DE102011077087A1 - Method for forming image of body part of patient in medical field, involves removing expected-proportion of scattering radiation portion from total radiation before concluding image processing - Google Patents

Abstract

The method involves determining a base scattering portion from radiation that is emitted on a radiation detector i.e. X-ray detector (6), where the radiation is produced by an X-ray source (4). A model calculation is made with an aid of the scattering radiation portion to form a model of an object to be examined. A scattering calculation is made with an aid of the formed model for determining expected proportion of the scattering radiation portion. The expected proportion of the scattering radiation portion is removed from the total radiation before concluding image processing. An independent claim is also included for an image forming device.

Description

The invention relates to a method for image generation, in particular in the field of medical technology, with the aid of a radiation source and a multipixel radiation detector, wherein the radiation generated by the radiation source impinges on an object to be examined and subsequently on the radiation detector. The invention further relates to an image-forming apparatus, in particular X-ray apparatus for carrying out this method.

In such an imaging method for the examination of objects, ie in the field of medical technology of a body part of a patient, the object texture or object structure is inferred from the portion of the radiation transmitted through the object, which is detected metrologically with a radiation detector.

However, a portion of the radiation directed to the object is scattered in the interaction with the object and accordingly does not pass directly to the radiation detector from a radiation source which generates the radiation. This scattered portion of the radiation detected by the radiation detector acts as a kind of background noise and therefore reduces the contrast resolving power of the imaging method.

In order to improve the image quality achievable with such an imaging method, it is possible to post-process the measurement signals generated by means of a radiation detector with the aid of a mathematical algorithm in order to reduce the effects caused by the scattering component. Here, a simple model of the object to be examined is first given, and subsequently the expected scattering of the radiation on this model is calculated. The calculated proportion of radiation is deducted as a result from the metrologically detected radiation. The effectiveness of this method of improving the image quality depends, among other things, on how well the object to be examined is represented by the given model. This is very difficult for a patient as an object because in order to avoid unnecessary radiation exposure for the patient, there is generally only a small amount of information available before the examination of the patient in order to determine a suitable model.

The invention has for its object to enable improved image formation.

This object is achieved by a method having the features of claim 1 and by an image-forming device, in particular X-ray apparatus for performing the method. The dependent claims include in part advantageous and in part self-inventive developments of this invention.

The method is used for image generation with the aid of a radiation source and a multipixel radiation detector, wherein the radiation generated by the radiation source initially strikes an object to be examined and subsequently the radiation detector. Within the scope of the method, a basic scatter proportion is determined from the radiation impinging on the radiation detector and thus detected by signal technology, with the aid of which a model calculation is carried out in order to form a model of the object to be examined. On the basis of the model, a scatter calculation for determining the expected scatter proportion of the radiation impinging on the radiation detector is subsequently carried out, and in advance of a final image preparation, the expected scatter proportion is subtracted from the radiation impinging on the radiation detector. In the final image processing then takes place, for example, depending on the application purpose, a conversion of the data generated by means of the method such that they can finally be displayed graphically on a monitor. Alternatively, these data serve, for example, as the basis for a reconstruction according to the published in the applicant's disclosure DE 102 38 322 A1 described principle. In that case, the reconstruction is part of the final rendering.

In the application of the method, therefore, a part of the information that is obtained in the examination of the object by means of radiation, first used to determine a suitable model for the scatter calculation. Thus, in this method much more information is used for the model than is the case with known methods. The model for the scatter calculation is determined from the actual measured values of the object to be examined. Accordingly, the model is much better suited to represent the object to be examined, which is why the scatter calculation based on this model gives much more realistic results. This ultimately leads to the fact that the post-processing of the measurement signals generated with the aid of the radiation detector is more effective and that the contrast resolution capability of the imaging method is further increased.

Multipixel radiation detector is generally understood to mean a digital detector, preferably an X-ray detector, with an array of sensor-active elements (pixels). The radiation source used is preferably an X-ray source.

According to a preferred process variant, the basic scatter proportion is the one Used radiation fraction which impinges on predetermined reference pixels of the multipixel radiation detector. As reference pixels in this case preferably those pixels are given, is incident on the only scattered radiation. It must be ensured that only a scattered radiation hits a suitable large number of pixels. This simplistic procedure makes it possible to keep the mathematical and technical effort required to realize the method low. This embodiment is based on the consideration that usually some pixels are only exposed to stray radiation, which can be clearly identified. For example, these pixels can be identified by means of pixel-specific measured values which are characteristic of the scattered radiation.

Usually, a beam-limiting object, such as a diaphragm and / or a collimator, is inserted in the beam path as an X-ray-absorbing obstacle which defines a radiation area on the detector surface. Measurement signals of pixels outside this radiation range are assigned in a preferred embodiment of the scattered radiation.

Preferably, therefore, an X-ray absorbing barrier between the radiation source and the radiation detector is generally used to define the reference pixels. The x-ray-absorbing obstacle is usually a collimator, ie in particular a component which can be manipulated by an operator with regard to its shape and / or position, so that, for example, the number of reference pixels can be changed for a given total number of pixels ,

Conveniently, therefore, those pixels are predetermined as reference pixels, on the basis of Geometric optics no radiation passes directly from the radiation source to the corresponding pixels, which are therefore outside the expected radiation range. In other words, it is assumed that the radiation was scattered for lack of direct access to the reference pixels, and accordingly that the total of the radiation attributable to such a pixel is part of the scattered fraction.

In one embodiment, for example, those pixels are determined as reference pixels, to which a measure of radiation impinges, which lies below a predetermined threshold value.

Preferably, however, the reference pixels are determined mathematically with the aid of a projection calculation. In this, the design-related and therefore known properties of the radiation source and the radiation detector and their arrangement to one another and the arrangement of the beam-limiting objects (aperture / collimator) are taken into account in order to calculate an expected shadowing on the radiation detector. The shadowing resulting from the beam-limiting objects can be determined by a simple projection. Due to this mathematical determination, false identifications of pixels as reference pixels, for example due to implants in the body part to be examined, are avoided.

According to a further development of the method, relative position and position changes between the radiation source and the radiation detector are taken into account in the projection calculation. As relative position changes are therefore in this context, especially those relative changes in position to understand that are not dictated by an adjustment. If, for example, the radiation source and the radiation detector are fixed relative to one another by a common holding device, the relative position changes in this case are, for example, relative positional changes which result from a distortion of the common holding device. Additionally or alternatively, it is also provided to take into account in this way relative changes in position, resulting for example by aging phenomena in the adjustment. By this measure, deviations from a desired positioning of the individual components are therefore taken into account, which occur due to, for example, torsions in the support system. Due to these distortions, a reliable calculation of the image of the collimators in the image is only possible to a limited extent.

In accordance with a suitable method variant, calibration data stored in order to take into account the relative position changes are used. These can be individual parameters, one or more calibration curves or comparable data sets which characterize the arrangement of radiation source and radiation detector in more detail. This is based on the consideration that for defined positions of the radiation source the relative positions of the further components such as collimator and X-ray detector are defined. Only when adjusting the X-ray system can variations in the relative position occur, but they are reproducible.

Preferably, therefore, a number of relative positions of the radiation source and of the radiation detector relative to one another are stored as calibration data as a function of position specifications. In this case, it is provided in particular, the arrangement of radiation source and radiation detector in the context of a calibration or To bring calibration measurement in discrete positions and to determine a determination of the relative positions of the radiation source and the radiation detector to each other for each discrete position.

According to a further development of this method variant, between 10 and 200 and preferably between 20 and 80 relative positions are provided as calibration data. At a position for which no calibration data is stored, then preferably relative positions are determined by interpolation. In this way, relative changes in position between the radiation source and the radiation detector can be considered with relatively little effort.

The invention will be explained in more detail with reference to a schematic drawing. The single FIGURE shows a side view of an X-ray machine.

In the embodiment described below, the method for image formation by means of an X-ray device 2 executed, which is preferably used in the medical field for examining a body region of a patient. The X-ray machine 2 includes for this purpose as a radiation source an X-ray source 4 and as a radiation detector, a multipixel X-ray detector 6 , which in the embodiment shown here opposite each other at each end to a C-arm 8th are attached. For shaping and manipulation of the X-ray source 4 X-ray generated is the X-ray source 4 a collimator 10 downstream, whose shape and position is set variable relative to the X-ray by an operator. In operation, an object to be examined (not shown), namely the patient, is between the x-ray source 4 and the X-ray detector 6 placed and penetrated by the X-ray during the examination.

For the C-arm 8th are provided in a manner not shown adjustment mechanisms for exercising an adjustment. The adjustment movement generally allows a suitable placement of the plant with respect to the object to be examined and, if necessary, also a scanning of the object to be examined. For a C-arm 8th is usually an orbital motion (rotational movement in the circumferential direction of the C-arm about an orbital axis) as well as an angular movement (rotational movement about the axis of rotation perpendicular to the orbital axis) possible.

In addition, the X-ray machine includes 2 an evaluation or electronic unit 12 , which signaling with the X-ray source 4 , with the X-ray detector 6 , with the adjustment mechanisms and depending on the version with other systems not shown, such as connected to a server. With the help of this electronic unit 12 Among other things, the X-ray source 4 , the X-ray detector 6 as well as the adjusting mechanisms supplied, controlled and read out.

With the X-ray machine, the body region to be examined is transilluminated. The on the x-ray detector 6 incident radiation is evaluated for imaging. The on the formed of several pixels X-ray detector 6 Incoming radiation generates measurement signals in the individual pixels, which are evaluated for image generation. Due to the material and material thickness-dependent absorption probability of X-radiation, different values for the intensity of the X-radiation striking a respective pixel result. From this information, an X-ray image is reconstructed.

A portion of the X-ray radiation is scattered in the interaction with the matter of the object to be examined, wherein this portion of the X-ray subsequently signal technology acts as background noise and thus reduces the image quality, more precisely the contrast resolving power.

For this reason, the X-ray detector 6 generated measurement signals by means of a mathematical algorithm in the evaluation unit 12 first reworked before they are then used for the final image editing. This mathematical algorithm thus acts as a kind of noise filter, which enhances the contrast resolving power of the imaging method.

The measurement signals caused by the scattered radiation at the X-ray detector 6 are determined with the help of the mathematical algorithm and calculated out in favor of improved image generation.

For mathematical determination of the scattered radiation, a mathematical model of the object to be examined is required. The more accurate the mathematical model, the better the calculated scattered radiation component and the better the reconstructed image.

In the method for image generation described here, the examination of the patient by the x-ray detector is initially carried out 6 recorded measurement signals determines the mathematical model. Then, based on the known parameters (beam parameters, geometry ...) of the X-ray system, the scattered radiation is calculated with the help of the thus determined model.

To determine the model, reference pixels are determined in a first step, to which only scattered radiation hits. This proportion of scattered radiation is referred to as the basic scatter proportion.

In the second step, the mathematical model is formed from this basic scatter proportion, in such a way that the base scatter proportion of the reference pixels calculated with the model at least largely coincides with the actually measured basic scatter proportion.

In the third step, finally, this determined model for the calculation of the expected scattered radiation for all pixels of the X-ray detector 6 determined and taken into account in the reconstruction of the image.

For the determination of the reference pixels in the first step, the circumstance is exploited that the collimator 10 an area on the x-ray detector 6 shields against the X-rays, so that on the X-ray detector 6 forming a kind of shadow in this area. The shadow forms on the X-ray detector 6 a frame that covers the area on the x-ray detector 6 bordered, on which the transmitted portion of the X-radiation impinges. The collimator 10 thus acts as an X-ray absorbing barrier that prevents X-ray radiation from directing away from the radiation source 4 reaches those pixels, which in the shadow area on the X-ray detector 6 lie. These pixels in the shadow area are considered the reference pixels according to the method.

The incident on the reference pixels X-ray radiation is based on geometric optics thus only scattered X-rays.

The scattered X-radiation or rather the measurement signals of the reference pixels are used in the process to generate the model of the object to be examined in the second step and finally based on the model in the third step, the scatter calculation to determine the expected Streu portion of the on the X-ray detector 6 make incident radiation.

The determination of the model in the second step is preferably carried out in an iteration method, in which initially a very simple stored base model is used to simulate its scattering behavior. The scattered radiation pattern thus calculated is then compared with the measurement signals of the reference pixels. In the following, the basic model is preferably adjusted step by step until there is a match of the scattered radiation pattern with the measurement signals of the reference pixels to a predetermined extent.

The model generated in this way is then used as a basis for the scatter calculation in the third step, as a result of which the scattered portion of the X-ray radiation calculated in this way represents the actual scattered portion of the X-radiation quite well. In this case, the corresponding scattering proportions are calculated for all pixels, that is to say not just for the reference pixels, and this calculated scatter proportion of each pixel is finally subtracted from the fraction of the radiation actually attributable to the corresponding pixel in advance of a final image preparation.

To determine those pixels in the first step, which are used as reference pixels of the determination of the model, the calculation of the expected shadowing on the X-ray detector is performed 6 with the help of a projection calculation. The properties of the X-ray source 4 , the X-ray detector 6 and the collimator 10 and their arrangement are by the construction of the X-ray machine 2 given and are therefore known. Therefore, it can be determined by a simple mathematical projection, which pixels of the X-ray detector 6 from the collimator 10 be shielded against X-rays.

The C-arm 6 as a real system shows - albeit slight - torsions during the adjustment, leading to a relative movement between the X-ray detector 6 and the X-ray source 4 to lead. This is indicated in the figure by an offset of the marked central ray of the ray bundle, which is off-center on the X-ray detector 6 incident. In order to further increase the accuracy of the projection calculation, it is therefore additionally provided for such relative positional changes between the X-ray source 4 and the X-ray detector 6 , for example, by a twist of the C-arm 8th during operation and depending on the relative position of the C-arm 8th to be considered.

For this purpose, different positions of the C-arm are used as part of a calibration or calibration measurement 8th (Position of the C-arm 8th in space) and for these the relative positions of the X-ray source 4 (with the collimator 10 ) and the X-ray detector 6 determined to each other. This is preferably done by generating an X-ray image without an object to be examined. In this case, the shadow area has a very sharp edge and it can be decided based on the measurements of each pixel, which pixel is in the shadow area and which in the image area. The calibration or calibration data generated hereby are stored in the evaluation unit 12 stored and are available in the sequence for a correction calculation in the projection calculation.

In particular, a projection calculation is determined for each of the calibration positions, namely as a rule a mathematical matrix which describes the projection.

Subsequently, when examining an object, a relative position of the C-arm 8th By default, for which no calibration or calibration data is available, in this case, the relative positions of the X-ray source become 4 and the X-ray detector 6 determined by interpolation. Therefore, the projection calculations of calibration positions adjacent to the current position are used. From these calibration projection calculations, a projection calculation for the current position, that of the C-arm, is calculated in particular by interpolation 8th occupies.

The invention is not limited to the embodiment described above. Rather, other variants of the invention can be derived therefrom by the person skilled in the art without departing from the subject matter of the invention. In particular, all the individual features described in connection with the exemplary embodiment can also be combined with each other in other ways, without departing from the subject matter of the invention.

LIST OF REFERENCE NUMBERS

2

X-ray machine

4

X-ray source

6

X-ray detector

8th

C-arm

10

collimator

12

Processor / electronics unit

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10238322 A1 [0007]

Claims (12)

Method for the production of images, in particular in the field of medical technology, with the aid of a radiation source ( 4 ) and a multipixel radiation detector ( 6 ), whereby the radiation source ( 4 ) generated radiation on an object to be examined and then on the radiation detector ( 6 ), characterized in that - from the radiation detector ( 6 ) is calculated using the basic Streuanteils a model calculation is made to form a model of the object to be examined, - that with the help of the model, a scatter calculation to determine the expected Streu portion of the on the Radiation detector ( 6 ) radiation is made and - that in the run-up to a final image processing of the expected Streu portion of the on the radiation detector ( 6 ) is stripped off incident radiation. Method according to Claim 1, characterized in that the radiation fraction which is applied to predetermined reference pixels of the multipixel radiation detector ( 6 ). A method according to claim 2, characterized in that an X-ray absorbing obstacle ( 10 ) between the radiation source ( 4 ) and the radiation detector ( 6 ) is used to define the reference pixels. A method according to claim 3, characterized in that as an X-ray absorbing obstacle ( 10 ) a collimator ( 10 ) is being used. Method according to one of Claims 2 to 4, characterized in that the reference pixel is a number of pixels to which, on the basis of geometric optics, no radiation is emitted directly from the radiation source ( 4 ) to the corresponding pixels. Method according to one of claims 2 to 5, characterized in that with the aid of a projection calculation those pixels are determined on the basis of Geometric optics no radiation directly from the radiation source ( 4 ) to the corresponding pixels and that these pixels are given as reference pixels. A method according to claim 6, characterized in that in the projection calculation relative changes in position between the radiation source ( 4 ) and the radiation detector ( 6 ). Method according to Claim 7, characterized in that calibration data stored in order to take account of the relative position changes are used. Method according to claim 8, characterized in that as calibration data a number of relative positions of the radiation source ( 4 ) and the radiation detector ( 6 ) depending on position specifications. A method according to claim 9, characterized in that between 10 and 200 and preferably between 20 and 80 relative positions are deposited. A method according to claim 9 or 10, characterized in that for position specifications for which no calibration data are stored, the relative positions are determined by interpolation. Imaging device ( 2 ), in particular X-ray apparatus ( 2 ) for the medical field, with a radiation source ( 4 ), a radiation detector ( 6 ) and an evaluation unit ( 12 ) for carrying out the method according to one of the preceding claims.

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