Patent Publication Number: US-9417194-B2

Title: Assessment of focal spot characteristics

Description:
BACKGROUND 
     The subject matter disclosed herein relates to focal spot evaluation in X-ray devices. 
     In modern medicine, medical professionals routinely conduct patient imaging examinations to assess the internal tissue of a patient in a non-invasive manner. Furthermore, for industrial applications related to security or quality control, screeners may desire to non-invasively assess the contents of a container (e.g., a package or a piece of luggage) or the internal structure of a manufactured part. Accordingly, for medical, security, and industrial applications, X-ray imaging techniques may be useful for noninvasively characterizing the internal composition of a subject of interest. 
     X-ray imaging techniques typically involves the generation of X-rays from a source, such as an X-ray tube. Such X-ray emitters typically utilize an emitter that emits electrons that are electro-statically or magnetically focused on a target that emits X-rays in response to the electron stream. In such contexts, the impact region of the electrons on the target is known as the focal spot. The characteristics (e.g., position, size, and so forth) of the focal spot may difficult to maintain within the desired tolerances or may otherwise vary during operation. It may be useful to know the characteristics of the focal spot in a real-time manner as these characteristics may impact the image quality of images generated using the emitted X-rays and/or may be useful to know in the reconstruction of such images. Similarly, such characteristics, when measured in real-time, may be used as part of a real-time feedback loop to maintain the focal spot within the desired tolerances. 
     BRIEF DESCRIPTION 
     In one embodiment, a CT system is provided. The CT system comprises an X-ray source comprising a target material. The X-ray source is disposed on a first side of an imaging volume. The CT system further comprises an imaging detector configured to generate a first set of electrical signals in response to a first portion of the X-rays emitted by the X-ray source. The imaging detector is disposed on a second side of the imaging volume opposite the first side. The CT system also comprises a reference detector positioned on the first side of the imaging volume. The reference detector is configured to generate a second set of electrical signals in response to a second portion of the X-rays emitted by the X-ray source. The CT system further comprises a data acquisition system configured to receive the first set of electrical signals from the imaging detector and the second set of imaging signals from the reference detector and a processing component configured to process the second set of imaging signals to generate measures of one or more characteristics of a focal spot on the target when X-rays are emitted by the X-ray source. 
     In a further embodiment, a reference detector is provided. The reference detector comprises an X-ray lens assembly. The X-ray lens assembly comprises at least one central aperture is configured to transmit X-rays emitted by the X-ray source and two or more slits or holes on opposing sides of the central aperture. Each slit or hole is configured to transmit X-ray for a localized sub-region of the focal spot. 
     In an additional embodiment, a method for characterizing an X-ray generation focal spot is provided. In accordance with the method, during operation of an X-ray source, localized intensity measurements are acquired from a reference detector. The localized intensity measurements are associated with a focal spot of the X-ray source. One or more characteristics of the focal spot are determined. The one or more characteristics of the focal spot are provided to a processing component or controller to adjust operation or collimation of the X-ray source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatical view of a CT imaging system for use in producing images, in accordance with aspects of the present disclosure; 
         FIG. 2  depicts a flowchart depicting control logic for characterizing a focal spot, in accordance with aspects of the present disclosure; 
         FIG. 3  depicts an exploded view of a source-side reference detector (SSRD), in accordance with aspects of the present disclosure; 
         FIG. 4  depicts a perspective view of a SSRD, in accordance with aspects of the present disclosure; 
         FIG. 5  depicts a plan view of a SSRD, in accordance with aspects of the present disclosure; 
         FIG. 6  depicts a combination perspective and cross-sectional view of an X-ray lens assembly and one use of such an X-ray lens assembly, in accordance with aspects of the present disclosure; 
         FIG. 7  depicts a cross-sectional view of an X-ray lens assembly describing the views through the various apertures of the assembly with respect to an X-ray emission site, in accordance with aspects of the present disclosure; 
         FIG. 8  depicts a cross-sectional view of a slit through an X-ray lens assembly along with descriptive parameters of the slit and assembly, in accordance with aspects of the present disclosure; 
         FIG. 9  depicts X-ray transmission as seen through a low aspect ratio slit, in accordance with aspects of the present disclosure; 
         FIG. 10  depicts X-ray transmission as seen through a high aspect ratio slit, in accordance with aspects of the present disclosure; 
         FIG. 11  depicts a perspective view of an X-ray lens assembly incorporating hole features, in accordance with aspects of the present disclosure; 
         FIG. 12  depicts a schematic view of a hole through an X-ray lens assembly along with descriptive parameters of the hole and assembly, in accordance with aspects of the present disclosure; 
         FIG. 13  depicts an X-ray lens assembly incorporating hole features and depicting sampling over an x-y deflection range, in accordance with aspects of the present disclosure; 
         FIG. 14  depicts a graphical example of intensities observed at different x-y locations through the X-ray lens assembly of  FIG. 13 , in accordance with aspects of the present disclosure; and 
         FIG. 15  depicts a flow diagram demonstrating determination of various characteristics of a focal spot, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to the use of a reference detector that generates localized intensity information that may be used to characterize an X-ray emission focal spot, such as based on the shape, size, or location of the focal spot. In certain embodiments, the reference detector discussed herein contains one or more openings or apertures (such as slits or holes) that may be used to acquire localized intensity information that may in turn be processed to derive the focal spot characteristics in question. As discussed herein, the reference detector may be separate from the primary, or imaging, detector used to acquire projection data used to reconstruct diagnostic images. Similarly, in certain such embodiments the reference detector is on the source-side of the imaged volume, as opposed to being on the detector-side of the volume. The derived focal spot characteristics can be used in real-time to control an X-ray imaging operation of a patient, such as by adjusting collimator blades used to shape and direct the X-ray emissions into the imaged volume. In such an implementation, real-time adjustment of the collimator blades in response to the derived focal spot characteristics can reduce or eliminate image artifacts that might otherwise result from the collimator blades not being properly directed to the emission focal spot (i.e., being misaligned). In addition, the focal spot characteristics may be used to control or adjust operation of the X-ray source, such as electrical parameters that influence focal spot size and/or focal spot position, and/or to adjust a reconstruction operation performed by the imaging system. 
     Measurement of focal spot characteristics may be particularly useful in certain contexts. For example, certain X-ray imaging systems may use a classical or more advanced emitter structure (as discussed in U.S. Patent Application No. 2011/0142193 A1, which is herein incorporated by reference in its entirety for all purposes) to generate the electron beam used in X-ray generation. Magnetic focusing or magnetic deflection may be utilized to steer or guide the electron beam. However, in such contexts focal spot size on the target may be highly sensitive to changes or variation in the electron beam current or the magnetic focusing and/or deflection currents, which can lead to image artifacts. For example, in extreme cases as little as a 1% change in magnet current can result in a 50% change in focal spot size. As discussed herein, real-time measurement of focal spot characteristics (such as size, location, and/or shape) may help to reduce the effects of such variation or may allow for control schemes to reduce, mitigate, or eliminate such focal spot variation. 
     With this in mind, an example of an imaging system  10 , such as a computed tomography (CT) system, suitable for use with the present focal spot assessment approaches is depicted in  FIG. 1 . Though a CT system is discussed with respect to  FIG. 1 , it should be appreciated that the system  10  and discussion related to CT imaging is provided merely to facilitate explanation by providing one example of a particular imaging context. However, the present approach is not limited to CT implementations and, indeed may be used in various other suitable imaging contexts where radiation is generated by using a focus beam (such as an electron beam). To facilitate explanation and to provide useful context and examples, the present discussion generally describes X-ray generation approaches where an electron beam is focused on some form of target material. However, it should be appreciated that the present approach is not limited to these contexts and may be used with other X-ray generation techniques, including techniques where no explicit target material is employed. For example, the present approach may also be useful in X-ray generation approaches where a laser beam and electron beam are collided in a cavity to generate X-rays. 
     Turning back to  FIG. 1 , in the depicted example, the imaging system  10  is designed to acquire X-ray attenuation data at a variety of view angles around a patient (or other subject or object of interest). In the embodiment illustrated in  FIG. 1 , imaging system  10  includes a source of X-ray radiation  12  positioned adjacent to a collimator  14 . The X-ray source  12  may be an X-ray tube or other source of X-ray radiation. 
     In the depicted example, the generated X-rays  8  may be emitted over an angular range wider than needed for imaging purposes. A collimator  14  may be provided that shapes the emitted X-rays  8  into a shaped beam  16  of X-rays that is allowed to pass through the imaging volume in which a patient  18  is positioned. For example, in practice, the collimator  14  may comprise a set of adjustable blades or apertures constructed of a highly attenuating material. In operation, some X-rays are allowed to pass through the collimator  14 , while others are blocked by the collimator  14 . In the depicted example, the collimated X-rays  16  are in a fan-shaped or a cone-shaped beam that passes through the imaged volume. A portion of the X-ray radiation  20  passes through or around the patient  18  (or other subject of interest) and impacts an imaging detector array, represented generally at reference numeral  24 . Detector elements of the array produce electrical signals that represent the intensity of the incident X-rays  20 . These signals are acquired and processed to reconstruct images of the features within the patient  18 . 
     In addition,  FIG. 1  depicts a source-side reference detector (SSRD)  22  provided on the source-side of the imaged volume (as opposed to the detector-side). In the depicted example, the SSRD  22  is positioned on or in conjunction with the collimator  14  so as to be impacted by emitted radiation  8  that would otherwise be blocked by the collimator  14  from passing into the imaged volume. That is, in the depicted example the SSRD  22  detects emitted radiation  8  that would not be otherwise be used in image generation but which would otherwise be blocked by the collimator  14  (i.e., would not be part of the shaped beam  16 ). In the depicted example, the SSRD  22  and detector  24  provide separate signals to the data acquisition system  30 . The SSRD  22  and its use are discussed in greater detail below. 
     In  FIG. 1 , the source  12  is controlled by a system controller  26 , which furnishes both power, and control signals for examination sequences. In the depicted embodiment, the system controller  26  controls the source  12  via an X-ray controller  28  which may be a component of the system controller  26 . In such an embodiment, the X-ray controller  28  may be configured to provide power and timing signals to the X-ray source  12 . 
     Moreover, the detector  24  is coupled to the system controller  26 , which controls acquisition of the signals generated in the detector  24 . In the depicted embodiment, the system controller  26  acquires the signals generated by the detector  24  and by the SSRD  22  using a data acquisition system  30 . The data acquisition system  30  receives data collected by readout electronics of the detector  24  and SSRD  22 . The data acquisition system  30  may receive sampled analog signals from the detector  24  and SSRD  22  and may convert the data to digital signals for subsequent processing by a processor  32  discussed below. Alternatively, in other embodiments the analog-to-digital conversion may be performed by circuitry provided on the detector  24  or SSRD  22  itself. The system controller  26  may also execute various signal processing and filtration functions with regard to the acquired image signals, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. 
     In the embodiment illustrated in  FIG. 1 , system controller  26  is coupled to a rotational subsystem  34 . A linear positioning subsystem  36  may also be present in certain contexts, such as where the system  10  is a CT system. The rotational subsystem  34  enables the image acquisition components to be rotated one or multiple turns around the patient  18 , such as rotated primarily in an x,y-plane about the patient (where the z-axis refers to the long axis of the patient). It should be noted that the rotational subsystem  34  might include a gantry or C-arm upon which the respective X-ray emission and detection components are disposed. Thus, in such an embodiment, the system controller  26  may be utilized to operate the gantry or C-arm. 
     The linear positioning subsystem  36 , when present, may enable the patient  18 , or more specifically a table supporting the patient, to be displaced, such as in the z-direction relative to rotation of the gantry or C-arm. Thus, the table may be linearly moved (in a continuous or step-wise fashion) to generate images of particular areas of the patient  18 . In the depicted embodiment, the system controller  26  controls the movement of the rotational subsystem  34  and/or the linear positioning subsystem  36  via a motor controller  38 . While the preceding discussion generalizes aspects of the various rotational and linear positioning systems that may be present, other positioning systems may be present and/or the linear or rotational positioning systems may include respective subsystems. 
     In general, system controller  26  commands operation of the imaging system  10  (such as via the operation of the source  12 , detector  24 , SSRD  22 , and positioning systems described above) to execute examination protocols and to process acquired data. For example, the system controller  26 , via the systems and controllers noted above, may rotate a gantry or C-arm supporting the source  12  and detector  24  about a subject of interest so that X-ray attenuation data may be obtained at a variety of views relative to the subject. In the present context, system controller  26  may also include signal processing circuitry, associated memory circuitry for storing programs and routines executed by the computer (such as routines for executing artifact reduction techniques described herein), as well as configuration parameters, image data, and so forth. 
     In the depicted embodiment, the image signals from the detector  24  and the reference signals from the SSRD  22  are acquired by the system controller  26  and provided to a processing component  32  for reconstruction of images. In certain embodiments, the system controller  26  may itself utilize the SSRD output (or measures generated from the SSRD output) to control operation of the X-ray source  12  and/or to control operation of the collimator  14 , such as to allow real-time focal spot size control. The processing component  32  may, in certain embodiments, be one or more conventional microprocessors, such as general purpose microprocessors, or may take the form of application specific integrated circuits (ASICs). The data collected by the data acquisition system  30  may be transmitted to the processing component  32  directly or after storage in a memory  40 . Any type of memory suitable for storing data might be utilized by such an exemplary system  10 . For example, the memory  40  may include one or more optical, magnetic, and/or solid state memory storage structures. Moreover, the memory  40  may be located at the acquisition system site and/or may include remote storage devices for storing data, processing parameters, and/or routines for artifact reduction, as described below. 
     The processing component  32  may be configured to receive commands and scanning parameters from an operator via an operator workstation  42 , typically equipped with a keyboard and/or other input devices. An operator may control the system  10  via the operator workstation  42 . Thus, the operator may observe the reconstructed images and/or otherwise operate the system  10  using the operator workstation  42 . For example, a display  44  coupled to the operator workstation  42  may be utilized to observe the reconstructed images and to control imaging. Additionally, the images may also be printed by a printer  46  which may be coupled to the operator workstation  42 . 
     Further, the processing component  32  and operator workstation  42  may be coupled to other output devices, which may include standard or special purpose computer monitors and associated processing circuitry. One or more operator workstations  42  may be further linked in the system for outputting system parameters, requesting examinations, viewing reconstructed images, and so forth. In general, displays, printers, workstations, and similar devices supplied within the system may be local to the data acquisition components, or may be remote from these components, such as elsewhere within an institution or hospital, or in an entirely different location, linked to the image acquisition system via one or more configurable networks, such as the Internet, virtual private networks, and so forth. 
     It should be further noted that the operator workstation  42  may also be coupled to a picture archiving and communications system (PACS)  48 . PACS  48  may in turn be coupled to a remote client  50 , radiology department information system (RIS), hospital information system (HIS) or to an internal or external network, so that others at different locations may gain access to the raw or processed image data. 
     While the preceding discussion has treated the various exemplary components of the imaging system  10  separately, these various components may be provided within a common platform or in interconnected platforms. For example, the processing component  32 , memory  40 , and operator workstation  42  may be provided collectively as a general or special purpose computer or workstation configured to operate in accordance with the aspects of the present disclosure. In such embodiments, the general or special purpose computer may be provided as a separate component with respect to the data acquisition components of the system  10  or may be provided in a common platform with such components. Likewise, the system controller  26  may be provided as part of such a computer or workstation or as part of a separate system dedicated to image acquisition. 
     With the foregoing discussion of a suitable implementation of an imaging system  10  in mind,  FIG. 2  depicts a flowchart  80  describing an algorithm for image reconstruction using data derived from one or more source-side reference detectors (SSRD)  22  as well as data derived from the imaging detector  24 . In this example, the imaging detector  24  acquires projection data  82  at a number of views about the imaged volume. The projection data  82  is reconstructed (block  84 ) to generate one or more reconstructed images  88 . In the depicted example, the reconstruction step  84  may leverage a variety of data acquired via the SSRD  22  to improve the reconstruction process. 
     For example, in one implementation, the SSRD  22  can acquire a set of localized measurements  90  at some or all of the views for which projection data is also acquired. These localized measurements  90  may vary depending on configuration of the SSRD  22  and will be discussed in greater detail below. However, for the purpose of this example, the localized intensity measurements may be used to determine (blocks  92 ,  94 ,  96 ) one or more characteristics of the focal spot, such as focal spot shape  100 , focal spot size  102 , and focal spot location  104 . To some extent, determination of one focal spot characteristic may be useful in determining another focal spot characteristic. For example, in the depicted implementation, focal spot size  102  may itself be an input to the focal spot location determination (block  96 ). One or both of the focal spot size  102  and the focal spot location  104  may be provided as inputs to the reconstruction step  84 . Likewise, focal spot shape, if determined, may be provided as an input to the reconstruction step  84 . While the present example demonstrates the use of focal spot characteristics to improve an image reconstruction process, as noted herein the focal spot characteristics may also be used, such as by a controller of the system controller  26 , to adjust or control operation of the X-ray source  12  and/or to adjust the collimators  14 . Indeed, such adjustments to the X-ray source operation or to collimator placement may occur essentially in real-time during an imaging operation to improve the quality of the projection data acquired. 
     With this in mind,  FIGS. 3-5  depict varying views of one example of an SSRD  22  suitable for use in accordance with the present disclosure. For example,  FIG. 3  depicts an exploded view of an SSRD  22  while  FIG. 4  depicts a perspective view of an assembled SSRD  22  and  FIG. 5  depicts a plan view of the assembled SSRD  22 . In the depicted example, the SSRD  22  includes an X-ray lens portion  120  that includes various apertures through which emitted X-rays may be filtered and/or directed onto the detection elements. For example, in the depicted example the X-ray lens  120  includes three central apertures through which X-rays may pass to impact the detection elements: an unfiltered reference normal aperture  122 , a first filtered aperture  124 , and a second filtered aperture  126 . In the depicted example, the first and second filtered apertures  124  and  126  are each differently filtered (such as by first KvP filter  128  and second KvP filter  130 ) to provide different spectral information with respect to the emitted X-rays. In this manner, overall X-ray emissions may be determined via the reference normal aperture  122  while X-ray emission at different wavelengths of interest may be determined at via the respective filtered apertures  124  and  126 . While the depicted example includes central apertures  124 ,  126 , and  128  (which, as noted, may be useful for flux normalization and/or kVp measurement), it should be understood that the depicted central apertures are not required for determination of focal spot characteristics (e.g., size, shape, or location) as discussed herein. Therefore, in other embodiments, the central apertures  124 ,  126 ,  128  may be absent or provided elsewhere, such as in a second SSRD. 
     Turning back to the figures, in the depicted example, slits  130  are provided to the sides of the respective apertures  122 ,  124 ,  126 . The slits  130  allow X-rays to pass through in a limited manner determined by the aspect ratio of the slits  130 . In certain implementations, the aspect ratio of the slits is greater than or equal to 20 and/or less than or equal to 100. This limited transmission may be detected by the detection elements to yield the localized intensity measurement data  90 , discussed herein, used to determine focal spot characteristics. In  FIGS. 3 and 4 , a single slit  130  is depicted in each side of the apertures  122 ,  124 , and  126 . In  FIG. 5 , a pair of slits  130  is depicted at each side of the apertures. In this example, therefore, the slits  130  are of two types, those having a first orientation (e.g., slits  132 ) and those having a second orientation orthogonal to the first (e.g., slits  134 ). The differently oriented slits  132  and  134  may be useful in providing localized intensity measurements  90  that provide information in two dimensions (e.g., x and y), which in turn may be useful in determining focal spot characteristics such as focal spot size  94  and focal spot shape  100  in each of those two dimensions. As will be appreciated, additional slits  130  may be provided in other embodiments relative to the four and eight slits depicted. Further, in other implementations other types of openings, such as holes, may be provided instead of slits  130  while still providing comparable localized intensity measurement data  90 . 
     In the depicted example, the X-ray lens  120  may be mounted on or formed contiguously with a mounting substrate  150  having alignment and mounting features  152  that may be useful for mounting the assembly onto an external support, such as a portion of the collimator  14 . The substrate  150  may have a corresponding aperture beneath where the X-ray lens  120  is mounted to allow X-rays passed by the X-ray lens  120  to reach detection elements, discussed below. In the depicted example, a blocker  154  (such as a 0.6 mm layer of tungsten) may also be provided to reduce the high X-ray flux present close to the X-ray source to the lower levels typically encountered by CT X-ray detectors. This reduction to the X-ray flux may allow the use of a standard CT detector assembly as the detecting element of the SSRD. 
     The respective apertures and slits of the X-ray lens  120  of the SSRD  22  (and the corresponding aperture of the substrate  150 ) allow some portion of the emitted X-rays to pass through the X-ray lens  120  and to reach detection elements positioned to receive the transmitted X-rays and to generate electrical signals in response to the transmitted X-rays. For example, in the depicted example, a layer  140  of scintillators is provided that convert incident X-rays to optical light photons. The optical light photons may then be detected by an array  142  of photodiodes (or other electronic light detecting elements) that may be readout by electrical circuitry and the acquired signal data communicated to downstream electrical components, such as via a suitably configured flex circuit  144  or other conductive path in communication with the data acquisition system  30 . 
     With this example of an SSRD  22  in mind,  FIG. 6  depicts a schematic view of an X-ray lens  120  of a source-side reference detector  22 . In the depicted example, the X-ray lens  120  is shown from a top perspective and includes various central apertures, including reference normal aperture  122 . As noted above, in other embodiments, the central apertures may be absent. In addition, a pair of slits  130  are provided on each side of the apertures, including, in the depicted example, four slits  132  that are orthogonal to the depicted a-a plane and four slits  134  that are orthogonal to the depicted b-b plane. In the depicted example, the respective slits  130  are used to measure focal spot properties in orthogonal directions, such as in the x and y dimensions, though the slits themselves may not mathematically be orthogonal to one another due to the respective angular paths they take through the lens assembly. For example, cross-sectional views taken along sight lines a-a and b-b are depicted beside the top perspective view of the X-ray lens  120  which show the respective paths taken by the apertures and the by the slits  132  and  134  through the X-ray lens  120 . In the depicted example, the apertures are straight passages through the X-ray lens  120  while the respective slits are angled relative to the apertures. 
     As discussed herein, X-rays passing through the angled slits provide localized intensity measurements  90  of X-ray incidence. In certain embodiments, the differential transmission of X-rays through slits of a given orientation (e.g., the x-orientation or the y-orientation in the depicted example) can provide useful information about the focal spot on the target associated with the emitted X-rays, such as the focal spot size or the focal spot location. By way of example, in  FIG. 6  localized intensity measurements  90  corresponding to X-ray transmission through slits  132  may be used to derive ratios that may in turn be used to derive focal spot size and focal spot location (e.g., deflection) based on known relationships between focal spot location and size (as depicted in graph  150 ). 
     Turning to  FIG. 7 , this concept is further developed by reference to a cross section through X-ray lens  120  depicting slits  130  and reference normal aperture  122 . In the depicted example, X-rays passing through the slits  130  and the reference aperture  122  impact pixilated detector elements  162  (which may include scintillator and photodiode elements as discussed above). The X-rays are emitted from a localized focal spot  160  on a target structure. In the depicted example, the focal spot  160  is represented as an intensity distribution  162  of emitted X-rays with respect to a line, wherein the distribution  162  reflects the location, amplitude, and size of the focal spot with respect to the line. As depicted in this example, non-localized X-ray emissions are detected though the reference normal aperture  122 . Such non-localized X-ray emission data may be useful for determining the overall amplitude of the curve  162  (i.e., of the focal spot  160  at the represented plane), assuming the peak of the curve  162  is visible within the aperture  122 . 
     Conversely, slits  130  due to their limited aperture and angled orientation, see only a portion of the curve  162  or, in some instance, none of the curve  162 , as denoted by lines  174 . Thus, measurements obtained at the detector elements  162  due to X-ray transmission through slits  130  constitute localized intensity measurements  90  corresponding to only a limited portion of the focal spot  160 . Analysis and/or comparison of these separate localized intensity measurements  90  (such as by determining the ratios of certain localized measurements may be useful in determining the location and size of the focal spot  160  (and potentially the amplitude) on the target relative to a desired size and location. In general, three measurements (e.g., a measurement through reference normal aperture  122  and two of the slits  130 ) are needed to determine amplitude, size, and location of the focal spot  160 . Thus, for each view, at least three slits  130  or at least two slits  130  and the reference normal aperture  122  should transmit X-rays emitted by the focal spot  160 , even for deflected positions. As will be appreciated, additional slits  130  provide redundancy and may improve the robustness of the measurements acquired for non-Gaussian focal spot profiles. 
     Turning to  FIG. 8 , an example of a one-dimensional calculation related to X-ray transmission through a slit  130  is provided. In this example, a cross section of a slit  130  is depicted along with a detector element  162  (e.g., a pixel of a detector). Various other elements related to the one-dimensional transmission calculation are also depicted, including: an emitting point, x e , of the focal spot from which an X-ray is emitted; dimensions x s . α s , and d of slit  130 ; height, h, of the X-ray lens  120 , and distance, L, between the focal spot and the X-ray lens  120 . In addition,  FIG. 8  depicts the x-coordinates of the edges of the slit  130  as xt1, xt2, xb1, and xb2 (i.e., a pair of top and a pair of bottom x coordinates). The detector element  162  is illuminated between max(α t1 , α b1 ) and min(α t2 , α b2 ). Assumptions made for the present calculation include that the detector element  162  collects all of the light passing through the slit  130  and that one-dimensional analysis is appropriate, such as in the case of an infinitely large slit. 
     In accordance with the depicted elements: 
     
       
         
           
             
               
                 
                   
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     The aspect ratio of the slit  130  is h/d. In practice, the lens height h may be limited by the available space where the SSRD  22  is installed. For example, in certain implementations, potential mechanical interference with the moving blades of the collimator  14  may limit the height h of the lens assembly  120 . The diameter d of the slit  130  may be limited by manufacturing capabilities. As a result of these various considerations, aspect ratios greater than 20 may require more complicated processing, such as using two parts instead of cutting a slit  130  into a single part. 
     With the foregoing in mind, the impact of aspect ratio on focal spot size determination is discussed. As will be appreciated, the slit aspect ratio can affect how difficult or easy it is to determine focal spot size, with lower slit aspect ratios generally leading to more difficulty in determining focal spot size and higher slit aspect ratios making it easier to determine focal spot size. Conversely, however, lower slit aspect ratios may be better for determining focal spot location than higher aspect ratios. In the present discussion, it should be appreciated that the absolute numeric value of the aspect ratio may, by itself, be insufficient to establish whether an aspect ratio should be considered high or low in a given context. In particular, a given aspect ratio should be considered in the context of the respective length between the X-ray lens, L, and the height, h, of the lens, i.e., L/h. With this in mind, the examples of a low aspect ratio of 20 and a high aspect ratio of 100 given below may be proper in the circumstance where L/h=10, however, for other values of L/h the aspect ratio providing the same results may be different. For example, a slit aspect ratio of 20 with L/h=10 would give same result as a slit aspect ratio of 40 with L/h=20. 
     For example, for a low slit aspect ratio (e.g., 20 with L/h=10), the penumbra region (i.e., the region on the target from which transmission to the detector pixel is between 100% and 0% of maximum transmission) associated with a respective slit  130  and associated detector element may be much larger than the focal spot size, and the observed measurements are more likely to fall within a region of linear decay of the transmission coefficient (which is indistinguishable for different spot sizes). If the focal spot is entirely within the penumbra region (which is more likely at low slit aspect ratios), the focal spot size has negligible effect on transmission integrated on the SSRD detector elements. Thus, focal spot size may not be determinable from the measured data at the SSRD due to the linearity of the signal as a function of spot location in the observed region. That is, the data associated with different spot sizes may only differ (and thus useful for distinguishing between different spot sizes) at a limited number of points (such as at the focal spot center and distribution tails), with other regions being co-linear at different focal spot locations, and thus unusable to differentiate different focal spot sizes. This is shown conceptually in  FIG. 9 , where curves  190 ,  192 , and  194  are shown depicting transmission as a function of focal spot center for three different spot sizes as seen through a slit  130  having a low aspect ratio. As depicted, the low slit aspect ratio is associated with a single large measurement region  198  that may be insufficient to distinguish the curves associated with the different focal spot sizes. However, the low slit aspect ratio, which sees a large portion of the curves, may be well suited for determining a location of the focal spot. 
     Conversely, for a comparably high slit aspect ratio (e.g., 100 with L/h=10), the focal spot size may be much larger than the respective penumbra region associated with the slit  130  and associated detector element, and the focal spot size may have a large effect on transmission integrated at the respective SSRD detector element. Thus, at higher slit aspect ratio, the measured signal is strongly dependent on focal spot size. Thus, focal spot size may be more readily determinable from the measured data at the SSRD due to the transmission characteristics of focal spots of different sizes not being on the same line (i.e., not overlapping) over an extended range of focal spot locations. Therefore, for higher slit aspect ratios, it may be easier to determine focal spot size due to the measurements not overlapping (over a range of focal spot locations) to a great extent. 
     This is shown conceptually in  FIG. 10 , where curves  190 ,  192 , and  194  are shown depicting transmission as a function of focal spot center for three different spot sizes as seen through a slit  130  having a high aspect ratio. As depicted, the high slit aspect ratio is associated with a narrow measurement region  200  that may be suitable for distinguishing between the curves associated with the different focal spot sizes, i.e., the measured signal within the narrow measurement region  200  may be strongly dependent on focal spot size. However, as may also be noted, the high aspect ratio slit “sees” only a narrow region (e.g., measurement region  200 ) of the target and, therefore, may be poorly suited for determining focal spot location. With this in mind, it may be desirable to provide additional slits  130  (yielding additional measurement regions  202 ) to accurately measure focal spot size at each possible deflection location and to also provide sufficient information to allow accurate determination of focal spot location. 
     It may also be noted that, in one embodiment, for focal spot size measurement where there is only a single slit  130  (or a single hole, as discussed below) in each direction, the focal spot may be intentionally deflected onto different locations of the target. Transmission may then be measured as a function of spot deflection. As deflection is presumably known (and measurable by the SSRD  22 ), focal spot size can still be determined, even with fewer slits or holes, as discussed herein. 
     While the preceding has described examples where localized intensity measurements are generated using slits  130 , in other embodiments, other types of openings may be employed. For example, turning to  FIGS. 11 and 12 , an X-ray lens  120  having holes  220  instead of slits is depicted, where the bottom of each hole (i.e., the hole opening facing the detector elements) is aligned to a detector element (e.g., detector pixel). Though the embodiment of  FIG. 11  depicts the holes  220  used in conjunction with the apertures  122 ,  124 ,  126 , in other embodiments the apertures are not present and the holes  220  provide sufficient information to determine the focal spot characteristics of interest. In one implementation, each hole  220  is a long, thin hole having an aspect ratio determined as with slits  130 . The holes  220  may be either tilted or angled with respect to the X-ray lens  120  body or may be vertical (i.e., straight through) the X-ray lens  120  such that the holes are perpendicular to the surface of the X-ray lens  120  facing the focal spot. As depicted in  FIG. 12 , an example of a hole  220  is depicted along with a corresponding sensitivity graph  230  depicting the maximum sensitivity (here shown along a single dimension, x) within a region that is the same size as the diameter d of hole  220 . Each hole  220  integrates over a small region of the x-y deflection range. The outer bound of the integrated region is:
 
 D= 2 *d*L/l   (5)
 
For example, when d=0.05 mm, L=100 mm, and l=10 mm, then D=1 mm.
 
     In one embodiment, depicted in  FIGS. 13 and 14 , each hole  220  is aligned to (i.e., points to) a different location  228  in the x-y deflection range 224. Instead of acquiring a line integral, the holes  220  allow acquisition of the intensity observed in small, circular regions. Thus, in embodiments employing holes  220 , the corresponding detector elements  162  simultaneously measure x and y deflection over a large x-y range, such as the entire x-y deflection range. 
     Turning to  FIG. 14 , which depicts intensity measurements taken at such circular regions along an x-y deflection range, it can also be seen how such measurements can be used to determine or characterize the shape of a focal spot. That is, a collection of such intensities taken along the x-y grid can be used to characterize not only the center or location of the focal spot and the size of the focal spot, but also the general shape or outline of the focal spot. This can also be seen in  FIG. 15 , where a flow diagram incorporating x-y measurement data is depicted. Instead of the graphical three-dimensional distributions of  FIG. 14 , the example of  FIG. 15  uses numerical measurements or values within the x-y range to facilitate explanation of certain aspects of the present approach. In this example, chart  250  (e.g., a 4×4 matrix) depicts measured beam intensity at different x-y locations measured by the holes  220  of an X-rays lens  120 . In one example, Gaussian beam intensity can be characterized as:
 
 I=I   0 *exp(− r   2   /r   0   2 )  (6)
 
where r is the distance from the center of the focal spot. From the intensity measurements, the distance from the center of the focal spot center can be determined:
 
 r/r   0 =√{square root over (ln( I   0   /I ))}  (7)
 
where I 0  is the value measured or observed by the reference normal region of the detector (i.e., observed through reference normal aperture  122 ). This is depicted at chart  260 , where measured beam center distance r/r 0  is shown for the corresponding x-y locations. In the depicted example, from this data, the most likely beam (i.e., focal spot) center  262  can be calculated (block  264 ) and the r 0  can be calculated (block  266 ) based on the distance of the data points from the calculated beam center. Alternatively, as depicted by dashed arrow  268 , the beam center may instead be calculated directly from the beam intensity data of chart  250  (without going through the intermediary step of calculating the beam center distance values of chart  260 ), such as by calculating the center of gravity of the intensity distribution  250  or the center of gravity of a subset of the intensity distribution with the highest numbers. As will be appreciated, based on the calculated beam center  262 , characteristics of the focal spot such as location and size may directly (or indirectly) derived. Likewise, based on the location and shape characteristics, characteristics of the focal spot shape may also be determined from the intensity data, for example, deviations from a circular Gaussian shape may be evident from unexpectedly high or low intensity values at the expected edges of the focal spot.
 
     Technical effects of the invention include real-time characterization of focal spot characteristics in an X-ray generating system. Further technical effects include real-time control of an X-ray generating apparatus based upon one or more measured focal spot characteristics, such as size, location, and/or shape of a focal spot used to generate X-rays. Additional technical effects include controlling operation of a collimator and/or an image reconstruction process based on one or more measured focal spot characteristics. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.