METHOD AND DEVICE FOR SLIT RADIOGRAPHY

Abstract: Radiography method, in which method a fan-shaped x-ray beam is produced with the aid of an x-ray source and a slit-type diaphragm placed in front of the x-ray source, with which beam a body to be examined is at least partially scanned to produce an x-ray shadow image on an x-ray detector placed behind the body, which fan-shaped x-ray beam comprises a number of adjacently situated sectors, and in which method the transmitted x-ray radiation is modified instantaneously for each sector during the scanning movement by means of a controllable beam sector modulation device which cooperates with the slit diaphragm, and in which method the hardness of the x-ray beam provided by the x-ray source is cyclically varied. The fan-shaped x-ray beam which varies in hardness is simultaneously cyclically modulated spatially in synchronism with the hardness variation in all the sectors.

Description: Title: Method and device for slit radiography The invention relates to a slit radiography method, in which method a fan-shaped x-ray beam is produced with the aid of an x-ray source and a slit-type diaphragm placed in front of the x-ray source, with which beam a body to be examined is at least partially scanned to produce an x-ray shadow image on an x-ray detector placed behind the body, which fan-shaped x-ray beam comprises a number of adjacently situated sectors, and in which method the transmitted x-ray radiation is modified instantaneously for each sector during the scanning movement by means of a controllable beam sector modulation device which cooperates with the slit diaphragm, and in which method the hardness of the x-ray beam provided by the x-ray source is cyclically varied. The invention also relates to a device for applying the method. Such a method and device are disclosed by Dutch patent application 8601678. According to the technique disclosed by Dutch patent application 8601678, the hardness of the x-ray beam is cyclically varied in an identical manner for all the sectors with the aid of electrical or mechanical means and the beam sector modulators are controlled in such a way that they cyclically transmit part of the x-ray radiation presented cyclically for each sector. The beam sector modulators are controlled for this purpose by control signals which are provided by a control signal generator which comprises a detector placed between the body to be examined and the x-ray detector. The detector, also referred to as a dosimeter, measures the radiation transmitted through the body for each sector of the x-ray beam. The control signals provided by the control signal generator control the beam sector modulators in such a way that, during the scanning of the soft parts of the body, they essentially transmit soft radiation, whereas harder radiation is also transmitted in the case of less transparent parts. The hardness is modulated by supplying the x-ray source with a ripple voltage and/or by means of mechanical means which comprise one or more x-ray- attenuating elements which are cyclically moved through the x-ray beam. The advantage of this technique is that equalized radiograms can be obtained with it, that is to say radiograms which have a good contrast both in the light parts and in the dark parts without harder radiation being used than is necessary. If, therefore, a radiogram is, for example, taken in this way of the upper body of a patient, the radiologist is able to find adequate information in one and the same radiogram for both the lungs and the abdominal cavity of the patient without the lungs being exposed to unnecessarily hard radiation. A drawback of the known technique is that the beam sector modulators have to operate at the same frequency as that at which the hardness is modulated, an amplitude modulation and/or phase modulation of the cyclic movement of the beam sector modulators also having to be ef ected in addition. The control needed for this purpose is relatively complicated and also imposes high requirements on the beam sector modulators themselves. This last aspect is of importance, in particular, if piezoelectric tongues, optionally provided with a separate x-ray- absorbing element, are used as beam sector modulators. The object of the invention is to eliminate the drawbacks outlined and in general to provide an accurate and reliable slit radiography method and device. For this purpose, according to the invention, a method of the type described above is characterised in that the fan-shaped x-ray beam which varies in hardness is simultaneously cyclically modulated spatially in synchronism with the hardness variation in all the sectors. A slit radiography device comprising an x-ray source, a slit-type diaphragm which is placed in front of the x-ray source and produces a fan-shaped x-ray beam with which a body to be examined can be at least partially scanned to produce an x-ray shadow image of the scanned part of the body on an x-ray detector placed behind the body, a control signal generator which, during operation, supplies a signal representing the trans¬ mission of the body for each sector of the x-ray beam to control means, a controllable beam sector modulation device which cooperates with the slit diaphragm and which is able to modify the x-ray beam for each sector under the control of signals from the control means, and hardness variation means for providing an x-ray beam which varies in hardness in a predetermined cyclic way, which slit radiography device is characterised according to the invention by beam modulation means which, during operation, cyclically modulate spatially the x-ray beam, which varies in hardness, in synchronism with the hard¬ ness variation. The invention will be described in greater detail below by reference to the accompanying drawing of an exemplary embodiment. Fig. 1 shows diagrammatically an example of a slit radiography device; Fig. 2 shows diagrammatically an example of a predetermined cyclic modulation of the x-ray beam; Figs. 3 to 7 inclusive show various embodiments of mechanical modulation means which can be used in the invention; Fig. 8 shows diagrammatically in perspective an exemplary embodiment of a part of a slit radiography device according to the invention; Fig. 9 shows diagrammatically an exemplary embodiment of a slit radiography device according to the invention; Fig. 10 shows diagrammatically a further embodiment of a device according to the invention in a so-called soft radiation phase; and Fig. 11 shows diagrammatically an embodiment of Fig. 10 in a so-called hard radiation phase. Fig. 1 shows diagrammatically an example of a slit radiography device comprising an x-ray source 1, a slit diaphragm 2 placed in front of the x-ray source and x-ray detection means 3, such as an x-ray screen or x-ray film. The slit diaphragm 2 transmits a fan-shaped x-ray beam 4 having a relatively small thickness. During operation, the x-ray source and/or the slit diagram are moved in such a way that the x-ray beam 4 scans the x-ray detector 3. For this purpose, the x-ray source may, for example, be swivelled together with the slit diaphragm around an axis extending perpendicularly to the plane of the drawing through the x-ray focus f , as indicated by an arrow 5. If a body 6 to be irradiated is situated between the x-ray source and the x-ray detector, it is possible to take a radiogram of (a part of) the body 6 in this way. It is pointed out that, instead of a stationary x-ray detector, a moving strip-type x-ray detector can also be used in the way described in Dutch patent application 8303156. In order to be able to modify the amount of x-ray radiation transmitted through the slit diaphragm for each sector of the fan-shaped x-ray beam so as to take an equalized radiogram, controllable attenuation elements 7 are provided which cooperate with the slit diaphragm and act as beam sector modulators. The attenuation elements can be constructed in various ways, as described, for example, in Dutch patent application 8400845. In the example shown in Fig. 1, the attenuation elements are tongue-shaped and the free ends of the tongues can be swivelled into the x-ray beam to a greater or lesser extent under the influence of suitable control signals. The attenuation elements may, however, also be of slide-type, as also described in Dutch patent application 8400845. If the attenuation caused by the irradiated body is large in a particular sector at a particular instant, the attenuation element associated with said sector is moved completely or largely out of the x-ray beam. If, on the other hand, the attenuation caused by the body is low in a particular sector at a particular instant, the associated attenuation element is brought further into the x-ray beam. The attenuation elements can be constructed as elements which completely absorb the incident x-ray radiation, or as elements which partly absorb and partly transmit the incident x-ray radiation. In the case where piezoelectric tongues are used, the tongues themselves can also be used as attenuation elements or the free ends of the tongues may be provided with separate attenuation elements which, again, may partly or completely absorb the incident x-ray radiation. To generate the control signals needed for the attenuation elements, a control signal generator in the form of an x-ray detector is provided which is situated beyond the body 6 to be irradiated and which detects the radiation transmitted by the body 6 for each sector of the x-ray beam and delivers corresponding electrical signals. The x-ray detector may consist of a row of light detectors which are situated behind the x-ray detection means 3 at the level of the incident beam and which detect the quantity of light generated by the x-ray screen 3 under the influence of the incident x-ray radiation. It is also possible to detect the x-ray radiation transmitted by the x-ray screen 3. The x-ray detector may also be situated in front of the x-ray detection means 3 and in that case may consist of an elongated dosimeter, such as is described, for example, in the Applicant's earlier Dutch patent applications 8503152 and 8503153. Such an x-ray detector is indicated diagrammatically in Fig. 1 at 8 and is concomitantly moved in synchronism with the scanning x-ray beam, as indicated by an arrow 9. The signals originating from the dosimeter are fed via an electrical conductor 10 to a control circuit 11 which produces the control signals for the attenuation elements. In the technique described hitherto, a constant spectrum and a constant intensity of the x-ray beam delivered by the x-ray source has been assumed before it has been modified by the attenuation elements. According to the technique disclosed by Dutch patent application 8601678, on the other hand, the radiation flux and/or the hardness of the x-ray beam is modulated in a predetermined fixed way, while the x-ray beam is also modified sector-by-sector in addition with the aid of the attenuation elements. The predetermined fixed modification of the x-ray beam can be effected in various ways. According to a first method, the high voltage of the x-ray tube is modulated with a fixed ripple voltage. If a ripple voltage having the mains frequency (50 Hz or 60 Hz) is used, the high-voltage supply for the x-ray tube can be relatively cheap because in that case no measures are necessary to eliminate the ripple associated with the mains frequency, always present in the supply voltage. Fig. 2 shows an example of a modulated supply voltage V B for the x-ray tube. Such a voltage can be obtained in a simple way by rectifying a normal sinusoidal alternating voltage. The value of the x-ray tube supply voltage determines the hardness of the x-ray radiation, and that specifically in such a way that the hardness of the x-ray radiation increases for a higher value of V B . An x-ray tube energised with a supply voltage of the type shown therefore delivers an x-ray beam whose hardness cyclically increases in synchronism with the supply voltage from a minimum value to a maximum value and then decreases again to the minimum value. As an alternative, the fixed modulation can be effected with the aid of mechanical means. Such mechanical means should comprise one or more devices which cyclically screen the slit of the slit diaphragm. A first embodiment of such mechanical modulation means is shown in Fig. 3. In the embodiment of Fig. 3, a plate-type device 40 is placed between the x-ray source, of which only the x-ray focus f is shown, and the slit diaphragm 2. Like Fig. 1, Fig. 3 shows a cross section of the device. The slit diaphragm 2 and the slit S extend in a direction perpendicular to the plane of the drawing. The fan-shaped x-ray beam 4 which is produced in this way has a small thickness dimension which is visible in Fig. 3. The dimensions of the beam 4 in the direction perpendicular to the plane of the drawing are much greater and reach a size of approximately 40 cm at the position of the body 6. At that point the thickness of the beam 4 is 2 to 4 cm. A plan view of the beam 4 and the slit S can be seen in Fig. 6. The plate-type device 40 (seen in cross section in Fig. 4) extends over the entire length of the slit S and is shown in the position which leaves the slit completely free. A position of the device 40 which screens the slit is shown by broken lines. The plate-type device can swivel or rotate with respect to a longitudinal edge 41 thereof. It is possible to cause the plate-type device to swivel cyclically to and fro between the two positions shown, but it is equally possible to cause the plate-type device to rotate about the edge 40 or about a spindle joined thereto and extending perpendicular to the plane of the drawing. In the first case, the plate-type device can advantageously be made of piezoelectric material, in which case the device swivels to and fro between the two positions shown with respect to the permanently fixed edge under the influence of a cyclic control voltage. In the second case, a plurality of plate-type blades extending radially with respect to an axis of rotation may be used, producing a structure in which the blades extend over the entire length of the slit and therefore modify all the sectors simultaneously and in the same way. Such a structure could be described as a blade cylinder. It is also possible to use a plate-type device which slides up and down in front of the slit S as shown at 42 in Fig. 4. It is pointed out that the way of modulating the x-ray beam 4 in a fixed manner is independent of the chosen embodiment of the attenuation elements operating for each sector. In Fig. 3, the attenuation elements are shown by way of example as blade wheels, as described in greater detail in Dutch patent application 8601678, while in Fig. 4 the attenuation elements are tongue-shaped. It is also pointed out that the mechanical modulation means may be situated in front of or behind the slit as desired. This also applies to the attenuation elements, so that the position of the mechanical modulation means and the attenuation elements can be interchanged with respect to the embodiment shown in Figs. 3 and 4, or may be situated on the same side of the slit. This also applies to the embodiments still to be described below. Figs. 5 and 6 illustrate an alternative embodiment of mechanical modulation means which can be used in a system according to the invention. Fig. 5 shows in front view, for example seen from the x-ray focus f, a sector wheel 45 constructed from a central hub 46 which can rotate around a spindle 47, as indicated by an arrow 48. The hub is provided with a number of radial arms 49 made of material which attenuates x-ray radiation. In the example shown, four arms 49 are used, but it is possible to use more or fewer arms. In principle, one arm may be adequate. The sector wheel is arranged in such a way that, during operation, the arms rotate over the slit S. For this purpose, as shown in Fig. 6, the spindle 47 extends perpendicularly to the plane of the slit diaphragm 2. Fig. 6 shows a plan view of a device according to the invention provided with such a sector wheel. The space between the arms which attenuate x-ray radiation may be filled up with material which is transparent to x-ray radiation in order to impart more rigidity to the sector wheel, but it may also be open. In order to make the effect of the arms rotating over the slit s the same over the entire length of the slit, the arms may advantageously be constructed in sector shape, as indicated by broken lines in Fig. 5. A sector wheel as described above may also be constructed with consecutive sectors of two materials which both modify the x-ray beam but in different ways. An example is shown in Fig. 7. The sectors 50 may, for example, consist of lead and the intervening sectors 51 of, for example, copper. Other material combinations may also be used, such as, for example, aluminium and copper or lead and aluminium. It is possible to use a sector wheel in conjunction with a fixed beam modulation obtained by varying the high voltage of the x-ray tube. During the crest of the varying high voltage (Fig. 2) , when the x-ray beam contains both hard and soft radiation, this offers the possibility of filtering out the soft radi- ation by using a sector wheel whose arms which are situated at that instant in front of the slit block soft x-ray radiation. If a blade cylinder is used, a similar effect can be obtained by making the blades alternately of different materials. It is pointed out that Fig. 6 shows the attenuation elements 7 as straight tongues which extend parallel to one another. The tongues may, however, be placed in a fan-shaped configuration with a point of convergence situated in or near the x-ray focus. The tongues in such a fan-shaped configuration may also be of tapered construction. A plurality of sets of tongues which are placed, for example, behind one another and/or partially above one another may also be used. Dutch patent application 8601678 is deemed to be incorporated here. In the combination involving the varying supply voltage, the position of the attenuation elements is controlled, according to the known technique, in such a way that each attenuation element is in the open position during the intervals when the tube voltage is low, whereas the attenuation elements are in principle in the closed position in the intervening intervals, with the result that the x-ray beam is essentially intercepted in those intervals. According to the invention, on the other hand, use is made of a separate, spatially cyclic screen which moves through the x-ray beam in synchronism with the hardness modulation and which simultaneously modifies all the sectors of the x-ray beam. An example of such a screen is shown diagrammatically in Fig. 8. Fig. 8 shows diagrammatically a spatially cyclic screen 120 which, in the example shown, consists of a slit 123 produced between two parallel longitudinal elements 121, 122. The longitudinal elements extend parallel to the slit 124, shown by broken lines, of a slit diaphragm 125, which corresponds to the slit diaphragm 2 shown in Fig. 1. The screen can also move to and fro in a direction perpendicular to the longitudinal direction of the two slits, as indicated diagrammatically by an arrow 126. The elements 121 and 122 are opaque to x-ray radiation. The height of the slit 123, i.e. the dimension of the slit 123 in the direction in which the screen 120 is moved to and fro is less than the corresponding dimension of the diaphragm slit 124. The distance over which the screen 120 is moved to and fro is preferably such that, in one extreme position, the upper edge of slit 123 coincides with the upper edge of slit 124 and that, in the other extreme position, the lower edge of slit 123 coincides with the lower edge of slit 124. Various techniques are used to cause the screen 120 to move in the way indicated. In the exemplary embodiment shown, use is made of two robust strips 127, 128 of piezoelectric material which are each mounted in a fixed holder 129 by means of one end which is remote from the slit diaphragm and whose other ends jointly support the screen 120. The screen can be moved cyclically over the slit 125 of the slit diaphragm in the way indicated by the arrow 126 by controlling the strips 127, 128 in a suitable way with electrical signals via conductors 130 and 131. The screen should be moved in synchronism with the modulation of the x-ray beam already present. The slit 123 of the screen may be situated, for example, at the instant when the x-ray radiation is softest, that is to say, at instance tl, t2, t3, t4, t5 etc. in Fig. 2, at the level of the lower side of the slit 124 of the slit diaphragm, or as an alternative, precisely at the upper side thereof. At the intervening times t6, t7, t8, t9, etc. , the x-ray radiation is hardest and the screen slit 123 is situated at the level of the opposite side of the diaphragm slit 124. The frequency of the hardness modulation and of the screen 120 may be, for example, 50 Hz, but other frequencies, preferably higher frequencies such as 100 Hz are also possible. The tongues 127 and 128 may be sturdy, robust tongues because said tongues are not just required to modify a narrow sector of the x-ray beam as is in fact the case for the tongues which form the beam sector modulators. The beam sector modulators are controlled in the normal way as described, for example, in Dutch patent application 8400845 or in Dutch patent application 8401411. Preferably, the beam sector modulators also comprise piezoelectric tongues, as already stated above. A beam sector modulator device which is shown by way of example in an embodiment which comprises a number of piezoelectric tongues is indicated in Fig. 8 by 132. It is pointed out that the invention is not restricted to a particular type of beam sector modulator, but that any known beam sector modulator can be used. The device 132 is situated in this example between the strips 127, 128 which are situated outside the region of the diaphragm slit 124. It is pointed out that, in the example shown, the screen 120 is situated at the side of the slit diaphragm 125 remote from the x-ray source, which is not shown in Fig. 8. As an alternative, the screen could be situated at the other side of the slit diaphragm. In that case, the strips 127, 128 or other drive means could also be situated at the other side of the slit diaphragm or could extend on either side of the slit diaphragm 124. The vulnerable beam sector modulators must not now be caused to vibrate relatively rapidly and continuously. If desirable, it is possible to vary the phase of the screen vibration with respect to the phase of the beam modulation already present. All this is shown diagrammatically in Fig. 9. Fig. 9 shows diagrammatically an example of a device according to the invention in a similar way to Fig. 1. The device shown in Fig. 9 differs from the device shown in Fig. 1 in the presence of a cyclically moving screen 120. The movement of this screen is controlled by a device 140 and is synchronised with the cyclical modulation of the x-ray beam which is effected by a device indicated diagrammatically by 141. The modulation device 141 may, as already stated above, act on the beam 4 before it reaches the slit diaphragm 2, as is symbolised diagrammatically by an arrow 142. Examples of such devices and effects have already been described above within the scope of Figs. 2 to 7 inclusive. The modulation device may also modify the supply voltage of the x-ray source 1 directly, as described in relation to Fig. 2. The synchronisation between the modulation device 141 and the control device 140 is diagrammatically indicated by the link 143. The control device 140 again receives control signals from the detector 8 and the phase of the screen movement with respect to the cyclic beam modulation may vary on the basis of the control signals. Like Figs. 1, 3 and 4, Figs. 10 and 11 again show a cross section of the device according to the invention. Figs. 10 and 11 again shown the section of the device around the slit diaphragm 2 having the slit S. The x-ray focus f and, very diagrammatically, the beam sector modulation device 132 are also shown. The device shown in Figs. 10 and 11 is a special embodiment of the device according to Fig. 9. The cyclically moving element 120 comprises an oblong element 151 to be described in greater detail and a drive device 152 for moving the element 151 to and fro in the direction of the arrow 153. The direction of the arrow 153 with respect to the x-ray beam 4 is the same as the direction of the arrow 126 in Fig. 8. Drive device 152 may, for example, again be a piezoelectric drive device, as described above within the scope of Fig. 8, but other drive devices may also be used. An example of another drive device is a continuously rotating motor with an output spindle, on which output spindle an eccentric is mounted, which eccentric drives a rod linked to the element 151. Many other embodiments of such a drive device for generating a to and fro movement are also obvious to a person skilled in the art. The reference numerals 140 and 143 indicate diagrammatically that drive device 152 ensures that the to and fro movement of element 151 is synchronised with the modulation of the x-ray beam 4. The element 151 has a length which is at least equally as great as the length of the slit S (the length direction is the direction perpendicular to the drawing plane of Figs. 10 and 11). The height h of the element 151 is at least twice as great as the height H of the slit (the height direction coincides with the direction in which the element 151 is moved to and fro) . The element 151 consists of three mutually joined strips of material 154, 155 and 156. The materials of the strips 154, 155 and 156 have the following properties. The material of strip 154 absorbs x-ray radiation to a large extent, examples being lead and tantalum. Strip 154 has a thickness such that, viewed practically, no x-ray radiation is transmitted. The material of strip 155 is chosen in such a way that it transmits virtually all the hard x-ray radiation but only a small amount of soft x-ray radiation. An example of such a material is aluminium. The thickness of the strip 155 is chosen in such a way that the hard x-ray radiation undergoes very little attenuation, whereas the soft x-ray radiation is attenuated to an appreciable extent. The material of the strip 156 is chosen in such a way that even soft x-ray radiation is still transmitted to a large extent. An example of such a material is palladium. The heights of the strips of material 154, 155 and 156 are consecutively chosen as 1/2 H, 1/2 H, and H. The operation of the device according to Figs. 10 and 11 is as follows. The x-ray beam 4 is cyclically modulated. As has already been described above, the modulation may be carried out at the x-ray source itself, between the x-ray source and slit diaphragm 2 or after the x-ray radiation has passed through the slit diaphragm 2. For the simplicity of the description, it is assumed in the following description of the operation of the device according to Figs. 10 and 11 that the x-ray beam 4 has already been modulated. The x-ray beam 4 is modulated in such a way that soft x-ray radiation on its own is alternated with hard and soft x-ray radiation simultaneously. The synchronisation, due to device 140, of the movement of the element 151 with the modulation of the x-ray beam 4 has been chosen as follows. During the time when soft x-ray radiation is present on its own in the x-ray beam 4, the element 151 is in the uppermost position shown in Fig. 10. In that position, the strip 156 is entirely in the x-ray beam 4 and transmits the soft x-ray radiation completely. As already described in the earlier Dutch patent application 8400845, the entire height H of the x-ray beam 4 is available for modulation by the beam sector modulators 7. It should be pointed out that a relatively large part of soft x-ray radiation is absorbed in human tissue and can give rise to tissue damage therein. In particular, this is important in examining the female breast, also referred to as mammography. Since the breast consists almost completely of soft tissue, the use of soft x-ray radiation is required in order to make tissue details in the breast visible. That is a consequence of the fact that the various tissues in the breast exhibit few absorption differences for the hard x-ray radiation and exhibit greater absorption differences for soft x-ray radiation. However, situated in the breast are also tissue structures which have such a large thickness that no acceptable image can be produced with an acceptable amount of soft x-ray radiation (in connection with the occurrence of tissue damage) . In this situation, the use of harder x-ray radiation is desirable. In a subsequent phase of the modulation of the x-ray beam 4, the beam contains hard x-ray radiation. That is shown in Fig. 11. The drive device 152 has also ensured that the element 151 is now in the lowermost position in which the uppermost half of the x-ray beam 4 is obstructed by the strip of material 154 and the lowermost half of the x-ray beam passes through the strip of material 155. The harder radiation does now come through the strip of material 155, but the soft radiation does not or comes through only to a small extent. In this way too, a device is therefore obtained in which not only does the x-ray beam 4 contain an alternatingly hard and soft radiation, but the hard radiation, viewed spatially, is available in a different part of the beam to the soft radiation and is therefore spatially modulated in synchronism with the hardness modulation. Still another embodiment of the cyclically moving screen is that in which, just as in Figs. 10 and 11, an element 151 is provided which also consists of three strips of material. However, the strips of material now have the sequence 155-154-156, i.e. transmitting only hard radiation, absorbing everything and transmitting soft radiation. Each of the strips then has a height of 1/2 H, so that the total height h is equal to 1.5 H. The total stroke length is 1/2 H, so that, in the uppermost position, the upper half of the x-ray beam 4 contains only hard radiation transmitted by strip 155 and the lower half contains no radiation (strip 154) . In the lowermost position, on the other hand, the upper half of the x-ray beam contains no radiation (strip 154) and the lower half contains soft radiation (strip 156) . It is pointed out that, after the above, various modifications are obvious to the person skilled in the art. Thus, for example, another type of beam sector modulation device can be used which does not contain any piezoelectric tongues. The screen can be driven in a way other than with the aid of piezoelectric strips. The screen may also have more than one slit. The screen may also be situated on the other side of the slit diaphragm to the beam sector modulators. If the screen and the beam sector modulators are both situated on the same side of the slit diaphragm, both these devices may be situated either on the x-ray source side or on the side of the slit diaphragm remote from the x-ray source. These and similar modifications are deemed to fall within the scope of the invention.

Claims: CLAIMS 1. Slit radiography method, in which method a fan- shaped x-ray beam is produced with the aid of an x-ray source and a slit-type diaphragm placed in front of the x-ray source, with which beam a body to be examined is at least partially scanned to produce an x-ray shadow image on an x-ray detector placed behind the body, which fan- shaped x-ray beam comprises a number of adjacently situated sectors, and in which method the transmitted x-ray radiation is modified instantaneously for each sector during the scanning movement by means of a controllable beam sector modulation device which cooperates with the slit diaphragm, and in which method the hardness of the x-ray beam provided by the x-ray source is cyclically varied, characterised in that the fan-shaped x-ray beam which varies in hardness is simultaneously cyclically modulated spatially in synchronism with the hardness variation in all the sectors. 2. Method according to Claim 1, characterised in that the spatial cyclic modulation is carried out by means of a screen which contains at least one longitudinal element which extends in the longitudinal direction of the diaphragm slit of the slit diaphragm and bounds at least one slit, which screen is moved to and fro over the diaphragm slit perpendicular to the longitudinal direction of the diaphragm slit. 3. Method according to Claim 1 or 2, characterised in that the phase of the spatial cyclic modulation is varied with respect to the cyclic hardness variation as a function of the x-ray radiation instantaneously transmitted by the body to be examined. 4. Slit radiography device comprising an x-ray source, a slit-type diaphragm which is placed in front of the x-ray source and produces a fan-shaped x-ray beam with which a body to be examined can be at least partially scanned to produce an x-ray shadow image of the scanned part of the body on an x-ray detector placed behind the body, a control signal generator which, during operation, supplies a signal representing the trans¬ mission of the body for each sector of the x-ray beam to control means, a controllable beam sector modulation device which cooperates with the slit diaphragm and which is able to modify the x-ray beam for each sector under the control of signals from the control means, and hardness variation means for providing an x-ray beam which varies in hardness in a predetermined cyclic way, which slit radiography device is characterised by beam modulation means which, during operation cyclically modulate spatially the x-ray beam, which varies in hardness, in synchronism with the hardness variation. 5. Device according to Claim 4, characterised in that the beam modulation means comprise a screen compris- ing at least one screen slit, which slit is situated near the diaphragm slit of the slit diaphragm and extends parallel thereto, and also drive means which, during operation, move the screen cyclically over the diaphragm slit perpendicularly to the longitudinal direction thereof. 6. Device according to Claim 5, characterised in that the drive means contain at least two strips of piezoelectric material which are clamped at one end, which can receive electrical control signals and which each have a free end situated near the slit diaphragm, the free ends jointly supporting the screen. 7. Device according to Claim 6, characterised in that the beam sector modulation device is placed between the piezoelectric strips. 8. Device according to one of Claims 4 to 7 inclusive, characterised by a control device 40 for the beam modulation means, which control device receives synchronisation signals from the hardness variation means. 9. Device according to one of Claims 4 to 7, characterised by a control device for the beam modulation means, which control device feeds synchronisation signals to the hardness variation means. 10. Device according to Claims 8 or 9, characterised in that the control device has an input which is connected to the control signal generator.

Inventors: GELUK RONALD JAN, GELUK, RONALD, JAN
Applicants: OPTISCHE IND DE OUDE DELFT NV, B.V. OPTISCHE INDUSTRIE "DE OUDE DELFT"
Application year: 1992
Main classification: G21K1
IPC codes: A61B6, G21K1