data represent the magnitude of a selected field parameter and are associated
with a given
19
CA 02571466 2006-12-22
location or position within the space. As the device 10 is hand-held and hand-
carried, the
positions in space at which the data are collected are variable and
unpredictable. However, the
methods and apparatus described herein overcome this problem.
Turning to Figure 4, the space 11 is shown partitioned into three-dimensional
cellular
regions or regions 11 a. The cellular regions 11 a are distinct regions of the
space. Preferably, for
computational ease, the cellular regions are defined so as to form a regular,
orthonormal lattice
structure as shown, but this is not essential.
Figure S shows a two-dimensional "slice" 21 of the space 11 shown in Figure 4.
The slice
21 is preferably taken laterally with respect to the patient 13 in Figure 2,
i.e., in the plan of the
Figure, though this is not essential. A path "C" represents the path of the
device 10 as it is
carned through the space 11. While the path "C" is shown lying entirely on the
slice 21 for
illustrative purposes, it should be understood that, in practice, the path "C"
would travel through
the three-dimensional space 11. The path "C" therefore represents a "pass"
through the space 11.
As the device 10 is moved along the path "C", the sampling portion 22 causes
samples of
the field parameter to be taken, i.e., data "A1," "A2," . . . "A12." These
data have associated
positions as indicated in Figure 5 by the placement of the data along the
path. The spacing
between the positions appears randomized due to the fact that the device,
which because it is
moved by hand, is not moved at a constant velocity.
The slice 21 is partitioned into' cellular areas 21 a that correspond to the
three-dimensional
cellular regions 11 a of the space 11 (Figure 4). Particularly, the cellular
areas 21 a may represent
faces of the cellular regions 11 a lying on the x-y axes.
A second path "D" is also shown representing a second pass through the space.
As the
zo
CA 02571466 2006-12-22
device 10 is moved by hand, the second path "D" is not coincident with the
path "C," and
samples of the field parameter are taken at different positions, resulting in
data "B1," "B2," . . .
"810."
According to the invention, all of the aforementioned data are spatially
averaged by
"quantizing" the positions of the data. As used herein, "quantizing" the
positions of data means
that all of the data taken within a given cellular region, which is used as a
fundamental unit of a
measurement space, are associated with and assigned to a single point or
position in the
measurement space.
In the two dimensions shown in Figure 5, the positions are quantized within
the
corresponding cellular areas 21a. For example, the position of the datum"'A1"
is quantized to a
point "P 1" corresponding to the cellular area 21 al (and the cellular region
11 al of Figure 4). The
positions of the data "A2" and "B 1" both fall within the cellular area 21 a2
and both are assigned
a quantized location "P2" corresponding to the cellular area 21a2. The
positions of the data
"A3,"B2," and "B3" each fall within the cellular area 21a4 and are all
assigned the quantized
1 S location "P3." It should be understood that the selection of the
quantization points within the
cellular areas and cellular regions is arbitrary; however, it is preferable to
choose points that are,
like the cellular areas and cellular regions, also regularly spaced from one
another.
It should be understood and emphasized that, while quantization within a slice
is shown
and described for illustrative convenience, in general, quantization is
performed for data
positions falling anywhere within the corresponding cellular region 11 a shown
in Figure 4. That
is, each cellular region 11 a has an associated point "P" at which data
positions falling within that
region are quantized. The data associated with paths "C" and "D" as described
above may
21
CA 02571466 2006-12-22
therefore be viewed as those data associated with the paths "C" and "D"
falling on the slice 21, so
that there are other data having positions falling at other locations within
the space 11 a off the
slice 21 that are not being shown in Figure 5.
In addition to spatially averaging the data positions, the data themselves are
averaged or
otherwise combined. Particularly, data associated with a given quantized data
position are
combined where, preferably, the combination is a simple average. For example,
the data "A2"
and "B1," which are each assigned the quantized position "P2," are averaged
together. A simple
such average would sum the data "A2" and the data "B 1" and divide the result
by two. Similarly,
the data "A3," "B2," and "B3" assigned the quantized position "P3" are
averaged together.
RF1D tag data present a special case that is discussed below in connection
with presence
detection and location determination.
As the data are acquired at randomized locations in the space 1 l, some
cellular regions
11 a will have no data associated therewith, some cellular regions may have
only a single datum
associated therewith, and other cellular regions may have a number of data
associated therewith.
Preferably, cellular regions for which no data are associated therewith are
identified and
discarded in any subsequent analysis. For convenience, hereinafter, it is
assumed that references
to cellular regions for purposes of analysis as described are cellular regions
for which data are
associated therewith.
According to the data collection methodology, discrete data that are
originally gathered at
randomized positions within the space 11 are transformed to combined or
averaged data
corresponding to specific, fixed positions in the space 11. The data thus
transformed and thereby
collected may be used according to the invention for (a) detecting the
presence of a disturbance,
22
CA 02571466 2006-12-22
or (b) locating the disturbance in the space 11.
It is important to appreciate that the data collection methodology provides
for high
sensitivity in the data by providing a novel framework for combining data.
Here, "high
sensitivity" in the data is distinguished from the high sensitivity of the
sensor, which is due to the
very high gain in the sensor's circuits and which makes the sensor highly
sensitive to noise. The
sensor's high sensitivity to noise is compensated for according to the
invention to produce high'
sensitivity in the data to signal by making a large number of passes of the
device through the
space, tolerating the randomness in the positions of acquired data caused by
moving the device
by hand through the use of spatial averaging, and accumulating a large number
of data to be used
in a data averaging which both reduces noise and increases signal. This novel
data collection
methodology thereby provides an unusual capability to "see" weak disturbances.
Even in cases
where the sensor's sensitivity is not or does not need to be exceptionally
high, these techniques
allow a hand-held device to detect reliably relatively weak disturbances
within a very noisy
environment.
Presence Detection
Presence detection according to the present invention may be performed for
both coded
and non-coded disturbances. A coded disturbance is a disturbance having a
specific, known
signature, such as the transmission ofa code from an RFID tag. A non-coded
disturbance is a
disturbance for which a field parameter is measured as a random variable.
Non-Coded Disturbances
For non-coded disturbances, the field for which a parameter is measured by the
sensing
portion 12 may be generated specially for detection purposes, but the
invention contemplates
23
CA 02571466 2006-12-22
collecting data, using as many passes of the device 10 through and about the
space 11 as is
desired to obtain a desired level of sensitivity in the presence of an ambient
or pre-existing field.
According to the data collection methodology, quantized data positions and
combined data are
associated with the cellular regions 11 a, characterizing the space 11 under
ambient conditions.
This characterization may be performed, for example, in an operating room with
the patient
present but before surgical procedures are commenced on the patient. All of
the passes
associated with a completed characterization is termed herein a "scan," and a
scan under ambient
conditions is termed herein a "baseline scan."
However, as surgery is being completed, it is now desired to detect the
presence of any
medical objects unintentionally left behind inside the patient. According to
the invention, the
space is re-characterized in a second, "detection scan" using the device 10 in
the same manner as
before to determine whether~there are any significant changes in field
parameters. For example,
with reference to Figures 6 and 7, there is assumed to be a cellular region 1
lal through which the
device 10 has been separately passed in a BASELINE SCAN, and in a DETECTION
SCAN,
respectively. In the exemplary BASELINE SCAN as shown in Figure 6, three data
points DBs
happen to have been obtained, namely DBS~, DBSZ, and DBS3. Each of the three
data points DB
have associated therewith a measured value (indicated in parentheses in the
Figures), e.g., the
value of the data point DBS~ may be assumed to be 3, the value of the data
point Dssz may be
assumed to be 2, and the value of the data point DBS3 may be assumed.to be 3.
These data are
quantized so as to be associated with a single quantization point Ps
associated with the region,
and combined (here averaged) so that, along with the point Ps, there is
associated a number (3 +
2 + 3)/3 = 2.67.
24
CA 02571466 2006-12-22
In the exemplary DETECTION SCAN as shown in Figure 7, only one data point DDs
happens to have been obtained, namely DDSs. Supposing that a disturbance that
was not present
during the BASELINE SCAN has occurred and is sensed by the device 10 during
the
DETECTION SCAN, the value of the data point Dos, may be assumed to be a
relatively large
value, e.g., 5. Since there is only one data point in the DETECTION SCAN, the
average
associated with the quantization point Ps of the data in this example is 5.00
Finally, with reference to Figure 8, a difference operation ("DIFFERENCE
RESULTS")
is performed between the combined data of the BASELINE SCAN and the combined
data of the
DETECTION SCAN. In the simplest example, the difference result is the
arithmetic difference
between the value associated with the region l lal for the BASELINE SCAN,
i.e., 2.67 in this
example, and the value associated with the same region for the DETECTION SCAN,
i.e., 5.00,
this difference being 2.33 as indicated at the quantization point Ps.
The difference results may be analyzed in a number of ways to decide whether a
disturbance is present. As one example, a disturbance can be detected if the
difference results for
one cellular region, irrespective of the difference results for other regions,
exceeds an absolute
threshold. As another example, the difference results for one cellular region
can be compared to
an average of the difference results for a number of other cellular regions
which can be used to
establish a noise floor. As still another example, difference results for a
cellular region that fail
to exceed the absolute threshold of the first example above may nevertheless
be considered to
indicate the presence of a disturbance if, in combination with exceeding a
decreased threshold,
the difference results for adjacent or nearby cellular regions also exceed a
threshold.
Coded Disturbances
CA 02571466 2006-12-22
Coded data have both a binary aspect and a magnitude aspect. That is, such a
disturbance
produces data that can be considered to be either valid or invalid, and the
signal that carries the
data has a magnitude or strength that diminishes with distance from the signal
source. The
magnitude aspect of coded disturbance data may and preferably is collected and
used in the same
manner as the non-coded disturbance data discussed immediately above. Further,
the magnitude
aspect is used for location determination as described in the section below.
However, a presence
indication for a coded disturbance determined in this manner may be
corroborated using the
binary aspect by identifying the disturbance to confirm the indication, and
therefore to further
enhance the accuracy of the method.
An exemplary and preferred coded disturbance for the medical purposes
discussed herein
is an RF>17 tag. Tn the binary aspect of RFID tag data, the correct code for
the tag is either
present or not. For example, no code may be received, a partial or
unintelligible code may be
received, or a code may be received that does not conform to a known or
expected code
corresponding to an object for which the search is being directed. For
example, a sponge used in
surgery may be tagged with an RF1D tag, but other objects or devices in the
operating room
might also be tagged. The surgeon is interested only in the presence of the
sponge, and so the
method is looking for a unique code.
Such binary aspects of the data may be combined or averaged by counting the
number of
incidences of valid data. For example, within a particular cellular region,
samples indicating that
a proper code is present may be counted to obtain a total presence count.
While instances of
invalid data are ordinarily not counted, such instances may be counted and
subtracted from the
presence count if desired.
26
CA 02571466 2006-12-22
As for the difference results for non-coded disturbances, the presence counts
rnay be
analyzed in a number of ways to decide whether a disturbance is present. As
one example, a
disturbance can be detected if the result of the presence count for one
cellular region, irrespective
of results for other regions, exceeds an absolute threshold. As another
example, the presence
count for one cellular region can be compared to an average of the presence
counts for a number
of other cellular regions which can be used to establish a noise floor. As
still another example, a
presence count for a cellular region that fails to exceed the absolute
threshold of the first example
above may nevertheless be considered to indicate the presence of a disturbance
if, in combination
with exceeding a decreased threshold, the presence counts for adjacent or
nearby cellular regions
also exceed a threshold.
Location Determination
For non-coded disturbances, locating a disturbance may be accomplished
according to the
invention by using the detection scan results as points for triangulating the
disturbance. The
same is true for coded disturbances, using the aforementioned magnitude aspect
of coded
disturbance data. For example, the magnitude aspects of RF)D tag data may be
treated as any
other data associated with a random variable or field parameter. That is, the
strength of the
signal conveying the code of an RF>I? tag has a range of possible values, and
combining or
averaging the data is accomplished in the ordinary manner for location
determination as
described below.
In addition to refining the data according to the novel methodology discussed
above,
preferably, though not necessarily, a triangulation is performed according to
the invention in two
other distinct ways: (1) data points used in a given triangulation are those
lying on a single slice;
27
CA 02571466 2006-12-22
and (2) an indeterminate data set is used for triangulation to generate,
instead of a point location
of the disturbance, a locus of points that are possible locations of the
disturbance. The number of
loci passing through a given cellular region is counted and those cellular
regions containing the
most loci are determined to be most likely locations of the disturbance.
It should be understood, however, that triangulation generally contemplates
any
methodology by which data associated with a plurality of points that are
spaced apart from one
another and that are responsive to a disturbance are used to infer the
location of the disturbance.
Triangulation of a disturbance depends on having data about the disturbance
that are
representative of distance from the disturbance. For fields such as electric
fields, field strength
falls inversely with the square of the distance from the disturbance. For
other disturbances, the
measured field parameter may have some other relationship to distance. For
some disturbances,
the actual distance may be determined from the measured field parameter, but
in general, only
relative distances can be determined. For example, if a field strength is
measured as "five units"
at a first location and "ten units" at a second location, and it is known that
the magnitude of the
measured parameter falls off inversely proportional to the square of the
distance from the
disturbance, then all that can be determined from these two measurements it
that the first location
is four times farther away from the disturbance than the second location. By
contrast, if it is
known how strong the disturbance is, each measurement by itself would be
sufficient to
determine distance from the disturbance.
In general, it is recognized herein that two points lying in a plane and
spaced apart fr om
one another provide a locus of points through triangulation that could
correspond to the location
of the disturbance. Figure 9 illustrates the principle. Assume a measurement
M1 is taken at a
28
CA 02571466 2006-12-22
point PM ~ and a measurement M2 is taken at a point PMZ of a disturbance "E"
that is located at a
point PE.
A number of concentric circles can be drawn around the points PMT and PMZ
respectively.
Each circle, by its radius, represents a possible value of the measured field
parameter caused by
the disturbance E were it located on that circle, For example, a circle C1
about the point PMT
having a radius Rl represents a first possible value of Ml if the disturbance
E lies on the circle
C1. A concentric ciicle C2 having a second radius R2 represents a second
possible value of Ml
corresponding to the disturbance E lying on the circle C2. Where R2 is twice
R1, for an inverse
square relationship to distance, values of M1 corresponding to the disturbance
being somewhere
on the circle C1 would be expected to be four times higher than values of M1
corresponding to
the disturbance being on the circle C2.
Applying a similar set of circles about the point PMZ there is the same
relationship
between the radii of these circles. Moreover, there is the same relationship
between the radius of
circles centered on the point PMT and the radius of circles centered on the
point PMZ~
Possible locations of the disturbance are identified as points where circles
centered on the
point PM, cross circles centered on the point PM2. Assume that the value of M1
is four times the
value of M2 and that there is an inverse square relationship to distance.
Thence, each circle
centered on the point PMT is related to a corresponding circle centered on the
point Paz in that the
radius of the circle centered on the point PMZ is twice the radius of the
corresponding circle
centered on the point PM,. For example, circles C1, C3, and CS centered on the
point PMT
correspond respectively to circles C2, C4 and C6 centered on the point PM2.
More particularly,
the radius of the circle C2 is twice the radius of the corresponding circle C
1; the radius of the
29
CA 02571466 2006-12-22
circle C4 is twice the radius of the corresponding circle C3; and the radius
of the circle C6 is
twice the radius of the corresponding circle C5. Where corresponding circles
cross, a continuous
locus ("LOCUS") of points ("LOC") is defined. Points lying on the locus are
all the possible
locations of the disturbance E according to the measurements Ml and M2.
S The lack of determinance of the location of the disturbance, i.e., the lack
of being able to
locate the disturbance "E" at the point PE, flows from the fact that only two
points were used in
the triangulation. If a third point were used, then the locus would collapse
to .a single point that
is within an error bound of the point PE . To locate the point PE with greater
accuracy, a number
of sets of three measurement points would be used to develop a scatter plot of
possible locations
of the point PE that would, assuming that the measurements exhibited only
random error,
converge on the point PE. In addition to showing convergence, the method
provides an indication
of the error, as the density (or standard deviation) of the results will fall
off with increasing
distance from the median or convergence point PE. Further, the method works
equally well to
discern convergence at multiple points, corresponding to multiple
disturbances.
It has been discovered by the present inventor through computer simulations
that using an
indeterminate set of points to obtain a corresponding locus in combination
with the methodology
described below provides less noisy results than triangulating with
determinate sets of the data.
As a brief review, data points for use in triangulation according to the
invention are
quantized spatially, and data corresponding to locations within a given
quantization space are
combined. For purposes of locating one or more disturbances according to the
invention,
difference results are next obtained on a cell-by-cell basis. The difference
results reflect, for each
cellular region, changes in the measured field parameter within that region
detected in the
CA 02571466 2006-12-22
detection scan with respect to the baseline scan.
Next, the difference results themselves are combined for triangulation in the
manner
described above. Particularly and preferably, various combinations of sets of
two difference
results, each set of two difference results corresponding to two different
cellular regions 1 la, are
used to generate corresponding loci. The loci are curvilinear lines passing
through specific
cellular regions 11 a. According to the invention, a histogram is developed
for each cell region
reflecting the number of counts that a locus passes through the region.
Cellular regions l la
having higher counts are cellular regions in which the location of a
disturbance is more likely.
Assuming that the difference results exhibit only random error, the cellular
region having the
highest count will be the most likely location of the disturbance, and the
decrease in likelihood
exhibited by nearby cellular regions having lesser counts provides an
indication of the error.
Graphical Visualization
Often, as in the case of locating medical objects in medical patients, the
location of a
disturbance is determined for the purpose of retrieving the object. According
to the invention,
the positions already obtained by the device 10 can be of use for this
purpose. Particularly, the
passes of the hand-held device 10 through the space 11 describe curvilinear
lines in the space that
may be used to define a three-dimensional surface. The three-dimensional
suxface can be
displayed on a graphics display device, along with a determined location (or
locations), the
surface providing a useful reference against which the location can be
visualized.
Cross-sections of such a three-dimensional surface may be identified on the
aforedescribed slices. Refernng to Figure 10, a single three-dimensional
surface 30 is defined by
corresponding paths PATH,, PATH2, . . . PATHN (generally "PATH") lying in
corresponding
31
CA 02571466 2006-12-22
parallel slices SLICE, SLICE2, . . . SLICEN (generally "SLICE") of the space
11.
Each PATH is determined by all of the paths lying on the corresponding SLICE.
For
example, Figure 4 shows two paths "C" and "D" corresponding to a SLICE
referred to above as
slice 21. According to the invention, a new PATH would be created for the
slice 21 that splits
the difference between the paths "C" and "D."
Turning to Figure 11, two paths "E" and "F" are shown lying on the slice
SLICE, of
Figure 10. The slice SLICEi is partitioned into cellular areas 21a. The paths
"E" and "F"
represent two separate passes of the hand-held device. Quantized positions for
the two paths
within the cellular areas are shown in Figure 12, the positions of the path
"E" being indicated by
the symbol ~ while the positions of the path "F" are indicated by the symbol
X. Up and down
arrows indicate how the difference between the two paths may be split. Where,
as for the cellular
areas CE, and CE2, corresponding to the path "E," and the cellular areas CE3
and CE4,
corresponding to the path "F," or as for the cellular areas CES and CEO, there
is not a cellular area
that splits the difference, a rule can be invoked to break the tie, e.g., to
select the closest lower-
most cellular area such as shown in Figure 13. Here, a new path PATHS is
created as indicated
by the symbol 0 by combining the paths "E" and "F." Where there are additional
positions
corresponding to additional paths, a weighting may be used. Paths may also be
combined in
other desired ways.
Similarly, additional PATHS can be generated for the remaining SLICES to
construct the
surface indicated in Figure 10. The three-dimensional surface can be used to
provide a graphic
indication of the location of the disturbance determined as described above.
Particularly, the
computer 8 may generate the surface and display, for example, either a
specific location for the
32
CA 02571466 2006-12-22
disturbance, or the histogram counts showing dispersion of the probability
associated with the
disturbance in relation to the surface.
The usefulness of the reference surface as a graphical visualization aid can
be increased
by intentionally moving the device 10 so that it conforms to physical
landmarks. For example,
where the passes of the device 10 are intentionally held in close conformance
to the body of the
patient 13 of Figure 2, e.g., within about 3 inches, the three-dimensional
surface provides a body
contour. A body contour, when displayed along with the location of an
indicated surgical
instrument left inside the same body, provides a best reference for the
surgeon to use in judging
where to enter the body to retrieve the instrument. Close conformance to a
target volume in
which a disturbance is located may be defined generally for this purpose as
being within about 10
- 20% of the extent of the target volume in the direction along which
conformance is being
measured.
All of the aforedescribed processing can be and preferably is performed
outside of the
hand-held device 10, in a remote computer or other data processing device 8
(see Figure 1 and
note that for graphics display, a suitable graphics display device is coupled
to the device 8). This
facilitates portability of the hand-held device. The hand-held device is or
can be coupled to the
computer 8 via a LINK that may be wired or wireless. While data collection and
storage of data
into the memory 26 in the hand-held device occurs in substantially real time,
the processing may
be performed in the computer 8 at a later time and over a longer duration if
necessary. However,
it should be understood that the computational and analytical functionality
described above could
be implemented in any desired combination of hardware and software.
It should be understood that, while a specific method and apparatus for
collecting data for
33
CA 02571466 2006-12-22
use in detecting and locating disturbances has been shown and described as
preferred, other
configurations and methods could be utilized, in addition to those already
mentioned, without
departing from the principles of the invention.
The terms and expressions which have been employed in the foregoing
specification are
used therein as terms of description and not of limitation, and there is no
intention in the use of
such terms and expressions to exclude equivalents of the features shown and
described or
portions thereof, it being recognized that the scope of the invention is
defined and limited only by
the claims which follow.
34
