METHOD AND APPARATUS FOR CALCULATING IMAGE POSITION AND PROJECTION ANGLE IN X-RAY IMAGING

Abstract: Method for the controlling of the taking of X-ray pictures at different occasions but of the same object, in particular skeleton parts, so that corresponding picture positions and picture angles are obtained, including the taking of two initial somewhat different pictures. The pictures are compared, and with as suggested a computer is calculating how a picture coinciding with the previous picture is to be taken. Advantageously Fourier analysis can be used to calculate correct picture angle and picture position.

Description: METHOD AND APPARATUS FOR CALCULATING IMAGE POSITION AND PROJECTION ANGLE IN X-RAY IMAGING Frequently when X-ray methods are used this is to monitor that no changes occur. This puts great demands on radiologists that consequently must be able to look at two pictures and see the differences. These differences are of two kinds, real changes in the anatomy of the patient or differences that depends on the pictures being taken with different directions of projection. As it is only anatomic differences that are of interest it is important not to interpret differences in direction of protection as real changes. This is difficult since one almost always is troubled by projection dependent differences. A known way to established precisely the differences between two pictures is to subtract these from each other, the rest remaining after the subtraction corresponds to the difference between the pictures. It is known to use this method at X-raying for instance to study blood vessels, where in this way it is possible to study the transport in the vessels. Since the issue here is pictures taken in sequence after each other there are no problems with other movements between the pictures. This however requires the pictures to be taken with exactly the same projection. If instead skeleton changes are to be monitored by the taking of X-ray pictures with even intervals, for instance every fourteenth day or once a month or even yearly, the problem arise with securing that each time a picture is taken with exactly the same projection as the original one. One way of carrying out this is described in the literature and includes the taking of a number of pictures with different directions of projection. Thereafter a computer is used to interpolate or analyze the pictures. Starting from the analysis the computer then constructs a picture coinciding with the previous exposure, whereafter the differences can be studied. This method however has a number of drawbacks: In the first place many pictures must be taken to enable the computer to construct the desired picture. This means an increased radiation load for the patient. Furthermore the errors or the unsharpness and the uncertainty in the picture are added so that the final picture will be unsharp. This in turn means that if the final quality is given the required radiation load and the occupation time for the X-ray equipment are increased which in turn will mean a poor use of the latter. At X-raying of skeleton parts it is further known to mount metallic pins or balls that can serves as position identification in the bone or bones that are of interest to monitor, for instance so that each time the same picture angles (projection directions) can be obtained. In the first place this however means an additional time consuming and possible painful measure for the patient. Furthermore additional special equipment is required for the measuring in of the correct position for the part of the patient that is to be X-rayed. Since as is realized a more efficient diagnostic would be gained at skeleton X-ray by improved possibilities to decide if real changes have occurred starting from pictures taken at different occasions there exist today a need for such a method and a corresponding device that is better than the known ones and that saves work for the radiologist. The invention has as its object to define such a method. The object of the invention is in accordance with the invention solved in a surprisingly simple way, at each taking of X-ray pictures after a first picture taking occasion, first two X-ray pictures are taken with an angle between them and starting from these position and direction of the apparatus is determined for the taking of the intended picture that is coincide with the one taken the last time before. The patient or the part that is to be X-rayed is at this preferably held still in relation to a support or the like while the X-ray equipment in itself moves between the two first measuring in positions and the third picture taking position. Preferably of course the calculation of the correct picture angle and picture position is done by means of a computer in a way that is similar to the above mentioned interpolation in a series of pictures. It is then close at hand to expect a likewise circumstantial and time and radiation consuming method. Surprisingly enough this is not the case. Furthermore the calculation process becomes simpler since less data have to be processed. The initial determining of desired picture angle can for instance be done by the use of Fourier analysis and synthesis. It has in praxis been found that a very good coincidence i angle adjustment and picture position can be obtained in this way, which in turn means that pictures can be taken at different occasions of one and the same bone or part thereof, the coincidence becoming so good that picture subtraction can be used for comparison of the pictures. This in turn means that an x-ray examination in accordance with the invented method can be carried out without a doctor if so should be desired. A computer connected to the x-ray equipment can thus establish if any change has taken place or not. No professional skill is needed to compare the pictures. This is not only of value from safety and saving points but also psychologically valuable for the patients that have no changes. They can immediately have a positive infor ¬ mation, even directly from the computer if so should be desired. In order to secure, if possible, that a picture when it is taken for the first time is taken at a suitable angle two measuring in pictures can be taken in the same way as has been described above, which are compared with a statistical material of corresponding pictures from a number of different patients of the same skeleton part. By using filters for the Fourier calculations corresponding to a bad resolution of the pictures it is possible to calculate position and picture angle so that the same standard angle can be used also for the first picture in a series, which among another things facilitates for the radiologist. The X-ray equipment according to the invention does however require an accurate positioning apparatus for the part that takes the X-ray picture, that is the picture detector or corresponding as well as the radiation source. The technique required for this is however today available with very great accuracy for the machine technique and constitutes therefore neither any greater problem nor cost. Preferably the X-ray equipment is entirely electronic that is no exposure of film take place but instead the picture is registered electronically on a digital picture detector. Further advantages and details of the invention are apparent from the below described and in the drawings shown embodiment. In these fig 1 shows schematically a device in accordance with the invention, fig 2a and 2b same device in more detail, fig 3 three X-ray pictures taken during the course of the invented method as well as fig 4, fig 5a and 5b focus comparisons, fig 6 measuring with two projections, fig 7 - 11 stereometric measuring. The X-ray device shown in fig 1 includes a projection system where an X-ray tube 1 and a digital detector 2 are rigidly connected with each other so that the center beam always hit the center of the detector at a right angle. This system is able to swivel in any direction around a fix point, origo. (Fig 2). The patient that is to be X-rayed has the reference number 3. The projection system in fig 2 with its two rotational axises X and Y is suspended in the pillar P. A rotation R x around the X-axis moves X-ray tube S and detector D along circles that run as meridians through the center of H that constitutes a pole point along the Y-axis. Rotation R y around the Y-axis moves radiation source and detector along circles that runs as latitudes perpendicular to the meridians. The entire system can also be translated along all three axises in relation to the patient, alternatively or even preferably one can translationally move the patient table with the patient. The object is to be able to place the origo of the system on the desired location in the patient with the desired picture angle or direction. The movements in the system and of the patient table are controlled by a computer that is programmed for this purpose. If we have a previous picture of skeleton part of a patient taken with the same apparatus, and takes a new picture, the two pictures will have slightly different projections. If the difference in projection directions of the two pictures is reasonable small, say less than 15°, it is possible, by Fourier analyses, to find the so called common line between the pictures (se for instance "3-D Fourier syntheses of a new X-ray picture identical in projection to a previous picture " by Per E Karlsson LIU-TEK-LIC 1993:51) despite the picture planes not being parallel. The procedure could then be the following. (Fig 3a the previous picture, fig 3b the new picture and 3c the previous picture rotated to parallellity with the new one). 1). The patient is placed on the examination table in the same way as in the previous examination and by hand an adjustment is made that roughly coincide with the previous picture BO and a new picture Bl is taken. 2). In both pictures the skeleton part that one wish to examine can be "contaminated" with details that not belong to the skeleton part in question, that is details that are not rigidly connected to the skeleton part. It is important that such details as well as it is possible are blocked from the pictures before one continues, possible by simply focusing on a smaller part of the skeleton part. In the following we call this method to clear the skeleton part. The cleared pictures BO and Bl are Fourier transformed and in the Fourier domain BO is rotated until it becomes parallel with Bl . 3). With the method that is described in the rapport L.TH-ISY-R4620 "syntes av rόntgenprojektion" . Metod att finna translationen av ett objekt mellan en tidigare bild och ett set av nya bilder" one finds the translation between the pictures and corrects for this. 4). Thereafter one finds the common line between the pictures BO and Bl, that we denote G(B0-1). Perpendicular to this to this, NB1 gives us a direction to the position of BO. 5). The patient table is now moved so that one compensates the translational movement between the pictures. 6). Thereafter focus is moved a distance along G(B0-1), and a new picture B2 is taken. 7). Between BO and B2 the above points 3 - 6 are repeated. From G(B0-2) we get a new direction, NB2 to BO. 8). Focus is moved to the intersection point for NB1 and NB2. This point should coincide with the position for BO. An additional picture B3 is taken from this position. 9). Between BO and B3 the above points 3 to 6 are repeated. If the position for B3 differ sufficiently from BO we obtain a Gl(B0-3). If not we can accept the position for B3 as the position for BO. B3 is then not only an identical projection to BO but also an identical picture since the picture planes now coincide with each other. Fig 4a shows BO and Bl and a common line Gl(BO-l) between them, while RPF denotes that the direction to the position of focus in the old picmre. Fig 4b: in the apparatus focus moves along a spherical area and in the figure this area is seen from above. The focus position of Bl is marked with the direction of the (unknown) position of BO. When B2 has been taken and treated in the same way as Bl we obtain an additional direction to the position of BO. The described method can be used iteratively. If Bl and BO together provide a Gl(BO-l) we have a bearing on BO and can principally achieve as accurate estimation of the position of BO as we wish, provided that BO and B3 are not to different to their content. At the processing of a projection that looks to be identical with a previous picmre at this suggested method, focus or the focal line lies with great accuracy on the same projection direction as for the previous picmre but there may be a small error in the focal distance to the object, see fig 5, that is the placing of the object between X-ray tube and picmre plate or detector. At this fig 5a shows a first picture and fig 5b an angularly identical projection but with a different distance to focus. The smaller the object is, the greater this error may become. A small error in distance between focus and object will only result in a small influence on the look (scale) of the picture. If however the object and the picmre are very small, the beams to the picmre will be almost parallel and the error can then be considerable. One way to eliminate this error is to use a combination of two fixed projections to the object both for the earlier examination and the new one, see fig 6, with a fixed intermediate angle MV between the projection directions. At the new examination one looks for the combination of two focus positions that are identical to the positions in the earlier examination. With two focus positions we can translationally move the object so that it receives the same position in relation to origo at both examinations. With two focus positions we also obtain a more accurate adjustment to the object in regard to its rotation. At both examinations the focus combinations are locked to the coordinate system of the object in the same way and we can use the focus combination as a definition of the coordinate system of the object. If the X-ray mbe is provided with a round aperture field the two pictures in a new picmre pair will be restricted in the same way as the picmres of the old picmre pair. This results in a far greater precision in the algorithms that are used to find the new picmre pair. When the new picmre pair has been found one has obtained not only a picture pair with projections that are identical to the old picture pair, it also has the same focus-object distance and the same restrictions. The new projection system can be used to calculate a change in position between two ridged object in the patient. Suppose that we have two skeleton parts that can change position in relation to each other. We symbolize these in a fig 7a and b with two cubes. We call the two objects A (white cube) and B (shaded cube). Dependent of their orientation or movement relative each other we consider it as if it is B that moves in relation to A. The following picmres of A and B and the projection system is shown in a constant projection in relation to an external coordinate system centered on origo. The two objects are X-rayed with the projection system before the movement (Rl) and after the movement (R2). At Rl with help of for instance transillumination the patient is adjusted so that A and B will be situated as close as possible to the origo of the system, and at the same time one tries to get the patient in such a position that the two object are "projected freely" as much as possible, that is they cover each other as little as possible. The skeleton of the patient is fixed and two pictures taken, RU and R12. The two picmres are taken with two different and known focus positions that constitute a large and controlled angle, say 60°, between their directions of projection (fig 10a). At R2 the patient is placed as carefully as possible in the same manner as at Rl and by means of transillumination the patient is adjusted so one has approximately the same pro¬ jection of A as in RU . A first picmre R21 is taken. In both RU and R21 A is cleared. The previously described method is now used to find the focus position of Rl 1. When this is done a new R21 is taken from this position and R22 from the fixed and known position of R12. In both R12 and R22 A is cleared. Object A should now be projected in the same way in these two picmres but is probably displaced somewhat in the projection direction of R21. This translation error is corrected and the two fixed projections are adjusted alternatingly until they are entirely identical with RU and R12. When this has been done a new R21 and a new R22 are taken that are now identical to RU and R12 regarding A, see fig 8a - c, that shows projections of object A. a) The two projections at Rl . b) The first projections at R2. Observe the translation error, c) The final projections at R2. Hereafter is established how B has moved in relation to A. We return to RU and R21 and clear B. By means of the new method the position that has been used at the exposure of B in Rl 1 is established . When this has been done (fig 9) R23 is taken from this position and R24 from the fixed position that has the same relation to R23 as R12 has to RU . The two fixed projections are adjusted until they are entirely identical to the picmres of B in RU and R12. The difference between the pair of positions R23 - R24 and the pair RU - R12 provides full information of the rotation B as undergone in relation to A between Rl and R2. The translations of the object that has been necessary between Rl and R2 correspond to the translations B has undergone in relation to A. After this we take a new R23 and a new R24. Fig 10 shows in a larger scale the center parts in fig 9. Only the beams through the origo of the system are shown. In fig 11 is finally shown the focus positions Rl 1 - R24 in the spherical coordinate system of the projection system. The total rotation of B in relation to A is the rotation that brings the combination R23 - R24 to coincide with the combination R21 - R22. At this RU - R12 are the common positions of A and B at the first examination. R21 - R22 the positions of A at the second examination and R23 - R24 the positions of B.

Claims: C L A I M S 1. X-ray method, characterized in that in a first step at least two picmres with different views are taken of the object that is to be X-rayed, the two picmres are compared with a picmre taken at a previous occasion and based on this comparison the picture angle is calculated that is to be used to obtain a picture entirely coinciding with the previously taken picmre, facilitating the comparing of these at different occasions taken picmres. 2. Method according to claim 1 , characterized in that the two initial picmres are compared by means of Fourier analysis. 3. Method according to claim 1 or 2, characterized in that the picmre taken at a previous occasion is constituted by a mean value picture of picmres taken of similar corresponding objects, as for instance the same skeleton parts, at which the two initial picmres may have a less good resolution than the actual picture that is then to be taken after adjustment, alternatively the mean value picmre can be correspondingly unsharp. 4. Method according to any of the preceding claims, characterized in that the pictures are compared with subtraction methods for the detection of changes. 5. X-ray device for the taking of each other corresponding X-ray picmres at different occasions with coinciding picmre angles and picture positions to facilitate the comparison of the picmres taken at different occasions, characterized in that it includes a radiation source and a picture registering picmre plane that are fixedly connected to each other and together adjustable in each angular position and translational position and a computer with a computer software that starting from at least two picmres with different views of the object that is to be X-rayed, can compare the two picmres with a previous picmre and based on this comparison compute the picmre angle that is then to be used to give a picmre entirely coinciding with the previously taken picmre in order to facilitate comparison. 6. Device according to claim 5, characterized in that the picmre registering picmre plane is constimted by an electronic picmre disc that is in contact with the computer.

Inventors: EDHOLM PAUL, EDHOLM, PAUL
Applicants: EDHOLM PAUL, EDHOLM, PAUL
Application year: 1997
Main classification: A61B6
IPC codes: G06T7