Gain calibration technique for digital imaging systems

A computer-implemented method for gain calibration is provided. The method includes sorting the calibration data of each pixel location from the offset-corrected X-ray image data into a sequence. The method also includes removing part of the calibration data from one end or both ends of the respective sequence for each pixel location. The method further includes averaging the calibration data remaining within each respective sequence to obtain an average pixel value for each pixel location. The method yet further includes generating a gain map based on the average pixel value for each pixel location.

BACKGROUND

A number of radiological and fluoroscopic imaging systems of various designs are known and are presently in use. Such systems generally are based upon generation of X-rays that are directed toward a subject of interest. The X-rays traverse the subject and impact a digital detector or an image intensifier. In medical contexts, for example, such systems may be used to visualize internal bones, tissues, and organs, and diagnose and treat patient ailments. In other contexts, parts, baggage, parcels, and other subjects may be imaged to assess their contents. In addition, radiological and fluoroscopic imaging systems may be used to identify the structural integrity of objects and for other purposes.

Increasingly, such X-ray systems use digital circuitry, such as solid-state detectors, for detecting the X-rays, which are attenuated, scattered or absorbed by the intervening structures of the subject. It will be appreciated that raw image data acquired via such X-ray systems may include a number of artifacts or other undesirable elements that may, if left uncorrected, result in visual artifacts in a reconstructed image based on the raw image data. In turn, these visual artifacts may negatively impact the ability of a user or computer to discern finer details in the image. For example, non-uniformity of various aspects of the X-ray system, such as the X-ray beam, diodes and/or data channels of a digital detector, and the like, may result in gain variation in the acquired raw image data. While certain approaches for performing gain calibration and correcting image data for such gain variation may be known, these approaches are not applicable to certain types of detectors such as complementary metal-oxide-semiconductor (CMOS) based detectors due to the presence of impulse-type noise generated by direct X-ray hits on the light imager. There is a need, therefore, for improved approaches to gain calibration for digital imaging systems that account for impulse-type noise.

BRIEF DESCRIPTION

In accordance with a first embodiment, a computer-implemented method for gain calibration is provided. The method includes sorting calibration data for each pixel location from offset-corrected X-ray calibration image data into a sequence. The method also includes removing part of the calibration data from one end or both ends of the sequence for each pixel location. The method further includes averaging the calibration data remaining within each respective sequence to obtain an average pixel value for each pixel location. The method yet further includes generating a gain map based on the average pixel value for each pixel location.

In accordance with a second embodiment, a computer-implemented method is provided. The method uses a digital X-ray system to execute a gain calibration process via the digital X-ray system. The gain calibration process includes sorting calibration data for each pixel location from offset-corrected X-ray image data into a sequence. The gain calibration process also includes removing part of the calibration data from one end or both ends of the sequence for each pixel location. The gain calibration process further includes averaging the calibration data remaining within each respective sequence to obtain an average pixel value for each pixel location. The gain calibration process yet further includes generating a gain map based on the average pixel value for each pixel location.

In accordance with a third embodiment, an imaging system is provided. The imaging system includes a radiation source, a digital detector configured to generate image data, and control circuitry configured to acquire the image data from the digital detector. The imaging system also includes processing circuitry configured to apply gain correction to the image data via a gain map, wherein the processing circuitry is configured to generate the gain map by executing code to perform the following acts. The acts include sorting calibration data for each pixel location from offset-corrected image data into a sequence and removing part of the calibration data from a respective sequence for each pixel location. The acts also include averaging the calibration data remaining within each respective sequence to obtain an average pixel value for each pixel location. The acts further include generating the gain map based on the average pixel value for each pixel location.

DETAILED DESCRIPTION

The present disclosure provides for methods and systems to perform gain calibration for radiographic detectors where impulse-type noise (i.e., noise generated by direct X-ray hits on the light imager of the detector) is present. For example, detectors such as complementary metal-oxide-semiconductor (CMOS) based detectors may experience impulse-type noise. However, the techniques discussed below may be applied to other types of detectors (e.g., amorphous silicon based detectors). The techniques discussed below remove part of the calibration data from the image data that may include impulse-type noise prior to generating a gain map. In addition, the techniques discussed below may reduce Gaussian noise (i.e., electronic noise) present in the remaining calibration data prior to generating the gain map. The techniques described below may be utilized in a variety of radiographic imaging systems, such as computed tomography (CT) systems, fluoroscopic imaging systems, mammography systems, tomosynthesis imaging systems, conventional radiographic imaging systems, and so forth. However, it should be appreciated that the described techniques may also be used in non-medical contexts (such as security and screening systems and non-destructive detection systems).

Turning now to the drawings,FIG. 1illustrates diagrammatically an imaging system10for acquiring and processing discrete pixel image data. In the illustrated embodiment, system10is a digital X-ray system designed both to acquire original image data and to process the image data for display in accordance with the present technique. The imaging system10may be a stationary system disposed in a fixed X-ray imaging room or a mobile X-ray system. In the embodiment illustrated inFIG. 1, imaging system10includes a source of X-ray radiation12positioned adjacent to a collimator14. Collimator14permits a stream of radiation16to pass into a region in which a subject, such as a human patient18is positioned. A portion of the radiation20passes through or around the subject and impacts a digital X-ray detector, represented generally at reference numeral22. The detector22may be portable or permanently mounted to the system10. In certain embodiments, the detector22may convert the X-ray photons incident on its surface to lower energy photons, and subsequently to electric signals, which are acquired and processed to reconstruct an image of the features within the subject. In other embodiments, such as in a direct conversion implementation, the incident radiation itself may be measured without an intermediary conversion process.

Source12is controlled by a power supply/control circuit24which furnishes both power and control signals for examination sequences. Moreover, detector22is coupled to a detector controller26which commands acquisition of the signals generated in the detector22. Detector controller26may also execute various signal processing and filtration functions, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. Both power supply/control circuit24and detector controller26are responsive to signals from a system controller28. In general, system controller28commands operation of the imaging system to execute examination protocols and to process acquired image data. In the present context, system controller28also includes signal processing circuitry, typically based upon a general purpose or application-specific digital computer; and associated manufactures, such as optical memory devices, magnetic memory devices, or solid-state memory devices, for storing programs and routines executed by a processor of the computer to carry out various functionalities (e.g., gain calibration and gain correction), as well as for storing configuration parameters and image data; interface protocols; and so forth. In one embodiment, a general or special purpose computer system may be provided with hardware, circuitry, firmware, and/or software for performing the functions attributed to one or more of the power supply/control circuit24, the detector controller26, and/or the system controller28as discussed herein.

In the embodiment illustrated inFIG. 1, system controller28is linked to at least one output device, such as a display or printer as indicated at reference numeral30. The output device may include standard or special purpose computer monitors and associated processing circuitry. One or more operator workstations32may be further linked in the system for outputting system parameters, requesting examinations, viewing images, and so forth. In general, displays, printers, workstations, and similar devices supplied within the system may be local to the data acquisition components, or may be remote from these components, such as elsewhere within an institution or hospital, or in an entirely different location, linked to the image acquisition system via one or more configurable networks, such as the Internet, virtual private networks, and so forth.

FIG. 2is a diagrammatical representation of functional components of digital detector22.FIG. 2also represents an imaging detector controller or IDC34which will typically be configured within detector controller26. IDC34includes a CPU or digital signal processor, as well as memory circuits for commanding acquisition of sensed signals from the detector. In one implementation, IDC34is coupled via two-way fiberoptic conductors to detector control circuitry36within detector22. In certain presently contemplated embodiments, other communications systems and technologies may be used, such as Ethernet communications protocols, and wireless communications devices and protocols. IDC34thereby exchanges command signals for image data within the detector during operation.

Detector control circuitry36receives DC power from a power source, represented generally at reference numeral38. Detector control circuitry36is configured to originate timing and control commands for row and column electronics used to acquire image data during data acquisition phases of operation of the system. Circuitry36therefore transmits power and control signals to reference/regulator circuitry40, and receives digital image pixel data from circuitry40.

In a present embodiment, detector22consists of a scintillator that converts X-ray photons received on the detector surface during examinations to lower energy (light) photons. An array of photodetectors then converts the light photons to electrical signals which are representative of the number of photons or the intensity of radiation impacting individual pixel regions of the detector surface. In certain presently contemplated embodiments, the X-ray photons may be directly converted to electrical signals. Readout electronics convert the resulting analog signals to digital values that can be processed, stored, and displayed, such as in a display30or a workstation32following reconstruction of the image. In a present form, the array of photodetectors is formed of silicon CMOS. The array elements are organized in rows and columns, with each element consisting of a photodiode and complementary and symmetrical pairs of p-type and metal oxide semiconductor field effect transistors (MOFSET). The cathode of each diode is connected to the source of the transistor, and the anodes of all diodes are connected to a negative bias voltage. The gates of the transistors in each row are connected together and the row electrodes are connected to the scanning electronics as described below. The drains of the transistors in a column are connected together and the electrode of each column is connected to an individual data channel of the readout electronics.

In the particular embodiment illustrated inFIG. 2, by way of example, a row bus42includes a plurality of conductors for enabling readout from various rows of the detector22, as well as for disabling rows and applying a charge compensation voltage to selected rows, where desired. A column bus44includes additional conductors for commanding readout from the columns while the rows are sequentially enabled. Row bus42is coupled to a series of row drivers46, each of which commands enabling of a series of rows in the detector. Similarly, readout electronics48are coupled to column bus44for commanding readout of all columns of the detector.

In the illustrated embodiment, row drivers46and readout electronics48are coupled to a detector panel50, which may be subdivided into a plurality of sections52. Each section52is coupled to one of the row drivers46, and includes a number of rows. Similarly, each column driver48is coupled to a series of columns. The photodiode and transistor arrangement mentioned above thereby define a series of pixels or discrete picture elements54which are arranged in rows56and columns58. The rows and columns define an image matrix60, having a height62and a width64.

As also illustrated inFIG. 2, each pixel54is generally defined at a row and column crossing, at which a column electrode (or data line)68crosses a row electrode (or scan line)70. As mentioned above, a couple of MOFSETS72is provided at each crossing location for each pixel, as is a photodiode74. As each row is enabled by row drivers46, signals from each photodiode74may be accessed via readout electronics48, and converted to digital signals for subsequent processing and image reconstruction. Thus, an entire row of pixels in the array is controlled simultaneously when the scan line70attached to the gates of all the transistors of pixels on that row is activated. Consequently, each of the pixels in that particular row is connected to a data line68, through a switch, which is used by the readout electronics to restore the charge to the photodiode74and measure an amount of charge depletion resulting from irradiation.

It should be noted that in certain systems, as the charge is restored to all the pixels in a row simultaneously by each of the associated dedicated readout channels, the readout electronics is converting the measurements from the previous row from an analog voltage to a digital value. Furthermore, the readout electronics may transfer the digital values from rows previous to the acquisition subsystem, which will perform some processing prior to displaying a diagnostic image on a monitor or writing it to film. In at least some embodiments, the digital detector22may include data processing circuitry66configured to perform some local processing of the data acquired via the detector panel50within the digital detector itself. For instance, as discussed in greater detail below, the digital detector22may be configured to perform gain calibration and gain correction (e.g., to reduce impulse-type noise and Gaussian noise) to the acquired data independent of a host processing system, such as the system controller28. Additionally, in one embodiment, the digital detector22may perform such calibration and apply such correction to the acquired data before outputting the data to other components of the system10.

The circuitry used to enable the rows may be referred to in a present context as row enable or MOFSET circuitry based upon the use of complementary and symmetrical pairs of p-type and metal oxide semiconductor field effect transistors for such enablement (row driving). The MOFSETs associated with the row enable circuitry described above are placed in an “on” or conducting state for enabling the rows, and are turned “off” or placed in a non-conducting state when the rows are not enabled for readout. Despite such language, it should be noted that the particular circuit components used for the row drivers and column readout electronics may vary, and the present invention is not limited to the use of MOFSETs or any particular circuit components.

Various functionality, including image data gain correction and calibration of the detector22described herein, may be performed by, or in conjunction with, a processor-based system76, which is generally depicted inFIG. 3in accordance with one embodiment. For example, the various controllers and circuitry discussed herein may include, or be partially or entirely embodied in, a processor-based system, such as that presently illustrated. The processor-based system76may be a general-purpose computer, such as a personal computer, configured to run a variety of software, including software implementing all or part of the functionality described herein. Alternatively, in other embodiments, the processor-based system76may include, among other things, a distributed computing system, or an application-specific computer or workstation configured to implement all or part of the presently described functionality based on specialized software and/or hardware provided as part of the system. Further, the processor-based system76may include either a single processor or a plurality of processors to facilitate implementation of the presently disclosed functionality.

In one embodiment, the exemplary processor-based system76includes a microcontroller or microprocessor78, such as a central processing unit (CPU), which executes various routines and processing functions of the system76. For example, the microprocessor78may execute various operating system instructions, as well as software routines configured to effect certain processes, stored in or provided by a manufacture including one or more computer readable-media (at least collectively storing the software routines), such as a memory80(e.g., a random access memory (RAM) of a personal computer) or one or more mass storage devices82(e.g., an internal or external hard drive, a solid-state storage device, a CD-ROM, a DVD, or another storage device). In addition, the microprocessor78processes data provided as inputs for various routines or software programs, such as data provided as part of the present subject matter described herein in computer-based implementations.

Such data may be stored in, or provided by, the memory80or mass storage device82. Alternatively, such data may be provided to the microprocessor78via one or more input devices84. The input devices84may include manual input devices, such as a keyboard, a mouse, or the like. In addition, the input devices84may include a network device, such as a wired or wireless Ethernet card, a wireless network adapter, or any of various ports or devices configured to facilitate communication with other devices via any suitable communications network, such as a local area network or the Internet. Through such a network device, the system76may exchange data and communicate with other networked electronic systems, whether proximate to or remote from the system76.

Results generated by the microprocessor78, such as the results obtained by processing data in accordance with one or more stored routines, may be provided to an operator via one or more output devices, such as a display86and/or a printer88. Based on the displayed or printed output, an operator may request additional or alternative processing or provide additional or alternative data, such as via the input device84. Communication between the various components of the processor-based system76may typically be accomplished via a chipset and one or more busses or interconnects which electrically connect the components of the system76. In one embodiment, the exemplary processor-based system76can be configured to, among other things, receive image data, generate a gain map or gain correction map, apply gain correction to the image data via one or more gain correction maps, and output the corrected image data.

FIG. 4illustrates a method90for operating the digital X-ray imaging system10(e.g., acquiring image data, gain calibration, and gain correction). The followings acts of method90may be performed by various control circuitry and processing circuitry of the system10(e.g., by the detector22and/or by components of the system10external to the detector22). The method90includes acquiring dark images or offset images92(i.e., images acquired when the detector22is not exposed to radiation from the source12) (block94) represented by

O{m)⁢=Δ⁢{oi,j{m}}.
O represents the offset images, m=0, 1, . . . M−1 with M being the number of dark images92available, and i=0, 1, . . . I−1 and j=0, 1, . . . J−1 with/and J, respectively, being the number of rows and columns of the detector22. The dark images92only include the Gaussian type of electric noise. The method90includes averaging the M available dark images92(block96) to obtain an offset map98, Ô, with minimum noise. The averaging of the dark images92to obtain the offset map98is described by the following:

The method90also includes acquiring X-ray images or X-ray image data100(block102) represented by

X{n)⁢=Δ⁢{xi,j{n}},
where n=0, 1, . . . N−1 with N being the X-ray images available. After obtaining the X-ray images100, the method90includes correcting the N X-ray images100by subtracting the obtained offset map98(block102) from each of the N X-ray images100

The offset-corrected images104may include a Gaussian-type of electric noise and/or impulse-type noise due to direct X-ray hits on the imager of the detector22. To minimize the impulse-type noise within the offset-corrected images104, the method90includes sorting pixels (block106), {{circumflex over (x)}i,j{n}, n=0, 1, . . . , N−1}, from the N offset-corrected images104based on their pixels values (e.g., calibration data) for each pixel location (i, j) to generate a sequence108{ . . . , ŝi,j{−L}, . . . , ŝi,j{−1}, ŝi,j{0}, ŝi,j{1}, . . . , ŝi,j{L}}, where ŝi,j{0}is the median pixel value of the sequence108for each respective pixel location and L is an experimentally determined parameter interceded to balance the performance between the impulse-type and Gaussian type of noises. The generated sequence108(e.g., of calibration data) for each pixel location may be an ascending sequence or a descending sequence.

Upon generating the sequences108for each pixel location (block106), the method90includes removing values (e.g., calibration data) that may contain the impulse-type of noise from the left and/or right hand sides (i.e., relative to the median pixel value) (block110) of each sequence108to generate a sequence of remaining pixels112(i.e., remaining calibration data or pixel values) for each pixel location. The calibration data may be removed from a beginning of a descending sequence, an end of an ascending sequence, or both from the beginning and end of the sequence108. The method90then includes averaging the pixel values (e.g., calibration data) of the 2L+1 middle pixels (i.e., remaining pixels) of each respective sequence112(block114) to obtain an average pixel value for each pixel location and to reduce the electronic noise (e.g., Gaussian noise) as described in the following:

S^⁢=Δ⁢{s^i,j}=12⁢L+1⁡[si,j{0}+∑l=1L⁢(s^i,j{-l}+s^i,j{l})],(3)
where {ŝi,j} represents the average pixel value for the middle pixels or the remaining pixels of the respective sequence112for each pixel location.

The method90further includes generating a gain map116(block118) from the average pixels values. The gain map116is obtained by

Technical effects of the disclosed embodiments include providing methods and systems to perform gain calibration and correction for radiographic detectors22where impulse-type noise is present. In particular, the disclosed embodiments include sorting pixels from offset-corrected image data for each pixel location into a sequence based on a value (e.g., calibration data) of each pixel and removing those values from the sequence that may include impulse-type noise. The disclosed embodiments further include averaging the values for the remaining data within the sequence for each pixel location to minimize Gaussian-type noise and to obtain the gain map. Upon obtaining the gain map, the offset-corrected images may be gain-corrected to generate gain-corrected images to improve the image quality of the images.