WO2012038857A1

WO2012038857A1 - Phase gradient unwrapping in differential phase contrast imaging

Info

Publication number: WO2012038857A1
Application number: PCT/IB2011/053995
Authority: WO
Inventors: Thomas Koehler; Ewald Roessl
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2010-09-20
Filing date: 2011-09-13
Publication date: 2012-03-29

Abstract

The present invention relates to recovering image data in differential phase- contrast imaging. In order to provide improved phase gradient unwrapping in case of insufficient image data, a method for phase gradient unwrapping in differential phase contrast imaging based on a quality check is provided with the following the steps: a) providing X-ray phase contrast image data; b) estimating (132) wrapped phase gradients (134) for at least sample image points (116) of the image data; c) determining (136) likelihood (138) that phase wrapping occurred at the sample image points and selecting (140) image points of the sample image points with medium likelihood (142, 150); d) adapting the data of the selected image points by applying predetermined filtering (144); e) replacing (146) the acquired data of the selected image points with the adapted data providing adapted image data (148), which adapted image data also comprises the acquired data of the non- selected image points; and f) recovering the image data on the basis of the adapted image data by phase-un-wrapping (152).

Description

PHASE GRADIENT UNWRAPPING IN DIFFERENTIAL PHASE CONTRAST IMAGING

The present invention relates to recovering image data in differential phase- contrast imaging, in particular to phase gradient unwrapping in differential phase-contrast imaging, a device for phase gradient unwrapping in differential phase-contrast imaging, a medical imaging system for phase gradient unwrapping, a computer program element and a computer-readable medium.

BACKGROUND OF THE INVENTION

Differential phase-contrast imaging is used, for example, to enhance the contrast of low absorbing specimen, compared to conventional amplitude contrast images. In WO 2004/071298 Al an apparatus for generating phase-contrast X-ray imaging comprises in an optical path an incoherent X-ray source, a beam splitter grating, a beam recombiner grating, an optical analyzer grating and an image detector. In differential phase-contrast CT, for example, phase wrapping occurs in medical application settings. Phase unwrapping techniques must be used.

SUMMARY OF THE INVENTION

However, phase unwrapping requires that the data are sampled appropriately, i.e., they have to be free of aliasing. Phase unwrapping further requires that the signal-to-noise ratio is high.

Hence, there may be a need to provide improved phase gradient unwrapping in case of insufficient image data.

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 device for phase gradient unwrapping, the medical imaging system, the method for phase gradient unwrapping, the computer program element and the computer-readable medium. According to an exemplary embodiment of the invention, a method for phase gradient unwrapping in differential phase-contrast imaging is provided comprising the following steps: a) providing X-ray phase-contrast image data; b) evaluating quality of at least sample image points; c) selecting image points with predetermined quality range; d) adapting selected image points; e) providing adapted image data; and f) recovering image data.

According to a further exemplary embodiment of the invention, a method is provided wherein step b) comprises estimating wrapped phase gradients for at least sample image points of the image data; wherein step c) comprises determining likelihood that phase wrapping occurred at the at least sample image points and selecting image points of the at least sample image points with medium likelihood; wherein step d) comprises adapting the data of the selected image points by applying predetermined filtering; wherein step e) comprises replacing the acquired data of the selected image points with the adapted data providing adapted image data, which adapted image data also comprises the acquired data of the non-selected image points; wherein step f) comprises recovering the image data on the basis of the adapted image data by phase unwrapping; and wherein a step g) is provided which step provides the recovered image data.

According to a further exemplary embodiment, in step c), the noise quality of the acquired data of the at least sample image points of the image data is determined, wherein the noise determination is based on predetermined threshold values.

For example, image points with a noise quality below a predetermined threshold are selected.

According to a further exemplary embodiment, the values of the at least sample points are broad into relation with at least one neighbourhood pixel in step c).

For example, the neighbourhood is a spatial and/or angular neighbourhood. According to a further exemplary embodiment, for applying the predetermined filtering in step d), low pass filtering is applied.

According to a further exemplary embodiment of the invention, a device for phase gradient unwrapping in differential phase-contrast imaging is provided, comprising a processing unit and an interface unit. The processing unit is adapted to evaluate the quality of at least sample image points, to select image points with predetermined quality; to adapt selected image points; to provide adapted image data and to recover image data. The interface unit is adapted to provide the X-ray phase-contrast image data. According to a further exemplary embodiment, the processing unit is adapted to estimate wrapped phase gradients for at least sample image points of provided X-ray phase- contrast image data, to determine likelihood that phase wrapping occurred at the at least sample image point to select image points of the at least sample image points with medium likelihood, to adapt the data of the selected image points by applying predetermined filtering, to replace the acquired data of the selected image points with the adapted data providing adapted image data, which adapted image data also comprises the acquired data of the non- selected image points; and to recover the image data on the basis of the adapted image data by phase unwrapping. The interface unit is adapted to provide the X-ray phase-contrast image data and to provide the recovered image data.

According to a further exemplary embodiment, a medical imaging system for phase gradient unwrapping in differential phase-contrast imaging is provided, comprising a differential phase-contrast X-ray image acquisition device, an object receiving device and a device according to one of the above-mentioned exemplary embodiments.

It can be seen as the gist of the invention to evaluate the provided image data with respect to the quality of the acquired data and to manipulate the value of only those image points which have been selected due to the evaluation step. Thus, the data provided for, for example, further image processing algorithms is adapted only where an improvement of the data can be achieved, whereas other image data is directly provided as measured.

These and other aspects of the present invention will become apparent from and elucidated with reference to the exemplary 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 schematically shows an example of a medical imaging system.

Fig. 2 schematically shows a detector arrangement of an X-ray system for phase-contrast imaging.

Fig. 3 schematically shows basic method steps of an exemplary embodiment.

Fig. 4 schematically shows method steps of a further exemplary embodiment. Fig. 5 schematically shows method steps of a further exemplary embodiment. Fig. 6 schematically shows a sinogram of differential data with pixels where phase wrapping was induced by noise.

Fig. 7 schematically shows a tomographic reconstruction of the object using differential data of Fig. 6 and the resulting artifacts.

Fig. 8 schematically shows a sinogram with adapted differential data according to the invention.

Fig. 9 schematically shows a reconstruction of the object using the recovered image data of Fig. 8.

Fig. 10 schematically describes a further exemplary embodiment of a method according to the invention.

Fig. 11 shows two graphs with exemplary neighbouring samples of measured phase gradient.

Figs. 12 to 15 show photographic images in greyscale of the sinograms and reconstructions of Figs. 6 to 9.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically shows an X-ray imaging system 10 with an examination apparatus 50 for generating phase-contrast images of an object. The examination apparatus comprises an X-ray image acquisition device 11 with a source of X-ray radiation 12 provided to generate X-ray radiation beams with a conventional X-ray source. A table 14 is provided to receive a subject to be examined, for example a patient. Further, an X-ray detector unit 16 is located opposite the source of X-ray radiation 12. The detector unit is sending data to a processing unit 18 via an interface unit 19 (not further shown) which is connected to the detector unit 16 and the radiation source 12. The processing unit is located underneath the table 14 to save space within the examination room. However, of course, it could also be located at a different place, such as a different laboratory. Further, a display 20 is arranged in the vicinity of the table 14 to display information such as the generated images to the person operating the X-ray imaging system. Further, a user interface 22 is arranged to input information by the user. It is noted that the example shown is of a so-called C-type X-ray image acquisition device. The X-ray image acquisition device comprises an arm in form of a C where the image detector is arranged at one end of the C-arm and the source of X-ray radiation is located at the opposite end of the C-arm. The C-arm is movably mounted and can be rotated around the object of interest located on the table 14. In other words, it is possible to acquire images with different directions of view.

It is further noted, that other forms of X-ray image acquisition devices are also possible, such as a gantry with a rotating pair of X-ray source and detector.

According to an exemplary embodiment, the processing unit 18 is adapted to evaluate the quality of at least sample image points, to select image points with predetermined quality, to adapt the selected image points, to provide adapted image data and to recover image data. The interface unit 22 is also adapted to provide the X-ray phase-contrast image data.

According to a further exemplary embodiment, the processing unit 18 is adapted to estimate wrapped phase gradients for at least sample image points of provided X- ray phase-contrast image data, to determine likelihood that phase wrapping occurred at the at least sample image points and to select image points of the at least sample image points with medium likelihood, to adapt the data of the selected image points by applying predetermined filtering, to replace the acquired data of the selected image points with the adapted data providing adapted image data, which adapted image data also comprises the acquired data of the non-selected image points and to recover the image data on the basis of the adapted image data by phase unwrapping. The interface unit is adapted to provide the X-ray phase-contrast image data and to provide the recovered image data.

The medical imaging system 10 for phase gradient unwrapping in differential phase-contrast imaging as shown in Fig. 1 comprises a differential phase-contrast X-ray image acquisition device, as also explained in relation with Fig. 2, an object receiving device, for example a table, and a device for phase gradient unwrapping in differential phase-contrast imaging with the processing unit and an interface, as described exemplary above.

Fig. 2 schematically shows a detector arrangement 24 of an X-ray system for generating phase-contrast images of an object 26. The object 26, for example a patient or a sample as shown in Fig. 2, is arranged between a source grating 28 and a phase grating 30. An analyzer grating 32 is arranged behind the phase grating 30. Further, a detector with a sensor 34 is provided behind the analyzer grating 32.

For examination of the object 26, an X-ray beam 36 of polychromatic spectrum of X-rays is provided by a conventional X-ray source 38. The X-ray radiation beam 36 is applied to the source grating 28 splitting the X-ray radiation such that at least partially coherent X-ray radiation is provided. The sp lifted beam, indicated with reference numeral 39 is applied to the phase grating 30 which is recombining the split beams in an analyzer plane. After recombining the split beams behind the phase grating 30, the recombined beam is applied to the analyzer grating 36. Then, the sensor 34 is recording raw image data while the analyzer grating 32 is stepped transversely over one period of the analyzer grating 32.

In Fig. 3, the basic steps of a first method 100 for phase gradient unwrapping in differential phase-contrast imaging is described, comprising the following steps: In a providing step 110, X-ray phase-contrast image data 112 is provided. In an evaluation step 114, quality of at least sample image points 116 is evaluated. In a selection step 118, image points with predetermined quality 120 are selected. In an adaption step 122, the selected image points are adapted. In a further providing step 124, adapted image data 126 is provided and in a recovering step 128, image data 130 is recovered.

According to a further aspect of the invention, the term„at least sample image points" can also refer to image point which see the object, i.e. in which the object is detected. In particular, image points, just outside the projection of the object are not selected since there is a-priori knowledge that the differential phase contrast projection of the object has a singularity at the boundary of the object and therefore, it is known that phase-unwrapping is impossible there.

According to a further aspect, the selection can also be based on the absorption projection recorded at the same time.

According to a further exemplary embodiment of a method, as shown in Fig. 4, in the providing step 110 X-ray phase-contrast image data 112 is provided. In step b), in an estimating procedure 132, wrapped phase gradients 134 are estimated for the at least sample image points 116 of the image data. For the selection step c), in a determining step 136, a likelihood 138 that phase wrapping occurred at the at least sample image points 116 is determined and in a selecting step 140, image points of the at least sample image points with medium likelihood 142 are selected. Further, in the adaption step 120, also referred to as step d), the data of the selected image points are adapted by applying predetermined filtering 144. For the providing step 124, in a replacement step 146, the acquired data of the selected image points is replaced with the adapted data providing adapted image data 148 which adapted image data also comprises the acquired data of the non-selected image points, which is indicated with an error coming from the lower part of the box of step d) entering the box of step e), indicated with reference numeral 150. For the recovering step 128, the image data is recovered on the basis of the adapted image data by phase unwrapping 152. Further, in a providing step 154, recovered image data 156 is provided, e.g., to a display unit or to a tomographic reconstruction unit.

For example, the image data provided in step a) is raw image data.

According to a further aspect of the invention, the raw image data is acquired by a sensor of a differential phase-contrast X-ray image acquisition device.

According to the present invention, the term "image points of the image data" relates to image pixels.

For example, the image pixels are related to pixels of the sensor of the detector.

According to a further aspect of the invention, the method is provided for phase gradient unwrapping in differential phase-contrast CT.

According to a further aspect of the invention, the wrapped phase gradients are estimated for all image points of the image data (not further shown).

According to a further exemplary embodiment, not shown, the noise quality of the acquired data of the at least sample image points of the image data is determined, wherein the noise determination is based on predetermined threshold values.

According to a further aspect of the invention, image points with a noise quality below a predetermined threshold are selected.

According to a further example, for determining the noise quality in step c), a signal-to-noise ratio is determined and compared with predetermined threshold.

According to a further aspect, for the non-selected image points of the at least sample image points, a very high or very low likelihood is determined.

According to a further aspect of the invention, in step c), the values of the at least sample points are brought into relation with at least one neighbourhood pixel.

According to a further exemplary embodiment, the neighbourhood is a spatial and/or angular neighbourhood. Thus, it is possible to provide correction for all directions of the acquired image data in X-, Y- and Z-axis.

According to a further exemplary embodiment, for applying the predetermined filtering in step d), low pass filtering is applied (not further shown). According to a further aspect of the invention, in step e) the image data comprises current pixel values of the non-selected image points and adapted pixel values of the selected image points (see also Fig. 10).

According to a further exemplary embodiment of the invention, for recovering the image data in step f) the following sub-steps are provided: In a calculating step 160, the differences of adjacent image points are calculated. Further, in a wrapping sub-step 162, the differences are wrapped. Still further, in an initializing step 164, the series of unwrapped data is initialized and in an accumulation step 166, the wrapped differences are accumulated.

According to a further aspect of the invention, the image data can be displayed or provided for further image processing purposes.

In differential phase-contrast CT, phase wrapping occurs in medical application settings, therefore, unwrapping techniques must be used. However, phase unwrapping requires that the data are sampled appropriately and that the signal-to-noise ratio, for example, is high, i.e., high above 1. However, the signal-to-noise ratio (SNR) varies strongly within the data to be unwrapped. The present invention provides consideration of this aspect. According to the invention, the points where the phase unwrapping might fail due an insufficient SNR is detected by a statistical analysis. The decision about the right phase difference for these points is then driven by the neighbourhood or the sample at hand, where the neighbourhood can mean a spatial neighbourhood or an angular neighbourhood or both, as already mentioned above.

In the following, some further aspects of the invention shall be described with reference to the equations that are provided according to an exemplary embodiment of the invention.

The quantity to be unwrapped is the local gradient of the phase front. It is derived from a set of x-ray transmission measurements obtained during the so-called phase stepping. Given a set m_k of N measurements, the phase gradient of a pixel is derived as

with

.

For the problem of phase unwrapping, a series of pixels/; is considered and the spatial variation of the gradient of the phase front is to be recovered. Then, the following algorithm can be applied.

1. Calculating differences:

A: = fi+i - fi <≡ [-2π : 2ττ]

2. Wrapping the differences

Ai = arctan (sin £)j/ cos A) G j— : IT]

3. Initializing the series of unwrapped data g_t

90 = fo

4. Accumulating the wrapped differences

In the following, it is assumed that the symbols above relate to the true, noiseless quantities, whereas the noisy actual data are indicated by a ~. In the presence of noise, two failures can happen:

1. There is no phase wrapping between samples z^'-l and z, but due to noise, a phase wrapping is induced in the measured data:

\D.; < π but \D{ \ > 7T

2. There is a true phase wrapping between samples z^'-l and z, but noise is caching it:

Each single measurement nit is subject to Poisson noise. The variance of the estimated local gradient / is then given by

A^r- 1

σ f Of

∑ -

Basically, the approach is to detect the two cases where there is a risk to fail by statistical means and to avoid this situation by local low pass filtering, as explained above.

In the following, the set of true, noiseless phase gradients is denoted as and the measured phase gradients as . Generally, the phase unwrapping algorithm looks for differences between adjacent measurements. Keeping in mind the two cases where phase unwrapping might fail, the following statistical criterion can be defined to check for the appearance of the problem and mitigate it using local low-pass filtering: apply low pass filter

apply low pass filter

The difference that is thus calculated is the variance of the data^. According to a further aspect, the variance of A is used, wherein

+ var( J).

An illustration of the noise induced failures of the phase unwrapping and the resulting artefacts after recovering the image and tomographic reconstruction is shown in Figs. 6 and 7. The effect of the proposed method on the data is shown in Figs. 8 and 9.

In Fig. 6, a sinogram 200 of differential data is shown, i.e., the differential image data from a multitude of different projection directions. Horizontal dark and bright lines, indicated with reference numerals 210a,b,c... start at pixels where an artificial phase wrapping was induced by noise. In Fig. 7 the resulting artefacts are shown in a tomographic reconstruction 212. A reconstruction step is indicated with an arrow 219. As can be seen, dark lines 214 as well as bright lines 216 are crossing the circular area in a random manner as well as bright areas 218 are shown.

In Fig. 8, a sinogram 200' with adapted differential data 210', i.e., with adapted image points, according to the invention is shown. The reconstruction step is indicated with an arrow 219' . As can be seen in Fig. 9, the reconstruction result, indicated with reference numeral 212' , is showing a circular area with no disturbing artefacts, such as the black and white lines or dark and bright lines of Fig. 7, due to the adaptive filtering according to the invention.

According to another exemplary embodiment, a method 300 is provided which is shown in Fig. 10. In a first step 310, raw data 312 is acquired. In a next step 314, phase gradients 316 are estimated. This can be computed for a part of the image points or image pixels of the raw data. According to one example, this can also be computed for all pixels. In a following step 318 the likelihood 320 that phase wrapping occurred at pixel (i,j) is determined.

For example, the determination divides pixels into a first group 322 with a very high or very low likelihood, which is indicated with a first arrow 324 leaving the box 318 in the lower left direction and a second group 326 with a medium likelihood, which is indicated with a second arrow 328 leaving the box 318 in the lower right direction. For the second group, in a further step 330, low pass filtering 332 is applied adapting the image data for the selected image points of the second group 326. The image points and their adapted values are then provided back to the determination box 318 in form of an adapted second group 326', indicated with a third arrow 334. From there, the adapted image points are added to the image points of the first group, i.e., they are also leaving the box 318 via the first arrow 324. The combination takes place in a combining step 336 to provide adapted image data 338, comprising the first group 324 and the adapted second group 326', which adapted image data is taken as current pixel value.

In a next step 340, a so-called Itoh-algorithm 342 is applied.

For example, once the above steps have been computed for all pixels.

A further aspect according to the invention is shown in Fig. 11 in which two graphs 410 and 412 shown. The upper graph 410 shows some exemplary neighbouring samples 414a, 414b, 414c, 414d and 414e of measured phase gradient if) 416, which are indicated with black dots. Each sample is shown with a first and a second point 414ai and 414a₂.

The lower graph 412 shows the difference (D) 418 of the samples 414a', 414b', 414c', 414d' and 414e'.

With respect to sample 414a, samples^ and^m are close, their difference almost 0, the likelihood that phase wrapping occurred is low.

With respect to sample 414b, essentially the same as case 414a is shown, illustrating that the SNR of the individual/ s is a bad indication for the likelihood.

With respect to sample 414c, samples are far away from each other; the likelihood that phase wrapping occurred is high, i.e. the left sample should be unwrapped to indicated position 414c_P.

With respect to sample 414d, it is unclear whether phase wrapping occurred or not since the error margin of the difference does not allow to tell for sure that the difference is larger or smaller than -π.

With respect to sample 414e, same samples are shown as in 414d, but error margins have been reduced by filtering. Now it can be decided that the difference is smaller than - π and that the second sample should be unwrapped to indicated position 414e_P. Further, Figs. 12 to 15 show photographic images in greyscale of the sinograms and the reconstructions of Figs. 6 to 9, provided with similar reference numerals. Thus, these are not further discussed.

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 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.

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 fulfill 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.

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 that, unless other 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.

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 recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

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. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A device (50) for phase gradient unwrapping in differential phase contrast imaging, comprising:

a processing unit (18); and

an interface unit (19);

wherein the processing unit (18) is adapted to evaluate quality of at least sample image points of provided X-ray phase contrast image data; to select image points with predetermined quality; to adapt the selected image points; to provide adapted image data; and to recover image data; and

wherein the interface unit (19) is adapted to provide the X-ray phase contrast image data.

2. Device according to claim 1,

wherein the processing unit (18) is adapted to estimate wrapped phase gradients for at least sample image points of provided X-ray phase contrast image data; to determine likelihood that phase wrapping occurred at the at least sample image points and to select image points of the at least sample image points with medium likelihood; to adapt the data of the selected image points by applying predetermined filtering; to replace the acquired data of the selected image points with the adapted data providing adapted image data, which adapted image data also comprises the acquired data of the non-selected image points; and to recover the image data on the basis of the adapted image data by phase-un- wrapping; and wherein the interface unit (19) is adapted to provide the X-ray phase contrast image data; and to provide the recovered image data.

3. A medical imaging system (10) for phase gradient unwrapping in differential phase contrast imaging, comprising a differential phase contrast X-ray image acquisition device (11); an object receiving device (14); and a device (50) according to claim 1 or 2.

4. A method (100) for phase gradient unwrapping in differential phase contrast imaging, comprising the steps of:

- a) providing (110) X-ray phase contrast image data (112);

- b) evaluating (114) quality of at least sample image points (116);

- c) selecting (118) image points with predetermined quality (120);

- d) adapting (122) the selected image points;

- e) providing (124) adapted image data (126); and

- f) recovering (128) image data (130).

5. Method according to claim 4,

wherein step b) comprises estimating (132) wrapped phase gradients (134) for at least sample image points (116) of the image data;

wherein step c) comprises determining (136) likelihood (138) that phase wrapping occurred at the at least sample image points and selecting (140) image points of the at least sample image points with medium likelihood (142, 150);

wherein step d) comprises adapting the data of the selected image points by applying predetermined filtering (144);

wherein step e) comprises replacing (146) the acquired data of the selected image points with the adapted data providing adapted image data (148), which adapted image data also comprises the acquired data of the non-selected image points;

wherein step f) comprises recovering the image data on the basis of the adapted image data by phase-un- wrapping (152); and

wherein in a step g) the recovered image data (1 6) is provided (1 4).

6. Method according to claim 4 or 5, wherein in step c), the noise quality of the acquired data of the at least sample image points of the image data is determined; wherein the noise determination is based on predetermined values.

7. Method according to claim 6, wherein in step c), image points with a noise quality below a predetermined threshold are selected;

8. Method according to claim 6 or 7, wherein for determining the noise quality in step c), a signal-to-noise ratio is determined and compared with a predetermined threshold.

9. Method according to claim 4, 5, 6, 7 or 8, wherein in step c), the values of the at least sample points are brought into relation with at least one neighbourhood pixel.

10. Method according to claim 9, wherein the neighbourhood is a spatial and / or angular neighbourhood.

11. Method according to one of the claims 4 to 10, wherein for applying the predetermined filtering in step d), low-pass filtering is applied.

12. Method according to one of the claims 4 to 11, wherein for recovering the image data in step f), the following sub-steps are provided:

- calculating (160) differences of adjacent image points;

- wrapping (162) the differences;

- initializing (164) the series of unwrapped data; and

- accumulating (166) the wrapped differences.

13. Computer program element for controlling an apparatus according to one of the claims 1 to 3, which, when being executed by a processing unit, is adapted to perform the method steps of one of the claims 4 to 12.

14. Computer readable medium having stored the program element of claim 13.