Note: Descriptions are shown in the official language in which they were submitted.
z~
¦ The present invention relates to radiography, and it
relates more especlally to that branch of radiography which has
become known as computerised axial tomography, or briefly C.A.T.
Apparatus for performing C.A.T. has the aim of evaluating the
absorption coefficient, with respect to the radiation used, at
each of a plurality of locations di`stributed over a planar slice
disposed in a body under examination.
The evaluation is usually performed by suitably process-
ing signals indicative of the absorption suffered by the radia-
tion on traversing each of many substantially linear beam pathsthrough the body in the plane of the slice. To obtain the
required si~nals, it is usual to scan a source of radiation
relative to the body and to detect the radiation emergent from
the side of the body opposite the source whilst the source
assumes many different positions relative to the body, as
described in one example given in United States Patent
No. 3778614 dated December 11, 1977, to G.N. Hounsfield.
If it is desired to a¢quire the signals rapidly, it is
convenient to use a source of a fan-shaped, planar spread of
radiation which encompasses at least a substantial part of the
slice; the planes of the spread of radiation and of the slice
being coincident. Such a spread may be a continuous fan of
radiation or may if desired be split up by collimators between
the body and the source. An array of detector devices is
disposed at the opposite side of the body to the source so that
; each detects the radiation emergent from the body along a
respective beam path, the paths being divergent, and the source
and the detector devices are rotated around the body about a
common axis substantially perpendicular to the planes of the
slice and of the spread of radiation~ so as to provide signals
:, . .. ~ ., ~. . .................................. . . . . ........... .
. . ., . . . .: : ,: :.,. . . : : :
"',.. "','',' ' ' ''; "' ,:,; .' .'.", " ':, ;'~ :. '': ~ '
iO7ZZ~Z
relating to the absorption suffered by the radlation on
traversing further groups of beam paths; signals relating to
many groups of beam paths being obtained on rotation of the
source and the detector devices through for example an angle
S exceeding 180 ~y about the angle of the fan of radiation.
Such a ...........................
,
:
.
., ,. , . , : . ~'
.: . . . :
: 2 ~ 1%~Z~2
technique i~ described and claimed ;n United States Patent Serial
No ~o35647~ dated July 12 1977~ to G.N. Hounsfield and D.J7 Gibbons.
Preferably the signals are sorted into sets relating to substantially
parallel beam paths and are processed~ a set at a time~ by the technique
disclosed and claimed in United States Patent ~o.392~129 dated December
2 1975, to C.A.G~ LeMay~ due allowance being made for the fact that the
parallel beam paths are not uniformly spaced across the sliceO It
~ill be understood that the data need not be sorted into sets of
parallel beam paths provided processing appropriate to fan diskributions
of beam path3 is used.
A difficulty arises, however, due to the tendency of different
detector device~ to drift in gain relative to one another during the
time taken to acquire the ~ignals~ i.e. the 4canning time~ Since a
given detector alway~ provides signals relating to beam paths a~ a
~5 constant perpendicular distance from the axis of rotation~ such drifting
causes the superposition of ring-shaped artifacts upon the av~aluated
coefficients.
It is an object of this invention to reduce the above-mentioned
difficulty.
According to the invention there i8 provided radiographic
apparatus~ for examining a body, including at least one source projectin~
a fan shaped distribution of penetrating radiation through a slice of the
body, means for ~ngularlymoving ~aid at least one source relative to the
body~ about an axis intersecting the slice~ to cause the source to
project the radiation through the body from a plurality of dlfferent
directions~ and a plurality of detector devices dispo~ed along a curved
path around said body, wheFein the detectors are fixed ~o as to be sub-
stantially prevented from angular movement around the body in the
direction of motion of the source~ and extend around the curved path
:~ .
,, -~,.
;
. . : : - : .. .: : . .
' ' ' ' ' , ' ' ' ;, . ' ~ ~, ' , ~ . ' , , :
. . . ..
....
: 3 :
to an extent which subtends at the axis an angle substantially equal
to or greater than 180°, where the means for angularly moving the said
at least one source moves the source angularly to an extent sufficient
to irradiate the detector devices with radiation which has traversed
the body and the extent of the fan-shaped distribution is sufficient
to cause each detector to receive, at least once during the examination,
radiation, from the source, which has not passed through the body.
In order that the invention may be clearly understood and
readily carried into effect, on embodiment thereof will now be
described, by way of example only, with reference to the accompanying
drawings of which
Figure 1 shows, in schematic from elevational view, apparatus
in accordance with one example of the invention,
Figure 2 shows the relationship of collimator baffle plates to
the detector devices, and
Figure 3 is a diagram used to explain the principles of the
collimator baffle plates.
Referring to the drawing, an X-ray tube 1, typically a rotating
anode tube of convention construction, is mounted on an angularly
movable ring 2 so as to irradiate a part 3 of a patient's body. The
tube 1 is arranged to produce a substantially planar, fan-shaped
spread 4 of X-radiation, and the body is positioned so that the part 3,
which represents a cross-sectional slice over which the absorption co-
efficients are to be evaluated is in the plane of the spread 4. The
angular motion of the ring 2 occurs about an axis 5 which is disposed,
in this example, substantially centrally of the body part 3 and is
perpendicular to the plane of the spread 4. The motive force for
effecting the angular movement of the ring 2 is an electric motor 6
which drives a gear wheel 7. The latter co-operates with gear teeth
: 3 :
~7~Z~
formed all around the inner periphery of the ring 2. Motor 6
is mounted on a stationary main frame 8 concentric with the
ring 2 and sufficiently large to enable the body to pass
therethrough in a supine position. The body is supported on
a bed 10, which itself is supported as at 11 on either side
of the scanning gantry, and secured thereto by means of a
strap 12. Packing material 13, which may contain water or
viscous or particulate material in one or more plastic bags,
is placed between the body and the bed 10 in the region of
examination so as to reduce the entrapment of air between
the body part 3 and the bed 10. The material 13 preferably
absorbs the X-radiation to a similar extent as does human
tissue.
The main frame 8 also supports a bank 14 of detector
devices; the devices being disposed on a circular path
concentric with, but of larger radius than, the ring 2, i.e.
centred on axis 5. The array extends over an angle which,
in this example, substantially equals the sum of 180 and the
fan angle. Since the angle of the fan-shaped spread 4 of
radiation is 40 in this example, the extent of the detector
array 14 is approximately 220. This extent i5 necessary in
order that signals may be obtained relatlng to sets containing
I equal numbers of parallel beam paths distributed over sub-
stantially 180 as is required for highly accurate operation
if the signals are to be processed in accordance with the
techni~ue described and claimed in the aforementioned United
States Patent No.3924129. If desired the detector array may
extend over the full 360.
: 4 :
: ~ ., . .. - .
-~ . . . :
:: ,. ' : ' . . .
. .
: . ~.- . :
" '.. ', ' ~ ' ~ ' ' ~ : '',.' . . ,
~ 2~2'~
,.
Each detector device in the array 14 typically com-
prises a scintillator crystal, for example thallium activated
caesium iodide, together with a light sensitive element such
as a photomultiplier tube or a photo diode. Between the det-
ector array 14 and the body is disposed a collimator arrange-
ment 15, 16, to reduce scatter incident upon~the detector
devicesO The element 15 of the collimator arrangement com-
prising a pair of plates disposed parallel to the plane of
the spread 4 of radiation and the element 16 comprising a
baffle consisting of a plurality of collimator plates which
are parallel to one another in one direction and inclined
to the junction lines between adjacent detector crystals as
will be explained in more detail hereinafter. The baffle 16,
while reducing the amount of incident scattered radiation,
; 15 does not define a precise angle of incidence for each indivi-
dual detector. This enables the detector devices to receive
radiation projected along vario~ls beams within the spread 4
as the radiation is scanned over the devices during the angu-
¦ lar movement of the ring 2. The pitch of the baffle plates
1 :
~,~ 20 ; is not necessarily related to the distance between correspond-
. I .
'~ ing parts of adjacent detector devices, however it is typi-
ij `
cally of the same magnitude as or less than the detector pitch.
The detector devic~s in some parts of the array 14
have to be capable of receiving radiation from any angle within i`
the spread 4 and thus each detector is arranged to view the ;
source through an aperture having a 40 field of view.
. : ~
.
j
: 5 :
.
.. . . ~ : , . ..
-` ~o~ z
I
It will be appreciated that allowance has to be made, in
determining the placing of the detector devices, for the fact
that the circular path upon which the detector devices are
located is of larger diameter than the trajectory of the effect-
ive point source of radiation. In one example, 660 detectors
are provided, angularly spaced by 1/3 in relation to axis 5.
In operation, the active scan commences with -the fan
in a position to irradiate a group of detectors at one extreme
of array 14 and the ring 2, and with it tha source 1, is angul-
arly moved around the body part 3 about axis 5. Clearly, as
the angular movement proceeds, the radiation sweeps around
the detector array 14: the output signals provided by the
devices of array 14 being sampled at a rate d~termined by
timing pulses produced by the co-operation of a photocell unit
17, mounted on the stationary frame 8, and a graticule 18
mounted on the ring 2. At regular intervals one detector
device at the rear end of the spread 4 is substituted by a
new detector device at the forward end of the spread 4, so
that samples are at all times provided by the same number o~
detectors. In order to save expense, detectors spaced apart
by more than the fan angle, i.e. detectors which cannot be
irradiated at the same time, can share photo-multipIiers
I and/or subsequent electrical circuits on a time division basis.
The scan is terminated when all detectors have been lrradiated
by radiation which has passed through the body.
.
; . :-;
: 6 :
, '
-,: . .,~: . . . ; . - . :
. . , ,. ~ .- , . . . - :
. , , . , ~ ., ~ .. , , . .: : : :,
- . : . :. ,. , : :
: . . , - , : ,, -
:lLQ~
¦ Such an arrangement i~ shown in the drawing; detectors spaced
apar~ in angle by more than 40 bein~ coupled7 ~ia fibre-optic li~ht
guides such a~ 19 to a common photomultiplier such aq 20 and each
photomultiplier being arranged to feed a respective channt~l ;
comprit~ing an amplifier ~uch a~ 21, an integrator such a~ 22 which
i~ read and re~t periodically by the aforementioned timing pul~es~
an analogue to digital convsrter circuit such as 23 and a logarith~ic
con~erter circuit such as 24. All of the logarithmic con~erter
circuit such as 24 feed a proce~sing circuit 25 which i.e arranged
to -~ort the ~it~nal~ applied thereto into ~ets relating to parallel
beam paths through the body part 3~ to adju-qt the signals ~o take
account of the a~ore~entioned non-u~i~or~ity of spacing of tha
parallel beam path~ and to proce~ thet-~ignals ~o ~orted and adjusted
in a~cordance ~ith the technique de~cribed and claimetd in the
~orementioned Unlted State~ Patent No~3924129 to evaluata the
ab~orption coef~icient at each o~ a plurality o~ loca*ion~
dlstributed o~er the~lice co~pri~ing the body part 3. Pr~ferably
the~ co~ficient~ ~o e~aluated are di~played on a ~isual di~play ~:
~uch as a cathode ~ay-tube 26, which ha~ facilitie~ gor photo-
-~raphin~ the~di pla~ there0n, and al~o supplied to a long term
store 270 ~tt~rc 27 i~ pre~erably a mag~etic tape or di~c ~tore.
The timet di~i~ion multiple~ing of the ~ariou~ photomultiplier~ and
~ubsequent~channel~ of electrical circuits i9 effect~d under the
influence of a timing circuit 2~ which recei~es the aforementioned
25. timing pulses and develops fu~*har timin~ si~nals whi d operate ~ate~
in the circuit ~5 to route the various signal~ to th~ir correct
locations.
.
~ : 7 : .
:' '.
~ ' ' -' ~
: . . . . , , .. , . , , ~ ~ . .:
:: , . . ' . . . : ~ ~
'' ' ' .. : . ' : ' .' : . :
.:
', ' . . ' , : :
.
JZ~%2
I
The arrangement of the ba~fles 16 is shown schematically
in plan view in Figure 2a and, in the same elevation as Figure
l, in Figure 2b. Part of the detector array 14 is also shown.
As in Figure 1, the baffles and detectors are each disposed
on circles centred on the axis 5 and the junctions between
individual detectors are on radii from that axis. The baffles
16 are, however, radial to the oxigin of the X-rays so that
they intercept directly transmitted radiation as little as
possible. For the same reason they are relatively thin. In
this way they allow direct radiation, such as 29, to pass to
¦ the detectors with little loss but tend to intercept scattered
radiation, such as 30. The ~unction between individual
detectors is intended to include a plane lying midway between
adjacent detectors which may not be in actual physical contact.
It is nevertheless not possible to prevent the baffles
16 intercepting at least some radiation directly transmitted
from the source. Furthermore the baffles 16, in the course
of rotation about axis 5, move relative to the detectors 14.
If the output readings from each detector are to be of equal
signifiçance it is necessary to ensure that each detector
loses the same proportion of radiation, to any baffles disposed
in its pathl in each sampling period of an integrator.
Clearly, for baffles paralLel to the junctions between detectors
(i.e. in a directlo~ perp~dicular to the paper in Figure 2b),
the timing of the integrators must be carefully regulated to
achieve that efect. In certain circumstances an error of
timing equivalent to a circumferential movement of the thick-
ness of one baffle plate could lead to an error which may
be unacceptable.
: 8 :
. ~: - . : . . . :. :. ,: : : , ,
,'' : : : . ,: , :' ' ' ' .: ~, ' , '
. . . ~ , . : . . . :
. , . , : : , .. . :
. - ~
¦ The arrangement of Figure 2 therefore disposes the
baffles 16 so that they are inclined to the junctions between
detectors in the said direction. The amount of baffle over-
lying each detector during an integration period is then
constant, despitetiming errors, provided thatthe inter baffle
spacing is not too large.
Figure 3 illustrates the relationship for one detector
crystal 14a, which is shielded by several baffles 16 to the
extent indicated by the solid lines. If the baffles move
relative to the detector to the position indicated by the .:
broken lines it can be seen that the total baffle length,
shielding the detector, is substantially the same.
It will be understood that other shapes and dispositions
of baffles may be used, for example S-shaped or chevron-shaped, .
provided the baffles present substantially the same exit aper-
ture for the radiation to each detector device despite their
relative movement. That effect :cequires that when the pro-
portion of any baffle overlying a detector device increases,
the overlying proportion of another baffle should decrease
to substa~tially the same extent. The pitch of the baffle
must be sufficiently short to give that ef~ect.
Of course baffles may be disposed parallel to the
junction lines between detectors of the integrator timing is
. .
~:~ I precisely controlled as indicated hereinbefore.
?.5 As the fan-shaped spread 4 of radiation is more than ~:
: : sufficient to embrace the breadth of the body part~ .the
: :
examination plane, each detector receives, at least once
: .,
duxing the examination, radiation directly from the source
1. The output signals obtained at these times are used as
calibration signals to chec~ the sensitivity of the detectors.
:. :
: . ' , : ~ ,, , '
,
'' :.", " ' . ~ '
,, : , . . . . . .
z~z
¦ If the body is too large in some or all dimensions to
permit the calibration referred to above to be effected for
all detector devices, an auxiliary source 31 can be mounted
on the ring 2 beyond one extreme of the fan of radiation
and used to irradiate the detector devices directly (i.e.
not through the body) to enable calibration signals to be
obtained. The auxiliary source 31 may be an X-ray tube or
a radioisotope source and may project radiation at the ~ -
detector devices along a single pencil-like beam or along a
fan-like s~read. It is, of course, necessary to take
account of the presence of the auxiliary source when deciding
which detector devices can share photomultipliers etc. If
necessary, the auxiliary radiation can be of different
energy distribution than the main source 1 so that information
relating to the two sources, if fed into a common channel,
can be separated on an energy basis, such separation being
well known in the art.
n a further embodiment of the invention more than one
X-ray source such as 1 may be provided to irradiate the entire
detector array in the course of a lesser angular motion.
t will be understood that suitable collimator arrange- -
ments different than that shown in ~igures 2 and 3~ can be used
in conjunction with this invention.
' I -'; :'
.: :
';
.. ~
.
... . ,; . . . , ., , , ,, . ::
: . . . : - , , : . : , . : . : , : .. .: :
