Aligning source-grating-to-phase-grating distance for multiple order phase tuning in differential phase contrast imaging

An X-ray imaging method includes acquiring a differential phase contrast imaging X-ray scan with an X-ray imaging system having an X-ray source, an X-ray detector, and a grating arrangement having a source grating, a phase grating and an analyzer grating. The source grating is misaligned in respect to an interferometer such that moiré fringes are detectable in the plane of the detector. A translation signal is computed for translating the source grating for achieving a predetermined moiré pattern. The positioning of the source grating is adjusted in an X-ray projection direction based on the translation signal such that at least 2 pi of phase changes are covered with the Moiré fringes over the width of the detector. And a further differential phase contrast imaging X-ray scan is acquired.

FIELD OF THE INVENTION

The present invention relates to an X-ray imaging system for differential phase contrast imaging, to a method for handling misalignment in an X-ray imaging system for differential phase contrast imaging, to a computer program element and to a computer readable medium.

BACKGROUND OF THE INVENTION

Differential phase contrast imaging (DPCI) is an emerging technology that has a potential to improve the diagnostic value of X-ray imaging. For example, one application of this technology is mammography. In a DPCI system, a setup is used with three gratings between the X-ray source and the detector. For image acquisition, several X-ray images at different relative positions of two of the gratings are provided. Since the gratings have pitches in the order of a few micrometers only, there are rather tight requirements on the accuracy of the stepping device that performs the relative movement of the gratings, and also for alignment of the system. For larger objects, for example when investigating a breast, the virtual phase stepping is provided by using a scan of the object relative to the imaging system, including a virtual phase stepping parallel to this scan direction. For example, either the imaging system is moved relative to the sample/object, for example as application in mammography known from the Philips-owned company Sectra, Sweden, or the object/sample is moved with respect to a fixed imaging system, for example for security screening or baggage inspection. However, a requirement for all these setups is that across all detector lines, over the width D, i.e. parallel to the scan direction X, a phase shift of at least one interference fringe period of the interferometer, i.e. the analyser grating G2and the phase grating G1, shows up. During the scan, each individual part of the object/sample successively passes the different detector lines, thus experiencing different phase states of the interferometer. The phase retrieval is then done by an evaluation of the detector line signal taken during the scan. As a requirement, the distance between the two gratings G1and G2, i.e. the phase grating and the analyser grating, has to be adjusted precisely. Further, also the distance between the source grating GO and the phase grating G1has to be aligned precisely in all cases. However, it has been shown that tuning and stabilizing such an interferometer in hospital environments, for example, may consume unnecessary time and be cost-intensive.

SUMMARY OF THE INVENTION

Thus, there may be a need to provide a reduction for the pre-tuning and adjustment requirements for manufacture and maintenance in a differential phase contrast imaging system.

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 X-ray imaging system for differential phase contrast imaging, and for the method for handling misalignment in an X-ray imaging system for differential phase contrast imaging, as well as for the computer program element and the computer readable medium.

According to a first aspect of the present invention, an X-ray imaging system for differential phase contrast imaging is provided, comprising a differential phase contrast setup with an X-ray source and an X-ray detector, a grating arrangement and a moving arrangement for a relative movement between an object under examination and at least one of the gratings of the grating arrangement. The grating arrangement comprises a source grating, a phase grating, and an analyser grating. The source grating is arranged between the X-ray source and the phase grating, and the analyser grating is arranged between the phase grating and the detector. Further, a processing unit and a translation arrangement are provided. The translation arrangement is provided for translating the source grating. The phase grating, the analyser grating, and the detector are provided as a rigid interferometer unit, in which the phase grating and the analyser grating are mounted in parallel to each other. The source grating is misaligned in respect to the interferometer unit such that moiré fringes are detectable in the plane of the detector. The processing unit is configured to detect moiré patterns in signals provided by the detector upon X-ray radiation. The processing unit is further configured to compute a translation signal for translating the source grating for achieving a predetermined moiré pattern. The translation arrangement is configured to adjust the positioning of the source grating at least in the X-ray projection direction, based on the value of the translation signal.

The distance between the source grating and the phase grating is referred to as distance L, and the distance between the phase grating and the analyser grating is referred to as distance D. The imprecise adjustment of the distance D is compensated by the adjustment of the distance L. Therefore, a misalignment in the distance D, or a pre-set detuned D, can be compensated by an adjustment of L. Here, a precision in the sub-millimeter region is sufficient. The interferometer unit may also be referred to as detection unit. The misalignment may also comprise a deviation of the source grating and the interferometer unit in relation to each other.

According to an exemplary embodiment, the translation arrangement is configured to tilt the source grating.

According to an exemplary embodiment, the translation arrangement comprises at least one actuator for aligning the X-ray source unit and/or the X-ray detection unit.

According to an exemplary embodiment, the at least one actuator is provided as piezo actuator, and/or as motor-driven micrometer-screw. The motor-driven micrometer-screw can also be provided as micrometer-head. The at least one actuators provides a movement in the range of approximately 1 micrometer up to approximation 10 millimeters. The alignment accuracy of the actuator is approximately plus/minus 0.1 micrometer, according to an example.

According to an exemplary embodiment, the source grating is misaligned such that at least 2 pi of phase changes are covered with the moiré fringes over the width of the detector array.

According to an exemplary embodiment, a moving arrangement for a relative movement between an object under examination and at least one of the gratings is provided. For example, the moving arrangement is provided as a stepping arrangement for stepping at least one of the gratings of the interferometer unit in the respective grating plane.

Alternatively, an object support is provided and a relative movement between the object support and the differential phase contrast setup is provided, wherein the gratings are provided in a constant alignment to each other during a scan for at least one image acquisition. According to a first example, the object support is provided stationary, and the differential phase contrast setup is moved in a direction transverse to an X-ray direction. According to a second example, the differential phase contrast setup is provided stationary, and the object support is moved in a direction transverse to the X-ray direction. For example, in case of the moving arrangement as stepping arrangement, a stepping arrangement for stepping the source grating or the interferometer unit in the respective grating plane is provided. If one of the gratings of the interferometer unit is stepped, this can be provided with an accuracy of less than plus/minus 0.1 micrometer.

According to a second aspect of the present invention, a method for handling misalignment in an X-ray imaging system for differential phase contrast imaging is provided, comprising the following steps:a) In a first step, at least a first differential phase contrast imaging X-ray scan is acquired with an X-ray imaging system for differential phase contrast imaging, comprising a differential phase contrast setup with an X-ray source, an X-ray detector, and a grating arrangement comprising a source grating, a phase grating, and an analyser grating. The source grating is misaligned in respect to the interferometer unit such that moiré fringes are detectable in the plane of the detector.b) In a second step, moiré patterns in signals provided by the detector upon X-ray radiation are detected.c) In a third step, a translation signal for translating the source grating for achieving a predetermined moiré pattern is computed.d) In a fourth step, the positioning of the source grating is adjusted at least in an X-ray projection direction based on the translation signal.e) In a fifth step, at least one further differential phase contrast imaging X-ray scan is acquired.

“Moiré fringes”, also known as “moiré pattern”, show up when superimposing two grids having nearly identical pitches either in the parallel as well as in an inclined configuration. For example, one grid in the phase contrast imaging set up is caused by the phase grating G1as interference pattern of the x-ray beam, the other grid is the analyzer grid G2.

According to an exemplary embodiment, in step a), a plurality of first differential phase contrast imaging X-ray scans is acquired for different projection angles, and the scans are provided as a reference pattern for adjusting the position of the source grating for each projection angle individually.

According to an aspect of the present invention, the number of tuning and adjustment procedures is reduced to a minimum, and the precision that is necessary for the mechanical adjustment and demand on mechanical stability is shifted from the sub-micrometer region preferably into the sub-millimeter region or even higher. This is achieved, for example, by providing a movement of the source grating G0. Therefore, a compact rigid interferometer unit with the planes of the gratings G1and G2can be provided mounted in parallel with respect to each other. For example, the parallelism of the grid lines of G1with respect to the G2structures has to be around 0.1 millirad or better for typical grid pitch values encountered in low to medium energy X-ray interferometry. An occurring misalignment may be responsible for the appearance of moiré fringe components perpendicular to the grid structure. The number of moiré fringes parallel to the direction of the grid structures is dependent on the distance between G1and G2as well as on the distance between G0and G1. More precisely, the number of moiré fringes is dependent on the quotient of the distance D to the distance L. Therefore, a misalignment in the distance D, or a pre-set detuned D, can be compensated by an adjustment of L. Here, a precision in the sub-millimeter region is sufficient. The total alignment that remains is the tuning of the distance between the grid G0, i.e. the source grating, and the interferometer as represented by the phase grating and the analyser grating. For example, this may be done by the aid of a linear translation stage, mounted to the rigid gantry that supports the X-ray tube, the interferometer and the detection unit. The distance L has to be tuned, for example, by the translation stage in such a way that at least one complete moiré fringe shows up across the width D of the detector. The number of moiré fringes may be further increased. However, the upper limit is reached by the number of detector lines per fringe falls below 4, for example.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1shows an X-ray imaging system10for differential phase contrast imaging, comprising a differential phase contrast setup12with an X-ray source14and an X-ray detector16. Further, a grating arrangement18is provided, comprising a source grating20, a phase grating22, and an analyser grating24. The source grating is arranged between the X-ray source and the phase grating, and the analyser grating is arranged between the phase grating and the detector. Further, a moving arrangement for a relative movement between an object under examination and at least one of the gratings is provided (not further shown). A dotted oval structure26indicates an object, and an X-ray beam28in a fan-shaped formation is also indicated, together with an X-ray projecting direction30. Further, a processing unit32is provided, and a translation arrangement34for translating the source grating. The phase grating, the analyser grating, and the detector are provided as a rigid interferometer unit36, in which the phase grating and the analyser grating are mounted in parallel to each other.

The source grating is misaligned in respect to the interferometer unit36such that moiré fringes are detectable in the plane of the detector16. The processing unit32is configured to detect such moiré patterns in signals provided by the detector16upon X-ray radiation. The processing unit32is further configured to compute a translation signal, indicated with an arrow38, for translating the source grating20for achieving a predetermined moiré pattern. A double arrow40indicates the translation in the X-ray projection direction30. The translation arrangement34is configured to adjust the positioning of the source grating20at least in the X-ray projection direction30, based on the value of the translation signal.

For example, not further shown, the translation arrangement34is configured to tilt the source grating20.

As indicated inFIG. 2, the translation arrangement34may comprise at least one actuator42for aligning the X-ray source unit and/or the X-ray detection unit, for example the source grating20can be moved by a number of piezo actuators or motor-driven micrometer-screws as the actuators42. Of course, as shown inFIG. 2B, it is also possible to provide actuators42for moving the interferometer unit in relation to the source grating20and the X-ray source14, as indicated with a second double arrow44.

A moving arrangement46for a relative movement between an object under examination and at least one of the gratings is provided, as shown inFIGS. 3A, 3B, and 3C. As shown inFIG. 3A, the moving arrangement is provided as a stepping arrangement48for stepping, for example, the phase grating of the interferometer unit36in the respective grating plane, as indicated with a third double arrow50. According to the example shown inFIG. 3A, the source grating20can also be moved, i.e. aligned, in the X-ray projection direction30, and indicated with the above-mentioned double arrow40.

As shown inFIG. 3B, the moving arrangement46can also be provided with an object support52, and a relative movement between the object support and the differential phase contrast setup12, wherein the gratings are provided in a constant alignment to each other during a scan for at least one image acquisition. The object support inFIG. 3Bis provided stationary; the differential phase contrast setup is moved in a direction transverse to an X-ray direction, for example by a pivoting movement, indicated with pivoting indication arrows54around the location of the X-ray source14. For example, such moving arrangement46can be provided for mammography. It must be noted that further key elements of a mammography investigation apparatus, such as breast compression paddles, are not further shown.

According toFIG. 3C, the moving arrangement46is provided with a stationary differential phase contrast setup, but a moving object support52′, for example a conveyer belt, for a movement in a direction transverse to the X-ray direction, as indicated with conveyer belt direction arrow56, for example for luggage inspection.

FIG. 4shows an example of a method100for handling misalignment in an X-ray imaging system for differential phase contrast imaging. In a first step110, at least a first differential phase contrast imaging X-ray scan is acquired with an X-ray imaging system for differential phase contrast imaging, comprising a differential phase contrast setup with an X-ray source, an X-ray detector, and a grating arrangement comprising a source grating, a phase grating, and an analyser grating. The source grating is misaligned in respect to the interferometer unit such that moiré fringes are detectable in the plane of the detector. In a second step112, moiré patterns are detected in signals provided by the detector upon X-ray radiation. In a third step114, a translation signal is computed for translating the source grating for achieving a predetermined moiré pattern. In a fourth step116, the positioning of the source grating is adjusted at least in an X-ray projection direction based on the translation signal. In a fifth step118, at least one further differential phase contrast imaging X-ray scan is acquired. The first step110is also referred to step a), the second step112as step b), the third step114as step c), the fourth step116as step d), and the fifth step118as step e).

According to a further example, not shown, in step a), a plurality of first differential phase contrast imaging X-ray scans is acquired for different projection angles, and the scans are provided as a reference pattern for adjusting the position of the X-ray source grating for each projection angle individually.

FIG. 5shows a further example of a differential phase contrast setup12, with a first starting point representing the X-ray source14, followed by the source grating20. A space58for receiving an object60, for example in a moving direction62is provided. The object60is shown for a first position with a straight line, and with a dotted pattern64for a second position upon being moved. Still further, the phase grating22and the analyser grating24are provided as a rigid unit, indicated with a dotted frame66. Still further, a detector structure indicates the detector16. The detector is characterized, among others, by the detector width, indicated with arrows68. Further, the phase grating22and the analyser grating24are provided with a distance70, and the phase grating22is provided in relation to the source grating20with a distance72. The detector width68is also referred to as width D, the distance70between the phase grating22and the analyser grating24is also referred to as width d, and the distance between the source grating and the interferometer unit is referred to as distance L. A double arrow74indicates the alignment movement of the source grating20with a delta76of +/− delta L. Due to the provision of the grating arrangement and the scanning direction, a detector flux78can be measured, indicated with a curved graph. A first arrow80relates to a maximum point among the graph, and a dotted arrow82relates to a minimum point in the graph78.

In an ideal system not employing phase contrast, each detector line would measure the same sonogram up no measurement noise. In a system as explained above, the different detector lines acquire different intensities due to the intentional misalignment in z between source and interferometer units. This misalignment causes the intensity measured by different detector lines to oscillate from one line to the next with a spatial period λ inverse proportional to this mismatch, a phenomenon called moiré fringes. In order to assure a homogenous phase acquisition the number of detector elements N, the distance between two detectors D and the moiré period λ have to obey the following relationship:
ND=nλ,
where n is the number of fringe period per entire detector array. The number of sampling points for the phase is thus given by λ/D=N/n and should at least be at least 4, hence, for N=20 detector lines, n should at most be 5, typically 2.

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.

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

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.