WO2014097026A1

WO2014097026A1 - Breast thickness measurement in mammography

Info

Publication number: WO2014097026A1
Application number: PCT/IB2013/060491
Authority: WO
Inventors: André GOOSSEN; Hanns-Ingo Maack
Original assignee: Koninklijke Philips N.V.; Philips Deutschland Gmbh
Priority date: 2012-12-21
Filing date: 2013-11-29
Publication date: 2014-06-26

Abstract

The present invention relates to measuring breast thickness in mammography. In order to provide improved height measurements with high accuracy, a mammography imaging system (10) is provided that comprises a breast support device (12), an X-ray imaging device (14), and a processing device (16). The breast support device comprises a support surface (18) and at least one compression element (20). A distance measuring device (22) is provided to determine the current distance between the support surface and the compression element, and to provide the current distance as a first breast height. The X-ray imaging device is configured to provide spectral X-ray image data of a breast arranged between the support surface and the compression element. The processing device is configured to perform spectral material decomposition based on the spectral image data, and to calculate a second breast height. Further, the processing device is configured to compare the first breast height and the second breast height to determine and to provide a height difference value.

Description

BREAST THICKNESS MEASUREMENT IN MAMMOGRAPHY

FIELD OF THE INVENTION

The present invention relates to measuring thickness of a breast in mammography, and relates in particular to a mammography imaging system, a method for calibrating breast thickness measurements for mammography, a computer program element, and a computer readable medium.

BACKGROUND OF THE INVENTION

In mammography, knowledge of the breast height between the two breast holding paddles is used, for example, for determining and defining the acquisition parameters, such as which X-ray spectrum will be used for an X-ray imaging procedure to acquire mammograms, for example. The acquired X-ray images may be used for providing density information of the tissue structure; the assessment of volumetric breast density is considered to be a key feature for detection and prediction of breast cancer. For example, WO

2012/080914 Al describes breast density assessment based on dual energy basis material decomposition. However, imprecise compression heights values negatively affect many quantitative algorithms applicable to mammograms. It has been shown that this may result in inferior, non-optimal image quality, since the acquisition parameters are set for an incorrect tissue thickness. SUMMARY OF THE INVENTION

There may be a need to provide improved height measurements with high accuracy.

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 mammography imaging system and the method for calibrating breast thickness measurements for mammography, as well as to the computer program element and the computer readable medium.

According to the present invention, a mammography imaging system is provided comprising a breast support device, an X-ray imaging device, and a processing device. The breast support device comprises a support surface and at least one compression element. A distance measuring device is provided to determine the current distance between the support surface and the compression element, and to provide the current distance as a first breast height. The X-ray imaging device is configured to provide spectral X-ray image data of a breast arranged between the support surface and the compression element. The processing device is configured to perform spectral material decomposition based on the spectral image data, and to calculate a second breast height. The processing device is configured to compare the first breast height and the second breast height to determine and to provide a height difference value.

Thus, a calibrated breast height or breast thickness, i.e. corrected and approved by two different height measurements, is provided meaning improved accuracy. The breast thickness can thus be used for further purposes, for example for determining acquisition parameters, leading to improved image quality and thus providing enhanced information about the object of interest, i.e. a patient's breast. In particular, the height difference value can be provided for further image acquisition procedures and for further image processing steps for post-image processing. Since the spectral material decomposition is provided by the processing device, and thus a manual calibration of the mammography imaging system is not required for the determining of the height difference value, the present invention provides a facilitated operating procedure in terms of calibration.

In an example, the provided X-ray image data is dual energy spectral X-ray image data.

In an example, the compression element is height-adjustable in order to adapt the mammography imaging system for different breast sizes.

The first breast height is also referred to as mechanical breast height. The second breast height is also referred to as spectral-based breast height. The compression element is also referred to as compression paddle. The breast support device may comprise a compression element as the support surface, i.e. breast support device may be provided by two adjustable compression elements. Further, the height of the breast support device may be adapted to different heights of the patient. When mentioning the term "height" in relation with the breast or the patient, this relates to a patient in a standing position. In one example, the mammography imaging system is provided for a standing-up position of the patient. In another example, the patient is arranged on a patient support, such as a patient table, to acquire images in a horizontal position, for example in a position where the breast is facing downwards, and may be temporarily fixated by two abutting paddles.

For example, the distance measuring device is provided as at least one of the group of a mechanical measurement readout device, an optical measurement device, an electromagnetic measurement device, and an ultrasound measurement device.

The mechanical measurement may be provided by the paddle adjustment mechanism. The mechanical measurement may also be provided as a separate measurement such as a flexible, i.e. bendable band ruler. The optical measurement device may be provided by optical signals, such as laser signals to determine distance between two surfaces. The electromagnetic measurement may be provided with integrated coils in the support stand to which the paddles are attached to and a counterpart in the paddle(s). The ultrasound measurement device may be provided in one of the paddles in an area where a breast does not hinder direct path between a signal generating device and a signal receiving device. For example, a transducer may be provided such that the distance measurement is based on sound wave reflection.

The performance of the spectral material decomposition is provided in one example according to "Spectral Volumetric Glandularity Assessment" by Andre GooBen, Harald Heese, Klaus Erhard and Bjorn Norell, IWDM 2012, LNCS 7361, pages 5 to 9 to 536, Springer- Verlag 2012.

According to an example, the processing device is configured to determine a calibration factor based on the first breast height and the second breast height. The distance measuring device is calibrated with the calibration factor for further measurements.

The term "to calibrate" comprises the provision of the height difference value to the X-ray imaging device. The system may be provided to be self-calibrating. The calibrating result can be used for thickness compensation. The calibrating result can also be used for skin line correction. In a further example, the calibration is used for image correction of the image that was the basis for the spectral material decomposition.

According to an example, a compression force measuring device is provided. The compression force measuring device is configured to detect the currently applied compression force, and to provide a compression force value to the processing device. The processing device is configured to determine a compression force related calibration factor for further distance measuring and/or height calculating based on the first breast height and the and second breast height and compression force value.

The compression force related calibration factor is used to calibrate the distance measuring device, for example. According to an example, the compression element is exchangeable with a plurality of compression elements, and an individual height difference value is provided for a number of the compression elements. The error factor can be stored or can be attached / associated otherwise to the respective compression element or paddle.

In one example, the distance measuring device is configured to perform the distance measuring for a plurality of locations across the flexible compression element's surface, and the processing device is configured to calculate the second breast height for a plurality of locations, and to generate a height difference profile for the compression element. It is noted that the height difference profile can be provided for flexible compression elements, as well as to non- flexible compression elements.

It must be noted that it is also possible to provide a distance measuring only for one location of the compression element's surface, but to perform the calculation of the second breast height for a plurality of locations and to generate a height difference profile for the compression element.

According to the present invention, a method for calibrating breast thickness measurements for mammography is provided, comprising the following steps:

a) measuring a current distance between a support surface and a compression element of a breast support device of a mammography imaging system, and providing the current distance as a first breast height;

bl) providing spectral X-ray image data of a breast arranged between the support surface and the compression element;

b2) performing spectral material decomposition based on the spectral image data; b3) calculating a second breast height; and

c) comparing the first breast height and the second breast height to determine and provide a height difference value.

The height difference value is used for determining breast thickness for further X-ray imaging, in one example. The height difference value may be used for determining breast thickness for other purposes, such as calculating an average glandular dose.

According to an example, the height difference value is used for correcting a breast thickness value.

According to an example, the determination of the first breast height and the calculation of the second breast height are provided for the same breast in the same compressed breast state.

The determination of the first breast height and the calculation of the second breast height are provided for the same breast location, i.e. they refer to the same breast location.

According to an example, the height difference value is used to determine at least one of the group of: i) a system offset between mechanical and spectral measurement; ii) an application error factor in dependency of applied compression force and breast size; and iii) a compression element error factor of a specific compression element.

The system offset may also be referred to as system error or system compression deviation factor. The application error factor may differ between different compression forces; for example, smaller compression forces lead to less deformation and thus less deviation between measured first breast height and calculated second breast height than larger compression forces. Another variation of the application error factor may be caused by breast size; for example, larger breasts show different compression resistance than smaller breasts. The compression element error factor may be caused by aging, change in the material behaviour, defects or wear over time.

According to an example, the distance measuring is performed for a plurality of locations across the compression element's surface. For example, the compression element is non- flexible. In another example, the compression element is a flexible compression element. The calculation of the second breast height is performed for a plurality of locations, and a height difference profile is generated for the compression element.

In one example, the height difference profile is a 2D profile, i.e. a line, curve or contour profile. In another example, the height difference profile is a 3D profile, i.e. a surface profile. The height difference profile is also referred to as height profile. The height difference profile considers paddle tilt and paddle deflection, i.e. paddle bending, e.g. either in chest wall to nipple direction or in perpendicular direction.

For example, instead of or in addition to the height difference profile, a mean height difference value is determined. The distance measuring and the calculation are performed for the same plurality of locations. In another example, the distance measuring and the calculation are performed for a plurality of locations arranged in rows and columns.

According to an example, a calibration factor is determined based on the first breast height and the second breast height, and the distance measuring device is calibrated with the calibration factor for further measurements.

According to an example, a currently applied compression force is detected and a compression force related calibration factor is determined for further distance measuring and/or height calculating based on the first breast height and the second breast height and compression force value.

According to an aspect of the present invention, the breast height in a current state or situation is measured by two different ways, for example by a mechanical height measuring and by the performance of a spectral material decomposition leading to a second breast height. By putting the two different height values into relation with each other, an error factor, or calibration factor is provided indicating the deviation between the two different measurements. Since the spectral material decomposition may be provided with a very high accuracy, for example up to less than 1 mm, the height difference value indicates the incorrectness or impreciseness of the first height measuring, for example the mechanical measuring. In addition it is possible to use calibration factors depending on external parameters, such as different compression elements, the size of the compression paddle, or the applied compression force.

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 shows a schematic setup of a mammography imaging system in one example in Fig. 1A and in another example in Fig. IB;

Fig. 2 shows a detail of a mammography imaging system with a compression force measuring device;

Fig. 3 shows a visualization of a height difference profile for a non- flexible paddle in Fig. 3A and for a flexible paddle in Fig. 3B;

Fig. 4 shows steps of an example of a method for calibrating breast thickness measurements for mammography;

Fig. 5 shows a detail of a further example of a method;

Fig. 6 shows a still further example of the method;

Fig. 7 shows an example of the method for calibrating breast thickness measurements for mammography with distance measuring at a plurality of locations;

Fig. 8 shows a further example of the method, in which a calibration factor is determined and used for calibration of the distance measuring in further measurements; and

Fig. 9 shows a further example of a method, in which the compression force is detected and a compression force related calibration factor is determined. DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a mammography imaging system 10 that comprises a breast support device 12, an X-ray imaging device 14, and a processing device 16. The breast support device 12 comprises a support surface 18, and at least one compression element 20. A distance measuring device 22 is provided to determine the current distance between the support surface 18 and the compression element 20, and to provide the current distance as a first breast height, for example to the processing device 16, as indicated with data connection line 17. The X-ray imaging device 14 is configured to provide spectral X-ray image data of a breast arranged between the support surface 18, and the compression element 20. The processing unit 16 is configured to perform spectral material decomposition based on the spectral image data, and to calculate a second breast height. The processing device 16 is configured to compare the first breast height and the second breast height to determine and provide a height difference value.

The X-ray imaging device 14 may comprise an X-ray source 24 generating X- ray radiation 26, and a detector 28. The breast support device 12 is arranged between the X- ray source 24 and the detector 28. For example, a breast is arranged between the support surface 18 and the compression element 20, as indicated with dotted line 30. The X-ray source 24 and the detector 28 may also be controlled by the processing device 16, as indicated with connecting lines 32 and 34.

Fig. IB shows a further example of the mammography imaging system 10. The breast support device 12 is movably connected to a stand 36, i.e. a structural and stable housing for a free standing apparatus. However, instead of the stand 36, also an attachment to a wall surface may be provided. A guiding arrangement 38 indicates the possible up- and downwardly oriented adjustment, indicated with double-arrow 40. The breast support device 12 may comprise a lower support surface 18 with an integrated detector 28 (not further shown). Thus, the X-ray source 24 may be provided above the breast support device 12 with the compression element 20 to be adjustable in a vertically oriented direction, as indicated with double-arrow 42.

The mammography imaging system may be controllable by a control unit 44, arranged on a separate or second stand 46. For example, a keyboard 48 may be provided in combination with a display 50, showing certain information 52.

The distance measuring device 22 may be provided integrated in the adjustment structure of the breast support device 12 in form of an integrated mechanical measurement readout device.

By combining the spectral material decomposition and a mechanical distance measuring, an improved knowledge about the breast height is provided, i.e. the dimension of the breast in the vertical direction in form of the stand-up arrangement shown in Fig. IB. It is noted that according to the present invention, also other forms of mammography imaging systems are provided, for example a horizontal arrangement in form of a patient table, on which the patient can lie in a downwardly facing direction such that the breast can be arranged in a hanging manner between two abutting surfaces of the breast support device (not further shown in detail).

Fig. 2 shows a further example, where a compression force measuring device 54 is provided. The compression force measuring device 54 is configured to detect the currently applied compression force F and to provide a compression force value to the processing device 16. The processing device 16 is configured to determine a compression force related calibration factor for further distance measuring and/or height calculating based on the first breast height and the second breast height and compression force value.

According to a further example, a distance measuring is performed for a plurality of locations across the compression element's surface. Further, the calculation of the second breast height is performed for a plurality of locations and a height difference profile is generated for the compression element.

Fig. 3A shows an example of a height difference profile 56 indicating a difference in thickness, for example in millimetres, provided on a vertical axis 58, indicating the respective value for the surface of, for example, a non-flexible paddle. On a first horizontal axis 60, different positions of rows R are indicated, whereas on a second horizontal axis 62, the location with respect to different columns C is indicated.

In a further example, shown in Fig. 3B, an example of a height difference profile 56 is indicated in the same manner for a flexible paddle.

The height profiles may be (colour) coded such that parts with the same deviation are also shown in similar manner. Further, the spatial orientation in the diagram itself also represents the bending or inaccuracy factor of the compression element.

Thus, a visualization of the existing error of factors or deviations of the mechanically measured breast height and the breast height provided by the spectral material decomposition is provided.

In a further example, not shown in detail, the distance measuring device 22 is provided as an optical measurement device. In a further example (also not shown), an electromagnetic measurement device is provided. In a still further example, an ultrasound measurement device is provided (also not shown in detail).

Fig. 4 shows an example of a method 100 for calibrating breast thickness measurements for mammography. The following steps are provided:

- In a first step 110, a current distance between a support surface and compression element of a breast support device of a mammography imaging system is measured. The current distance is provided as a first breast height 112.

In a second step 114, spectral X-ray image data of a breast arranged between the support surface and the compression element is provided.

- In a third step 116, spectral material decomposition, indicated with reference numeral 118 is provided based on the spectral image data.

In a fourth step 120, a second breast height 122 is calculated.

In a fifth step 124, the first breast height 112 and the second breast height 122 are compared to determine and provide a height difference value 126.

The first step 110 is also referred to as step a), the second step 114 as step bl), the third step 116 as step b2), the fourth step 120 as step b3), and the fifth step 124 as step c).

It must be noted that also other orders of the steps may be provided, where applicable.

The height difference value 126 is used, for example, for determining breast thickness for further X-ray imaging. The height difference value 126 may also be used for determining breast thickness for other purposes, such as calculating an average glandular dose.

Fig. 5 shows an example of a method, where the height difference value 126 is used for correcting 128 a breast thickness value.

For example, the determination of the first breast height 112 and the calculation of the second breast height 122 are provided for the same breast in the same compressed breast state.

Fig. 6 shows an example of the method, in which the height difference value 126 is used to determine at least one the group of a system offset 130 between mechanical and spectral measurement, an application error factor 132 in dependency of applied compression force and breast size, and a compression element error factor 134 of a specific compression element.

Fig. 7 shows an example of the method, in which the distance measuring is performed for a plurality of locations across the compression element's surface, as indicated with a plurality of frames 136. The calculation of the second breast height is performed for a plurality of locations, as indicated with a plurality of frames 138. Further, a height difference profile is generated for the compression element, indicated with frame 140. For example, a height difference value is provided for the plurality of locations, as indicated with a plurality of frames 142. However, the height difference value determination is provided in one example. In another example, the height difference value determination is not provided, and thus rather the height difference profile 142 is provided.

The height difference profile 142 may be provided as a 2D profile, i.e. a line, curve or contour profile. In another example, the height difference profile 142 is a 3D profile, i.e. a surface profile, for example shown in Figs. 3A and 3B.

As shown in Fig. 8, a further example is provided, in which a calibration factor 144 is determined in a further step 146, wherein the calibration factor 144 is determined 146 based on the first breast height 112 and the second breast height 122. The distance measuring device is calibrated 148 with the calibration factor 144 for further measurements, for example the measuring of the first step 110.

Fig. 9 shows a further example of the method where a currently applied compression force is detected in a detection step 150. For example, a compression force related calibration factor 152 is determined 153 for further distance measuring, for example the measuring 110, as indicated with a loop-like feedback arrow 154, or in addition, or alternatively, for further height calculating, for example the calculating step 120.

The compression force related calibration factor 152 is based on the first breast height 112 and the second breast height 122 as well as the compression force value 155.

According to an aspect of the present invention, a spectral method is used to measure the amount and thus the height of breast tissue along an X-ray regardless of the breast's composition. Comparing the mechanical measurement of compression height, i.e. readout from the gantry, for example, to the retrospectively determined breast height via spectral material decomposition, it is possible to determine a system error, as mentioned above, an error depending on compression force, or an error due to paddle aging, for example.

According to an example, for each acquired mammogram, the determination of a compression element's identification number of identification tag is provided. Further, breast thickness via mechanical measurement is provided in addition to breast thickness via spectral decomposition, h, based on the following equation: ø_¾ + -. L -+- (_¾ίί - ¾£² t ( iLB ÷ (x T

h i lji) low energy counts/ alue

high energy couuts/value

The coefficients a_n, n=0..5, and p_m, m=1..2, of the rational polynomial can be determined via calibration with phantoms of different compositions and known heights.

Still further, a self-training calibration for each configuration, i.e. for each specific compression element, is provided by relating mechanical measurement, spectral measurement, and/or compression force.

Upon new acquisitions, the calibration can be applied to correct mechanical measurement. Still further, the calibration can be continuously updated to capture changes, such as aging, defects, etc. In a further embodiment, defect or aged paddles are detected by inspecting calibration glitches.

In another embodiment, the height measurement is not only corrected with respect to a single value, e.g. the body part thickness written into for example the

corresponding DICOM (digital-imaging-and-communication-in-medicine standard) tag, but a height profile is derived based on the calibration, i.e. including paddle tilt and deflection.

Besides providing the combination of the mechanical measurement with the spectral material decomposition in a mammography imaging system, the invention is also provided in tomosynthesis systems.

According to the invention, the image impact of a flexible paddle is reduced. For example, the "wedge contribution" of the paddle can be removed. In a further example, the skin line correction itself, reducing the final depiction of the skin line in the processed image on very dark shades of grey, may be facilitated and thus improved in terms of accuracy.

As a further example, the average glandular dose (AGD) can also benefit from the present invention, as the computation of AGD relies on a precise breast thickness.

In another exemplary embodiment of the present invention, a computer program or a computer program element is provided that is characterized by being adapted to execute the method steps of the method according to one of the preceding embodiments, on an appropriate system.

The computer program element might therefore 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 mammography imaging system (10), comprising:

a breast support device (12);

an X-ray imaging device (14); and

a processing device (16);

wherein the breast support device comprises a support surface (18) and at least one compression element (20);

wherein a distance measuring device (22) is provided to determine the current distance between the support surface and the compression element, and to provide the current distance as a first breast height;

wherein the X-ray imaging device is configured to provide spectral X-ray image data of a breast arranged between the support surface and the compression element;

wherein the processing device is configured to perform spectral material decomposition based on the spectral image data, and to calculate a second breast height; and wherein the processing device is configured to compare the first breast height and the second breast height to determine and to provide a height difference value.

2. System according to claim 1 , wherein the processing device is configured to determine a calibration factor based on the first breast height and the second breast height; and

wherein the distance measuring device is calibrated with the calibration factor for further measurements.

3. System according to claim 1 or 2, further comprising:

a compression force measuring device (51);

wherein the compression force measuring device is configured to detect the currently applied compression force, and to provide a compression force value to the processing device; and

wherein the processing device is configured to determine a compression force related calibration factor for further distance measuring and/or height calculating based on the first breast height the and second breast height and compression force value.

4. System according to claim 1, 2 or 3, wherein the compression element is exchangeable with a plurality of compression elements; and

wherein an individual height difference value is provided for a number of the compression elements.

5. A method (100) for calibrating breast thickness measurements for

mammography, comprising the following steps:

a) measuring (110) a current distance between a support surface and a compression element of a breast support device of a mammography imaging system, and providing the current distance as a first breast height (112);

bl) providing (114) spectral X-ray image data of a breast arranged between the support surface and the compression element;

b2) performing (116) spectral material decomposition (118) based on the spectral image data;

b3) calculating (120) a second breast height (122); and

c) comparing (124) the first breast height and the second breast height to determine and provide a height difference value (126).

6. Method according to claim 5, wherein the height difference value is used for correcting (128) a breast thickness value.

7. Method according to claim 5 or 6, wherein the determination of the first breast height and the calculation of the second breast height are provided for the same breast in the same compressed breast state.

8. Method according to claim 5, 6 or 7, wherein the height difference value is used to determine at least one of the group of:

i) a system offset (130) between mechanical and spectral measurement;

ii) an application error factor (132) in dependency of applied compression force and breast size; and

iii) a compression element error factor (134) of a specific compression element.

9. Method according to one of the claims 5 to 8, wherein the distance measuring is performed (136) for a plurality of locations across the compression element's surface;

wherein the calculation of the second breast height is performed (138) for a plurality of locations; and

wherein a height difference profile (142) is generated (140) for the compression element.

10. Method according to one of the claims 5 to 9, wherein a calibration factor (144) is determined (146) based on the first breast height and the second breast height; and

wherein the distance measuring device is calibrated (148) with the calibration factor for further measurements.

11. Method according to one of the claims 5 to 10, wherein a currently applied compression force is detected (150); and

wherein a compression force related calibration factor (152) is determined (153) for further distance measuring and/or height calculating (154) based on the first breast height and the second breast height and compression force value.

12. A computer program element for controlling an apparatus according to one of the claims 1 to 4, which, when being executed by a processing unit, is adapted to perform the method steps of any one of claims 5 to 11.

A computer readable medium having stored the program element of claim 12.