JP2009171990A - System and method for calibrating x-ray detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the validity of the calibration of a detector in an X-ray radiography system wherein an X-ray source and the detector are not fixed to each other or the like. <P>SOLUTION: The method and system for generating high frequency components and low frequency components (62, 64) for the pixel (20) of a detector array (18) are provided. The method includes the operation of forming a gain map image (56) comprising the gain coefficient of one, two or more pixels (20) of the detector array (18). Conversion based a frequency is applied to the gain map image (56) and the high frequency components (62) and low frequency components (64) of the gain map coefficient are generated for each pixel (20). The high frequency components and low frequency components (62, 64) can be applied so as to be different to the processing of images. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates generally to imaging techniques, and more specifically to calibration of X-ray detectors.

  Non-invasive imaging broadly encompasses techniques for forming images of structures or areas inside an object or human body that are otherwise difficult to obtain for visual inspection. One of the best known uses of non-invasive imaging is in the medical technology field, where these techniques are used to form images of organs and / or bones in the patient that are otherwise invisible To do. Examples of such non-invasive imaging modalities include X-ray radiography and other X-ray imaging techniques such as tomosynthesis.

  For example, medical x-ray radiography systems typically operate by projecting x-rays from an x-ray source through an imaging volume. Some of the x-rays pass through the patient's part, such as the chest or arms or legs, and are attenuated by such part. Attenuated x-rays are detected by an array of detector elements that generate a signal representative of the attenuation of the incident x-rays. The signal is processed and reconstructed to form an image of the imaging area.

  For example, a digital detector may be composed of a single monolithic scintillator or an array of individual photodetectors that are placed underneath individual scintillators. Scintillators typically generate visible light when struck by x-rays. The photodetector then detects visible light and generates responsive electrical signals that can be read and used to form an image based on the location of the photodetector on the panel. it can. In such a system, the degree of the output signal generated by the photodetector in response to a given x-ray input is known as the photodetector gain.

  However, the photodetector may vary in its ability to detect visible light and / or generate an output signal that responds. As a result, not all photodetectors in the detector array will produce equivalent output signals in response to the same x-ray dose, i.e., individual photodetectors may have different intrinsic gain functions. In systems where the X-ray source and detector have a fixed geometric configuration, i.e. the system where the source and detector do not move relative to each other, the calibration will cause the light detector to respond to known X-ray exposure. These gain differences are addressed by providing a correction factor to each photodiode so that the gain differences can be compensated. For example, an array of photodetectors can be exposed in a uniform x-ray field and corrected for each photodetector so that each photodiode produces a uniform signal after application of each correction factor. Factors can be determined. In this manner, each photodetector can be corrected to produce a uniform signal in the presence of such a uniform field.

  However, in systems where the x-ray source and detector are not fixed relative to each other, the effectiveness of the calibration is low. Specifically, in such a system, the output difference between the photodetectors is not only the result of the difference due to the photodetector itself, but also the relative relationship between the X-ray source and the detector during the irradiation event. It can also be the result of a geometric configuration. Thus, the correction factor derived in one source / detector geometry may not properly correct for the photodetector output difference in the other source / detector geometry. In such a system, it is desirable to distinguish between the portion of the photodetector output difference due to the source / detector geometry and the portion due to the photodetector itself.

  A method for generating an image correction factor is provided. The method includes an act of forming a gain map image composed of gain correction factors for one or more pixels of the detector array. A frequency based transform is applied to the gain map image to generate a high frequency component of the gain map coefficient and a low frequency component of the gain map coefficient for each pixel. Also provided are claims corresponding to a tangible machine-readable medium containing executable code for performing these operations.

  A method for processing an image is also provided. The method includes an act of acquiring an image using an imaging system that includes a source and a detector array. One or more low frequency components determined for the pixels of the detector array are adjusted to take into account the position of the source relative to the detector array at the time of image acquisition. The image is corrected using the one or more adjusted low frequency components. Also provided are claims corresponding to a tangible machine-readable medium containing executable code for performing these operations.

  In addition, an imaging system is provided. The imaging system includes a detector array that includes a plurality of detector elements and a radiation source configured to irradiate radiation toward the detector array. The source is movable relative to the detector array. The imaging system also includes a system controller configured to control the operation of at least one of the detector array or source and the operation of the image processing component. The image processing component is configured to process signals generated by the detector array in response to the emitted radiation to form an image. In addition, the image processing component adjusts the one or more low frequency components determined for the pixels of the detector array to take into account the position of the source relative to the detector array at the time of image acquisition, The image is corrected using one or more adjusted low-frequency components.

  The foregoing features, aspects and advantages of the present invention as well as other features, aspects and advantages will be more fully understood when the following detailed description is read with reference to the accompanying drawings in which: Like numerals represent like parts throughout the drawings.

  FIG. 1 shows diagrammatically an imaging system 10 that acquires and processes projection data in accordance with the techniques of the present invention to form a radiographic image. In the illustrated embodiment, the system 10 is a mobile X-ray that is designed to both acquire original image data and process the image data for display and analysis in accordance with the techniques of the present invention. It is an imaging system. In other embodiments, the system 10 is a tomosynthesis system or other system in which images are acquired over a limited number of view angles. The system 10 includes an x-ray source 12 that is configured to emit x-rays 14. In one exemplary embodiment, the x-ray source 12 is an x-ray tube having a rotating anode and a thermionic electron source. In other embodiments, the X-ray source 12 may be a stationary anode X-ray tube having a solid state electron source or a thermionic electron source, or other X-ray radiation source suitable for medical image acquisition, Also good.

  X-rays 14 pass through an area where an object such as a patient's arm 16 is located. A portion of the x-ray radiation 14 passes through the object or passes around the object and impinges on the detector array 18. The detector elements 20 or pixels of the array 18 generate an electrical signal representative of the intensity of the incident x-ray 14. These signals are acquired and processed to form an image of the internal features of the object, such as arm 16, in the illustrated example. In one embodiment, the detector array 18 is a flat panel detector such as a monolithic detector array comprising a large area and / or continuous scintillation surface that covers the surface of a light detection assembly such as an array of photodiodes. Is included.

  The source 12 is controlled by a system controller 22 that provides both power and control signals for radiographic examination. In the illustrated embodiment, the system controller 22 controls the source 12 via an x-ray controller 24 that may be a component of the system controller 22. In such embodiments, the x-ray controller 24 may be configured to provide power and timing signals to the x-ray source 12 and / or otherwise control the activation and operation of the x-ray source 12.

  Further, the detector 18 is coupled to a system controller 22 that commands acquisition of signals generated at the detector 18. In the illustrated embodiment, the system controller 22 acquires a signal generated by the detector 18 using a data acquisition system 26. Data acquisition system 26 receives the data collected by the readout electronics of detector 18. In one embodiment, the data acquisition system 26 receives the sampled analog signal from the detector 18 and converts this data into a digital signal for subsequent processing by the image processing component 30. In an alternative embodiment, the readout circuitry of detector 18 converts the signal to digital form before providing the signal to data acquisition system 26. The data acquisition system 26 can perform various signal processing and filtering operations on the acquired image signal, such as initial adjustment of dynamic range and interleaving of digital image data.

  In general, the system controller 22 commands the operation of the imaging system 10 (such as via the operation of the source 12 and detector 18) to execute the examination protocol and process the acquired data. In this context, the system controller 22 is also typically a signal processing circuit based on a general purpose or application specific digital computer, as well as programs and routines executed by the computer (described herein). A routine for executing the image processing method and the reconstruction method, etc.), an attached memory circuit for storing the configuration parameters and the image data, and an interface circuit.

  In the illustrated embodiment, the image signal acquired and processed by the system controller 24 is provided to the image processing component 30 for image formation. The processing component 30 may consist of or include special purpose processors such as one or more conventional microprocessors or graphics coprocessors. Data collected by the data acquisition system 26 can be sent to the processing component 30 directly or after storage in memory. It should be understood that any type of memory suitable for storing large amounts of data may be utilized by such an exemplary system 10. In addition, the memory may be located at the location of the acquisition system and may include remote components that store data, processing parameters and routines for image processing and reconstruction.

  The processing component 30 is configured to receive commands from an operator and output images to the operator, typically via an operator workstation 32 with a keyboard and other input devices. . An operator can control the system 10 via an input device. In this way, the operator can observe the acquired image and / or otherwise operate the system 10 via the operator workstation 32. For example, the formed image can be observed by using a display provided in the operator workstation 32 to control imaging. In addition, images can be printed on a printer, which may be a component of the operator workstation 32 or may be coupled to the workstation 32.

  In addition, processing component 30 and operator workstation 32 may be coupled to other output devices that may include standard or special purpose computer monitors and associated processing circuitry. One or more operator workstations 32 can be further coupled to the system to output system parameters, request inspection, view images, and so forth. In general, the displays, printers, workstations and similar devices supplied in the system may be located locally with respect to the data acquisition components or remote to these components. Located in the same facility or other location in the hospital or in a completely different location and connected to the image acquisition system via one or more configurable networks such as the Internet and virtual private networks May be.

  It should be further noted that the operator workstation 32 can be coupled to an image archiving communication system (PACS). Such a PACS can be coupled to a remote client, a radiology information system (RIS), a hospital information system (HIS), or an internal or external network, where third parties at different locations can receive images, image data, and Optionally, access to distributed data can be obtained.

  While the above discussion dealt with the various exemplary components of the imaging system 10 separately, those skilled in the art will recognize that some or all of these various components may be internal or interconnected within a common platform. It will be appreciated that can be provided at. For example, the processing component 30, memory, and operator workstation 32 may be provided together as a general purpose or special purpose computer or workstation configured to operate in accordance with the techniques of the present invention. Similarly, a system controller 22 may be provided as part of such a computer or workstation.

  In one embodiment of the present technique, the system 10 of FIG. 1 is calibrated and the calibrated system is used to acquire radiographic data to form useful medical images. For example, FIG. 2 shows an example of operations for calibrating and using the system 10 of FIG. In this embodiment, the detector panel 18 is uniformly illuminated by the x-ray source 12 at various voltage and current settings of the x-ray source 12 (block 40). The resulting flat field image is used to calculate the gain 44 of each pixel 20 at each of the current and voltage settings used to generate the X-rays 14 in the irradiation step of block 40 (block 42). These pixel gain values 44 are then used to calculate a correction factor 50 for each pixel 20 for each voltage and current setting of the X-ray source 12 (block 48).

  In the illustrated embodiment, pixel gain 44 and correction factor 50 are used to derive corresponding slope and offset values for the gain function associated with each respective detector pixel 20. For example, if the gain function is linear, the slope and offset calculations may simply correspond to calculating a line representing the gain function. If the gain function is non-linear, the gain function may include a quadratic term. Each of the calculated gain factors is plotted for each pixel 20 (block 54) to form a two-dimensional gain map image 56. For example, the gain map image 56 may be a two-dimensional representation of each coefficient value shown at the corresponding pixel space location.

  A two-dimensional Fourier transform, or other suitable frequency based transform, is applied to the gain map image 56 (block 60) to obtain a high frequency component 62 and a low frequency component 64 of the gain function coefficient for each pixel 20. For example, in one embodiment, the high frequency component 62 and the low frequency component 64 are weighted to a two-dimensional fast Fourier transform image to generate the high frequency component 62 and a function that generates the low frequency component 64 using an inverse function. Generated by. Specifically, in one embodiment, the suitability of this approach is evidenced by the bimodal distribution found in the Fourier transform. As will be appreciated by those skilled in the art, each mode (mode) or peak of the distribution can be separated by use of appropriate functions and inverse functions as described above, respectively, high frequency component 62 or low frequency component 64. Can be appropriately specified as including.

  The high frequency component 62 is generally considered to correspond to an electrical difference or other response difference inherent to each pixel 20 of the detector array 18 due to, for example, scintillator and / or photodiode variations. Therefore, the high frequency component 62 is considered to be independent of the position of the X-ray source 12. Conversely, the low frequency component 64 is generally due to the fact that it is generally impossible to form a uniform field from the X-ray source 12 in the detector array 18 in clinical operation. It is considered to be partially or completely dependent on the geometric configuration. For example, when the X-ray source 12 and the detector array 18 can be moved relative to each other, the distance between the X-ray source 12 and the various pixels 20 of the detector array 18 depends on the position of the X-ray source 12 at the time of image acquisition. Resulting in a response difference due to the source / detector geometry. In addition, the rotating anode x-ray tube has a “heel” effect due to the geometry of the x-ray target and can result in the presence of such low frequency components 64. Accordingly, as will be appreciated by those skilled in the art, for each pixel 20, a high frequency gain component 62 due to the pixel itself is calculated along with a low frequency gain component 64 generally due to the position of the X-ray source 12. . To determine an appropriate function to separate the low frequency component 64 from the position independent high frequency component 62, a series of flat field images can be obtained from various source locations. This data can then be used to separate the position independent high frequency component 62 from the position dependent low frequency component 64 as described above.

  Considering each high frequency component 62 and low frequency component 64 derived for the detector 18 of the imaging system 10, the image acquired by the system 10 is appropriately processed to account for gain changes between the respective pixels 20. It can be corrected. For example, in the illustrated embodiment, a diagnostic image 70 is acquired by the system 10 (block 72). The acquisition of image 70 is associated with a corresponding source / detector geometry 74 that describes the relative position of source 12 and detector array 18 relative to each other at the time of image acquisition. Based on the source / detector geometry 74, the low frequency component 64 determined for the detector array 18 is adjusted to take into account the geometry 74 (block 76). In the illustrated embodiment, using the adjusted low frequency component 78 and the high frequency component 62 (which may be adjusted based on other factors rather than being adjusted based on the source / detector geometry 74). The diagnostic image 70 is corrected (block 80), and a gain-corrected diagnostic image 82 is formed. The gain corrected diagnostic image 82 can then be printed and / or displayed at the operator workstation 32 of FIG. 1 for consideration by a technician.

  While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, while the discussion herein is in the context of medical imaging using a radiography system, one skilled in the art will recognize an X-ray source that the technique of the present invention can move relative to a tomosynthesis system or a detection device. It will be appreciated that the non-medical imaging applications used are equally applicable. For example, the technique of the present invention is also applied to security management applications in fields such as baggage and parcel screening, manufacturing quality control and security screening, and non-invasive imaging techniques and / or non-destructive imaging techniques used for quality control applications. be able to. Therefore, it is to be understood that the claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Further, the reference numerals in the claims corresponding to the reference numerals in the drawings are merely used for easier understanding of the present invention, and are not intended to narrow the scope of the present invention. Absent. The matters described in the claims of the present application are incorporated into the specification and become a part of the description items of the specification.

1 is a diagram of an exemplary imaging system in the form of a radiographic x-ray imaging system used to form an image in accordance with an aspect of the present technique. 2 is a flow diagram illustrating exemplary operations for processing an image acquired using the system of FIG. 1 in accordance with an aspect of the present technique.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Imaging system 12 X-ray source 14 X-ray 16 Arm 18 Detector array 20 Detector element / pixel 22 System controller 24 X-ray controller 26 Data acquisition system 30 Image processing component 32 Operator workstation 40 Detector panel 42 Gain value 48 Calculate the correction factor 50 Correction factor 54 Plot the gain coefficient of each pixel 56 Gain map image 60 Apply 2D conversion 62 High frequency Component 64 Low frequency component 70 Diagnostic image 72 Acquire diagnostic image 74 Source / detector geometry 76 Adjust low frequency component 78 Adjusted low frequency component 80 Correct diagnostic image 82 Gain corrected diagnostic image

Claims (10)

Forming a gain map image (56) composed of gain correction factors of one or more pixels (20) of the detector array (18);
A frequency based transformation is performed on the gain map image (56) to produce a high frequency component (62) of the gain map coefficient and a low frequency component (64) of the gain map coefficient for each pixel (20). The steps to apply to
A method for generating an image correction factor comprising: Determining an appropriate low frequency component based on the source / detector geometry (74) associated with the imaging system (10) at the time of acquisition of the image (70);
Correcting the image (70) with the low frequency component when the system (10) is in a similar geometric configuration;
The method of claim 1 comprising:   A step (80) of correcting the image (70) using the high frequency component (62) and the low frequency component (78), wherein the low frequency component (78) is obtained when the image (70) is acquired. A correction step (80) is included which is adjusted (76) before correcting the image (70) to take into account the position of the X-ray source (12) relative to the detector (18). The method according to 1. Acquiring (72) an image (70) using an imaging system (10) comprising a source (12) and a detector array (18);
Determined for the pixels (20) of the detector array (18) to take into account the position of the source (12) relative to the detector (18) at the time of acquisition (72) of the image (70). Adjusting one or more low frequency components (64) (76);
Correcting (80) the image (70) using the one or more adjusted low frequency components (78);
A method of processing an image comprising:   The method of claim 4, comprising correcting (80) the image (70) using one or more high frequency components (62). A detector array (18) comprising a plurality of detector elements (20);
A radiation source (12) configured to emit radiation (14) toward the detector array (18) and movable relative to the detector array (18);
A system controller (22) configured to control operation of at least one of the detector array (18) or the source (12);
An image processing component (30) configured to process signals generated by the detector array (18) in response to the emitted radiation (14) to form an image (70). The pixel (20) of the detector array (18) to take into account the position of the source (12) relative to the detector array (18) at the time of acquisition of the image (70). One or more low frequency components (64) are adjusted (76) and the image (70) is corrected (80) using the one or more adjusted low frequency components (78). An image processing component (30),
An imaging system (10) comprising:   The imaging and imaging device of claim 6, wherein the image processing component (30) is further configured to correct (80) the image (70) using one or more high frequency gain components (62). System (10).   The imaging system (10) of claim 6, wherein the detector array (18) comprises a flat panel detector.   The imaging system (10) according to claim 6, comprising one of a medical imaging system, a non-destructive inspection system, or a screening system suitable for imaging baggage or parcels.   The imaging system (10) of claim 6, including one of a radiographic x-ray imaging system or a tomosynthesis imaging system.

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