CA1059654A - Radiography - Google Patents
RadiographyInfo
- Publication number
- CA1059654A CA1059654A CA255,210A CA255210A CA1059654A CA 1059654 A CA1059654 A CA 1059654A CA 255210 A CA255210 A CA 255210A CA 1059654 A CA1059654 A CA 1059654A
- Authority
- CA
- Canada
- Prior art keywords
- radiation
- detectors
- source
- paths
- beams
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Landscapes
- Analysing Materials By The Use Of Radiation (AREA)
- Apparatus For Radiation Diagnosis (AREA)
Abstract
Radiographic apparatus is described for evaluating the absorption coefficient of a body at each of a plurality of locations distributed over a planar slice disposed crosssectionally of the body. A source is arranged to produce a fan-shaped, planar beam of radiation which is directed through the body, in the plane of said slice, and the source is orbited around the body about an axis perpendicular to the plane of said slice. Detectors are provided, and orbited in synchronism with the orbital motion of the source, each to detect the radiation emergent from the body along a plurality of paths. In order to render the paths substantially linear despite the orbital motion, a lateral scan is imposed upon the source which is sufficient, over a predetermined time period, to substantially cancel the orbital motion and replace it by a lateral motion.
Description
~ S9~S~
This invention relates to a method of and apparatus for constructing a representation of the variation of absorption of a planar slice of a body with respect to penetrating radiation such as X- or y- radiation.
One method of and apparatus for constructing such a representation is described in Canadian Patent Specification No.
949,233. According to one example given in that specification a scanning movement is imparted to a suitable source of radiation to provide a plurality of substantially parallel pencil beams of radiation at each of a plurality of inclinations in the plane of the slice. A suitable detector is scanned in a corresponding manner to provide a measure of the absorption suffered by each of the beams in passing through the body. These measurements of absorption are then processed by suitable means to provide a distribution of linear absorption coefficients for the planar slice. To provide the required plurality of beams the source and detector are reciprocated in the plane of the slice and orbited in steps about a common axis normal to that plane.
An alternative processing method involving a form of convolution is further described in the Canadian App~ication No. 198,145.
In our co-pending Canadian Application No. 207,597 there is described a variation of the apparatus of the said ~ana~ian Patent, for the same purpose, having a source arranged to produce a fan shaped beam of radiation having a wide angular spread in the plane of examination. Collimators are provided to divide that beam into a plurality of pencil beams and an array of detectors is provided to detect thé intensity of each of those pencil beams after passage through the body. Scanning , ~ . .
.
~VS9~iiS4 motions as described are further imposed on the source-detector unit. As a result of the lateral scanning movement the array of detectors provides absorption information for a plurality of sets of parallel beams of radiation, the sets being angularly spaced by the angular separation of the beams. Thus the orbital step between each lateral movement is through a relatively large angle. That variation of the apparatus is therefore capable of providing a faster scanning movement than that of the said British Patent. However, for the examination of certain parts of the body it is desirable to further increase the scanning rate.
To that effect our co-pending Canadian Application No.
201,517 describes a method of and apparatus for constructing the said representation in which the angular spread of the fan shaped beam is sufficient to include the whole region of interest in the plane of the body so that a complete scan can be effected solely by orbiting the source and detectors about the common axis.
For both of the arrangements of the said Canadian Applications Nos. 201,517 and 207,597 it is preferable for the orbital motion to be continuous rather than being stepped and occurring between determinations of absorption. Unfortunately, however, such continuous motion results in data being obtained for beam paths which are distorted, as a result of motion in the course of a detector reading, and, in the case of the arrangement of Canadian Application No. 207,597, for sets of beam paths obtained in the course of a lateral scan which are not parallel as is desired but are distributed in the form of a fan.
It is an object of this invention to provide an improved ~L~S9654 arrangement allowing continuous orbital movement for the scanning methods of both of said application.
According to the invention there is provided an apparatus for examining a substantially planar section of a body by means of penetrating radiation such as X- or r- radiation including a source of a fan of radiation lying in the plane of the said section and originating substantially from a point, a plurality of detectors for detecting the radiation after passage through the body along a plurality of beams at different angles within the fan, means for angularly displacing the source and detectors about a common axis perpendicular to the said plane and means for subjecting the said point of origin to a periodic displace-ment, relative to the detectors and in the said plane, such that during each of a series of increments of said angular displacement, said periodic displacement causes a further angular displacement of said beams, substantially equal and opposite to said first mentioned angular displacement to cause each of said beams to remain parallel to its position at the start of the respective increment.
In order that the invention may be clearly understood and readily carried into effect examples will now be described with reference to the accompanying drawings of which:-Figure 1 shows one example of an X-ray apparatus incorporating an embodiment of the invention, Figure 2 shows an X-ray source suitable for use with the invention, Figure 3 illustrates the scanning movements of one example of the invention, Figure 4 illustrates the scanning movements of another .. . . . . ..
.: - ., . : .:, , . : . , ~. .
: . . : . , .
:. : . . i .
5~59~
example of the invention.
Figure 5 i5 a diagram used to explain the organisation of data derived from the example of Figure 4, Figure 6 shows another example of an X-ray apparatus incorporating an embodiment of the invention and Figure 7 shows a simplified Eorm of another embodiment of the invention.
Referring to Figure 1 there is shown therein an apparatus, of the type described in the aforesaid Canadian Application No.
201,517 incorporating one example of the invention. A body 1 to t be examined, shown in transverse section, is supported on a suitably shaped bed 2, also shown in transverse section. A
material 3, having an absorption to the radiation similar to that of body tissue, is positioned between the body 1 and the bed 2, to substantially exclude air from the gap therebetween, and is extended partly about the body to provide an approximately circular cross-section to the radiation. The body is retained firmly in the desired position by means such as a retaining strap 4. If desired a more rigid retaining ring, such as the two part ring described in co-pending Canadian Application No. 201,517 -~
may be used.
The bed 2 and the body 1 are inserted into an aperture 5 in a rotatable member 6 so that a desired part of the body is centred in the aperture. The rotatable member 6 is arranged to rotate about an axis 7, longitudinal of the body 1 and perpendicular to the paper, central to the aperture 5.
For that purpose it is supported by three gear wheels 8 a, b, c, which engage with gear teeth, not shown, cut into the periphery of member 6. The gear wheels 8 are journalled in a main frame 8d of the apparatus which may take any form suitable to support the apparatus and to allow the necessary rotation. Gear wheel !. . ' . .,'; , .,' . i 1 ' ; ' : . ' , .
`` 1~59~5~
8a is driven by a synchronous electric motor 9, also mounted on the main frame, the operation of which will be described hereinafter.
The rotatable member 6 also carries a source of X-rays 10, a bank of detectors 11 and associated collimators 12. The detectors, which in a typical embodiment number 200, can be of any suitable type, for example scintillation crystals with associated photomultipliers or photodiodes.
The source 10 is of the type which includes an elongated target/anode 13, which will be discussed further hereinafter, and provides a fan shaped spread 14 of X-rays from a substantially point origin in plan which can be scanned by electronic means from the position 14a to the position 14b shown. In this example the corresponding scan of the origin of the X-rays along target 13 is of the order of 10 cm although it may be less if desired.
The collimators 12 have longitudinal axes which intersect at the centre of the anode 13/ the axes being angularly spaced by about 1/3 from each other.
In this example the X-ray source 10 is placed of the order of 40cm from the central axis 7 with the detector 11 being placed a further 80cm on the opposite side of axis 7 so as to intercept the radiation of fan 14 for any position of the point of origin of the X-rays in its lateral scan along target 13. The detectors and source preferably lie along arcs of a single circle which, in this example, is not concentric with axis 7. It should be understood that collimators 12 are of dimensions which allow such interception while preventing the reception of scattered reception to the greatest degree practically possible. Although in the example the distance between source 10 and axis 7 is half :~ .
. . .
- .. . . . ..
:: : : . . .. . . .
~. . - . ~ ~ . , ~5~;S~
of that between detector 11 and axis 7, the relationship is for the purpose of obtaining a particularly beneficial result which will be explained hereinafter. If desired the source and detectors may be placed equidistant from the axis or in any other desired relationship.
Disregarding for the moment the rotary motion referred to hereinbefore, the arrangement is such that the point of origin of the X-rays is scanned steadily along target 13, taking the fan of X-rays from 14a to 14b, and is rapidly returned to the starting point before repeating the scan. During the time of one such scanning movement each detector of array 11 provides an output indicative of the intensity of radiation incident thereon. These outputs are amplified in amplifier 15 and then input to integrators 16. There the outputs are integrated over periods determined by a series of pulses from pulse forming circuits 17.
In this example the timin~ of the pulses is such that there are eleven integration periods in the time of one lateral scan of X-ray fan 14 from 14a to 14b. Thus each detector measures radiation in effect along eleven narrow beams joining that detector with eleven equally spaced positions along target 13.
Hereinafter the word beam will be used to denote a beam of radiation incident on a detector and scanned with the source and detectors.
Conversely the path through the body irradiated by a beam, and fixed in relation to the body, will be termed a beam path.
The paths are, of course, of width determined by the integration intervals and are of a shape determined by the geometry of scanning movements in those intervals. For the purposes of illustration, however, they may be considered to be represented 9t;~
by single lines which are in fact their centre lines. The lines illustrating the extremes of fan 14 are in fact the centre lines of the extreme beams of the fan. Signals representing the intensity of radiation received along such paths are converted to digital form in converters 18 and to logarithmic form in converters 19 for output at 20 for further processing. It will be understood that one amplifier 15, integrator 16, A/D converter 18 and log converter 19 is provided for every detector, all operated in synchronism. The processing is effective to sort the signals into sets representing absorption along sets of parallel paths, as will be further explained hereinafter, for processing by a suitable method such as that described in our co-pending Canadian Application No. 198,145 to provide the desired represen-tation. The circuits 15 to 19 are of well known construction.
In order to achieve the effect of the present invention, ' which will be described in detail hereinafter, motor 9 provides a continuous motion of rotat~e member 6 and all the equipment mounted thereon, in the direction shown by the arrow about axis 7 and therefore about the body 1 of the patient on bed 2. The rotary motion and the lateral scanning of X-ray fan 14 must be in a strict relationship to achieve the desired result.
Synchronous motor 9 is driven by a periodic sinusoidal voltage from a power supply 21 and, after a suitable period of time, stabilises in synchronisation with that sinusoidal voltage. It will be appreciated that, when under load, the motion of motor 9 lags the phase of the sinusoidal voltage but this is not significant provided the load does not change and therefore the lag lS constant. The sinusoidal voltage from supply 21 is supplied to a time base generator 28 (Pigure 2) where it provides . , . . .. . i , -: .
. . : - , : .
.. . . ~ ~ .
- : :
. . . . . . .
. . , 9~
a periodic sawtooth waveform voltage, to operate the scanning of source 10, and also to unit 17 which converts it to square pulses of the same phase and generates therefrom the series of pulses, in strict phase relationship with the sinusoidal voltage, to clear and read integrators 16 as explained herein-before. Pulse forming circuit 17 operates in a conventional manner by any suitable means known in the art. Flyback of the sawtooth waveform takes place during selected resetting periods of the integrators.
The X-ray source 10 is shown in greater detail in Figure
This invention relates to a method of and apparatus for constructing a representation of the variation of absorption of a planar slice of a body with respect to penetrating radiation such as X- or y- radiation.
One method of and apparatus for constructing such a representation is described in Canadian Patent Specification No.
949,233. According to one example given in that specification a scanning movement is imparted to a suitable source of radiation to provide a plurality of substantially parallel pencil beams of radiation at each of a plurality of inclinations in the plane of the slice. A suitable detector is scanned in a corresponding manner to provide a measure of the absorption suffered by each of the beams in passing through the body. These measurements of absorption are then processed by suitable means to provide a distribution of linear absorption coefficients for the planar slice. To provide the required plurality of beams the source and detector are reciprocated in the plane of the slice and orbited in steps about a common axis normal to that plane.
An alternative processing method involving a form of convolution is further described in the Canadian App~ication No. 198,145.
In our co-pending Canadian Application No. 207,597 there is described a variation of the apparatus of the said ~ana~ian Patent, for the same purpose, having a source arranged to produce a fan shaped beam of radiation having a wide angular spread in the plane of examination. Collimators are provided to divide that beam into a plurality of pencil beams and an array of detectors is provided to detect thé intensity of each of those pencil beams after passage through the body. Scanning , ~ . .
.
~VS9~iiS4 motions as described are further imposed on the source-detector unit. As a result of the lateral scanning movement the array of detectors provides absorption information for a plurality of sets of parallel beams of radiation, the sets being angularly spaced by the angular separation of the beams. Thus the orbital step between each lateral movement is through a relatively large angle. That variation of the apparatus is therefore capable of providing a faster scanning movement than that of the said British Patent. However, for the examination of certain parts of the body it is desirable to further increase the scanning rate.
To that effect our co-pending Canadian Application No.
201,517 describes a method of and apparatus for constructing the said representation in which the angular spread of the fan shaped beam is sufficient to include the whole region of interest in the plane of the body so that a complete scan can be effected solely by orbiting the source and detectors about the common axis.
For both of the arrangements of the said Canadian Applications Nos. 201,517 and 207,597 it is preferable for the orbital motion to be continuous rather than being stepped and occurring between determinations of absorption. Unfortunately, however, such continuous motion results in data being obtained for beam paths which are distorted, as a result of motion in the course of a detector reading, and, in the case of the arrangement of Canadian Application No. 207,597, for sets of beam paths obtained in the course of a lateral scan which are not parallel as is desired but are distributed in the form of a fan.
It is an object of this invention to provide an improved ~L~S9654 arrangement allowing continuous orbital movement for the scanning methods of both of said application.
According to the invention there is provided an apparatus for examining a substantially planar section of a body by means of penetrating radiation such as X- or r- radiation including a source of a fan of radiation lying in the plane of the said section and originating substantially from a point, a plurality of detectors for detecting the radiation after passage through the body along a plurality of beams at different angles within the fan, means for angularly displacing the source and detectors about a common axis perpendicular to the said plane and means for subjecting the said point of origin to a periodic displace-ment, relative to the detectors and in the said plane, such that during each of a series of increments of said angular displacement, said periodic displacement causes a further angular displacement of said beams, substantially equal and opposite to said first mentioned angular displacement to cause each of said beams to remain parallel to its position at the start of the respective increment.
In order that the invention may be clearly understood and readily carried into effect examples will now be described with reference to the accompanying drawings of which:-Figure 1 shows one example of an X-ray apparatus incorporating an embodiment of the invention, Figure 2 shows an X-ray source suitable for use with the invention, Figure 3 illustrates the scanning movements of one example of the invention, Figure 4 illustrates the scanning movements of another .. . . . . ..
.: - ., . : .:, , . : . , ~. .
: . . : . , .
:. : . . i .
5~59~
example of the invention.
Figure 5 i5 a diagram used to explain the organisation of data derived from the example of Figure 4, Figure 6 shows another example of an X-ray apparatus incorporating an embodiment of the invention and Figure 7 shows a simplified Eorm of another embodiment of the invention.
Referring to Figure 1 there is shown therein an apparatus, of the type described in the aforesaid Canadian Application No.
201,517 incorporating one example of the invention. A body 1 to t be examined, shown in transverse section, is supported on a suitably shaped bed 2, also shown in transverse section. A
material 3, having an absorption to the radiation similar to that of body tissue, is positioned between the body 1 and the bed 2, to substantially exclude air from the gap therebetween, and is extended partly about the body to provide an approximately circular cross-section to the radiation. The body is retained firmly in the desired position by means such as a retaining strap 4. If desired a more rigid retaining ring, such as the two part ring described in co-pending Canadian Application No. 201,517 -~
may be used.
The bed 2 and the body 1 are inserted into an aperture 5 in a rotatable member 6 so that a desired part of the body is centred in the aperture. The rotatable member 6 is arranged to rotate about an axis 7, longitudinal of the body 1 and perpendicular to the paper, central to the aperture 5.
For that purpose it is supported by three gear wheels 8 a, b, c, which engage with gear teeth, not shown, cut into the periphery of member 6. The gear wheels 8 are journalled in a main frame 8d of the apparatus which may take any form suitable to support the apparatus and to allow the necessary rotation. Gear wheel !. . ' . .,'; , .,' . i 1 ' ; ' : . ' , .
`` 1~59~5~
8a is driven by a synchronous electric motor 9, also mounted on the main frame, the operation of which will be described hereinafter.
The rotatable member 6 also carries a source of X-rays 10, a bank of detectors 11 and associated collimators 12. The detectors, which in a typical embodiment number 200, can be of any suitable type, for example scintillation crystals with associated photomultipliers or photodiodes.
The source 10 is of the type which includes an elongated target/anode 13, which will be discussed further hereinafter, and provides a fan shaped spread 14 of X-rays from a substantially point origin in plan which can be scanned by electronic means from the position 14a to the position 14b shown. In this example the corresponding scan of the origin of the X-rays along target 13 is of the order of 10 cm although it may be less if desired.
The collimators 12 have longitudinal axes which intersect at the centre of the anode 13/ the axes being angularly spaced by about 1/3 from each other.
In this example the X-ray source 10 is placed of the order of 40cm from the central axis 7 with the detector 11 being placed a further 80cm on the opposite side of axis 7 so as to intercept the radiation of fan 14 for any position of the point of origin of the X-rays in its lateral scan along target 13. The detectors and source preferably lie along arcs of a single circle which, in this example, is not concentric with axis 7. It should be understood that collimators 12 are of dimensions which allow such interception while preventing the reception of scattered reception to the greatest degree practically possible. Although in the example the distance between source 10 and axis 7 is half :~ .
. . .
- .. . . . ..
:: : : . . .. . . .
~. . - . ~ ~ . , ~5~;S~
of that between detector 11 and axis 7, the relationship is for the purpose of obtaining a particularly beneficial result which will be explained hereinafter. If desired the source and detectors may be placed equidistant from the axis or in any other desired relationship.
Disregarding for the moment the rotary motion referred to hereinbefore, the arrangement is such that the point of origin of the X-rays is scanned steadily along target 13, taking the fan of X-rays from 14a to 14b, and is rapidly returned to the starting point before repeating the scan. During the time of one such scanning movement each detector of array 11 provides an output indicative of the intensity of radiation incident thereon. These outputs are amplified in amplifier 15 and then input to integrators 16. There the outputs are integrated over periods determined by a series of pulses from pulse forming circuits 17.
In this example the timin~ of the pulses is such that there are eleven integration periods in the time of one lateral scan of X-ray fan 14 from 14a to 14b. Thus each detector measures radiation in effect along eleven narrow beams joining that detector with eleven equally spaced positions along target 13.
Hereinafter the word beam will be used to denote a beam of radiation incident on a detector and scanned with the source and detectors.
Conversely the path through the body irradiated by a beam, and fixed in relation to the body, will be termed a beam path.
The paths are, of course, of width determined by the integration intervals and are of a shape determined by the geometry of scanning movements in those intervals. For the purposes of illustration, however, they may be considered to be represented 9t;~
by single lines which are in fact their centre lines. The lines illustrating the extremes of fan 14 are in fact the centre lines of the extreme beams of the fan. Signals representing the intensity of radiation received along such paths are converted to digital form in converters 18 and to logarithmic form in converters 19 for output at 20 for further processing. It will be understood that one amplifier 15, integrator 16, A/D converter 18 and log converter 19 is provided for every detector, all operated in synchronism. The processing is effective to sort the signals into sets representing absorption along sets of parallel paths, as will be further explained hereinafter, for processing by a suitable method such as that described in our co-pending Canadian Application No. 198,145 to provide the desired represen-tation. The circuits 15 to 19 are of well known construction.
In order to achieve the effect of the present invention, ' which will be described in detail hereinafter, motor 9 provides a continuous motion of rotat~e member 6 and all the equipment mounted thereon, in the direction shown by the arrow about axis 7 and therefore about the body 1 of the patient on bed 2. The rotary motion and the lateral scanning of X-ray fan 14 must be in a strict relationship to achieve the desired result.
Synchronous motor 9 is driven by a periodic sinusoidal voltage from a power supply 21 and, after a suitable period of time, stabilises in synchronisation with that sinusoidal voltage. It will be appreciated that, when under load, the motion of motor 9 lags the phase of the sinusoidal voltage but this is not significant provided the load does not change and therefore the lag lS constant. The sinusoidal voltage from supply 21 is supplied to a time base generator 28 (Pigure 2) where it provides . , . . .. . i , -: .
. . : - , : .
.. . . ~ ~ .
- : :
. . . . . . .
. . , 9~
a periodic sawtooth waveform voltage, to operate the scanning of source 10, and also to unit 17 which converts it to square pulses of the same phase and generates therefrom the series of pulses, in strict phase relationship with the sinusoidal voltage, to clear and read integrators 16 as explained herein-before. Pulse forming circuit 17 operates in a conventional manner by any suitable means known in the art. Flyback of the sawtooth waveform takes place during selected resetting periods of the integrators.
The X-ray source 10 is shown in greater detail in Figure
2 and in this example comprises an electron gun 22, powered by a conventional supply now shown, providing a beam of electrons 23 which is incident on target/anode 13 to provide X-ray fan 14. In Figure 2 the elongation of target 13 is perpendicular to the paper so that the X-ray fan 14 is also perpendicular to the paper. Source collimator 24 is provided, as shown, to restrict the X-rays substantially to the plane of the fan, shown dotted at 25 and that is then the plane of a section of the body 1 to be examined. The electron gun and target are enclosed in an evacuated envelope 26 havlng a neck section around which are disposed scanning coils 27. In operation, a suitable time (to allow motor 9 to settle in speed) after power supply 21 is switched on by switch 21s in Figure 1, the time base generator 28 is switched on by a delayed signal from power supply 21. This signal also switches on electron gun 22. The sawtooth voltage from generator 28 scans the points of incidence of the electron beam 23 along target 13 from one end in a direction perpendicular to the paper to scan the X-ray point as shown in Figure 1. Although a pencil - - : :: , . . ; ~ . - . : . . .. . . . .
~q~59~i54 beam of electrons is indicated it will be understood that it may be a ribbon shaped beam used in conjunction with a suitable shape of target 13. Furthermore oil cooling of target 13, although not shown, is preferably provided in conventional manner. Although scanning coils have been shown in Figure 2, deflection pl~es may be used if desired; any configuration of source 10 capable of achieving the scanning of the X-ray fan 14 being suitable for use with the invention. Alternatively any other suitable arrangement for scanning the X-ray fan, in accordance with the principles outlined herein, may be employed.
As described hereinbefore, time base generator 28 provides the scanning sawtooth voltage in conventional manner in phase with the sinusoidal voltage provided by synchronous motor power supply 21 and this maintains the desired relationship between lateral scan and rotary motion. The exact relationship ................................................................
- . ~. . .
`` ~059ti5~
obtained is determined by the gearing of motor 9~ turning member 6 through a predetermined angle for each cycle of the sinu~oidal voltage. Since the sinusoidal voltage is also supplied to pulse forming circuits 179 the inte~ration times are retained in the de~ired relationship with the scanning of X-ray ~an 14 to provide the required effective beam paths.
It has been mentioned that proces~ing~ ~uitable for use with X ray apparatus of the type de~cribed~ such as that disclosed in copendin~ Canadian Application No. 198145 operates preferably on data representing the absorption along a plurality of sets of beam paths in the plane of examination the sets con~eniently being sets of parallel paths. The manner in which the present invention provides such data~ despite the continuous orbital motion inYolved will now be described with reference to Figures 3 and 4. ;
Figure 3 shows in simplified form a scanning arrangement in which the detectors 11 and source anade 13 lie on the same circle which~ unlike the example of Figure 1 is centred on the axis 7~ ~nd are tharefore equidistant from that axis. Thirty four detectors are shown for this simplified arrangement, it bein~ understood that beam path~ of radiation incident on those detector~ will be represented by their centre lines. The source and detectors are illustrated at what may be considered to be an arbitrary starting po ition for the scan, at which the source spot of the X-rays i~ at the extreme right of anode 13 to provide fan 14a as in Figure 1. Con~idering now the effect of the orbital motion and lateral motion of the -- 10 _ '~ - ', "' '. ' , ,., '' , , ,' ' ' ' : , .' ;' ' ', . "' ", ', i' .' ' ' ' ' ' )59~S4 X-ray Yource spot on anode 13 the relationship between them i~ arranged to be as shown by the b~oken lines. In the time in which the spot on anode 13 traverses from extreme right to extreme left of the anode the rotation of rotary member 6 bring~ anode 13 to the po~ition 13' shown broken so that fan 14b emana*e~ from a point, in relation to a fixed frame of reference such ag body 1, which was initially at the centre of anode 13.
The identical orbital motion is, of course applied to the detector~ 11 taking them to the position of the broken line~ ~o that each detector still intercept~ the same beam of the fan 14.
It will be observed that the two extreme beams of fan 14~ and those intermediate but not shown have mo~ed laterally but remain parallel to their original po~itions. Thus they are two beam paths of the parallel set of beam paths which this invention provides. In view of the chosen number of integration intervals~
- in the time of a latera} scan of the X-ray ~ource spot9 a further plurality o~ such parallel beam paths are pro~ided between those shown.
As described hereinbefore the X-ray source ~pot i3 then subject to a rapid 'flyback' taking it to the extreme right of the anode at 13' to provide the fan of X-rays ~ho~n by the chain dotted line. The X-ray source spot i~ then at *he position~which it would have reached in the course of rotation, without the lateral can, and is therefore inclined. It will be ~een however that, while the extreme left hand bea~ takes a new inclination another beam~ in this example ~he fifth from the left identified by reference numeral 29, takes up a po~ition parallel to tho~e previou~ly taken by the extreme bea~. A similar change applie~
to all other beam in the fan. In the cour~e of the followin~
.; .
~4~`~
. j . .... ~ ~ . -- 11 ~ , - :
-.: :- .. ~ .
.:
.
` ~.059~;~4 lateral X-ray spot scan and orbital movement, the fan 1~ moves to the position from which fan 14a originated but at the new inclination due to the inter~ening movement of detectors 11.
In that time beam 29 move~ to the position of the e~treme left hand beam of fan 1~a remaining at the same inclination as deffcribed hereinbefore. Thus beam 29 provide~ data for further beam paths of the parallel set started by the e~treme left hand beam~ In the course of further lateral scans of the X-ray source sp~t other detectors will contribute data to this~? and other, parallel sets extending them, as required, completely across the region including body 1.
It will be observed that at junctions between parts of a parallel set, such as that between the paths examined by beam 29 and those examined by the extreme left hand beam, a path iff examined twice by two beams. In practice, because of the finite time of a flyback of the source spot the two beams will not exa~ine exactly the same path thus denying a valuable check between the sensitivities of the different detectors.
In order to provide the overlap, between portions of a parallel set provided by dif~erent detectors, which allows such comparison~
between detectars, the preferred embodiment of Figure 1 employs the unequal spacing of source and detector ? which is shown in simplified form in Figure ~.
In that Figure the arrangement has been ~hown to be substantially identical to that of Figure 3 except that the source anode 13 and detectors lies on a circle whose centre 7' is displaced from axis 7~ The di~tance between axis 7 and the detector is then twice that between axiff 7 and the source, i.e. as shown in Figure 1. In practice to provide cleara~ce of the body ~'? 1 it is likely that the detectors would be moved to twice the radius ~f : . ; . . , -)59~iS~
a Figure 3 arrangement. However the difference from ~igure 4 would then merely be one of scale. The scanning motion~ of the Figure l~
arrangement are sub~tantially the sam~ as those explained in relation to Figure 3, the overlap being produced entirely as a result of the geometry adopted. In the course of the initial orbital motion the X-ray source spot i~ scanned from the extreme right to the extreme lef* of anode 13 to maintain the beam~ of fan 14 at the same inclination to the fixed frame of reference, despite the orbital motion~ as explained hereinbefore. If the detector~
are at the same radius a~ in Figure 3 the apgular motion to be compensated will be substantially the ~ame. The source anode 13 will now be at half the radius of the ànode of Figure 3 and it will not move through the same distance for the same angular changeO Consequently instead of the source spot from the left hand end of the anode1 at 13', moving a distance equal to half of the anode length, as in Figure 3, it now move~
a distance equal to two third~ of the anode length, namely to po ition 30. As before a parallel set of data has been provided by the traverse and orbit combined.
As before the spot 'flies back~ to the extreme right of the anode, at 13' 9 giving a new fan, indicated by the wide chain dotted lines and at a new inclination. Also as before one beam, 29 which in this case is the fourth from the left is parallel to the previous position~ of the extreme lefthand beam and commences a new portion of the parallel set. However the spot now reaches the end of tha anode two thirds of the way along po~ition 13'~ namely at 30' to pro~ide a b~am along path 29' indicated by the fine shain dotted line, to the fourth detector w~lich is displaced a~ shown. As before flyback follows this to .
~ .
.
, ~ , .
:, ~ . - '. , 96~i~
` - 14 -start the next of the series of lateral scanning movements. It will be apparent however that the contributions to the parallel set between the extreme left hand beam paths of 14a-and b are o~erlapped by those between 29 and 29'. Thus the sensitivities of, in this example, the first and fourth detectors on the left can be compared for all beam paths between 29l and the extreme left hand beam of l4a~ The same is true for all other detectors of the fan and all other contributions to the parallel sets by further lateral scans of the source spot.
As for the Figure 3 arrangement finite time flyback will prevent the full attainment of the overlap shown in the Figure but in thi~ example overlap of several beams is still provided. In the course of many lateral source spot scan~ the arrangeme~t provides that all beam paths, except some at extremes of e~ch parallel set~ are sxamined by two detectors and th~
detailed geometry may readily be adJusted to provide this despite the finite flyback time. The motion of source and detectors can be described as a rotation about the centre 7' of the circle on which they lie, plus a precession of that centre about axi~ 7. It will be understood that~ because of this precession of source and detector circle, caused by t~e off-centre axis~ the beam path~ described as identical will be different at the extremes of each baam near to the source and detectors. However slnce they will be substantially identical in the region of the body l the conditions required are suitably satisfied. The precession does not otherwise afPect the scanning motion.
If required the source may be placed intermediate the positions of Figures 3 and 4. It will be apparent that placing the source of Figure 3 at a slightly reduced distance from axis 7 will give sufficlent overlap to negate the effects on that arran~ement of the finite flyback time.
~ - ` ` ~ i!` ~ ;
-` ~o596b4 It will ~e seen from the arrangements of Figures 3 and ~ that many other arrangements, of source and detector po~itions, detector numbers fan spread angle etc. may be provided such that the ba~ic principle of the invention is satisfied, namely that the lateral scan of the source spot i~ sufficient~ over a predetermined number of inte~ration intervals to substantially cancel the orbital motion of the X-ray fan so that the orbital motion is replaced in effect by a lateral displacement.
It should be understood that the arrangement of Fi~ure 4 is simplified for the purpose~ of explanation although the relative radii of source and detectors correspond to the preferred embodiment of ~i~ure 1. The numbers and relative spacing of the detectors in Figure l~
are not those of the preferred embodiments.
The geometry of the relati~nships of the two scanning motions of the apparatus are predetermined, although provision may be made to vary them if desired. Consequently the beam paths which will be examined by the detectors in the course of a complete scan are known in advance and are supplied to predetermined location~ of a store, in this example a random acce~s memory (RAM) store, in response to the signals, from timing circuits 17, which determine the integration intervals. Each location in the store, representing a beam path may be identified by a suitable co-ordinate Aystem, such as r~ defined as the distance of the beam path from axis 7 along a perpendicular to the path~ and ~, defined as the angle which that perpendicular makes with an arbitrary zero.
The store is indicated schematically at 31 in Figure 50 The data supplied at 20 on individual circuit path~ from log con~erters 19 are applied to an address selector 32 which~ in response to the pulses from circuits 17 supplies them to the appropriate addresses in the predetermined sequence.
., . . : ... , ~ , . . ...
' . .: ' , ~ ,', , .: . " , , :: . , ; :
~` ~05~
appropriate to a beam path and arranged in terms of the appropriate co-ordinates r and e for the purposes of illustrating the distribution of data. It will be understood, however~ that the data need not, in practice, retain any particular physical distribution in store 31 provided the respecti~e locations are known. For the purposes of illustration five integration intervals have been assumed for each lateral scan of the source spot~ instead of the larger number used in practice. In view of the finite flyback time of the sourca spot a complete overl~p is not achieved. The flyback time is, however, arranged to be equal to one integration interval so that the beams of successi~e parts of a parallel set are displaced by an exact number of beam paths and remain in registration in the parts which do overlap. The data have been identified by the numbers of detectors 11 by which they are derived, the numbering starting at detectors 111 at the extreme left in ~igure 4.
In the course of the first scan of the source spot detector 11~ provides data for five beam paths at constant 0 but at increasing r, for the co-ordinates chosen, and the data are ~ead by address selector 32 into the appropriate five locations in succession as they are derived. Simultaneou~ly the data for the other detectors are read into locations for other sets at constant 0, displaced from those of 111 as shown. Those for detectors beyond 114 cannot be seen in the part of the storage matrix illustrated in Figure 5. During the next scan of the source spot~ detector 114 provides data for beam paths at th~
angle 0 appropriate to paths previously examined by 111 but - 16 _ -`- ~V~9S5a~
.
start mg at a higher value of r. These are al~o read into their appropriate locations in qequence, as indicated by 11l'.
It will be seen that two of these locations already contain data for 111. The new data are added to the previous data by any appropriate means, such as recirculation and adding of the previous data. Simultaneously data for the other detectors are read into the location~ indicated by the primes. At the next lateral scan of the source spot a further change by three detector~, for a value of e, i~ made a~ shown by 117" and 118".
It will be observed that, a~ a result of the displacement of beam path~ provided by the finite flyback time, each beam path will be examined by two detectors, with the exception of some at the end of the scan. In the absence of flyback delay, those beam paths in the region of overlap would have been examined by three de~ectors. The data for each beam path may merely be combined if desired. Data for these paths being examined only once may be doubled~ discarded or their acqui~ition may be avoided by suitable tailoring of the start and finish of the scan. Alternatively data added into an occupied location may be averaged with those already stored.
The provi~iion of data for two detectorq for each beam path allow~ the reduction of errors due to relative differences of ~enqit-ivities of adjacent detectors. Furthermore, since the dete~tors used change sequentially acroi~s a parallel qet~ differences acrosq a parallel set tend to be smoothed out to the benefit of the finally derived represeDtation. However, as a further refinement the fact that two detector2 examine the same path~
': : -- . ';
.: : . . ....................... . . ; : . ...
': ' , '' " '. ' ', ': ' ' ~` ' ' ' `' ~ ~ .
)59654 .
for which they should give the 3ame value~ allol~ the possibility of adjusting the gains of their respective amplifiers to equate their ~ensitivities. Such equalising may be carried right across the bank of detectors 11 by virtue of the successive overlap~.
At the conclusion of the complete scanning motions the store 31 contain~ data for a pl~rality of parallel set~ of beam pathsi at different inclination~, as required~ These are then suitable for the aforementioned proiressing and are read out in sequence to a proce~sing computer 33 for processing for example as described in co-pending Canadian Application No. 198145.
It has been mentioned that the invention can also be used with an apparatus of the form described in co-pending Canadian application No. 207597 to eliminate the stepping of the orbital motion deYcribed in that application. The arrangement described therein employs a fan of radiation of relatively narrower angle together with a mechanical lateral motion of source and detectors to scan the fan across the body to achieve a required number of beam paths. It will be apparent that such a lateral scan, - 20 considered in rela$ion to the motion of a single beam of the fan merely imposes a motion at constant e. Thus the principle of the invention, that the scan of the X-ray source spot temporarily negates the change of ~ imposed by the orbital motion, is not affected by the extra source-and detector lateral scan. The description of the relationship according to the invention given hereinbefore is therefore still appropriate to this other form of the apparatus although the effects of the superimposed con~tant ~ motion should be considered when allocating the derived A
~ 6~i~
data to their appropriate storage locations.
There is shown in Figure 6 an example of an apparatus according to the said Canadian Application No. 207,597 in which features common to the arrangement of Figure 1 have been indicated by the same reference numerals. The rotary member 6, in this example, carries also two compensating members 34 and 35. These members are arranged to provide a substantially uniform absorption to the radiation for all beam paths of a source detector scan despite the substantially circular cross-section provided by the body 1. Thus it is ensured that anyvariations of absorption are caused substantially only by variations in the body 1. Such compensating members could also be provided for the Figure 1 arrangement if desired.
Also secured to the member 6 is a reversible motor 36 which drives a toothed belt 37 by means of a drive shaft 38 journalled in member 6. The belt 37 also passes over an idler wheel 39 also journalled in member 6. Secured to the belt 37 is the source 10, which is of the type described hereinbefore. The source is driven to and fro laterally by the belt 37, being mounted on a bearing travelling in a track 40. A counter balance weight 41 is fixed to the opposite side of belt 37 to compensate for out of balance forces during the lateral movement.
Linked to the source 10 by a light weight but rigid yoke 42 is the collimator/detector unit 12/11. The detectors 11 and collimators 12 also move on a bearing on a track 43 on member 6.
As an alternative to the Figure 1 arrangement for controlling the relative motions there is provided on turntable 6 a graticu~e 44 (only shown in part) comprising a translucent strip carrying engraved lines. The lines interrupt a light path between a , . - :, ; :
`~
65~
light source and a photocell in unit 45 to provide pulses indicating the progress of the orbital movement. These pulses are used to cause periodic reversals o~ motor 36 in accordance with the required relationship. Yoke 19 carries a similar graticule 46 cooperating with light source and photocell unit 47 to provide signals indicative of the progress of the lateral scan of source 10 and detectors 11. These pulses are applied to the integrators 16 to control the integration intervals and to source control/time base generator 28 to control the scan of the source spot along anode 13 in the required relationship.
The pulses may also be applied to a counter, not shown, to record the progress of the scan for the purpose of any processing or timing required.
It will be apparent that a photocell graticule combination could also be used to provide timing pulses for the Figure 1 arrangement in which case pulses from a combination such as 44/45 monitoring the rotary motion would be applied directly to integrators 16 and source control 28.
In a further embodiment of the invention Figure 7 shows a means for obtaining the required motion of the X-ray beams with-out the use of a scanning anode X-ray source such as has been described hereinbefore. ~-In Figure 7 the detectors 11 are shown in the form of individual scin*illator crystals 48 and photomultipliers 49.
The X-ray source 10 is in the embodiment a single point source of X-rays incident on the crystals 48. For simplicity only four typical detectors have been shown. A plurality of collimators 50 are provided on a ring 51 which rotates about an axis 52 which is fixed in relation to the source and detectors.
5~ 4 In the case of an apparatus such as that shown in Figure 1 axis 52 is identical with axis 7. However for an apparatus such as that shown in Figure 6 these are not the same, as shown in figure 7, since axis 52 moves with the yoke 42.
Each collimator 50, of which only two are shown, is of a width suitable for defining a penc:il beam, although in this embodiment each detector is significantly larger. The collimator ring 14 is arranged to rotate in the opposite direction to the rotation of the source and detectors and at such a speed that it matches the orbital motion of the source. Thus while each collimator passes across a detector it remains in a relationship with source 10 such thatthe pencil beam defined is subject to no rotation but a translational motion is substituted. It will be appreciated that this is the same effect described hereinbefore for the scanning anode source. As the collimator moves to the next detector the following collimator takes its place. Thus there is a sudden angular change corresponding to the flyback of the scanning anode source.
It will be appreciated that the collimators at the opposite side of the ring 51 can pass between the source 10 and the body 1. The ring 51 must, therefore, be slightly inclined in relation to the plane of the slice so that such interference is avoided.
It should further be noted that other collimators which are fixed relative to X-ray source 10 can be provided as close as possible to the body 1 so that as far as possible the body is not subjected to radiation which will not be accepted by collimators 50.
Alternatively collimators 50 may be placed so as to pass ~5~6S~
between the body and the source instead of between the body andthe detectors. In that case the relative motion described would achieve the same effect, however, the body would not be subjected to radiation which is not to be detected.
- -: . . ~ . . .. . : . . .
~q~59~i54 beam of electrons is indicated it will be understood that it may be a ribbon shaped beam used in conjunction with a suitable shape of target 13. Furthermore oil cooling of target 13, although not shown, is preferably provided in conventional manner. Although scanning coils have been shown in Figure 2, deflection pl~es may be used if desired; any configuration of source 10 capable of achieving the scanning of the X-ray fan 14 being suitable for use with the invention. Alternatively any other suitable arrangement for scanning the X-ray fan, in accordance with the principles outlined herein, may be employed.
As described hereinbefore, time base generator 28 provides the scanning sawtooth voltage in conventional manner in phase with the sinusoidal voltage provided by synchronous motor power supply 21 and this maintains the desired relationship between lateral scan and rotary motion. The exact relationship ................................................................
- . ~. . .
`` ~059ti5~
obtained is determined by the gearing of motor 9~ turning member 6 through a predetermined angle for each cycle of the sinu~oidal voltage. Since the sinusoidal voltage is also supplied to pulse forming circuits 179 the inte~ration times are retained in the de~ired relationship with the scanning of X-ray ~an 14 to provide the required effective beam paths.
It has been mentioned that proces~ing~ ~uitable for use with X ray apparatus of the type de~cribed~ such as that disclosed in copendin~ Canadian Application No. 198145 operates preferably on data representing the absorption along a plurality of sets of beam paths in the plane of examination the sets con~eniently being sets of parallel paths. The manner in which the present invention provides such data~ despite the continuous orbital motion inYolved will now be described with reference to Figures 3 and 4. ;
Figure 3 shows in simplified form a scanning arrangement in which the detectors 11 and source anade 13 lie on the same circle which~ unlike the example of Figure 1 is centred on the axis 7~ ~nd are tharefore equidistant from that axis. Thirty four detectors are shown for this simplified arrangement, it bein~ understood that beam path~ of radiation incident on those detector~ will be represented by their centre lines. The source and detectors are illustrated at what may be considered to be an arbitrary starting po ition for the scan, at which the source spot of the X-rays i~ at the extreme right of anode 13 to provide fan 14a as in Figure 1. Con~idering now the effect of the orbital motion and lateral motion of the -- 10 _ '~ - ', "' '. ' , ,., '' , , ,' ' ' ' : , .' ;' ' ', . "' ", ', i' .' ' ' ' ' ' )59~S4 X-ray Yource spot on anode 13 the relationship between them i~ arranged to be as shown by the b~oken lines. In the time in which the spot on anode 13 traverses from extreme right to extreme left of the anode the rotation of rotary member 6 bring~ anode 13 to the po~ition 13' shown broken so that fan 14b emana*e~ from a point, in relation to a fixed frame of reference such ag body 1, which was initially at the centre of anode 13.
The identical orbital motion is, of course applied to the detector~ 11 taking them to the position of the broken line~ ~o that each detector still intercept~ the same beam of the fan 14.
It will be observed that the two extreme beams of fan 14~ and those intermediate but not shown have mo~ed laterally but remain parallel to their original po~itions. Thus they are two beam paths of the parallel set of beam paths which this invention provides. In view of the chosen number of integration intervals~
- in the time of a latera} scan of the X-ray ~ource spot9 a further plurality o~ such parallel beam paths are pro~ided between those shown.
As described hereinbefore the X-ray source ~pot i3 then subject to a rapid 'flyback' taking it to the extreme right of the anode at 13' to provide the fan of X-rays ~ho~n by the chain dotted line. The X-ray source spot i~ then at *he position~which it would have reached in the course of rotation, without the lateral can, and is therefore inclined. It will be ~een however that, while the extreme left hand bea~ takes a new inclination another beam~ in this example ~he fifth from the left identified by reference numeral 29, takes up a po~ition parallel to tho~e previou~ly taken by the extreme bea~. A similar change applie~
to all other beam in the fan. In the cour~e of the followin~
.; .
~4~`~
. j . .... ~ ~ . -- 11 ~ , - :
-.: :- .. ~ .
.:
.
` ~.059~;~4 lateral X-ray spot scan and orbital movement, the fan 1~ moves to the position from which fan 14a originated but at the new inclination due to the inter~ening movement of detectors 11.
In that time beam 29 move~ to the position of the e~treme left hand beam of fan 1~a remaining at the same inclination as deffcribed hereinbefore. Thus beam 29 provide~ data for further beam paths of the parallel set started by the e~treme left hand beam~ In the course of further lateral scans of the X-ray source sp~t other detectors will contribute data to this~? and other, parallel sets extending them, as required, completely across the region including body 1.
It will be observed that at junctions between parts of a parallel set, such as that between the paths examined by beam 29 and those examined by the extreme left hand beam, a path iff examined twice by two beams. In practice, because of the finite time of a flyback of the source spot the two beams will not exa~ine exactly the same path thus denying a valuable check between the sensitivities of the different detectors.
In order to provide the overlap, between portions of a parallel set provided by dif~erent detectors, which allows such comparison~
between detectars, the preferred embodiment of Figure 1 employs the unequal spacing of source and detector ? which is shown in simplified form in Figure ~.
In that Figure the arrangement has been ~hown to be substantially identical to that of Figure 3 except that the source anode 13 and detectors lies on a circle whose centre 7' is displaced from axis 7~ The di~tance between axis 7 and the detector is then twice that between axiff 7 and the source, i.e. as shown in Figure 1. In practice to provide cleara~ce of the body ~'? 1 it is likely that the detectors would be moved to twice the radius ~f : . ; . . , -)59~iS~
a Figure 3 arrangement. However the difference from ~igure 4 would then merely be one of scale. The scanning motion~ of the Figure l~
arrangement are sub~tantially the sam~ as those explained in relation to Figure 3, the overlap being produced entirely as a result of the geometry adopted. In the course of the initial orbital motion the X-ray source spot i~ scanned from the extreme right to the extreme lef* of anode 13 to maintain the beam~ of fan 14 at the same inclination to the fixed frame of reference, despite the orbital motion~ as explained hereinbefore. If the detector~
are at the same radius a~ in Figure 3 the apgular motion to be compensated will be substantially the ~ame. The source anode 13 will now be at half the radius of the ànode of Figure 3 and it will not move through the same distance for the same angular changeO Consequently instead of the source spot from the left hand end of the anode1 at 13', moving a distance equal to half of the anode length, as in Figure 3, it now move~
a distance equal to two third~ of the anode length, namely to po ition 30. As before a parallel set of data has been provided by the traverse and orbit combined.
As before the spot 'flies back~ to the extreme right of the anode, at 13' 9 giving a new fan, indicated by the wide chain dotted lines and at a new inclination. Also as before one beam, 29 which in this case is the fourth from the left is parallel to the previous position~ of the extreme lefthand beam and commences a new portion of the parallel set. However the spot now reaches the end of tha anode two thirds of the way along po~ition 13'~ namely at 30' to pro~ide a b~am along path 29' indicated by the fine shain dotted line, to the fourth detector w~lich is displaced a~ shown. As before flyback follows this to .
~ .
.
, ~ , .
:, ~ . - '. , 96~i~
` - 14 -start the next of the series of lateral scanning movements. It will be apparent however that the contributions to the parallel set between the extreme left hand beam paths of 14a-and b are o~erlapped by those between 29 and 29'. Thus the sensitivities of, in this example, the first and fourth detectors on the left can be compared for all beam paths between 29l and the extreme left hand beam of l4a~ The same is true for all other detectors of the fan and all other contributions to the parallel sets by further lateral scans of the source spot.
As for the Figure 3 arrangement finite time flyback will prevent the full attainment of the overlap shown in the Figure but in thi~ example overlap of several beams is still provided. In the course of many lateral source spot scan~ the arrangeme~t provides that all beam paths, except some at extremes of e~ch parallel set~ are sxamined by two detectors and th~
detailed geometry may readily be adJusted to provide this despite the finite flyback time. The motion of source and detectors can be described as a rotation about the centre 7' of the circle on which they lie, plus a precession of that centre about axi~ 7. It will be understood that~ because of this precession of source and detector circle, caused by t~e off-centre axis~ the beam path~ described as identical will be different at the extremes of each baam near to the source and detectors. However slnce they will be substantially identical in the region of the body l the conditions required are suitably satisfied. The precession does not otherwise afPect the scanning motion.
If required the source may be placed intermediate the positions of Figures 3 and 4. It will be apparent that placing the source of Figure 3 at a slightly reduced distance from axis 7 will give sufficlent overlap to negate the effects on that arran~ement of the finite flyback time.
~ - ` ` ~ i!` ~ ;
-` ~o596b4 It will ~e seen from the arrangements of Figures 3 and ~ that many other arrangements, of source and detector po~itions, detector numbers fan spread angle etc. may be provided such that the ba~ic principle of the invention is satisfied, namely that the lateral scan of the source spot i~ sufficient~ over a predetermined number of inte~ration intervals to substantially cancel the orbital motion of the X-ray fan so that the orbital motion is replaced in effect by a lateral displacement.
It should be understood that the arrangement of Fi~ure 4 is simplified for the purpose~ of explanation although the relative radii of source and detectors correspond to the preferred embodiment of ~i~ure 1. The numbers and relative spacing of the detectors in Figure l~
are not those of the preferred embodiments.
The geometry of the relati~nships of the two scanning motions of the apparatus are predetermined, although provision may be made to vary them if desired. Consequently the beam paths which will be examined by the detectors in the course of a complete scan are known in advance and are supplied to predetermined location~ of a store, in this example a random acce~s memory (RAM) store, in response to the signals, from timing circuits 17, which determine the integration intervals. Each location in the store, representing a beam path may be identified by a suitable co-ordinate Aystem, such as r~ defined as the distance of the beam path from axis 7 along a perpendicular to the path~ and ~, defined as the angle which that perpendicular makes with an arbitrary zero.
The store is indicated schematically at 31 in Figure 50 The data supplied at 20 on individual circuit path~ from log con~erters 19 are applied to an address selector 32 which~ in response to the pulses from circuits 17 supplies them to the appropriate addresses in the predetermined sequence.
., . . : ... , ~ , . . ...
' . .: ' , ~ ,', , .: . " , , :: . , ; :
~` ~05~
appropriate to a beam path and arranged in terms of the appropriate co-ordinates r and e for the purposes of illustrating the distribution of data. It will be understood, however~ that the data need not, in practice, retain any particular physical distribution in store 31 provided the respecti~e locations are known. For the purposes of illustration five integration intervals have been assumed for each lateral scan of the source spot~ instead of the larger number used in practice. In view of the finite flyback time of the sourca spot a complete overl~p is not achieved. The flyback time is, however, arranged to be equal to one integration interval so that the beams of successi~e parts of a parallel set are displaced by an exact number of beam paths and remain in registration in the parts which do overlap. The data have been identified by the numbers of detectors 11 by which they are derived, the numbering starting at detectors 111 at the extreme left in ~igure 4.
In the course of the first scan of the source spot detector 11~ provides data for five beam paths at constant 0 but at increasing r, for the co-ordinates chosen, and the data are ~ead by address selector 32 into the appropriate five locations in succession as they are derived. Simultaneou~ly the data for the other detectors are read into locations for other sets at constant 0, displaced from those of 111 as shown. Those for detectors beyond 114 cannot be seen in the part of the storage matrix illustrated in Figure 5. During the next scan of the source spot~ detector 114 provides data for beam paths at th~
angle 0 appropriate to paths previously examined by 111 but - 16 _ -`- ~V~9S5a~
.
start mg at a higher value of r. These are al~o read into their appropriate locations in qequence, as indicated by 11l'.
It will be seen that two of these locations already contain data for 111. The new data are added to the previous data by any appropriate means, such as recirculation and adding of the previous data. Simultaneously data for the other detectors are read into the location~ indicated by the primes. At the next lateral scan of the source spot a further change by three detector~, for a value of e, i~ made a~ shown by 117" and 118".
It will be observed that, a~ a result of the displacement of beam path~ provided by the finite flyback time, each beam path will be examined by two detectors, with the exception of some at the end of the scan. In the absence of flyback delay, those beam paths in the region of overlap would have been examined by three de~ectors. The data for each beam path may merely be combined if desired. Data for these paths being examined only once may be doubled~ discarded or their acqui~ition may be avoided by suitable tailoring of the start and finish of the scan. Alternatively data added into an occupied location may be averaged with those already stored.
The provi~iion of data for two detectorq for each beam path allow~ the reduction of errors due to relative differences of ~enqit-ivities of adjacent detectors. Furthermore, since the dete~tors used change sequentially acroi~s a parallel qet~ differences acrosq a parallel set tend to be smoothed out to the benefit of the finally derived represeDtation. However, as a further refinement the fact that two detector2 examine the same path~
': : -- . ';
.: : . . ....................... . . ; : . ...
': ' , '' " '. ' ', ': ' ' ~` ' ' ' `' ~ ~ .
)59654 .
for which they should give the 3ame value~ allol~ the possibility of adjusting the gains of their respective amplifiers to equate their ~ensitivities. Such equalising may be carried right across the bank of detectors 11 by virtue of the successive overlap~.
At the conclusion of the complete scanning motions the store 31 contain~ data for a pl~rality of parallel set~ of beam pathsi at different inclination~, as required~ These are then suitable for the aforementioned proiressing and are read out in sequence to a proce~sing computer 33 for processing for example as described in co-pending Canadian Application No. 198145.
It has been mentioned that the invention can also be used with an apparatus of the form described in co-pending Canadian application No. 207597 to eliminate the stepping of the orbital motion deYcribed in that application. The arrangement described therein employs a fan of radiation of relatively narrower angle together with a mechanical lateral motion of source and detectors to scan the fan across the body to achieve a required number of beam paths. It will be apparent that such a lateral scan, - 20 considered in rela$ion to the motion of a single beam of the fan merely imposes a motion at constant e. Thus the principle of the invention, that the scan of the X-ray source spot temporarily negates the change of ~ imposed by the orbital motion, is not affected by the extra source-and detector lateral scan. The description of the relationship according to the invention given hereinbefore is therefore still appropriate to this other form of the apparatus although the effects of the superimposed con~tant ~ motion should be considered when allocating the derived A
~ 6~i~
data to their appropriate storage locations.
There is shown in Figure 6 an example of an apparatus according to the said Canadian Application No. 207,597 in which features common to the arrangement of Figure 1 have been indicated by the same reference numerals. The rotary member 6, in this example, carries also two compensating members 34 and 35. These members are arranged to provide a substantially uniform absorption to the radiation for all beam paths of a source detector scan despite the substantially circular cross-section provided by the body 1. Thus it is ensured that anyvariations of absorption are caused substantially only by variations in the body 1. Such compensating members could also be provided for the Figure 1 arrangement if desired.
Also secured to the member 6 is a reversible motor 36 which drives a toothed belt 37 by means of a drive shaft 38 journalled in member 6. The belt 37 also passes over an idler wheel 39 also journalled in member 6. Secured to the belt 37 is the source 10, which is of the type described hereinbefore. The source is driven to and fro laterally by the belt 37, being mounted on a bearing travelling in a track 40. A counter balance weight 41 is fixed to the opposite side of belt 37 to compensate for out of balance forces during the lateral movement.
Linked to the source 10 by a light weight but rigid yoke 42 is the collimator/detector unit 12/11. The detectors 11 and collimators 12 also move on a bearing on a track 43 on member 6.
As an alternative to the Figure 1 arrangement for controlling the relative motions there is provided on turntable 6 a graticu~e 44 (only shown in part) comprising a translucent strip carrying engraved lines. The lines interrupt a light path between a , . - :, ; :
`~
65~
light source and a photocell in unit 45 to provide pulses indicating the progress of the orbital movement. These pulses are used to cause periodic reversals o~ motor 36 in accordance with the required relationship. Yoke 19 carries a similar graticule 46 cooperating with light source and photocell unit 47 to provide signals indicative of the progress of the lateral scan of source 10 and detectors 11. These pulses are applied to the integrators 16 to control the integration intervals and to source control/time base generator 28 to control the scan of the source spot along anode 13 in the required relationship.
The pulses may also be applied to a counter, not shown, to record the progress of the scan for the purpose of any processing or timing required.
It will be apparent that a photocell graticule combination could also be used to provide timing pulses for the Figure 1 arrangement in which case pulses from a combination such as 44/45 monitoring the rotary motion would be applied directly to integrators 16 and source control 28.
In a further embodiment of the invention Figure 7 shows a means for obtaining the required motion of the X-ray beams with-out the use of a scanning anode X-ray source such as has been described hereinbefore. ~-In Figure 7 the detectors 11 are shown in the form of individual scin*illator crystals 48 and photomultipliers 49.
The X-ray source 10 is in the embodiment a single point source of X-rays incident on the crystals 48. For simplicity only four typical detectors have been shown. A plurality of collimators 50 are provided on a ring 51 which rotates about an axis 52 which is fixed in relation to the source and detectors.
5~ 4 In the case of an apparatus such as that shown in Figure 1 axis 52 is identical with axis 7. However for an apparatus such as that shown in Figure 6 these are not the same, as shown in figure 7, since axis 52 moves with the yoke 42.
Each collimator 50, of which only two are shown, is of a width suitable for defining a penc:il beam, although in this embodiment each detector is significantly larger. The collimator ring 14 is arranged to rotate in the opposite direction to the rotation of the source and detectors and at such a speed that it matches the orbital motion of the source. Thus while each collimator passes across a detector it remains in a relationship with source 10 such thatthe pencil beam defined is subject to no rotation but a translational motion is substituted. It will be appreciated that this is the same effect described hereinbefore for the scanning anode source. As the collimator moves to the next detector the following collimator takes its place. Thus there is a sudden angular change corresponding to the flyback of the scanning anode source.
It will be appreciated that the collimators at the opposite side of the ring 51 can pass between the source 10 and the body 1. The ring 51 must, therefore, be slightly inclined in relation to the plane of the slice so that such interference is avoided.
It should further be noted that other collimators which are fixed relative to X-ray source 10 can be provided as close as possible to the body 1 so that as far as possible the body is not subjected to radiation which will not be accepted by collimators 50.
Alternatively collimators 50 may be placed so as to pass ~5~6S~
between the body and the source instead of between the body andthe detectors. In that case the relative motion described would achieve the same effect, however, the body would not be subjected to radiation which is not to be detected.
- -: . . ~ . . .. . : . . .
Claims (14)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for examining a substantially planar section of a body by means of penetrating radiation such as X- or .gamma.-radiation including a source of a fan of radiation lying in the plane of the said section and originating substantially from a point, a plurality of detectors for detecting the radiation after passage through the body along a plurality of beams at different angles within the fan, means for angularly displacing the source and detectors about a common axis perpendicular to the said plane and means for subjecting the said point of origin to a periodic displacement, relative to the detectors and in the said plane, such that during each of a series of increments of said angular displacement, said periodic displacement causes a further angular displacement of said beams, substantially equal and opposite to said first mentioned angular displacement to cause each of said beams to remain parallel to its position at the start of the respective increment.
2. An apparatus according to claim 1, in which said source is an X-ray source incorporating an elongated anode arranged to provide X-rays in response to electrons incident thereon, said periodic displacement being the result of periodic displacement of said electrons on said anode.
3. An apparatus according to claim 1, wherein said source is disposed closer to said axis than said detectors.
4. An apparatus according to claim 3, wherein the distance between said source and said axis is substantially half of the distance between said detectors and said axis.
5. An apparatus according to claim 1, including means for laterally displacing said source and said detectors in a common direction in said plane.
6. An apparatus according to claim 1, wherein in one of said increments are of said beams irradiate a series of beam paths at least some of which are substantially identical to beam paths irradiated by another of said beams in another of said increments.
7. An apparatus for examining a substantially planar section of a body by means of penetrating radiation such as X- or .gamma.-radiation including a source of a fan of radiation such lying in the plane of the said section and originating substantially from a point, a plurality of detectors for detecting the radiation after passage through the body along a plurality of beams at different angles within the fan, means for angularly displacing the source and detectors about a common axis perpendicular to the said plane and means for periodically angularly displacing the said beams, relative to the said detectors and in the said plane, such that during each of a series of increments each of said beams remain at the same inclination in relation to said body for a predetermined lateral displacement.
8. An apparatus, for examining a body by means of penetrating radiation, such as X-radiation, including:
source means arranged to irradiate a slice of the body with a spread of the radiation originating substantially from a point, a plurality of detectors including detectors arranged to detect the radiation after passage through the body along a plurality of beams angularly distributed in the spread, means for deriving, from the detectors, output signals indicative of the amount of absorption of the radiation along paths traversed by the respective beam, means for angularly displacing the source means and detectors about an axis intersecting the said slice, and means for additionally and periodically displacing the point of origin of the radiation to cause each beam of the spread, during each period of the additional displacement, to traverse the body along a plurality of paths defining a linear zone in said slice, the arrangement being such that each zone defined by paths traversed by one beam overlaps, at least in part, at least one other zone defined by paths traversed by a different beam from substantially the same mean direction.
source means arranged to irradiate a slice of the body with a spread of the radiation originating substantially from a point, a plurality of detectors including detectors arranged to detect the radiation after passage through the body along a plurality of beams angularly distributed in the spread, means for deriving, from the detectors, output signals indicative of the amount of absorption of the radiation along paths traversed by the respective beam, means for angularly displacing the source means and detectors about an axis intersecting the said slice, and means for additionally and periodically displacing the point of origin of the radiation to cause each beam of the spread, during each period of the additional displacement, to traverse the body along a plurality of paths defining a linear zone in said slice, the arrangement being such that each zone defined by paths traversed by one beam overlaps, at least in part, at least one other zone defined by paths traversed by a different beam from substantially the same mean direction.
9. An apparatus according to claim 8, wherein said source means is disposed closer to said axis than said detectors.
10. An apparatus according to claim 8, wherein the two said displacements are arranged to cause each detector to receive radiation along at least some paths previously examined by another detector.
11. An apparatus according to claim 10, including means for providing a single absorption value for each beam path examined by more than one detector to take account of differences in the sensitivities of the detectors.
12. An apparatus according to claim 11, in which said means for providing a single absorption value comprises means for combining the output signals of said different detectors.
13. An apparatus for examining a body by means of penetrating radiation, such as X-radiation, including: source means arranged to irradiate a slice of the body with a spread of the radiation, a plurality of detectors including detectors arranged to detect the radiation after passage through the body along a plurality of beams angularly distributed in the spread, means for deriving, from the detectors, output signals indicative of the amount of absorption of the radiation along paths traversed by the respective beams through the slice, means for displacing the beams of radiation to direct them along different paths through the body such that each beam traverses, at different times, several paths of each of a plurality of sets of parallel paths in the slice and such that, for each such set, each region of the slice defined by the parallel paths traversed by a respective beam overlaps, at least in part, at least one region defined by paths substantially parallel to them but traversed by another beam.
14. An apparatus, for examining a body by means of penetrating radiation such as X-radiation, including: source means arranged to irradiate a slice of said body with a fan-shaped distribution of said radiation; a plurality of detectors arranged to detect the radiation after passage through the body along a plurality of beams at different angles within said distribution; scanning means for angularly displacing the source means about an axis intersecting said slice, so as to cause said fan-shaped dis-tribution to assume successively different angular dispositions relative to the body; output means for deriving output signals from said detector, each detector providing output signals relating to radiation received thereby along a plurality of beam paths passing through said body in many different positions relative thereto, said scanning means being arranged to produce relative movement between the detectors and said fan-shaped distribution of radiation to displace each beam path across a region of interest in the slice, to cause said output means to derive from the detectors sets of output signals each set representing the radiation received by a respective detector along a succession of parallel paths wherein groups of said sets of output signals, each set provided by a different detector, each relate to a plurality of paths in said slice, all of which are parallel to each other, distributed substantially across the whole of said region; and means for deriving from the output signals a representation of the variation of attenuation of the penetrating radiation with position in said slice.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA255,210A CA1059654A (en) | 1976-06-18 | 1976-06-18 | Radiography |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA255,210A CA1059654A (en) | 1976-06-18 | 1976-06-18 | Radiography |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1059654A true CA1059654A (en) | 1979-07-31 |
Family
ID=4106236
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA255,210A Expired CA1059654A (en) | 1976-06-18 | 1976-06-18 | Radiography |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1059654A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5620436A (en) * | 1994-09-22 | 1997-04-15 | Chiron Technolas Gmbh Ophthalmologische Systeme | Method and apparatus for providing precise location of points on the eye |
| US5645550A (en) * | 1994-04-08 | 1997-07-08 | Chiron Technolas Gmbh Ophthalmologische System | Method and apparatus for providing precise location of points on the eye |
-
1976
- 1976-06-18 CA CA255,210A patent/CA1059654A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5645550A (en) * | 1994-04-08 | 1997-07-08 | Chiron Technolas Gmbh Ophthalmologische System | Method and apparatus for providing precise location of points on the eye |
| US5620436A (en) * | 1994-09-22 | 1997-04-15 | Chiron Technolas Gmbh Ophthalmologische Systeme | Method and apparatus for providing precise location of points on the eye |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4010370A (en) | Computerized tomography apparatus with means to periodically displace radiation source | |
| US4031395A (en) | Radiography | |
| EP0586692B1 (en) | Improved x-ray volumetric ct scanner | |
| US5291402A (en) | Helical scanning computed tomography apparatus | |
| CA1135878A (en) | Method of and apparatus for tomographic examination of structures by x-ray or gamma ray scanning | |
| US4101768A (en) | Apparatus for computerized tomography having improved anti-scatter collimators | |
| US4115698A (en) | Radiography | |
| US4114041A (en) | Radiography | |
| US4138611A (en) | Fan beam CT apparatus with post-processing weighting of picture element signals | |
| US4081681A (en) | Treatment of absorption errors in computerized tomography | |
| JPS6147539B2 (en) | ||
| US4066900A (en) | Technique for cat utilizing composite beam paths which are wider at their ends than therebetween | |
| US4398251A (en) | Radiography | |
| US4097744A (en) | Radiographic apparatus having repetitive movement of the origin of the radiation | |
| IL128035A (en) | Methods and apparatus to desensitize incident angle errors on a multi-slice computed tomograph detector | |
| CA1059654A (en) | Radiography | |
| US4114040A (en) | Radiography | |
| US4076985A (en) | Computerized tomographic scanner with beam distribution control | |
| US4091285A (en) | Traverse and rotate CT scanner having rotational step through a multiple of the fan angle | |
| US4206360A (en) | Radiography | |
| GB1589469A (en) | Radiography | |
| US4843618A (en) | Radiography | |
| US4117336A (en) | Computerized radiography with means to process only selected signals | |
| US5218624A (en) | Radiography | |
| GB2066017A (en) | Compensation for x-ray halo effects in computerized tomography |