System and method for image compression in X-ray imaging systems

An imaging system includes an analog-to-digital converter configured to convert an analog pixel value into a first digital pixel value. The imaging system also includes an index value source configured to receive the first digital pixel value from the analog-to-digital converter and to generate a digital index value based on a comparison of the first digital pixel value to a digital reference value. In addition, the imaging system includes a transmitter in communication with the index value source and configured to transmit the digital index value. Further, the imaging system includes an image processing component configured to receive the digital index value and to generate a second digital pixel value based at least in part on the received digital index value and a lookup table of the image processing component.

BACKGROUND

The subject matter disclosed herein relates to X-ray imaging systems and more particularly to image compression over a communication channel in X-ray imaging systems.

Digital X-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital X-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons that are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. The detector communicates the encoded output signals to a host computer, which processes the image received based on the digital pixel values. This communication takes place over a communication channel, which may include a tether or a wireless link, between the detector and the computer. Unfortunately, it often takes a long time to send the digital pixel values over the communication channel because of the size of the digital representation needed to maintain sufficient gray levels of the X-ray image. Accordingly, it is now recognized that there is a need for reducing image transfer time across the communication link, without an undesirable loss of gray levels or increase in noise.

BRIEF DESCRIPTION

In one embodiment of the present disclosure, an imaging system includes an analog-to-digital converter configured to convert an analog pixel value into a first digital pixel value. The imaging system also includes an index value source configured to receive the first digital pixel value from the analog-to-digital converter and to generate a digital index value based on a comparison of the first digital pixel value to a digital reference value. In addition, the imaging system includes a transmitter in communication with the index value source and configured to transmit the digital index value. Further, the imaging system includes an image processing component configured to receive the digital index value and to generate a second digital pixel value based at least in part on the received digital index value and a lookup table of the image processing component.

In another embodiment of the present disclosure, a method of operating an imaging system includes converting an analog pixel value to a first digital pixel value using a linear analog-to-digital converter. The method also includes generating a digital index value based on a comparison of the first digital pixel value to a digital reference value using a digital comparator. Further, the method includes mapping the generated digital index value to a second digital pixel value via an image processing component. The second digital pixel value has a larger bit-width than the digital index value.

In a further embodiment of the present disclosure, a digital X-ray detector for use in an imaging system includes a linear analog-to-digital converter configured to convert an analog pixel value received from a detector element to a digital pixel value. The digital X-ray detector also includes an index value source configured to receive the digital pixel value and to generate a digital index value corresponding to the received digital pixel value. The digital index value has a smaller bit-width than the digital pixel value. The digital X-ray detector is configured to communicate the digital index value to a separate image processing component.

DETAILED DESCRIPTION

Present embodiments are directed to systems and methods for reducing image transfer time over a communication channel between a digital X-ray detector and an image processing component within an X-ray imaging system. The X-ray detector includes an analog-to-digital converter to change a detected analog signal, which is representative of an image pixel, to a digital representation. The digital representation may be a certain bit-width (e.g., 14-bit) to maintain a desired number of gray levels in the image. Gray levels refer to a gradient of different pixel intensities within an image, between 0% intensity (e.g., black) and 100% intensity (e.g., white). The X-ray detector also includes an index value source that uses a digital comparator and a lookup table to generate an index value for each of the digital pixel values. The index values may have a smaller bit-width than the digital pixel values they are based upon, allowing for reduced transfer time across the communication channel. The lookup table may include a mapping of the digital pixel values to the digital index values, and a similar lookup table may be resident in the image processing component for converting the digital index values back to a pixel representation with a larger bit-width. This mapping may be at least partially quadratic, based on a relationship between X-ray quantum noise in the signal and quantization noise due to the image compression.

Referring generally toFIG. 1, an imaging system, in particular an X-ray system is represented and referenced generally by reference numeral10. In the illustrated embodiment, the X-ray system10is a digital X-ray system. The X-ray system10is designed both to acquire original image data and to process the image data for display in accordance with present techniques. In the embodiment illustrated inFIG. 1, the X-ray system10includes an imager system12. The imager system12includes an overhead tube support arm14for positioning a radiation source16, such as an X-ray tube, and a collimator18with respect to a patient20and a portable digital X-ray detector22. In one embodiment, the imager system12may be used in consort with one or both of a patient table26and a wall stand28to facilitate image acquisition. Particularly, the table26and the wall stand28may be configured to receive the detector22. For instance, the detector22may be placed on an upper, lower, or intermediate surface of the table26, and the patient20(more specifically, an anatomy of interest of the patient20) may be positioned on the table26between the detector22and the radiation source16. The wall stand28may include a receiving structure30also adapted to receive the detector22, and the patient20may be positioned adjacent the wall stand28to enable the image data to be acquired via the detector22. The receiving structure30may be moved vertically along the wall stand28.

Also depicted inFIG. 1, the imager system12includes a systems cabinet31that includes a workstation32and display34. In one embodiment, the workstation32may include or provide the functionality of the imager system12such that a user, by interacting with the workstation32, may control operation of the source16and detector22. The detector22may be in communication with the workstation32as described below. The workstation32may house systems electronic circuitry that acquires image data from the detector22and that, where properly equipped (e.g., when the workstation32includes processing circuitry), may process the data to form desired images. In addition, the systems electronic circuitry both provides and controls power to the X-ray source16. The workstation32may include buttons, switches, or the like to facilitate operation of the X-ray source16and detector22. In one embodiment, the workstation32may be configured to function as a server of instructions and/or content on a network36of the medical facility, such as a hospital information system (HIS), a radiology information system (RIS), and/or picture archiving communication system (PACS). In certain embodiments, the workstation32and/or detector22may wirelessly communicate with the network36.

In present embodiments, the detector22includes circuitry for processing the image data received through the detector22before communicating the image data to the workstation32. The detector22may convert analog signals from detector elements to digital pixel values. The detector22may include an index value source for generating digital index values with a smaller bit-width than the digital pixel values. This reduces the digital representation of the image data collected by the detector22, thereby reducing the data transfer time between the detector22and the workstation32.

In one embodiment, the imager system12may be a stationary system disposed in a fixed X-ray imaging room, such as that generally depicted in and described above with respect toFIG. 1. It will be appreciated, however, that the presently disclosed techniques may also be employed with other imaging systems, including mobile X-ray units and systems, in other embodiments.

For instance, as illustrated in the medical imaging system10(e.g., X-ray system) ofFIG. 2, the imager system12may be moved to a patient recovery room, an emergency room, a surgical room, or any other space to enable imaging of the patient20without requiring transport of the patient20to a dedicated (i.e., fixed) X-ray imaging room. The X-ray system10includes a mobile imager or mobile X-ray base station50and a portable digital X-ray detector22. As above, the illustrated X-ray system10is a digital X-ray system. In one embodiment, a support arm52may be vertically moved along a support column54to facilitate positioning of the radiation source16and collimator18with respect to the patient20. Further, one or both of the support arm52and support column54may also be configured to allow rotation of the radiation source16about an axis. In addition, the X-ray base station50has a wheeled base58for movement of the station50.

The patient20may be located on a bed60(or gurney, table or any other support) between the X-ray source24and the detector22and subjected to X-rays that pass through the patient20. During an imaging sequence using the digital X-ray system10, the detector22receives X-rays that pass through the patient20and transmits imaging data to a base unit56. The detector22is in wireless communication with the base unit56. The base unit56houses systems electronic circuitry62that acquires image data from the detector22and that, where properly equipped, may process the data to form desired images. In addition, the systems electronic circuitry62both provides and controls power to the X-ray source16and the wheeled base58. The base unit56also has the operator workstation32and display34that enables the user to operate the X-ray system10. The operator workstation32may include buttons, switches, or the like to facilitate operation of the X-ray source16and detector22. In one embodiment, the workstation32may be configured to function as a server of instructions and/or content on the network36of the medical facility, such as HIS, RIS, and/or PACS. In certain embodiments, the workstation32and/or detector22may wirelessly communicate with the network36.

Similar to the X-ray system10inFIG. 1, components of the imager system12(e.g., base unit56) and the detector22are configured to reduce a size of the digital representation of the image communicated between the detector22and the base unit56. These components may decrease the digital representation size of an image collected by the detector22and transmit the compressed digital image over a communication channel. When the compressed image reaches the base unit56, the electronic circuitry62may increase the digital representation to a desired bit-width for the display34.

Regardless of the differences between the X-ray systems10shown inFIGS. 1 and 2, certain features internal to the X-ray system10remain consistent across different embodiments. These components are illustrated diagrammatically inFIG. 3. The imager system12includes the X-ray source16of radiation. The X-ray source16is controlled by a power supply70, which furnishes both power and control signals for examination sequences. In addition, in mobile imaging systems the power supply70furnishes power to a mobile drive unit72of the wheeled base58. The power supply70is responsive to signals from a system controller74. In general, the system controller74commands operation of the imaging system to execute examination protocols and to process acquired image data. In the present context, the system controller74also includes signal processing circuitry, typically based upon a general purpose or application-specific digital computer, associated memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth. The system controller74may include or may be responsive to a processor76. The processor76receives image data from the detector22and processes the data to reconstruct an image of a subject.

The processor76is linked to a wireless communication interface80that allows wireless communication with the detector22. Further, the processor76is linked to a wired communication interface82that allows communication with the detector22via a tether (e.g., a multi-conductor cable). The imager system12may also be in communication with a server. The processor76is also linked to a memory84, an input device86, and the display34. The memory84stores configuration parameters, calibration files received from the detector22, and lookup tables used for image data processing. The input device86may include a mouse, keyboard, or any other device for receiving user input, as well as to acquire images using the imager system12. The display34allows visualization of output system parameters, images, and so forth.

The detector22includes a wireless communication interface88for wireless communication with the imager system12, as well as a wired communication interface90, for communicating with the detector22when it is tethered to the imager system12. The detector22may also be in communication with a server. The wireless communication interfaces80and88, as well as the wired communication interfaces82and90, define a communication channel91between the imager system12and the detector22, over which digital X-ray images are transmitted. It is noted that the wireless communication interface88may utilize any suitable wireless communication protocol, such as an ultra wideband (UWB) communication standard, a Bluetooth communication standard, or any 802.11 communication standard. Moreover, the detector22is coupled to a detector controller92which coordinates the control of the various detector functions. For example, the detector controller92may execute various signal processing and filtration functions, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. The detector controller92is responsive to signals from the system controller74, as well as the detection circuitry78. The detector controller92is linked to a processor94. The processor94, the detector controller92, and all of the circuitry receive power from a power supply96. The power supply96may include a battery. Alternatively, the detector22, including the power supply96, may receive power from the power supply70when tethered to the imager system12.

Also, the processor94is linked to detector interface circuitry98. The detector22converts X-ray photons received on its surface to lower energy photons. The detector22includes a detector array100that includes an array of photodetectors to convert the light photons to electrical signals. Alternatively, the detector22may convert the X-ray photons directly to electrical signals. These electrical signals are converted to digital values by the detector interface circuitry98which provides the values to the processor94to be converted to imaging data and sent to the imager system12to reconstruct an image of the features within a subject. Alternatively, the imaging data may be sent from the detector22to a server to process the imaging data.

Further, the processor is linked to a memory104. The memory104may store various configuration parameters, calibration files, and detector identification data. In addition, the memory104may store one or more lookup tables used by the processor94to quantize image data before communicating the data over the communication channel91. In certain embodiments, the lookup tables stored in the memory104of the detector22may also be stored in the memory84of the imager system12, in order to process the compressed image data received from the detector22.

In order to reduce image transfer time across the communication channel91, present embodiments of the detector22include image processing components (e.g., processor94) for reducing a size of the image for transmission across the communication channel91. This is accomplished without reducing the number of pixels of the image, as this would compromise the resolution of the X-ray imaging system10. Instead, the image processing components are configured to alter the digital representation of the pixels themselves. That is, the image processing components reduce the representation of each pixel from a 14-bit binary word to smaller bit-width.

Such compression of the digital pixel representation may lead to an increase in quantization noise in the X-ray image data. This quantization noise contributes to the total noise within the X-ray image data according to the following equation:
σT=√{square root over (σQ2+σE2+σδ2)}.  (1)

In the equation above, σTrepresents total noise, σQrepresents X-ray quantum noise, σErepresents electronic noise, and σδrepresents analog-to-digital (A/D) quantization noise. The electronic noise is an inherent level of noise relating to the electronic components operating in the detector22and the imager system12. The electronic noise level does not change throughout use of the X-ray system10, and the electronics are generally designed in a way to minimize this electronic noise. The X-ray quantum noise is due to the quantum nature of the X-rays, and is proportional to the square root of the number of X-ray photons arriving at the detector22. Consequently, the X-ray quantum noise is higher when more photons hit the detector22. Since the number of photons arriving at each detector element may vary, the amount of X-ray quantum noise for each pixel of the X-ray image may vary proportionally.

The A/D quantization noise is related to the step size used in quantizing each pixel into a digital representation. An increase in the step size of a digital quantizer reduces the digital representation of the pixels to a lower bit-width, but it also increases the quantization noise. In order to reduce the digital representation of pixels without increasing the total noise level beyond an acceptable limit, the step size of the quantizer may be chosen such that the A/D quantization noise is maintained below the X-ray quantum noise for each pixel. Thus, the quantizer uses a smaller step size when the X-ray dose is low and a larger step size when the X-ray dose is high. In this way, the total noise is dominated by the quantum noise and is basically independent of the quantization noise. To determine the appropriate step size for quantizing each pixel, the detector22may include certain image processing components (e.g., implemented by the processor94) that make up the quantizer. Throughout the following discussion, the term “quantizer” may be used interchangeably with the term “index value source”.

These image processing components are illustrated in detail inFIG. 4. The detector22includes, for example, an analog-to-digital (A/D) converter110, an index value source112(or quantizer), and a transmitter114. The A/D converter110is configured to receive an analog pixel value115from a detector element116of the detector array100, and convert the analog pixel value115into a digital pixel value117. In the illustrated embodiment, the A/D converter110is a linear A/D converter configured to convert the analog pixel value115into a 14-bit digital pixel value. The index value source112receives the digital pixel value117from the linear A/D converter110and generates a digital index value119based on a comparison of the digital pixel value117to a digital reference value, as discussed in detail below. This digital index value119may have a smaller bit-width than the digital pixel value117received by the index value source112. In this way, the index value source112acts as a quantizer to compress the X-ray image for communication over the communication channel91.

In the illustrated embodiment, the index value source112includes a comparator118and a lookup table120used to determine the digital index value119appropriate for the digital pixel value117received from the A/D converter110. The lookup table includes a mapping of digital index values122to digital reference values124, and the comparator is designed to compare the incoming digital pixel value117with the digital reference values124. As illustrated, the digital index values122may include integers that go from 0 to 1 to 2, all the way through nI−1, where nIis a total number of values that can be represented with log 2(nI) binary bits. Similarly, the digital reference values124may include integers that go from 0 to nG−1, where nGis a total number of gray levels that can be represented with log 2(nG) binary bits. In certain X-ray systems10, the total number nGof gray levels is 16384, which can be expressed in 14 bits. In present embodiments, nGis greater than nI. This ensures that the digital index value119output from the index value source112has a smaller bit-width than the input digital pixel value117. Consequently, the size of the image representation transferred over the communication channel91is significantly reduced.

In certain embodiments, the mapping of digital index values122to digital reference values124is at least partially non-linear. The mapping is representative of the relationship between the increasing quantum noise σQand the increasing quantization noise σδ. Again, as the number of photons hitting the detector element116increases, the quantum noise σQincreases. This increase in the quantum noise σQenables an increase in the quantization noise σδaccording to the formula described above, and this amount of allowable quantization noise corresponds with a given number nIof steps. The lookup table120representing the relationship between nGand nImay be determined by the detector controller92in order to reduce the total noise σTin the system while allowing the fewest number nIof steps (or digital index values122).

The comparator118is used to determine the digital index value119that corresponds with the digital pixel value117received from the A/D converter110. Specifically, the comparator118receives the digital pixel value117, and one of the digital reference values124at a time. As the digital index value122tracks up from 0 to nI−1, the corresponding digital reference value124goes up gradually, according to the relationship maintained in the lookup table120. When the digital reference value124reaches the same level as the digital pixel value117in the comparator118, the comparator118locks its output with the digital index value122corresponding to the current digital reference value124. Thus, the comparator118performs the quantization of the digital pixel value117of each detector element116. In certain embodiments, the comparator118includes two or more stages for performing this quantization, as described in detail below. The comparator118, which is a digital comparator, may be implemented with the processor94, a digital computer, a microcontroller, or the like.

The quantization process described above may be performed for each detector element116within the detector array100before the determined digital index values119for the entire image are transmitted over the communication channel91. In other embodiments, the detector22may send the digital index values119corresponding to each of the detector elements116as each one becomes available. In either case, the digital index values119are transferred from the detector22to an image processing component126used to process the image data collected by the detector22. As illustrated, the detector22may communicate the digital index values119to an entirely separate image processing component126. In such instances, the image processing component126may be resident in the imager system12and representative of certain components implemented through the processor76.

The transmitter114of the detector22may send the determined digital index values119from the index value source112to a receiver128of the image processing component126over the communication channel91. The communication channel91may be a tether or a wireless link, as described with reference toFIG. 3. In embodiments having a tether for such communication, the transmitter114functions as the wired communication interface90and the receiver128functions as the wired communication interface82. Similarly, in wireless embodiments, the transmitter114functions as the wireless communication interface88(e.g., wireless transmitter), and the receiver128functions as the wireless communication interface80.

Upon receiving the digital index value119from the transmitter114, the image processing component126is designed to generate another digital pixel value127based at least in part on the received digital index value119and a lookup table130of the image processing component126. This lookup table130includes a mapping between the digital index values122and digital pixel values132for the final X-ray image. In the illustrated embodiment, the lookup table130is the same as the lookup table120used in the index value source112. That is, the index value source112generates the digital index value119based on the lookup table120, and the image processing component126maps the generated digital index value119to the digital pixel value127using the same lookup table120. This may be desirable when the number of gray levels for the final X-ray image is the same as the number of gray levels nGused in the detector22. In some embodiments, the lookup table130of the image processing component126may be adjusted from the lookup table120of the index value source112to make a mean of the quantization error approximately equal to zero. This adjustment may account for an amount of offset in the mapping of digital index values to digital reference values. In such embodiments, the detector controller92may provide the lookup table120to the processor76via the communication channel91and detection circuitry78. By reducing the bit-width of the image representation sent over the communication channel91and increasing the bit-width once the image is received by the image processing component126, the imaging system10allows for reduced image transfer time without an undesirable loss in image quality.

By using the lookup table130, the image processing component126may generate the digital pixel value127having a bit-width larger than the bit-width of the received digital index value119. Again, this provides a decreased amount of data transferred over the communication channel91without an undesirable decrease in image quality. The generated digital pixel value127may be stored in a memory (e.g.,84) of the image processing component126. In addition, the generated digital pixel value127may be provided to a display (e.g.,34) of the image processing component126, allowing a user to view the image relatively quickly after image detection takes place.

As mentioned above, the comparator118may include multiple stages, such as two or more digital comparators in series.FIG. 5is a diagrammatical representation of a two-stage comparator140for use in the X-ray system10. The illustrated two-stage comparator140includes a coarse comparator142and a fine comparator144. The coarse comparator142is used to generate a coarse digital index value146, and the fine comparator144is used to generate a fine digital index value148. The coarse and fine digital index values146and148may both be transferred from the detector22to the image processing component126, and the image processing component126may be configured to generate the digital pixel value127based on the coarse and fine digital index values146and148.

The following description is one example of the use of coarse and fine comparators142and144to compress an image for transmission. The digital pixel value117, which is generally denoted in the illustrated embodiment as si, is received by the coarse comparator142. The coarse comparator142then compares the digital pixel value117with coarse digital reference values150. The coarse digital reference values150may be maintained in a lookup table with associated coarse digital index values, similar to the lookup table120ofFIG. 4. The coarse comparator142steps through the coarse digital reference values150, comparing these to the digital pixel value117until the coarse digital reference value150reaches or exceeds the digital pixel value117. When this occurs, the coarse comparator142locks in the coarse digital index value146(denoted as ci) associated with the coarse digital reference value150.

After determining the coarse digital index value146, the comparator118calculates (152) a weighted difference between the coarse digital index value146and the digital pixel value117. That is, the coarse digital index value146may be multiplied by a constant and subtracted from the digital pixel value117, and the calculated difference is input to the fine comparator144. The fine comparator144is configured to compare the calculated difference with fine digital reference values154in order to determine the fine digital index value148(denoted as fi). The digital index value119sent to the image processing component126includes both the coarse digital index value146received from the coarse comparator142and the fine digital index value148received from the fine digital comparator144.

In one embodiment, the digital pixel value117input to the two-stage comparator140includes a value that can be expressed in 14 bits (e.g., 1 to 16384), while the output digital index value119can be expressed in 8 bits. To accomplish this, the two-stage comparator140may determine two 7-bit index values ciand fito represent the 14-bit digital pixel value117. The coarse and fine comparators142and144may be linear, the coarse comparator142having a step size of 128 and the fine comparator144having a step size of 1. The weighted difference input to the fine comparator144may be calculated, as indicated by reference numeral152by the following expression: si−ci×27.

Since the digital index values146and148each range from 0 to 128, the digital representation sent across the communication channel91may be 8-bits (7-bits plus 7-bits), instead of the original 14-bits. In other words, the combined bit-width of the coarse and fine digital index values146and148is smaller than the bit-width of the first digital pixel value117. The image processing component126may generate the digital pixel value127represented by the 8-bit transmission according to the following expression: ci×27+fi.

It should be noted that other types of multi-stage comparators140may be used in the index value source112of the detector22in the X-ray system10. The multi-stage comparator140may compress the incoming digital pixel value117into digital index values with other desired bit-widths, instead of 7-bits each. In addition, other embodiments of the multi-stage comparator140may include non-linear mappings between the coarse and fine digital reference values150and154and their respective index values. Many other types and configurations of digital comparators118may be utilized within the detector22to reduce a size of the image representation and, consequently, image transfer time over the communication channel91.

The mapping between digital index values122and digital reference values124maintained in the lookup table120of the index value source112and/or the image processing component126may be non-linear. That is, unlike the digital reference values150and154described inFIG. 5, the digital reference values124may have varying step sizes.FIG. 6illustrates a plot170modeling a linear-quadratic mapping between the digital index values122and the digital pixel values132, which may be maintained in the lookup table130. This linear-quadratic mapping may be used to generate the digital pixel value127from the digital index value119received by the image processing component126. The plot170illustrates digital pixel values132(ordinate) against digital index values122(abscissa), and the plot170includes a linear section172and a quadratic section174. The linear section172extends from zero to linear index value limits176and178. This type of mapping may be especially useful when the digital pixel value127contains an offset. This offset could represent any number of offsets to the first digital pixel value117across a range of temperatures, frame rates, and part to part variations within the system components, among other things. For example, when the X-ray system10is operating under certain temperature conditions, the dynamic range of the digital pixel values117received from the detector element array100may be different or offset from the desired dynamic range for the digital pixel values127to be stored/displayed. The quadratic section174represents the relationship between the digital pixel values132and the digital index values122outside of this offset. As the digital index values122increase, the step size between subsequent corresponding digital pixel values132increase as well. This quadratic section174extends from the linear index value limits176and178to quadratic index value limits180and182, respectively. In certain embodiments, the quadratic section174extends across nIvalues along the axis of digital index values122, and across nGvalues along the axis of digital pixel values132. The illustrated linear-quadratic mapping may be modeled with the following two-part equation:
yn=b·xnwhenxn≦nLand  (2)
yn=b·nL+c·(xn−nL)2whenxn>nL.  (3)

In the above equation, b and c are constant coefficients determined based on the configuration of the particular X-ray system10. xnis the input, which may represent the digital index value119generated by the index value source112and communicated to the image processing component126. ynis the output, which may represent the digital pixel value127generated based on the received digital index value119. nLis the number of linear levels within the model, based on the amount of pixel value offset in the digital index values122. This equation may be utilized to map the digital index values122with their corresponding digital pixel values132and/or digital reference values124. Again, this mapping may be determined by the detector controller92based on the relationship between X-ray quantum noise σQ, which increases with respect to the detected analog pixel value115, and quantization noise σδ.

FIG. 7is a process flow diagram of a method190for operating the X-ray system10. The method190includes converting (block192) the analog pixel value115to the digital pixel value117using the linear A/D converter110. The method190also includes generating (block194) the digital index value119based on a comparison of the digital pixel value117to one or more of the digital reference values122using the digital comparator118. This may be accomplished through the use of the lookup table120mapping the digital index values122to the digital reference values124. In some embodiments, generating the digital index value119includes generating the coarse digital index value146and the fine digital index value148, as discussed in reference toFIG. 5. In certain embodiments, the method190includes transmitting (block196) the digital index value119from the index value source112having the comparator118to the image processing component126via the transmitter114. Further, the method190includes mapping (block198) the generated digital index value119to the digital pixel value127via the image processing component126, where the digital pixel value127has a larger bit-width than the digital index value119. This may be accomplished by using an equation that is at least partially quadratic, such as the equation described in reference toFIG. 6. The mapping (block198) may be based on the same lookup table120used to map digital index values122to digital reference values124in the index value source112. The method190also may include displaying (block200) an image on the display34of the image processing component126, where the image includes the digital pixel value127generated by the image processing component126.

Technical effects of the disclosed embodiments include the ability to, among other things, decrease image transfer time across the communication channel between the X-ray detector and the imager system. The use of a quadratic mapping between pixel values and index values may allow for substantially more compression of the image than would be possible with a linear mapping, due to the relationship between quantum noise and quantization noise. That is, the higher the X-ray intensity for a given pixel, the higher the step size selected for quantizing the pixel value to an index value. The use of a multi-stage comparator may allow smaller representations of the digital pixel values for communication through two or more relatively simple, linear mappings of pixel values to coarse and fine index values that can be communicated to the image processing component. The linear A/D converter may convert the analog signal to a full-sized digital representation before the digital value is quantized, saving processing time within the system.