Patent Description:
In an X-ray based imaging system, such as a computed tomography (CT) imaging system, an X-ray beam is emitted towards an object such as a patient or item (e.g., package, manufactured item, and so forth) to image a region of interest in the object. The beam is typically attenuated as it passes through the object. Subsequently, the attenuated beam is incident on a radiation detector having an array of detector elements. In response to the attenuated beam, the detector elements of the array generate respective electrical signals representative of internal information of the object. These electrical signals are processed by a data processing unit to generate an image representative of the region of interest in the object.

Reconstruction of images from the acquired data is generally based upon the assumption that X-ray photons have traveled in a straight path from an X-ray emission focal spot to the detector element at which the respective photon is detected. However, mis-alignment or movement of the X-ray focal spot with respect to one or more collimating elements or plates (e.g., a post-patient anti-scatter grid) may result in image artifacts that are detrimental to clinical use of imaging systems, such as CT imaging systems. This effect may be more significant in systems where the collimator blade pitch is larger than the channel (i.e., pixel) pitch, such that different channels may be effected to different degrees by the "shadow" case by the respective collimator blades.

D1 (<CIT>) discloses - The techniques disclosed may be used to detect and correct channel gain errors resulting from X-ray focal spot mis-alignment during the course of a scan. One benefit of the present invention relative to conventional techniques is that additional hardware is not required for detection of focal spot drift. Instead, the static mis-alignment of each blade is taken into account as part of estimating and correcting X-ray focal spot drift or mis-alignment. In this manner, the risk of image artefacts due to focal spot motion is reduced and the need for costly hardware solutions to detect focal spot motion is avoided.

D2 (<CIT>) discloses - Systems and methods for focal spot motion correction are provided. One system includes a radiation source configured to project radiation from a first focal spot onto an object and a plurality of radiation detectors disposed around at least a portion of the object. The plurality of radiation detectors measure received radiation along a path projected from the first focal spot to the plurality of detectors. The imaging system further includes an imaging region from which the detectors provide image information for image reconstruction and a plurality of collimators positioned between the object and the plurality of radiation detectors. At least one collimator at a first end of the plurality of collimators and at least one collimator at second end of the plurality of collimators are aligned to a second focal spot different than the first focal spot and having a different location.

These embodiments are not intended to limit the scope of the claimed subject matter, but rather these embodiments are intended only to provide a brief summary of possible embodiments.

In one embodiment, a method for estimating motion of an X-ray focal spot is provided. The acts of the method include acquiring image data by causing X-rays to be emitted from the X-ray focal spot of an X-ray source toward a radiation detector comprising multiple channels, wherein a subset of the channels each have a collimator blade positioned above the respective channel. The acts of the method also include independently estimating X-ray focal spot motion in an X-direction for the X-ray focal spot relative to an isocenter of the radiation detector and in a Y-direction along a direction of the X-rays for the X-ray focal spot relative to the isocenter based on respective channel gains for a first channel and a second channel of the subset of the channels.

In another embodiment, an imaging system is provided. The imaging system includes an X-ray source configured to emit X-rays from an X-ray focal spot during operation, a collimator including multiple collimator blades, and a radiation detector, including multiple pixels, each pixel corresponding to a channel of the radiation detector, wherein a subset of the channels each have a collimator blade positioned above the respective channel. The imaging system also includes processing circuitry configured to perform acts. The acts include acquiring image data by causing X-rays to be emitted from the X-ray source toward the radiation detector. The acts also include independently estimating X-ray focal spot motion in an X-direction for the X-ray focal spot relative to an isocenter of the radiation detector and in a Y-direction for the X-ray focal spot relative to the isocenter based on respective channel gains for a first channel and a second channel of the subset of the channels.

In a further embodiment, a non-transitory computer-readable medium, the computer-readable medium including processor-executable code that when executed by a processor, causes the processor to perform acts. The acts include acquiring image data by causing X-rays to be emitted from an X-ray focal spot of an X-ray source toward a radiation detector including multiple channels, wherein a subset of the channels each have a collimator blade positioned above the respective channel. The acts also include simultaneously measuring a respective channel gain for a first channel and a second channel of the subset of channels disposed on opposite sides of an isocenter of the radiation detector. The acts further include independently estimating X-ray focal spot motion in an X-direction for the X-ray focal spot relative to the isocenter and in a Y-direction for the X-ray focal spot relative to the isocenter based on the respective channel gains for the first channel and the second channel.

When introducing elements of various embodiments of the present subject matter, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. Furthermore, any numerical examples in the following discussion are intended to be nonlimiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments.

While aspects of the following discussion may be provided in the context of medical imaging, it should be appreciated that the present techniques are not limited to such medical contexts. Indeed, the provision of examples and explanations in such a medical context is only to facilitate explanation by providing instances of real-world implementations and applications. However, the present approaches may also be utilized in other contexts, such as tomographic image reconstruction for industrial Computed Tomography (CT) used in non-destructive inspection of manufactured parts or goods (i.e., quality control or quality review applications), and/or the non-invasive inspection of packages, boxes, luggage, and so forth (i.e., security or screening applications). In general, the present approaches may be useful in any imaging or screening context or image processing field mis-alignment of an X-ray emission point may be mis-aligned with an array of detector elements having associated anti-scatter or collimation elements (e.g., blades).

As discussed herein, reconstruction of images from the acquired X-ray transmission data is generally based upon the assumption that X-ray photons have traveled in a straight path from an X-ray emission focal spot to the detector element at which the respective photon is detected. However, mis-alignment or movement of the X-ray focal spot with respect to one or more collimating elements or plates (e.g., a post-patient anti-scatter grid) may result in image artifacts that are detrimental to clinical use of imaging systems, such as CT imaging systems. This effect may be more pronounced in systems where the collimator blade pitch is larger than the channel (i.e., pixel) pitch.

The techniques disclosed may be used to detect and correct channel gain errors resulting from X-ray focal spot mis-alignment during the course of a scan, which may result in the noted image artifacts. One benefit of the techniques described herein relative to conventional techniques is that they do not require additional hardware for detection of focal spot drift. Instead, the methods described herein take into account the static mis-alignment of each blade during manufacturing as part of estimating and correcting X-ray focal spot drift or mis-alignment. In this manner, the risk of image artifacts due to focal spot motion is reduced and the need for costly hardware solutions to detect focal spot motion is avoided.

With the preceding discussion in mind, <FIG> illustrates an embodiment of an imaging system <NUM> for acquiring and processing image data in accordance with structures and approaches discussed herein. In the illustrated embodiment, system <NUM> is a computed tomography (CT) system designed to acquire X-ray projection data and to reconstruct the projection data into volumetric reconstructions for display and analysis. The CT imaging system <NUM> includes one or more X-ray sources <NUM>, such as one or more X-ray tubes or solid state emission structures which allow X-ray generation at one or more energy spectra during an imaging session.

In certain implementations, the source <NUM> may be positioned proximate to a pre-patient collimator and/or filter assembly <NUM> that may be used to steer the X-ray beam <NUM>, to define the shape (such as by limiting off-angle emissions) and/or extent of a high-intensity region of the X-ray beam <NUM>, to control or define the energy profile of the X-ray beam <NUM>, and/or to otherwise limit X-ray exposure on those portions of the patient <NUM> not within a region of interest. In practice, the filter assembly or beam shaper <NUM> may be incorporated within the gantry, between the source <NUM> and the imaged volume.

The X-ray beam <NUM> passes into a region in which the subject (e.g., a patient <NUM>) or object of interest (e.g., manufactured component, baggage, package, and so forth) is positioned. The subject attenuates at least a portion of the X-ray photons <NUM>, resulting in attenuated X-ray photons <NUM> that impinge upon a pixelated detector array <NUM> formed by a plurality of detector elements (e.g., pixels) arranged in an m × n array. In the depicted example, the attenuated X-ray photons <NUM> pass through a collimator <NUM> (e.g., an anti-scatter grid) prior to reaching the detector array <NUM>. As discussed herein, the collimator <NUM> may consist of a plurality of blades or other elements aligned substantially perpendicular to the surface of the detector array <NUM> and formed from an attenuating material that limit or prevent X-ray photons <NUM> traveling at off-angles (e.g., scattered X-rays) from reaching the detector array <NUM>. The electrical signals reaching the detector array <NUM> are detected and processed to generate one or more projection datasets. In the depicted example, the detector <NUM> is coupled to the system controller <NUM>, which commands acquisition of the digital signals generated by the detector <NUM>.

A system controller <NUM> commands operation of the imaging system <NUM> to execute filtration, examination and/or calibration protocols, and may process the acquired data. With respect to the X-ray source <NUM>, the system controller <NUM> furnishes power, focal spot location, control signals and so forth, for the X-ray examination sequences. In accordance with certain embodiments, the system controller <NUM> may control operation of the filter assembly <NUM>, the CT gantry (or other structural support to which the X-ray source <NUM> and detector <NUM> are attached), and/or the translation and/or inclination of the patient support over the course of an examination.

In addition, the system controller <NUM>, via a motor controller <NUM>, may control operation of a linear positioning subsystem <NUM> and/or a rotational subsystem <NUM> used to move the subject <NUM> and/or components of the imaging system <NUM>, respectively. For example, in a CT system, the radiation source <NUM> and detector <NUM> rotate about the object (e.g., patient <NUM>) to acquire X-ray transmission data over a range of angular views. Thus, in a real-world implementation, the imaging system <NUM> is configured to generate X-ray transmission data corresponding to each of the plurality of angular positions (e.g., <NUM>°, <NUM>° + a fan beam angle (α), and so forth) covering an entire scanning area of interest.

The system controller <NUM> may include signal processing circuitry and associated memory circuitry. In such embodiments, the memory circuitry may store programs, routines, and/or encoded algorithms executed by the system controller <NUM> to operate the imaging system <NUM>, including the X-ray source <NUM> and/or filter assembly <NUM>, and to process the digital measurements acquired by the detector <NUM> in accordance with the steps and processes discussed herein. In one embodiment, an algorithm be stored in the memory circuitry and executed by a processor to X-ray focal spot motion in both an X-direction relative to an isocenter of the detector <NUM> and in a Y-direction relative to the isocenter of the detector <NUM>. In one embodiment, the system controller <NUM> may be implemented as all or part of a processor-based system.

The source <NUM> may be controlled by an X-ray controller <NUM> contained within the system controller <NUM>. The X-ray controller <NUM> may be configured to provide power, timing signals, and/or focal spot size and spot locations to the source <NUM>. In addition, in some embodiments the X-ray controller <NUM> may be configured to selectively activate the source <NUM> such that tubes or emitters at different locations within the system <NUM> may be operated in synchrony with one another or independent of one another or to switch the source between different energy profiles during an imaging session.

The system controller <NUM> may include a data acquisition system (DAS) <NUM>. The DAS <NUM> receives data collected by readout electronics of the detector <NUM>, such as digital signals from the detector <NUM>. The DAS <NUM> may then convert and/or process the data for subsequent processing by a processor-based system, such as a computer <NUM>. In certain implementations discussed herein, circuitry within the detector <NUM> may convert analog signals of the detector to digital signals prior to transmission to the data acquisition system <NUM>. The computer <NUM> may include or communicate with one or more non-transitory memory devices <NUM> that can store data processed by the computer <NUM>, data to be processed by the computer <NUM>, or instructions to be executed by image processing circuitry <NUM> of the computer <NUM>. For example, a processor of the computer <NUM> may execute one or more sets of instructions stored on the memory <NUM>, which may be a memory of the computer <NUM>, a memory of the processor, firmware, or a similar instantiation. By way of example, the image processing circuitry <NUM> of the computer <NUM> may be configured to generate a diagnostic image. In one embodiment, the diagnostic image is a real-time image obtained using image reconstruction techniques applied to the plurality of signals obtained from the plurality of pixels <NUM> and corrected for X-ray focal spot motion or mis-alignment. In one embodiment, the diagnostic image is a CT image corrected for X-ray focal spot motion or mis-alignment and displayed on a display device <NUM> for assisting a medical practitioner.

The computer <NUM> may also be adapted to control features enabled by the system controller <NUM> (i.e., scanning operations and data acquisition), such as in response to commands and scanning parameters provided by an operator via an operator workstation <NUM>. The system <NUM> may also include a display <NUM> coupled to the operator workstation <NUM> that allows the operator to view relevant system data, imaging parameters, raw imaging data, reconstructed data or images, and so forth. Additionally, the system <NUM> may include a printer <NUM> coupled to the operator workstation <NUM> and configured to print any desired measurement results. The display <NUM> and the printer <NUM> may also be connected to the computer <NUM> directly (as shown in <FIG>) or via the operator workstation <NUM>. Further, the operator workstation <NUM> may include or be coupled to a picture archiving and communications system (PACS) <NUM>. PACS <NUM> may be coupled to a remote system or client <NUM>, radiology department information system (RIS), hospital information system (HIS) or to an internal or external network, so that others at different locations can gain access to the image data.

With the preceding discussion of an overall imaging system <NUM> in mind, and turning to <FIG>, an example of a prior detector <NUM> and collimator <NUM> arrangement is shown in a cut-away side view. In this example, the detector <NUM> is shown as including an array of pixels <NUM> each corresponding to a readout channel. In one such example, the pixel pitch may be approximately <NUM>. A set of collimator blades <NUM> are shown associated with the array of pixels <NUM> such that each pixel is separately collimated. The blades <NUM> are shown as being placed at where pixels are joined, such that shadowing attributable to the blades <NUM> is primarily at these joins, leaving the majority of the active area of the pixels <NUM> relatively free of shadows produced by the blades. In this manner, each pixel <NUM> is effected relatively consistently and uniformly by the collimator blades <NUM>. In particular, if an X-ray emission focal spot is mis-aligned, the differential gain change attributable to the mis-alignment between adjacent channels is relatively small.

Turning to <FIG>, an example of a higher spatial resolution detector <NUM> having smaller pixels <NUM> (e.g., a pixel pitch less than <NUM>). Due to the smaller pixels, each channel may not be separated by respective collimator blades. Instead as shown, each collimator blade <NUM> may provide collimation for multiple pixels <NUM> (i.e., channels), with some pixels <NUM> touched by or immediately adjacent a blade <NUM> and others not adjacent a blade <NUM>. Correspondingly, in the event of an X-ray focal spot mis-alignment the differential gain change due to X-ray focal spot mis-alignment between adjacent channels may be large due to the different placement of the relevant blade <NUM>. That is, X-ray focal spot mis-alignment may result in large gain changes in high resolution detectors.

Conceptually, this is illustrated on <FIG>, where a side-by-side comparison of an aligned (left) and mis-aligned (right) X-ray focal spot <NUM> are illustrated in the context of a collimated detector <NUM>. As shown in the left figure, when the X-ray focal spot <NUM> is aligned (as denoted by longitudinal axis <NUM> extending through the blade <NUM>) with the blade <NUM>, the shadow <NUM> cast by the blade <NUM> is generally symmetric and minimized. Conversely, as shown on the right, when the X-ray focal spot <NUM> is misaligned with respect to the blade <NUM>, the shadow <NUM> cast by the blade <NUM> is not symmetric with respect to different pixels <NUM> (denoted here as channels (CH) -<NUM>, -<NUM>, -<NUM>) and may be increased in size relative to when the X-ray focal spot <NUM> is aligned.

With the preceding discussion in mind, the mis-alignment of the collimator blade <NUM> and the X-ray focal spot <NUM> may result in image artifacts that are detrimental to clinical image quality. In particular, the effect of mis-alignment at the detector level is the introduction of small, but impactful changes in the gain of individual channels due to collimator blade shadowing of the X-ray focal spot, as shown in <FIG>. That is, incremental change to the collimator blade shadow on the respective detector channels may lead to differential changes in channel gain, which can result in image artifacts. As illustrated with respect to <FIG>, this effect may be more significant in contexts where the pitch of the collimator blades <NUM> is greater that the pixel (i.e., channel) pitch such that there are pixels with a collimator blade above them and pixels without such a blade above them. If not detected and corrected, these changes due to X-ray focal spot mis-alignment may be wrongly interpreted as changes in object attenuation, thereby leading to image artifacts.

In practice, X-ray focal spot misalignment may be of two types. Static mis-alignment, as used herein, may be understood to be due to manufacturing tolerances, such as with respect to the deflection or tilt of collimator blades, and can be corrected to some extent by detector calibration. However, dynamic mis-alignment, may occur during the course of a scan due to thermal and mechanical forces generated during operation. Dynamic misalignment can be difficult to detect and, correspondingly, challenging to correct.

With the preceding in mind, the techniques discussed herein may be used to detect and correct channel gain errors attributable to X-ray focal spot misalignment during the course of a scan, including in higher resolution type system, as shown in <FIG>. In particular, the techniques disclosed herein may be performed without additional hardware for the detection of X-ray focal spot drift.

As illustrated in <FIG> and <FIG>, pixels on which a collimator blade is present are affected by X-ray focal spot misalignment (e.g., motion). <FIG> depicts the X-ray focal spot <NUM> (represented as S (x, source to detector distance (SDD)) and the tilted collimator blade <NUM> disposed over a pixel or channel <NUM>. When the X-ray focal spot <NUM> shifts from the left of the tilted collimator blade <NUM> (as depicted in left figure) to the right of the tilted collimator blade <NUM> (as depicted in the right figure), a shadow <NUM> (e.g., signal lost by pixel <NUM>) cast by the blade <NUM> changes in size and shape (which means the pixel or channel gain also changes). For a point focal spot the pixel gain, g, experienced by the pixel <NUM> with the tilted collimator blade <NUM> is represented by the following X-motion gain sensitivity function g(x, θ), where w represent plate width, h represents plate height, p represents pixel or channel pitch, and θ represents the tilt of the collimator blade <NUM>: <MAT> <MAT> and <MAT>.

<FIG> depicts a graph <NUM> of the X-motion gain sensitivity function for a particular pixel having a collimator blade disposed over it at different tilt angles (θ). The graph <NUM> includes a Y-axis <NUM> representing gain sensitivity and an X-axis <NUM> representing X-ray focal spot shift in the X-direction. For the plots <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, the collimator blade is at different tilt angles (θ) (represented as fraction of a minute). As depicted, pixel gain is a linear function of X-ray focal spot location for large shifts in the X-direction (represented by the sloped or inclined portions of the plots <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>), while the pixel gain is constant for smaller shifts in the X-direction represented by the plateaus of the plots <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. The shapes are the same for the plots <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> but the plots <NUM>, <NUM>, <NUM>, <NUM>, <NUM> shift based on the plate tilt angle. The slope of the plots <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are independent of the plate tilt angle.

As depicted in <FIG>, for certain pixels or channels, motion of the X-ray focal spot in the Y-direction (indicated by arrow <NUM> and along a direction of the X-rays) relative to an isocenter (ISO) of the detector may experience apparent motion in the X-direction (indicated by arrow <NUM> and along an arc of the detector) relative to an isocenter. Channels further away from the isocenter (e.g., channels i and j in <FIG>), such as those channels closer to an edge of the detector, may experience the apparent motion in the X-direction. Channels closer to the isocenter may be experience negligible apparent motion in the X-direction. In <FIG>, channels i and j are equidistant from the isocenter at fan angles -γ and +γ. A channel at the isocenter may be designated channel ISO. Channels i, j, and ISO include tilted collimator blades disposed over them. The induction of apparent motion in the X-direction due to motion in the Y-direction is proportional to the sine of the fan angle, γ, and may be represented by the following equation: <MAT> where x represents the apparent motion in the X-direction and y represents the motion in the Y-direction for the X-ray focal spot.

<FIG> depicts a graph <NUM> of apparent motion in the X-direction of an X-ray focal spot experienced by channels of a detector in response to motion in the Y-direction of the X-ray focal spot utilizing equation (<NUM>). The graph <NUM> includes a Y-axis <NUM> representing apparent motion of the X-ray focal spot in the X-direction and an X-axis <NUM> representing the channels of a detector (e.g., with an isocenter being near channel <NUM>). Plots <NUM>, <NUM>, and <NUM> represent different amounts of motion of the X-ray focal spot in the Y-direction. As depicted in graph <NUM>, the y-motion induced apparent X motion of the X-ray focal spot (as determined via equation (<NUM>)) is an asymmetric function. As depicted in graph <NUM>, in the presence of Y-motion of the X-ray focal spot, the apparent X-motion increases as the distance from the isocenter increases (both to the left and right of the isocenter). In the presence of Y-motion of the X-ray focal spot, the apparent X-motion is the same at equal distances to the left and right of the isocenter but different in direction. Also, as depicted in graph <NUM>, as the magnitude of Y-motion of the X-ray focal increases, the apparent X-motion increases for a given channel.

Returning to <FIG>, X-ray focal spot motion in both the X-direction and the Y-direction relative to the isocenter will result in a net or total focal spot motion (x) in the X-direction for the channels i and j, respectively, represented by the following: <MAT> The asymmetric dependence of Y motion induced channel gain, g, on the distance of the channel from the isocenter can be utilized to extract both X motion and Y motion of the X-ray focal spot by the following: <MAT> and <MAT>.

This asymmetric dependence of Y motion induced channel gain, g, on the distance of the channel from the isocenter to extract or detect both X motion and Y motion of the X-ray focal spot is illustrated in <FIG>. Graph <NUM> depicts the Y-motion induced X motion (apparent X-motion) for channels i and j and channel at the isocenter, channel ISO, of the detector in <FIG>. Graph <NUM> includes a Y-axis <NUM> representing X motion and an X-axis <NUM> representing the channels of a detector (e.g., with an isocenter being near channel <NUM>). The graph <NUM> illustrates the Y motion induced X motion xj, xo, and xj for channels i, ISO, and j, respectively. Graph <NUM> depicts the gain sensitivity (and linear gain response) for channels i,j, and ISO in response to the Y-motion induced X-motion shown in graph <NUM>. Graph <NUM> includes a Y-axis <NUM> representing gain sensitivity and an X-axis <NUM> representing the focal spot shift in the X-direction. The gain sensitivities for the channels i, j, and ISO are gj, go, and gi, respectively. The linear gain response of the channels with titled collimator plates (e.g., channels i, ISO, and j) may be utilized in detecting X-motion and Y-motion of the X-ray focal spot.

Based on these relationships discussed above, an algorithm may be utilized to detect X-motion and Y-motion of the X-ray focal spot. In particular, X-motion and Y-motion may be estimated independently from two simultaneous measurements. <FIG> illustrates some of the definitions utilized in the following discussion by relating the schematic of <FIG> to a graph <NUM> that illustrates the linear tilted gain responses for channels i and j in response to total X-ray focal spot shift in the X-direction. The respective total X-motion for channels i and j are <MAT> and <MAT> where xX and xY represent intrinsic X-motion and apparent X-motion, respectively. The respective tilted blade gain responses (e.g., linear gain responses) for channels i and j (shown as plots <NUM> and <NUM>, respectively, in graph <NUM>) are <MAT> and <MAT> where m represents the slope and c the vertical intercept. These equations can be further simplified due to symmetry. For example, intrinsic x-motion is same for ∀ i, thus: <MAT> and <MAT>.

Also, Y-motion of the X-ray focal spot is anti-symmetric, thus: <MAT> and <MAT>.

Further, the slopes mj and mi are identical, thus, <MAT>.

Based on these equations, both the X-motion and the Y-motion of the X-ray focal spot may be estimated independently. The X-motion may be estimated utilizing the following: <MAT>.

The Y-motion may be estimated utilizing the following: <MAT> where <MAT>.

In certain embodiments, the channels with tilted blade gain responses utilized in determining X-ray focal spot motion may have the same tilt angles but opposite orientations since the channels are located on opposite sides of the isocenter of the detector. In other embodiments, as indicated in graph <NUM>, the channels (e.g., channels i and j) with the tilted blade gain responses utilized in determining X-ray focal spot motion may have different tilt angles as well as opposite orientations. Although it is preferable to utilize channels with collimator plates that are equidistance from the isocenter of the detector (e.g., at fan angles -γ and +γ), in certain embodiments, the channels with tilted blade gain response utilized in determining X-ray focal spot may not be equidistant from the isocenter of the detector.

<FIG> depicts a flow chart of a method <NUM> for detecting and correcting for X-ray focal spot motion. One or more steps of the method <NUM> may be performed by the CT imaging system <NUM> of <FIG>. The method <NUM> includes acquiring image data (e.g. CT scan data) by causing X-ray to be emitted from an X-ray focal spot of an X-ray source toward a radiation detector including multiple channels or pixels (block <NUM>). A first subset of the channels each have a collimator blade positioned above a respective channel, while a second subset of channels are unobstructed by collimator blades. The method <NUM> also includes simultaneously measuring respective channel gain for a first channel and a second channel of the subset of channels having a collimator blade disposed over them (block <NUM>). The measuring of the channel gain may occur as image data is being acquired. The first channel and second channel are disposed on opposite sides of an isocenter of the detector. In certain embodiments, the first and second channels are disposed equidistant from the isocenter of the detector. In other embodiments, the first and second channels are not equidistant from the isocenter of the detector. In order to measure the respective channel gain, the method <NUM> includes determining a respective total X-ray focal spot motion in the X-direction for both the first channel and the second channel from which the channel gain can be determined. The total X-ray focal spot motion includes a summation of intrinsic X-ray focal spot motion in the X-direction and apparent X-ray focal spot motion (inducted by Y-motion of the X-ray focal spot) in the X-direction. The method <NUM> further includes independently estimating X-ray focal spot motion in both the X-direction and the Y-direction relative to the isocenter based on the respective channel gains for the first channel and the second channel (block <NUM>). The method <NUM> even further includes calculating a focal spot motion correction factor(s) based on the estimates X-ray focal spot motion in the X-direction and the Y-direction (block <NUM>). The method <NUM> still further includes using the focal spot motion correction factor(s) as part of the image reconstruction or post-reconstruction process, to correct or remove artifacts or other image irregularities (block <NUM>). By way of example, in certain embodiments signal correction may be limited to those channels associated with a pixel on a collimator plate is positioned, i.e., those channels where signal change may be due to misalignment of the collimator blades. Alternatively, in certain embodiments, based on the estimates of X-ray focal spot motion in the X-direction and the Y-direction, the X-ray focal spot location may be actively controlled or corrected (e.g., via magnets that steer the electron beam toward an anode).

Technical effects of the disclosed embodiments include providing a CT imaging system capable of reducing the effects of X-ray focal spot motion during an imaging operation, such as by reducing or eliminating image artifacts attributable to X-ray focal spot motion. Estimation and/or correction of X-ray focal spot motion effects are achieved without additional hardware.

The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as "means for [perform]ing [a function]. " or "step for [perform]ing [a function]. ", it is intended that such elements are to be interpreted under <NUM> U. However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under <NUM> U.

Claim 1:
A method for estimating motion of an X-ray focal spot (<NUM>), comprising the acts of:
acquiring image data by causing X-rays to be emitted from the X-ray focal spot (<NUM>) of an X-ray source (<NUM>) toward a radiation detector (<NUM>) comprising a plurality of channels (<NUM>), wherein a subset of the channels (<NUM>) each have a collimator blade (<NUM>) positioned above the respective channel (<NUM>); and
independently estimating X-ray focal spot motion in an X-direction along an arc of the radiation detector (<NUM>) for the X-ray focal spot (<NUM>) relative to an isocenter of the radiation detector (<NUM>) and in a Y-direction along a direction of the X-rays for the X-ray focal spot (<NUM>) relative to the isocenter based on respective channel gains for a first channel (<NUM>) and a second channel (<NUM>) of the subset of the channels (<NUM>),
wherein a respective total X-ray focal spot motion in the X-direction is estimated to measure the respective channel gains for the first channel (<NUM>) and the second channel (<NUM>), and
wherein the respective total X-ray focal spot motion comprises a summation of an intrinsic X-ray focal spot motion in the X-direction and apparent X-ray focal spot motion in the X-direction, the apparent X-ray focal spot motion in the X-direction being induced by Y-motion of the X-ray focal spot.