Method and system for correcting cone beam scanning CT intensity attenuation

Abstract

A method and system for correcting cone beam scanning CT intensity attenuation. The method includes determining an intensity compensation for the selected scan configuration and applying the intensity compensation to the reconstructed volume of interest to produce an intensity compensated volume for scaling the volume of interest.

Description

[0001] Related applications

[0002] This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 63 / 551,730, filed on February 9, 2024, the entire contents of which are incorporated herein by reference. Background Art

[0003] X-ray microtomography systems provide high-resolution, non-destructive imaging of a sample's internal structure. These systems are used in a variety of industrial and research applications, such as mining, manufacturing, materials science, clinical research, and failure analysis. These systems provide the ability to visualize features within a sample without cutting or slicing the sample.

[0004] The X-ray microtomography system includes an X-ray projection system that generates sample projection data and a computer system that reconstructs a tomographic imaging volume of the sample based on the projection data. In operation, the X-ray projection system scans the sample at different angles to generate projection data. During the scanning process, X-rays are directed toward the sample and are absorbed or scattered by the sample as the X-rays pass through the sample. The X-rays that are not absorbed or scattered are transmitted through the sample and modulated by the sample. The detector system receives the transmitted X-rays and creates an image representation of the received X-rays in units of pixels. A series of X-ray projections at different angles generated by the scan form the projection data of the sample. The computer system then accesses the projection data and applies a tomographic reconstruction algorithm to the projection data to reconstruct a volumetric data set of the sample. The volumetric data set is a three-dimensional (3D) representation of the entire sample, and a slice is a two-dimensional (2D) cross-sectional image of the sample based on the volumetric data set.

[0005] A circular scanning configuration / acquisition geometry is commonly used. In this arrangement, the specimen is typically rotated about a vertical axis, and a series of projections are acquired at different angles. The specimen's height is then varied, resulting in a new set of projections. This has the advantage of mechanical simplicity. However, there is a data shortage issue. This scanning configuration will not acquire enough raw data to accurately recover the attenuation coefficients of the 3D specimen object. A noticeable image artifact is the attenuation of intensity along the longitudinal direction in high cone beam regions. Therefore, it exhibits image artifacts related to axial variations in the cone beam and the object's anatomy. Therefore, when more accurate images without cone beam artifacts are required, non-circular scanning geometries such as a circle plus a line, two circles, a spiral, or a circular sinusoid are often employed, despite the increased mechanical complexity.

[0006] The most commonly used reconstruction algorithms belong to a class of reconstruction techniques known as analytical reconstruction. Their goal is to find a closed-form solution to the problem of reconstructing the internal structure of an object from its projections. The most common analytical method is filtered back projection (FBP). The projections are first processed in the frequency or spatial domain using a high-pass filter (typically a ramp filter). Each filtered projection is then "smeared" back onto the imaging plane, as if each data point were uniformly projected back in the shape of the original beam. These back projections for all angles are superimposed to produce a reconstructed volume that approximates the internal structure of the object. The filtering and back projection operations together contribute to a more accurate and less blurry reconstruction of the original object. One type of filtered back projection is FDK, which is often used with X-ray microtomography systems to reduce artifacts associated with typical cone beams. See Feldkamp, LA, Davis, LC, and Kress, JW (1984) Practical Cone-Beam Algorithm. Journal of the Optical Society of America A, 1, 612-619. Summary of the Invention

[0007] The present invention relates to a method and system for correcting intensity attenuation image artifacts caused by different trajectories in a cone-beam CT scanner.

[0008] Cone-beam circular trajectories are very popular in medical and industrial CT scanning due to their convenience and simple acquisition geometry.

[0009] The present invention can also be applied to other scanning trajectories, such as circle plus line, sine wave, variable magnification (elliptical trajectory), two circles. In this article, the present invention can be used to process image artifacts caused by insufficient raw data.

[0010] In a preferred embodiment, a coefficient map is created for intensity compensation. This coefficient map is inverted and then multiplied by the reconstructed volume of interest. This increases the volume of the undersampled portion.

[0011] In general, according to one aspect, the invention features a method for intensity correction of cone-beam CT. The method includes determining an intensity compensation for a selected scan configuration by performing a forward projection using the selected scan configuration, performing reconstruction of the forward projection data and generating the intensity compensation using the reconstruction data, and applying the intensity compensation to a reconstructed volume of interest to generate an intensity-compensated volume.

[0012] In this embodiment, the preferred scanning configuration is a circular scanning configuration, but other suitable scanning configurations (circular and linear, elliptical, sinusoidal, double circular, etc.) can also be selected. For example, the orthographic projection of the instantiated volume, which fills each voxel with a constant, should have the same dimensions and voxel size as the reconstructed volume of interest. A reconstruction of the orthographic projection data is then performed, and the intensity compensation is generated using the reconstructed data.

[0013] In the example shown, the voxel-by-voxel intensity compensation is generated by inverting the reconstructed forward projection data. The reconstruction of the reconstructed volume of interest and the forward projection data is performed using FDK.

[0014] In general, according to another aspect, the invention features an x-ray tomography system comprising an x-ray system including an x-ray source system for generating an x-ray beam and a detector system for detecting the x-ray beam after transmission through a sample to generate projection data, a computer system for determining an intensity compensation for a selected scan configuration by performing a forward projection using the selected scan configuration, performing reconstruction of the forward projection data and generating the intensity compensation using the reconstruction data, and applying the intensity compensation to a reconstructed volume of interest to generate an intensity-compensated volume.

[0015] The above and other features of the present invention, including various novel construction details and combinations of parts, as well as other advantages, will now be described in more detail with reference to the accompanying drawings, and pointed out in the claims. It should be understood that the specific methods and devices embodying the present invention are shown by way of illustration and not as limitations of the present invention. The principles and features of the present invention may be employed in various and numerous embodiments without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the accompanying drawings, reference characters denote the same parts throughout the different views. The drawings are not necessarily drawn to scale; emphasis is instead placed upon illustrating the principles of the invention. In the drawings:

[0017] Figure 1 is a schematic diagram of an X-ray microtomography system to which the method of the present invention is applicable;

[0018] Figure 2A and Figure 2B is a slice along the longitudinal direction of the water phantom and a graph showing the change of gray value along the AA line with distance, showing the intensity decay;

[0019] Figure 3 A flow chart showing a method based on FDK (Filtered Back Projection Reconstruction) for correcting intensity attenuation image artifacts caused by the circular trajectory of a cone-beam CT scanner;

[0020] Figure 4A and Figure 4B1 is a slice along the longitudinal direction of the water phantom and a graph showing the grayscale value versus distance plotted along line AA showing the reduced intensity attenuation associated with the present invention. DETAILED DESCRIPTION

[0021] The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments described herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

[0022] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Furthermore, unless expressly stated otherwise, the singular and the articles "a," "an," and "the" also include the plural. It should be further understood that the terms: includes, comprises, including, and / or comprising, when used in this specification, specify the presence of the features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof. Furthermore, it should be understood that when an element comprising a component or subsystem is referred to as and / or shown as being connected or coupled to another element, it may be directly connected or coupled to the other element, or there may be intervening elements.

[0023] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It should also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0024] Figure 1 FIG. 1 is a schematic diagram of an X-ray microtomography system 100 to which the method and workflow of the present invention are applicable.

[0025] Generally, the X-ray microtomography system 100 combines an X-ray microscope system 101 and a computer system 200 to receive projections and calculate a volumetric dataset from these projections.

[0026] The X-ray microscope system includes an X-ray source system 102 that generates an X-ray beam 104 and a rotating stage 110 having a sample holder 112 to hold and rotate a sample 114 in the X-ray beam 104 from the X-ray source system 102. Images, or X-ray projections, are captured by a detector system 118. The X-ray source system 102, the rotating stage 110, and the detector system 118 are mounted to a base 108 of the X-ray CT system 100. A computer system 200 typically receives and processes these projections and provides overall control of the system 100. The computer system 200 or another computer typically uses the X-ray projections to perform tomographic reconstructions to create a volumetric dataset.

[0027] In one example, the X-ray source 102 is a laboratory X-ray source because of their ubiquity and relatively low cost. Such sources typically produce a cone-beam, polychromatic X-ray beam.

[0028] Common laboratory X-ray sources include X-ray tubes, in which electrons are accelerated in a vacuum by an electric field and fired into a target piece of metal, emitting X-rays as the electrons decelerate in the metal.

[0029] In one example, X-ray source 102 is a microfocus source with a tungsten target. Target materials including molybdenum, gold, platinum, silver, or copper may also be used. Preferably, a transmission target configuration is used for X-ray source 102, in which the electron beam strikes thin target 103 from the back side. X-rays emitted from the other side of target 103 are then used as beam 104.

[0030] When the sample 114 is exposed to the X-ray beam 104 , the X-ray photons transmitted through the sample form an attenuated X-ray beam 106 that is received by a detector system 118 .

[0031] In the most common configuration of the detector system 118, a magnified projected image of the sample 114 is formed on the detector system 118 at a geometric magnification that is equal to the inverse ratio of the source-to-sample distance and the source-to-detector distance. Typically, the geometric magnification provided by an X-ray table is between 2x and 100x, or higher. In this case, the resolution of the X-ray image is limited by the focal spot size, or virtual size, of the X-ray source system 102.

[0032] To achieve high resolution, embodiments of the X-ray microtomography system 100 also utilize an extremely high resolution detector 124-1 of the detector system 118 and position the sample 114 near the X-ray source system 102. In embodiments of the high resolution detector 124-1, a scintillator is used in conjunction with a microscope objective to provide additional magnification in the range of 2x to 100x or greater.

[0033] Other possible detectors may be included in the illustrated X-ray CT system 100 as part of the detector system 118. For example, the detector system 118 may include a lower resolution detector 124-2. In an example, this may be a flat panel detector or a detector with a lower magnification microscope objective. Configurations of the detector system 118 with one, two, or even more detectors 124 are possible.

[0034] Typically, two or more detectors 124 - 1 , 124 - 2 are mounted on a rotating turret 122 of the detector system 118 so that they can be alternately rotated into the path of the attenuated beam 106 from the sample 114 .

[0035] Typically, based on operator-defined parameters, the controller 210 of the computer system 200 instructs the rotation stage 110, via the control interface 130, to position the sample 114 and the detector 124 for the desired CT setup. Upon completion, the controller 210 rotates the sample 114 relative to the beam 104 to perform a CT scan of the sample 114 and saves the projection data 262 to the data store 260. The scan configuration commanded by the controller 210 to the rotation stage can be circular or another scan configuration, such as a circle and line, an ellipse, a sine wave, a double circle, etc.

[0036] In one example, the computer system 200 includes a graphics or other accelerated processor 220 that analyzes the X-ray projections and may perform the calculations required to create tomographic reconstructions based on the X-ray projections. A display device 240 connected to the computer system 200 displays information from the X-ray CT system 100, typically including projection data, volume data, and slices. An input device 250, such as a touch screen, keyboard, and / or computer mouse, enables interaction between an operator, the computer system 200, and the display device 240.

[0037] In one example, an operator defines / selects a CT scan or calibration via a user interface application executed on the computer system 200 and displaying its interface on the display device 240. These include settings for the X-ray acceleration voltage, and settings for the X-ray energy spectrum that define the scan and exposure time on the X-ray source system 102. The operator also typically selects other settings, such as the number of X-ray projection images to create for the sample 114, the angle to rotate the rotation stage 110, including the scan configuration (i.e., circle, circle and line, ellipse, sine wave, double circle, or other).

[0038] Computer system 200, with the aid of its image processor 220, receives image or projection information associated with each rotation angle of sample 114 from detector system 118. Typically, image processor 220 combines the projection data using a reconstruction algorithm to create a 3D tomographic reconstructed volume of information (volume dataset) of the sample.

[0039] The computer system 200 will typically execute a reconstruction application 254, such as one that implements the FDK algorithm, for reconstructing projection data 262. A forward projection program 256 is also provided. Both the reconstruction application 254 and the forward projection program 256 are stored in a non-transitory storage medium, such as an electrical or magnetic storage medium.

[0040] The reconstruction application 254 runs on top of an operating system 252. The operating system 252 is in turn executed by a central processing unit (CPU) 250 of the computer.

[0041] Figure 2A and Figure 2B Basic issues of circular scanning configurations and similar configurations are explained.

[0042] Figure 2A This is a longitudinal slice through a water phantom generated using a circular scan configuration. There is a noticeable drop-off in intensity at each corner. For example, see highlighted region R.

[0043] Figure 2B The grayscale value along the AA line varies with distance. In the highlight area, the intensity attenuation R is obvious.

[0044] Figure 3 is a flow chart illustrating a method for correcting intensity attenuation image artifacts caused by trajectories in cone-beam CT scanners (such as the X-ray microtomography system described above) and medical CT scanners in which a patient is supported by a source and a detector system rotates around the patient.

[0045] In step 310, a 3D image volume V1 is instantiated, which has the same dimensions and voxel size as the reconstructed 3D image volume. In this initialized volume, each voxel is filled with a constant, denoted by C, which corresponds to the desired reconstructed field of view (FOV) of the system geometry and is based on the selected scan configuration. In the embodiment shown, the selected scan configuration is a circular scan configuration, but other scan configurations that exhibit intensity falloff due to the conical x-ray beam can also be selected, such as scan configurations: circle and line, ellipse, sine wave, double circle, etc.

[0046] In step 320, a constant-filled image volume V1 is forward projected using the same geometry for each view as the original scan, performed on a computer using a forward projection computer program 254. These forward projected digital images are denoted by P1.

[0047] In step 330, the forward projection data P1 is then applied to the FDK method for reconstruction using the analysis reconstruction application 254. The reconstructed volume is denoted by V2.

[0048] In step 340, an intensity-compensated volume Vc is created by inverting the volume V2 element-by-element (voxel). Specifically, for each element, Vc=C / V2.

[0049] In step 350, the analysis and reconstruction application 254 reconstructs the raw projection data acquired during the circular or other appropriate (circular and linear, elliptical, sine wave, double circle) scan configuration / acquisition geometry of the sample 114 using the FDK method. This creates a reconstructed volume of interest for the sample 114. The reconstructed volume data set of the sample 114 is denoted by Vr. It should be noted that, particularly in this step, the order is not important. In fact, in practice, such data acquisition of the sample is typically performed at the beginning (before step 310), and then the correction is generated according to the acquisition geometry outlined in steps 310-340.

[0050] Finally, in step 360, the original reconstructed volume Vr is multiplied by the intensity compensation volume Vc element / voxel to obtain the final intensity compensated reconstructed volume Vrc, corrected for intensity attenuation.

[0051] Figure 4A and Figure 4B Intensity falloff correction for a circular scanning configuration is shown.

[0052] Figure 4A A longitudinal slice through a water phantom generated using a circular scan configuration but corrected for intensity falloff. Corners no longer show intensity falloff.

[0053] Figure 4B Here is a graph of the grayscale value along line AA as a function of distance. Intensity decay no longer exists.

[0054] While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention as encompassed by the appended claims.

Claims

1. A method for cone-beam CT intensity correction, comprising: determining an intensity offset for the selected scan configuration by performing a forward projection using the selected scan configuration; performing a reconstruction of the forward projection data and using the reconstructed data to generate the intensity compensation; as well as The intensity compensation is applied to the reconstructed volume of interest to generate an intensity compensated volume.

2. The method according to claim 1, wherein The selected scanning configuration is a circle or other suitable scanning configuration (circle and line, ellipse, sine wave, double circle, etc.).

3. The method according to claim 1 or 2, wherein: The orthographic projection has the same dimensions and voxel size as the reconstructed volume of interest.

4. The method according to any one of claims 1 to 3, wherein Performs an orthographic projection of the instanced volume filling each voxel with a constant value.

5. The method according to claim 4, wherein Voxel-by-voxel intensity compensation is generated by inverting the reconstructed forward projection data.

6. The method according to any one of claims 4 or 5, wherein The reconstruction of the reconstructed volume of interest and forward projection data is performed using FDK.

7. A computer software product for cone-beam CT intensity correction, implementing the method according to any one of claims 1 to 6.

8. Computer software product according to claim 7, stored in a non-transitory storage medium such as an electronic or magnetic storage medium.

9. An X-ray tomography system comprising: an x-ray system comprising an x-ray source system for generating an x-ray beam and a detector system for detecting the x-ray beam after transmission through a sample to generate projection data; as well as A computer system for determining an intensity compensation for a selected scan configuration by performing a forward projection using the selected scan configuration, performing reconstruction of the forward projection data, generating the intensity compensation using the reconstruction data, and applying the intensity compensation to a reconstructed volume of interest to produce an intensity compensated volume.

10. The system according to claim 9, wherein: The selected scanning configuration is a circle or other suitable scanning configuration (circle and line, ellipse, sine wave, double circle, etc.).

11. The system according to claim 9 or 10, wherein: The orthographic projection has the same dimensions and voxel size as the reconstructed volume of interest.

12. The system according to any one of claims 9 to 11, wherein: Performs an orthographic projection of the instanced volume filling each voxel with a constant value.

13. The system according to claim 12, wherein: The computer system generates voxel-by-voxel intensity compensation by inverting the reconstructed forward projection data.

14. The system according to any one of claims 12 or 13, wherein: The reconstruction of the reconstructed volume of interest and the forward projection data is performed using FDK.

Images