WO2013068957A1

WO2013068957A1 - Automatic measurement of beam quality parameters

Info

Publication number: WO2013068957A1
Application number: PCT/IB2012/056254
Authority: WO
Inventors: Hanns-Ingo Maack
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2011-11-11
Filing date: 2012-11-08
Publication date: 2013-05-16

Abstract

The present invention relates to a detector unit, an imaging system, and a method of processing radiography data. In order to provide a detector unit for an X-ray imaging system which permits an improved image processing, taking into account the current physics of the X-ray beam, a detector unit (210) for an X-ray imaging system is provided, comprising a detector device (220) with a detector surface (230), and at least one reference structure (240), and a processing unit (250). The reference structure (250) has predetermined radiation attenuation properties and is located in front of the detector surface (230). The detector device (220) is configured to detect X-ray radiation and to provide respective radiography image signals to the processing unit (250). The processing unit (250) is configured to determine at least one parameter associated with the irradiation of said reference structure (240), and to process radiography image data based on the at least one determined parameter.

Description

AUTOMATIC MEASUREMENT OF BEAM QUALITY PARAMETERS

FIELD OF THE INVENTION

The present invention relates to a detector unit, an imaging system, a method of processing radiography data, a computer program element and a computer readable medium.

BACKGROUND OF THE INVENTION

In radiography system, for example in mammography systems, the physics of the X-ray beam is taken into account when converting the tube charge (mAs) into a dose by using parameters derived from factory settings. In conventional radiography systems these parameters are not re-assessed on a regular basis. In mammography systems, re-assessment of theses parameter may be provided by the use of calibration phantoms. However, this reassessment may be used only for a periodic calibration of the device affecting image quality, often leading to the necessity of additional image acquisition. SUMMARY OF THE INVENTION

There may be a need to provide a detector unit for an X-ray imaging system which permits an improved image processing, taking into account the current physics of the X-ray beam.

The object of the present invention is solved by the subject-matter of the independent claims, wherein further embodiments are incorporated in the dependent claims.

It should be noted that the following described aspects of the invention apply also for the detector unit, the imaging system, the method of processing radiography data, the computer program element and the computer readable medium.

According to a first aspect of the present invention, a detector unit for an X- ray imaging system is provided, comprising a detector device with a detector surface, and also comprising at least one reference structure and a processing unit. The reference structure has predetermined radiation attenuation properties and is located in front of the detector surface. The detector device is configured to detect X-ray radiation and to provide respective radiography image signals to the processing unit. The processing unit is configured to determine at least one parameter associated with an irradiation of said reference structure, and to process radiography image data based on the at least one determined parameter.

According to an exemplary embodiment, the reference structure comprises a plurality of regions, each region having a different attenuation property.

According to a further exemplary embodiment, the reference structure has a circular structure and the plurality of regions is arranged in concentric circles.

According to a further exemplary embodiment, the reference structure comprises at least one aluminium part having thickness about 1 mm, 3.7 mm, or 4.7 mm, or at least one part of human-tissue equivalent attenuation material.

According to a further exemplary embodiment, the detector device is provided with a detector housing and the reference structure is located in the proximity of the edge of the detector housing.

For example, the reference structure is attached to a compression paddle of a mammography system, i.e. the reference structure is not directly connected with the detector itself. However, the compression paddle may be part of the detector unit. Of course, the reference structure may also be directly connected to the detector, for example to a compression plate or surface on the detector.

For example, the determination of at least one parameter comprises determining at least one of: i) the signal from the direct radiation next to the reference structure, ii) at least one signal from behind the reference structure, iii) at least one attenuation coefficient associated with the reference structure, or iv) the half value layer of the radiation (HVL). The term "behind" the reference structure refers to the area, i.e. the volume, behind the reference structure and not only to the actual rear side of the structure itself.

For example, the determination of the signal next to the reference structure may be that high that the detector response would be "clipped" and considered as not usable. However, according to an example of the preset invention, the reference structure still provides usability of such signals that otherwise would lead to not-usage.

According to a further exemplary embodiment, processing radiography image data comprises at least one of post-processing a radiography image or pre-processing radiography image signals.

According to a second aspect of the invention, an imaging system is provided, comprising an X-ray source and a detector unit as described above. The X-ray source is configured to provide X-ray radiation towards the detector unit. According to a further exemplary embodiment, the X-ray source is arranged to be controlled based on the at least one determined parameter as described above.

According to a further exemplary embodiment, the X-ray imaging system is a mammography system comprising at least two compression surfaces. For example, more than two paddles are used, such as up to a number of ten paddles. The at least one reference structure is located on at least one of the two compression surfaces.

For example, the at least one reference structure is arranged on the at least one of the two compression surface so that it can be flipped to both sides of the at least one of the two compression surfaces.

At least one of the two compression surfaces may be provided as an adjustable compression paddle.

According to a further exemplary embodiment, in the above described imaging system, the detector comprises two opposite edges, wherein the focal spot of the X-ray source is located closer to one of the two edges, and wherein the reference structure is located closer to the other one of the two edges.

According to a third aspect of the invention, a method of processing radiography data is provided, comprising the following steps:

a) providing a reference structure with predetermined radiation attenuation properties located in front of the detector unit of a radiography apparatus;

b) determining at least one parameter associated with an irradiation of said reference object; and

c) processing radiography image data based on the at least one determined parameter.

According to a further exemplary embodiment, the determination of at least one parameter comprises determining at least one of: i) the signal from the direct radiation next to the reference structure; ii) at least one signal from behind the reference structure; iii) at least one attenuation coefficient associated with the reference structure; and/or iv) the half value layer of the radiation (HVL).

According to a further exemplary embodiment, the above described method is further comprising a step d) of controlling an X-ray source of the radiography apparatus based on the at least one determined parameter.

According to a fourth aspect of the invention, a computer program element is provided, which, when being executed by a processing unit is adapted to carry out the above described method. According to a fifth aspect of the invention, a computer readable medium is provided having stored the above described program element.

According to an aspect of the invention, a detector for an X-ray radiography system is provided, the detector having a detection area on which X-ray signals are received, a sample absorber element and a processor. The sample absorber element has known absorption characteristics and is placed so as to receive X-ray signals which are directed to the detection area of the detector. The detector is arranged to detect X-ray radiation and to convert the detected radiation into respective radiography image signals which are provided to the processor. The processor unit is configured to automatically calculate at least one beam quality parameter associated with a measure of the radiation absorbed by the sample absorber and to perform a corresponding automatic image processing of radiography image data based on the at least one calculated parameter.

According to a still further aspect of the invention, an X-ray source is adjusted based on the calculated beam quality parameter.

For example, the X-ray source is adjusted for an upcoming radiation.

According to a further aspect, the physical parameters with which the direct radiation value can be computed, for example, can be adjusted. Also, the parameters which are used in the breast density assessment can be adjusted.

According to a further aspect of the invention, the reference structure is used for spectral imaging, where spectral physical models are used which can be assured more precisely by the HVL-measurement.

According to still a further aspect of the invention, automatic image processing of radiography image data comprises at least one of post-processing an already obtained radiography image, or pre-processing the radiography image signals to be used for obtaining a radiography image.

According to still a further aspect of the invention, the above described invention can be used for upgrading an existing radiography apparatus or X-ray imaging system by providing it with a reference structure as described above in front of the detector unit and by providing an upgrade of the software of the radiography apparatus or X-ray imaging system by using the computer program element or computer readable medium described above.

These and other aspects of the present invention will become apparent from and be elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in the following with reference to the following drawings.

Fig. 1 illustrates an imaging system according to an exemplary embodiment of the invention.

Fig. 2 schematically illustrates a detector unit for an X-ray imaging system according to an exemplary embodiment of the present invention.

Fig. 3 shows a reference structure according to an exemplary embodiment of the invention.

Fig. 4 shows basic steps of a method according to an exemplary embodiment of the present invention.

Fig. 5 shows a further example of a method according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 illustrates an imaging system 120 according to an exemplary embodiment of the invention, containing an X-ray source 130 and a detector unit 110 according to the invention. Although Fig. 1 is showing an example of a mammography system as an imaging system 120 according to the present invention, the system of the present invention can be any imaging system comprising an X-ray source and a detector, e.g. a C-arm system or any other movable or non-movable X-ray system.

Before further describing the imaging system 120, it is referred to Fig. 2, showing a detector unit 210 according to the present invention for the imaging system 120. The detector unit 210 comprises a detector device 220 with a detector surface 230, and at least one reference structure 240, and a processing unit 250. The reference structure 240 has predetermined radiation attenuation properties. For example, the reference structure has determined absorption characteristics. The reference structure 240 is located in front of the detector surface 230.

According to a further example (not shown), the reference structure 240 is located below a detector cover surface. In other words, although being in front of the detector, the reference structure 240 can be provided such that it is not directly visible by the user, but visible by the detector. The detector cover surface is at least more or less X-ray transparent. The term "in front of relates to the viewing direction defined by the X-ray radiation.

For example, the reference structure 240 is placed so as to receive X-ray signals which are directed to the detection area of the detector.

The detector unit 210 further comprises a processing unit 250. The detector device 220 is configured to detect X-ray radiation and to provide respective radiography image signals to the processing unit 250. The processing unit 250 is configured to determine at least one parameter associated with an irradiation of said reference structure 240, and to process radiography image data based on the at least one determined parameter.

For example, the present invention provides the advantage that image processing is based on the automatic measurement of updated beam quality parameters which takes into account the actual conditions, unlike calibration factors which are merely factory settings and, thus, not fully matching the actual values of a specific system.

According to the invention, the reference structure ensures that the respective image data can always be used, since the reference structure provides reference values that are inherent to the image data.

For example, the data portion of the image data relating to the position or area where the reference structure is located, is used to determine further data processing.

According to another example, the data portion of the detected image parameter values relating to the position or area where the reference structure is located, is used for the actual generation of the image data. In other words, the signals provided by the detector itself are provided into image data based on the values relating to the reference structure.

Fig. 3, shows examples of the reference structure 310, 320. For example, the reference structure 310, 320 comprises a plurality of regions 310A, 310B, 3 IOC, 320A, 320B, 320C, each region having a different attenuation property. For example, the attenuation properties can be the result of each region 310A, 310B, 3 IOC, 320A, 320B, 320C having different object thickness or absorption characteristics. In Fig. 3, the reference structure 320 has a circular structure and the plurality of regions 320A, 320B, 320 C are arranged in concentric circles. However, although reference structures of circular and rectangular construction are shown, the reference structure can have any other shape or construction.

The choice of a circular shape 320 may be provided in the context of mammography, as the focal point of the radiation is usually located above the middle of one edge of the detector. As a consequence there may be a gradient in the radiation profile. In order to avoid the influence of this gradient, concentric test object patterns are provided, because using the average of the ring-shaped zones 320A, 320B, 320 C will help in eliminating such gradient.

For example, the reference structure 240 can be of made of aluminium (Al) The reference structure 240 can comprise at least one aluminium part having portions with a thickness of about 1mm, 3.7 mm or 4.7 mm. A sample-object of 1 mm thick Al can be chosen in order to match the order of magnitude of the Half- Value-Radiation (HVL) of the radiation. A sample object of 3.7 mm thick Al can be chosen in order to provide a signal up to a factor 10 above the clipping dose of the detector. A sample-object of 4.7 mm thick Al can be chosen in case of strong over exposure.

The reference structure 240 can also comprise, in addition or in alternative, at least one part of human-tissue equivalent attenuation material. Other materials that are equivalent in attenuation to fatty and dense tissue can directly be used to quantify the amount of dense breast within a fatty background. For example the test object can consist of some (for example, 2 to 5) absorber sample materials.

According to a further example, as schematically shown in Fig. 2, the detector device 210 is provided with a detector housing 260 and the reference structure 240 is located in proximity of the edge of the detector housing 260. Mammography systems for X-ray cassettes may include sliders with labels used as image markers. For example, the reference structure 240 can be located at the detector housing 260 at a position, which corresponds to the former position of those sliders. The reference structure may be provided in a way that it can be moved out of the field of view of the detector.

According to the present invention, the determination of at least one parameter comprises determining at least one of: the signal from the direct radiation next to the reference structure, at least one signal from behind the reference structure, at least one attenuation coefficient associated with the reference structure, or the Half Value Layer of the radiation (HVL). The term "behind" the reference structure refers to the area, i.e. the volume, behind the reference structure and not only to the actual rear side of the structure itself.

In the following, some illustrative examples refer to parameters or measurement values particularly used in mammography. However, these examples are merely for further clarification and are not limiting for the teaching of the present invention, which is more general and can be applied to any radiography system.

In mammography systems, the Mean Glandular Dose (MGD) is calculated from the mAs of the image using calibration factors that take object thickness and absorption characteristics into account. Knowing the Half- Value-Layer (HVL) of aluminium allows a more precise measurement. This also allows a more accurate calculation of the Volumetric Breast Density (VBD) from the image data.

Another physical parameter is the value of the un-attenuated radiation 10 that can be measured next to an object. This value is needed for Tomosynthesis reconstruction and scatter correction. In images with high exposure, it cannot be measured due to detector clipping and must be extrapolated from the mAs of the image. Automatic measurement of such beam quality parameters to be used for image processing based on physical models is performed by placing the reference structure according to the above examples in proximity of the detector, for example on the corner of the detector.

One example of the principle used for the calculation of the parameters using sample objects located in front of the detector surface can be as follows:

10 = the signal from the direct X-ray, next to the sample objects;

11 = the signal from behind the thinnest object Al , having thickness hi ;

12 = the signal from behind the object A2, having thickness h2;

13 = the signal from behind the object A3, having thickness h3.

The thickness dependent attenuation coefficient μ can be calculated from the data:

Ii = 10 * EXP (-μϊ*Μ)

μί = 1/hi * In (I0/Ii)

The Half Value Layer of the radiation, which represents the thickness of an absorber which attenuates the air karma of non-monochromatic X-ray beams by half, can be calculated as:

HVL = In (2) / μί = 0.693 / μί .

The above determination can be made with a transmission T = 90%, 50%,

10%, 5%.

Using the sample objects, the determination of 10 can be done also with detector clipping.

The above determination permits a more accurate Mean Glandular Dose (MGD) calculation.

For example, using two Al absorbers of 300 μιη and 400 μιη, the HVL of the radiation can be determined in accordance to the European Reference Organization for Quality Assured Breast Screening and Diagnostic Services (EUREF).

As outlined above, according to the present invention, radiography image data is processed based on the at least one determined parameter.

For example, processing radiography image data comprises post-processing a radiography image which has already been obtained.

Alternatively or in addition, processing radiography image data comprises pre- processing radiography image signals before a radiography image is obtained.

Fig. 1 describes an imaging system 120 according to the present invention, comprising an X-ray source 130 and a detector unit 110 as described above, wherein the X- ray source 130 is configured to provide X-ray radiation towards the detector unit 110. For example, the X-ray source 130 is arranged to be controlled based on the at least one determined parameter above described. This provides an improved control of the X-ray source 130 based on actual parameters.

According to a further example, the X-ray imaging system 120 is a

mammography system comprising at two compression surfaces 140, 150 and the at least one reference structure is located on at least one of the two compression surfaces.

For example, the at least one reference structure is arranged on the at least one of the two compression surfaces 140, 150 so that it can be flipped to both sides of the at least one of the two compression surfaces. The compression surfaces 140, 150 can be for example those of compression plates of the mammography system. The objects at the edge of the compression plate will always be in the image and, as the breasts are typical semi-circular shaped, they will not be in conflict with such samples.

The mammography system may be adaptable in its height of the detector unit 110, as indicated with first double arrow 160. Further, the upper compression surface may also be adaptable in its height, as indicated with second double arrow 170.

In Fig. 1, a first box 180 indicates a control unit, e.g. comprising a processor, and a second box 190 indicates a display unit. Further, one or more interface units are provided (not shown).

With reference to Fig. 2, in the above described imaging system, the detector 220 is comprising two opposite edges, wherein the focal spot of the X-ray source is located closer to one of the two edges, and wherein the reference structure 240 is located closer to the other one of the two edges.

According to a further example, as shown in Fig. 4, a method 400 of processing radiography data is provided, comprising the following steps: In a first step 410, a reference structure with predetermined radiation attenuation properties located in front of the detector unit of a radiography apparatus is provided. In a second step 420, at least one parameter associated with an irradiation of said reference object is determined. In a third step 430, radiography image data is processed based on the at least one determined parameter.

According to a further example, the determination of at least one parameter comprises determining at least one of: the signal from the direct radiation next to the reference structure; at least one signal from behind the reference structure; at least one attenuation coefficient associated with the reference structure; and/or the half value layer of the radiation (HVL).

According to a further example, shown in Fig. 5, the above described method may comprise a fourth step 440 of controlling an X-ray source of the radiography apparatus based on the at least one determined parameter. For example, as indicated with a dotted line 450, the controlling can be provided for a further step of radiating the detector.

The first step 410 is also referred to as step a), the second 420 step as step b), the third step 430 as step c) and the fourth step 440 as step d).

According to a further example, a computer program element is provided, which, when being executed by a processing unit is adapted to carry out the above described method.

According to a further example, a computer readable medium is provided having stored the above described program element.

The upgrade of an existing radiography apparatus or X-ray imaging system can include any suitable method for providing the apparatus or system with upgraded software. For example upgraded software can be downloaded from a memory element or from a disk or any other suitable support means. Alternatively, the software can be downloaded from a server or any other computer device.

The computer program element might be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. A computer program may be loaded into a working memory of a data processor. The data processor may thus be equipped to carry out the method of the invention.

This exemplary embodiment of the invention covers both, a computer program that right from the beginning uses the invention and a computer program that by means of an up-date turns an existing program into a program that uses the invention.

Further on, the computer program element might be able to provide all necessary steps to fulfil the procedure of an exemplary embodiment of the method as described above.

According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM, is presented wherein the computer readable medium has a computer program element stored on it which computer program element is described by the preceding section.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

However, the computer program may also be presented over a network like the

World Wide Web and can be downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform a method according to one of the previously described embodiments of the invention.

It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application.

However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A detector unit (210) for an X-ray imaging system comprising:

a detector device (220) comprising a detector surface (230);

at least one reference structure (240); and

a processing unit (250);

wherein the reference structure (250) has predetermined radiation attenuation properties and is located in front of the detector surface (230);

wherein the detector device (220) is configured to detect X-ray radiation and to provide respective radiography image signals to the processing unit (250); and

wherein the processing unit (250) is configured to determine at least one parameter associated with the irradiation of said reference structure (240), and to process radiography image data based on the at least one determined parameter.

2. Detector unit according to claim 1, wherein said reference structure (310, 320) comprises a plurality of regions (31 OA, 310B, 3 IOC, 320A, 320B, 320C), each region having a different attenuation property.

3. Detector unit according to claim 1 or 2, wherein the reference structure (320) has a circular structure; and

wherein the plurality of regions (320A, 320B, 320C) are arranged in concentric circles.

4. Detector unit according to one of the preceding claims, wherein the reference structure (240) comprises at least one of the group of:

at least one aluminium part having thickness about 1 mm, 3.7 mm or 4.7mm; or

at least one part of human-tissue equivalent attenuation material.

5. Detector unit according to one of the preceding claims, wherein the detector device (220) is provided with a detector housing (260) and the reference structure (240) is located in proximity of the edge of the detector housing (260).

6. Detector unit according to one of the preceding claims, wherein processing radiography image data comprises at least one of the group of:

post-processing a radiography image; or

pre-processing radiography image signals.

7. An imaging system (120), comprising:

an X-ray source (130); and

a detector unit (110) according to any of the preceding claims; wherein the X-ray source (130) is configured to provide X-ray radiation towards the detector unit (110).

8. Imaging system (120) according to claim 7, wherein the X-ray source is arranged to be controlled based on the at least one determined parameter.

9. Imaging system (120) according to claim 7 or 8, wherein the system is a mammography system comprising two compression surfaces (140, 150); and

wherein the at least one reference structure is located on at least one of the two compression surfaces (140, 150).

10. Imaging system (120) according to claim 7, 8 or 9, wherein the detector (110) is comprising two opposite edges;

wherein the focal spot of the X-ray source (130) is located closer to one of the two edges; and

wherein the reference structure is located closer to the other one of the two edges.

11. A method (400) of processing radiography data, comprising the following steps:

a) providing (410) a reference structure with predetermined radiation attenuation properties located in front of the detector unit of a radiography apparatus; b) determining (420) at least one parameter associated with an X-ray irradiation of said reference object; and

c) processing (430) radiography image data based on the at least one determined parameter.

12. Method according to claim 11, wherein the determination of at least one parameter comprises determining at least one of:

i) the signal from the direct radiation next to the reference structure;

ii) at least one signal from behind the reference structure;

iii) at least one attenuation coefficient associated with the reference structure; iv) the half value layer of the radiation (HVL).

13. Method according to claim 11 or 12, further comprising the step of:

d) controlling (440) an X-ray source of the radiography apparatus based on the at least one determined parameter.

14. A computer program element, which, when being executed by a processing unit is adapted to carry out the method according to of any of the claims 11 to 13.

15. A computer readable medium having stored the program element of claim 14.