Method and system for X-ray CT imaging

Methods and systems for performing x-ray computerized tomographic (CT) reconstruction of imaging data on a rotatable portion of the system, such as a ring-shaped rotor. The rotor may include an x-ray source, and x-ray detector system and a processor, coupled to the detector system, for performing tomographic reconstruction of imaging data collected by the detector system.

BACKGROUND

Conventional 3D computed tomography (CT) x-ray scanning systems are large, fixed-bore devices that are typically located in the radiology department of a hospital or other medical facility. In a typical device, the patient is loaded into the bore through the front or rear of the device, and a rotatable component, such as a large drum to which imaging components are secured, is rotated around the patient to collect imaging data. The collected imaging data is exported off of the rotating portion, such as via a cable or slip ring system, to an external computer or workstation, where the collected data may be processed using a suitable tomographic algorithm to produce a three-dimensional tomographic reconstruction of a region of interest of the patient.

SUMMARY

Various embodiments include methods and systems for performing x-ray computerized tomographic (CT) reconstruction of imaging data on a rotatable portion of the system, such as a ring-shaped rotor. The rotor may include an x-ray source, and x-ray detector system and a processor, coupled to the detector system, for performing tomographic reconstruction of imaging data collected by the detector system.

Embodiments include methods for generating an x-ray CT reconstruction with an imaging system including an x-ray source and a detector mounted to a rotatable rotor, the method including generating an electronic representation of image data received at a plurality of detector elements of the detector while the rotor rotates, sending the electronic representation of the image data to a processor located on the rotor while the rotor is rotating, performing tomographic reconstruction of the image data using the processor located on the rotor, and transmitting the reconstruction from the rotor to an entity off the rotor.

Further embodiments include an x-ray CT system that includes an x-ray source, a detector, a memory, and a processor coupled to the memory and configured with processor-executable instructions for performing tomographic reconstruction of image data received from the detector, wherein the x-ray source, the detector, the memory and the processor are located on a rotor that rotates around an object being imaged.

Further embodiments include an x-ray CT system including means for generating an electronic representation of image data received at a plurality of detector elements of the detector while the rotor rotates, means for sending the electronic representation of the image data to a processor located on the rotor while the rotor is rotating, means for performing tomographic reconstruction of the image data using the processor located on the rotor, and means for transmitting the reconstruction from the rotor to an entity off the rotor.

Further embodiments include non-transitory computer-readable storage media having stored thereon processor executable instructions configured to cause a processor of a first detector module of an x-ray CT imaging system to perform operations including receiving an electronic representation of image data from a second detector module, appending an electronic representation of image data received at a plurality of detector elements of the first detector module to the electronic representation of image data received from the first detector module to generate a combined image data set, and transmitting the combined image data set from the from the first detector module.

DETAILED DESCRIPTION

Various embodiments include methods and systems for performing x-ray computerized tomographic (CT) reconstruction of imaging data on a rotatable portion of the system, such as a ring-shaped rotor. The rotor may include an x-ray source, and x-ray detector system and a processor, coupled to the detector system, for performing tomographic reconstruction of imaging data collected by the detector system.

FIG. 1is a schematic cross-sectional view of an imaging system100according to one embodiment. The imaging system may include a rotatable portion101and a non-rotatable portion103. The rotatable portion101may rotate around an image bore16within which an object, such as a human or animal patient, may be positioned, to obtain x-ray image data (e.g., raw x-ray projection data) corresponding to the object. The rotation of the rotatable portion101with respect to the non-rotatable portion is schematically illustrated by an arrow inFIG. 1. The rotatable portion101may comprise a rotor41. The rotor41may be a rigid, ring-shaped component that may be located within a gantry (not illustrated). The gantry may be a substantially O-shaped housing that defines the bore16, and may include a protective outer shell that defines an internal cavity within which the rotor41may rotate. The outer shell of the gantry may not rotate, and thus may be part of the non-rotatable portion103of the system.FIG. 1schematically illustrates a number of components of the rotating portion101, including an x-ray source43, detector system45, and computer46, which may be mounted to rotor41. Other components may be provided on the rotor41, such as a high-voltage generator, a power supply (e.g., battery system), rotor drive system, and a docking system, which are not illustrated for clarity.

During an imaging scan, the rotor41rotates around an object positioned within the bore16, while the imaging components such as the x-ray source43and detector system101operate to obtain imaging data (e.g., raw x-ray projection data) for an object positioned within the bore of the gantry, as is known, for example, in conventional X-ray CT scanners. The collected imaging data may be fed to an on-board computer46, preferably as the rotor41is rotating, for performing x-ray CT reconstruction, as will be described in further detail below.

Various details of embodiments of an imaging system can be found in the above-referenced U.S. application Ser. Nos. 12/576,681, 13/025,566, 13/025,573, 13/441,555, 61/658,650, 61/659,609, and 61/664,437, which have been incorporated herein by reference. It will be understood that these embodiments are provided as illustrative, non-limiting examples of imaging systems suitable for use in the present methods and systems, and that the present systems and methods may be applicable to imaging systems of various types, now known or later developed.

The detector system45may include a plurality of x-ray sensitive detector elements, along with associated electronics, which may be enclosed in a housing or chassis303(FIG. 3). In one embodiment, the detector chassis has a width of 7¾ inches, a depth of between about 4-5 inches and a length of about 1 meter or more, such as about 43 inches. The detector chassis303may be a rigid frame, which may be formed of a metal material, such as aluminum, and which may be formed by a suitable machining technique. The detector system45may be mounted to the rotor41opposite an x-ray source43, as is shown inFIG. 1. A plurality of x-ray-sensitive detector elements may be provided in the interior of the detector chassis303so that the detector elements face in the direction of the x-ray source43. The detector chassis303may form a protective air- and light-tight shroud around the detector elements, so that unwanted air and light may not contaminate the sensitive components housed within the detector system45.

In various embodiments, the individual detector elements may be located on a plurality of detector modules107.FIG. 3illustrates a plurality of detector modules107arranged within a chassis303of detector system45. Each individual detector element, which may be for example, a cadmium tungstate (CdWO4) material coupled to a photodiode, represents a pixel on a multi-element detector module107. The modules107may be 2D element array, with for example 512 pixels per module (e.g., 32×16 pixels).

The detector system45may include one or more detector modules107mounted within the detector chassis103. The module(s)107may be arranged along the length of the chassis103to form or approximate a semicircular arc, with the arc center coinciding with the focal spot109of the x-ray source43(seeFIG. 1). In one embodiment, the detector system45includes thirty-one two-dimensional detector modules107positioned along the length of the chassis103, and angled relative to each other to approximate a semicircular arc centered on the focal spot of the x-ray source. Each module107may be positioned such that the detector module107surface is normal to a ray extending from the x-ray focal spot109to the center pixel of the module107.

It will be understood that the detector system45may include any number of detector modules107along the length of the detector. As shown inFIG. 3, for example, a detector may include “m” modules107, where “m” may be any integer greater than or equal to 1. Further, each detector module107may include an arbitrary number of individual elements (pixels) in the module. Larger and/or a greater number of detector modules107may allow a larger diameter “backprojection” area around the isocenter of the imaging system, and thus may allow a larger cross-section of the object to be reconstructed.

As shown in the embodiment ofFIG. 1, each module107may be electronically connected to a computer46which may be located on the rotatable portion101of the system (e.g., mounted to the rotor41). The computer46may include a memory104and a processor102coupled to the memory, as is known in the art. The processor102may be configured to perform tomographic reconstruction of image data that is sent to the computer46from the detector modules107. The computer46may also include a transmitter/transceiver106, which may provide a wireless link to an external entity108. The computer46may wirelessly transmit tomographic reconstruction data (e.g., 3D images of the object) to the external entity108, which may be another computer, such as an external workstation, or a separate computer on the imaging system100(e.g., a computer on a gimbal that supports the gantry). In other embodiments, the computer46may transmit tomographic reconstruction data to another entity using a wired link (e.g., via a slip ring or cable connection to the non-rotating portion103, or via a data dock to the non-rotating portion103in between scans).

FIG. 2is a process flow diagram illustrating an embodiment method200for generating 3D CT reconstructions. The method200may be performed using an imaging system100such as described and illustrated in connection withFIG. 1. In block202of method200, an electronic representation of the image data received at the plurality of detector elements on detector45may be generated while the rotor41rotates. In embodiments, each detector element may include a x-ray sensitive element, such as a cadmium tungsten crystal, coupled to a photodiode, which may produce an electric charge corresponding to the number of x-ray photons incident on the detector element. Each detector module107may include associated electronic components that may be configured read out this charge signal from the photodiode at regular intervals (e.g., 480 Hz). In embodiments, each detector module107may further include components, such as one or more analog-to-digital converters, for converting the detector signals to digital signals. In other embodiments, the detector elements may be photon counting type detectors that may directly produce a digital representation of the incident x-ray radiation without requiring separate A/D converters.

In block204of method200, the electronic representation of the image data is sent to the processor102located on rotor41while the rotor41is rotating. In the embodiment ofFIG. 1, for example, the electronic representation may be sent to a computer46containing a processor102and memory104which is located on the rotor41. In various embodiments, the detector modules107may include associated electronics for converting the raw image data from each detector element into a form suitable for sending the data to the processor102. As discussed above, the detector modules107may include A/D converter(s) for converting analog signals from the detector elements into digital signals. In embodiments, the digital signals may be provided to the computer46and/or processor102as a digital video signal, such as in LVDS or camera link format. In some embodiments, the image data signals may be provided to the computer46and/or processor102in another format, such as gigabit Ethernet. The imaging system100may further include a frame grabber, which may be integrated with the computer46and may be implemented in hardware, software, or a combination of both. The image data received from the detector45may be stored in memory104in the form of a plurality of image frames, each of which may represent a combined image of the object from all detector elements/modules in the detector system45.

In block206of method200, the processor102on the rotor41may perform tomographic reconstruction of the image data. In embodiments, the processor102may be coupled to memory containing processor-executable instructions to perform tomographic reconstruction of the image data received from the detector45. A variety of tomographic algorithms are known which may be implemented by a processor, as is known in the art. In embodiments, the processor102may be a parallel processor comprising a plurality of processing cores for performing the tomographic reconstruction process in parallel. In embodiments, the processor102may be a graphics processing unit (GPU), which may be located on a graphics card. The GPU may include a large internal memory (e.g., up to 8 gigabytes or more, such as 2-4 gigabytes) and a plurality of processing cores (e.g., up to 4096 cores or more, such as 2048 cores) for performing parallel processing of the imaging data. The image data (e.g., image frames) from memory104may be copied to the GPU memory, and the GPU processor(s) may implement a tomographic algorithm to generate a 3D CT reconstruction of the object.

It will be understood that the processor102may include any suitable processing device, such as one or more of a GPU, a CPU, an FPGA, ASIC, etc.

In various embodiments, the tomographic reconstruction at block206of method200may be performed, at least in part, while the rotor41rotating (e.g., during the imaging scan as the detector45is acquiring image data). This may save substantial time in generating the reconstruction.

In block208of method200, the reconstruction from processor102may be transmitted off of the rotating portion101(i.e., the rotor41) to another entity. As discussed above, the computer46may wirelessly transmit tomographic reconstruction data (e.g., 3D images of the object) to the external entity108, which may be another computer, such as an external workstation, or a separate computer on the imaging system100(e.g., a computer on a gimbal that supports the gantry). In other embodiments, the computer46may transmit tomographic reconstruction data to another entity using a wired link (e.g., via a slip ring or cable connection to the non-rotating portion103, or via a data dock to the non-rotating portion103in between scans).

In various embodiments, the computer46and/or processor102may pass the fully reconstructed image off the rotor as soon as the imaging scan is completed. In embodiments, the computer46and/or processor102may begin passing the reconstruction off the rotor while the imaging system is still scanning. In embodiments, the computer46and/or processor102may pass the reconstruction off the rotor41while the rotor is rotating.

Since the data may be reconstructed while the system100is scanning, the most recent images from the reconstruction may be passed off wirelessly while the system is still scanning. The wireless transfer rate may be at least about 300 megabits per second. In embodiments, each image in the reconstruction may be about 4 megabits, thus at least about 75 reconstructed images may be passed off the rotor41per second. In one embodiment, the system100may scan at least about 24 images per second, and may reconstruct at least about twice that rate (e.g., 48 images per second, or twice real time scan speed). Thus, the system100may pass the 24 reconstructed slices per second of scan over a wireless link essentially in real time.

In embodiments, the reconstruction data may be transmitted off the ring via a data dock which may be selectively engaged to provide a connection between the rotating101and non-rotating103portions of the system100when the rotor41is not rotating (e.g., between scans). The docking system may include a connector for carrying power between the rotating and non-rotating portions. In embodiments, the docking system may be used to provide power to a power source (e.g., rechargeable battery system) on the rotor41such that the power source may be charged using power from an external power source (e.g., grid power). The docking system may also include a data connection to allow data signals to pass between the rotating and non-rotating portions. Further details of a suitable docking system are described in U.S. application Ser. No. 13/441,555, filed Apr. 6, 2012, which has been incorporated herein by reference. In embodiments, a docking system may include, for example, a gigabit Ethernet connection, or similar data connection, that may be used to transmit CT reconstructions off the rotor41once the scan is completed and the docking system is engaged. In embodiments, the reconstruction data for an average scan may be about 1000 megabits, so a data dock having a gigabit Ethernet connection may transfer the completely reconstructed data off the rotor41in about 1 second for a typical scan.

In embodiments, the system100may include a slip ring system that may be configured to pass reconstruction data off the rotor41while the system scans. A typical slip ring system may have a data transfer rate that is faster than the scan and reconstruction rates, and thus may pass reconstruction data off the rotor41essentially in real time.

In various embodiments, the system100may be used to pass “scout” scan data from the rotor41in real-time. A scout scan may be performed while the rotor41is not rotating to provide a series of scan lines of the patient (e.g., as the source and detector translate along the patient axis), which may be useful, for example, in choosing a subregion to perform a full 3D scan. The scan lines may be provided from the detector45to processor102, as described above, which may transmit the scan lines in real time to an external entity (such as a workstation or other computer) for displaying a 2D image of the patient in real-time.

FIG. 3illustrates an alternative embodiment system300for implementing the method200ofFIG. 2, in which the 3D tomographic reconstruction may be performed within the detector45itself, such as within the detector chassis303. In this embodiment, a processor102and memory104may be located within the detector45, and a separate computer46may not be required. Image data may be fed from the detector modules107to the memory104and processor102as described above. The memory104and processor102may be provided on a graphics card with a GPU, for example. The 3D reconstruction data produced by processor102may be transmitted off the rotor41from the detector45, or may be sent to a separate component (e.g., a transmitter/transceiver) outside of the detector45for transmission of the rotor41.

In embodiments, multiple processors102with associated memory104may be provided in the detector. For example, each module107or a subset of modules107may include a processor102and memory104for performing tomographic reconstruction of a portion of the image data (e.g., each module or module subset may backproject its own data). The partially reconstructed data may then be summed, which may be done at a separate processor102, to provide the full reconstruction, which may then be transmitted off the rotor41.

It will be understood that in addition to on-board computer46and detector45, the processing device102for performing the reconstruction may be at any location on the rotating portion101(e.g., rotor41).

FIGS. 4-6illustrate further features in accordance with various embodiments.FIG. 4is a schematic side view of a detector module107and associated electronics according to one embodiment, andFIG. 5is a schematic end view illustrating a plurality of detector modules107in a daisy-chain configuration.FIG. 6is a schematic illustration of an imaging system600for performing tomographic CT reconstruction on the rotating portion of the system in accordance with one embodiment.

As shown inFIG. 4, a detector module107may include an array of photosensitive elements402which may be electrically and optionally physically coupled to a circuit board404that may include one or more electronic components. In embodiments, the detector element array402may plug into a circuit board404using a suitable electronic connection. The circuit board402may be configured to couple the raw analog signals from each detector element in the array402into an analog-to-digital converter406for converting the signal to a digital signal. In the embodiment ofFIG. 4, the circuit board402includes four A/D converters406. Each detector element may provide its analog signal over a separate channel into the A/D converters406. For example, where the array402includes 512 pixels, four 128-channel A/D converters406may be provided to convert the analog signal from each element into a digital signal.

The A/D converters406may include a “double buffering” configuration, such that while a first plurality (e.g., frame) of image data accumulates in one buffer, a second plurality (e.g., frame) of digital image data may be read out. The A/D converters406may further output the converted digital data in a suitable digital video format, such as LVDS. In one embodiment, the A/D converters406may comprise ADAS1128 analog-to-digital converters from Analog Devices, Inc. of Norwood, Mass.

The circuit board402may include a processor410, which may be, for example, an FPGA. The processor410may receive the digital image data from the A/D converters408, which may be in a digital video format, such as LVDS, and may be programmed to assemble the data into a single image. The processor410may be configured to convert the image data to a different digital video format, such as Camera Link. In embodiments, the processor410may convert the image data into another suitable format, such as gigabit Ethernet. The processor410may also be programmed to receive image data from one or more other detector modules107, which may be combined with the image data from the A/D converter(s)406and passed off of the module107in a daisy-chain configuration, as is discussed in further detail below. In preferred embodiments, the processor410may receive and transmit the image data in a Camera Link digital video format.

FIG. 5illustrates three adjacent detector modules107n−1,107n, and107n+1. Each module may include a circuit board402and processor410(e.g., FPGA) as discussed above in connection withFIG. 4. Each circuit board402may also include a pair of connectors502, which may be digital video connectors, such as Camera Link digital video connectors. A suitable electrical connection504, such as a ribbon connector, may be provided between the connectors502of each adjacent module107. Camera Link format may be advantageous due to the small size of the connectors and for clocking issues, although other suitable formats for transmitting the image data, including other digital video formats, may be employed.

FIG. 6illustrates an imaging system600according to one embodiment. A detector45includes a plurality of detector modules1071through107m, which may be as described above in connection withFIGS. 4 and 5. Each detector module may be connected to its adjacent modules via connectors502, which may be digital video (e.g., Camera Link) connectors. The first module1071may be similarly connected to a separate circuit board602(a “headboard”), which may include a processor (e.g., FPGA). The last module107mmay be similarly connected to a separate circuit board604(a “tailboard”), which may also include a processor (e.g., FPGA). The processor of the headboard602may generate signals, such as clock signals (e.g., Camera Link clock signals) which may be sent over the digital video connector and propagate down the line of modules107in a daisy chain fashion to tailboard604. The processor of tailboard604may similarly generate signals that may propagate back through the line of modules107to headboard602. As discussed above, the processor410of each module107, in response to receipt of a clock signal from headboard602and/or in response to receiving image data from another detector module107, may read out its own image data and transmit the data, which may be in a digital video format such as Camera Link format, to the next detector module107in the line. Where the processor410of a module107receives image data from a prior module107in the line, the processor410may be configured to combine its own image data with the data of one or more prior modules107before passing the combined image data to the next detector module107in the line in a daisy-chain configuration.

The combined image data may be received at tailboard604, which may include a processor configured to transmit the combined data to a computer46having a memory104and processor102and which may be located on the rotatable portion101of the system (e.g., mounted to the rotor41), as is described above in connection withFIGS. 1-3. The processor102may be configured to perform tomographic reconstruction of image data that is sent to the computer46from the detector modules107. The tailboard604may send the combined image data to the computer46in a video signal format, such as Camera Link, or in another format, such as gigabit Ethernet. In embodiments, a video transmitter device606, such as the iPORT from Pleora Technologies of Ottawa, ON, may be connected to the tailboard604for converting the digital video image signal (e.g. Camera Link) into a gigabit Ethernet signal for transmission to the computer46.

FIG. 7is a process flow diagram illustrating a method700for performing tomographic CT reconstruction according to one embodiment. The method700may be performed using an imaging system such as described and illustrated in connection withFIGS. 4-6. In block702of method700, a clock signal may be received at a processor of a first detector module1071indicating that a set of imaging data is to be collected. The clock signal may be generated by a headboard602, as described above. In block704, the processor of the first detector module1071may transmit digital image data to an adjacent detector module (e.g.,1072). The image data may be transmitted in a digital video format, such as Camera Link. In block706, the digital image data from the first module1071is received by the processor of the adjacent module1072. In block708, the processor of the second module1072may append its own digital image data to the digital image data from the first module1071to generate a combined digital image data set. The combined digital image data set may be transmitted in a digital video format, such as Camera Link. If there are additional modules with imaging data to transmit (i.e., block710=Yes), then the combined digital image data set may be sent to the next adjacent module (e.g.,1073) in block704. This process may then be repeated for each detector module (e.g.,107n−1,107n,107n+1, etc.) along the line of detector modules. The combined digital image data set transmitted by each module may be in a digital video format, such as Camera Link. When the last module107mhas appended its own image data to the combined image data set, there are no additional modules to which the combined image data set may be transmitted (i.e., block710=No). The last module107mmay then transmit the combined image data set to a processor102for performing tomographic reconstruction at block712. As described above, the last module107mmay transmit the combined image data set to the processor102via a tailboard604and/or a video transmitter device606(e.g., iPort). The combined image data set may be transmitted in a digital video format (e.g., Camera Link), and optionally converted into a different format (e.g., gigabit Ethernet) before being received at processor102/computer46. The tailboard604may optionally send a return signal back through the detector modules107to headboard604indicating that the combined video image data set has been transmitted to the processor102. The headboard604may then issue another clock signal (e.g., block702of method700), and the entire process may repeat for new image data (e.g., a new frame) collected by the detector modules107. The process may be repeated at a regular frequency (e.g., 480 Hz) for the entirety of an image scan (e.g., x-ray helical or circular CT scan). The transfer rate of the detector may be variable, and may be more or less than 480 Hz in various embodiments. The clock or frame rate may vary based on the speed of rotation of the rotor41. For example, for a system that scans at a rate of 1 rotation every two seconds, with 960 frames per rotation, the transfer rate of the detector may be 480 Hz. However, with a faster or slower rotation speed of the rotor41the transfer rate of the detectors may be more or less than 480 Hz. In embodiments, between about 500 and 1500 frames may be recorded per rotation of the rotor and the clock or frame rate may be dependent on the speed of rotation of the rotor.

It will be understood that the number of modules (m) in the detector45may vary, and modules may be added or removed as needed. In various embodiments, changing the number and/or types of detector modules does not require a new or modified “backplane” electronics board, for example. Also the clock signal (e.g., a Camera Link clock signal) may be variable to provide more or less image frames per second.

As shown inFIG. 6, a reference detector608may be provided at the x-ray source43to measure the flux of the photons leaving the x-ray tube before the photons impinge on the object being imaged. The reference detector608may be a single x-ray sensitive element (e.g., a scintillator, such as a cadmium tungstate crystal), and may be identical to the x-ray sensitive elements in each of the detector elements of the detector system45. A fiber optic cable610may be coupled to the reference detector608to transmit an optical signal from the reference detector608to an electronics module612. The electronics module612may be located in a temperature-controlled location on the rotor41(e.g., in a location where heat from the x-ray source43does not interfere with operation of components, such as a photodiode, of the electronics module612). The reference detector608, fiber optic cable610and electronics module612may be potted (e.g., with carbon-filled epoxy) to prevent unwanted light from contaminating the optical signal. The electronics module612may include a photodiode that generates an electronic signal in response to the incident optical signal from the reference detector608, and associated electronics (e.g., A/D converter, FPGA, etc.) that may convert the electronic signal into a digital signal that may be fed to the processor102for use in performing the tomographic reconstruction. The reference detector signal may be sent in a digital video format, such as Camera Link. In embodiments, the digital reference detector signal from the electronics module612may be sent to the detector45, where the signal may be embedded within the digital image data from the detector modules107before it is transmitted to the processor102for reconstruction. For example, the reference detector signal may be sent to the headboard602of the detector45. The headboard602may then send the signal to the first detector module1071, such as with its clock signal, and the reference detector signal may be appended to the digital image data from the first detector module1071when it is transmitted to the next module1072along the line. The reference detector signal may thus propagate down the line of detector modules107in a daisy-chain fashion, and may then be fed to the processor102for tomographic reconstruction.

The reference detector608may also include a temperature sensor, such as a resistance temperature detector (RTD) that may generate an electronic signal indicative of the temperature within the x-ray source43. The temperature signal may be a digital signal that may be embedded within the image data stream that is sent to the processor102for tomographic reconstruction in the manner described above for the reference detector signal.

FIG. 8illustrates a reference detector608and fiber optic cable610assembly according to one embodiment. The reference detector608may be embedded in a housing, which may be a brass housing having a hole for x-ray photons to enter. An RTD may also be provided in the housing. The fiber optic cable610may have a polished first end that is bonded to a polished end of the reference detector608(e.g., scintillator crystal) for receiving incident light from the reference detector608. The subassembly of reference detector608and fiber optic cable610may inserted into the housing (along with the RTD) and potted within the housing, which may be a brass housing. The fiber optic cable610may have a polished second end that may be bonded to a photodiode. One or more wire leads may couple the RTD output to an electronics module (e.g., circuit board).

FIGS. 9A and 9Billustrate the reference detector608and fiber optic cable610assembly within an x-ray source43. As is illustrated inFIGS. 9A and 9B, the reference detector608may be positioned proximate to an edge of the x-ray beam outlet port, such that the reference detector608does not cast a “shadow” on the object being imaged. The reference detector608may be positioned behind a collimator so that it may measure the flux of the x-ray photons prior to the photons being collimated.