CA1072222A - Axial tomographic apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In a computerized tomographic radiographic apparatus a source directs a fan distribution of radiation through a patient to be detected by a bank of detector devices. The detector devices are disposed on a circular path centred on an axis about which the source orbits. The detectors are fixed relative to the body so that the radiation is incident on different detectors as the source orbits. Each detector devices receive radiation in different orientations, in the fan distribution, at different times.

Description

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¦ 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 :, . .. ~ ., ~. . .................................. . . . . ........... .

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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.

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: 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 :~ .

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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 :

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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.
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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.

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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.

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¦ 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.

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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.
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¦ 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 ~:
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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.

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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.
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Claims (12)

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The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Radiographic apparatus, for examining a body, including at least one source projecting a fan-shaped distribution of penetrating radiation through a slice of the body, means for angularly moving said 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 different directions, and a plurality of detector devices disposed along a curved path around said body, wherein the detectors are fixed so as to be substantially prevented from angular movement around the body, in the direction of motion of the source, and extend around the curved path 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 device 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.

2. An apparatus according to claim 1 in which each detector device is a scintillator crystal and including a plurality of light sensitive devices co-operating with the scintillators to provide electrical output signals each representing the intensity of radiation incident on one detector device.

3. An apparatus according to claim 2 in which each light sen-sitive device is arranged to receive light emitted by a group of more than one of said crystals, in response to radiation incident thereon; the arrangement being such that only one of said crystals in each such group is irradiated at any time.

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4. An apparatus according to any of claims 1 to 3 in which the said at least one source is arranged to move along a circular path concentric with but of smaller radius than the first mentioned circular path so that it can move between the said axis and some of the detector devices.

5. An apparatus according to Claim 1 including collimator means, arranged to move about the said axis in a fixed relation with said at least one source to reduce the incidence on said detector devices of radiation transmitted from said at least one source along indirect paths.

6. Apparatus according to claim 1 wherein the extent of the fan-shaped distribution is such that the radiation extends beyond the body to provide, at the edge of the distribution, radiation which has not passed through the body and which is successively received by the detectors, as the angular movement proceeds, to provide detector outputs suitable for use in calibration.

7. Apparatus according to claim 1 including collimators moving angularly around the body together with the source relative to the detectors to reduce the incidence on the detectors of radiation transmitted along indirect paths.

8. Radiographic apparatus according to claim 7 in which the at least one source of radiation is a single collimated X-ray tube and the means for angularly moving the source orbits the X-ray tube around the body to be examined.

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9. Radiographic apparatus, for examining a body, including at least one X-ray tube projecting a fan-shaped distribution of X-rays through a slice of the body, means for angularly moving the at least one X-ray tube relative to the body about an axis intersecting the slice to cause the X-ray tube to project radiation through the body from a plurality of different directions, and a plurality of detector devices disposed along a curved path around the body, to an extent which subtends at the axis an angle substantially equal to or greater than 180° and fixed so as to be substantially prevented from angular movement around the body, in the direction of motion of the source, wherein the means for moving moves the said at least one X-ray tube to an extent sufficient to irradiate each of the detector devices with radiation which has traversed the body and the extent of the fan-shaped dis-tribution 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, and including collimators moving angularly around the body together with the X-ray tube relative to the fixed detectors to reduce the incidence on the detectores of radiation transmitted through the body along indirect paths.

10. A medical radiographic system for examining a patient comprising a circular array of X-ray detectors surrounding the patient and an X-ray tube which moves around the patient along a circular path concentric with the detector array and is collimated to emit a fan beam of X-radiation which passes through the patient and simultaneously illuminates a number of said X-ray detectors, : 13 :

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the spread of said fan being more than sufficient to embrace the patient to thereby cause each of said detectors to receive, at least once during examination of a patient, X-radiation directly from the X-ray tube rather than through the patient, said X-ray detectors providing, in response to said directly received X-radiation, : 13a :

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output signals for use as calibration signals for checking the sensitivity of the X-ray detectors.

11. A medical radiographic system as in claim 10 including collimators located between the patient and the X-ray detectors and moving around the patient with the X-ray tube, said collimators reducing the incidence of scattered X-radiation on the X-ray detectors.

12. Radiographic apparatus, for examining a body, including at least one source projecting a fan-shaped distribution of penetrating radiation through a slice of the body, means for angularly moving said at least one source relative to the body, about an axis inter-secting the slice, to cause the source to project the radiation through the body from a plurality of different directions, and a plurality of detector devices disposed along a curved path around said body, wherein the detectors are fixed so as to be substantially prevented from angular movement around the body, in the direction of movemant of the source, and extend around the curved path to an extent which subtends at the axis an angle substantially equal to or greater than 180°, and wherein the extent to which the means for angularly moving moves the at least one source and the extent of the fan-shaped distribution are such that individual detector devices receive, at different times, radiation from the source along paths which traverse the body and paths which do not traverse the body.

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