CA1061478A

CA1061478A - Device for measuring the distribution of the absorption or the emission of radiation in a layer of a body

Info

Publication number: CA1061478A
Application number: CA252,097A
Authority: CA
Inventors: Gunter Kowalski
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1975-05-13
Filing date: 1976-05-10
Publication date: 1979-08-28
Also published as: FR2311358B1; BE841756A; IT1061971B; DE2521171A1; FR2311358A1; ES447778A1; JPS51146884A; GB1553857A

Abstract

ABSTRACT:

In devices for reconstructing the absorp-tion distribution in a layer of a body on the basis of the measuring values obtained by means of an (Emi) scanner, the measuring values or the values derived therefrom are "spread" along strips which occupy the same position relative to the image plane as the strips along which the measuring value has been obtained relative to the layer. This "spreading"
is effected in that for each image element it is measured to what extent it is covered by this strip the measuring values, or the values derived therefrom assigned to those strips being proportionally distri-buted across the calculated surfaces in order to cal-culate the absorption. This requires a comparatively long calculating time and very fast and hence expen-sive multiplier circuits. The inventionoffers a simpler possibility of assigning the measuring values, or the values derived therefrom, to the individual image elements, the calculation of weighting factors being eliminated by the sub-division of each image element into smaller image points (9 to 16 times smaller).

_ ?? _

Description

10~478 "Device for measuring the distribution of the absorp-tion or the emission of radiation in a layer of a body"
The invention relates to a device for determining the spatial distribution of the absorp-tion or the emission of radiation in a layer of a body, the absorption or the emission of the body being measured in a large number of measuring series in a large number of directions in the layer by means of a radiation source and a detector, each measuring series producing a number of measuring values of the absorption or the emission in strips which extend at least approximately parallel relative to each other, the absorption or the emission being calculated and displayed in image points of the layer on the basis of the said measuring values. This device is prefer-ably used for X-ray diagnosis or nuclear medicine.
A device of this kind is known from United States Patent 3,778,612, Hounsfield, December 11, 1973. The absorption in a (human) body is measured by means of a radiator which is displaced, together with a radiation detector which measures the radiation behind the body, perpendicularly to the direction of the radiation, the detector measuring a series of measuring values ~measuring series) which is a measure for the ab-sorption of the radiation along the straight lines through the body which extend parallel to each other and which are determined by the position of the radiator and the detector. After such a measuring series, the radiator/detector system is rotated and a further measuring series is completed at a different angle

- 2 -j PHD.75-o64 ~ ~0~1478 23-4-1976 relative to the body~ etc. The absorption in the individual points or regions in the layer covered by the measurement cannot be simply reconstructed from thc measuring values obtained, because the measuring values do not represent a measure for the absorption in individual points, but rather for the absorption in a straight line or strip through the body to be examines which is-obtained during the ¦ measurement. Erom an arithmetical point of view, this implies that the value of a function (absorption, emis-~ sion, density, etc.) in individual points of the ¦ layer defined by the straight lines must be calculated from the traject integrals of this function along a large number o~ intersecting straight lines.
This problem is also encountered in ~the measurement of the radioactivity distribution in radioactively marked biological objects, in the calculation of layers of macromolecules (viruses and the like) measured by means of an electron micros-cope, and also in the examination of layers of technical objects (for example, rnaterials test) by - means of penetrating radiation. The reconstruction of the absorption in the layer is effected in known ~ ~QP)i/~
devices in that the layer to be ex~m-ilff~-is sub-divided into a matrix of square image elements, the dimension of which corresponds approximately to the width of a strip. Each image element is assigned the measuring value (or a value derived therefrom and from the other measuring values of the measuring series) of each measuring series which has been measured in the strip in which the image element is situated. ~f it is assumed that the image element and , 1~;147~

the strip have approximately the same width, an image element can be influenced by as much as three measuring values (or measuring values derived therefrom) which represent the absorption of the rad~ation in three parallel strips. The measuring value or the value for a strip derived therefrom, conse~uently, is multiplied by a weighting factor for the calculation of an image element, the said factor corresponding to the common plane of the strip and the image element.
This method, performed in a computer, involves very long computing times and a very expensive computer, notably for the calculation of the weighting factors.
In order to realize a shorter computing time by means of a simple device, it has already been proposed to superpose the values derived from the measuring values by a convolution process on the target of a charge storage tube along adjoining strips, the position and the direction of the said strips corresponding to the position and the direction of the strips during the formation of the assigned measuring values. However, serious problems are then encountered as regards the signal-to-noise ratio.
The invention has for its object to realize fast processing of the measuring values by simple means without deterioration of the signal-to-noise ratio.
According to this invention there is provided a device for determining the spatial distribution of the absorp-tion or the emission of radiation in a layer of a body, the absorption or the emission of the body being measured in a large number of measuring series in a large number of directions in the layer by means of a radiation source and a detector, each measuring series producing a number of measuring values of the absorption or the emission in strips which extend at least approximately parallel relative to each other, the absorption or the emission being calculated and displayed in image points of the layer on the basis of the said measuring values characterized in that there is provided an allocation device which assigns the measuring value, or a value derived , .

10~1478 therefrom, to each image point of the layer to be examined for each measuring series, the said - 4a -,~.
.

PIID.75-064 1061478 23-l~-1976 valuc hnving been obtained in a strip in which the relevant image point is situated, the values assigned to the image points from all measuring series being stored and superposed in a storage and summing device, the superposed values being displayed by means of a display device, each time the mean value of the absorption or the emission in different adjacent image points in the region determined by these image points being displayed.
The invention will be described in detail hereinafter with reference to the drawing.
Figure 1 shows the geometrical lay-out of the strips and the layer to be examined, Figure 2 shows a first embodiment in accordance with the invention~
Figure 3 shows the geometrical lay-out of the strips, the layer to be examined and the image points for the embodiment shown in Figure 4, Figure 4 shows a further embodiment, and Figure 5 shows the appearance in the time of the clock pulses in the device shown in Figure 4.
The absorption of the object 0 in the layer E to be examined is determined by way of a large number of meas~ring series, the absorption being measured each time along a number of parallel ex-tending strips. Figure 1 shows these strips 1...6 for a measuring series. The absorption in each of these strips is represented by a measuring value.
This measuring value is subsequently subjected to a convolution process, the value M1.. M6 thus calculated including, besides the measuring value measured in the strip, the weighted sum of all other measuring P~ID.75-OG4 values of this measuring series.
The calculation of the absorption from these values M1...M6 is effected in that the values - are "spread" across the region of the layer to be examined, the said region being covered by the asso-ciated strip; for example, the value M2 is assigned to the part of the layer which is covered by the strip 2. Consequently, the position of the point determines which value is assigned to a point of the layer E. Subsequently, the values derived from a further measuring series are assigned, the said values being superposed on the previously assigned values.
The assignment of a point of the layer to a strip is effected by means of a computer which calculates, on the basis of the position of a point in a fixed x, y system of co-ordinates, its relation-ship with one of the strips. The association of a point of the layer with a given strip is dependent of the distance 7 (see Figure 1) between a straight line which passes through the point, parallel to the strip, and the co-ordinate origin (x = 0, y = 0), the said distance being calculated from the formula:
~ = x . cos e + y.sin e (1) Therein, x and y represent the position of this point in the x, y system of co-ordinates, whilst e is the angle at which the strips intersect the x-axis. The distance ~ is calculated for each individual point of the layer, it then being possible to calculate directly the strip whose value (for examp1e, M5) is to be assigned to the point (the relationship be-tween the distance ~ and the strlp whose value M
is to be assigned to points at the distance ~ is Pl]~.75-oG4 23-l~-1976 clearly illustrated by Flgure 1).
Figure 2 shows a device in accordance with the invention which is constructed on the basis of the foregoing considerations, The device comprises two sawtooth generators 10 and 11 which generate two sawtooth signals UX and uy of different frequency.
If it is assumed that UX and u are proportional to the distance x and y, respectively, from the co-ordi-nate origin, the output signals UX and u of the sawtooth generators 10 and 11 represent a quantity which line-wise scans the layer to be examined (Figure 1 shows only two lines ~ and 9).
It is important that the distance between two adjacent lines is substantially smaller, for example, by a factor 3 or 4, than the width of a strip. In the configuration shown in Figure 1, in which the width of the layer corresponds to four strip widths, this means that the layer must be scan-ned by approximately 12 to 16 lines. This implies that the period of the signal u must be 12 to 16 times larger than the period of the signal ux. In practice, the layer is not scanned in six rather wide strips, as is shown in Figure 1, but along a large number of strips, for example 150 strips. The number f lines must then be accordingly larger (450 to 600) so that the horizontal and verticai deflection gene-rations (the latter generators scanning 625 lines) commonly used in a television apparatus are suitable for use as the sawtooth generators 10 and 11.
The output signals UX and uy of the sawtooth generators 10 and 11, respectively, are each applied to the input of a multiplier circuit 12 PI I D, 7 5 ~
10~1478 23-4-1 97G
and 13, respectively, the other input of said multi-plier circuits carrying a voltage which is proportion~l to cos e and sin e, respectively. The output signals of the multiplier circuits 12 and 13 are added in an adding circuit 14 in which an additional value ~ 0 is added so that, taking into account the equation (1), the output of the adding circuit 14 carries a signal which is proportional to ~ + ~ . This signal is applied to an analog-to-digital converter 15 which converts the output signal into a digital number. When the proportions are suitably chosen, the digital output signal represents the number of the strip or the address of the store in which the measuring value obtained in this strip or the value M derived there-from is stored. This will be illustrated on the basis of the below calculating example.
It is assumed that the distance y or x = 1 fromthe co-ordinate origin corresponds to the width of the strip. It is also assumed that the value x or y = 1 corresponds to the signal UX or uy = 1 Y, and that no further proportionality factors are introduced by the multiplier circuits 12 and 13 and the adding circuit 14, so that, for example, for e = o, x = 0.5, a voltage of 0.5 V is present on the output of the adding circuit 14, the value correspon-ding to ~ 0, assumed to be 4 V, being added thereto yet. If a voltage of, for example~4.5 V has thus been generated, it will be converted into the numerical term 4.5 by the analog-to-digital converter 15, how-ever, this converter 15 is constructed so that the last decimal position is eliminated, so that on the output of the converter 15 the number 4 is present PI-II) 75-o64 106~478 23-4-1976 which, if the values M1...M6 are correctly assigned to the storage position, denotes the address of the storage position in which the value M4 assigned to the strip 4 is stored. Because the sawtooth voltage UX
increases, the output voltage always increases and _ exceeds the voltage value 5V at a given instant, for example, when for e = 0 the value x becomes larger than 1. The number 5 then appears on the output of the analog-to-digital converter 15, which means that the address of the store in which the measuring value M5 assigned to the fifth strip is stored is addressed.
Generally, the analog signal is converted into a binary value for which the above considerations are also valid.
The addressing device formed by the analog-to-digital converter 15 controls an inter-mediate store 16, the various storage positions of this store storing the values measured in the indi-vidual strips or the values (M1...M6) derived there-from, so that each time the contents of the addressed storage position are available on the output of the intermediate store 16.
The intermediate store 16 can be re-placed by controlled multiplex access to data present in analog form. The value each time.fetched from the intermediate store 16 in this manner is written on a disc store 17 which simultaneously synchronizes the sawtooth generators 10 and 11. The measuring values, or the values M1.,.M6 of a single measuring series derived therefrom, assigned to the individual points, are written on a single track in this disc store 17.
For the next measuring series, the PIID 75-o64 10~1478 strips then intersecting the layer to be examined at a different angle e, for example, a process computer introduces new values sin e and cos e (for this reason, the multiplier circuits 12 and 13 are preferably con-structed as multiplying analog-to-digital converters), and the measuring values, or the values derived there-from, which are obtained in this series and which usually deviate from the measuring values, or the values M1...M6 derived therefrom of the preceding series are written in the intermediate store 16. These new measuring values, or the values derived therefrom, are assigned to the layer one point after the other, and are stored accordingly in the next track of the disc store 17.
This is repeated for all measuring series, so that the number of tracks of the disc store 17 should correspond at least to the number of measuring series recorded.
When all measuring series have been processed and recorded on different tracks in the disc store 17 in this manner, all tracks are simul-taneously read during a read operation. The signals read are added in an adding circuit 18 and are applied to a display apparatus 19, for example, a television monitor. This display apparatus 19 has a limited resolution so as to achieve the blurring of the various lines to form an image element. The limit of the resolution could possibly be imposed by a strip width. This means that the width of the line whereby the layer is scanned is smaller, for example, by the factor 3 to 4, than the resolution of the display apparatus 19.

--1 O~

1'111~. 75_ofil1 23~1~_1976 Thus, far, it has been assumed that the absorption is measured in exactly parallel strips within a measuring series. However, there are also devices for measuring the absorption in a layer of a body in which a large number of detectors cover the flared radiation of the radiator behind the object.
In these devices, the strips in which the absorption is measured diverge, viewed from the radiator. In ~` such a case, the equation (1) must be replaced by 1Q ~ (x,~) = 1-+~S(x s~iYn-~in y cos e) (2) Therein, ~ describes the divergence of the radiation beam and amounts to zero in the border case of paral-lel projection. In the case of such flared spreading of the measuring values or the values derived there-from, the adding circuit 14 must be replaced by an analog computing network which reproduces the formula (2).
In the described embodiment in accor-dance with the invention, each point of the layer to be examined is continuously assigned the value (M1...M6) measured along the strip in which the re-levant image point is situated. The formation of the mean value of the absorption in different adjacent image points and the display of this mean value in the region determined by these image points is also continuously effected, in that the-;mage, superposed by means of the adding circuit 18 which adds the video signals of the individual tracks, is displayed on a display device 19 having a limited resolution.
Figures 3, 4 and 5 show a further em-bodiment in accordance with the invention.
In order to assign the strips obtained ~061478 PHD 75-o64 by the "spreading" of the measuring values to an image matrix representing the layer to be examined,(Figure

3 is based on an image matrix of only 4 x 4 4uadratic elements~ each element, whose dimensions correspond approximately to the width of a strip, is again sub-- divided into a matrix of points as shown in Figure 3 which illustrates this sub-division for one element.
These points are assigned, without interpolation, to the strip in which they are situated. As a result, the exact movement is replaced by an approximative movement as is indicated in Figure 3 by the heavy, stepped line 20. Generally, a sub-division of an element into 3 x 3 points already suffice.s. The number of image points whereto a measuring value or a value derived therefrom is to be assigned is thus increased by a factor 9, but each point is assigned each time to only one strip. Calculation of any weighting fac-tors is thus avoided.
Figure 4 shows a device for reali~ing a fast reconstruction of the absorption values in the layer to be examined; Figure 5 shows the asso-ciated clock signals. The device mainly consists of two units, one of which effects the assignment of a measuring value or a value derived therefrom to a point of the layer, whilst the othe~ unit calculates on the basis of the values assign~ to the individual .
points of an element, the mean value of the absorption for this element and, after this has been done for all measuring series, this unit superposes and dis-plays the mean values assigned to this image element.
The assignment of an image point to a strip, i.e. to a measuring value or to a value derived -12~

rllD.75-ef~
1061478 23-4-1'~76 thererrom, is defined by the cquation (1) when parallel projections are assumed. In the case of a flared projection, the distance is calculated in accordance with the equation (2). If the strip width ~ ~ = 1 is chosen by appropriate standardization, the result of the equation (1) merely need be rounded off to an integer number in order to produce the number (address) of the intermediate store position in which the measuring value (or the value derived therefrom) measured in the strip at this distance is stored. If the constant value 0.5 is added to the numerical value of the equation (1), the rounding off in the upward or downward direction is replaced simply by elimination of the positions behind the decimal point. Figure 4 can be described on the basis of this consideration.
The addends ~ x cos e and ~ y sin ~
are prepared by two shift registers 21 and 22 whose contents are cyclically advanced per image by a clock signal, ~ x representing the distance between two points in the x-direction, and ~ y representing the distance between two adjacent points in the y-direc-Jc tion. However, these addends can also be calculated by a process computer. This is particularly advan-tageous when the angles at which the radiation passes through the layer to be examined during the measuring series have not been predetermined.
When all points are sequentially pro-cessed line-wise, the value ~ for each new point must be increased by the addend ~ x cos e and for each new line by the addend ~ y sin e, starting from initial 1061478 23_i~_1976 values xO, yO which correspond to the co~ordinates of the first point of a line or of an image and which have been written in the mono-cell interrnediate stores 23 and 24' by the clock signals ty and tB. This is effected by means of three adding circuits 24, 25 and 26. In the adding circuit 24, in conjunction with the store 23 whose output is fed back to an input of the adding circuit 24, the clock pulse tx generates a (preferably digital) signal whose instan-taneous value varies in accordance with the co-ordi-nates of thepoints on a line. This step-wise increas-ing signal is returned to the initial value xO by the clock signal ty after each line. In the adding circuit 25, the same clock signal generates, in con-junction with theintermediate store 24', a signal which increases in phase and which corresponds to the y-co-ordinate of the relevant line (y perpendicu-lar to the line direction). Each time when all image points of the layer have thus been treated, the store 24' is returned to its initial value yO by the signal tB-The initial values xO and yO are depen-dent of the angle e; however, an initial value can always be chosen at random, for example xO = 0, it then being necessary to change the other ~alue accor-dingly. This other initial value can be given in advance by the device which also supplies the addends ~ x cos e and ~ y sin e, i.e. by a shift register 22' which comprises a number of storage cells which corresponds to the number of measuring series and whose contents are cyclically circulated by the clock pulse tB.

rl ll) , 7 5 - o G 1~
10~1478 23-ll-1976 The output signal of the intermediate stores 23 and 24' is applied to both inputs of an adding circuit 26, the output signal of which controls an addressing device 27. In the case of a digital out-put signal of the adder - obtained in the described - manner - the least significant bits of a digital signal can be omitted if correct standardization and generation of the address are chosen. If the signal which represents the distance ~ is present in analog form, the addressing device must comprise an analog-to-digital converter as has already been described with reference to Figure 2.
The device to be described below could in principle be constructed as shown in Figure 2.
The embodiment shown in Figure 4, however, digitally processes the values M1...M6, so that the construc-tion is different.
The address formed by the addressing device 27 is used for directly actuating the random access intermediate store 28 in which the values M1...M6 are stored and whose output supplies the desired value for further processing.
The subsequent device sums the values assigned to the individual points of an element, superposes the measur~ng values obt~ined from the individual measuring series, and s~ores the recon-structed image. The values assigned to three adjacent points are each time added in a loop between an adding device 29, having an input which is connected to the output of the intermediate store 28, an intermediate loop 30 comprising a single storage position, and a switch 31, under the control of clock signals t1 and t2 l'llD.75-o61~
1061478 23-4-197~
(see Figures 4 and 5). l~hen t~le first value is taken over, provided that the switch 31 is in the correct position, previous values are also added, i.e, the value assigned to a set of three points o~ the same element in one or two previous lines, or the value assigned to an element on the basis of preceding measuring series. The value added is dependent of the position of a second switch 32 which is connected to a contact of the switch 31 and whose other contact is connected to the output of a shift register 33 which is connected behind the intermediate store 30 which may also be considered as a shift register ~com-prising one storage position).
When the value of a line of points has thus been obtained and taken over, under the control of the clock signal t2, in the subsequent shift register 33 which comprises m-l-storage positions (m = number of elements of a line; so 4 in the present example) three successive lines are added by means of the clock signals t2 and t3 in that the value of the previous line is fed back to the adding device 29 via the switches 31 and 32 The combination of the 3 x 3 points to form one image element is thus per-formed without an additional storage position being used. -~
The complete line of image elements is stored in a shift register 34 (number of storage posi-tions: n-m; n = number of elements of the image matrix) by means of the clock signal t3. A-t the beginning of each line of elements, that is at the beginning of each third line of points, a stored value is returned from the shift register 34 to the adder 29, via the Pllr). 7s-oG
23~ 1976 10~1478 switches 31 and 32 and under the control of the clock signals t2, t3 and t3 in order to superpose the ab-sorption distribution obtained from the new measuring series on the absorption distributions obtained from previous measuring series. After completion of the superposition, the absorption distribution derived from the relevant measuring series is stored in the network formed by the shift registers 30, 33 and 34 (possibly after superposition of the previously ob-tained absorption distributions) and can be read.
Before the beginning of a new reconstruction,that is to say when completely new measuring series have to be evaluated, obviously, the stores 30, 33 and 34 must be set to zero. This is not separately shown in the circuit diagram.
Figure 5 shows the variation in time of the clock signals tx, ty, tB, t2 and t3. The clock signals tB~ t2 and t3 correspond to the signals tB~
t2 and t3, respectively; however, the individual pulses are wider, i.e. they start earlier and terminate later than in the clock signals without a stroke. The clock signal t1 corresponds to the clock signal tx, but has been delayedwith respect thereto, because the reading of a value Ml...M6 from a line of the intermediate store 28 cannot be effected simultaneously with the supply of the address of this storage position.
The references I-1 and I-2 in the time diagram of ~igure 5 denote two successive periods of the reconstruction of two complete images; L-1, L-2..... L-4 denote the periods of the reconstruction of a line of the image I-1 or I-2, and P denotes the reconstruc-PIID,75-~)GII
1061478 23~ 976 tiOIl period of an image element.
Referring to the device shown in Figure

4 and the diagram shown in Figure 5,it is to be noted that the operation is only diagrammatically illustrated.
In order to ensure a simple operation of the stores, normal steps known in the digital technique must be taken. For example, a shift register comprising charge-coupled elements must be actuated by two clock pulse series, supplied to different inputs. The feeding back of a store output to the preceding adding device (like, for example, the components 29 and 30) ~equires an additional intermediate store (not shown in the drawing) in order to prevent, in the case of a modification of an output quantity, the unambiguous state of the adding device from being disturbed be~ore it is taken over in the store. These problems, however, occur in all logic circuits, so that it not necessary to elaborate this aspect.
The device shown has the advantage that the sub-division of an element into different points is controlled only by clock signals. For exam-ple, if an ~lement is to be sub-divided into a dif-ferent number of points, for example, 4 x 4 points instead of 3 x 3 points, merely the clock drive need be modified.

. .

Claims

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

1. A device for determining the spatial distribution of the absorption or the emission of radiation in a layer of a body, the absorption or the emission of the body being measured in a large number of measuring series in a large number of directions in the layer by means of a radiation source and a detector, each measuring series producing a number of measuring values of the absorption or the emission in strips which extend at least approximately parallel relative to each other, the absorption or the emis-sion being calculated and displayed in image points of the layer on the basis of the said measuring values, characterized in that there is provided an allocation device (10...16; 21...27) which assigns the measuring value or a value derived therefrom (?1...?6) to each image point of the layer to be examined for each uring series, the said value having been obtained in a strip (1..6) in which the relevant image point is situated, the values (?1...?6) assigned to the image points from all measuring series being stored and superposed in a storage and summing device, the super-posed values being displayed by means of a display device, each time the mean value of the absorption or the emission in different adjacent image points in the region determined by these image points being displayed.

2. A device as claimed in Claim 1, characterized in that the number of storage positions corresponding to the number image points, exceeds the number of measuring values by at least a factor four.

3. A device as claimed in Claim 1, characterized in that the allocation device comprises a computer which calculates a distance ( ? ) between a straight line in the layer to be examined, parallel to the strips (1...6), and a reference point (x = o, y = o), an intermediate store being provided which stores the values measured in the individual strips or the values derived therefrom (?1...?6), an addressing device being provided which forms, on the basis of the calculated distance, the address of an intermediate storage position and which assigns the contents thereof to the point.

4. A device as claimed in Claim 1, 2 or 3, characterized in that the values (?1...?6) of each measuring series, assigned to the individual image points of the layer to be examined (E), are stored in a number of storage positions in the storage device, the values to be superposed of each measuring series in the various storage positions being simultaneously read and summed by means of a reading and summing device (18) the summed values of each point being displayed by means of a display device (19), the image points displayed at least partly overlapping the neighbouring image points.

5. A device as claimed in Claim 3, characterized in that the calculating device comprises: two sawtooth generators (10, 11) for generating output voltages (ux,uy) which correspond to the position of an image point of the layer to be examined in a rectangular system of co-ordinates, two multiplier circuits (12, 13) for multiplying the output voltages (ux, uy) by the angle-dependent factors (sin .theta., cos .theta.) which are determined by the angular position (.theta.) of the strips, an adding circuit (14) being connected to the outputs of the two multipliers (12, 13).

6. A device as claimed in Claim 5, characterized in that a constant element (?O) is added to the output signals of the multiplier circuits (12, 13) by means of the adding device (14).

7. A device as claimed in Claim 6, characterized in that the addressing device (15) is an analog-to-digital converter for converting the analog signal representing the distance ( ? ) into a digital signal and for eliminating the least-significant bits of the digital signal.

8. A device as claimed in Claim 1, by means of which the absorption for discrete points can be calculated, characterized in that it comprises an adding device (29) for adding the measuring values, or the values derived therefrom (?1...?6), assigned to the adjacent image points, a storage device (30,...34) assigning the sum value thus formed to the element of the layer to be examined which is defined by the image points, the display device ( 35) displaying the absorption or the emission in the individual regions of the layer on the basis of the sum values.

9. A device as claimed in Claim 8, characterized in that three shift registers (30, 33, 34) are connected to an output of the adding device, a first input of the adding device (29) being connected to the output of the intermediate store, a second input of the adding device ( 29) being connectable at option to the output of one of the shift registers.

10. A device as claimed in Claim 1, 2 or 3, characterized in that the allocation device comprises two step generators (23, 24; 25, 24' ) which generate a stepped signal, the step amplitude of the two generators being variable independent of each other and each step generator consists of an adding circuit (24; 25) and a subsequent auxiliary store (23; 24' ), the output thereof being fed back to the input of the adding circuit, it being possible to apply an initial value (x0, y0) to the input of the said adding circuit at the beginning of a line and/or at the beginning of an image.

11. A device as claimed in Claim 1, 2 or 3, in which the strips extend parallel relative to each other, characterized in that the allocation device comprises two step generators (23, 24; 25, 24') which generate a stepped signal, the step amplitude of the two generators being variable independent of each other and the step generators are followed by a further adding circuit (26) which forms the address in the intermediate store (28) on the basis of the sum of the output signals of the step generators.

12. A device as claimed in Claim 1, 2 or 3, characterized in that the allocation device comprises two step generators (23, 24; 25, 24') which generate a stepped signal, the step amplitude of the two generators being variable independent of each other and the step amplitude of the output signals of the step generators and the initial values thereof can be controlled for each measuring series by a calculating device in dependence of the direction (.theta.) of the strips along which the measuring series have been measured.

13. A device as claimed in Claim 1, 2 or 3, characterized in that the allocation device comprises two step generators (23, 24, 25, 24') which generate a stepped signal, the step amplitude of the two generators being variable independent of each other and each of the steps of the output signals of the step generators and the initial values thereof each stored for all measuring series in a cyclical shift register whose contents are shifted through one position for each image or for each measuring series.

14. A device as claimed in Claim 9, characterized in that one of the shift registers comprises a mechanical drum or disc store, the clock signals being synchronized to the store.