WO2015044238A1 - Détermination de phase de référence dans l'imagerie de contraste de phases à balayage différentiel - Google Patents

Détermination de phase de référence dans l'imagerie de contraste de phases à balayage différentiel Download PDF

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Publication number
WO2015044238A1
WO2015044238A1 PCT/EP2014/070426 EP2014070426W WO2015044238A1 WO 2015044238 A1 WO2015044238 A1 WO 2015044238A1 EP 2014070426 W EP2014070426 W EP 2014070426W WO 2015044238 A1 WO2015044238 A1 WO 2015044238A1
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phantom
detector
radiation
different
phase
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PCT/EP2014/070426
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English (en)
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Thomas Koehler
Ewald Roessl
Gerhard Martens
Udo Van Stevendaal
Heiner DAERR
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Koninklijke Philips N.V.
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/48—Diagnostic techniques
    • A61B6/484—Diagnostic techniques involving phase contrast X-ray imaging
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
    • A61B6/4291—Arrangements for detecting radiation specially adapted for radiation diagnosis the detector being combined with a grid or grating
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/58—Testing, adjusting or calibrating thereof
    • A61B6/582—Calibration
    • A61B6/583—Calibration using calibration phantoms
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/44—Constructional features of apparatus for radiation diagnosis
    • A61B6/4476—Constructional features of apparatus for radiation diagnosis related to motor-assisted motion of the source unit
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/52—Devices using data or image processing specially adapted for radiation diagnosis
    • A61B6/5211—Devices using data or image processing specially adapted for radiation diagnosis involving processing of medical diagnostic data

Definitions

  • the invention relates to a method of calibrating an X-ray scanner, to an X-ray scanner system; to a phantom body, to a computer program element, to a computer readable medium.
  • the object to be imaged is scanned by movement of the imaging system's detector or by moving the object to be imaged.
  • Some of these scanning imaging systems include an interferometer arrangement that allows grating based phase contrast imaging. It has been observed that, when a fixed interferometer setup is used, i.e., one where the gratings are fixed with respect to each other during image acquisition, calibration proves remarkably cumbersome.
  • C Kottler et al describe in "Grating interferometer based scanning setup for hard x-ray phase contrast imaging", REVIEW OF SCIENTIFIC INSTRUMENTS 78, 043710 (2007) a scanning system where the object to be imaged is moved past a detector in a scanning motion.
  • a method of calibrating an X-ray scanner having differential phase contrast imaging equipment with a phantom body residing in an examination region of the scanner between the scanner's X-ray source for emitting radiation and the scanner's detector for receiving radiation, the phantom having a jagged surface defined by a plurality of different, pre-defined slopes, the method comprising:
  • the detector pixel assumes the different positions because of the movement of the detector and the different positions are aligned with the different slopes of the phantom in the sense that at each position, the pixel is opposite a different one of the different phantom slopes.
  • the proposed method allows addressing the problem of determining the reference phase of a Moire pattern, i.e., the phase of a flat field (a flat field is a detector response when no object is in the X-ray beam of the X-ray source), in fixed grating interferometers where the phase gradient is retrieved from the data set by using information from several different detector pixels, which makes it more difficult to calibrate the system properly.
  • the proposed method allows dispensing with a dedicated scan with "phase stepping" where relative movement between interferometric gratings is required.
  • the proposed method allows calibrating the X-ray scanner and at no stage relies on phase stepping. System cost and complexity can be reduced.
  • calibration and object scan can be done one after the other without appreciable time in between by an essentially seamless "handover" between the two scans where object and phantom body may together reside in the scanner's examination region during both scans. Because there is essentially no delay between calibration scan and the object scan, the likelihood for undesirable drift (e.g. thermal) in respect of the reference phase and other setting parameters is appreciable lowered. Users can therefore reasonably rest assured that the reference phase established in the calibration scan matches the reference phase that holds during the actual object scan.
  • the fitting operation includes a maximum- likelihood optimization step, in particular a least squares fitting, but other numerical fitting techniques are likewise envisaged.
  • the object to be imaged resides in the examination region alongside the phantom (PH), wherein the phantom is positioned, relative to a scanning direction, in front of or behind the object.
  • a second phantom there is a second phantom, with both phantoms residing in the examination region alongside the object to be imaged, with one of the two phantoms positioned, relative to the scanning direction, in front of the object and the other behind the object. This allows for quality check for the calibration phase.
  • the grating components remain stationary relative to each other during the calibration.
  • the phantom body as used in the calibration method has at least three different slopes.
  • the phantom body has a profile with sections, each section having the same average height.
  • the thickness of the phantom is such that a radiation attenuation caused by the phantom body is substantially equal to that of the object. Both embodiments allow keeping beam hardening effects low.
  • the phantom has a second surface opposite the first surface, the second surface jagged in a manner complementary to the first surface.
  • the phantom is formed from Poly-methyl- methacrylate, PMMA or aluminum or any other suitable materials.
  • the method and phantom may find useful application in phase contrast imaging with scanning mammography systems but use in other scanning system is also envisaged.
  • the scanning system is implemented by having the detector move relative the object to be imaged
  • the reverse situation is also envisaged where it is the object-to be-imaged that is made to move in suitable structure such as a frame relative to stationary detector.
  • Fig. 1 shows a radiography scanner
  • Fig. 2 shows phase contrast interferometric equipment as used in the scanner of Fig 1;
  • Fig. 3 shows a radiation wave front incident on a prism of a calibration phantom
  • Fig. 4 shows interaction of a deflected radiation wave with a system of gratings
  • Fig. 5 illustrates motion of a detector
  • Fig. 6 shows a signal processing apparatus as used in Fig. 1;
  • Fig. 7 shows embodiments of a phase reference calibration phantom
  • Fig. 8 shows further embodiments of a phase reference calibration phantom
  • Fig. 9 shows further embodiments of a phase reference calibration phantom
  • Fig. 10 is a flow chart for a radiography scanner calibration method.
  • Mammography imaging system MIS includes a frame FR which is either wall mounted or freestanding. On said frame FR, a rigid imager gantry IC is mounted so as to be slidable along a vertical axis y. The gantry IC is movable by a suitable actuator-controller arrangement for precise positioning along said axis to so accommodate the imaging system to height requirements of a patient whose breast BR is to be imaged.
  • the imaging system MIS is connected via suitable interfaces means OUT and across a communication network to a workstation WS.
  • workstation is a computing system with which a clinician ("user") is able to control operation of the imaging system.
  • there is also a display unit or monitor M which is controlled by work station WS and which allows displaying of images that are acquired by the imaging system.
  • Workstation WS runs an operating system which in turn controls execution of a number of modules whose operation will be explained in more detail below.
  • the gantry IC receives a control signal from work station WS and is thereby instructed to rotate into a desired angular position ⁇ relative to the examination region and the gantry moves into a height appropriate y-position. Patient is then asked to introduce the relevant breast BR into the examination region. Compression plate CP then slides downwardly and into contact with breast BR to gently compress breast BR against a breast support BST arranged between plate CP and detector D to ensure image quality.
  • Compression plate CP and breast support are so arranged that gantry IC can rotate about same whilst both, plate CP and breast support BST, remain stationary.
  • X-ray source XR is then energized to emit from a focal spot an X-ray beam XRB whose central ray passes through the breast tissue at main projection direction
  • the X-ray source XR remains stationary during the image acquisition(that is, whilst the radiation beam is being emitted) but, as will be explained in more detail below, the detector is moved in an arc segment about said main direction in a scanning motion.
  • the X-ray source XR rotates around a pivot located in the focal spot) so as to follow the scanning motion of the detector.
  • said X-ray beam XRB experiences attenuation and, when considered as a wave, its wave front experience a certain phase shift. If the wave front associated with the x-ray beam propagates through matter with complex refractive index n, attenuation is related to the imaginary part of n, while the phase shift is related to the real part of n.
  • Each detector pixel opposite to the focal spot receives a certain number of x-ray photons and responds by issuing a corresponding electric signal. The collection of said signals is then translated by a data acquisition system DAS (not shown) into a respective digital value.
  • the phase retrieval operation also yields conventional attenuation projection images or small scatter images if desired.
  • the size (measured in rows and columns of pixels) of the projection image equals the size of the detector's image effective sensitive surface during the scan. Due to cost restraints, the detector's size affords a rather restricted field-of-view (fov) so the projection image for one exposure, is not normally rendered for view in the display although of course in principle this could be done if the user so requests.
  • the mammography imager is of the scanner type, that is, its detector D is movable along an arc- type scan path and its motion is along a track system ST or similar and the motion is effected by a suitable actuator AC such as a stepper motor or similar (see Fig 3).
  • a suitable actuator AC such as a stepper motor or similar
  • the scan path is not an arc-type path but a straight line segment.
  • the imager IMA operates to perform a scan movement with the scan path run under the breast BR, that is, the breast always remains between X-ray source and detector surface.
  • the detector moving mechanism that is scanning track ST and actuator AC including its control circuitry, is in one embodiment housed in the housing H and is not visible from the outside.
  • detector D During irradiation of breast BR and whilst the detector is moving along the scan path detector D's pixels detect for each angular position on the scan path a certain signal intensity I so the detector repeatedly receives radiation as it travels along the scan path. In this manner, a series of image raw data sets (with each set indexed by the respective detector position) are then produced by the detector D during the scan movement.
  • phase grating 204 Arranged after object PAT and spaced apart from both the detector D and an analyzer grating G 2 206 (having pitch q) by distance d, is phase grating 204 (having a pitch p).
  • the interferometric gratings Gl, G2 are integrated in the detector D's housing and are fixedly mounted and arranged between the detector's radiation sensitive surface (made up of the pixels px) and the breast support.
  • the source grating element 202 is integrated in the egress portion of x-ray source XR.
  • the Talbot interferometer described by Weitkamp is not of the fixed type as proposed herein but includes movable elements such an actuator element to laterally displace analyzer grating G 2 206 relative to X-ray tube XR with source grating GO 202, phase grating Gl 204 and X-ray detector D. Said actuator is used in
  • Image acquisition that is, exposure to x-ray beam XRB
  • lateral displacement is repeated, e.g., four, six, or eight times, for acquiring a plurality of phase contrast projections, constituting together a phase stepping series.
  • MIS there is no actuator element to effect relative motion between gratings Gl and G2.
  • said gratings Gl and G2 are fixedly mounted in opposed spatial relationship in a bracket frame structure onto the detector plate D to face same and for sliding with detector D along the scan path track ST.
  • the relative lateral position between gratings Gl and G2 may not be known because the relative positing may still change, albeit unwanted.
  • there is no mechanical or electric actuator element envisaged herein to cause a deliberate relative motion between the two gratings as in the classical phase stepping approach there nevertheless is a slight fluctuation between said relative position between Gl and G2 which may be caused by thermal deformations
  • the fixed grating imaging system MIS needs to be calibrated now and then in order to establish current relative positioning between gratings Gl, G2.
  • the mutual relative position of the two gratings Gl, G2 can be defined by a distance e by which one grating has been shifted relative the other for whatever reason.
  • the local relative position e can also be described in terms of a quantity called the "reference phase a" that defines signal phase of an interference pattern that can be observed when there is radiation incident on grating Gl with wave front parallel to the grating Gl .
  • the jagged surface of the phantom PB is defined by a plurality of mutually different and mutually unique (i.e. +/- ⁇ /4, +/- ⁇ /2 and +/- 3 ⁇ /4) slopes having nonzero inclination (i.e. +/- ⁇ /4, +/- ⁇ /2 and +/- 3 ⁇ /4) relative to a bottom surface of the phantom PB.
  • the phantom PB furthermore comprises a segment having a slope with zero inclination relative to a bottom surface of the phantom PB.
  • Figs 3,4 Interaction between the wave front and phantom body is now briefly outlined in Figs 3,4 where for ease of exposition, two prism section of phantom body are shown with the understanding that the following explanation is applicable to each of the different prism sections of phantom body PB.
  • the heavy arrow at the top of the figure shows the direction of the normal of incoming wave fronts (shown as horizontal lines) of the incident radiation XRB is emitted by x-ray source XR.
  • the wave front After passage of the wave front through a prism section of phantom PB (shown as a triangle), the wave front is inclined because of refraction by a refraction angle ⁇ .
  • the offset quantity e denotes the spatial distance between maxima of the interference pattern and the walls of grating G2 and is thus a measure for the relative positioning between the two gratings Gl, G2.
  • reference phase a is in general a local property o3 ⁇ 4 of the gratings G1,G2 caused by different parts of the on or both of the gratings Gl and/or G2 being twisted relative to each other.
  • operation of signal processor is broadly as follows: In the calibration phase, the phase of wave front for each individual detector pixel is derived , from which the reference phase a can be computed. Then, in the object scan, the breast BR scan data is acquired. Phase shift caused by object BR is then derived in a curve fitting based phase retrieval operation using said reference phase a. As an alternative, the object scan may be done first and then calibration scan.
  • the second, optional phantom PB positioned to the left of breast BR is then scanned to derive a second reference phase a' which is used to check that the phase of the wave front has not varied during the scan. If a, a' are found to be sufficiently close, the reference value a or a' are accepted and applied to process the breast image data into a projection image.
  • Other variants of workflow are also envisaged: for instance, in a different embodiment, the object PAT is scanned first and then the phantom PB is scanned to derive the calibration data, in particular reference phase a.
  • the phantom body PB is used in a calibration step for obtaining the reference phase gradient a
  • the data obtained in the phantom imaging step may also furnish information on the reference intensity signal and coherence.
  • Synchronization is achieved in one embodiment by a tracker TR that interrogates internal state variables as generated by the actuator AC whilst detector D is travelling along the scan path on track ST.
  • a calibration module CM includes a switch element to this effect which communicates with suitable pick up circuitry S/H that picks up the intensity signals as detected by the detector pixels (for instance along rows of detector pixels) as detector D journeys along its scan path. Operation of tracker TR allows indexing the pixel measurements with the respective detector position.
  • the calibration signals fi captured in the phantom sector are then processed by the solver SOL into inter alia the reference phase a which is then passed on along with the scan data g j from the object scan to object image processor OIP where the final projection image is generated.
  • the projection image can then be displayed on screen M or stored in database DB or otherwise post-processed.
  • phase sampling results in a signal intensity curve / to be picked up by any given pixel px, but from different positions i along the scan path.
  • fi A(l + V cos(a + Vi )) (1)
  • ⁇ used in the phase sampling of the radiation wave front after passage through the phantom and the gratings.
  • a phantom is used that causes the following interference pattern shifts: ⁇ ⁇ ⁇ ⁇ 3 ⁇ 3 ⁇
  • Each of the different sample phases correspond to respective phase shifts of the interference pattern of the wave front that were caused by refractions at the respective prism slopes of the jagged phantom body surface as explained above in relation to Figs 3,4.
  • the above fitting problem is over-determined and can be solved by using maximum likelihood methods, in particular least square methods if a Gaussian or Poisson distribution is assumed to underlie the detector measurements fi.
  • the measured data fi represents the different intensity signals as picked up by a detector pixel pxj from a plurality of positions i as the detector D (with the fixed gratings arrangement Gl .G2) travels under the phantom PB on its scan path ST.
  • This is an over-determined system of equations for the three unknown parameters p ⁇ , pi, and /3 ⁇ 4 ip pi are the line integrals for the attenuation and visibility, respectively, and /3 ⁇ 4 is the shift of the fringe phase of the intensity signal caused by the object PAT from which the phase gradient can be recovered) that can be solved for the three unknowns by using maximum-likelihood methods where, as mentioned before with respect to equation (1),
  • a user configuration module is run by the workstation and provides user interfaces, text-based or graphical (GU) so the user can supply the configuration variable which is then stored in a memory for access by the processor when needed. If unacceptable deviations are detected this fact is indicated by a suitable indicator signal to the user to invite him to run another phantom scan or take any other suitable remedial action.
  • GUI graphical
  • Aspect ratio (hight /z/halfwidth w) (see Fig 4), of each prism in the phantom should be in the order of 1 : 10 to ensure that the entire dynamic range of the interferometer can be covered, the dynamic range being a function of qld (q being the pitch of G2 und d the distance between Gl and G2).
  • qld the pitch of G2 und d the distance between Gl and G2
  • troughs and crests can be formed by cutting or milling or sawing, diamond polishing other suitable surface processing techniques.
  • the phantom body is assembled from geometrical primitives such as a flat plate module to which different triangular prism profiles are fastened by gluing or otherwise.
  • the phantom body may be formed from opaque or translucent Poly-methyl-methacrylate (PMMA) or from aluminum or from any other suitable material such as polycarbonate or TeflonTM, depending on the application.
  • PMMA Poly-methyl-methacrylate
  • the phantom material is a high- ⁇ (has comparably high refraction index) and comparably low attenuation.
  • the phantom has a weight from anywhere between 100-s of grams to double digit kilogram figures such as 10 kilograms or more. Manufacture allowances of about 1/100 - 2/100 mm proved sufficient.
  • Figure 7B affords a cross-sectional profile view of the phantom body in figure 7A.
  • this exemplary embodiment there are seven (which is non-limiting and exemplary only) sloped surfaces interconnected by troughs and crests to form the jagged surface by having effectively a plurality of prims arranged side by side and connected in one
  • the dimensions are preferably so chosen, that the resulting aspect ratio in each prism should be so chosen that the highest slope still allows covering the dynamic range of the interferometer.
  • tracker T is configured to associate, when the detector is located in calibration sector SI or S3, each position of the detector on the scan path with the respective phantom body section.
  • calibration module CM can then associate each detected intensity signal with the respective sample phase that corresponds to the body section at which the receiving pixel is currently positioned at.
  • the phantom body may have ridges extending from both of its opposing sides, either lengthwise or crosswise.
  • the ridges are receivable in respective tracks formed on the outside of the detector housing. In this manner the phantom body's ridges are first carefully threaded into the tracks and body is then "slid" into place onto the housing in correct alignment.
  • the Phantom's bodies are fixedly installed an integrated between the breast support and the interferometer gratings Gl and G2.
  • the average height h that is, its extension in radiation propagation direction of the phantom is constant for each section.
  • the equalization gap GP between prism sections 5,6 (or between sections 1 ,2). For instance, although some segments of prism 6 are higher than those of adjacent prism 5, this is off-set by prism 6 having a lower hight at gap GP than prism 5. In other words, the average attenuation over each section is
  • the imager+phantom body system as proposed herein allows execution of imager calibration and object image acquisition essentially in one "sweep", that is, calibration scan and object scan transition into each other without appreciable interruption which increases comfort for the patient and allows easier integration in the clinical work flow.
  • Signals detected at detector after issuance of said switch signal are then interpreted by signal processor IDP as radiation signals caused by interaction with the to be imaged object rather than with the Phantom, and the radiation signals g j are gated to object image processor OIP where the phase contrast image is computed as per equation (2) above.
  • object image processor OIP object image processor
  • the calibration kit comprises several different phantom's PB each having a different thickness or height h.
  • the user Prior to the calibration scan, the user either picks the phantom having the right thickness or attaches a suitably thick one or a suitable number of extension plates with the same or different thicknesses to the "base" phantom PB so that the thickness of the overall phantom body thickness corresponds to the thickness and or material composition (in particular the density) of the BR to be scanned.
  • both proximal and distal surfaces are jagged as shown in Figure 8 thereby defining an upper and lower (in propagation direction) prism.
  • the ray will need to pass through two sloped surfaces one being sloped upwardly the other downwardly one being the mirror symmetric image of the other relative to a horizontal symmetry plane.
  • a symmetric phantom body PB as per Fig 8 makes it easier to achieve precise angular alignment of the phantom with respect to the x-ray beam to effect the desired interference pattern phase shift.
  • the symmetric phantom is of figure 8 can likewise be extended vertically by adding extension plate PLx to achieve a desired attenuation strength that corresponds to the constitution of the object to be imaged.
  • the extension plate is sandwiched in between the upper and lower prism. In an alternative embodiment it is only the distal surface that is jagged.
  • the sloped surfaces of the sections are necessarily contiguous as in figure 7B.
  • FIG 9a there is shown in upper Fig 9a another embodiment for a phantom prism for use in phantom body PB. Phantom prism 9a is of the Fresnel type.
  • the Fresnel type prism of Fig 9b can be used with either of the embodiment of Figs 7,8 which yields a phantom body made up from a plurality of different Fresnel type prism sections, each similar to the one of Fig 9a.
  • Fig 10 there is shown a flow chart that summarizes the basic steps of the phantom body based calibration method as proposed herein. The method allows computing a reference phase for a radiation wave front that is emitted by a radiography scanner.
  • the radiography scanner has DCPI capabilities so includes interferometric gratings.
  • the gratings are of the fixed type, that is, are not moved during the imaging acquisition with respect to each other (but together with the detector) and phase stepping is achieved by effecting a relative motion between the detector and the phantom body.
  • the relative motion is achieved by having the radiography scanner's detector move along a scanning path that is run under the DCPI grating.
  • the detector remains fixed and it is the object to be imaged that is moved instead to trace out the scanning path.
  • the wave front is distorted after its passage through the phantom body with the distortion caused by refraction interaction of the different wave front portions with different ones of each of the slope gradients.
  • the refraction interaction includes respective phase changes at the different wave front portions which in turn cause deflection of the respective wave front portions at a plurality of different predefined refraction angles that correspond to the pre-defined slopes of the phantom body surface.
  • the detected intensity signals are a consequence of the local phase shifts of an interference pattern caused by the presence of the grating components Gi, G 2 interposed between detector and the phantom body. From the known phase shifts due to the phantom, the local intensity A, local fringe phase a and local visibility V can be determined.
  • the curve is fitted to the plurality of different, pre-defined sample oscillation phases ( ⁇ ,) to obtain calibration parameter such reference phase (a) and/or attenuation A and/or visibility V for the current relative grid positions.

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Abstract

L'invention concerne un procédé et un corps fictif apparenté (PB) pour l'étalonnage d'un système de balayage à rayons X (MIS) avec capacité d'acquisition d'image en contraste de phases (DPCE). Le système de balayage à rayons X a un détecteur mobile (D) et des réseaux interférométriques fixes (G0, G1, G2) où aucun mouvement relatif entre les réseaux (G1, G2) n'est prévu. Le procédé utilise un corps fictif (PB) avec une surface déchiquetée pour permettre l'étalonnage du système de balayage (MIS) pour une phase de référence des réseaux interférométriques (G1, G2).
PCT/EP2014/070426 2013-09-27 2014-09-25 Détermination de phase de référence dans l'imagerie de contraste de phases à balayage différentiel WO2015044238A1 (fr)

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US10172580B2 (en) 2013-12-17 2019-01-08 Koninklijke Philips N.V. Phase retrieval for scanning differential phase contrast systems
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CN107567640B (zh) * 2015-05-07 2022-04-05 皇家飞利浦有限公司 用于扫描暗场和相位对比成像的射束硬化校正
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