X-ray imaging apparatus and image acquisition method thereof

Abstract

An X-ray imaging apparatus (XI) comprises an X-ray source (XS) having a cathode (C) and an anode (A). The source (XS) is to generate a beam (XB) of X-radiation. An X-ray detector (XD) detects X-radiation after interaction with an object (OB) being imaged. The beam (XB) has different spectra at its anode side (AS) and cathode side (CS) caused by a heel effect when the X-ray source (XS) is in operation. The X-ray imaging apparatus (XI) has a heel effect utilization (HH) mechanism configured to alternate exposure of pixels (PX) of the detector (XD) to both the anode side (AS) and the cathode side (CS) of the beam (XB).

Description

TECHNICAL FIELD

[0001] The present invention relates to an X-ray imaging apparatus, an X-ray image acquisition method, a computer program element, a computer readable medium and use of a heel effect in X-ray imaging. BACKGROUND

[0002] X-ray imaging is still the backbone for a large number of diagnostic and therapeutic tasks in medical imaging.

[0003] Conventional X-ray imaging is energy integrating, wherein spectral information in the X-ray radiation is not utilized. The X-ray detector is only used to detect the integrated energy as a line integral to calculate an attenuation image. This conventional energy integrating X-ray imaging typically does not allow material differentiation. For example, it can not be possible to distinguish whether a certain attenuation is due to a small thickness of a high density material or due to a larger thickness of a low density material. In both cases the same attenuation would be observed in energy integrating conventional X-ray imaging.

[0004] Spectral X-ray imaging changes this, wherein it utilizes the spectral information in the received X-radiation as detected at the X-ray detector. Spectral X-ray imaging allows to calculate material specific "maps", i.e. images that are given contrast by a specific material type. Spectral X-ray imaging can also be used to calculate mono- energy images as another application.

[0005] However, spectral X-ray imaging requires new investments, since existing X-ray imagers are typically not suitable for spectral imaging. Dedicated hardware is required, such as dual layer detectors, fast switching X-ray sources, dual X-ray source imagers, etc. SUMMARY

[0006] Therefore, there can be a need for alternative ways to perform spectral X-ray imaging.

[0007] The object of the present invention is solved by the subject matter of the embodiments of the present invention. It should be noted that the following described aspects of the present invention equally apply to the X-ray image acquisition method, the computer program element and the computer readable medium.

[0008] According to a first aspect of the application, there is provided an X-ray imaging apparatus comprising: an X-ray source having a cathode and an anode, configured to generate an X-radiation beam; and an X-ray detector for detecting X-radiation after interaction with an object being imaged, the beam having different spectra on its anode side and cathode side when the X-ray source is in operation caused by a heel effect; and a heel effect exploiting mechanism configured to cause pixels of the detector to be alternately exposed to both the anode side and the cathode side of the beam by controlling an actuator in coordination with the timing of readout activities at a data acquisition unit to change the imaging geometry, the actuator acting on at least one of the X-ray source, a patient examination table and the detector.

[0009] The proposed method allows using existing X-ray imagers without the need for additional hardware components to perform spectral imaging.

[0010] The heel effect exploiting mechanism comprises in embodiments only standard components, such as the X-ray source and the X-ray detector. The components are coordinated in operation by a control unit to implement a new imaging protocol. The control unit can be an existing control unit that can be reprogrammed to implement the new protocol. Thus, existing energy integrating X-ray imagers can be operated in spectral imaging mode. No new hardware components are required in these embodiments.

[0011] In more detail, in embodiments, the heel effect exploiting mechanism changes the imaging geometry such that each geometric pixel is exposed to different parts of the beam, the anode side and the cathode side, and thus to different spectra due to the heel effect, in turn.

[0012] In embodiments, the heel effect exploiting mechanism causes a relative motion between the object and the X-ray beam.

[0013] In embodiments, the motion is caused by a motion of at least part of a support on which the object being imaged resides during imaging, and / or by a motion of the X-ray source.

[0014] In embodiments, the motion of the X-ray source is transversal and / or rotational such that the respective positions of the anode and cathode are changed, in particular and in embodiments (exchanged).

[0015] In embodiments, the heel effect exploiting mechanism causes a motion of the support and / or X-ray source in order to increase the distance between the X-ray source and the object.

[0016] In embodiments, the plurality of pixels is exposed so as to obtain two sets of intensity readings for the plurality of pixels, at least one set for each of the two spectra, the apparatus further comprising a spectral imaging processor configured to compute a spectral image for the object based on the two sets of intensity readings.

[0017] While the above described embodiments can be used without additional hardware, certain such additional hardware is still envisaged in alternative embodiments now referred to. In particular, according to a further aspect, there is provided an X-ray imaging apparatus comprising: an X-ray source configured to generate an X-radiation beam; and an X-ray detector for detecting X-radiation after interaction with an imaged object; and a movable filter arranged between the X-ray source and the object and movable by a control unit so as to change the spectrum of the beam at least locally to a first spectrum and a second spectrum, thereby causing pixels of the detector to be alternately exposed to X-radiation at the two spectra.

[0018] The additional hardware is envisaged in the form of a filter element. However, in contrast to the above described dedicated spectral hardware option as an expensive solution, this filter is cheaper and easier to install / retrofit.

[0019] In embodiments, the filter comprises sub-elements comprising different materials, respectively, wherein at least two of the sub-elements are alternately brought one at a time into (perspective) registration with the pixels, caused by the filter motion.

[0020] The two materials are to cause different spectral modulation. Envisaged combinations include Al and Cu or Al and a polymer, such as PE. One of the elements can be formed as a void, in which case air is one of the materials.

[0021] In embodiments, the sub-elements are arranged in a checkerboard pattern.

[0022] In another aspect, there is provided a related method comprising an image acquisition method for an X-ray imaging apparatus, the apparatus comprising: an X-ray source having a cathode and an anode configured to generate an X-radiation beam; and an X-ray detector for detecting X-radiation after interaction with an imaged object, the beam having different spectra on its anode side and cathode side caused by a heel effect when the X-ray source is in operation, the method comprising:

[0023] causing pixels of the detector to be alternately exposed to both the anode side and the cathode side of the beam by controlling an actuator acting on at least one of the X-ray source, a patient examination table and the detector in line with the timing of readout activities at the data acquisition unit, the imaging geometry being changed.

[0024] In another aspect, an image acquisition method for an X-ray imaging apparatus is provided, the apparatus comprising an X-ray source for generating an X-radiation beam and an X-ray detector for detecting X-radiation after interaction with an object being imaged, the method comprising:

[0025] alternating exposure of the pixels of the detector to X-radiation at a first spectrum and a second spectrum by moving a moveable filter arranged between the X-ray source and the object so as to change the spectrum of the beam at least locally to the first and second spectrum.

[0026] In embodiments, the alternating exposure causes two measurements to be detected at different spectra at the pixels, and wherein the method further comprises processing the two measurements by a spectral imaging algorithm.

[0027] In embodiments, the spectral imaging algorithm is implemented as a machine learning algorithm.

[0028] In another aspect, a computer program element is provided, which, when being executed by at least one processing unit, is adapted to cause the processing unit to perform any of the above described methods.

[0029] In another aspect, a computer readable medium having stored the program element is provided.

[0030] In another aspect, a use of a heel effect for spectral X-ray imaging is provided.

[0031] Definitions

[0032] A“user” relates to a person operating the imaging apparatus or supervising the imaging procedure, such as a medical personnel or other person. In other words, the user is typically not the patient.

[0033] “Object” is used herein in a general sense to include animate“objects”, such as a human or animal patient or an anatomical part thereof, but also inanimate objects, such as baggage items in security checks or products in non-destructive testing. However, the proposed system will be mainly discussed herein with reference to the medical field, so we will refer to the“object” as a“patient” and the region of interest ROI is a specific anatomical structure or group of anatomical structures of the patient.

[0034] “Motion” as used herein includes a change in position and / or orientation with respect to a world coordinate system.

[0035] “(Filter) material” as used herein includes air, so one of the filter elements can be formed as a hole. In this context, exactly two different material types can be sufficient, as these materials can be arranged spatially in a spatially alternating manner, such as in a checkerboard manner of multiple sub-filter elements. In contrast, one-dimensional layouts are also conceivable, and so are filter layouts with more than two material types. The different materials are to cause different spectral modulations. BRIEF DESCRIPTION OF DRAWINGS

[0036] Exemplary embodiments of the present application will now be described with reference to the following drawings (which are not to scale), in which:

[0037] Figure 1 A schematic diagram showing components of an X-ray imaging apparatus is shown;

[0038] Figure 2 A schematic cross-sectional view of an X-ray source is shown;

[0039] Figure 3 Spectral X-ray imaging as envisaged herein is illustrated;

[0040] Figure 4 An illustration of the heel effect is shown;

[0041] Figure 5 Different spectra of the heel effect due to a function of take-off angle are shown;

[0042] Fig. 6 shows different embodiments of heel utilization mechanisms that can be used in an X-ray imaging apparatus;

[0043] Fig. 7 shows a filter element for use in spectral imaging as envisaged herein;

[0044] Figure 8 A flowchart of a method of spectral imaging according to a first embodiment is shown; and

[0045] Figure 9 A flowchart of a method of spectral X-ray imaging according to a second embodiment is shown. DETAILED DESCRIPTION

[0046] Reference Figure 1 is a schematic diagram of an X-ray imaging apparatus XI. Embodiments for this include a C-arm imaging apparatus, a CT scanner, a mammography apparatus, a dental X-ray imager or radiographic apparatus or other configured to acquire X-ray images of an object OB.

[0047] Broadly speaking, the components of the X-ray imaging apparatus XI include an X-ray source XS and an X-ray sensitive detector XD. In use, an object OB is positioned in an examination region within the X-ray source XS and the X-ray detector XD. To facilitate this, in some embodiments a table T is provided on which the patient OB resides during imaging.

[0048] As envisaged herein, some or all of the mentioned components are used in a new imaging protocol managed by the control unit CU to allow spectral imaging. No additional hardware components are required for this in embodiments, and this will be discussed more fully after explaining the operation of the components in more detail below.

[0049] In use, the X-ray source XS is energized to produce an X-ray beam XB that passes through the examination region and thus through at least a region of interest of the object OB. The X-radiation interacts with the matter (e.g. tissue, bone, etc.) of the object OB. After the interaction, the radiation is incident on the X-ray detector XD.

[0050] The impinging X-radiation is detected by the detector XD as intensity in the form of electrical signals. The electrical signals are converted to digital image values by suitable conversion circuitry in a data acquisition unit DAQ, which can then be processed into X-ray images by a signal processing subsystem SPS. The signal processing subsystem SPS includes a spectral image processor SP, which can be used when the imager is requested by a user through a suitable user interface to operate in a spectral imaging mode. For example, the user interface can include a button or a touch screen widget in a display associated with an operator console. The spectral image processor SP implements desired spectral imaging algorithms, such as material decomposition, material mapping, mono- energy imaging, material-contrast images, effective atomic number images and electron density images, etc. Each of these types of images will be referred to herein as a “spectral image”, see for example RE Alvarez et al. “Energy-selective Reconstructions in X-ray Computerized Tomography” in Phys. Med. Biol. Vol. 21, No. 5, pp. 733-744 (1976).

[0051] Generally, spectral (or multi-energy) imaging utilizes two intensity readings to resolve the photoelectric and Compton contributions that make up the mass attenuation coefficient of the scanned material, and then identifies unknown materials by their photoelectric and Compton contribution values. This method works very well for materials with a k-edge energy close to the average of the diagnostic energy range, such as iodine. Since any two linearly independent sum of two basis functions spans the entire attenuation coefficient space, any material can be represented by a linear combination of two other materials, often referred to as basis materials, such as water and iodine.

[0052] The spectral x-ray image can represent details inside the imaged object OB, in particular the presence of different materials etc. This can help in the diagnosis and treatment or other examination of the imaged object OB. The spectral imagery can then be used in a visualizer VZ with suitable visualization software to enable display on one or more display devices DD, such as monitors etc. The images can also be stored in a repository, or can be further processed in other ways.

[0053] Figure 2 is a schematic cross-sectional view of the x-ray source XS. Broadly, the x-ray source XS comprises a pair of electrodes, an anode A and a cathode C. A high voltage potential is established between the cathode C and the anode A. This can be done by connecting the cathode and the anode to suitable power supplies PS + , PS - as shown. For this purpose, suitable electrical connections CON are provided at the source XS. In use, the anode A and the cathode C are held at a high voltage potential of approximately 150KV (referred to herein as the "tube voltage" or "operating voltage") with respect to ground. Figure 2

[0054] The cathode C and the anode A are arranged in opposing relationship in space in a housing H to define a driftway between the cathode C and the anode A. The anode A and the cathode C and the driftway are encapsulated in a vacuum glass tube (not shown) within the housing H.

[0055] Preferably, but not necessarily, the x-ray source XS is of the rotating type, in which the anode is arranged as a disc (shown in cross-sectional side view in Figure 2 ), which is rotatably journaled in suitable bearings B and driven by a suitably powered motor. X-ray sources with a stationary anode are also envisaged.

[0056] The cathode C comprises an emitter (not shown). An electrical current (referred to herein as the "emitter current") is generated by a power supply PSH. The emitter current passes through the emitter during use. It will be understood that the three power supplies PSH, PS - , PS + are shown in Figure 2 as separate, independent entities, and this is indeed envisaged in some embodiments. However, this does not exclude alternative embodiments herein in which some or all of the mentioned power supplies are integrated into a single power supply. A positioning tool, such as a cathode cup CC, is arranged to hold the emitter at a distance from a peripheral portion of the anode A. The peripheral portion can be formed as a beveled edge, especially when the anode is as Figure 2 ​The illustrated rotating type. The oblique angle β is typically about 15°, but other oblique angles are also envisaged, such as for example 9°, 12°, 15°, 18° or any other. When the emitter current is applied, the emitter heats up to a temperature of about 2400° and electrons evaporate from the emitter surface in thermal emission.

[0057] Due to the high potential difference +V, -V between the cathode C and the anode A, the evaporated electrons form an electron beam which is accelerated towards the anode A and impinges at a focal spot FS on the surface of the anode.

[0058] In case of a rotating anode, the focal spot is located at the oblique edge of the anode disc. It will be understood that due to the rotation, the focal spot FS traces a trajectory around the edge of the anode disc AN. The anode AN is formed of a high density material such as molybdenum, tungsten or other high-Z metal / material.

[0059] When impinging at the focal spot FS, the electron beam decelerates and the energy drop is partly transformed into heat, and a fraction (about 1%) is transformed into a beam XB of X-radiation irradiating away from the focal spot FS.

[0060] The housing H is radiation blocking, for example by having a layer of lead (or other suitable high-Z material) to prevent X-radiation from escaping outside the housing, safe for an exit window EW formed of a non-radiation opaque material such as glass. The X-radiation beam XR generated inside the X-ray source XS then exits substantially undisturbed through the exit window EW to propagate towards the detector XD (whose relative position is indicated with “X” in Figure 2

[0061] Downstream of the X-ray source XS, an optional collimator COL can be arranged to form the X-radiation into a directional beam XB having a desired, typically diverging, shape (e.g. a cone beam, a fan beam). Alternatively, a more conventional parallel beam geometry is also envisaged herein.

[0062] The overall control of the imaging procedure is through a user-operable computerized operator console OC. A control unit CU, which can be integrated into the operator console OC, controls the interaction of the various components and allows to select operational settings such as cathode voltage, anode current, etc. Typically, the control unit controls the way the X-ray beam XB irradiates the object OB and the detector XD.

[0063] Some or all of the components can be movable in motion by respective actuators AC1-AC3 to enable different imaging geometries. The term “imaging geometry” as used herein describes the spatial geometrical relationship between the X-ray beam XB and the object OB to be imaged and / or the detector XD position / orientation.

[0064] ​The actuators AC1-AC3 by which the imaging geometry can be changed can be controlled automatically by the control unit CU or by a user. In one embodiment, one actuator AC1 allows a translation of the X-ray source XS parallel to the object OB to be imaged and / or parallel to the examination table T. The translation of the source can be along one or both spatial dimensions in the plane.

[0065] Additionally or alternatively, the imager IX can allow for further degrees of freedom. In particular, in embodiments, the X-ray source itself can be rotatable by an actuator AC1 (or by a different actuator) around a rotation axis which passes through the drift path between the anode A and the cathode C towards the X-ray detector XD. In embodiments, the rotation axis passes through the anode, in particular through the focal spot, towards the X-ray detector XD as indicated. Thus, the position or orientation of the anode and the cathode with respect to the world coordinate system can be changed continuously or in steps of 90° or in any other angular increment. In other words, with the rotation axis passing through the drift path between the anode A and the cathode C, a rotation by 180° would exchange the positions of the anode A and the cathode C as shown in the side view of Figure 1 Figure 1

[0066] In other embodiments, the rotation axis of the rotatable X-ray source can pass through the cathode C instead. With the rotation axis passing through the anode A or the cathode C, respectively, one of the respective electrodes changes its orientation, while the other electrode changes its orientation and position.

[0067] In addition or alternatively to any of the above mentioned degrees of freedom, the examination table T can be translatable in a plane in one or, preferably, two dimensions. It is not required that the entire table is translated. For the present purposes, it can be sufficient that a part of the table, e.g. a support surface of the table on which the patient resides, is translatable by an actuator AC2.

[0068] An additional actuator AC3 can be used to change the SID ("source to image distance"), i.e. the distance along the optical axis OX (not shown in Figure 1 but in Figure 6D ) between the detector XD and the focal spot FS of the X-ray source XS.

[0069] Instead of using three separate actuators as discussed above, fewer or more can be used. In embodiments operated by a suitable gear system, a single or two actuators can be used such that all of the above mentioned movements can be achieved.

[0070] Turning now in more detail to the spectral imaging capabilities of the X-ray imager XI as presented herein, reference is now made to Figure 3 . The X-ray beam XB can be conceptualized as a bundle of geometric rays GR Figure 3 ​​a focal spot FS, through the object OB to be imaged, and terminates at a pixel position. In this sense, each geometric ray has a certain pixel associated with it. This pixel position can be referred to herein as a geometric pixel position, which can also be referred to herein as "GR", but to be distinguished from a certain detector pixel position PX.

[0071] Along each geometric ray, X-ray photons are travelling and interacting with the object, and then detected at the detector PX. The distribution of energies in between the photons travelling along each of the geometric rays defines a local ray spectrum. Different groups of geometric rays with their respective photons travelling along them define different beam portions, each having a certain local spectrum as a function of the different local ray spectra.

[0072] Each geometric ray GR interacts with the object to be imaged in a tissue in path length IPL. For spectral imaging, it must be ensured that each tissue in path length in the region of interest is traversed or exposed to beam portions of different spectra. The different spectra are indicated in the figure as differently length arrows S1, S2. This exposure should occur in order, such that first exposure to radiation having a first spectrum S1, and then exposure to radiation photons having a second spectrum S2. These sequential exposures at different spectra will be referred to herein as "multi-spectral (X-ray) exposures".

[0073] The respective intensity readings at different spectra need to be read out and registered separately at the respective pixels PX associated with the geometric rays and thus at the tissue in path lengths of interest. It is proposed herein to change the imaging geometry by controlling the actuators in line with the timing of the readout activities at the data acquisition unit DAQ to achieve the respective multi-spectral exposures for each pixel of interest. More particularly, it is proposed herein to program the control unit CU to implement a spectral multi-shot imaging protocol which exploits the heel effect to achieve the multi-spectral exposure of each geometric pixel GR / tissue in path length IPL.

[0074] "Shot" as used herein is defined as a discrete period of exposure to X-radiation, which can be temporally interrupted and separated from previous or subsequent shots by switching off the X-ray source, applying a switch grid voltage to interrupt the electron beam and / or by closing the collimator or any other suitable fast exposure control scheme. It is envisaged herein that each tissue in path length or geometric pixel position GR is exposed at least twice to ensure the multi-spectral X-ray exposure. The control unit CU also operates to coordinate the readout sequencing in the readout circuitry of the data acquisition unit DAQ when capturing the respective exposures at different spectra.

[0075] Reference is now made to Figure 4To briefly explain the heel effect. Due to the heel effect, the distribution of the spectrum in the X-ray beam XB is anisotropic. The part of the beam closer to the anode and with an exit angle γ larger than the anode's inclination β needs to pass through more anode material and thus experiences a modulation of its spectrum. There is also an intensity modulation. The spectral and intensity modulation is illustrated in Figure 3

[0076] The change of the spectrum from the cathode side to the anode side does not happen across a hard border, but gradually, and the spectral modulation starts at an exit angle of about corresponding to the anode's inclination β. In the illustrated example, the inclination is 15°, and accordingly, there is a significantly different spectrum for exit angles from the anode side larger than 15° compared to the beam part at the cathode side, here shown as from about 0°, which corresponds to the optical axis OX, to about 15° angled towards the cathode. Figure 4

[0077] Thus, a pixel pXC located at the cathode side of the beam will experience a different spectrum than an anode side pixel pXA. In other words, the heel effect, which is usually compensated and considered a nuisance, is used herein as a source for multi-spectral exposure to implement spectral imaging.

[0078] Figure 5 is an illustration of the different spectra due to the heel effect. Figure 5 illustrates a spectral diagram, where the flux Φ is plotted against the energy E. Three spectra are shown: one for the central ray at 0°, one (in dashed line) for an exit angle of about 4°, and one for an exit angle at about 10° (shown in dotted line). The spectrum at 10° exit angle is anode side relative to the relatively more cathode side 4° exit angle.

[0079] The proposed control unit CU preferably and only operates with the regular components of the X-ray imager XI to implement a heel utilization mechanism HH for spectral imaging purposes. The heel utilization mechanism HH allows an operator to produce spectral imaging with a regular X-ray imager, preferably without using any additional hardware components.

[0080] ​​Now first referring to Fig. 6, different embodiments of the heel effect utilizing mechanism HH are described. Broadly, the heel effect mechanism HH as contemplated herein uses the heel effect, and thus different spectra of different portions of the X-ray beam, to sequentially expose a given pixel to said different spectra. This is done by changing the imaging geometry in line with the DAQ readout timing to ensure that each pixel of interest receives a multi-spectral exposure. The control unit CU operates to change the imaging geometry, trigger the appropriate shots and coordinate the appropriate signal capture at the readout circuit in the DAQ to ensure that each geometric pixel position (and thus the in-organ path length) is properly associated with an intensity measurement at at least two different spectra. In embodiments, the CU also ensures that it keeps track of which two spectra each pixel is exposed to. The spectral processing of the spectral imaging processor SP will be explained in more detail below.

[0081] Now first turning to the hardware aspect of the utilizing mechanism HH, now referring to Figure 6A In this embodiment, the X-ray source and / or the table move with translational movement to substantially scan the object, ensuring that each in-organ path length is sequentially exposed to the anode side portion of the beam AS and the cathode side of the beam at least once, respectively.

[0082] In Figure 6B Another embodiment is illustrated in which the rotation of the X-ray source around its rotation axis is used to achieve this multi-spectral exposure. This embodiment works by Figure 6B illustrating a 180° source XS rotation between Figure 6C , where the anode side and the cathode side of the beam change position. Due to the source rotation, the geometric pixel GR measured at the detector pixel PX is exposed to the anode side of the beam and thus to the first spectrum in one shot according to the imaging geometry of Figure 6B In the next shot and after rotating the X-ray source by 90° or 180°, the geometric pixel GR (still measured at the detector pixel PX) is now exposed to the cathode side of the beam in order to receive photons at the second spectrum as Figure 6C illustrated. The X-ray source can be rotated in angular steps of 90° or any other angular increment, each followed by a corresponding shot to ensure a multi-spectral exposure of each geometric pixel GR of interest. In this way, pixel positions in a circular area around the rotation axis of the X-ray source will receive a multi-spectral exposure. However, this leaves a central portion around the rotation axis which does not necessarily receive such a multi-spectral exposure. In order to also cover these central pixels, the X-ray source and / or the patient can be subjected to a lateral motion such as Figure 6A illustrated and then repeat the protocol as described in Figure 6B , 6C to also expose the central pixels to a sequential multi-spectral exposure.

[0083] In the embodiment ofFigure 6D In another embodiment shown, the divergent properties of the X-ray beam XB are utilized. For this purpose, the SID is made large enough that the corresponding anode-side and cathode-side portions cover the entirety or at least the relevant portion of the X-ray detector plane XD. In this embodiment, all or all related sequential multispectral exposures can be achieved in two images, one targeting the anode side, such as... Figure 6D As shown, and another time in which as Figure 6B The X-ray source described in C is rotated 180° to the cathode side before taking the picture.

[0084] As mentioned, there is no hard boundary in the spectral change from the anode side AS to the cathode side CS, but this spectral change is gradual. However, for the purposes of this study, the portion of the beam tilted towards the anode A and corresponding to an exit angle at least as large as the angle of inclination to the anode can be considered the anode side, while the remaining angular range can be considered the cathode side. Other definitions can be used as long as there is sufficient difference between the two spectra.

[0085] Turning now to Figure 7, this illustrates an extension of the embodiment described above that does not require additional hardware components. In this embodiment, an additional filter (FL) component is present. However, this additional filter (FL) can be manufactured relatively inexpensively and can be easily retrofitted from existing imagers at low cost.

[0086] exist Figure 7A In embodiments B, the additional hardware component, filter element FL, comprises at least two sub-filter elements E1, E2 of different types, each comprising a different material. The filter is essentially planar and can have any shape, such as square, rectangular, or curved, such as spherical or others. Only two different material types are required in the layout, such as the one shown, wherein multiple such elements of both types are arranged in a layout with varying material types. One-dimensional (1D) or two-dimensional (2D) designs are envisioned in the embodiments. Figure 7 shows a 2D layout of one embodiment. Preferably, elements E1, E2 are arranged in a checkerboard pattern as shown in Figure 7, although this is not required in all embodiments. Typically, any two adjacent different filter sub-elements E1, E2 are made of different materials. The filter elements may or may not have the same shape. The filter elements may not necessarily be square, but can be rectangular, circular, or any other geometry. However, square or rectangular shapes are preferred because they allow for better coverage. Two materials E1, E2 are chosen to induce different spectra. Suitable material combinations envisioned herein include Al (aluminum) and Cu (copper) or Al and polyethylene.

[0087] One of the sub-components can be simply implemented as a hole, thus leaving air as another material. If using, for example...Figure 7A , the chessboard pattern shown in B, rotated by 900 degrees around the central axis (which extends into the drawing plane of Fig. 7) results in Figure 7A 、 7B the two configurations in C. The rotation allows each pixel GR to be exposed in turn to different spectra as caused by the respective filter elements E2, E1.

[0088] The rotation of the filter elements takes place between shots, and only requires two shots for this embodiment. The filter rotation can be managed by the control unit CU by means of a suitable actuator that rotates the filter around a rotation axis that passes through the layout plane of the filter. The filter elements are arranged between the X-ray source and the object OB. In particular, it can be arranged at the exit window EW after the collimator, if any. It is ensured that each filter element E1, E2 is perspective-registered with the respective pixel GR. In other words, each element E1 or E2 intersects with a geometric ray and is thus associated with a respective pixel GR further downstream.

[0089] Turning now in more detail to the signal processing of the spectral image processor SP, it is required to ensure that the correct spectral information is used for each pixel. A synchronizer (not shown) can operate the shots for each shot by using information about the current imaging geometry to register the respective spectral exposures. In other words, the respective spectrum for each pixel and each shot is registered in a suitable data structure. The result of the synchronized readout is two sets of projection spectral data π1 and π2. Each entry in one of the matrices corresponds to an intensity reading of one of the two spectra.

[0090] As mentioned previously, the transition of the spectrum from the cathode side to the anode side is not hard, but gradual. Also, the spectrum of the anode side of the beam XB differs with the exit angle. In other words, in the proposed scheme there is typically a range of respective anode side spectra, rather than a single anode spectrum. However, for spectral imaging, the two spectra of each pixel GR should be known, at least close to within error bounds. With this in mind, different embodiments for spectral image processing by the spectral image processor SP are envisaged.

[0091] In one embodiment, the different spectra in the anode are simply averaged, and then each pixel is assigned the same anode side spectrum, the same operation being performed for the cathode side, although the cathode side is expected to be more uniform anyway.

[0092] The averaging can be suitable for simple spectral imaging tasks. The averaging can introduce some artifacts. In another embodiment, a theoretical spectral distribution model is used. In theory, since the material and the path length through the anode are known, and the imaging geometry is also known, the model can be used to pre-compute the respective spectral pair for each pixel GR, respectively. In the previously mentioned case where the spectral information is averaged, or in the case where each pixel is assigned a specific spectral pair based on the theoretical model, the spectral image can be processed by a classical, i.e. analytical, spectral imaging algorithm as discussed by Alvarez (as mentioned above), where a system of linear equations is solved to compute the desired type of spectral image.

[0093] Instead of these explicit analytical schemes, implicit schemes based on machine learning models or algorithms can be used instead. In this approach, a machine learning algorithm is used, where the model is trained based on previous data. One such model is a neural network, in particular a convolutional neural network CNN with one or preferably multiple hidden layers (deep learning).

[0094] Supervised learning can be used, where the training data comprises pairs of training images, each pair comprising a training input image and an associated target. The training input image comprises two images taken at different spectral or spectral features, and the target image is the associated real spectral image of the type that is desired to be computed. The target can be a material map image. The spectral resolution can be better than the spectral resolution encountered during deployment, i.e. after training of the machine learning model. In an embodiment, the training input data can be generated from existing energy-resolved projection data sets.

[0095] Different models can be trained for different types of spectral images. The training input images are fed into the model. The parameters of the model are adjusted based on the output of the model and the target. The parameters are adjusted to improve a target function that measures the mismatch between the output and the target.

[0096] The training images can be generated by acquiring the above described multi-spectral exposure images π 1,2 of a multi-material phantom, such as a Shepp-Logan phantom, in an experimental phase. The neural network is trained on real spectral images used as targets to perform thickness estimation or material decomposition.

[0097] In an embodiment, the phantom is first imaged by an imager with spectral imaging hardware (dual layer detector, dual X-ray source, fast switching X-ray source, etc.). The resulting image then forms the target for training. The same phantom is then imaged using the heel effect or filter FL based imaging protocol described above to obtain the associated training input images. The heel effect or filter FL based imaging protocol can be run on a different standard imager or on the same spectral imager as for the target but without the spectral imaging hardware.

[0098] In a further embodiment, the target can be obtained as a geometrically measured decomposition of a spectral phantom (such as a Shepp-Logan phantom) or other phantom. Instead of using a real, physical phantom, the training images can be obtained partly or entirely by simulation.

[0099] The machine learning algorithm, such as a neural network, once trained on real spectral images, can then compute in deployment from any new pair of spectral projection images π1and π2(desired spectral images in image domain).

[0100] It is advantageous to use such a machine learning algorithm because the approach does not require knowledge of the exact two spectra for each geometric pixel position. For each geometric pixel, it is sufficient to simply have two intensity readings at different spectra, as can be obtained by the heel effect utilization protocol discussed above.

[0101] It should also be noted that with the spectral scheme mentioned above, it is also possible to produce regular energy integrated images. To this end, the operator console can include the user interface mentioned above, such as a button, which allows the user to select to operate the imager XI in regular energy integrated mode or the described spectral mode.

[0102] Reference is now made to Figure 8 which shows a flowchart of a method for X-ray based spectral imaging to implement the protocol as discussed above in Fig. 6, wherein the heel effect is used. However, it will be understood that the computer implemented method steps described below are not necessarily related to the architecture discussed above in Figure 1 -6.

[0103] At step S810, the pixels, groups of pixels or all pixels of the X-ray detector are exposed to the anode side or the cathode side of the X-ray beam. Due to the heel effect in the X-ray source used to generate the X-ray beam, the anode side and the cathode side of the beam have different spectra.

[0104] At step S820, the imaging geometry is changed so that the pixels or groups of pixels are now exposed to the other side of the X-ray beam, i.e. the cathode side or the anode side, respectively.

[0105] Steps S810 and 820 are repeated so that all the geometric pixels of interest have received sequential multispectral exposure of the anode side and the cathode side of the X-ray beam.

[0106] At step S830, a spectral imaging algorithm is used to process the two sets of projection images obtained in steps S810 and S820. The spectral images thus derived can then be processed or displayed or stored.

[0107] The proposed method can be used in the projection domain, as in planar radiography, but also in tomographic images in the imaging domain after CT reconstruction from such projection images.

[0108] Reference is now made to the flowchart in Figure 9 which represents the steps involved when using the multi-filter element filter FL described in figure 7 above.

[0109] In this embodiment, at step S910, the pixels, groups of pixels or all the pixels of the X-ray detector are exposed to X-radiation after the X-ray beam has passed through the multi-element filter. The filter is arranged between the X-ray source and the object to be imaged. The filter comprises at least two different filter elements made of different materials. The filter is movable and is held in a first position so that the pixels or groups of pixels are exposed to radiation at a first spectrum due to spectral modulation by the elements of the first material type.

[0110] At step S920, the filter is then moved into a second position or configuration and the exposure is repeated so that now, due to spectral modulation by the filter element(s) of the second type, the pixels or groups of pixels are now exposed to radiation at a second spectrum.

[0111] At step S930, the two sets of projection images thus obtained in steps S910 and S920 are then processed by a spectral imaging algorithm.

[0112] In a two-step embodiment, different sub-elements of the filter of different material types are sequentially brought into respective registration with the geometric pixels or groups of pixels.

[0113] The motion causing this spectral change at step S920 can be rotational or transversal or both.

[0114] The filter can consist exactly of two elements of different materials and then the filter is scanned over the object. However, this is less preferred as it involves a significant time delay.

[0115] Preferably, the chessboard-based pattern is formed by a plurality of elements of at least two (in embodiments exactly two) different materials (types) to cover the entire field of view. Motion can then be minimized and only one element or shift of rotation 900 is required, depending on the layout of the filter elements, to sequentially register two elements of the two different material types with each geometric pixel of interest.

[0116] The components of the image / signal processing system SPS can be implemented as software modules or routines in a single software package and run on a general purpose computing unit PU, such as a workstation associated with an imager XI or a server computer associated with a group of imagers. Alternatively, the components of the image processing system SPS can be arranged in a distributed architecture and connected in a suitable communication network.

[0117] The control unit CU can be integrated in the operator console or can be located remote therefrom. The control unit can be arranged in hardware or software or both. Preferably, the control unit CU can be implemented by reprogramming an existing control unit of the imagers XI to implement the above described heel effect exploitation protocol.

[0118] Alternatively, some or all components of the control unit can be arranged in hardware, such as a suitably programmed FPGA (field programmable gate array) or a hardwired IC chip.

[0119] One or more features disclosed herein can be configured or implemented as / within circuitry and / or combinations thereof encoded within a computer readable medium. Circuitry can include discrete and / or integrated circuitry, application specific integrated circuits (ASICs), system on a chip (SOC), combinations thereof, machines, computer systems, processors and memories, computer programs.

[0120] In another exemplary embodiment of the present application, 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.

[0121] The computer program element might therefore be stored on a computer unit, which can also be part of an embodiment of the present application. This computing unit can be adapted to perform or induce a performing of the steps of the method described above. Moreover, it can be adapted to operate the components of the apparatus described above. The computing unit can be adapted to operate automatically and / or to execute the orders of a user. A computer program can be loaded into a working memory of the data processor from a suitable data storage unit or from a suitable data carrier. The data processor is thereby equipped to execute the method of the present application.

[0122] This exemplary embodiment of the present application encompasses both computer programs using the present application from the outset and computer programs which transform existing programs into programs using the present application by means of an update.

[0123] Further on, the computer program element might be able to provide at least part of the functionality of the present application described above.

[0124] According to a further exemplary embodiment of the present application, a computer readable medium, such as a CD-ROM, having stored the computer program element is presented.

[0125] A computer program can be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid state storage medium supplied together with or as part of other hardware, but can also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

[0126] However, the computer program can 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 application, a medium for making a computer program element available for downloading is provided, wherein the computer program element is arranged to perform a method according to one of the previously described embodiments of the present application.

[0127] It has to be kept in mind that the embodiments of the present application have been described with reference to different subjects. In particular, some embodiments have been described with reference to claims of the method type, while other embodiments have been described with reference to claims of the device type. However, a person skilled in the art will readily recognize from the above and the following description that any combination of features belonging to one type of subject can be combined with features belonging to the other type of subject, unless otherwise indicated. However, all features can be combined to provide synergistic effects which go beyond the simple sum of the features.

[0128] While the application has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The application 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 practising the claimed application, from a study of the drawings, the disclosure, and the appended claims.

[0129] In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality. A single processor or other unit can 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. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An X-ray imaging apparatus (XI) comprising: An X-ray source (XS) having a cathode (C) and an anode (A), the X-ray source being configured to generate an X-radiation beam (XB); and an X-ray detector (XD) for detecting X-radiation after interaction with an object under imaging (OB), the beam (XB) having different spectra on an anode side (AS) and a cathode side (CS) of the beam caused by a heel effect when the X-ray source (XS) is in operation; and a heel effect utilization (HH) mechanism configured to cause pixels of the detector (XD) to be alternately exposed to both the anode side (AS) and the cathode side (CS) of the beam (XB) by controlling actuators acting on at least one of the X-ray source, a patient table and the detector in concert with timing of readout activities at a data acquisition unit to change imaging geometry.

2. The X-ray imaging apparatus of claim 1, wherein, The heel effect utilization (HH) mechanism causes relative motion between the object (OB) and the X-radiation beam (XB).

3. The X-ray imaging apparatus of claim 2, wherein, The motion is caused by motion of at least part of a support (TB) on which the object under imaging (OB) resides during imaging and / or by motion of the X-ray source (XS).

4. The X-ray imaging apparatus of claim 3, wherein, The motion is caused by the motion of the X-ray source (XS), the motion of the X-ray source (XS) being lateral and / or rotational such that respective positions of the anode (A) and the cathode (C) are changed.

5. The X-ray imaging apparatus of claim 1 or 2, wherein The heel effect utilization (HH) mechanism causes motion of a support (TB) and / or the X-ray source (XS) in order to increase a distance between the X-ray source and the object (OB).

6. The X-ray imaging apparatus of claim 1 or 2, wherein The plurality of pixels are so exposed to obtain two sets of intensity readings for the plurality of pixels, at least one set of intensity readings for each of the two spectra, the apparatus further comprising a spectral imaging processor (SP) configured to compute a spectral image for the object (OB) based on the two sets of intensity readings.

7. An image acquisition method for an X-ray imaging apparatus, the apparatus (XI) comprising: An X-ray source (XS) having a cathode (C) and an anode (A), the X-ray source being configured to generate an X-radiation beam (XB); and an X-ray detector (XD) for detecting X-radiation after interaction with an object under imaging (OB), the beam (XB) having different spectra on an anode side (AS) and a cathode side (XB) of the beam caused by a heel effect when the X-ray source (XS) is in operation, the method comprising: causing pixels of the detector (XD) to be alternately exposed to both the anode side (AS) and the cathode side (CS) of the beam (XB) by controlling actuators acting on at least one of the X-ray source, a patient table and the detector in concert with timing of readout activities at a data acquisition unit to change imaging geometry.

8. An image acquisition method for an X-ray imaging apparatus, the apparatus (XI) comprising: An X-ray source (XS) for generating an X-radiation beam (XB); and an X-ray detector (XD) for detecting X-radiation after interaction with an object under imaging (OB); the method comprising: by moving a moveable filter (FL) arranged between the X-ray source (XS) and the object (OB) to alternately expose the pixels (PX) of the detector (XD) to X-radiation at a first spectrum and a second spectrum in order to change the spectrum of the beam (XB) at least locally to the first spectrum and the second spectrum.

9. The image acquisition method of claim 7 or 8, wherein, The alternating exposure is such that two measurements are detected at different spectra at the pixels and wherein the method further comprises processing the two measurements by a spectral imaging algorithm.

10. The image acquisition method of claim 9, wherein, The spectral imaging algorithm is implemented as a machine learning algorithm.

11. A computer program element, which, when being executed by at least one processing unit (CU), is adapted to cause the processing unit (CU) to perform the method according to any one of claims 7-10.

12. A computer readable medium having stored thereon the program element of claim 11.

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