METHOD FOR CORRECTING IMAGE DISTORTION IN AN X-RAY IMAGE

Abstract: Method for correcting image distortion in an X-ray image which is obtained with the aid of an X-ray image device with an X-ray source mounted on a rotatable arm, an electrooptical image intensifier device positioned opposite the X-ray source and a digital image processor, comprising a calibration phase, in which deviations of the image are recorded with the aid of a calibration phantom and a correction phase, in which a correction is carried out on the basis of the recorded deviations, wherein the total image distortion is split into a geometrical component and a magnetic component; the geometrical component is split into a part caused by cushion-shaped distortion and a part associated with pixel dimensions; in the calibration phase, the pixel dimensions are calibrated and on the basis of an algorithm which contains exclusively geometrical parameters, the cushion-shaped distortion is determined and corrected; for a number of predetermined calibration positions of the rotatable arm, the residual image distortion is then recorded in a magnetic deformation table valid for at least a number of pixels and the X-ray image is separately corrected for the geometrical deformation and on the basis of the magnetic deformation table in the correction phase.

Description: METHOD FOR CORRECTING IMAGE DISTORTION IN AN X-RAY IMAGE The invention relates to a method for correcting image distortion in an X-ray image which is obtained with the aid of an X-ray image device which is provided with at least one X-ray source mounted on a rotatable arm, an electrooptical image intensifier device positioned opposite the X-ray source and a digital image processor, comprising a calibration phase, in which deviations of the image with respect to a calibration phantom are recorded with the aid of an image, obtained with the X-ray image device, of the phantom, and a correction phase, in which a correction of an image formed with the X-ray image device is carried out based upon the recorded deviations. The invention furthermore relates to an X-ray image device provided with means for using the method according to the invention. Correction of the image distortion is desired because an image true to reality is generally desirable, for example, in order thus to be able to make preparations for the treatment of a patient as well as possible. Images true to reality also make it possible to join together satisfactorily images of parts of a body situated next to one another or, for example, to form two-dimensional or spatial images from strip-like images with the aid of tomography procedures. In order to make possible a good assessment of an object to be examined, or in the case of a medical application, a good planning of a treatment for a patient, the accuracy of the X-ray image should be as great as possible. For medical applications, it is required in practical situations that the deviation between the reality and the X-ray image is less than 1 mm per 20 cm. Such an accuracy is achievable without problems if use is made of photographic imagings in which the X-ray radiation leaving a body to be examined is used to expose a photographic film. Use of film is, however, troublesome and time-consuming. An appreciable time elapses between the instant of exposure of the film and the instant at which the developed and dried film is available for the doctor. An image obtained electrooptically offers more and/or simpler possibilities of processing, storing and combining images. There is therefore a need for a method for obtaining X-ray images with the aid of electrooptical means, which X-ray images have the same order of accuracy as X-ray images obtained with the aid of photographic film. Since image distortions are unavoidable when electrooptical means are used, correction is necessary in this connection. A method of the type described above is disclosed in the US Patent Specification 4 736 399. This literature reference describes a method for correcting deformations in an output image obtained with the aid of an X-ray image system. The X-ray image system described comprises an image intensifier, which converts an X-ray shadow image obtained with the aid of an X-ray source into a visible image; an optical system; an image processing device and an image reproduction device. According to the known method, a raster pattern of lines intersecting one another at predetermined points is used to measure to what degree and in what direction the points of intersection of the lines in the final output image of the raster pattern are displaced with respect to the original raster pattern. A displacement vector is then calculated for each point of intersection and also for pixels situated between the points of intersection. Once the displacement vector is known for each pixel in the output image, the pixels in the output image can then be moved back in any radiogram in accordance with the displacement vectors to the corrected positions, which correspond to the original positions of the element of the object associated with each pixel in the output image . An advantage of this known procedure is that, in this way, all the image distortions which occur in an X- ray image device can be corrected in one operation once the displacement vectors are determined for all the pixels. A problem which arises in the known procedure is that the distortions in an output image provided by an X- ray image system are dependent on the position on earth where the X-ray image system is sited. This is a consequence of the fact that one of the causes of the image distortion is formed by the earth's magnetic field, which has a different value from place to place. In addition, the influence of a magnetic field is dependent on the position of an image intensifier with respect to the field lines of the earth's magnetic field. The same applies to any other electrooptical devices in which electrons travel through a free space between a cathode and an anode. Since X-ray image devices usually comprise at least one rotatable arm, one end of which carries an X-ray source and the other end of which carries an X-ray image intensifier, separate calibrations ought to be carried out for each position of the arm in order to be able to calculate the displacement vectors. This means that very many time-consuming calibrations are necessary in a practical situation. The object of the invention is to eliminate the drawback outlined and, in general, to provide a rapid and reliable distortion correction method for images obtained with the aid of an X-ray image device having at least one image intensifier tube. In this connection, use is made of the insight that the total distortion can be seen as the superposition of a geometrical distortion which can be completely calculated mathematically beforehand and which is associated with the geometry of the X-ray image device and of a magnetic distortion which can only be determined experimentally. A method of the type described above is therefore characterized according to the invention in that the total image distortion is split into a geometrical component and a magnetic component; in that the geometrical component is split into a part caused by cushion-shaped distortion and a part associated with pixel dimensions; in that, in the calibration phase, the pixel dimensions are calibrated and in that, on the basis of an algorithm which contains exclusively geometrical parameters, the cushion-shaped distortion is determined and corrected; in that, for a number of predetermined calibration positions of the rotatable arm, the residual image distortion is then recorded in a magnetic deformation table valid for at least a number of pixels; in that, in the correction phase, an X-ray image is separately corrected for the geometrical deformation and on the basis of the magnetic deformation table. An important advantage of the method according to the invention is that the result is thereby achieved that the distortion to be corrected is split into a location- independent geometrical component and a location- dependent, interpolatable magnetic component. Owing to the interpolatability obtained as a result of the splitting, the magnetic deformation has only to be determined for a limited number of (rotation) positions of the X-ray image device without thereby adversely affecting the reliability of the corrections obtained. It is pointed out that hereinafter the term "cushion-shaped distortion" or "cushion deformation" also means barrel-shaped distortion or barrel deformation, unless this is explicitly excluded or is absurd in the associated context. An X-ray image device comprising at least one X- ray source attached to a rotatable arm and at least one electrooptical image intensifier device mounted opposite the X-ray source for, optionally via an analogue/digital converter coupled to the image intensifier delivering the output image of the image intensifier device in the form of digital signals to a digital image processor is characterized, according to the invention, in that the image processor is equipped for the separate correction of deformations, occurring in the device, of geometrical or magnetic origin. The invention will be described in greater detail below with reference to the attached drawing. Figure 1 shows diagrammatically in perspective an example of an X-ray image device in which the invention can be used; Figure 2 shows an undistorted raster pattern and two examples of a raster pattern with cushion deformation; Figure 3 shows an example of magnetic deformation; Figure 4 shows diagrammatically a part of an X- ray image device; Figure 5 shows an example of a block diagram of a device according to the invention. Figure 1 shows diagrammatically in perspective an example of an X-ray device which is suitable for using the invention. The X-ray device comprises a fixed frame 2 , on which an arm 4 extending transversely to an axis 3 is mounted around the diagrammatically indicated, essentially horizontal axis 3. The arm 4 extends on either side of the axis 3 and has two unsupported ends. A transverse arm 6 extending from the frame is mounted in a cantilever fashion near the one unsupported end 5, which is normally the uppermost end of the arm 4 which is vertical when at rest. A transverse arm 8 which extends essentially parallel to the transverse arm 6 and which also points away from the frame 2 is mounted near the normally undermost end 7 of the arm 4. The arm 4 forms, together with the transverse arms 6 and 8 , a U-shaped structure which is placed on edge and whose arm 4 forms the base and the two transverse arms 6 and 8 of which form the limbs. The position of the transverse arms 6 and 8 can be adjusted along the arm 4 by means of adjustment devices which, in the example shown, comprise threaded spindles 9 and 10. Structures such as those formed by the arms 4, 6 and 8 are also known in other embodiments, such as, for example, C-shaped arms in which a separate distinction cannot be indicated between 'arms 4, 6 and 8' because there is a single, smooth curve. The transverse arm 6 carries at the unsupported end an X-ray source 11 which is directed at an image recording device 12 having an image intensifier tube which is situated at the unsupported end of the transverse arm 8. With the aid of a patient table 13 mounted in cantilever fashion, a patient can be positioned between the X-ray source 11 and the image recording device 12 in order to form an X-ray image of a part of the patient. The patient table is mounted in a known manner on a base 16 in such a way that the table 13 can be moved both in the longitudinal and in the transverse direction. In addition, the patient table can be adjusted in height as is normal. The patient table can also rotate around a vertical axis which preferably coincides with the connecting line between the centre of the X-ray focus of the X-ray source 11 and the centre of the input window 17 of the image recording device 12. Said connecting line is indicated at 18. With the aid of a device of the type shown in Figure 1 and described above, X-ray images of a (part of a) patient to be examined, for example for the purpose of simulating and planning radiotherapy, can be made from many different angles. To simulate and plan brachytherapy, use is often made of a device having a rotatable C-shaped arm whose two unsupported ends can be positioned on either side of a patient. Devices are also known in which the arm 6 again carries at the unsupported end a rotatable C-shaped arm with an X-ray source at the one end and an image recording device at the other end. The arm 8 is then either absent or present as a result of mechanical balancing and without image recording means. The invention can equally be used in such devices and other similar devices. Generally, the invention can be used in any X-ray image device which comprises electrooptical means for processing and reproducing an X- ray image . The invention is based on the insight that the image distortion encountered in an X-ray image device can be split into two types of components. The first type of component is a consequence of the imaging of an object plane having a first curvature on an image plane having a second curvature which differs from the first curvature. Thus, an important cause of distortion is the fact that, in the standard image intensifiers, the input phosphorus screen (the cathode) is curved. Another cause of image distortion is the curved electron wavefront which occurs in standard image intensifier tubes and which ultimately forms an output image on a plane output window or an output window curved with a radius of curvature which is different from the electron wavefront. The image distortion encountered as a result of these and similar effects is of the type which is usually described as cushion-shaped distortion. Said cushion- shaped distortion has a geometrical cause and is completely determined by the design and adjustment of the associated X-ray image device. The cushion-shaped distortion is, however, independent of the orientation of the X-ray image device and of the geographical location on earth where the X-ray image device is installed. Figure 2 shows diagrammatically a raster pattern at a, the same raster pattern with cushion-shaped distortion at b and the same pattern with barrel deformation at c. Depending on the geometry of an image processing device, cushion-shaped distortion or the complement thereof, i.e. barrel-shaped distortion may occur. The second type of component is a consequence of magnetic fields which influence the path of electrons in an image intensifier tube (Lorentz forces) . Magnetic fields result in image rotation, or S-shaped or spiral-shaped image distortion, respectively. Figure 3 shows diagrammatically an undistorted raster at a and an image distorted by magnetic deformation at b with image rotation and spiral- shaped distortion. The influence of a magnetic field on the track followed by an electron in an electrooptical device is dependent to a considerable degree on the direction of the magnetic field with respect to the track followed by said electron and therefore on the mutual position of the electrooptical device and the magnetic field. An X-ray image device having a rotating C- and/or U-shaped arm may be active in a very large number of operating positions. The image rotation or the spiral- shaped distortion, respectively, is different in each of said positions. In addition, an important and sometimes the only component of the magnetic field is the earth's magnetic field. The earth's magnetic field varies, however, with the geographical position of a location and is therefore different from place to place. Otherwise than is the case in the image correction method disclosed in US Patent Specification 4 736 399, according to which, after comparing a standard raster pattern with the image obtained therefrom by means of the associated X-ray image device and calculating displacement vectors for all the pixels, all the distortion components are corrected together in one operation, according to the invention, the image obtained by means of an X-ray image device is corrected separately for the mathematically completely precalculable geometrical deformation and for the magnetic deformation. The geometrical deformation comprises two types of cushion-shaped distortion, viz. the cushion-shaped distortion which is a consequence of the curvature of the input window of the image intensifier tube of the X-ray image device and the electrooptical deformation (cushion or barrel deformation) which is a consequence of the curved electron wavefront which travels towards the anode screen in the image intensifier tube during operation. The first type of cushion-shaped distortion is dependent on the distance L (Figure 4) between the X-ray focus f of the X-ray source of the X-ray image device and the input window of the image intensifier tube and on the radius of curvature R of the input window of the image intensifier tube. If the values of L and R are known, the displacement of each image point as a consequence of the first type of cushion-shaped distortion can be calculated with the aid of known formulae. Such formulae are described, for example, in an article entitled "Correction of Abberation in Image-Intensifier Systems" by E. Pietha and H. K. Huang, published in "Computerized Medical Imaging and Graphics", July-August 1992, Vol. 16, No. 4. If the displacement of each pixel resulting in the deformed image is known, the way in which the pixels of the deformed image should be moved back to obtain a corrected image is, of course, also known. In a specific device, the value of the parameter R is known from the specifications of the image intensifier tube used. The value of the parameter L is adjusted by the user every time a radiogram is made. This value is always known in the system, for example because it is entered from an operating panel 33 (Figure 5) or, for example because a detection device 37 known per se detects the mutual position of the arms 6 and 8. The value of the parameter L can be used together with the value of R to calculate correction values for each pixel. For this purpose, use is made of an image processor which is to be discussed in greater detail and to which the fixed value of R and the adjustment value of L are to be fed as input quantities. The electrooptical deformation has in fact the same cause as the first type of cushion-shaped distortion and is represented by a parameter R eo which is specific for each type of image intensifier tube. If said parameter is not stated by the manufacturer of an image intensifier tube, R eo can be calculated or determined empirically with the aid of test recordings of a phantom. A method of calculation for R eo can be derived from the book "Image tubes" by P. Czorba 1985 ISBN 0-672-22023.7. Figure 4 shows an example of a part of an X-ray image device having an X-ray source 11 with X-ray focus f, an image intensifier tube 12 having a curved cathode window or input window 20 with a radius of curvature R and a flat anode window or output window 21. The electron wavefront 22 moving towards the anode of the image intensifier tube during operation has a radius of curvature R eo . Said radius is dependent on the voltage prevailing at the focusing electrodes such as the electrode 23 and on the shape of the electrode 23. In this connection, it is pointed out that it is possible in the case of a number of image intensifier tubes to adjust the effective size of the anode screen by a suitable choice of focusing electrodes in the tube. Philips, for example, markets an image intensifier tube having a 15 inch anode screen which can also be adjusted in a 7 inch or 10 inch mode. Associated with each of these modes is a specific R eo and a specific cushion-shaped distortion (or barrel deformation) . Figure 4 shows an object 0 which is exposed to the X-ray source 11 during operation and which is imaged as image O τ in a plane 24 which touches the central point of the curved input window 20 of the image intensifier tube. In practice, the plane 24 is often referred to as the film plane by analogy with the situation in which use is made of photographic procedures in order to form an X- ray image on a film in said plane. Produced on the cathode screen 25 situated on the inside of the curved input window is a curved image 0 XI with cushion-shaped distortion, which is converted into an electron wavefront image 0 XII having a radius of curvature R eo . The image 0 ιτι is finally converted via the anode screen 25 and the output window 21 into an optical image 0 IV . The image 0 IV is deformed yet again with respect to the image 0 II∑ . According to the invention, the relationship between the actual position of a point in the image 0 τ in the film plane 24 and the position of this same point in the deformed image 0 IV can be approximated with adequate accuracy by the following formula: h' = h ( i + ( α - β ) h 2 ) in which h is the distance of an image point of the image O j in the film plane 24 from the central axis H of the image intensifier tube 12 and h 1 is the distance of the corresponding image point of the image 0 IV from the central axis H of the image intensifier tube 12; and in which a = 3 R + L ; and R^L β = 6 R eo . With the aid of this formula, the processor of the device can correct the cushion deformation. The above formula is based on the situation in which the central axis H of the image intensifier tube coincides with or is parallel to the connecting line between the X-ray focus f and the central point of the input window of the image intensifier tube. If both lines enclose an angle with one another, which is the case, for example, if a peripheral part of a patient has to be viewed in greater detail, the following formula of the same kind can be derived: h' = h ( 1 + Δ*γ*h + ( α - β + γ) h 2 ) in which h, h', and β are defined as above, in which the image intensifier tube is displaced over a distance Δ perpendicular to the direction of the central axis H and in which: γ = 2 RL The same applies if the anode screen is not flat but has a certain curvature. In the correction of the geometrical deformation, apart from the cushion-shaped distortion already discussed, the pixel dimensions also play a part. Proceeding from the deformed image of a suitable calibration phantom and, more particularly, from certain dimensions thereof in the central portion of the image, the pixel dimension is determined both in the X- direction and the Y-direction. These dimensions are then used in the correction of the image for all the pixels of the image. A suitable calibration phantom comprises a brass plate provided with bores made in accordance with a raster. The diameter of the bores may be, for example, 1.5 mm and the centre-to-centre spacing between two adjacent bores may be, for example, 20 mm. The pixel size can be determined by measuring the number of pixels between two points situated at a distance from one another in the (deformed) image ultimately formed. The points should be chosen in such a way that the associated spacing on the phantom is known. If the final image is displayed by way of example by means of a video camera or the like directed at the anode screen of the image intensifier, as is indicated in Figure 5 at 30, an A/D converter 31, a digital processor 32, a D/A converter 35 and a display device 36, in which case the total number of pixels is known both in the X- direction and the Y-direction of an image on the image screen of the display device, the number of pixels between two arbitrarily chosen points in the image of the display device can easily be determined. If the display device is, by way of example, an image screen coupled to a personal computer, the number of pixels between two points can easily be determined with the aid of a cursor preferably controlled by a mouse in conjunction with software known per se. If a phantom is used as described above, for example, the number of pixels between the imagings of two bores situated at a distance can be determined. Since the distance of the bores of the phantom is accurately known, the pixel size, defined as the actual dimensions in the phantom for each pixel, of the final image is consequently known. The pixel size is preferably determined in the X-direction and in the Y-direction and, specifically, in the centre of the image because the centre of the image is the least influenced by cushion deformation. The pixel size in the X-direction can then be made uniform with the aid of suitable software known per se. The same applies to the Y-direction. According to the insight on which the invention is based, the pixel size is dependent on the radius of curvature R of the image intensifier input plane, the radius r of the output image of the image intensifier or the radius of the field which is used for the output image of the image intensifier, and the distance L. In an existing device, only L is variable. During the pixel size calibration, the pixel size P is determined for a fixed value L 0 of the distance L, for example 1.5 m or 1.24 m. For another value of L, the following formula applies: P o* F o = P r' F r in which P 0 = the pixel size for the fixed value L 0 of L; F Q = 1 + α Q .r 2 and a Q = (3 R + L 0 ϊr 2 6 0 R 2 P r = the new pixel size for the actual value of L, viz. L r ; F r = 1 + α.r 2 where α r = (3 R + L r )r 2 6 L r R It may appear from the above that, when making an X-ray image recording, the geometrical deformation can automatically be corrected, including the determination and equalization of the pixel size, on the basis of the system parameters which are known at that instant and are used above in the various formulae and of which only L is normally variable. The equalization of the pixel size also provides at least in part a correction for an X-ray beam incident somewhat obliquely. "Obliquely" means in this connection that the connecting line between the X-ray focus f and the centre of the image intensifier input plane makes an angle with the centre line H of the image intensifier tube. To correct the magnetic deformation, use is made of tables which are obtained with the aid of the calibration phantom and which are stored in the memory of the processor of the X-ray image device. The calibration phantom is placed in the film plane 24 and recordings are then made of the phantom in a number of different rotation positions of the arms 4, 6 and 8. In this connection, the output images of the image intensifier 12 are digitized in one of the ways known therefor, for example in the way shown in Figure 5 with the aid of a video camera 30 or the like, optionally via an optical system and an A/D converter 31 and fed to an image processor 32. The adjusted value of L is fed to the image processor 32 from an operating device 34 provided with an operating panel 33. The value of L may, however, also be fed directly from the detection device 37 to the image processor 32. The values of R and R eo may either be stored in the operating device 34 or in the image processor 32 itself. The image processor 32 performs the geometrical correction, including the correction of the pixel dimensions, on the digitized image. The image thus corrected is converted to analogue form by means of an A/D converter 35 and displayed on a display device 36. The image displayed still contains therefore the magnetic distortion, but not the cushion-shaped distortion. The correction and display referred to here may take place on the basis of a program which is automatically processed by the image processor 32 on the basis of instructions entered for this purpose via an operating panel 38 or on the basis of separate instructions which are given to the image processor 32 via the panel 38. Since the dimensions and configuration of the calibration phantom are very accurately known, the magnetic distortion, which is the deviation with respect to the original, that is to say the calibration phantom itself or an undeformed imaging thereof can be determined for each pixel in the image of the display device. In this connection the deviations of the pixels at the position of the bores in the phantom could, if desired, be determined by direct comparison and the deviations of the pixels situated in between could be calculated by interpolation. The deviations are determined for each pixel, for example, in the X-direction and Y-direction. A deviation <SX(i, j) and 5Y(i, j) is then associated with pixel P'(i', j') in the deformed image. Said deviations are stored in the memory of the processor 32 in a magnetic deformation table or deformation matrix for each of a number of rotation positions of the arms 4, 6, 8. In a practical situation, for example, tables or matrices for, for example, eight rotation positions (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°) or even for sixteen rotation positions (each 22.5°). It sometimes occurs that a separate correction coil is already present, in which case the correction values of 0° and 360° differ. In that case, the tables or matrices may contain separate values for the deviations at 0° and 360°. If the X-ray image device is then used in one of the calibrated positions to make a recording of an unknown object or a patient, a direct correction of the magnetic distortion is possible with the aid of one of the tables. If a working position is used which is situated between two calibrated positions, a correction table or correction matrix is calculated by interpolation, proceeding from the tables or matrices associated with the respective adjacent calibration positions. As may appear from the above, according to the invention, the correction of image distortion in an X-ray image device which comprises at least one image intensifier tube comprises two phases. The first phase is a calibration phase and the second phase is the actual correction phase. During the calibration phase, the pixel dimensions are determined with the aid of a suitable phantom and the magnetic distortion vector is determined for a number of predetermined operating positions of the image intensifier tube. This last calibration should be carried out while the X-ray image device is installed at the definitive operational location. During the correction phase, the geometrical correction, including the adjustment of the correct pixel dimensions, is first performed. This is done with the aid of a relatively simple routine which can be performed rapidly on the basis of system parameters already known beforehand (R and R eo ) , a parameter (L) likewise known at the instant that a radiogram is made, and possibly the adjusted field diameter (2r) of the X-ray image intensifier. The magnetic correction for each pixel is then performed proceeding from tables determined during the calibration phase for a number of predetermined operating positions of the image intensifier tube, corresponding to certain rotation positions of the arms of the X-ray image device. If the actual position of the image intensifier tube during a recording does not correspond to one of the calibration positions, the correct correction table is derived by interpolation from two tables associated with adjacent calibration positions. It has been found that the separate corrections used according to the invention for the geometrical distortion and the magnetic distortion result in a very accurate final result, even if the magnetic correction table has to be obtained by means of interpolation. Both types of correction are also relatively simple and, consequently, can be performed relatively rapidly by the image processor. The corrected image is consequently available virtually immediately after a radiogram has been made. It is pointed out that the correction method according to the invention is very suitable for use in X- ray image installations for medical use. The method is, however, equally suitable for industrial X-ray image installations. Moreover, the above description has proceeded from a device having a single image intensifier tube. The invention is, however, equally of use in a device having more than one image intensifier tube and/or another electrooptical device. It is pointed out, moreover, that, during normal operation, the object is situated at some distance from the input window of the image intensifier tube, as can be seen in Figures 4 and 5. As a result, the projection 0 τ which is formed by the input image of the image intensifier tube is a linear enlargement of the object. If desired, this linear enlargement can be compensated for in the image processor or the display device by a corresponding linear reduction.

Claims: Claims 1. Method for correcting image distortion in an X- ray image which is obtained with the aid of an X-ray image device which is provided with at least one X-ray source mounted on a rotatable arm, an electrooptical image intensifier device positioned opposite the X-ray source and a digital image processor, comprising a calibration phase, in which deviations of the image with respect to a calibration phantom are recorded with the aid of an image, obtained with the X-ray image device, of the phantom, and a correction phase, in which a correction of an image formed with the X-ray image device is carried out on the basis of the recorded deviations, characterized in that the total image distortion is split into a geometrical component and a magnetic component; in that the geometrical component is split into a part caused by cushion-shaped distortion and a part associated with pixel dimensions; in that, in the calibration phase, the pixel dimensions are calibrated and in that, on the basis of an algorithm which contains exclusively geometrical parameters, the cushion-shaped distortion is determined and corrected; in that, for a number of predetermined calibration positions of the rotatable arm, the residual image distortion is then recorded in a magnetic deformation table valid for at least a number of pixels; in that, in the correction phase, an X-ray image is separately corrected for the geometrical deformation and on the basis of the magnetic deformation table. 2. Method according to Claim 1, characterized in that, to correct the magnetic deformation of an X-ray image formed at a position of the rotatable arm which does not correspond to a calibration position, a magnetic deformation table is calculated by interpolation of the tables associated with the two adjacent calibration positions. 3. Method according to Claim 2 , characterized in that, to correct the cushion deformation, an algorithm is used which is based on a formula in which the distance between the X-ray focus of the X-ray source and the input plane of the electrooptical image intensifier device occurs as a variable parameter. 4. Method according to Claim 3 , characterized in that, moreover, the formula contains as parameters the radius of curvature of the input plane of the electrooptical image intensifier device and the radius of curvature of the electron wavefront which travels in the electrooptical device between a cathode and an anode when in operation. 5. Method according to one of the preceding claims, characterized in that the calibration phantom used is a brass plate which is provided with a raster pattern of bores and which is situated immediately in front of the input window of the electrooptical image intensifier device in the calibration phase. 6. Method according to one of Claims 3 to 5 inclusive, characterized in that the algorithm for correcting the cushion deformation is based on the formula h = h' (l + ( α - β ) h 2 ) where: h = distance of an image point of the input image of the image intensifier device from the central axis H of the image intensifier device; h' = the distance of the corresponding image point of the output image of the image intensifier device from the axis H; α = 3 R + L 6 R^L 6 R eo ^ R = radius of curvature of the input window of the image intensifier device; R eo = radius of curvature of the electron wavefront in the image intensifier device; and L = the distance between the focus (f) of the X- ray source and the input window of the image intensifier device. 7. Method according to one of the preceding claims, characterized in that the pixel size is calibrated for at least one value L of the distance L between the focus (f) of the X-ray source and the input window of the image intensifier device, and in that, for a value L r other than the value L Q of the distance L, the pixel size is calculated from the following formula: P 0 .F 0 = P r .F r , in which P 0 = pixel size at L ( F 0 = 1 + α 0 .r .2 w IΪUhCe1r.Ce ια 0 _ —= I(3.R + Q )r^ 6 -R R = radius of curvature of the input window of the image intensifier device r = radius of the image field at the output side of the image intensifier device P r = pixel size at L r F r = 1 + α r .r 2 where α r = (3R + L r )r 2 6L r R 8. Method according to one of the preceding claims, characterized in that the correction formulae needed for the geometrical component of the image distortion are determined and fed into the digital image processor, prior to the siting of the X-ray image device at the operational location, and in that the calibration of the magnetic deformation is carried out at the position of the operational location. 9. X-ray image device, comprising at least one X-ray source attached to a rotatable arm and at least one electrooptical image intensifier device mounted opposite the X-ray source for, optionally via an analogue/digital converter coupled to the image intensifier device, delivering the output image of the image intensifier device in the form of digital signals to a digital image processor characterized in that the image processor is equipped for the separate correction of deformations, occurring in the device, of geometrical or magnetic origin. 10. X-ray image device according to Claim 9, characterized in that the image processor is provided with a memory in which magnetic deformation tables, on the basis of which correction of the deformation of magnetic origin can be performed, are stored during operation for at least a number of pixels of the X-ray image to be formed and for at least a number of predetermined rotation positions of the rotatable arm. 11. X-ray image device according to Claim 9 or 10, characterized in that the image processor is provided with a memory in which an algorithm based on geometrical parameters of the device is stored during operation to correct deformation of geometrical origin. 12. X-ray image device according to one of Claims 9 to 11 inclusive, characterized by a calibration phantom which comprises a brass plate provided with bores, having a predetermined diameter, made according to a predetermined pattern. 13. X-ray image device according to one of Claims 9 to 12 inclusive, characterized in that the image processor is connected to an operating device of the X- ray image device for feeding to the image processor the value of at least one adjusted geometrical parameter.

Inventors: SCHUURMAN JEROEN ANTONIUS, SCHUURMAN, JEROEN, ANTONIUS
Applicants: DELFT INSTR INTELLECTUAL PROPE, SCHUURMAN JEROEN ANTONIUS, DELFT INSTRUMENTS INTELLECTUAL PROPERTY B.V., SCHUURMAN, JEROEN, ANTONIUS
Application year: 1998
Main classification: G06T5
IPC codes: G06T5, H04N3, H04N5, H05G1