GB1579694A

GB1579694A - Measuring radiation absorption

Info

Publication number: GB1579694A
Application number: GB9458/77A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1976-03-10
Filing date: 1977-03-07
Publication date: 1980-11-19
Also published as: JPS52110582A; CA1072688A; IL51613A0; FR2344032B1; SE7702484L; DE2609925C2; NL7702394A; IL51613A; FR2344032A1; DE2609925A1

Description

(54) MEASURING RADIATION ABSORPTION (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMITED, of Abacus House, 33 Gutter Lane, London EC2V 8AH a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a device for measuring the spatial distribution of the absorption of radiation in a planar section body, said device comprising a source of penetrating radiation for generating in cooperation with a diaphragm, a fan shaped radiation beam which irradiates the body section, an array of adjacently arranged detectors for measuring radiation in a large number of different rotary positions of the source relative to a centre of rotation which is situated between the source and the detectors, an attenuating body situated between the source and the location of a body under examination in order to attenuate radiation within the fan-shaped radiation beam in a predetermined manner across said beam, and a computer for determining the distribution of local absorption of radiation in the irradiated body section from measurements performed in a plurality of respective rotary positions of said source. Such a device will be referred to herein as a device of the kind set forth.

One form of a device of this kind is known from UK Patent Specification Number 1,478,123. The measurement described therein can be performed comparatively quickly, because a large number of measurment values can be obtained simultaneously and because the measurement of absorption in the slice only requires the rotation of a source and detector system about an axis which is perpendicular to the sectional plane of examination and which preferably extends through the body to be examined.

Therein, an attenuating body is arranged between the source and the body to be examined (also between the body to be examined and the detectors). The attenuating body serves to attenuate the radiation in all directions of the fan-shaped radiation beam each time by approximately the same factor, so that the output signals of the detectors have approximately the same order of magnitude. As a result, all detectors can operate in the most advantageous region of their response characteristic.

It is stated in UK Patent Specification Number 1,479,123 that for diagnosis it is often not necessary for the absorption to be measured exactly in relation to all points of the body section, but rather that in many cases it is sufficient to measure the absorption exactly only in a predetermined region, namely the diagnostic region. Therefore, the detectors which sense radiation after passing through the region about the centre of rotation in the known device, have a substantially smaller sensing surface area that the detectors which are situtated further outwards. As a result, the spatial resolution in this central region can be increased. If a region outside the centre of the body to be examined is to form the diagnostic region, the body must be shifted so that this region coincides with the centre of rotation.

It has been found that this device, and also the other device mentioned before, have the following drawback. The fan-shaped radiation has approximately the same intensity in all parts of the irradiation region, disregarding deviations caused by the differential spatial radiation characteristic of the source.

Because the section of the body must be completely irradiated in order to ensure a correct measurement of the absorption, all regions of the body in the irradiated section are exposed to approximately the same large intensity of radiation. As has already been stated, generally the measurement of the absorption throughout the entire sectional region of the body is not considered important, the important measurement taking place only in a predetermined region (diagnostic region) which it is desired to examine in detail. Thus in the prior arrangement, not only the diagnostic region but also the remainder of the body wihin the sectional region will be exposed to a high radiation dosc.

The invention has for an object to provide an improved device of the kind set forth in which the radiation dose reccived by on that part of the body section situated outside the diagnostic region can be reduced.

According to the invention there is provided a device for measuring the spatial distribution of the absorption of radiation in a planar section of a body, said device comprising a source of pcnctrating radiation for generating in cooperation with a diaphragm, a fan shaped radiation beam which irradiates the body section. an array of adjacently arranged detectors for measuring radiation in a large number of diffcrcnt rotary positions of the source relative to a centre of rotation which is situated between the source and the detectors, an attentuating body situated between the source and the location of a body under examination in order to attenuatc radiation within the fan-shaped radiation beam in a predetermined manner across said beam, and a computer for determining the distribution of local absorption of radiation in the irradiated body section from mcas urements performed in a plurality of rcspective rotary positions of said source, wherein the attenuating body compriscs two parts which move with the source and which are arranged so that in all said rotary positions of said source, thcy do not. or substantially do not. attenuate the radiation passing through a predetermined diagnostic region within a body section under cxamintion. whilst the radiation passing outside the diagnostic region is attenuated. but not fully suppressed.

the distance between the two parts dctcrmining the extent of the diagnostic region.

Thus. the two parts of the attenuating body are shaped so that the radiation passing through the diagnostic region is not attenuated or is only slightly attenuated, whilst the radiation passing through regions adjoining the diagnostic region to one side or the other of the ray path through the diagnostic region.

substantially reduced. Substantially reduced is to he understood to mean herein a reduction of the irradiation intensity by a factor 10 100. The radiation passing through these adjacent regions should in no case be completcly suppressed. because the dctcrmination of the local absorption by means of known calculating methods would then be made difficult if not impossible. However. if the radiation directed at the regions outside the diagnostic region is simply reduced, for example, by a factor 100, the effect would only be that of a deterioration of the signalto-noise ratio resulting from the quantum nature of the radiation. Tests have demonstrated that an increased noise level during the measurement of measurment values not associated with the diagnostic region has no substantial effect on the determination of the local absorption in the diagnositc region.

In principle it is not necessary for both parts of the attenuating body to be displaceable. On the contrary, a device constructed in accordance with the invention can operate with the parts of the attenuating body moving together with the radiator, but being arranged to be fixedly located and symmetrically disposed relative to a connecting line passing through the point of origin of radiation from the source and the centre of rotation. The diagnostic region will then always be centrally situated about the centre of rotation, so that it must in that case be possible to displace the patient relative to the centre of rotation if a region other than the centre of the patient (in sectional view), is important for diagnosis. However, if the extent of the diagnostic region is to be made adjustable, the two parts can be made displaceable substantially at right angles to the connecting line between the source and the centre of rotation, i.e. in the present example, in an opposite sense relative to one another.

An embodiment of a device constructed in accordance with the invention is characterized in that the two parts of the attenuating body are displaccable along a circular path having the source (assumed to be a pointsourcc) at its centre. In the case of a displacement of the two parts of the attenuating body along the arc of a circle, the attentuation of radiation passing through the attenuating body is then only slightly changed. A prefcrrcd embodiment of a device constructed in accordance with the invention is characterized in that both parts of the attenuating body have a cylindrical shape, the centre of each cylindrical part being coincident with the point of origin of radiation from said source so that the radiation emitted by the source is directed perpendicularly to the surface of the attenuating body. In this way such an embodiment can ensure that all the radiation passing through a given attenuating portion of the attenuating body, will be subject to the same attenuation from the said portion because of the constant thickness and homogeneity of composition of that portion of the attenuating body. If end faces of the attenuating body extend parallel to the direction of the radiation path. the attenuate n X ill not change continuously at each cnd f;. re, but rather as a discontinuous stcp-function, and this can substantially simplify the calculation needed for the reconstruction of the distribution of the local absorption in the body section from the measurement values obtained.

A further embodiment of a device constructed in accordance with the invention is characterized in that the thickness in the irradiation direction of each of the two parts of the attenuating body, increases in a step wise manner in a direction transverse to the irradiation direction when viewed from a straight line passing through the source and the centre of rotation. Alternatively, use can be made of respective parts each of whose thickness increases in a wedge-like manner across the fan-beam. Such shaping of the two parts takes into account the fact that for the reconstruction of the distribution of local absorption in the diagnostic region on the basis of the measurement values obtaned, the measurement values measured from radiation passing through the immediate vicinity of the diagnostic region are more important than those obtained from more distant regions. Therefore, these measurment values must be mesured more accurately than the measurement values measured using radiation passing through the body in a region which is further removed from the diagnostic region. This means that the signal-to-noise ratio of the former measurement values must be better, which implies that the attenuation of the radiation provided by each of the two parts of the attentuating body must be less in the region adjacent the beam path through the diagnostic region than that in regions situated further outwards.

It can be demonstrated that, if the diagnostic region does not coincide with the centre of rotation, the two parts must be displaced during a measurement scan, i.e. during the rotation of the radiator around the body to be examined. Only in this manner can it be ensured that the diagnostic region is always exposed to unattenuated radiation, whilst the remaining region is exposed to radiation attenuated by the attenuating body. Accordingly, a further embodiment of a device constructed in accordance with the invention is characterized in that the two parts of the attenuating body are displaceable independently of one another and in dependence on the rotary position of the source, each part preferably being displaceable by means of a correspondingly controlled step motor. The step motors may be driven, for example, by means of a numerical control system which controls the step motors and hence the position of the two parts of the attenuating body in dependence on the rotary position of the source, on the position of the diagnostic region relative to the centre of rotation, and on the lateral extent of the diagnostic region.

Even though, as has already been stated, the absorption ratios outside the diagnostic region must also be known for determining the absorption within the diagnostic region, it is not necessary for the absorption outside the diagnostic region to be calculated and displayed (one point after the other). Such calculations may even be omitted, and this offers the advantage that the calculation time can be substantially reduced, because the local absorption only needs to be calculated for some of the image points in the overall sectional region, i.e. the points in the diagnostic region. On the other hand, the radiologist must at least be able to recognize, in a generalised manner, absorption structures being outside the diagnostic region in order to faciliate a determination of the orientation of image features within the diagnostic region and also the evaluation of the absorption behaviour therein.

Another embodiment of a device in accordance with the invention is characterized in that the measurement values affected by the two parts of the attenuating body, are subjected to a smoothing operation, and that for the computed reconstruction of the local absorption values that region situated outside the diagnostic region, the local absorption is calculated from measurement values only for each nth point of a notional computational output image matrix in the direction of the rows and for each nth point in the direction of the columns (n being an integer greater than 1, preferably between 2 and 5), the local absorption values for the n.(n-1) other points in said region are calculated by interpolation between the said local absorption values computed from measurement values.

The smoothing operation ensures low-pass filtering of the spatial frequencies of the absorption distribution, which means that steep transitions between two adjoining points are smoothed. Smoothing operations of this kind, providing low-pass filtering for the spatial frequency of the absorption distribution, are knowoperse (see, for example, German Offenlegungsschrift 25 21 889).

Because the absorption differences between two adjacent points are thereby substantially removed, it would not be sensible to calculate the local absorption value for each point from the measurement values so that in the said embodiment the local absorption values are calculated from the measurement values, for example, only for each fourth point of each row and of each column of the output image matrix, so that the calculating time is substantially reduced, and the radiologist can still generally recognize large absorption structures outside the diagnostic region. The low-pass filtering at the same time improves the signal-to-noise ratio. Overall, a reduction of the calculating time and a reduction of the signal-to-noise ratio with respect to the region outside the diagnostic region can thus be obtained, and this can make the reduction of the spatial resolution in the region outside the diagnostic region more acceptable.

A further device utilizes a row of adjacently arranged detectors in addition to the source. the number of detectors (for example, 30) not being sufficicnt, however, for obtaining all measurement values simultaneously, so that a translatory movement is required in this device. A variation of the radiation intensity produced by the source would simultaneously influence the meas urement values of all detectors. This is disadvantageous, because the radiation between the source and the detectors passes partly through the diagnostic region and partly outside this region when the source and detector system is situated in given parts of the translatory trajectory. The described attenuating body can thus also be advantageously used in this device when the radiation load outside the diagnostic region is to be reduced.

An embodiment in accordance with the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, of which :- Fig. l shows an embodiment in accordance with the invention where the diagnostic region is situated centrally about the centre of rotation.

Fig. 2 shows an embodiment in two different rotary positions of the source and detector system, the diagnostic region being situated outside the centre of rotation. and Fig. 3 shows a preferred embodiment of the parts of the attentuating body.

Figure 1 shows a source 1 of penetrating radiation, which irradiates a body section 2 with a radiation beam which is limited to be fan-shaped hy a collimator and a diaphragm 3. the boundary rays of the fan beam are denoted by the references 4 and 5. Radiation, after attcnuation by the body 2. is measured by means of an array 6 of detectors which are arranged on an are of a circle about the point of origin of radiation from the source 1 and which are only diagrammati cally shown. A scattered radiation grid 7 which is arranged between the body 2 and the row of detectors 6 suppresses radiation scattered by the body 2. The reference 8 denotes an attenuating body which consists of two parts which are symmetrically arranged relative to the connecting line between the radiator l and a centre of rotation 12 about which the soruce and detector system is rotated for obtaining measurement values in the various positions. The references 9 and 1 0 denote the two boundary rays of that part of the beam which is not attenti- ated by the attentiating body. The circle 11 about the centre of rotation 12 which is enclosed thereby is referred to as the diagnostic region which is always exposed to unattenuated radiation. The absorption of the remaining part of the body section will be measured with less accuracy. because measurements performed on radiation after passing through this part of the body section, will have a higher noise level.

The attenuating body is constructed so that the attenuation increases in a step-wise manner, viewed transversely from a connecting line between the source 1 and the centre of rotation 12 ; this has been found to be advantageous in two respects. Due to the comparatively small amount of attenuation of the radiation provided by the attenuating body 8, when measured directly adjacent the boundary rays 9 and 10, the signal-to-noise ratio of measurements carried out in this region is substantially better than the signal-tonoise ratio of the measurement values measured along paths in the vicinity of the outer boundary rays 4 and 5. This is important because, for the reconstruction of the distribution of local absorption values in the diagnostic region 11 those measurements performed in the directly adjoining region, and any errors incurred in the performance of these measurements will be more important than those measurements performed in the outlying regions. As a result of employing a step-wise change in attenuation, the calculation will be simplified because the same attenuation factor can be used for each of a group of measurements.

It the two parts of the attenuating body 8, which rotate about the centre of rotation 12 together with the soruce 1, cannot be displaced relative to one another, the diagnostic region will always be centrally situated relative to the centre of rotation 12. This is also applicable if the parts can be shifted only in opposite directions. each by the same amount. relative to the connecting line between the source 1 and the centre of rotation 12, in order to change the radius of the diagnotic region. In an embodiment of this kind, therefore. the patient must be shifted relative to the centre of rotation 12 if the centre of the region of the body 2 of the patient which is important for diagnosis. docs not coincide with the centre of rotation 12. This asymmetrical position of the patient relative to the centre of rotation 12. however, necessitates a corresponding width of the radiation beam which is bounded by the boundary rays 4 and 5, so that the absorption of all parts of the patient can still be measured in one operation.

An embodiment in which the use of such a wide fan-beam is unnecessary, is shown in Figure 2. in which the source and detector system 1. 6 is shown, together with the attenuating body which is rigidly connected to the system in known manner, in two positions which are shifted 90" relative to each other. It is clcarly shown that in the case of an eccentric location of thc diagnostic region 11, the parts 81 and 82 of the attenuating body must change thcir positions relative to the soruce 1 in order to ensure that the same region 11 is always exposed to the unattenuated radiation. To achieve this, the flat parts 81 and 82 of the attenuating body are slidable along the stright line 13, independently of each other, for example, by means of two step motors with a toothed rack as shown in another embodiment in Figure 3.

The distance between the centre of rotation 12 and the centre 110 of the diagnostic region 11 is denoted by the symbol d, and the radius of the diagnostic region 11 is denoted by the symbol r. Furthermore, it is assumed that the row of detectors 6 is arranged on an arc of a circle having the source 1 as its centre. The ratio of the distances between the source 1 and the line 13 and between the source 1 and the centre of rotation 12 is denoted by the symbol V (not shown in Figure 2), and the angle between the connecting line between the source 1 and the centre of rotation 12 and the straight line between the centre of rotation 12 and the centre 110 of the diagnostic region is denoted by the symbol 6. The position which must then be occupied by the attenuating part 82 to ensure that only the diagnostic region is exposed to unattenuated radiation is given approximately by the relation k2 = (-r + d sin h). V, where k2 is the distance from the inner edge of the part 82 and the line connecting the source 1 to the centre of rotation 12, and the position of the attenuating part 81 is then given approximately by the relation k, = (d sin < + r) . V, where kl is the distance from the inner edge of the part 81 and the line connecting the source 1 to the centre of rotation 12. The respective distance k, or k2 will equal 0 when the connecting line between the source 1 and the centre of rotation 12 is tangential to the diagnostic region 11. This is possible only if r < d. The given approximation becomes more valid as the distance between the source 1 and the centre of rotation 12 is made larger in comparison with the distance between the centre of rotation 12 and the centre 110. However, the position of the rays 9 and 10 between the two facing edges of the parts 81 and 82 of the attenuating body, can also be accurately calculated as a function of the rotary position of the source and detector system if the position of the diagnostic region 11 relative to the centre of rotation 12 as well as the radius thereof is given. For this purpose use can be made, for example, of a numerical control system which receives, respectively, the position relative to the centre of rotation 12 and the radius of the diagnostic region from the operator, and which calculates the position of the parts 81 and 82 as a function of the rotary position and controls the step motors (in Figure 3) to displace the parts 81 and 82, respectively.

Figure 3 shows a preferred embodiment of an attentuating body having parts 181 and 182. The two parts 181 and 182 correspond to the parts of a hollow cylinder, the wall thickness of which step-wise increases, in a direction away from the facing edges of the parts. The two parts 181 and 182 are arranged so that the centre of curvature thereof coincides with the centre of the source 1, and the parts are slidable about the source 1 on an arc of a circle 130 which passes through the parts. A step-wise change in the attenuation for each corresponding region of the radiation beam offers the advantage that only one attenuation factor which is constant for that angular region, needs to be employed; to achieve this, the inner face and the step side-faces are each arranged parallel to the radiation propagation direction.

As a result of the step-wise variation of attenuation of the radiation across the fan beam, as has already been stated, the whole of each corresponding distinct angular region of the fan shaped radiation beam will be attenuated by the same factor, so that the output signals from a corresponding group of detectors arranged to measure radiation in that region, will be reduced by a given amount relative to measurements made in a region in which unattenuated radiation is used, and this can be compensated for by the addition of an amount which is constant for a group of detectors because of the logarithmic nature of the signals.

Thus, it is not necessary to measure the attenuation due to the attenuating body, separately for each detector.

It is known that the attenuation A experienced by the X-radiation along a given measurement path through the body under examination can be determined from the relation A = tn Io- en I. Therein, 1o is the intensity of the radiation passing along the path at a point prior to entering the body, and I is the intensity of radiation in the path beyond the body which is measured by one of the detectors of the detector array 6. (The measurement of the primary intensity is described, for example, in UK Patent Specification Number 1,283,915). When the attenuating body is introduced into the measurement path, the value 1o must be replaced by a value Io/D, D being the factor whereby the primary radiation is attenuated by the attenuating body. When use is made of an attenuating body of this kind. the absorption of radiation by the body under examination is obtained in accordance with the rela tion : A = -ln D + In 1o - In I. In order to compensate for the effect of the attenuation by the attenuating body, therefore, the component ln D must be added. The quantity D is dependent on the thickness and on the material of the attenuating body an on the position of the attenuating body relative to the measurement path along which the cor rcsponding intensity 1 beyond the body is measured. In the device shown in Figure 3 the quantity D is constant for each group of measurement values.

WHAT WE CLAIM IS: 1. A device for measuring the spatial distribution of the absorption of radiation in a planar section of a body, said device comprising a source of penetrating radiation for generating in cooperation with a diaphragm, a fan shaped radiation beam which irradiates the body section, an array of adjacently arranged detectors for measuring radiation in a large number of differen rotary positions of the source relative to a centre of rotation which is situated between the source and the detectors, an attentuating body situated between the source and the location of a body under examination in order to attenuate radiation within the fan-shaped radiation beam in a predetermined manner across said beam, and a computer for determining the distribution of local absorption of radiation in the irradiated body section from meas urements performed in a plurality of rcspective rotary positions of said source, wherein the attenuating body compriscs two parts which move with the source and which are arranged so that in all said rotary positions of said source, they do not. or substantially do not, attenuate the radiation passing through a predetermined diagnostic region within a body section under examination, whilst the radiation passing outside the diagnostic region is attenuated, but not fully suppressed.

the distance between the two parts determining the extent of the diagnostic region.

2. A device as claimcd in Claim 1, wherein the two parts of the attenuating body are arranged to he displaccable along a path which lies at right anglcs to a straight line passing through the point of origin of radiation from <RTI ID=6.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. position of the attenuating body relative to the measurement path along which the cor rcsponding intensity 1 beyond the body is measured. In the device shown in Figure 3 the quantity D is constant for each group of measurement values. WHAT WE CLAIM IS:

1. A device for measuring the spatial distribution of the absorption of radiation in a planar section of a body, said device comprising a source of penetrating radiation for generating in cooperation with a diaphragm, a fan shaped radiation beam which irradiates the body section, an array of adjacently arranged detectors for measuring radiation in a large number of differen rotary positions of the source relative to a centre of rotation which is situated between the source and the detectors, an attentuating body situated between the source and the location of a body under examination in order to attenuate radiation within the fan-shaped radiation beam in a predetermined manner across said beam, and a computer for determining the distribution of local absorption of radiation in the irradiated body section from meas urements performed in a plurality of rcspective rotary positions of said source, wherein the attenuating body compriscs two parts which move with the source and which are arranged so that in all said rotary positions of said source, they do not. or substantially do not, attenuate the radiation passing through a predetermined diagnostic region within a body section under examination, whilst the radiation passing outside the diagnostic region is attenuated, but not fully suppressed.

2. A device as claimcd in Claim 1, wherein the two parts of the attenuating body are arranged to he displaccable along a path which lies at right anglcs to a straight line passing through the point of origin of radiation from slid source and said centre of rotation.

3. A device as claimed in Claim 1.

wherein the two parts of the attenuating body are arranged to be dispalccahle along a circular path having the source at its centre.

4. A device as claimcd in Claim 3, wherein the two parts of the attenuating body are cylindrical in shapc. the centre of each cylindrical part being coincident with the point of origin of radiation from said source.

5. A device as claimcd in any one of the preceding claims. including displacemcnt means for displacing the two parts of the attenuating body transversely to the irradiation beam direction independently of one another and in dependence on the rotary position of the source.

6. A device as claimed in Claim 5.

wherein said displtcement means comprise for each said part a correspondingly controlled stepping motor.

7. A device as claimed in any one of the preceding claims, wherein the thickness in the irradiation direction of each of the two parts of the attenuating body, increases in a step-wise manner in a direction transverse to the irradiation direction when viewed from a straight line passing through the point of origin of radiation from said source and said centre of rotation.

8. A device as claimed in any one of the preceding claims, wherein measurement values obtained from radiation attenuated by either of the two parts of the attenuating body, are subjected to a smoothing operation, and for the computed reconstruction of the local absorption values in that region situated outside the diagnostic region, the local absorption is calulated from measurement values for each ntt point of a notional computational output image matrix in the direction of the rows and for each nth point in the direction of the columns (n being an integer greater than 1, preferably between 2 and 5), and the local absorption values for each of the n .(n-1) other points in said region are calulated by interpolation between the said local absorption values calculated from measurement values.

9. A device for measuring the spatial distribution of the absorption of radiation in a planar section of a body, substantially as hercin described with reference to the accompanying drawings.