Patent Publication Number: US-10758202-B2

Title: Spectral imaging phantom and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2016/072570, filed Sep. 22, 2016, published as WO 2017/050914 on Mar. 30, 2017, which claims the benefit of European Patent Application Number 15186385.9 filed Sep. 23, 2015. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a spectral imaging phantom, a spectral imaging system and towards a spectral imaging method using the spectral imaging phantom. 
     BACKGROUND OF THE INVENTION 
     One of the advantages of spectral imaging, such as 2D or 3D x-ray imaging, for instance spectral Computed Tomography (CT), is that it may provide quantitative material images after material decomposition. For the material decomposition various methods may be contemplated, including, for instance, a dimensional look-up table approach (for instance known from Alvarez, R. E, Estimator for photon counting energy selective x-ray imaging with multi-bin pulse height analysis, Med. Phys., 2011, 38, 2324-2334), a theoretical forward model (for instance known from Roessl, E., Proksa, R, K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors, Phys. Med. Biol., 2007, 52, 4679-4696) or application of a semi-analytic approach (for instance known from Schirra, C. et al, Towards In-vivo K-edge Imaging Using a New Semi-Analytical Calibration Method, Proc. SPIE 9033, Medical Imaging 2014: Physics of Medical Imaging, 90330N, 19 Mar. 2014). 
     These and other known methods face the same problem that the outcome becomes distorted in case the status of the scanner and object of interest differ from the status during calibration. The status of the detector may change due to polarization of the sensor or the x-ray spectrum from the tube might change, e.g. due to heating during scanning An approach utilizing a look-up table (even with three or more materials) suffers from the fact that the human body does not consist exactly out of the materials used in the calibration. Artefacts like rings, bands, or cross-talk between the material images occurs due to the energy dependency of the attenuation of different materials, the different detector spectral responses and x-ray spectrum for different detector parts. It is likely that spectral forward models or semi-analytical approach never exactly matches the measurement. Scatter has a spectral footprint and changes the detected counts differently in different bins. This complicates the model even further. The mismatch in prediction and measurement produces cross-talk and artefacts in the material images. A better forward model or semi-analytical approach reduces the artefacts but it is not clear if the ultimately resulting images quality is good enough for quantitative medical imaging. 
     Besides the previously described systematic errors, the quality of the (temporal) stability of detector might frequently make a new spectral calibration necessary. It would be desirable to assess the status of the CT scanner in order to recommend a spectral calibration. 
     All mentioned image artefacts depend on the chosen scan protocols, especially on the x-ray tube voltage. After best processing, it would be preferable to provide a user, such as an operator or a physician, in a qualitative and a quantitative way about the remaining errors in the images. In best case the user is informed about the absence of significant errors. 
     US2012/0155617 A1 discloses a spectral CT calibration phantom that has inserts which allow for filling with a liquid) material that mimic the attenuation of different parts of a body. 
     WO2008/046498 A1 discloses a calibration method for two or more spectra tomography to determine material decomposition coefficients. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, amongst others, to overcome the previously mentioned problems. 
     In the present invention this is achieved by a spectral imaging phantom comprising a phantom body that includes a low attenuation material and at least a first insert comprising a material with a first Compton scatter and a first photo-electric absorption and a second insert with a second Compton scatter and a second photo-electric absorption, wherein a ratio of the first photo-electric absorption and the first Compton scatter is selected to be different from a ratio of the first photo-electric absorption and the first Compton scatter. Such an imaging phantom simulates the response of a subject, such as a human body. In a preferred embodiment Spectral imaging phantom according to claim  1 , wherein the first insert and the second insert have substantially the same attenuation profile, preferably exactly the same attenuation profile, which allows for distinguishing even more precise between the spectral properties of different inserts, irrespective of the attenuation of the inserts themselves. In a preferred embodiment the low attenuation material is a water equivalent material. 
     In a preferred embodiment at least one further insert comprising a material comprising or corresponds to the contrast agent at a further concentration, a further contrast agent at a first and/or a further concentration and/or a further material with a further Compton scatter and a further photo-electric absorption, including combinations thereof. As such, the response of more sections of the subject may be simulated. 
     In a preferred embodiment the first (and further) inserts are chosen such that they mimics attenuation characteristics of a part of a subject to be imaged, for instance the first and/or further inserts comprise a material comprising or corresponding to iodine or gadolinium contrast agents in one or more different concentrations or Teflon, PMMA, Polycarbonate, Polyethylene, Sodium Hydrogen Phosphate or Potassium Hydrogen Phosphate. 
     In a preferred embodiment the phantom body has flat shape or an arched shape, such that it can be placed under, on or over a subject to be scanned. Preferably the shape is elongated to cover a larger area, for instance a full subject or at least a part of interest of the subject. Also it might be more comfortable for a human to be ingredients. 
     In a further preferred embodiment the phantom body and inserts are solid. This allows for not having to fluidly isolate the inserts and the body to prevent leakage. 
     In a further embodiment the inserts are permanently fixed in the phantom body. This allows for ease of use, since no preparation is necessary before each procedure, which also allows for better reproducibility between procedures. 
     Embodiments of the present invention are further directed towards a spectral imaging system comprising a spectral x-ray imaging device, preferably a spectral computed tomography imaging device, and a spectral imaging phantom according to the invention. The spectral imaging phantom is preferably embedded in a subject support of the spectral x-ray imaging device, which allows for ease of use and a fixed position that allows for always generating the same data with respect to the imaging device. 
     Embodiments of the present invention are further directed towards a spectral imaging method including the steps of inserting a spectral imaging phantom according to present invention in an examination region of a computed tomography imaging device, performing a spectral computed tomography scan including at last a scan of the spectral imaging phantom, thereby obtaining spectral imaging data of at least the spectral computed tomography phantom and using the obtained imaging data of the spectral computed tomography phantom as input for obtaining improved further imaging data of a subject of which a spectral computed tomography scan is performed subsequent to or simultaneous with the spectral computed tomography scan of the spectral imaging phantom. Preferably the spectral imaging phantom is used as input for obtaining improved image data by using it as input for imaging data correction, for providing a recommendation and/or for further data processing. Imaging data of the spectral phantom may be used to improve image quality or reliability which assist physicians in their diagnosis, for instance of an area of interest of the subject. 
     In a preferred embodiment the spectral scan of the subject is performed simultaneously with the spectral scan of the spectral phantom. As such, imaging data of the subject and the phantom are taken at the same time and data from the phantom is therefore much better reflects actual irradiation conditions received by the subject. 
     In a preferred embodiment the spectral imaging method further comprises the steps of quantifying a crosstalk in material decomposition of the imaging data, correcting the obtained image data for the quantified crosstalk; and, optionally, determining at least one system parameter and use said at least one system parameter in the correction of the quantified crosstalk. By correcting for the crosstalk, more reliable images are obtained. 
     In a preferred embodiment the spectral imaging method further comprises the steps of comparing the obtained imaging data of the spectral phantom with reference imaging data, said reference data comprising theoretical data, data measured with other sources, previously measured data of a subject to be imaged, and/or data of the spectral phantom obtained during a previous scan of the spectral phantom, preferably a previous scan shortly after a latest calibration procedure; and providing a recommendation for a special action, for instance a recommendation for performing a new calibration when a difference between the obtained imaging data and the imaging data obtained during or shortly after the latest calibration exceeds a predetermined threshold. A user may decide to recalibrate the imager and thereby improve reliability of subsequently obtained subject image data. 
     In a preferred embodiment the spectral imaging method further comprises the step of reconstructing the imaging data by using the obtained imaging data of the spectral imaging phantom as input for a reconstruction algorithm, preferably an iterative reconstruction algorithm. In this way ‘calibration’ data from the phantom is used in the reconstruction of the image, which helps to improve the reliability of the modeled imaging data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following drawings: 
         FIG. 1A-E  show five schematically depicted embodiments for a spectral calibration phantom according to the present invention. 
         FIG. 2A-E  show various embodiments of a spectral calibration phantom according to the present invention depicted on a subject to be imaged. 
         FIG. 3  shows a flowchart illustrating a spectral imaging method according to the present invention. 
     
    
    
     The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention. To better visualize certain features may be omitted or dimensions may be not be according to scale. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present invention is explained using CT imaging as an example, but the present invention would also be suitable, with modifications a skilled person would know how to implement, for other spectral (x-ray) imaging systems, such as 2D or 3D (e.g. C-arm) spectral x-ray imaging systems. 
     Calibration phantoms for calibrating x-ray imagers, such as CT systems, are known. Phantoms are artificial constructs with precisely known attenuation properties, which are normally scanned (often with a stationary) gantry to determine detector response properties in a controlled, reproducible manner for calibration purposes or detector diagnostics. 
     Known CT phantoms are not optimal for use with spectral CT, since the phantom must simulate the response of the human body for Compton scatter and photo-electric absorption at all x-ray wavelengths used in the scan. This may be achieved by equipping the spectral phantom body  10  with at least two inserts, each selected to have a different ratio between photo-electric effect and compton scattering. This allows for optimizing the spectral information. In a preferred embodiment the at least two inserts are selected to have the same attenuation characteristics, but with a different ratio between photo-electric effect and compton scattering. This allows for distinguishing the spectral information irrespective of the attenuation. 
     The proposed spectral phantom  10  preferably comprises a bulk of a low attenuation material and at least one insert positioned therein. 
     It is preferable to use materials for the bulk and the inserts that mimic attenuation characteristics of one or more parts of a patient&#39;s body, such as organs, bones, or body fluids. The low attenuation material is preferably a water equivalent material, such as Delrin. As such the detector response is determined using a phantom that mimics a patient to be imaged, which therefore more likely results in a scanner that is optimally calibrated for the patient&#39;s body. 
     The inserts may be constructed using frequently used contrast agents, such as iodine or gadolinium, at different concentrations. The inserts may comprise actual contrast agent materials or ‘contrast agent-like’ materials that respond the same or very similar to contrast agents. As a non-limiting example, the spectral phantoms  10  depicted in  FIGS. 1A to 1E  have a first insert CA- 1 - 1  comprising iodine at a first concentration, a second insert CA- 1 - 2  comprising iodine at a second concentration, a third insert CA- 2 - 1  comprising gadolinium at a first concentration and a fourth insert CA- 2 - 2  comprising gadolinium at a second concentration. More or less species of contrast agents and/or concentrations may also be used. 
     Alternatively or additionally, inserts may also be constructed using materials with a different ration of photo-electric absorption and Compton scatter. As a non-limiting example, the spectral phantoms  10  shown in  FIGS. 1A to 1E  comprise six of such inserts with a different ration of photo-electric absorption and Compton scatter. These inserts P-C- 1 , P-C- 2 , P-C- 3 , P-C- 4 , P-C- 5 , P-C- 6  respectively comprise polytetrafluoroethylene (Teflon), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethelene (PE), sodium hydrogen phosphate (Ca 2 HPO 4 ) and potassium hydrogen phosphate (K 2 HPO 4 ). Less, other and more materials may be contemplated as well. 
     Preferably the spectral phantom according to the present invention comprises at least one, but preferably more, contrast agent inserts and at least one, preferably more, photo-electric absorption and Compton scatter inserts. 
     The spectral phantom  10  of the present invention may have any shape that fits in a spectral scanner. The spectral phantom of the present invention may be scanned together with a subject  1 , such as a patient, preferably close to an area of interest  2  of the subject. The shape and the design of the phantom may be adapted to accommodate this. The area of interest may in the context of the invention also be understood be a three-dimensional volume in the subject, such as an organ or a delimited section of the patient. 
     In a preferred embodiment he phantom body and inserts are solid, such as crystalline materials, (plastic) foils, etcetera. Known phantoms often use liquid materials, that may cause leakage resulting in severe damage to the phantom, the imaging system or may harm a patient. 
     In a further embodiment the inserts are permanently fixed in the phantom body. Many known inserts need to be prepared (particularly fluid inserts), which is cumbersome and needs to be done very precise. Using fixed inserts allows for ease of use, since no preparation is necessary before each procedure anymore, which also allows for better reproducibility between procedures. Obviously, the inserts must be chosen such that they are well-tuned for a wide range of subjects and objects of interest. It may be necessary to keep various phantoms in stock with different, permanently fixed inserts. 
       FIGS. 1A and 1B  show a spectral phantom  10  that may be placed under a subject to be scanned. The phantom of  FIG. 1A  is arched, which improves patient comfort, while the phantom of  FIG. 1B  is flat, which is easier to construct. Both these embodiments show an elongated shape, which allows the subject to lie on top of it. More compact solutions are possible as well, which reduces manufacturing cost and storage space requirements, but these may hinder patient comfort. Inserts (P-C, CA) may be positioned within the phantom body  10  in various manners, either grouped together or spaced apart. Preferably the inserts (P-C, CA) are positioned along the main length axis of the phantom, which allows for imaging all inserts in one imaging slice in a CT imager (as shown in the top images of  FIGS. 1A and 1B ). But it is also possible to have the inserts spread across the phantom body or in the width axis (as shown in the bottom images of  FIGS. 1A and 1B ). 
     An advantage of placing the phantom under the subject is that, when scanned together with the subject, a position of the phantom is not influenced by patient movement.  FIGS. 2A and 2B  show a spectral imaging system according to the present invention using the embodiments of respectively  FIGS. 1A and 1B  in an imaging procedure together with a patient. The embodiments of  FIGS. 2A and 2B  show the phantom embedded into the patient support of an imaging device, which allows for a particularly compact design. Furthermore it always has exactly the same position for each subsequent scan. 
       FIGS. 1C, 1D and 1E  show embodiments that are placed on or above a subject to be scanned. An advantage of this is that, when scanned together, no attenuation of radiation within the patient has occurred, so the phantom receives a full emitted dose of radiation. Also, these embodiments may be positioned near an area of interest within the subject. 
       FIG. 1C  shows a spectral phantom  10  with a block-like shape which may be attached on the body of or close of a patient  2  with attachment means  11 , as is shown in  FIG. 2C  for a patient  2  lying on a patient support  3  about to be moved through a gantry  4  of a CT scanner for a scan. The attachment means  11  may comprise clips, tape or any other known attachment means. This embodiment is depicted as a rectangular block, but it may also have different shapes, for instance such as a rectangular, circular or elliptical plate or as an arch or a ring. 
       FIG. 1D  depicts a spectral phantom  10  that is incorporated in a belt, which may be tied around a subject  2  to be scanned, as is shown is  FIG. 2D . The belt may be closed using attachment means  11 , which may take any form, such as belt buckles, buttons, Velcro, etc. An advantage is that there is less chance of the spectral phantom sliding off the patient compared to the embodiment of  FIG. 1C , for instance for particularly mobile subjects, such as children, animals or physically or mentally challenged subjects. Also it is potentially less uncomfortable than the embodiment of  FIG. 1C  when attached directly to the skin of the patient. Also, it may be attached tight to the patient  2 , and therefore potentially closer to the area of interest  2 . 
       FIG. 1E  shows an arch-shaped spectral phantom  10  that may be positioned over the subject  2  to be scanned, as is shown in  FIG. 2E . An advantage of this compact embodiment is that it is always at the same relative distance from the detector and it does not need to be attached to the patient  2  and is therefore not influenced by patient movement. Also, it receives direct unhindered emitted radiation without radiation attenuated by the subject. 
     In all described embodiments the spectral phantom  10  and/or subject  1  may be optimally placed, such that the data obtained from scanning the phantom  10  corresponds spatially with the area of interest  2 . 
       FIG. 3  depicts a flow chart of a spectral scanning method according to the present invention. In step  101  a phantom according to the present invention is placed in a CT device, to be scanned in spectral scanning step  104  to obtain spectral imaging data of the spectral phantom. In context of the present invention the phrases ‘placed in a device’ or inserted in an device&#39; may be interpreted as being placed inside an examination region of the gantry, but also includes being placed on, in or over a support of the imaging device outside the examination region. During scanning  104  the phantom may be stationary within the gantry or may be moved translated through the gantry, while the gantry may also be stationary or it may be rotating. 
     It is particularly advantageous to scan  104  the phantom together with the subject to be scanned, preferably close to the area of interest of the subject. To achieve this the subject to be scanned is also placed  102  in the CT device and the spectral phantom is positioned  103  on or near the subject, preferably near the area of interest. This includes placing the phantom on a desired position, but also includes positioning the patient relative to a (stationary) phantom. The term ‘near the area of interest’ in context of the present invention means close to the area of interest, preferably as close to the area of interest as physically possible or comfortable, while also not hindering the scan of the area of interest. This may in some case mean next to, under, on or over the area of interest. By scanning the phantom and the subject together the imaging data for the spectral phantom and the further spectral imaging data of the subject is temporarily and spatially optimally aligned. 
     After the scanning step  104  the spectral imaging data of the phantom may be used to determine detector response for a potential calibration or diagnostics step  108 , as is the common use for calibration phantoms. 
     However, in the present invention the phantom is not a calibration phantom in the traditional sense, but an imaging phantom in which imaging data is specifically used  105 ,  106 ,  107  to improve further image data, for instance of a subject to be scanned simultaneously with or subsequent to the scan of the spectral phantom. The further image data may be improved by the spectral imaging data of the phantom by using  105  the spectral imaging data as input for data correction of the further imaging data, or by using  106  the spectral imaging data of the spectral phantom to generate a recommendation to a user regarding a scan to generate the further image data, or by using  107  the spectral imaging data of the phantom as additional input for image reconstruction of the further spectral imaging data. 
     Using  105 ,  106 ,  107  the spectral imaging data may for instance be done in various manners, for instance by using systematic cross-talk or bias that is inherent in any material decomposition method for spectral CT imaging. Systematic errors in the material decomposition of spectral CT data manifests in image artefacts like rings or bands and in cross-talk between the material images. These artefacts can also arise or change during scanning 
     The systematic cross-talk inherent in the material decomposition of the spectral imaging data of the phantom may be quantified  105 - 1  and dedicated correction algorithms may then be applied as an input parameter to correct  105 - 2  for the quantified cross-talk in the further imaging data. Especially if the imaging data of the phantom and the further imaging data were acquired simultaneously, a relevant and accurate correction of the further imaging data may be achieved. Additionally changes of some system parameters may also be detected and appropriate input parameter(s) for the correction algorithm may be adapted to achieve even further optimized further imaging data. 
     The decomposed material images of the spectral CT phantom of the current scan may be compared  106 - 1  to images acquired right after the last spectral calibration. In case the differences between the images or the errors in the latest images are too large (measured in appropriate metrics), a special action may be recommended  106 - 2 , for instance a suggestion to perform a new energy calibration, switching the high voltage on an off or performing a material calibration or other spectral calibrations. As such, the CT device is optimized for scanning the subject and improved further image data of a subject may be obtained. 
     Alternatively or additionally, the residual amount of cross-talk or absolute errors of the CT values in the final material images can be assessed by comparing  106 - 1  the outcome of the spectral CT phantom with the ground truth and providing the results  106 - 2 , e.g. by displaying them, for supporting the viewer in reading the images and potentially to prompt the viewer to take an action to obtain improved further image data. 
     The decomposed material images of spectral CT phantom may also be used  107  during iterative reconstruction to tune the parameter for each slice. For reference, the standard iterative multi-channel reconstruction may be denoted by equation (1): 
     
       
         
           
             
               
                 
                   
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     In equation (1) i indexes all projections, A i  is the forward projection operator for projection angle i; p, S, and k are the photoelectric, the scatter, and the k-edge material image, respectively, and m i,p , m i,S  and m i,k  are the decomposed, measured line integrals for projection direction i. The matrix C i  is the noise covariance matrix of the projections m i,p , m i,S , and m i,k . Finally, R is a regularization term weighted with a regularization parameter λ(lambda) that enforces some additional constraints, e.g. smoothness, in order to stabilize the solution. It was empirically found that the crosstalk can be reduced by a heuristic linear model, such as shown in equation (2): 
                     (             m   ~       i   ,   p                   m   ~       i   ,   S                   m   ~       i   ,   k             )     =       T   i     ⁡     (           m     i   ,   p                 m     i   ,   S                 m     i   ,   k             )               (   2   )               
For instance, if the linearly transformed measurements are used for reconstruction. The spectral “instability” of the system is reflected in the fact that the transformation matrix T depends on the projection index. Nevertheless, T is close to the unity matrix. Now with the proposed phantom being inside the field of view, it becomes possible to estimate the elements of T during iterative reconstruction by optimizing the cost function also with respect to the free parameters of T i , as is shown in equation (3):
 
     
       
         
           
             
               
                 
                   
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     An additional regularization term R 2  is added in the cost function (which takes as argument all T i  matrices) in order to include prior knowledge about T. Possible prior knowledge to be included may contain temporal smoothness, row-wise normalization symmetry, distance to the unity matrix, and alike. The last term that is added in the cost function is prior knowledge about the phantom. This is included in form of a mask operator M, which masks the location of the phantom, and in the form of the a-priori known values Q p , Q S , and Q k , of the photo-electric effect, the Compton scattering, and the k-edge material within the phantom respectively. 
     Other known reconstruction algorithms may be similarly adapted used to obtain improved images from the further image data. Computer program products may execute the algorithms. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. For instance, the spectral imaging phantom and spectral imaging method were explained using spectral CT imaging, but a skilled person would know how to adapt the invention for use in other spectral imaging methods, e.g. 2D or 3D spectral x-ray imaging. 
     Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims and description. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.