Patent Grant
11246559
The project leading to this application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 830294.
The proposed technology generally relates to x-ray imaging, and more particularly to a calibration of x-ray imaging systems, including calibration phantoms and corresponding calibration procedures.
There is a general need for improved calibration of x-ray imaging systems to enable enhanced image reconstruction.
For example, there is a need for calibration of so-called Photon-Counting Spectral Computed Tomography (PCSCT) systems to be able to perform accurate material basis decomposition in the projection domain. PCSCT systems concern CT systems based on spectral (i.e. energy-discriminating) information from a photon-counting x-ray detector. This means that not only does the system respond to the counted number of individual photons, but also the energy of these individual photons is taken into account.
In this context, calibration is normally understood as a mapping between different x-ray spectra falling on a detector and the corresponding detector output. The, at least intermediate, output of a PCSCT system includes x-ray counts allocated in energy bins, typically stored as values in an n-dimensional vector (in a later processing stage these counts could be processed with a log-function). The photon-counting x-ray detector in such a system is often referred to as a photon-counting multi-bin detector. Assuming the x-ray anode is fed by constant voltage and current, and the beam filter is never changed, the difference in spectrum observed by the detector depends on what materials and thicknesses have been in the x-ray path. For example, for a 5-bin system, an example calibration mapping between some varying thicknesses of polyethylene (PE) and poly-vinyl-chloride (PVC) might look like this:
Material basis decomposition in the projection domain is the process to determine the equivalent pathlength in centimetres of some reference materials, typically two or three, that the x-ray photons of an exposure have passed through. This is typically achieved by some type of inversion of the calibration data or a model of the calibration data. Note that during calibration, several exposures of a stationary phantom can be acquired. The result will be close to noise-less data. When applied to individual projections, for example when constructing an image of a dose sensitive human patient, the limited dose and resulting Poisson noise will make the inversion problem more difficult.
Practical Considerations
There are two major practical concerns when constructing a calibration phantom for use in an x-ray imaging system such as a clinical PCSCT system. The first is that a large-enough part of the material-space has to be sampled (all combinations of PE and PVC above, or some other combination of clinically relevant materials) for each of the detector elements of the system. The second is that this must be possible to perform in practice.
A typical conventional approach to sampling a large-enough part of the material space for prototype systems is to use a step-wedge phantom with different pathlength combinations as illustrated schematically in
The simple concept of planar step wedge phantoms is not easily extended to multi-slice systems with large z-coverage for a multitude of reasons. Here we will briefly mention three: 1) size, affecting handling and manufacturing cost, 2) the required accuracy of placement, and 3) scatter profiles.
To avoid partial volume effects as the phantom is stepped in the z-direction, the entire extent of the beam hitting a detector must lie within a single material combination. For wide coverage detectors, with 8 cm coverage in the iso-center and a typical source-to-iso distance of 50 cm, the cone angles τ range from −0.08 to 0.08 radians. Even if a margin for the placement of the phantom is not considered, which could also result in partial volume measurement and thus corrupt calibration data, the length of the step would have to be at least the detector element size+L2×tan τmax. For L2=2 cm and a pixel width in the z-direction of 0.5 mm in iso-center, this indicates that the step size in a step wedge phantom has to be at least 0.21 cm. Adding a small margin yields 0.25 cm per step. Even for a small 3-material phantom with 6 different pathlengths of each material the extent of the step wedge in the z-direction quickly becomes very large, 6*6*6*0.25 cm=54 cm. Since one ideally wants to sample even more material combinations, it is clear that the step wedge approach quickly becomes unattractive from a weight and cost perspective.
A step wedge furthermore places strict requirements on positioning of the phantom. If the phantom is allowed to be somewhat tilted in the z-direction, added margin and larger step sizes are needed to avoid partial volume effects (as the tilt angle would add to the cone angle in the tan τ-factor.
Finally, the strong non-homogeneity of the phantom in the z-direction would result in an unequal amount of scattered radiation hitting each detector element from the positive and negative z-directions. This would not be representative of imaging a real object.
It should be understood that there is a lot of general prior art on phantoms for CT, but most of them are focused on either Quality Control (QC), geometric calibration or the optimization of beam hardening correction methods. There is also a group of phantoms intended for use in the system simultaneously with the patient, for example U.S. Pat. No. 9,420,983B1, intended to improve tissue characterization by comparison with real-time data from known materials.
None of these are designed to generate a mapping between different x-ray spectra falling on a detector and e.g. the corresponding x-ray counts allocated in energy bins for subsequent use in material basis decomposition.
Just for a more complete understanding, QC testing of CT scanners typically includes measurement of CT numbers in a reconstructed image of a CT phantom using a standardized protocol. CT number values are normally expressed in terms of Hounsfield Units (HU), and they are clinically relevant in determining the composition of various tissues in the body. Effective quality control requires that tolerance ranges of CT/HU values are defined: a measured value outside the range indicates the need for further investigation and possible recalibration of the scanner. Normally, the measurement results are used to define manufacturer- and kVp-specific tolerance ranges for the CT numbers of each material in the phantom. Quality control phantoms are supplied to the users by the vendors and by independent third-party suppliers. Other uses of QC phantoms include ensuring that the geometry of acquisition system is correctly determined (otherwise artefact will occur) as for example WO 2016/168292A1.
Calibration phantoms for the determination of detector-element specific beam hardening or scatter correction schemes include EP1475039B1, U.S. Pat. No. 7,056,018B2 and U.S. Pat. No. 8,121,250B2. A common feature of these technical solutions is that the phantom is either large enough to cover the entire radiation field or is shifted around at multiple positions to cover a multitude of different pathlength. Typically, the phantoms consist of a single piece of a homogenous material (with attenuation properties similar to water), but solutions with different materials and multiple pieces have been proposed (U.S. Pat. No. 8,121,250B2). Also, typically the placement of the phantom needs to be very precise, although solutions where placement insensitivity is achieved by using the x-ray data to determine the actual location have been proposed (U.S. Pat. No. 8,121,250B2).
However, none of the methods or phantoms are optimized for generating an optimal mapping between material pathlengths and attenuation data necessary for photon counting system designed to perform material basis decomposition in the projection domain. For example, U.S. Pat. No. 8,121,250B2 focuses on generating calibration corrections for beam hardening and scatter. None of the mentioned patent references focus specifically on phantoms or methods specifically tailored to the need of photon counting x-ray imaging systems, e.g. intended to perform material basis decomposition in the projection domain.
There is thus a general need for improved calibration phantoms and procedures for x-ray imaging systems such as photon-counting spectral x-ray imaging systems.
It is a general object to provide an improved calibration phantom for x-ray imaging systems.
It is also an object to provide an improved calibration procedure for x-ray imaging systems.
It is a specific object to provide a calibration phantom for an x-ray imaging system.
It is another object to provide a method for calibration of an x-ray imaging system.
These and other objects may be achieved by one or more embodiments of the proposed technology.
According to a first aspect, there is provided a calibration phantom for an x-ray imaging system having an x-ray source and an x-ray detector. The calibration phantom comprises a combination of geometric objects of at least three different types and/or compositions including:
According to a second aspect, there is provided a method for calibration of an x-ray imaging system having an x-ray source and an x-ray detector. The method comprises:
In this way, it is possible to calibrate an x-ray imaging system such as a photon-counting spectral CT system to allow it to perform improved and/or artefact-free material basis decomposition and/or image reconstruction.
The present invention addresses at least some of the issues with the prior art solutions by a unique phantom that is not highly sensitive to erroneous placement. The novel phantom does not have to change shape in the z-direction, thereby minimizing any potential bias introduced by uneven scattered radiation.
The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:
A basic idea is to provide a calibration phantom for an x-ray imaging system having an x-ray source and an x-ray detector, where the calibration phantom comprises a combination of geometric objects of at least three different types and/or compositions including:
It should be understood that the term “periphery” normally refers to and/or corresponds to the outside boundary, outside parts and/or outside surface of an object.
By way of example, at least a subset of the third objects may be arranged around the periphery of the first object in-between at least a subset of the second objects.
It should though be understood that the third objects or a subset thereof may be arranged around the periphery of at least a subset of the second objects, without being in contact with the first object, e.g. placed on the outer bound of the overall structure and not placed in-between the first object and the second objects.
It should also be understood that the objects do not have to be cylinders or rods, as illustrated in the particular examples of
By way of example, the first object, the second objects and the third objects may include at least one of cylinders, cuboids, and prisms (see also
In a particular example, as illustrated in
It should be understood that the term “cylinder” includes a circular cylinder, elliptical cylinder and/or any other type of cylinder.
For example, at least a subset of the smaller cylinders 3 may be arranged around the periphery of the middle cylinder 1 in spacings in-between at least a subset of the medium-sized cylinders 2, e.g. as schematically illustrated in
Preferably, the middle cylinder 1 has a larger diameter than the medium-sized cylinders 2, and the medium-sized cylinders 2 have a larger diameter than the smaller cylinders 3.
As an example, the plurality of second objects may all be of the same second material.
Alternatively, the plurality of second objects may include at least two types of second objects, of different materials and/or different sizes.
For example, a first subset of the second objects may be made of the second different material and a second subset of the second objects may be made of a different material such as the first material, e.g. as schematically illustrated in
It is also possible to select the size and number of the second objects so as to achieve a snug fit of the second objects around the periphery of the first object.
When it comes to material choices, the first material may be selected to mimic soft human tissue, and the second material may have a higher attenuation than the first material.
In a particular example, the first material mimics soft human tissue, the second material mimics bone, and the third material mimics a contrast agent.
As an example, the first material may comprise polyethylene (PE), and the second material may comprise poly-vinyl-chloride (PVC) or other plastic or resin.
Optionally, the calibration phantom is intended for use in a Computed Tomography (CT) system with a photon-counting multi-bin x-ray detector to enable calibration for material basis decomposition.
In a photon-counting multi-bin detector, each registered photon generates a current pulse which is compared to a set of thresholds, thereby counting the number of photons incident in each of a number of energy bins.
By way of example, the calibration phantom may be intended for use in the CT system to enable calibration for accurate material basis decomposition based on mapping between i) path length determinations through each of the first, second and third materials for each of a number of rotation angles of the CT system and each of a number of detector elements of the x-ray detector and ii) corresponding detector responses of the photon-counting multi-bin x-ray detector.
In this context, the x-ray source and the x-ray detector may be arranged on a support that is able to rotate around a subject or object to enable x-ray exposure at a set of projections at different view or rotation angles.
For a better understanding, the proposed technology will now be described in more detail with reference to non-limiting examples.
Reference can once again be made to
By way of example, the calibration phantom 5 may be constant in the scan direction, meaning all slices of the system see the same attenuation profile, with the exception of a possible correction based on the cone angle of the slice.
The reason for placing the medium-sized cylinders 2 and small-sized cylinders 3 at the periphery or outer bound of the phantom is to obtain a larger sample space of the material combinations.
If small rods with different materials are inserted in the interior part of the phantom, for instance following US20120155617A1, pathlengths of the material constituting the small rods (typically mimicking iodine) will only be seen in combination with relatively long pathlengths of the surrounding material. This is not beneficial from a basis decomposition or image reconstruction point of view.
For material basis decomposition in the projection domain, the inventors have recognized that it is beneficial to obtain more variable attenuation data (for instance combinations of iodine will very little PE or PVC). Therefore, the inventors proposed the novel, unique design of the calibration phantom as claimed herein.
During calibration, the x-ray source and detector may rotate like during a regular scan, possibly with more revolutions at each location to gather more statistics. Each detector element will see different pathlength combinations of the first, second and/or third materials at different view or rotation angels.
Benefits of the Novel Phantom Design May Include One or More of the Following:
If the phantom 5 is smaller than the full field of view, edge detectors will only see air should the phantom be placed only in the iso-center, as schematically illustrated in
In the example of
It is clear that a smaller but similar phantom can be placed on top of the first one (or used independently) to more accurately sample the material space for combinations that occur in imaging tasks of smaller objects (infants, heads etc) as schematically illustrated in
As a final example we show how the space of material pathlengths may be sampled using the phantom. For a central cylinder of 25 cm in cm, having 20 medium and small sized cylinders placed around it, and moved imaged at 11 different positions (in iso-center and 10 steps out to a maximal offset of 20 cm), the resulting pathlength combinations are shown in
In the particular example of
Some of the Main Benefits/Features of the Invention May Include One or More of the Following:
In a particular example, a calibration procedure based on the novel calibration phantom may be using the following features and actions:
In a particular example, the detector response may at least partly be represented by the projection data, such as counts in energy bins, also referred to as photon count information.
Optionally, the step of determining path lengths may involve generating an image of the phantom or at least part thereof, at least partly based on acquired projection data, such as photon count information, and determining the path lengths from the generated image. Ring artefacts may be present in such an image, and although these artefacts are normally not tolerated in a clinical image of a patient, they may be acceptable for the purpose of determining geometric objects of the phantom from the image and determining path lengths for each of the considered materials of the phantom.
In the following, a more detailed step-by-step description of a non-limiting example of a calibration procedure will be given:
For completeness, it may be useful to provide a brief overview of an illustrative example of an overall x-ray imaging system, with reference to
In this non-limiting example, the x-ray imaging system 100 basically comprises an x-ray source 10, an x-ray detector system 20 and an associated image processing device 30.
The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the x-ray detector system. Since some materials absorb a larger fraction of the x-rays than others, an image may be formed of the subject or object.
In general, the x-ray detector system 20 is configured for registering radiation from the x-ray source 10 that may have been focused by optional x-ray optics and passed an object or subject or part thereof. The x-ray detector system 20 is connectable to the image processing device 30 via suitable analog processing and read-out electronics (which may be integrated in the x-ray detector system 20) to enable image processing, such as basis material decomposition and/or image reconstruction by the image processing device 30.
An example of a commonly used x-ray imaging system is a Computed Tomography (CT) system, which may include an x-ray source that produces a fan or cone beam of x-rays and an opposing x-ray detector system for registering the fraction of x-rays that are transmitted through a patient or object. The x-ray source and detector system are normally mounted in a gantry that rotates around the imaged object.
Accordingly, the x-ray source 10 and the x-ray detector system 20 illustrated in
In this example, the x-ray detector system 20 is a photon-counting multi-bin detector, and the image processing device 30 may receive photon count information from the x-ray detector 20 as input for basis material decomposition and/or image reconstruction as described herein.
In this example, the x-ray source 10 and x-ray detector system 20 are mounted in a gantry 12 that rotates with respect to an iso-center 15.
In this non-limiting example, the various controllers include an x-ray controller 31 for controlling the x-ray source, e.g. for switching it on and off, and for controlling the mode of operation such as kV-switched mode. The system 100 also includes a gantry controller 32 and a table controller 33, e.g. for controlling the movements and rotation of the gantry and the table, respectively. There is also a detector controller 34 for controlling the operations of the photon-counting multi-bin detector 20 including read-out of photon count information and other possible detector output.
In this embodiment, also, the x-ray detector system 20 is connectable to the image processing device 30 via suitable analog processing and read-out electronics and analog and/or digital data paths to enable image processing, basis material decomposition and/or image reconstruction by the image processing device 30.
The system 100 may also include an operator console 35 with an associated display for allowing an operator to interact with the system.
It will be appreciated that the methods and devices described herein can be combined and re-arranged in a variety of ways.
For example, specific functions may be implemented in hardware, or in software for execution by suitable processing circuitry, or a combination thereof.
The steps, functions, procedures, modules and/or blocks described herein may be implemented in hardware using any conventional technology, such as semiconductor technology, discrete circuit or integrated circuit technology, including both general-purpose electronic circuitry and application-specific circuitry.
Particular examples include one or more suitably configured digital signal processors and other known electronic circuits, e.g. discrete logic gates interconnected to perform a specialized function, or Application Specific Integrated Circuits (ASICs).
Alternatively, at least some of the steps, functions, procedures, modules and/or blocks described herein may be implemented in software such as a computer program for execution by suitable processing circuitry such as one or more processors or processing units.
Examples of processing circuitry includes, but is not limited to, one or more microprocessors, one or more Digital Signal Processors (DSPs), one or more Central Processing Units (CPUs), video acceleration hardware, and/or any suitable programmable logic circuitry such as one or more Field Programmable Gate Arrays (FPGAs), or one or more Programmable Logic Controllers (PLCs).
It should also be understood that it may be possible to re-use the general processing capabilities of any conventional device or unit in which the proposed technology is implemented. It may also be possible to re-use existing software, e.g. by reprogramming of the existing software or by adding new software components.
Basically, the method comprises:
S1: placing a calibration phantom of any of the claims 1 to 16 in the beam path of the x-ray imaging system;
S2: turning on the x-ray source and beginning a calibration sequence based on controlled movement of the calibration phantom;
S3: acquiring projection data for a set of projections based on the output of the x-ray detector;
S4: determining path lengths through the different materials of the calibration phantom for each of the projections, at least partly based on acquired projection data; and
S5: generating a mapping between the path lengths and detector response of the x-ray detector.
For example, the mapping may be used for calibrated image reconstruction.
By way of example, the determining step S4 comprises determining the path lengths through each of the first, second and third material of the calibration phantom for each of a number of rotation angles and each of a number of detector elements of the x-ray detector.
In a particular example, the x-ray detector is a photon-counting multi-bin x-ray detector, and the generating step S5 comprises determining a detector-element specific mapping of path lengths of the different materials to corresponding registered photon counts of the photon-counting multi-bin x-ray detector.
By way of example, the detector response may at least partly be represented by the projection data.
For example, the projection data may include photon count information.
Optionally, the step of determining path lengths may include generating an image of the phantom or at least part thereof at least partly based on acquired projection data, and determining the path lengths from the generated image, e.g. as previously exemplified.
As an example, the x-ray imaging system may be a Photon-Counting Spectral Computed Tomography (PCSCT) system with a photon-counting multi-bin x-ray detector.
In this particular example, the system 200 comprises a processor 210 and a memory 220, the memory comprising instructions executable by the processor, whereby the processor is operative to perform computer-implementable steps and/or actions described herein. The instructions are typically organized as a computer program 225; 235, which may be preconfigured in the memory 220 or downloaded from an external memory device 230. Optionally, the system 200 comprises an input/output interface 240 that may be interconnected to the processor(s) 210 and/or the memory 220 to enable input and/or output of relevant data such as input parameter(s) and/or resulting output parameter(s).
The term ‘processor’ should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform a particular processing, determining or computing task.
The processing circuitry including one or more processors is thus configured to perform, when executing the computer program, well-defined processing tasks such as those described herein.
The processing circuitry does not have to be dedicated to only execute the above-described steps, functions, procedure and/or blocks, but may also execute other tasks.
The proposed technology also provides a computer-program product comprising a computer-readable medium 220; 230 having stored thereon such a computer program.
By way of example, the software or computer program 225; 235 may be realized as a computer program product, which is normally carried or stored on a computer-readable medium 220; 230, in particular a non-volatile medium. The computer-readable medium may include one or more removable or non-removable memory devices including, but not limited to a Read-Only Memory (ROM), a Random Access Memory (RAM), a Compact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray disc, a Universal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storage device, a flash memory, a magnetic tape, or any other conventional memory device. The computer program may thus be loaded into the operating memory of a computer or equivalent processing device for execution by the processing circuitry thereof.
Method flows or relevant parts thereof may be regarded as computer action flows, when performed by one or more processors. A corresponding device, system and/or apparatus may be defined as a group of function modules, where each step performed by the processor corresponds to a function module. In this case, the function modules are implemented as a computer program running on the processor. Hence, the device, system and/or apparatus may alternatively be defined as a group of function modules, where the function modules are implemented as a computer program running on at least one processor.
The computer program residing in memory may thus be organized as appropriate function modules configured to perform, when executed by the processor, at least part of the steps and/or tasks described herein.
Alternatively, it is possibly to realize the modules predominantly by hardware modules, or alternatively by hardware. The extent of software versus hardware is purely implementation selection.
The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.
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