Abstract:
In some embodiments, an optical mask for a CT detector is disclosed. In some embodiments, the mask is intercalated between a photodiode array and a scintillator array forming the CT detector. In some embodiments, the optical mask may extend along one or more axes and differentially absorbs and/or reflects light emitted from the scintillators at edges of photodiodes forming the diode array, with less absorption or reflection at edges of tiled die forming the diode array than in central portions of each of the die. Through selective absorption and/or reflection, transference of light photons from a scintillator to the photodiode corresponding to a neighboring scintillator is spatially modified, at least partially compensating for spatial differences in crosstalk signals between adjacent pairs of photodiode/scintillator cell elements. This reduction in differential crosstalk reduces artifacts in a reconstructing data descriptive of internal portions of a subject, which improves diagnostic value of the data.

Description:
FIELD OF THE DISCLOSURE 
   This disclosure relates generally to detector arrays for imaging technology, and in particular to a detector array providing improved signal detection capabilities and a computed tomography (CT) X-ray system incorporating the detector array. 
   BACKGROUND 
   Many medical diagnostic, surgical and interventional procedures rely on imaging tools to provide information descriptive of status of visually perceived representations of portions or organs of a patient. In part as a result of increasing sophistication of medical tools in general, and imaging apparatus in particular, more types of imaging devices are being adapted for application in the context of medical diagnostics and procedures. 
   In many instances, medical tools capable of rendering images of organs or tissues have found great utility and have been adapted to facilitate a broad variety of medical needs. These applications have resulted in development of a gamut of specialized imaging tools, including X-ray, CT and fluoroscopic visualizing aids, and many different types of optical imaging devices. 
   In many imaging applications, pixelated detectors are increasingly employed to realize electronic digital representations of image data. Some types of systems employ an array of scintillation cells and an associated array of photodiodes formed from a sheet of semiconductive material, where the scintillation material in each cell converts incident X-radiation to visible photons suitable for detection by the one diode in the array that is intended to be optically coupled to that cell. Mechanisms which degrade the signals from the diode array can cause machine-to-machine data instability, or reduce measurement or imaging repeatability, and may give rise to data distortion causing imaging defects such as ring artifacts, bands or smudges in the resultant data, when it is employed to form a visible image, or engender inaccuracy and/or reduced repeatability in the context of automated characterization of tissues. 
   In turn, digital techniques provide great imaging flexibility, such as, for example, overlay or direct comparison, on the fly, of various aspects and views from various times. For example, pre-surgery images can be available, in real time, in the operating room scenario, for comparison to images reflective of the present status of the same tissues. Many other types of special-purpose enhancements are now also possible. In some instances, imaging aids, such as contrast-enhancing agents, are introduced into the subject or patient to aid in increasing available data content from the imaging technique or techniques being employed. 
   However, regulatory changes, increasingly sophisticated measurement needs and other factors combine to place new demands on pixelated detectors for computed tomography applications, among others. Recent desire to even further reduce the total dose of X-radiation delivered to the subject, to reduce the energy of the X-rays on a per-photon basis and to achieve increased contrast parameters within the resulting images collectively demand greater linearity and sensitivity of the photodetector arrays used in such visualization tools, together with reduced image noise and artifacts of various sorts. 
   Signal artifacts resulting from the photodetector array itself also may pose some fundamental limits on overall system performance. Examples of mechanisms known to potentially give rise to crosstalk artifacts include: (i) charge carriers generated in one diode resulting in a signal in another diode, via carrier diffusion and/or inter-diode capacitance; (ii) scattering of X-rays from one scintillator cell to a neighboring scintillator cell, followed by conversion to a photon and detection of that photon by a diode coupled to the neighboring cell; (iii) leakage of light from a scintillator cell to a photodiode associated with another scintillator cell; and (iv) scattering of photons generated in the target scintillator cell into a neighboring scintillator cell through intercell septa, and thus to a photodiode associated with the neighboring cell. In many situations, where photodiodes are co-fabricated on a common substrate, diffusion of carriers from one photodiode to another contribute a dominant component to interdiode crosstalk. 
   In turn, these various artifacts present characteristics which vary linearly and nonlinearly with both X-ray fluence and operating parameters. Additionally, achieving alignment of the scintillator cell array with the photodiode array presents difficulty in manufacturing, with unwanted signal characteristics or artifacts being associated with residual imprecision in that process. 
   For the reasons stated above, and for other reasons discussed below, which will become apparent to those skilled in the art upon reading and understanding the present disclosure, there are needs in the art to provide improved photodiode/scintillator photodetectors in support of increasingly stringent and exacting performance and economic standards in settings such as medical imaging. 
   BRIEF DESCRIPTION 
   The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following disclosure. 
   In one aspect, a computed tomography detector system includes a photodiode array formed from multiple tessellated die each having a plurality of photodiodes formed thereon, and a scintillator array formed as a multiplicity of scintillation cells separated by septa. Each of the multiplicity of scintillation cells is associated with a respective one of the plurality of photodiodes to form a detector element. An optical mask that differentially spatially modifies transmission of light from each of the multiplicity of cells to a respective associated one of the plurality of photodiodes is intercalated between the photodiode array and the scintillator array. The optical mask provides different light transmission modification for detector elements at boundaries of each of the die than for detector elements in central portions of the die. 
   In another aspect, a computed tomography imaging system includes a patient table, an X-ray illumination source placed on one side of the patient table and a detector assembly comprising a plurality of detector elements placed on an opposed side of the patient table and oriented towards the X-ray illumination source. The imaging system also includes a computer control system controlling motion of the patient table and exposure of the plurality to X-rays which have passed through a patient, and forming spatial descriptions of internal aspects of the patient from data obtained from the detector assembly. The imaging system further includes an optical modulator formed in the detector assembly. The optical modulator differentially spatially modifies transmission of light from a scintillator cell associated with one detector element to a photodiode in a neighboring detector element. 
   In yet another aspect, a process for reducing differential crosstalk in a photodetector array includes optically masking first edges of first photodiodes to a first degree. The first edges form boundaries between adjacent photodiodes realized on a common die. The process also includes optically masking second edges of second photodiodes to a second degree that is less than the first degree. The second edges occur at die boundaries. 
   In a further aspect, a process for reducing differential crosstalk in an array of photodetector elements is described. The array is formed from a plurality of photodiodes, each associated with one of a multiplicity of scintillator cells. The process includes inhibiting lateral diffusion of photocarriers within a die across mutual photodiode edges. The process also includes optically coupling adjacent photodiodes formed on neighboring die to balance optically-induced crosstalk with photocarrier-induced crosstalk. 
   In a still further aspect, an array of photodetector elements includes multiple tiled die, each including a plurality of photodetectors, and a scintillator array including a multiplicity of scintillator cells separated by opaque septa. Each of the multiplicity is associated with a respective one of the plurality. The array also includes a crosstalk modification grid associated with the multiple tiled die, to differentially modulate crosstalk between photodetector elements formed in central portions of the multiple tiled die relative to crosstalk between adjacent photodetector elements formed on different ones of the multiple tiled die. 
   Systems and processes of varying scope are described herein. In addition to the features and benefits described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an overview of a system configured to improve the display of images from an imaging apparatus. 
       FIG. 2  is a simplified block diagram illustrating a pixelated detector system that is useful in the context of the system of  FIG. 1 . 
       FIG. 3  is a simplified block diagram illustrating a detector element that is useful in the context of the pixelated detector system of  FIG. 2 . 
       FIG. 4  is a graph showing how crosstalk may vary across boundaries between tiled die that each include an array of photodiodes. 
       FIG. 5  is a simplified block diagram illustrating an array of four tiled photodiode die and optical masking associated with the tiled photodiode die, in accordance with the teachings of the presently-disclosed subject matter. 
       FIGS. 6 through 9  are simplified side views, in section, taken along section lines shown in  FIG. 5 , depicting simplified block diagrams of photodetector assemblies or subassemblies capable of utility in the system of  FIG. 1 , in accordance with the teachings of the presently-disclosed subject matter. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown, by way of illustration, specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized, and that logical, mechanical, electrical and other changes may be made, without departing from the scope of the embodiments. 
   Ranges of parameter values described herein are understood to include all subranges falling therewithin. The following detailed description is, therefore, not to be taken in a limiting sense. 
   The detailed description is divided into four sections. In the first section, a system level overview is provided. In the second section, an example of a pixelated photodiode array is described. In the third section, embodiments of improvements in tiled detector assemblies are described. The fourth section provides a conclusion which reviews aspects of the subject matter encompassed in the preceding segments of the detailed description. A technical effect of the systems and processes described herein includes reduction of crosstalk-induced artifacts in images formed using tesselated arrays of photodiode/scintillator assemblies. 
   I. System Overview 
     FIG. 1  is a simplified diagram of an overview of a modified system  100  configured to improve X-ray imaging operations. The system  100  optionally includes a gantry  102  or other support for an illumination source  104 , such as an X-ray illumination source, capable of providing illumination  106 , such as X-rays or other non-destructive internal imaging illumination, and may optionally include a test subject support  108  that is transmissive with respect to the illumination  106  and that is positioned above a scintillator  109  and diode array  110  that is also opposed to the illumination source  104 . The scintillator  109  and diode array  110  collectively form a CT detector system. 
   In one embodiment, components of the system  100  and a test subject  112  are maintained in a defined geometric relationship to one another by the gantry  102 . A distance between the illumination source  104  and the diode array  110  may be varied, depending on the type of examination sought, and the angle of the illumination  106  respective to the test subject  112  can be adjusted with respect to the body to be imaged responsive to the nature of imaging desired. 
   In one embodiment, the test subject support  108  is configured to support and/or cause controlled motion of the test subject  112 , such as a living human or animal patient, or other test subject  112  suitable for non-destructive imaging, above the scintillator  109 /diode array  110  so that illumination  107  is incident thereon after passing through the test subject  112 . In turn, information from the detector array  110  describes internal aspects of the test subject  112 . 
   The scintillator  109  may be a conventional scintillator  109 , optically coupled to a two-dimensional array of photodiodes or any other form of diode array  110  suitable for use with the type or types of illumination  106  being employed, such as X-rays. The detector elements are typically tessellated in a mosaic. The scintillator  109  converts incident photons comprising electromagnetic radiation, such as X-rays, from high-energy, high-frequency photons  107 , into lower-energy, lower-frequency photons corresponding to spectral sensitivity of the detector elements, in a fashion somewhat analogous to fluorescence, as is commonly known in the context of many visible-light sources in use today. Materials useful in scintillator layers  109  include ceramics formed from materials such as gadolinium oxysulfide activated with rare earths, such as terbium (GOS:Tb), cadmium tungstate, yttrium gadolinium oxide with suitable activation components, cesium iodide, etc. 
   In some modes of operation, such as CT, the gantry  102  and test subject support or table  108  cooperatively engage to move the test subject  112  longitudinally, that is, along an axis extending into and out of the plane of  FIG. 1  and within an opening  114 . In some modes of operation, the gantry  102  rotates the X-ray source  104  and diode array  110  about the axis  116 , while the support  108  moves longitudinally, to provide a helical series of scans of the test subject  112 . 
   There are many different possible ways for achieving reduced signal artifacts from detectors  110 , providing improved robustness and repeatability of measurements and characterizations possible via the system  100 , and of achieving other benefits of the subject matter disclosed herein. The apparatus of  FIGS. 2  and those following  FIG.2  described below in more detail with reference to Section II, provide but a few examples for addressing these various needs. 
   II. Exemplary Pixelated Detectors 
     FIG. 2  is a simplified block diagram illustrating a pixelated detector system  200  that is useful in the context of the system  100  of  FIG. 1 . The pixelated detector system  200  includes a photodetector array  210  (e.g., part of the diode array  110  of  FIG. 1 ), which in this example is assumed to be an N×M array, where N and M represent integers describing a number of rows and columns in the photodetector array  210 . For example, a die might include an array of 16×64 photodiodes, although other sizes of die may be employed.  FIGS. 2 and 3  also employ “i”, “j”, “n” and “m” to represent integers, where i varies over a range {1, N}, and j varies over a range {1, M}. 
   The detector array  210  comprises a matrix or mosaic of detector elements  215  or pixel elements  215 , i.e., detector element PDE  215 (1, 1) through detector element PDE  215 (n, m), each having a first dimension  217  and a second dimension  219 . In the example of  FIG. 1 , the detector elements PDE  215 (i, j) thus each have an area that is equal to a product a×b, where the first dimension  217  is represented as “a” and the second dimension  219  is represented as “b”. The first  217  and second  219  dimensions typically range between 800 micrometers and one millimeter, and the first dimension  217  need not be chosen to be equal to the second dimension  219 . In other words, the detector elements PDE  215 (i, j) need not be square, and may be rectangular or may have shapes which are neither square nor rectangular. The detector elements PDE  215  are typically arranged along respective rows and columns as illustrated. 
     FIG. 3  is a simplified block diagram  300  illustrating a detector element PDE  215 (i, j) that is useful in the context of the pixelated detector system  200  of  FIG. 2 . The diodes  365  are fabricated to each include a relatively large photosensitive surface area (a×b,  FIG. 2 ), ensuring that the diodes  365  are capable of intercepting a representative portion of optical excitation  370 , responsive to illumination  107  that has passed through the test subject  112 . 
   In order to acquire an X ray image using the detector array  210 , the system  100  may perform a variety of sequences. One exemplary sequence is as follows. Exposure of the scintillator elements to X-rays  106 ′ selectively attenuated by passage through particular portions of the test subject  112  results in an amount of light  370  proportional to the intensity of the X-rays  106 ′ incident on that photodetector element PDE  215 (i, j). In turn, that photodetector element PDE  215 (i, j) passes a current I j  that then is directed through the column signal lines  230 (j), and thus to a respective transimpedance amplifier  385 (j) having a current-to-voltage transfer ratio of K j , and thereby converting the current I j  into a voltage V j  manifested on output line  387 (j). 
   Channel-to-channel variations in linearity of response of the photodetectors degrade accuracy of data acquired by tomographic scanners. Crosstalk between adjacent channels may also compromise dynamic range and other properties of the signals. Differential crosstalk, that is, a difference in crosstalk from a j−1 TH  channel to a j TH  channel with crosstalk from the j+1 TH  channel to the j TH  channel, also may be a significant source of error in measured data. 
   Further, variations in differential crosstalk also impact the achievable precision and accuracy of data collection. Generally, reducing both crosstalk and differential crosstalk results in improved spatial resolution and in increased dynamic range, fewer anomalies or artifacts in the tomographically-obtained data and/or in automated assessments of lesions or in reconstructed CT images. 
     FIG. 4  is a graph  400  showing how crosstalk may vary across multiple tiled die, where each die includes a plurality of photodiodes. The graph  400  has an abscissa  442 , labeled “Channel” and an ordinate  444 , labeled “Crosstalk”, both expressed in arbitrary units. For example, the abscissa  442  might correspond to a range of 200 channels, while the ordinate  444  might correspond to values ranging from five percent at the low end shown in the illustration, to a value of nine percent at the upper end shown in  FIG. 4 . 
   CT detectors, such as the detector system  200  of  FIG. 2 , meet tight performance requirements in order to enable the generation of high quality and artifact-free CT images and to be able to provide robust quantitative data for other purposes, such as automated computation of tumor size. The detector system  200  provides a response that is linearly related to incident X-ray intensity. Some other requirements are stability of the detector system  200  over time and temperature, sensitivity to focal spot motion, light output variation (sensitivity changes) over a useful life of the detector system  200 , etc. In CT scanners  100  of the types being currently developed and deployed, response behavior of adjacent channels or detector elements are intended to be nearly identical in order to reduce serious ring artifacts (usually defined as channel to channel non-linearity variation). This variation might be affected by the scintillator behavior from one pixel to its neighbor, by the collimator and by the diode pixel response. Generally, if these requirements are not met, ring artifacts, bands or smudges might appear in images. 
   In  FIG. 4 , plateaus  451  each correspond to a linear group of photodiodes (each corresponding to a respective one of adjacent channels), for example, eight photodiodes, all formed on one die, while dips  457  correspond to lateral separations or gaps between individual die boundaries. The dips  457  arise because photo-induced charge carriers cannot diffuse across lateral gaps between die boundaries, and, as a result, a component of crosstalk which is associated with adjacent photodiodes in an array of photodiodes formed on the same die is absent from diodes at die boundaries. As a result, there is non-uniformity in crosstalk response, which, in turn, detracts from robustness of data from the detector array and may give rise to undesirable distortion or artifacts in images formed from data from the detector array. 
   Diode-to-diode electrical crosstalk between the detector pixels is mainly driven by the lateral diffusion of photon-induced charge carriers in the semiconductor material forming the die. The amount of electrical crosstalk presented is dependent on, among other things, the thickness of the diode layer and the properties of the semiconductor material. Lateral diffusion of photocarriers generally leads to an effective photoactive area that is larger than the geometric area of the photodiode collection junction. Diffusion of photocarriers to adjacent photodiodes leads to crosstalk because some photocarriers diffuse out of the target diode in the pixel collection site in which they are generated and are collected by the diode in the neighboring pixel collection site, giving rise to a current in the adjacent photodiode. This effect is more pronounced in back-illuminated diodes, because the thickness of the diodes increases the diffusion length before collection. 
   However, in the example of  FIG. 4 , the fact that the crosstalk between adjacent diodes which are on different die is not zero allows differences in crosstalk between adjacent diodes on one die and adjacent diodes on separate die to be compensated, to some extent, by modification of optical masking or shielding for diode edges at die edges in comparison to masking or shielding between adjacent diodes on the same die. 
   The subject matter to follow describes apparatus and processes for reducing unwanted signal artifacts due to crosstalk between adjacent photodiodes formed on the same die. In turn, spatially modulating that reduction in crosstalk magnitude may also reduce differential crosstalk, that is, a difference between crosstalk originating on one side of a photodiode and crosstalk originating on an opposed side of that photodiode. This is discussed below in more detail in Section III below, with reference to  FIGS. 5 through 9 . 
   III. Embodiments 
     FIG. 5  is a simplified block diagram  500  showing pixelated detector elements  515 (i, j) arrayed in a planar fashion, for example in a plane defined by X  540  and Z  542  axes, illustrating a tesselation of four photodetector diode die  550 (1, 1) through  550 (2, 2). Optical masking  562 (x) and  562 (z) elements are shown in  FIG. 5  as forming a rectilinear grid structure and are collectively associated with the tiled photodiode die  550 (1, 1),  550 (1, 2),  550 (2, 1) and  550 (2, 2), in accordance with the teachings of the presently-disclosed subject matter. In one embodiment, the optical mask elements  562  reduce transmissivity of light therethrough. In one embodiment, the optical mask elements  562  comprise opaque masking materials. As seen in  FIG. 5 , the diodes  515  (pixels) located at boundaries do not have mask  562  along the die edges in order to balance the crosstalk and compensate for the boundary effect. 
   The optical mask elements  562  form a grid to selectively reduce the active area of the photodiodes in the photodetector elements  515 (i, j) by inhibiting photons  370  ( FIG. 3 ) from incidence at outer edges of the photodiodes in the photodetector elements  515 (i, j). The diffusion length for free carriers in semiconductors is finite. As a result, limiting the active area of the photodiodes in the photodetector elements  515 (i, j) to the more central portions of the photodiodes reduces carrier diffusion to photodiodes in adjacent photodetector elements  515 (i, j). 
   The illustration shown in  FIG. 5  also depicts lateral gaps  557 (x) at boundaries of the die  550  having width  558 (x).  FIG. 5  also depicts gaps  557 (z) at boundaries of the die  550  having width  558 (z). The lateral gaps  557  correspond to the dips  457  representing reduced crosstalk, as shown in  FIG. 4 . Optical masking elements  562  are arranged along coordinates which may be orthogonal (as illustrated) or which may conform to other coordinate systems. In the example of  FIG. 5 , the optical mask elements  562  are not placed on boundaries of the die  550 . As a result, crosstalk is increased between adjacent first photodiodes formed on neighboring die  550  relative to second photodiodes formed in central regions of the die, and the increased crosstalk due to optical effects for the first photodiodes offsets the diffusion-related component of crosstalk in the second photodiodes. 
   In other words, the optical mask elements  562  selectively spatially modulate crosstalk between adjacent photodetector elements  515 (i, j), depending on the position of the photodetector elements  515 (i, j) on the die  550  to provide different degrees of transmission modification between elements distributed along a common axis. Put another way, photodetector elements  515 (i, j) within an interior portion of the die (i.e., having neighboring photodetector elements  515 (i, j) on all sides) are surrounded on all sides by optical masking elements  562  and thus experience one degree of optical masking, while photodetector elements  515 (i, j) along boundaries of the die  550  have at least one edge which experiences a different (reduced) degree of optical masking by the optical masking elements  562 . 
   The masking elements  562 (z) are illustrated as having uniform widths, while the masking elements  562 (x, 1) (upper portion of  FIG. 5 ) are shown as having a first width, comparable to the width of the vertical masking elements  562 (z), and the masking elements  562 (x, 2) (lower portion of  FIG. 5 ) are shown as having a second width that is greater than that of the other masking elements  562 (z) and  562 (x, 1). The upper and lower portions of  FIG. 5  thus represent two different embodiments of the presently-disclosed subject matter. 
   Typically, individual photodetector elements  515 (i, j) might have dimensions ranging from circa eight hundred micrometers on a side, to one millimeter by one millimeter, however, other sizes and other arrangements, such as rectangular photodetector elements  515 (i, j), are possible. The lateral gaps  557 (x) and  557 (z) may have respective widths  558 (x) and  558 (z) on the order of fifty to about one hundred micrometers, although narrower or broader lateral gaps  557  may be employed. 
   In the upper portion of  FIG. 5 , widths of the vertically-oriented masking stripes  562 (z) are represented as being comparable to widths of the horizontally-oriented masking stripes  562 (1, x). When the extent of an individual photodetector element  515  in one dimension, such as parallel to the X axis  540 , differs from the extent of the same element  515  in another dimension, such as along the Z axis  542 , it may be appropriate to employ different widths of masking stripes  562  along different axes, for example, as shown by the wider horizontal stripes  562 (x, 2) depicted in the lower portion of  FIG. 5 . The larger dimension (the long side of the rectangle, for example) would tend to give rise to higher crosstalk due to diffusion of photo-generated mobile charge carriers and thus a greater width for the optical mask elements  562  running along those longer edges may be appropriate. 
   Other configurations of optical modulation elements  562  are possible and useful. For example, in one embodiment, masking elements  562 (z) might not be present, with masking elements  562  such as either masking elements  562 (1, x) or  562 (2, x) being employed, and with edges or perimeters of die  550  being masked to a different extent than interior portions, or not being masked at all. Such an arrangement provides different optical masking  562  at the edges or perimeters of the die  550  than in central regions (photodetector elements  515  having neighbors on all sides, for example), and also differentially modulates crosstalk effects along the X 540 and Z 542 axes. 
   In an analogous manner, in one embodiment, masking elements  562 (x) might not be present, with masking elements such as masking elements  562 (z) being employed, resulting in differential modulation of crosstalk along the X  540  and Z  542  axes, together with edges or perimeters of die  550  being masked to a different extent than interior portions, or not being masked at all. 
   Also, any of a variety of methods for creation of suitable masks  562  may be employed. In one embodiment, a mechanical grid may be separately formed and emplaced atop the die  550  to realize a suitable optical mask  562 . In one embodiment, screen printing may be employed to apply an optical mask  562  to the tesselated die  550 , or to the scintillator array, or both. In one embodiment, photolithographic techniques may be used to pattern a layer of applied material formed on the tesselated die, the scintillator array, or both, to realize a suitable optical mask  562 . 
   The optical mask  562  reduces the effective gain of the photodiodes because the light collecting area (a×b, described above with reference to  FIG. 2 ) is reduced. Additionally, because the area reductions are not equal for photodiodes at boundaries of the die  550  and photodiodes in central portions of the die  550 , gain equalization may be desired. 
   In one embodiment, the masking elements  562  or optical crosstalk inhibitors  562  comprise light absorbing materials having a thickness consistent with providing low transmissivity for incident visible light, or photons having energies near the visible range. Light absorbing materials may include light absorbent silicon, black polyimide, or other low-albedo materials. 
   In one embodiment, the masking elements  562  may comprise light reflective materials, such as metallic layers. For example, a layer of aluminum of suitable thickness may form a high-albedo layer having very low transmissivity for visible light, for photons having energies near the visible range. Reflective mask elements  562  may act to reduce effective gain loss by reflecting photons back to the target photodiode. 
     FIGS. 6 through 9  are side views, in section, taken along section lines shown in  FIG. 5 , depicting simplified block diagrams of photodetector assemblies  600 ,  700  or subassemblies  800 ,  900  capable of utility in the system  100  of  FIG. 1 , in accordance with the teachings of the presently-disclosed subject matter. The examples of  FIGS. 6 through 9  are not necessarily mutually exclusive and are not drawn to scale. 
   The embodiments  600  through  900  are depicted in configurations involving or compatible with “back-illuminated” photodiode array arrangements. The term “back-illuminated” refers to photodiodes structured to respond to illumination (such as photon  370  of  FIG. 3 ) incident on a semiconductor surface which is opposed to a surface adjacent a p-n or other diode junction. 
   For simplicity of illustration and ease of understanding, some conventional components used in forming CT photodetector arrays have not been depicted in these FIGS. By way of example, conductors associated with photodiodes are not depicted in  FIGS. 6 through 9 , and not all layers forming portions of scintillators are necessarily illustrated. 
     FIG. 6  is a side view, in section, showing the photodetector assembly  600  in cross-section, taken along section lines VI-VI of  FIG. 5 , and illustrating a group of four photodetector elements  615 (N). The photodetector elements  615 (N) are formed via a scintillator array  630  comprising scintillator cells  632  separated by conventional septa  634  each having a width  636 , such as one hundred micrometers, although larger or smaller septa  634  may be employed. A photodiode array  650  is represented by two semiconductor die  652  and  653  which include a plurality of photodiodes  654 (N). 
   More specifically, the die  652  includes adjacent or neighboring doped regions or photodiodes  654 (1) and  654 (2). The die  653 , neighboring the die  652 , includes doped region or photodiode  654 (3) adjacent the photodiode  654 (2) and also includes adjacent doped region or photodiode  654 (4). Channel stops  656  formed from dopants introduced into the die of semiconductive material  652 ,  653  to at least partially electrically separate doped regions or photodiodes  654 (N) that are adjacent one another on a single die  652  or  653 , while a physical lateral gap  657  having a width  658  separates adjacent or neighboring doped regions or photodiodes  654 (N) formed on different adjacent die  652  and  653 . 
   In one embodiment, the die  652  and  653  are formed from single-crystal silicon which is doped to be n-type (i.e., forms a cathode), while doped regions  654 (N) are counterdoped to be p-plus-type (heavily doped) regions (i.e., to form an anode). In one embodiment, the channel stops  656  may also be counterdoped to be p-type or p-plus-type regions. 
   An optical modulation region  660  including an optical mask  662  and optically transmissive or optical coupler portions  664  is intercalated, inserted, formed or sandwiched between the scintillator array  630  and the photodiode array  650 . In other words, optical crosstalk inhibitors  662  (i.e., analogous to optical mask elements  562  of  FIG. 5 ) are interstitially layered between the photodiode array  650  and the scintillator array  630 , with light (i.e., optically) transmissive regions  664  formed between the optical mask elements or stripes  662 . The light transmissive regions  664  act as optical couplers for guiding photons from the scintillator cells  632  to the associated photodiodes  654 . In one embodiment, the light transmissive regions  664  comprise vertical air gaps, however, it will be appreciated that other materials (such as epoxy) having suitable contrast capabilities relative to the mask elements  662  may be employed. Although the regions  664  are intended to be transmissive, the regions  664  contribute a degree of optical masking. For example, a small degree of reduction of transmissivity might be due to reflections etc. 
   The optical mask elements  662  shown in  FIG. 6  have a uniform width dimension  668 . In one embodiment, the width  668  of the reflective or absorbing mask elements  662  is larger than a width  636  of the septa  634  or lateral gap  634  between active scintillator elements  632 . 
   The optical mask elements  662  thus extend laterally outward of the septa  632  and consequently reduce or inhibit photons  670  from impinging on or immediately adjacent to edge portions of the photodiodes  654  which comprise boundaries shared by neighboring photodiodes  654 . As a result, the active area of the photodiodes  654  associated with such optical masking elements  662  is reduced, which reduces the gain or sensitivity of the photodiodes  654  by inhibiting photon-induced generation of mobile carriers (aka “photocarriers”) immediately adjacent the shared boundary. Introducing a controlled, additional degree of separation between active portions of adjacent photodiodes  654  also reduces diffusion of photocarriers between adjacent photodiodes  654  and thus reduces crosstalk between neighboring pixel in the photodetector. 
   The optical mask elements  662  are shown beneath the septa  634 (1) and  634 (3), but no optical mask element  662  is shown in  FIG. 6  atop the lateral gap  657  between the die  652  and  653 . As a result, the reduction in crosstalk between adjacent diodes  654 (1),  654 (2) on die  652 , and between neighboring diodes  654 (3),  654 (4) on die  653  is offset by the crosstalk from mechanisms other than free carrier diffusion. 
   In operation, an optical photon  670  results when a high energy photon  672 , such as an X-ray, is incident on one of the scintillator cells  632 . Many of the optical photons  670  then travel to the associated target photodiode  654 (3). However, as noted earlier, some of the incident high energy photons (such as X-rays)  672  are scattered into adjacent scintillator cells  632 ; some of the optical photons  670  are scattered into adjacent photodiodes  654 ; and some of the charge carriers produced in the intended or target photodiode  654  by incident optical photons  670  diffuse into neighboring photodiodes  654  formed on the same die  652  or  653 . These diverse mechanisms combine in formation of electrical signals representing crosstalk between adjacent or neighboring photodetector elements  615 . 
     FIG. 7  is a side view, showing the photodetector assembly  700  in cross-section, taken along section lines VII-VII of  FIG. 5 , illustrating a group of four photodetector elements  715 (N) formed from a scintillator array  730  comprising scintillator cells  732  separated by conventional septa  734  each having a width  736 , as described above. 
   A photodiode array  750  is represented by two semiconductor die  752  and  753 , having photodiodes  754 (N) formed thereon and separated within each die by channel stops  756 . A physical lateral gap  757  having a width  758  separates adjacent or neighboring doped regions or photodiodes  754 (N) formed on different adjacent die  752  and  753 . 
   An optical modulation region  760  includes optical mask elements  762  and  763 , and optically transmissive or optical coupler portions  764 , which are collectively interposed between the scintillator array  730  and the photodiode array  750 . The optical mask elements  762  shown in  FIG. 7  have a first width dimension  768 . The optical mask element  763  has a second width dimension  774  which is less than the first width dimension  768 . The second width dimension  774  is larger than the width  736  of the septa  734  or lateral gaps  734  between scintillator elements  732 . 
   The optical mask elements  762  having the first width  768  are shown beneath respective septa  734 (1) and  734 (3). An optical mask element  763  is depicted in  FIG. 7  atop the lateral gap  757  between the die  752  and  753 . As a result, the reduction in crosstalk between adjacent diodes  754 (1),  754 (2) on die  752 , and between neighboring diodes  754 (3),  754 (4) on die  753  due to the respective optical mask elements  762 (N) is partially offset by the crosstalk from mechanisms other than free carrier diffusion. 
     FIG. 8  is a side view, showing the photodetector subassembly  800  in cross-section, taken along section lines VIII-VIII of  FIG. 5 , illustrating a scintillator array  830  comprising scintillator cells  832  separated by conventional septa  834  each having a width  836 , as described above. 
   An optical modulation region  860  including optical mask elements  862  and  863  and optically transmissive or optical coupler portions  864  is shown on a bottom or lower surface of the scintillator array  830 . The optical mask elements  862  shown in FIG.  8  have a first width dimension  868 , while the optical mask element  863  has a second width dimension  874  which is less than the first width dimension  868 . The second width dimension  874  is larger than the width  836  of the septa  834  or the septa or lateral gaps  834  between and separating active scintillator elements  832 . 
   The optical mask elements  862  having the first width  868  are shown beneath respective septa  834 (1) and  834 (3). An optical mask element  863  is depicted in  FIG. 8  at a location which later will be atop a lateral gap between adjacent die, as shown and described above with reference to  FIGS. 6 and 7  and associated text. 
     FIG. 9  is a side view, showing the photodetector subassembly  900  in cross-section, taken along section lines IX-IX of  FIG. 5 , illustrating a group of four photodetector elements  915 (N) formed from a photodiode array  950 . The photodiode array  950  is represented by two semiconductor die  952  and  953 , having photodiodes  954 (N) formed thereon and separated within each die by channel stops  956 . A physical lateral gap  957  having a width  958  separates adjacent or neighboring doped regions or photodiodes  954 (N) formed on different adjacent die  952  and  953 . 
   An optical modulation region  960  including optical mask elements  962  and  963  and optically transmissive portions or optical couplers  964  is formed atop the photodiode array  950 . The optical mask elements  962  shown in  FIG. 9  have a first width dimension  968 . The optical mask element  963  has a second width dimension  974  which is less than the first width dimension  968 . The second width dimension  974  is larger than widths of septa between scintillator elements, such as widths  636 ,  736 ,  836  of respective septa  634 ,  734  or  834  between active scintillator elements  632 ,  732  or  832 , of  FIGS. 6 ,  7  and  8 , respectively. 
   The optical mask elements  962  having the first width  968  are positioned between adjacent photodiodes  954 (1),  954 (2) and  954 (3),  954 (4), respectively. The optical mask element  963  having the second width  974  is depicted in  FIG. 9  atop the lateral gap  957  between the die  952  and  953 . 
   In the embodiments  700 ,  800  and  900  of  FIGS. 7 ,  8  and  9 , respectively, the respective gaps  757 ,  857  and/or  957  may be optically transmissive, or may be optically transparent. The gaps  757 ,  857  and/or  957  may be air gaps. Alternatively, the gaps  757 ,  857  and/or  957  may be filled with material (such as a suitable epoxy) forming an optical coupler between respective neighboring diode pairs  754 ,  854  and/or  954  formed on respective adjacent die  752 ,  753 ;  852 ,  853  and/or  952 ,  953 . In these embodiments, the optical mask elements  662 ,  762 ,  763 ,  862 ,  863 ,  962 ,  963  may have respective widths  668 ,  768 ,  774 ,  868 ,  874 ,  968 ,  974  in a range of from about one hundred to about three hundred micrometers. In other words, the optical mask elements  662 ,  762 ,  763 ,  862 ,  863 ,  962 ,  963  may have respective widths  668 ,  768 ,  774 ,  868 ,  874 ,  968 ,  974  in a range of from about fifty to about two hundred micrometers greater than widths  636 ,  736 ,  836  of the respective septa  632 ,  732 ,  832 . 
   In one embodiment, deep diffusion of traps, or carrier killing dopants, for example in regions  656  ( FIG. 6 ),  756  ( FIG. 7 ) or  956  ( FIG. 9 ), in semiconductive materials, can also be employed to selectively inhibit diffusion of light-induced free charge carriers from one photodiode to a neighboring photodiode. 
   Differential crosstalk includes at least two components. One component arises from misalignment of scintillator cells vis-a-vis the associated photodetectors. Another component is due to differences in the physical environments, and thus the physical phenomena giving rise to crosstalk, for photodiodes lacking a neighbor on at least one side, relative to photodiodes that are surrounded by nearest neighbors. 
   As a result, an optical mask having opaque or relatively non-transmissive optical properties that is wider than septa between scintillator cells may reduce the performance impact of misalignment with respect to those photodiodes which are surrounded by nearest neighbor photodiodes. As a further result, spatially modulating the degree of masking to provide less masking along diode edges not abutted by nearest neighbor photodiodes permits crosstalk from one set of physical phenomena to be balanced against crosstalk derived from another set of physical phenomena to realize reduced differential crosstalk. 
   IV. Conclusion 
   The disclosed examples of the preceding sections combine a number of useful features and present advantages in contemporary CT scanner applications. These examples reduce variations in crosstalk between adjacent or neighboring imaging elements, and thus provide more robust data for image formation or for quantitative estimate (e.g., tumor size) purposes. Additionally, these examples relax need for precision control of scintillator mechanical dimensions as well as tolerance requirements of diode-scintillator alignment in assembly. 
   The disclosed subject matter reduces crosstalk between adjacent photodiodes formed on the same die and also employs spatially modulated reduction in crosstalk magnitude to reduce differential crosstalk, that is, a difference between crosstalk originating on one side of a photodiode and crosstalk originating on an opposed side of that photodiode. The present disclosure describes a variety of approaches to reducing signal artifacts arising from both crosstalk and differential crosstalk, resulting in improved spatial resolution and in increased dynamic range 
   An optical modulator formed from elements having opaque or relatively non-transmissive optical properties that are aligned with and wider than septa between scintillator cells may reduce the performance impact of misalignment with respect to those photodiodes which are surrounded by nearest neighbor photodiodes. Also, spatially modulating the degree of masking permits crosstalk derived from different physical phenomena to offset one another and thus to reduce differential crosstalk. 
   Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any adaptations or variations. For example, although described in procedural terms, one of ordinary skill in the art will appreciate that implementations can be made in a procedural design environment or any other design environment that provides the required relationships. 
   In particular, one of skill in the art will readily appreciate that the names or labels of the processes and apparatus are not intended to limit embodiments. Furthermore, additional processes and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments. One of skill in the art will readily recognize that embodiments are applicable to future communication devices, different file systems, and new data types. The terminology used in this disclosure is meant to include all object-oriented, database and communication environments and alternate technologies which provide the same functionality as described herein.