An X-ray tomography system and method

Abstract

Most image reconstruction algorithms are common to DT and CT, require a reconstructed volume assignment, and are based on ray tracing techniques. Only after the entire volume has been processed and the algorithm has completed is the reconstructed 3D image available. The present invention, in the following manner, reconstructs each slice one by one without waiting for the algorithm to complete: back-projecting each pixel in each attenuation image towards the emitter generating the image to a selected reconstruction slice and determining the overlapping ratio of the grid cells in the slice to obtain a weighting factor, thereby calculating the average back-projection intensity of each grid cell in the selected reconstruction slice.

Description

Technical Field

[0001] The present invention generally relates to digital tomography and has been found to be particularly useful in X-ray tomography, but this is not the only use. Background Art

[0002] Digital tomosynthesis (DT) is a limited-angle tomography that provides the advantages of 3D imaging. Very similar to computed tomography (CT), DT allows for better detection of 3D structures by viewing one slice at a time. High planar resolution, three-dimensionality, and low radiation dose make DT an attractive alternative to CT in many medical imaging applications.

[0003] Compared to CT, the DT projection dataset is incomplete, which violates the tomography sufficiency condition and results in limited angular artifacts in the reconstructed image. Although DT is a volume imaging technique and provides information about the internal structure of an object, it is not possible to reconstruct the entire 3D information about the object.

[0004] Most image reconstruction algorithms are common to DT and CT, and the most popular image reconstruction algorithms include simultaneous algebraic reconstruction, filtered backprojection, cone-beam reconstruction, and their variants. Image reconstruction can also be formulated as solving an optimization problem. All of the above methods require a reconstructed volume assignment and are based on ray-tracing techniques. The reconstructed 3D image is only available after the entire volume has been processed and the algorithm has completed.

[0005] Emitter panels consisting of a small array of X-ray emitters rather than a single source are advantageous because no physical movement is required, avoiding the cost of a motorized mover and avoiding motion blur by electronically switching between emitters. Summary of the Invention

[0006] According to the present invention, there is provided a method of generating a tomogram, the method comprising the steps of: providing an X-ray detector panel including a plurality of pixels; providing an X-ray emitter panel spaced apart from the X-ray detector panel, the emitter panel including a plurality of X-ray emitters; identifying the relative positions of the emitters and the pixels relative to each other; emitting a respective cone of X-ray radiation from each emitter towards the detector panel; generating a respective attenuation image at the detector panel in response to each respective cone of X-ray radiation impinging on the detector panel; and reconstructing a density function indicative of X-ray radiation attenuation by: selecting a reconstruction slice corresponding to a plane located between the detector and emitter panels; providing an array of grid cells on the reconstruction slice; in response to selecting the reconstruction slice: for each respective attenuation image, back-projecting each pixel in the respective attenuation image towards the respective emitter for that attenuation image onto the reconstruction slice and determining the proportion of overlap of the back-projected pixel with each grid cell to obtain a plurality of weighting factors, the plurality of weighting factors including a respective weighting factor for each grid cell of the back-projected pixel; using the plurality of weighting factors for each back-projected pixel in each attenuation image to calculate an average back-projection intensity for each grid cell in the selected reconstruction slice to produce an average intensity image of the selected reconstruction slice; and convolving the average intensity image with a ramp filter to obtain the density function within the selected reconstruction slice.

[0007] Thus, each slice can be reconstructed one by one, where the slices are acquired in a plane parallel to the first plane. One slice at a time can be reconstructed interactively rather than waiting for the algorithm to complete. This method is much faster than previous methods and requires much less computer memory. This method is particularly effective when only a portion of the entire reconstructed volume is of interest.

[0008] The method may explicitly include the step of generating a tomogram based on the reconstructed density function.

[0009] Surprisingly, the present invention allows for the generation of images with higher contrast, particularly strong contrast images of relatively thick objects being scanned.

[0010] Identifying the relative positions of the emitters and the pixels relative to each other may include receiving an indication from hardware (such as a firmware reading), receiving position information from alignment pins on the emitter and / or detector panels, receiving position information from an alignment armature or similar device for manipulating the emitter and detector panels, receiving an indication of the relative orientation of the panels, etc.

[0011] The attenuation image may represent the Radon transform (e.g., Beer-Lambert law) of the spatial distribution of the transmission coefficient.

[0012] Selecting the reconstruction slice may include the operator selecting the slice or may be automatically selected by the computer; for example, the selection of the slice may be predefined.

[0013] Reconstructing the density function indicative of X-ray radiation attenuation may further include selecting additional reconstruction slices. The additional reconstruction slices may be selected in a similar manner and the reconstruction may be performed in a similar manner. In this way, the operator can view the first reconstruction slice and determine which next reconstruction slice to select based on these results. Alternatively, this further selection may be performed automatically (e.g., predefined). Subsequent selections of the reconstruction slices may be performed in a similar manner until the entire region has been reconstructed.

[0014] The plane may be substantially flat; i.e., planar. The plane may be oriented parallel to the detector panel; however, in alternative embodiments, the plane may be tilted with respect to the detector panel (i.e., in any direction). Similarly, the plane may be oriented parallel to the emitter panel or may be tilted with respect to the emitter panel.

[0015] Providing an array of grid cells on the reconstruction slice may be predefined, selected by the operator, or may be calculated. The array of grid cells on each slice may be the same as the array of grid cells on each other slice, alternatively or additionally, they may be different. Furthermore, the size of the grid cell array may be different from the size of the pixel array on the detector panel such that the pixel size and the number of pixels of the back-projected image may not match the pixel size and the number of pixels of the detector panel.

[0016] Providing an array of grid cells on the reconstruction slice may include orthogonally projecting a plurality of pixels onto the reconstruction slice to obtain a two-dimensional array of corresponding grid cells.

[0017] Alternatively, providing an array of grid cells on the reconstruction slice may include orthogonally projecting a plurality of emitters onto the reconstruction slice to obtain a two-dimensional array of corresponding grid cells. However, any other computational method may be employed, depending on the resolution of the reconstruction slice required and / or possible within the hardware limitations.

[0018] The pixel values may be back-projected along the rays connecting the pixel corners and the emitters to a specified height of the plane, thereby mapping the pixels of the detector panel onto the reconstruction slice. This mapping may be achieved by emitting imaginary rays from each of the four corners of each pixel of the detector panel towards a given emitter such that these four rays form a pyramid. The intersection points between the pyramid and the selected reconstruction slice may then define the orthogonal projection. Since the size of the grid cell array may be different from the size of the pixel array on the detector panel, the orthogonal projection may only cover a portion of some of the grid cells.

[0019] Determining the overlap ratio of the back-projected pixel with each grid cell may include: for each grid cell covered by the orthogonal projection, determining the proportion of the grid cell covered by the orthogonal projection, and then this area can be further divided by the area of the grid cell to obtain a weighting factor.

[0020] The pixel values of the corresponding attenuation images are weighted by this factor and added to the values of the grid cells. This operation can be repeated for all attenuation images to obtain a composite value of the grid cells, and then it can be normalized with respect to the total number of overlapping pixels in the reconstruction plane to obtain an average value.

[0021] Convolving the average intensity image with a ramp filter to obtain the density function within the selected reconstruction slice may include evaluating the reconstruction formula:

[0022]

[0023] where:

[0024] f is the density function representing the attenuation of X-ray radiation;

[0025] x, y are the Cartesian coordinates in the reconstruction plane;

[0026] x’, y’ are the Cartesian parameters used in the integration;

[0027] in the average intensity determined for the reconstruction plane; and

[0028] h is the ramp filter.

[0029] The ramp filter may include any suitable ramp filter known in the art. For example, the ramp filter may be given by:

[0030]

[0031] where k is the wave number along the x-axis; and / or

[0032] for n = 0,

[0033] for odd n,

[0034] for non-zero even n,

[0035] where n is the distance χ or y in the pixel, and λ is the size of the pixel. λ may be an approximation of the wavelength of the selected reconstruction grid being used.

[0036] The method may further include identifying for the reconstruction plane A discontinuity in the determined average intensity, such as caused by the boundary of a flat panel detector. For example, this can be achieved by determining for each grid cell the number of attenuation images whose backprojection contributes to the corresponding grid cell and comparing the corresponding numbers of attenuation images of adjacent grid cells. In response to identifying a different number of attenuation images, for example, along the x-axis in adjacent grid cells, an integration profile (such as a straight line) can be defined as being discontinuous at χ c (where we have a jump), or in response to identifying a different number of attenuation images, for example, along the y-axis in adjacent grid cells, the integration profile can be defined as being discontinuous at y c . Other methods of defining χ c and / or y c are also contemplated. Further discussion of the contour will refer only to the x-axis, although application to the y-axis is also possible and desirable (although not required) and can be applied in the same manner.

[0037] Convolving the average intensity image with a ramp filter to obtain a density function within a selected reconstructed slice can include evaluating the reconstruction formula:

[0038]

[0039] where: when x' is close to x c , can be equal to and when x' is far from x c , can be equal to

[0040] is the derivative of, and

[0041] Sign(x - x c ) is the sign (plus or minus) of the expression x - x c .

[0042] x being close to x c can mean within a predetermined number of pixels, particularly between 3 and 30, more particularly between 5 and 25, for example between 10 and 20, for example 15.

[0043] x being far from x c can mean outside a predetermined number of pixels, particularly between 3 and 30, more particularly between 5 and 25, for example between 10 and 20, for example 15.

[0044] That is, replacing the average intensity determined for the reconstruction plane with a function close to the contour. The above method equally applies to contours parallel to the x-axis (such as at yc In the case of , if it is necessary to correct the contours parallel to the x-axis and the y-axis, substitutions can be made in the two integrals.

[0045] In extreme cases, one of the integrals may be lost (i.e., calculated as zero), resulting in:

[0046]

[0047] In the case of x, and a similar statement in the case of y, the necessary changes have been made.

[0048] According to a second aspect of the present invention, there is provided a system for generating tomographic images, the system comprising: an X-ray emitter panel including a plurality of X-ray emitters, each emitter configured to emit a respective cone of X-ray radiation; an X-ray detector panel including a plurality of pixels, the X-ray detector panel being spaced apart from the X-ray emitter panel and configured to generate a respective attenuation image in response to each cone of X-ray radiation, wherein the cone of X-ray radiation impinges on the detector panel from the X-ray emitter panel; a spatial position system for identifying the relative positions of the emitters and the pixels with respect to each other; and a processor for reconstructing a density function indicative of X-ray radiation attenuation by: selecting a reconstruction slice corresponding to a plane located between the detector and emitter panels; providing an array of grid cells on the reconstruction slice; in response to selecting the reconstruction slice, for each respective attenuation image, back-projecting each pixel in the respective attenuation image towards the respective emitter for that attenuation image onto the reconstruction slice and determining the proportion of overlap of the back-projected pixel with each grid cell to obtain a plurality of weighting factors, the plurality of weighting factors including a respective weighting factor for each grid cell of the back-projected pixel; using the plurality of weighting factors for each back-projected pixel in each attenuation image to calculate an average back-projection intensity for each grid cell in the selected reconstruction slice to produce an average intensity image of the selected reconstruction slice; and convolving the average intensity image with a ramp filter to obtain the density function within the selected reconstruction slice. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The above and other features, characteristics, and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which illustrate the principles of the invention by way of example. The description is provided by way of example only and does not limit the scope of the present invention. The following reference numerals refer to the drawings.

[0050] Figure 1 is a side view of the back-projection geometry.

[0051] Figure 2 is a plan view of the pixel projected onto the grid cell.

[0052] Figure 3This is an example of determining the average intensity after applying the weighting factor. Detailed implementation

[0053] The present invention will be described with reference to certain drawings, but the present invention is not limited thereto and is only limited by the claims. The described drawings are merely illustrative and non - restrictive. Each drawing may not include all features of the present invention and should not be considered as an embodiment of the present invention. In the drawings, for illustrative purposes, the dimensions of some elements may be exaggerated and not drawn to scale. The dimensions and relative dimensions do not correspond to the actual reduction in the practice of the present invention.

[0054] In addition, the terms first, second, third, etc. in the specification and claims are used to distinguish similar elements and are not necessarily used to describe an order in terms of time, space, sequence, or any other manner. It should be understood that the terms so used are interchangeable under appropriate circumstances and can operate in a different order than described or illustrated herein. Similarly, method steps described or claimed in a particular order can be understood to operate in a different order.

[0055] In addition, the terms top, bottom, above, below, etc. in the specification and claims are used for descriptive purposes and are not necessarily used to describe relative position. It should be understood that the terms so used are interchangeable under appropriate circumstances and can operate in a different orientation than described or illustrated herein.

[0056] It should be noted that the term "comprising" used in the claims should not be construed as being limited to the means listed thereafter; it does not exclude other elements or steps. Thus, it is construed as specifying the presence of the recited features, integers, steps, or components, but does not exclude the presence or addition of one or more other features, integers, steps, or components or groups thereof. Thus, the scope of the expression "a device comprising devices A and B" should not be limited to a device consisting only of components A and B. This means that for the purposes of the present invention, the only relevant components of the device are A and B.

[0057] Similarly, it should be noted that the term "connected" used in the description should not be construed as being limited to direct connection. Thus, the scope of the expression "device A connected to device B" should not be limited to a device or system in which the output of device A is directly connected to the input of device B. This means that the path between the output of A and the input of B can be a path that includes other devices or means. "Connected" may mean that two or more elements are in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but still cooperate or interact with each other. For example, consider a wireless connection.

[0058] References throughout this specification to "one embodiment" or "one aspect" mean that the particular features, structures, or characteristics described in conjunction with that embodiment or aspect are included in at least one embodiment or aspect of the present invention. Therefore, the phrases "in one embodiment" or "in one aspect" appearing throughout this specification do not necessarily all refer to the same embodiment or aspect, but may refer to different embodiments or aspects. In addition, it will be apparent to one of ordinary skill in the art that the particular features, structures, or characteristics of any one embodiment or aspect of the present invention may be combined in any suitable manner with any other particular features, structures, or characteristics of another embodiment or aspect of the present invention.

[0059] Similarly, it should be understood that in the description, various features of the invention are sometimes combined together in a single embodiment, figure, or description thereof, with the purpose of simplifying the content of the invention and aiding the understanding of one or more of the various inventive aspects. However, this method of disclosure should not be interpreted as indicating that the claimed invention requires more features than those explicitly stated in each claim. In addition, any individual drawing or aspect description should not be considered an embodiment of the invention. On the contrary, as reflected in the following claims, the inventive aspects lie in less than all the features of a single previously disclosed embodiment. Therefore, the claims following the specific embodiment are hereby expressly incorporated into the specific embodiment, and each claim stands alone as a separate embodiment of the invention.

[0060] In addition, as will be appreciated by those skilled in the art, although some embodiments described herein include some features included in other embodiments, the combination of features of different embodiments is intended to be within the scope of the present invention and form additional embodiments. For example, in the following claims, any claimed embodiment may be used in any combination.

[0061] In the description provided herein, many specific details are set forth. However, it should be understood that embodiments of the present invention can be practiced without these specific details. In other cases, well-known methods, structures, and techniques are not shown in detail in order not to obscure the understanding of this description.

[0062] In the discussion of the present invention, unless indicated to the contrary, the disclosure of alternative values ​​for the upper or lower limits of the permitted range of a parameter, together with an indication that one of the values ​​is more preferred than another, should be interpreted as implicitly indicating that each intervening value of the parameter between a more preferred and a less preferred value in the alternative is itself preferred over the less preferred value and every value between the less preferred value and the intervening value.

[0063] In some cases, the use of the term "at least one" may mean only one. In some cases, the use of the term "any" may mean "all" and / or "each".

[0064] The principles of the present invention will now be described by way of a detailed description of at least one drawing related to exemplary features. It is clear that other arrangements can be configured according to the knowledge of those skilled in the art without departing from the basic concepts or technical teachings, and the present invention is limited only by the terms of the appended claims.

[0065] Figure 1 is a side view of the back-projection geometry, where the emitter panel 1 is located above the detector panel 3. The emitter panel 1 includes a plurality of emitters (m, m-1, m-2). The detector panel 3 includes a plurality of pixels 5. For each attenuation image (produced by emitter m) rays trace back from each pixel to emitter m. For example, for the attenuation image (produced by emitter m) ray 7 traces back from pixel 5b to emitter m, and for the attenuation image (produced by emitter m-1) ray 9 traces back from pixel 5b to emitter m-1.

[0066] These rays 7, 9 intersect various reconstruction planes 11, 13, 15 located between the emitter panel 1 and the detector panel 3 and are shown parallel in the figure (but this may not necessarily be the case in alternative embodiments). In this way, pixel 5b can be projected onto these planes, for example, onto plane 11 for emitter m at region 17a for the large row projection, and onto plane 11 for emitter m-1 at region 17b.

[0067] For each emitter m, the back-projected image in the selected reconstruction plane 11 can then be determined as

[0068] Figure 2 is Figure 1 a plan view of pixels 5a and 5b (and additional adjacent pixels 5e and 5f), which are projected onto the grid cells of plane 11 for a single emitter m as regions 17a, 19a, 21a, and 23a, respectively.

[0069] Figure 3 is an example of the average intensity in plane 11 of the back-projected pixel regions 17c, 17d, 17e determined from emitters m = 1, m = 2, and m = 3, respectively, once the weighting factors are applied. of.

Claims

1. A method for generating a tomogram, the method comprising the steps of: providing an X-ray detector panel including a plurality of pixels; providing an X-ray emitter panel spaced apart from the X-ray detector panel, the emitter panel including a plurality of X-ray emitters; identifying the relative positions of the emitters and pixels relative to each other; emitting a respective cone of X-ray radiation from each emitter towards the detector panel; generating a respective attenuation image at the detector panel in response to each respective cone of X-ray radiation impinging on the detector panel; and reconstructing a density function indicative of the attenuation of the X-ray radiation by: selecting a reconstruction slice corresponding to a plane located between the detector and the emitter panel; providing an array of grid cells on the reconstruction slice; in response to selecting the reconstruction slice, for each respective attenuation image, back-projecting each pixel in the respective attenuation image towards the respective emitter for the attenuation image onto the reconstruction slice and determining the proportion of overlap of the back-projected pixel with each grid cell to obtain a plurality of weighting factors, the plurality of weighting factors including respective weighting factors for each grid cell for the back-projected pixel; using the plurality of weighting factors for each back-projected pixel in each attenuation image to calculate an average back-projection intensity for each grid cell in the selected reconstruction slice to produce an average intensity image of the selected reconstruction slice; and convolving the average intensity image with a ramp filter to obtain the density function within the selected reconstruction slice.

2. A system for generating a tomogram, the system comprising: an X-ray emitter panel including a plurality of X-ray emitters, each emitter configured to emit a respective cone of X-ray radiation; an X-ray detector panel including a plurality of pixels, the X-ray detector panel being spaced apart from the X-ray emitter panel and configured to generate a respective attenuation image in response to each cone of X-ray radiation, wherein the cone of X-ray radiation impinges on the detector panel from the X-ray emitter panel; a spatial position system for identifying the relative positions of the emitters and pixels relative to each other; and a processor for reconstructing a density function indicative of the attenuation of the X-ray radiation by: selecting a reconstruction slice corresponding to a plane located between the detector and the emitter panel; providing an array of grid cells on the reconstruction slice; in response to selecting the reconstruction slice, for each respective attenuation image, back-projecting each pixel in the respective attenuation image towards the respective emitter for the attenuation image onto the reconstruction slice and determining the proportion of overlap of the back-projected pixel with each grid cell to obtain a plurality of weighting factors, the plurality of weighting factors including respective weighting factors for each grid cell for the back-projected pixel; using the plurality of weighting factors for each back-projected pixel in each attenuation image to calculate an average back-projection intensity for each grid cell in the selected reconstruction slice to produce an average intensity image of the selected reconstruction slice; and convolving the average intensity image with a ramp filter to obtain the density function within the selected reconstruction slice.

Images