EP0348433A1

EP0348433A1 - Method and apparatus for contrast equalization of an x-ray image.

Info

Publication number: EP0348433A1
Application number: EP88903228A
Authority: EP
Inventors: Hugo Vlasbloem
Original assignee: Optische Industrie de Oude Delft NV
Current assignee: Optische Industrie de Oude Delft NV
Priority date: 1987-04-02
Filing date: 1988-03-28
Publication date: 1990-01-03
Anticipated expiration: 2008-03-28
Also published as: IN169731B; WO1988007807A1; JP2651229B2; IL85959A0; EP0348433B1; NL8700781A; DE3877749T2; CN1032234C; DE3877749D1; CN88101717A; JPH02502868A; IL85959A

Abstract

Le procédé décrit sert à égaliser les contrastes de photographies prises aux rayons X avec un appareil de radiographie à diaphragme à fente. Un dispositif d'absorption commandable (8) agit en interaction avec le diaphragme à fente (2) de façon à influer sur un faisceau de rayons X en éventail (3) par secteur. Le dispositif d'absorption (8) est commandé (10) en tant que fonction de la quantité de rayonnement instantanément transmise (11) par secteur à travers un corps (7), de sorte que la quantité de rayonnement transmise dans un secteur particulier à travers le dispositif d'absorption diminue au fur et à mesure qu'augmente, à partir d'une première valeur seuil en direction du haut une valeur correspondant à la transmission du corps se produisant instantanément dans ledit secteur. Au-dessus de la valeur seuil, une valeur de transmission supérieure a pour conséquence de produire dans une mesure prédéterminée une quantité essentiellement supérieure de rayonnement transmis à travers le corps, au moins dans les régions d'importance pour la photographie aux rayons X.The method described serves to equalize the contrasts of X-ray photographs taken with a slit diaphragm radiographer. A controllable absorber (8) interacts with the slit diaphragm (2) to influence a fan-shaped X-ray beam (3) per sector. The absorber device (8) is controlled (10) as a function of the amount of radiation instantly transmitted (11) per sector through a body (7), so that the amount of radiation transmitted in a particular sector to through the absorption device decreases as a value corresponding to the transmission of the body occurring instantaneously in said sector increases, starting from a first threshold value in the direction of the top. Above the threshold value, a higher transmission value results in producing to a predetermined extent a substantially greater amount of transmitted radiation through the body, at least in the regions of importance for X-ray photography.

Description

Title: Method and apparatus for contrast equalization of an X-ray image

The invention relates to a method for contrast equalization of X-ray photographs of a body having a locally varying transmission for X-ray radiation, made with an apparatus for slit radiography which is provided with a slit diaphragm, by means of which a body is scanned with a flat fan-shaped X-ray beam, and with at least one controllable absorption apparatus which interacts with the slit diaphragm and with which the fanshaped X-ray beam is influenced per sector, which absorption apparatus is controlled as a function of the quantity of radiation instantaneously transmitted per sector through the body in a manner such that the quantity of radiation transmitted in a particular sector through the absorption apparatus is reduced with an increasing value of the transmission of the body occurring instantaneously in said sector from a first threshold value upwards, and also an apparatus for slit radiography equipped for taking equalized X-ray photographs.

A general problem in taking X-ray photographs is that the dynamic range of the radiation incident on the X-ray detector is larger than the dynamic range of the available image-forming means, in particular of X-ray film.

In taking, for example, X-ray photographs of the thorax, the variation in the transmission of the thorax for X-ray radiation is much greater than can be reproduced by the X-ray films usual for X-ray photographs. The consequence is that, in many cases, several

X-ray photographs are necessary to be able to carry out a good investigation of the various parts of the thorax. If a photograph is taken which is such that the lungs are shown with a good contrast reproduction, it is usually almost impossible to distinguish the abdominal region any longer. If the photograph is taken, however, in a mannersuch that both the lungs and the abdominal region are clearly visible, the contrast reproduction is then unsatisfactory. All of this is also related to the choice of film, the method of development, and the setting of the X-ray tube.

Taking several photographs obviously also results in a higher exposure to radiation for the patient.

In the past attempts have already been made to eliminate the problem described above. Thus, for example, Dutch Patent Application 8401411 describes an apparatus for slit radiography, by means of which the thorax of a patient, or another part of the body or object to be examined, is scanned with a flat fan-shaped X-ray beam, transmitted by a slit diaphragm, in a direction which is transverse to the longitudinal direction of the slit of the slit diaphragm. Furthermore, an absorption apparatus is provided which interacts with the slit diaphragm and which has absorption elements which, while the X-ray photograph is being taken, are instantaneously controlled under the influence of electrical signals generated by detection means which detect the quantity of radiation transmitted by the patient at every instant in every sector in order to match the quantity of radiation, which reaches the patient or the object, locally to the patient transmission at that point.

In this manner, the dynamic range in the radiation transmitted by the patient (or the specimen under investigation) can be matched to the dynamic range of the available image-forming means. This technique is in fact termed contrast equalization.

According to the technique described in the Dutch Patent Application 8401411, the absorption elements are controlled in such a manner that, above a certain value of the quantity of X-ray radiation transmitted locally through the patient or the specimen, the associated absorption element(s) is (are) always brought to a state which corresponds to the exposure of the image-forming means associated with said particular value.

In a practical situation, this means that the exposure of the image-forming means, such as an X-ray film, increases in proportion to the patient transmission. starting from a low value until a predetermined value of the patient transmission is reached, and remains constant with further increase in the values of the patient transmission. This produces X-ray photographs in which the averaged picture halftone of every region corresponding to an absorption element is essentially equal to that of every other region corresponding to an absorption element. In such a photograph, details which fall within a region instantaneously covered by an absorption element are clearly visible, but contrast differences which occur over larger regions and which, for example, are encountered in pneumothorax and some large tumours are difficult to distinguish.

Such photographs are also somewhat unnatural because the local blackening is no longer proportional to the local patient transmission.

There is therefore a need for an equalization method which results in natural photographs and in photographs which make it possible to satisfactorily detect contrast differences over relatively large regions.

The object of the invention is to meet said need and, in general, to provide a reliable and expedient method for contrast equalization of an X-ray image. For this purpose, according to the invention, a method of the type described is characterized in that, above the threshold value, a higher transmission value results to a predetermined extent in an essentially higher quantity of radiation transmitted through the body, at least in region of relevance for the X-ray photograph. An apparatus for slit radiography equipped for taking equalized X-ray photographs and comprising a combin tion of an X-ray source and a slit diaphragm for forming a flat fan-shaped X-ray beam by means of which a body can be scanned, an X-ray detector for collecting the radiation transmitted through the body, at least one absorption apparatus which is situated near the slit diaphragm and which can influence the quantity of X-ray radiation transmitted through the slit diaphragm instantaneously per sector of the fan-shaped beam under the influence of suitable regulating signals, detection means which detect the quantity of X-ray radiation instantaneously transmitted through the body per sector of the fan-shaped beam and deliver an input signal to a regulating apparatus which forms output signals acting as regulating signals for the absorption apparatus, which regulating apparatus is equipped to form, starting from a first threshold value of the quantity of radiation transmitted through the body in a particular sector, a regulating signal which controls the absorption apparatus in a manner such that the quantity of radiation presented in said sector is partially absorbed, is characterized according to the invention in that, above the threshold value of the quantity of radiation transmitted through the body, a higher value corresponds to a predetermined extent to an essentially higher transmission at that point, at least in regions of relevance for the X-ray photograph.

The invention will be explained in more detail below with reference to the accompanying drawing. Figure 1 shows diagrammatically an apparatus for slit radiography in side view;

Figure 2 shows an example of a suitable control circuit for controlling according to the invention absorption elements shown in Figure 1; Figure 3 shows a graph of the relationship between screen dosage and patient transmission, or film blackening, to illustrate the invention;

Figures 4 and 5 show details of a modification of an apparatus according to the invention. Figure 1 shows diagrammatically an apparatus for slit radiography in side view. The apparatus shown comprises an X-ray source 1 with an X-ray focal point F. Placed in front of the X-ray source is a slit diaphram 2 by means of which a fairly flat, fan-shaped X-ray beam 3 which is directed at an X-ray detector 4 is formed. As shown in Figure 1, although the X-ray beam 3 is somewhat wedge-shaped in side view, the height at the position of the X-ray detector is small, for example 3 cm, while the width of the beam perpendicular to the plane of the drawing may be, for example, 40 cm, so that, in general, reference is made to a flat X-ray beam.

The X-ray source and the slit diaphragm are able to move together in a manner such that the X-ray beam performs a scanning movement transversely to the width direction of the beam, that is to say, vertically in the plane of the drawing, as indicated by a double arrow 5. Such a scanning movement can be achieved in a simplemanner by causing the combination of X-ray source and slit diaphragm to swivel about an axis extending transversely to the plane of the drawing through the X-ray focal point F as indicated by an arrow 6. A flat fan-shaped beam which performs a scanning movement can, however, also be obtained in another manner, such as, for example, specified in the Dutch Patent Application 8401411.

In the example shown, the X-ray detector 4 is a conventional large-image cassette which is exposed stripwise in the vertical direction during the scanning movement of the X-ray beam. Instead of such a stationary large-image . cassette, it will also be possible to use a strip-type

X-ray detector which converts the incident X-ray radiation into a strip-type light image which is used in turn to expose a photographic film. An example of such an application of a strip-type X-ray detector is shown in the Dutch Patent Application 8401411.

In order to be able to regulate the quantity of X-ray radiation which is supplied to a patient or a specimen 7 under examination at a particular instant and in a particular sector of the X-ray beam, as a result of which the exposure of the corresponding section of the

X-ray detector is also regulated, an absorption apparatus 8 is placed near the slit diaphragm 2 in the X-ray beam. The absorption apparatus is equipped in a manner such that the quantity of radiation transmitted per sector of the X-ray beam and at every instant can be regulated under the influence of suitable regulating signals.

Some examples of suitable absorption apparatuses have been described in the Dutch Patent Application 8400845. By way of example, an absorption apparatus comprising a number of tongues 9 placed next to each other, one of which can be seen, is shown in Figure 1. The tongues have free ends which, under the influence of regulating signals can be introduced into the X-ray beam to a greater or lesser extent in order to absorb part of the X-ray radiation.

The regulating signals for the absorption apparatus are provided by a regulating circuit 10. The regulating circuit 10 receives input signals from a detection apparatus 11 which instantaneously detects the amount of X-ray radiation transmitted through the patient or the specimen 7 per sector of the fan-shaped X-ray beam and delivers corresponding electrical output signals.

The detection apparatus may be situated between the patient or the object and the X-ray detector 4, as shown in Figure 1 but in principle it can also be situated behind the X-ray detector 4. In both cases the detection apparatus may respond either directly to incident X-ray radiation or to light radiation generated by the X-ray detector in response to incident X-ray radiation.

If the detection apparatus is situated between the patient or the specimen 7 and the X-ray detector 4, the detection apparatus should be as' transparent as possible for X-ray radiation so that the final X-ray image is influenced as little as possible by the detection apparatus. Suitable detection apparatuses are, for example, described in the Dutch Patent Application 8503152 and the Dutch Patent Application 8503153.

Figure 2 shows an example of a suitable regulating circuit 10 for application of the invention. The regulating circuit 10 forms a connection between the detection apparatus and the absorption apparatus and comprises in principle an associated sub—circuit for each set of corresponding sections of the detection apparatus and the absorption apparatus. Of said sub-circuits only one is shown and this will be termed regulating circuit below for the sake of simplicity. Attention is drawn to the fact that in practice some parts of the regulating circuit can be used jointly for all the sub-circuits by means of multiplex techniques.

The regulating circuit shown in Figure 2 is essentially identical to the regulating circuit shown in the Dutch Patent Application 8401411. The regulating circuit 10 receives, on the one hand, input signals from a section of the detection apparatus via the conductor 20 and provides, on the other hand, output signals to a corresponding section of the absorption apparatus via a conductor 21. The regulating circuit comprises a comparator circuit, which in this example comprises a reference amplifier 23. A signal proportional to the input signal of the regulating circuit is fed to one input of the reference amplifier via a conductor 24 and a reference signal which is provided in the example shown by a potentiometer 25, is fed to the other input. The output signal of the amplifier 25 corresponds at least in polarity to the difference between the signals supplied to the two inputs. The output of the amplifier 23 is connected to the input of an amplifier 26 which may be a voltage amplifier or a current amplifier depending on the type of absorption apparatus used and which forms a suitable output signal for the control of the appropriate section of the absorption apparatus. This output signal controls the absorption apparatus in a manner such that the difference between the input signals of the reference amplifier is reduced to zero. In the example shown, the regulating circuit 10 furthermore comprises in addition an input amplifier 27, the output signal of which is supplied to the reference amplifier 23. The regulating circuit described hitherto corresponds to the circuit shown and described in the Dutch Patent Application 8401411. Regardless of the transmission, use of such a circuit results in a constant average picture halftone (optical density) of the final X-ray photograph over every region influenced by a particular section of the absorption apparatus. Said picture halftone is dependent on the setting of the potentiometer 25. If the potentiometers 25 of all the sub-circuits have the same setting, the final X-ray photograph consequently has this picture halftone averaged over the whole image area.

As has already been pointed out above, in certain situations there exists the need to be able to detect contrast differences between relatively large parts of the X-ray photograph satisfactorily. According to the invention this requirement can be satisfied by controlling the absorption elements of the absorption apparatus in a manner such that the difference between the input signals of the reference amplifier is not completely eliminated but a certain proportionality continues to be maintained between the local patient transmission and the local picture halftone of the X-ray photograph. For a more detailed explanation reference is made to Figure 3. Figure 3 shows in the right-hand part the relationship between the patient transmission (or specimen transmission) T for X-ray radiation plotted along the horizontal axis and the screen dosage S plotted along the vertical axis. The screen dosage is the quantity of Xray r a d i a t i o n which reaches the X-ray detector. The left-hand part of Figure 3 shows the relationship between the screen dosage and the picture halftone resulting therefrom (optical density D) of an image on an X-ray film. The graph shown relates to a so-called reversal film which forms a faint image for a low exposure (after the film is developed) and which forms a dark image for a high exposure, and consequently for a high X-ray dosage. The invention is, however, equally applicable for the use of other types of film or other imageforming means.

In the right-hand part of Figure 3, a thorax is furthermore also depicted diagrammatically for the purpose of illustration. The lung region is indicated by I and the abdominal region is specified by II. The lungs are the most transparent for X-ray radiation and the abdomen is the least transparent. For the purpose of illustration, the associated patient transmission ranges or the optical density ranges are also indicated by I and II in the graphs.

The right-hand part of Figure 3 shows a number of characteristic curves 30, 31, 32 and 33. The characteristic curve 30 indicates the relationship between the patient transmission and the screen dosage if any form of influencing of the scanning X-ray beam is absent while a photograph is taken. The characteristic curve 30 is therefore essentially a straight line. It can be seen that the patient transmission in the abdominal region (hatched region II) corresponds to a screen dosage S₁ which in turn corresponds to a density range D₁ of the film used. Although the range D₁ does not fall completely within the optimum working range, indicated by W, of the film in which the film characteristic curve 34 shown is essentially linear, it is clear that the range S ₁ corresponds to a relatively wide range D₁ so that the contrast reproduction in this range is good.

The patient transmission in the lung region (hatched region I) is, on the other hand, much greater for the same setting of the X-ray source and corresponds to a screen dosage range S₂ which in turn corresponds to a density range D₂. The screen dosage range S₂ is situated, however, far outside the optimum working range of the film so that, with a screen dosage in this range, overexposure of the film occurs. The result thereof is a very dark photograph with a very poor contrast reproduction in addition, as should be evident from the relatively small width of the range D₂.

From the graphs shown it should be evident that, as a consequence of the large difference in transparency of the lungs and the abdomen, it is not readily possible to show both the lungs and the abdomen sufficiently clearly and with sufficient contrast on one and the same photograph. The solution to this problem described in the

Dutch Patent Application 8401411 is represented by the curve 31 which consists of a section 31a which approximately coincides with the straight line 30 and a section 31b which is situated beyond a point of inflection 31c and which essentially extends horizontally.

The section 31a corresponds to a range of low patient transmission in which the absorption apparatus is not, or virtually not, in operation. Beyond the point of inflection 31c, however, the regulating loop formed by the detection apparatus, the regulating circuit, the absorption apparatus and the X-ray beam aims at an averaged picture halftone determined by the setting of the reference signal generator 25 (Figure 2). If the sections of the absorption apparatus (and the sections of the detection apparatus) were to be infinitely small, such a regulation would result in a uniformly grey image. The sections of the absorption apparatus, however, each influence at any instant a region on the X-ray detector of dimensions which are not negligible, for example 4 x 4 cm. As a result thereof, contrast differences within such regions remain clearly visible in the final image. As a result of this contours also remain clearly visible. Contrast between larger regions, such as, for example, a difference in density between the left lung and the right lung such as may occur in pneumothorax are, however, not visible. Reference is in fact made to complete equalization. In this situation, furthermore, the part of the final X-ray photograph showing the lungs is not very natural, and this can be felt to be a drawback. On the other hand, overexposure is eliminated in this method of regulation.

According to the invention, use is therefore made of a regulation curve such as is shown at 32, the section of which beyond a point of inflection, which may again be the point of inflection 31c, has a slope which is between that of the straight line 30 and the horizontal part 31b of the curve 31. If such a regulation curve is used, the natural character of the final X-ray photograph is maintained because a larger patient transmission results in a greater film darkening, while no overexposure can nevertheless occur. The patient transmission range corresponding to the lung region I therefore corresponds in the case of curve 32 to a screen dosage range S₃ which falls within the working range W of the film and results in a film blackening in the density range D₃.

Such a regulation curve is obtained by not regulating the difference between the input signals of the reference amplifier completely to zero in the regulating circuit of Figure 2. All this can be achieved in practice by constructing the amplifier 26 with a gain control device 28. The set gain determines the slope of the regulation curve 32 beyond the point of inflection. More generally, the slope of the regulation curve 32 may be adjusted by a suitable adjustment of the loop gain in the circuit formed by the X-ray beam, the detection apparatus, the regulating circuit and the absorption apparatus.

Furthermore, the X-ray source should obviously be adjusted in such a manner that even in the least transparent part of the patient or of the specimen a screen dosage still occurs which is such that this part and the contrast occurring therein can still be satisfactorily reproduced by the film. This means in practice that in taking an equalized thorax photograph, a higher X-ray tube current is employed than is normally used.

For example, 125 mA instead of 30 mA.

It is pointed out that the function of the regulating circuit described can also be fulfilled by a suitably programmed microprocessor.

The regulating method described can also be modified in a manner such that the patient transmission range corresponding to the lung region I of a patient is reproduced within the working range W of the film with a better contrast reproduction than is the case for the regulation curve 32.

In Figure 3 it can clearly be seen that the contrast reproduction of the lung region I improves as the regulation curve 32 becomes steeper. The associated screen dosage range S₃ is then therefore greater and as a result, so is the associated density range D₃. If, however, to achieve this effect, a greater angle of slope of the curve 32 is chosen by a suitable adjustment of the gain regulation of the amplifier 26, the corresponding screen dosage range S₃ is shifted outside the working range W of the film. A steeper gradient of the regulating curve within the patient transmission range corresponding to the lung region I therefore has to be achieved in another manner.

According to the invention, for this purpose, the absorption apparatus can be constructed in a manner such that the absorption elements are able to influence the slit of the slit diaphragm only over a predetermined partof the height of the slit. This can be achieved, for example, by using a mechanical stop for the absorption elements which prevents the absorption elements completely shutting off the slit diaphragm. A similar effect can also be achieved in an electronic manner or by suit- able programming of a microprocessor controlling the absorption element.

A suitable mechanical stop can be constructed in many ways depending on the type of absorption elements used. If pivoting tongue-like absorption elements or sliding elements are used, use may be made of a cord or the like which is connected to an absorption element and limits the deflection thereof. All this is shown in Figure 4. The cord is indicated by 40 in the stretched state and by 40' in the rest state. In Figure 4 it can be seen that the slit S of the slit diaphragm 2 always remains clear over a part "a" of the total height.

A similar effect can be obtained by means of a stop 50, as shown in Figure 5. Since the stop is situated in front of the slit S, the stop has to be transparent to X-ray radiation. The stop may be constructed, for example, from perspex.

By using such a mechanical stop or the electrical equivalent thereof, a regulation curve of the type indicated at 33 in Figure 3 is produced. This curve comprises a first section 31a which coincides with that of the curve 31 in which the absorption apparatus is still not, or almost not, in operation and in which the screen dosage increases in proportion to the patient transmission. Past the point of inflection, there follows a sloping section 33a in which the absorption apparatus is in operation. The angle of slope of the section 33a can be adjusted in the manner already described.

Past a stop point 33b, there then follows a section 33c in which the absorption apparatus always has the maximum effect and the screen dosage again increases proportionately with increasing patient transmission as a consequence of the clear part of the diaphragm slit. The section 33c is thus in principle parallel to the straight line 30. If the stop point 33b as shown in

Figure 3 is suitably chosen, that is to say with a value of the patient transmission which is lower than the patient transmission in the lung region I, the screen dosage increases in proportion to the patient transmission in the entire lung region. The X-ray photograph has therefore a natural character both for the patient transmission values encountered in the abdominal region II and for the patient transmission vaIues encountered in the lung region I, at least if the X-ray film used is able to process the associated screen dosage values satisfactorily.

This latter is in fact the case. The screen dosage range S₁ has already been discussed earlier and the screen dosage range S₃' which corresponds to the point of intersection of the curve 33 with the range of patient transmission values associated with the lung region I essentially falls within the working range W of the film. A good contrast reproduction is therefore ensured.

It is pointed out that the position of the range S₃' can be adjusted by the choice of the point of inflection 31c by adjusting the angle of slope of the curve 33 beyond the point of inflection 31c and by the choice of the stop point 31b (adjustment of the stop 40 or 50). It is pointed out, furthermore, that the angle of slope of the section 33a could be adjusted in a manner such that the section 33a coincides with the section 31b of the curve 31. In fact a system of the type described in the Dutch Patent Application 8401411 is then produced, a stop point for the absorption elements being provided electrically or mechanically.

In Figure 3, the section 33c of the curve 33 is drawn parallel to the straight line 30. Theoretically it is possible to give the section 33c a different angle of slope. A less steep gradient can be obtained by effecting a certain increase in the operation of the absorption apparatus beyond the stop point 33b. If a mechanical stop device is used, this could be achieved by dividing the mechanical stop device into sections which correspond to different absorption elements and by setting up the stop sections in a sprung manner so that displacement is possible under the influence of a force exerted by an absorption element. All this can be achieved in the configuration shown in Figure 4 by including a (tension) spring in the connection 40. In a similar manner, for example, a (compression) spring 51 can be used in the configuration of Figure 5.

It is also possible to use a second absorption apparatus with associated regulating circuit for the part "a" of the slit S.

If an electrical stop point is used, all this can be achieved by altering the gain of the amplifier 26 for a screen dosage which is higher than the screen dosage associated with the stop point 33b or by changing the operation of a microprocessor.

In general it is, however, of more importance to allow the part 33c to run more steeply than the straight line 30 because the region S₃' is then increased, which leads to increase in the associated density range and, consequently, to an improvement in the contrast reproduction.

A steeper gradient of the part 33c can be obtained by causing the influence of the absorption apparatus, which is a maximum at the point 33b, to decrease again for screen dosage values which are higher than those associated with the stop point 33b. This effect can be achieved, for example, by means of a suitably programmed microprocessor or in an electronic manner.

The same effect is obtained by using a second absorption apparatus which normally shuts off the slit S over a certain height but, beyond the stop point of the first absorption apparatus, gradually opens with increasing screen dosage. It is pointed out that the preceding diverse modifications of the invention, for example of the circuit 10 or of the stop apparatuses described, are obvious to those skilled in the art.

Thus, in particular, if a microprocessor is used for regulating the absorption apparatus, it is possible to achieve a regulating curve which has more than one point of inflection and stop point. A similar effect can be obtained by using one or more additional absorption apparatuses which are controlled with separate regulating signals. A second absorption apparatus could be placed, for example, at the other side of the slit S and could have a point of inflection situated beyond the stop point 33b. Past such a point of inflection, a stop point could then also be created again. Such modifications are considered to fall within the scope of the invention.

Claims

C L A I M S

1. Method for contrast equalization of X-ray photographs of a body having a locally varying transmission for X-ray radiation, constructed with an apparatus for slit radiography which is provided with a slit diaphragm, by means of which a body is scanned with a flat fan-shaped X-ray beam, and with at least one controllable absorption apparatus which interacts with the slit diaphragm and with which the fan-shaped X-ray beam is influenced per sector, which absorption apparatus is controlled as a function of the quantity of radiation instantaneously transmitted per sector through the body in a manner such that the quantity of radiation transmitted in a particular sector through the absorption apparatus is reduced with an increasing value of the transmission of the body occurring instantaneously in said sector from a first threshold value upwards, characterized in that, above the threshold value, a higher transmission value results to a predetermined extent in an essentially higher quantity of radiation transmitted through the body, at least in regions of relevance for the X-ray photograph.

2. Method according to Claim 1, characterized in that the absorption apparatus is controlled in a manner such that, above a second threshold value of the local transmission which is higher than the first value, the corresponding sectors of the fan-shaped beam are not influenced by the absorption apparatus more than to a predetermined maximum extent, a pre determined part of the fan-shaped beam still being transmitted freely in each sector.

3. Method according to Claim 2, characterized in that the absorption apparatus is controlled in a manner such that, with a local transmission which increases starting from the second threshold value, the influencing of the corresponding sectors of the fan-shaped beam decreases starting from the said maximum extent.

4. Method according to Claim 1, characterized in that the absorption device is controlled in a manner such that, with a local transmission which increases starting from a second threshold value which is higher than the first value, the influencing of the corresponding sectors of the fan-shaped beam increases to an extent other than for transmission values lower than the second threshold value.

5. Method according to one of the Claims 1 to 4 incl., characterized in that the predetermined extent to which a higher transmission value leads to an essentially higher quantity of radiation transmitted through the body can be adjusted.

6. Method according to one of the Claims 2 to 5 incl., characterized in that the second threshold value is adjustable.

7. Apparatus for slit radiography equipped for taking equalized X-ray photographs and comprising a combination of an X-ray source and a slit diaphragm for forming a flat fanshaped X-ray beam by means of which a body can be scanned, an X-ray detector for collecting the radiation transmitted through the body, at least one absorption apparatus which is situated near the slit diaphragm and which can influence the quantity of X-ray radiation transmitted through the slit diaphragm instantaneously per sector of the fan-shaped beam under the influence of suitable regulating signals, detection means which detect the quantity of X-ray radiation instantaneously transmitted through the body per sector of the fan-shaped beam and deliver an input signal to a regulating apparatus which forms output signals acting as regulating signals for the absorption apparatus, which regulating apparatus is equipped to form, starting from a first threshold value of the quantity of radiation transmitted through the body in a particular sector, a regulating signal which controls the absorption apparatus in a manner such that the quantity of radiation presented in said sector is partially absorbed, characterized in that, above the threshold value of the quantity of radiation transmitted through the body, a higher value corresponds to a predetermined extent to an essentially higher transmission at that point, at least in regions of relevance for the X-ray photograph.

8. Apparatus according to Claim 7, characterized in that the regulating apparatus comprises a suitably programmed microprocessor.

9. Apparatus according to Claim 7 or 8, characterized by means for adjusting the loop gain in the circuit formed by the X-ray beam, the detection means, the regulating apparatus and the absorption apparatus.

10. Apparatus according to Claim 7 or 9, characterized in that the regulating apparatus comprises a comparator circuit which is able to compare an input signal with a predetermined reference signal that represents the said first threshold value, and in that the output of the comparator circuit is connected to an amplifier, the gain factor of which determines the said predetermined extent of absorption.

11. Apparatus according to Claim 9 or 10, characterized in that the gain factor is adjustable.

12. Apparatus according to one of the Claims 7 to 11 incl., characterized in that the regulating device is equipped to control the absorption apparatus, starting from a second threshold value of the quantity of X-ray radiation transmitted in a sector through the body which is higher than the first threshold value in a manner such that, in said sector, not more than a fixed maximum portion of the X-ray radiation is absorbed and in that the remainder of the radiation presented in said sector is freely transmitted.

13. Apparatus according to Claim 12, in which the absorption apparatus comprises absorption elements which interact with a slit of the slit diaphragm, the absorption elements being capable of being moved in front of the slit under the influence of the signals from the regulating apparatus to a greater or lesser extent in a direction transverse to the longitudinal direction of the slit, characterized in that the said second threshold value corresponds to an extreme position of an absorption element, the slit remaining free over a predetermined height.

1 4 . A p p a r a t u s a c c o r d i n g t o o n e o f t h e p r e c e d i n g Claims 7 to 13 incl., characterized by means which, at least in particular ranges of the value of the quantity of radiation transmitted through a body in a particular sector, the relationship between the regulating signals and the influencing of the X-ray beam caused by the absorption apparatus in response to the regulating signals can be altered.

15. Apparatus according to Claim 13 or 14, characterized in that the absorption apparatus is provided with at least one stop element which determines the extreme position of an absorption element.

16. Apparatus according to one of the Claims 7 to 11 incl. or 14, 15, characterized in that the regulating apparatus is equipped to control the absorption apparatus, starting from a second threshold value of the quantity of X-ray radiation transmitted through a body which is situated higher than the first threshold value, in a manner such that from the quantity of radiation presented in said sector, the portion which corresponds to the second threshold value is absorbed to a predetermined extent and in that the remaining radiation presented in said sector is absorbed to a predetermined second extent.

17. Apparatus according to Claim 15, characterized in that the stop element for each absorption element comprises a separate stop which can be displaced with a predetermined resistance by the absorption element.

18. Apparatus according to Claim 17, characterized in that the stop element comprises, for every absorption element, an element which is transparent for X-ray radiation, which is forced under the influence of spring force into a predetermined rest position which corresponds to a specific deflection of the absorption element and which can be moved out of the rest position by the absorption element against the spring force.

19. Apparatus according to Claim 17, characterized in that the stop element for every absorption element has a sprung element comprising an elastic connection between a fixed point and the absorption element.

20. Apparatus according to one of the Claims 12 to 15 incl., characterized in that the regulating apparatus is equipped to control the absorption apparatus in a manner such that, with a value of the quantity of X-ray radiation transmitted in a sector through the body which increases starting from the second threshold value, the absorption decreases proportionally, starting from the second threshold value corresponding to the maximum portion.

21. Apparatus according to one of the Claims 7 to

20 incl., characterized by at least one second absorption apparatus which is controlled by the regulating apparatus.

22. Apparatus according to Claim 21, characterized in that the regulating device is equipped to put the second absorption apparatus into operation, starting f r o m a rest position, if the quantity of X-ray radiation transmitted through a body in a particular sector is greater than a predetermined third threshold value.

23. Apparatus according to Claim 22, characterized in that the third threshold value is identical to the second threshold value.