CA1069222A - Device for measuring radiation absorption - Google Patents

Abstract

ABSTRACT:

In X-ray scanners operating with a large number of detectors ("fan scan") the problem exists that reconstruction errors occur in and about the centre of rotation, which errors are caused by the fact that the radiation absorption in said area is substantially always measured by the same detectors and that the sensitivity of all detectors is not exactly the same.
The invention provides an improvement in that the measurement is carried out with a larger number od detectors than is neces-sary for the measurement of the scanning field and that the de-tector arc pivots about its centre of curvature during the measurement. As a result of this the absorption in the centre of rotation is measured by different detectors of which the different sensitivities, with a sufficiently large number of extra detectors, can be averaged to a considerabl extent.

Description

1~69Z2Z

The invention relates to a device for measuring differences in radiation absorption in a plane of a body while using a source of radiation of which a fan-like diaphragmed radiation beam passes through said plane of the body and is measured by a large number of detectors arranged on an arc of a circle in said plane, in which an arc of a circle provided detectors and longer than is necessary for measuring the radiation behind the object performs a pivoting motion about a centre of curvature during the measure-ment, and in which the system source-detectors is rotated with respect to the body during the measurement.

German Offenlegungsschrift 24 27 418, which was published on January 9, 1975 in the name of Hounsfield, for example, discloses devices for measuring the absorption of a radiation in a plane of a body comprising a source of which a fan-like diaphragmed radiation beam passes through said plane of the body and is measured by a number of detectors arranged in an arc of a circle in said plane and in which the system source-detectors is rotated with respect to the body during the measurement.
The advantage of such a device as compared with a device having only one detector ~for example German Offenlegungsschrift 19 41 433, which was published on February 5, 1970 in the name of Hounsfield, et al,) is that the measurement can be performed more rapidly because a large number of measured values are obtained simultaneously and because for the measure-ment of the absorption in the said plane the system source-detectors has to be rotated only about an axis extending perpendicularly with respect to the plane of examination and preferably extends through the body to be examined. On the other hand, the drawbacks of this device is that as a result of the different sensitivities of the sources errors may occur which ;;
~ may appear in the reconstruction of the image Erom the absorption values ; in the plane of the body. ~ -~
German Offenlegungsschrift 25 03 978, which was published on August 7, 1975 in the name of LeMay, discloses a device of this kind and has for its object to reduce or remove said reconstruction errors. In this device, the system source-detectors ~irst performs a complete revolution

-2-~692Z2 during a measurement. At the beginning and at the end of said revolution the detectors measure the absorption of the body each time along the same stripe through the body. If the output signals of the various detectors nevertheless differ from each other, the periodic change in the sensitivity can be calculated therefrom. The detectors are then rotated about the centre ~.. . ..
of curvature of the arc of the circle on which they are arranged, so that each detector in a subsequent, second revolution measures the absorption along stripes through the body along which the absorption was measured in the preceding measurement by one of the two adjoining detectors. When the measured values supplied by a detector during the second revolution are compared with the measured values supplied by the adjoining detector during the first revolution, differences in sensitivity of adjoining detectors may be derived therefrom and be used for correcting of the measured values.
The detectors are then returned again to the initial position which they assumed during the first measurement, after which the system source-detectors is rotated for a third time and the absorption is measured again from which, by comparison with the measured values obtained during the second revolution, differences in the sensitivity , . . .

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~,, -3-P~ID.76-002 69ZZZ 2~ 2- l976 of ~djoining detectors and, by comparison of the mea-sured va-lues obtained at the beginning and at the end of the revolutlon, periodic fluctuations in t]he sensitivity of the individual de-tectors can again be determined.
The operation ~f this device is thus based on the fact that the absorption along each stripe through the body is measured several times. As a result of this, o~ course, the measuring time to obtain all the measured values ic corres-pondingly extended and the dose which has to be supplied to the body so as to obtain said measured values also becomes cor-respondingly larger. Because this apparatus is used in parti-cular ~or examining patients, this is a serious drawback.
Another drawback of the known device is that the measured values change when the examined body varies its position during the measurement, These changed measured values are wrongly evaluated by the device as a different sensitivity of adjoining detectors and as a drift phenomenon of the sensitivity of the individual detectors, respectively; as a result of this, extra errors occur in the reconstruction of the absorption in the plane of examination of the body.
The invention is also of interest ~or the . .
j problem of avoiding and reducing, respectively, errors in the reconstruction occurring in a device having one source and a large number of detectors. However, in the first instance it concerns avoiding and removing, respectively, errors which occur to an intensified extent in the proximity of the centre of rotation of the system source~detectors, which centre of radiation is as a rule present in the plane to be examined~of the body. As investigations which have also led to the present invention have demonstrated, these reconstruction errors are _ 4 _ .
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also determined by the different sensitivities of the individual detectors.
Accordingly, it is the object of the invention to avoid or at least reduce the reconstruction errors in the proximity of the centre of radiation of the system source-detectors without extending the measuring time or the dose for the patient and without ex*ra reconstruction errors occurring in the case of movement of the body during the measurement.
Thus, according to the present invention, there is provided in a device for measuring radiation absorption differences in a plane of a body wherein a source of radiation produces a fan like beam of radiation which passes through the body and is measured by a first plurality of detectors in a detector group which is disposed on an arc of a circle; the detector group àlso comprising a second plurality of detectors which are disposed on an excess length of the arc and do not measure radiation passing through the body; the source and group forming a system which rotates around the body so that the beam passes through the body from a plurality of angular orientations;
the improvement wherein the number of detectors in the second plurality of detectors in the group is substantially equal to or greater than the number -of detectors in the first plurality and further comprising means for pivoting the group about a center of curvature with a motion which is coupled to the rotation of the system so that individual detectors progressively move into and out of the radiation passing through the body at progressive angular positions and each detector in the detector group measures radiation passing through the body at some angular portion.
The expression "arc of a circle" may also be understood to be a straight line.
The invention wi-ll now be described in greater detail with reference to the drawing, in which:
Figures la to lc show a device according to the invention in various phases of the rotational and pivoting movements, respectively. `
Figures 2 and 3 show dlagrammatically a first embodiment of a device according to the invention.
Figure 4 is a diagrammatic representation of a further embodiment ,~; ~ ' ` .
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Referring now to the drawings, refe~ence numeral 1 denotes a source, or example and X-ray tube. For . -.' .

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: _ ' ' 6~Z;~2 ~.76-002 2~-12-1976 simplicity, only a focus of the source is shown. The source com-prises a diaphragm device not sho-~n which diaphragms a beam of radiation which in the direction perpendicular to the plane of the drawing i~ narrow and in the plane of the drawing is fan-like. The extreme limits of said beam are denoted by solid linesstarting from the focus of the source 1. The radiation passes through a body 2 of which the absorption distribution in the plane of the drawing and the plane of examination, respectively, is to be measured. The fan-like beam is measured by a group of detectors 3 which are arranged on an arc of a circle. The centre of curvature of the arc of a circle on which the group of detectors 3 is arranged may coincide with the focus of the source 1 but may also be situated closer to the detector group or farther away therefrom dependent on the curvature of the arc of a circle. It is obvious from the drawings that the arc of I the circle on which the detector group is arranged is consider-ably longer than is necessary for the measurement of the ra-diation behinf the object and for the measurement of the maximum examination area, respectively, said length being defined by the extreme lines of the fan-like beam.
- During the measurement, the source 1 and the group of detectors 3 perform a radiation about an axis of ro-tation 4 which extends perpendicularly to the plane of the drawing an<l passes through the examination area. Superimposed upon said rotary movement, as denoted by the arrow 5, is an extra pivotlng movement about a plvoting axis which also ex-tends at riLght angles to the plane of the drawing and through the centre of curvature of the arc of the circle, in which movement, however, only the group of detectors 3 ta~e part.
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lO~9~Z2 PH~jls_oo6 Fig. 1a shows the position of the device at the beginning of a measurement, Fig. lb sho-~s that in which the measuremen-t has progressed half, the source being rotated 90 abour the a~is of rotation 4, and Fig. 1c shows that at the end of the measurement. It may be seen that in Fig. lb the ra-diation is measured by a few detectors of the group which in the initial phase shown in Fig. la did not yet takc part in the measurement, while a corresponding region of detectors in the central phase of Fig. lb does no longer take part in the measurement. This is continued until, as is shown in Fig. 1c, the extreme right detector also takes part in the measurement.
As a result of the pivoting movement it is obtained that the radiation, viewed with respect to the source, behind the centre of rotation 4 in the various positions of ` 15 the source 1 is no longer measured by a single detector but b~ a large number of detectors. ~or example, if the number of the detectors necessary for measuring the fan-like beam i8 equal to 100 and if the detector group comprises 200, prefer-ably equally large, detectors,then the radiation behind the , 20 centre of rotation in the individual phases of the measurement j is measured by 100 different detectors, at any rate when during `, ~ the measurement at least a hundred times a measurement occurs from angular positions of the source divlded over equal angles.
The differ,ant sensitivltles of the lndivldual detectors are~
summed in the reconstruction of the radiation absorption in , the area of the centre of rotation and provide a conslderable reductlon lD~ the reconstructlon error ln the vlclnlty of the rotation centre, because the deviations of their sensitlvities from the e:~act value ln general compensate each other at least PHD.76-002 ~Q6922~ 2l~-12-1~76 partly. This error becomes smaller according as the number is larger o~ the detectors placed on the arc of the circle and not each time necessary for the measurement compared with the number of the detectors necessary for the measurement, or ac-cording as the part of the arc of the circle not each timenecessary for the measurement is larger in comparison with the part necessary for the measurement. The leng-th of this part and the number of the detectors not each time necessary for the measurement, respectively, would therefor have to lie in the same order of magnitude as the length o~ the arc of the circle necessary for the measurement and the number of the de-tectors each time necessary for the measurement, respectively.
If said length is considerably smaller, for example, only a few detector widths, no significant improvement is obtained.
~` 15 In the reconstruction of the absorption in the plane of examination of the body from the resulting measured results it should be taken into account, of course, in the devicQ according to the invention that the absorption of the path of radiation which pasSeS through the centre of rotation was measured by different detectors and consequQntly measured values provided by different detectors shouls be assigned to said path of radiation. In the paths of radiation outside the centre of rotation, but at a required distance ther~from, this is also the case and this should also be taken into account.
In the embodiment according to the invention shown in Figs. 2 and 3 the source 1 is secured to an annular support 6 which is` journalled on three bearings in such manner that it oan be rotated about the axis 4 by means of a drivlng ; meohanism (not shown). The obJect 2 is also positioned in the j!

PHD.7G-002 ~69ZZZ 2~-12-l97~

vicinity of said axis. The group of detectors 3 is secured to a yoke 8 which is pivotable about the pivoting shaft 10 which extends at right angles to the plane of the drawing, passes through the centre of curvature of the arc ~f the circle on 5 IYhich the detectors are ar:ranged, and is secured to the annular support 6 by means of a dr:iving mechanism 9. The driving me-chanisms for the yoke 8 and for the annular support 6, res pectively, are matched to each other in such manner that the yoke and the support need each time the same period of time 10 for the performance of the necessary pivoting and the rotation necessary of the measurement, respectively.
In the embodiment sho~n in Figs. 2 and 3 it is advantageous when using collimators before the detectors for suppressing the parasitic radiation l~hen, contrary to what 15 is shol~n in the drawing, the pivoting shaft and the centre of curvature, respeotively, coinclde with the focus of the source.
As a matter of fact, said collimators remain focused on the source in spite of the pivoting movement of the yoke.
~When the detectors are arranged on an aro of 20 a circle whose centre of curvature coincides with the axis of f rotation 4, only one driving motor for the rotary movement and the pivotlng movement is necessary because the source and the detectors are rotated and pivoted, respectively, only about the axis of rotation although at different angular velocities. F:or 25 that purpose, they may be coupled to the common driving motor -via drivin,g mechanisms having di~ferent transmissions.
It is furthermore favourable if the source is rotated about an axis extending perpendicularly to the plane of the dra~ing through the focus of the source synchronously ~ . _ g _ : :

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PHD.76-002 i~92Z2 2~1-12-1976 with the pivoting movement. Xeconstruction errors, if any, pro-duced by a non-ideal source may be prevented. As a matter of fact, the radiation emitted by the source in the plane of the drawlng does not have exactly the same intensity in all di-rections. Reconstruction errors caused by said inhomogeneousintensity distribution can be avoided in the known devices, B see for example the German Offenlegungsschrift 24 ~ 418, in that the sensitivity of the individual detectors is each time adjusted so ~r afterwards turned back that the product of the detector sensitivity and the intensity of the part of the ra-diation measured by the detector is the same for all detectors.
When according to the invention the group of detectors is pi-voted without correspondingly rotating the radiation cone, the individual detectors measure different parts of the radiation cone without it being possible to adapt the sensitivity of the detectors to the radiation cone. By the rotation of the source about the focus it can be achieved that the individual detectors always cover the same area of the radiation cone so that the sensitivity of the detectors can be matched to that. It is favourable when the centre of curvature of the detector arc .
and the pivoting shaft, respectively, about which point sald arc is pivoted, coincides with the focus of the source. The angular ve:Locity of the rotary movement of the source about the focus thereof and the angular velocity of the arc of the circle about the pivoting shaft, that is about the focus, then are ; equal. In the embodiment shown in Figs. 2 and 3 this has been achieved iIl that the source (1) is rigidly mounted on the~yoke (8) so tha1; the foous and the axis (10j coinclde.
, ~ ', -~ . ; 1 -:: , ' , ~ 22 2L~ -1976 Fig. l~ shows another embodiment according -to the invention which is specially suitable for devices in which the radius of curvature of the arc of the circle is very large tfor example, when the detectors are located on a straight line and the radius o~ curvature thus is infinite). In this case also the source 1 is secured again to an annular support 6 which can be rotated about its centre 4 by means of the bearings 7 and a driving mechani-sm not sho~n. On its side opposite to the source the annular support 6 has a bearing surface 11 which has the same curvature as the arc of the circle on which the group of detectors 3 is arranged. The group of detectors 3 is moved over the bearing surface 11 during the measurement by means of the bearings 12 and a driving mechanism 13 sho~rn diagrammatic-e, l ly .

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a device for measuring radiation absorption differences in a plane of a body wherein a source of radiation produces a fan like beam of radiation which passes through the body and is measured by a first plurality of detectors in a detector group which is disposed on an arc of a circle;
the detector group also comprising a second plurality of detectors which are disposed on an excess length of the arc and do not measure radiation passing through the body; the source and group forming a system which rotates around the body so that the beam passes through the body from a plurality of angular orientations; the improvement wherein the number of detectors in the second plurality of detectors in the group is substantial-ly equal to or greater than the number of detectors in the first plurality and further comprising means for pivoting the group about a center of curva-ture with a motion which is coupled to the rotation of the system so that individual detectors progressively move into and out of the radiation pass-ing through the body at progressive angular positions and each detector in the detector group measures radiation passing through the body at some angular portion.

2. A device as claimed in claim 1 wherein the pivoting and rotating movements occur continuously.

3. A device as claimed in claim 1, wherein the source is secured to a support which is rotatable about an axis of rotation perpendicular to the plane and the group of detectors is secured on a yoke which is journalled perpendicularly with respect to the plane, and is pivotable about a pivoting shaft connected to the support or a part rotated in the same sense.

4. A device as claimed in claim 1, wherein the source is secured to a support which is rotatable about an axis which is perpendicular with respect to the body plane, the support or a part connected thereto having a bearing surface which has the same curvature as the arc of the circle, in a second plane extending parallel with respect to the body plane and a part supporting the group of detectors is movable over said bearing surface.

5. A device as claimed in claim 3, wherein the center of curvature of the arc of the circle and the pivoting shaft, about which the detectors are pivoted during the measurement, coincides with a focus of the source.

6. A device as claimed in claim 1, wherein the source is rotatable about an axis extending perpendicular with respect to the body plane through a focus of the source, the angular velocity of the pivoting movement being chosen to be so that the part of a radiation cone emitted by the source and measured by the individual detectors does not vary as the detectors pivot.

7. A device as claimed in claim 2, wherein the pivoting and rotating movements have constant angular velocities.