Patent Publication Number: US-2007096034-A1

Title: Edge effects treatment for crystals

Description:
REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. patent application Ser. No. 10/993,012, filed Nov. 19, 2004, which claims priority of U.S. Provisional Patent Application Ser. No. 60/523,765 filed Nov. 20, 2003, the entire content of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to improved apparatus and methods relating to reduction or elimination of edge effects in optical elements, for example, scintillators.  
     BACKGROUND OF THE INVENTION  
      A scintillation material, or scintillator, produces light in response to incident radiation, typically ionizing radiation such as gamma rays or x-rays. Such a material can be a crystal such as thallium-doped sodium iodide, NaI(T1), or a non-crystalline material such as a plastic.  
      Radiation detectors using scintillators include gamma cameras, x-ray detectors, and other radiation imaging or sensing devices. The scintillator generally has a radiation receiving face on which radiation such as gamma rays are incident. The scintillator has a light output face from which light emerges in response to incident radiation, and also an outside edge. For example, a disk-shaped scintillator would have a cylindrical edge surface. An optical window, such as glass, is typically bonded to the light output face of the scintillator. An array of light sensors, such as photomultiplier tubes, receive scintillation light emitted from the light output face of the scintillator, and transmitted through the window to the detectors. A plastic “light pipe” is sometimes interposed between the window and the light sensors.  
      The light is generated within the scintillator as pulses (also termed scintillation events). The positions of light pulses generated within the scintillator are used in imaging applications, such as SPECT (single photon emission tomography) and PET (positron emission tomography) imaging. Such uses may be generally referred to as scintillation imaging.  
      A problem in scintillation imaging is that reflections from the edge of the scintillator will reduce the measured positional accuracy of imaging data, particularly for scintillation events occurring close to the edge of the scintillator. Light from scintillation events occurring near an edge will be reflected from that edge, causing a serious reduction in ability to accurately determine the position of such events. In practice, a dead zone may exist around a peripheral region, proximate to the edge of the scintillator or window, from which meaningful positional data cannot be collected. This dead zone or “edge effect” reduces the effective usable portion of the scintillator. In addition, the presence of this unusable region at the periphery of the detector prevents the detector from being positioned optimally in many medical applications, for example breast and brain scintigraphy. The increase in detector bulk caused by the wasted portion of the detector may also make it difficult to position the detector sufficiently close to the patient in applications such as heart scintigraphy and many forms of SPECT.  
      In some detector designs, it is desirable to have more than one scintillator element in close proximity to another. In this situation, the junctions between elements tend to act as reflecting edges causing an unusable dead zone of edge effect on each side of the junctions.  
      This problem is well recognized in the field. For example, a previous attempt to solve this problem is described in U.S. Patent Application Publication 2003/0034455 to Schreiner et al., which suggests segmenting the scintillator into a number of triangular segments. However, such segmentation adds to the cost of a device, is difficult to fabricate and may cause problems if the scintillator absorbs moisture from the air. For example, it is well known that sodium iodide should be protected from atmospheric moisture.  
      Another attempted solution is described by U.S. Pat. No. 4,284,891 to Pergale et al., which suggests providing a diffused light reflector around the periphery of the optical window. However, it can be difficult to provide a true diffused reflector, as reflection properties of many materials and the crystal edge will change with time and environmental conditions. In addition, such diffused edge treatments have been found in practice to provide an unsatisfactory degree of improvement to the problem.  
      Hence, there is a need for improved scintillators and radiation detectors which reduce or eliminate the undesirable effects of edge reflection.  
     SUMMARY OF THE INVENTION  
      Methods and apparatus are provided to reduce edge effects, such as loss of positional accuracy due to edge reflection, in optical elements such as scintillators. For example, one or more light guides can be provided in a peripheral region of a scintillator or optical window close to an edge.  
      A radiation detector according to an example of the present invention comprises a scintillator having a light-emitting face, a radiation receiving face, and a perimeter extending between the light-emitting face and the radiation receiving face, the perimeter including an edge. The scintillator emits scintillation light from the light emitting face in response to radiation incident on the radiation receiving face. One or more light guides are formed within a peripheral region proximate to the edge, a light guide redirecting a portion of scintillation light before it emerges from the light emitting face. The radiation detector may further comprise a number of light sensors receiving scintillation light from the scintillator.  
      Light guides provided within the peripheral region can improve the positional accuracy of the radiation detector. There may be a plurality of spaced apart light guides formed only within the peripheral region. The light guides may include grooves having a groove depth which decreases as the distance of the groove from the edge increases.  
      The light emitting face of the scintillator may have a non-peripheral region, such as a central region, in which there are no light guides. Depending on the application, the non-peripheral region may be larger, sometimes substantially larger, than the peripheral region.  
      The peripheral region is a region within a certain distance of the edge. The distance may be some multiple of an edge thickness, such as less than ten times, for example within eight times the edge thickness of the edge. The distance may also be the approximate diameter of a light sensor.  
      A light guide may comprise a groove formed in the light emitting face and/or the radiation receiving face of a scintillator. In other examples, light guides may be provided in a window between a scintillator and a detector or detectors. A light guide may comprise a groove, reflecting film, an interface or other boundary between two regions of different refractive indices, or other structure providing an internal reflection or refraction of light within the scintillator. The light guide can be substantially parallel to the edge.  
      Another example of an improved radiation detector comprises a scintillator, an array of sensors in optical communication with a light emitting face of the scintillator, a window between the scintillator and the array of light sensors, the scintillator and/or the window having one or more grooves formed in a face thereof. The grooves may be formed only within a peripheral region of the scintillator and/or window.  
      In examples discussed below, the term “crystal” is often used for convenience to refer to a scintillation material. However, any example discussed here equally applies to non-crystalline scintillators. The methods and apparatus described can also be adapted for use in other optical elements, as will be clear to those skilled in the optical arts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a side view of a pair of crystals, each having a number of grooves cut in respective peripheral regions proximate to crystal edges;  
       FIG. 2  shows a side view of a crystal, showing grooves cut in the light output face of the crystal within a peripheral region proximate to the crystal edge;  
       FIG. 3  is a side view of a pair of crystals, each having grooves cut in a peripheral region of the radiation receiving face;  
       FIGS. 4A and 4B  show top views of a crystal;  
       FIG. 5  shows a side view of a crystal, having both a window and an optical transmission element, the latter having grooves cut in the lower face;  
       FIG. 6  is a side view of a radiation detector, including a crystal and a window having grooves cut in the lower face of the window;  
       FIG. 7  is a side view of a radiation detector, having both a window and an optical transmission element, each having grooves;  
       FIG. 8  is a side view of a radiation detector, having grooves in both the crystal and the window;  
       FIG. 9  is a side view of a radiation detector having grooves in the crystal and the window, the grooves not being in register;  
       FIG. 10  shows a radiation detector, having grooves in the upper and lower faces of both the crystal and window;  
       FIG. 11  shows a radiation detector, the window/light pipe having an upper surface shaped so as to direct light to a plurality of sensors, the window also having grooves in a peripheral region;  
       FIG. 12  shows a curved crystal having grooves in a peripheral region;  
       FIG. 13  shows a circular crystal having circular grooves;  
       FIG. 14  shows a radiation detector configuration including a computer and display;  
       FIG. 15  shows a radiation detector having two crystals and a metal housing;  
       FIG. 16  shows a cross section of a scintillation crystal with light guides formed in the peripheral and non-peripheral regions; and  
       FIG. 17  shows a cross section of another scintillation crystal with light guides formed in the peripheral and non-peripheral regions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In examples discussed below, the term “crystal” is used for convenience to refer to a scintillation material. However, examples discussed here apply equally to non-crystalline scintillators. Also, in examples discussed below, light guides are provided by grooves cut into one or more surface of the crystal (or associated window). As discussed in more detail below, other forms of light guide can also be used, for example, reflective films.  
       FIG. 1  shows a portion of a first crystal  10 , a portion of a second crystal  12 , a crystal gap  14  between the two crystals, a plurality of sensors such as sensor  16 , a plurality of grooves in the light emitting face of each crystal, such as grooves  18   a - 18   d,  a first crystal edge  20 , and a second crystal edge  22 .  
      As drawn, the lower surface of the crystal  10  is the radiation receiving face, and the upper surface of the crystal is the light emitting face. This convention will be followed (for convenience only) through the various figure descriptions.  
      The light emitting face and radiation receiving face can be generally parallel and coextensive, and interconnected by a perimeter, the perimeter defining an edge. The crystal can be in the form of a cuboid, having a length, width, and thickness, the thickness being the edge thickness. The thickness may be substantially less than the length and/or width of the crystal.  
      Sensors may be in different positions from those shown. A sensor may, for example, span the gap between two crystals. Sensors may be provided in a linear or two-dimensional array, or other configuration. Scanning imaging methods may also be used.  
       FIG. 2  shows another view of crystal  10 . The star symbol at  28  represents a pulse of scintillation light (a scintillation event) produced by radiation interacting with the crystal. For convenience, the term “scintillation light” will often be abbreviated to “tight”. The zigzag arrow labeled L represents a possible path of light out of the crystal. For convenience, not all grooves are shown.  
      Light will tend to propagate in all directions within the crystal  10  from the scintillation event (the production of a pulse of light). However, the arrows show the groove  18   a  and crystal edge  20  operating cooperatively to provide a light guiding effect, whereby light is reflected from the crystal edge and from the inner surface of the groove so as to take a path remaining proximate to the edge, and to emerge from the light emitting face within a portion of the light emitting face between the groove and the crystal edge. Hence, the groove  18   a  and the crystal edge  20  provide partial optical conferment, or a waveguide effect.  
       FIG. 3  shows a first crystal  40 , a second crystal  42 , a separation gap  44 , grooves such as groove  46  in the radiation receiving face of each crystal, and a plurality of sensors such as sensor  48 . The sensors are in optical communication with the upper face, or light emitting face, of each crystal. This figure illustrates that grooves may be provided in the radiation receiving face of the crystal, as well as or instead of grooves in the light emitting face.  
       FIG. 4A  shows a top view of a crystal, showing a rectangular light emitting face, having a surface (such as the light emitting face)  50 , and a plurality of grooves such as  56  and  58  around the periphery of the crystal, proximate to the edge. The edge is shown as rectangular, including sides  52  and  54 . The grooves may have equal depth, or in other examples the groove depth is greatest for the groove closest to the edge (groove  58 ), and shallowest for the groove furthest for the edge (groove  56 ). The depth of grooves can be correlated with the distance from the edge, for example inversely proportional to the distance, as discussed in more detail below.  
      The crystal may also have a rectangular cross section, so as to have a cuboid form. The orthogonal edge regions of the crystal, as illustrated in  FIG. 4A , may be designated as sides or edges, as preferred. Analogous groove patterns may be formed in one or more faces of the crystal and/or window.  
       FIG. 4B  shows a top view of another example, in which grooves formed in the surface  59  form a cross-hatched pattern in the comers. Other details may be the same as discussed above in relation to  FIG. 4A .  
       FIG. 5  shows a side view of a radiation detector comprising a radiation-side optical transmission element  60 , a crystal  62 , a window  64 , an optical matching medium  66 , a plurality of sensors such as sensor  68 , and a plurality of grooves such as  70  and  72 . In this example the grooves are formed in the upper side of the optical transmission element  60 , proximate to the radiation receiving face of the crystal  62 .  
      In another example which may appear similar to that illustrated in  FIG. 5 , the radiation detector comprises a crystal and lower and upper optical transmission elements or window layers supported by the light emitting face of the crystal. The grooves can be formed in the light emitting face of the crystal.  
       FIG. 6  represents a side view of a radiation detector, comprising a crystal  80 , a window  82 , a plurality of sensors such as sensor  84 , and a plurality of grooves such as grooves  86  and  88 . In this example, the grooves are formed in the lower side of the window, the side of the window proximate to the light emitting face of the crystal. It should be recalled that the crystal and the window, both typically being transparent, may be considered as a single optical element., i.e. together they form a continuous medium for transmission of light. Thus, placement of groves in the window provides a reduction in edge effect in the same manner, although to a lesser extent, as do grooves in the crystal itself. It should also be appreciated that light reflections occur from both the edge of the crystal and from the edge of the window. Reflections from the edge of the window may further degrade spatial accuracy beyond the effects of edge reflections in the crystal itself. An existing radiation detector may be improved by the simple process of replacing the existing window with a window having a plurality of grooves within the side of the window proximate to the crystal. In another example, the grooves may be formed in the upper face of the window, the face most distant from the crystal.  
       FIG. 7  shows a radiation detector comprising a radiation side window or optical transmission element  100 , a crystal  102 , a window  104 , a sensor  106 , grooves such as  108  and  110  in the radiation-side window proximate to the radiation receiving face of the crystal, and grooves such as  112  and  114  in the lower surface of the window proximate to the light emitting face of the crystal.  
      In another example, which may appear similar to that illustrated in  FIG. 7 , a radiation detector comprises a crystal, a lower window, and an upper window. In this example, grooves are provided within the crystal and within one or two window layers transmitting light from the crystal to the sensor,  
       FIG. 8  illustrates a radiation detector comprising a crystal  120 , a window  122 , and a sensor  124 , with grooves such as  128  provided in the lower surface of the window. Grooves such as  126  are provided in the upper surface of the crystal. In this example the grooves in the crystal and the grooves in the window are substantially in register.  
       FIG. 9  shows a radiation detector comprising a crystal  140 , a window  142 , a sensor  144 , grooves such as  146  in the upper surface of the crystal, and grooves such as  148  in the lower surface of the window. In this example the grooves in the crystal and grooves in the window are substantially out of register.  
       FIG. 10  shows a radiation detector comprising a crystal  160 , a window  162 , a sensor  164 , grooves such as  166  in the lower face of the crystal, grooves such as  168  in the upper surface of the crystal, grooves such as  170  in the lower surface of the window, and grooves such as  172  in the upper surface of the window.  
       FIG. 11  shows a radiation detector comprising a crystal  180 , a window  182 , and a sensor  184 , the window having an upper surface topography  186  designed so as to direct light towards the light sensitive regions of the sensors. Grooves such as  188  are shown provided in the lower surface of the window, but alternatively or additionally could be in either surface of the crystal or the upper surface of the window.  
      A series of triangular indentations  190  are provided in the upper surface of the window so as to prevent light being lost to dead spaces within or between the sensors, which would otherwise not be detected Alternatively, structure  182  may be composed of two elements, a window proximate to the crystal and a “light pipe” interposed between the window and the light sensors. Other surface topographies can be used, as discussed further below.  
       FIG. 12  shows a curved (arcuate) crystal  200  having grooves  202  within a peripheral region, proximate to one edge of the crystal. In this example the grooves are substantially radial extending from the lower surface of the crystal towards the interior.  
       FIG. 13  is a top view of a circular crystal  220 , having a circular edge  222 , and grooves  224  and  226  within a peripheral region proximate to the edge.  
       FIG. 14  shows a radiation detector comprising a radiation source  240 , a crystal  242 , a window  244 , a light sensor array  246 , a computer  248 , a display device  250 , a data port (for example, a data input device)  252 , and an analysis circuit  254 . A computer program running on the computer  248  can be used to extract position information provided by the light sensor array. Algorithms may be provided to provide edge corrections, depth corrections and other corrections as well known in the art. The analysis circuit may comprise noise reduction circuitry, and the like, and may be integrated with the computer into a single device. The radiation source may be a mammal under diagnosis.  
       FIG. 15  shows a side view cross section of a radiation detector comprising a housing  260 , a first crystal  262 , a second crystal  264 , substantially in abutment to the first crystal, and a window  266 . The housing  260  can be a metal such as aluminum, and may provide protection of the crystal from atmospheric moisture or other sources of degradation. A detector array  270  includes a plurality of light sensors such as  272  as well as associated positioning circuitry. Grooves such as  268  are shown within peripheral regions of each crystal so as to reduce edge effects from reflections, and improve the accuracy of positional data provided by the sensor array  272  provided above the window.  
      In other examples, two windows can be provided in register with the crystals, and grooves provided in the windows close to the ends.  
      Light Guides (Grooves)  
      The term “light guide” can be used to refer to any structures that may be provided within a crystal to provide internal redirection of light. The light guide may be a groove (such as a cut in the surface of the crystal), and the term “groove” is used elsewhere for convenience to represent light guides. The term groove includes structures such as cuts, slots, and the like.  
      A light guide may include a groove, an interface between media of substantially different refractive indices, a reflective film, bubbles, defects, crystal defects such as crystal grain boundaries, fracture films, or other structure or components that provide redirection of light within the crystal before the light emerges from the light emitting surface. Light guides may also comprise embedded fibers, plastic or metal films, or other materials.  
      A groove can be air filled, or filled with fill material such as a liquid, plastic, glass, reflective film (such as a plastic or metal film), multilayer reflective film, fibers, spheroids (for example, forming a photonic band-gap reflector), interferometric structure, inert gas, vacuum (if the scintillator is in a sealed housing), or other material.  
      A light guide can be substantially parallel to a proximate edge region, and/or substantially normal to a surface in which it is formed, or nearby surface. A plurality of spaced apart light guides can be formed within a peripheral region. The depth or other extent of each light guide can be inversely correlated with the distance of the light guide from the edge (the distance being measured between the light guide and the most proximate region of the edge). The light guides may not extend entirely to any surface of the crystal or window, but may instead be disposed inside the volume of the crystal or window.  
      In other examples, light guides can be provided across the fill extent of a surface, not just in a peripheral region. As for peripheral light guides, the depth (or analogous extent) of the light guide can be inversely correlated with the distance from the nearest edge (less when further from the edge, the relationship can be linear or nonlinear). In one example, peripheral light guides have a depth that is inversely correlated with the distance from the nearest edge, and light guides in a middle region of the surface can all have an equal depth. This example is illustrated in  FIG. 16 , where the body of scintillation material is illustrated at  300 . A plurality of light guides  302  are formed in the body  300 . Peripheral region  304  is illustrated between outboard edge  306  and line P-P. An opposing peripheral region is illustrated at  308 . The light guides in the peripheral region all have depths greater than the depths of the guides in the non-peripheral region  310 . In this example, the light guides in the non-peripheral region all have the same depth. In another example, illustrated in  FIG. 17 , the light guide depths are inversely correlated with the distance from an edge, across the entire surface.  
      A light guide can provide partial optical confinement of scintillation light between the light guide and either another light guide or an edge. The partial optical confinement can improve the positional accuracy of a radiation detector using the scintillator.  
      If the scintillator has an elongated form having a uniform cross-section, having a first end and a second end, light guides can be formed in peripheral regions proximate to one or both ends.  
      The number of light guides proximate to an edge may be a number within the range 1-20 (inclusive), such as in the range 1-10 (inclusive), for example, one, two, three, four, five, six, seven, eight, nine, or ten. Example scintillators were made with 5-7 grooves, which were found to Improve positional accuracy and dramatically reduce the edge effect dead zone near crystal edges.  
      Groove spacing may be regular (equal spacing), or non-equally spaced. Graduations in groove depth can be linear or non-linear with distance from the edge, or all grooves can be the same depth.  
      It is preferred that the light guide depth near the edges be graduated. Equal-depth grooves or light guides in a periphery region may be beneficial for some applications, but they tend to produce a zone of edge effect inside the innermost groove, i.e. the innermost groove acts like an edge. A groove extending only partway through the crystal does produce less of an edge effect than a full edge however, and is therefore somewhat useful.  
      The preferred embodiment, however, is the progressively graduated grooves or light guides becoming shallower as one moves inward from the edge. Since the grooves are deeper on one side of the “waveguide” than the other, they limit light spread more in one direction than the other. This produces a gradual effect rather than a sharp edge, thus eliminating all or more of the dead zone. This is due to a “one-way” diffusion aspect of the graduated depth arrangement. This can be most easily explained by reference to one of the simplest arrangements of the present invention, where grooves or light guides are provided in the light emission side of the crystal only. Light can diffuse beyond the confines of the “waveguide” at the bottom. Light that exits a waveguide at the bottom can go either toward the edge or away from the edge. Since the open path (space between bottom of light guide and bottom surface of crystal) towards the edge is smaller than the open path away from the edge, light is more likely to diffuse away from the edge. This “pushes” the emitted light away from the edge. The intensity of light reaching the light sensors, instead of being a bell shaped curve is now skewed, with a wider spread away from the edge. This “pushing” of the light emission profile away from the edge also happens for light reaching the bottom of the next waveguide further from the edge. This includes both light that traveled down that waveguide plus light that traveled to that point from the adjacent waveguides. This causes the emission profile to be skewed even further away from the edge. This skewing effect becomes less and less, however, as one looks at waveguides further and further from the edge. This is because, proportionally, the relative openings toward and away from the edge become more and more equal. For the innermost waveguide, the chance of a light photon going to the right is almost as great as its chance of going to the left. Thus, the emitted light is pushed away from the edge effect dead zone, but the degree to which it is pushed away gets less and less, the further one moves from the edge. This causes the edge effect to be blurred out and spread over a wide region.  
      In addition to the above, the resolution enhancing effects of the light grooves both improve resolution and mitigate the resolution reducing effects of “smearing” the edge reflections over the whole peripheral region.  
      The positional accuracy of an imaging device can be increased by providing more closely spaced grooves. The groove spacing may be, for example, a fraction of a sensor diameter, such as a spacing within the range 0.01-1 times the sensor diameter, such as in the range 0.05-0.5 of the sensor diameter. The groove spacing may also be a fraction of the edge thickness, such as in the range 0.01-0.5 times the edge thickness.  
      If the grooves have variable groove depth, such as groove depths inversely correlated with distance from the edge, in some examples the shallowest groove may be approximately 1 mm, and the deepest groove approximately equal to half the edge thickness. In some examples, the grooves may be curved.  
      Peripheral Region  
      The peripheral region can be defined in terms of the sensor width, for a radiation detector including a plurality of sensors. The sensor width may be for example, the outer diameter of a sensor, or the average spacing distance of sensors. The peripheral region can be defined as a region proximate to the edge of the scintillator, and not more than a distance approximately equal to the sensor width from the edge. Alternatively, the peripheral region can be defined as a region proximate to the edge of the scintillator, and not more than a distance approximately equal to the half the sensor width from the edge. If the sensor is a photomultiplier tube, the sensor width may be termed the tube width.  
      The peripheral region can also be defined in terms of a fraction of the overall dimensions of a surface. For example, the peripheral region of a crystal or window may be a region proximate to the edge, and not more than a certain fraction of the distance from the center to that edge. The certain fraction may be, for example, 5 percent, 10 percent, 15 percent, or 20 percent.  
      The peripheral region can also be defined as a region proximate to the edge, and not more than six to eight times the thickness of the slab from the edge. Alternatively, the region may be less, such as 3-4 times the thickness. The peripheral region can also be defined as a region proximate to the edge, and not more than approximately the thickness from the edge. In one example, the crystal has a thickness of ¼″ to ⅜″, the phototubes have a diameter of 2″-3″, and the peripheral region is 1″-1.5″.  
      In one example, a radiation detector can include a scintillator in the form of a cuboid slab having a slab length, a slab width, and a slab thickness (equal to an edge thickness), with the slab length and slab width both being substantially greater than the slab thickness. The peripheral region, for some examples, may be an outer region of the slab within approximately three to eight times or approximately equal to the edge thickness from the edge.  
      Manufacture of Light Guides  
      Light guides (such as grooves) may be formed by a variety of mechanical, chemical, optical, ultrasonic, or other means. For example, a saw can be used to cut grooves in one or more surfaces of a crystal. Grooves may also be formed by a high pressure jet of fluid. It is known that sodium iodide, a common crystal material, is soluble in water. In this case a non-aqueous fluid could be used, such as an oil, supercritical carbon dioxide, or other fluid. Water can also be used, and the surface dried quickly after jet cutting. A high pressure gas jet such as carbon dioxide or nitrogen can also be used to provide grooves in a surface.  
      A laser can be used to ablate or otherwise provide grooves or cuts in the surface of a crystal. For laser cutting, it is advantageous to use a different wavelength from that of the light produced by radiation within the crystal, as the crystal will presumably be substantially transparent to that wavelength; for example, x-ray, U-V or IR wavelengths may be used if the scintillation light is in the visible region. The crystal may include additives so as to absorb laser radiation within a predetermined range of wavelengths to facilitate crystal processing.  
      Inclusions, bubbles, or defects may also be used to provide redirection of light within the crystal. For example a laser focused within the bulk of the crystal may be used to vaporize part of the crystal so as to provide an air bubble. Such defects may be provided in a regular array or pattern so as to provide the desired light guiding effect.  
      Bubbles may also be injected into the molten medium from which the crystal is formed. The crystal may also include other materials having a substantially different refractive index so as to provide refractive light guiding. Light guides, such as grooves, may also be formed by molding (for example of plastic or molten materials, for example during crystal growth), stamping, drilling, other mechanical processes, chemical etching, ion bombardment, electron beams, atomic beams, lithographic processes, and the like.  
      Crystal Geometry  
      A crystal may have a cuboid shape, having a light emitting face, an opposed radiation receiving face, and a rectangular edge (including first and second opposed ends, and first and second opposed sides). Any pair of opposed ends may be alternatively designated as sides, or vice versa.  
      The crystal has a crystal thickness, defined as the distance between the light emitting face and opposed radiation receiving face. The two faces may be parallel, providing a uniform thickness.  
      The light emitting face extends between the first and second ends, and between the first and second sides. Similarly, the radiation receiving face, opposed to the light emitting face, can be substantially parallel to the light emitting face and also extending between the first and second ends.  
      In other examples the crystal may be curved (actuate), for example either being formed initially in a curved or actuate shape, bent after crystal formation, or bent after heating or other softening process. The bending process may take place during heating of a crystal. The curve may be in a single plane, or may be in three dimensions so as to provide, for example, a spherical section.  
      Grooves may be formed generally normal to either the light emitting face or radiation receiving face. Grooves may be parallel to the portion of the edge to which they are proximate, for example parallel to the first or second end.  
      The grooves may be equally spaced, or provided at irregular intervals. The depth of the groove within the crystal, the depth being defined as the distance between the face into which the groove is cut and the distal end of the groove, can be correlated from the distance of the groove from the nearest end. For example, the correlation may be linear, exponential, quadratic, or other mathematical form.  
      Hence, an improved radiation detector includes a scintillator having the form of a slab, the radiation receiving face and the light emitting face being generally parallel, the slab having a slab width and a slab thickness, the slab thickness being equal to the edge thickness, the slab width being substantially greater than the edge thickness, the scintillator having a peripheral region, the peripheral region being proximate to the edge, the scintillator including one or more light guides formed only within the peripheral region. A plurality of grooves are formed in the peripheral region, the grooves acting as light guides, the depth of each groove being inversely correlated (such as inversely proportional) to a distance of the groove from the edge.  
      The crystal and window thicknesses can be approximately the same. The crystal and window thicknesses can be different relative thicknesses than shown in the Figures. A typical crystal (or edge) thickness may be in the range 0.125-3 inches, such as in the range 0.25 inches-0.5 inches, such as in the range of 0.25 to 0.375 inches. In some examples, the deepest groove can less than or equal to one half the edge thickness.  
      Some crystals, especially for PET work, may be substantially thicker, such as having a thickness greater than a width. The present invention may be used with these thicker crystals as well.  
      Scintillation Materials  
      Scintillation materials may comprise halides (such as sodium iodide, cesium iodide), oxides (such as bismuth germinate (BGO), cadmium tungstate, gadolinium orthosilicate (GSO), cerium doped yttrium orthosilicate (YSO), cerium doped lutetium orthosilicate (LSO), and the like), other inorganic materials (for example, as inorganic crystals), organic crystals, other organic materials, and other materials. Scintillation materials may include an activator and a host material, in which the activator is dispersed or otherwise disposed. The activator may be a transition metal, such as a rare earth metal. Scintillation materials can be crystalline or non-crystalline. Non-crystalline scintillation materials may comprise, for example, polymers, glasses, and other materials providing light in response to incident radiation.  
      In this specification, examples are provided referring to crystals, where the term crystal is used to refer to the scintillation material, such as a scintillation crystal. However, the methods and apparatus described herein can be used with any scintillation material, such as crystal or non-crystal scintillators, and also with other materials that produce light in response to non-ionizing radiation, such as fluorescent materials, or other optical elements in which edge reflections are a problem.  
      Windows  
      A window generally comprises a material substantially transparent to scintillation light. For example, a window may be bonded to, abutting, or proximate to the light output face of a crystal. The window can provide protection of the crystal from degradation, for example by protecting from scratches, moisture, fracture, and the like.  
      The window may be formed from any material substantially transparent to the scintillation light. Examples include glass, polymers (such as acrylic polymers, for example PMMA), transparent oxides, or other materials.  
      The topography of the upper surface of the window may include triangular indentations, pyramids, truncated pyramids, cones of conic sections such as frustoconical shapes, lenses, microlens arrays, Fresnel lens patterns, or other surface features operable to guide light towards light sensitive regions of the sensor. Equivalently, a window may be slab shaped, with a separate layer in optical communication with the upper surface providing light guiding. In the field of nuclear medicine, this separate layer is often referred to as the “light pipe”.  
      The window may have a thickness in the range 0.1-0.375 inches, though this is not limiting. If the window has a surface topography within the peripheral region, for example to direct light to sensors, grooves can be formed through such features. The grooves may be normal to the average plane of the upper surface, may be parallel to a proximate edge, or otherwise provided.  
      Light guides (such as grooves) can be provided in the window material, so as to reduce edge effects due to reflections from the edge of the window. The grooves in the window can be in addition to, or instead of, grooves in the crystal.  
      Analogous geometries can be used to the crystal examples described previously, and vice versa. Grooves in the crystal can be combined with grooves in the window.  
      The window may overhang the crystal edges, which may reduce edge effects due to the crystal, and the addition of grooves in the window may further reduce edge effects due to reflections from the window edges.  
      Provision of grooves in the window can advantageously increase positional accuracy of an imaging device. The improvement may not be as great as grooves formed in a crystal. However, even if it is not possible to replace the crystal of an imaging device, the positional accuracy of the device can be improved by replacing a non-grooved window with a grooved window.  
      Housing  
      If a humidity sensitive scintillator crystal such as sodium iodide is used, a housing may be necessary to protect the crystal from the effects of atmospheric moisture. A housing may also be provided to increase robustness, hold together multiple crystals, or other purpose. The housing preferably does not substantially attenuate the incident radiation, but need not be transparent to scintillation light. Example housing materials include aluminum, other metals, plastics and the like.  
      Reflectors  
      To prevent light escaping from the radiation receiving face of the crystal, and therefore not being detected by the sensor array, reflectors may be provided. For example, a reflector may be provided by the inner reflective surface of an aluminum housing. A reflective film, such as a separate metal film or photonic band gap layer, may also be provided. For example, a reflective metal film may be evaporated onto the radiation receiving surface of a crystal. Interferometric reflectors and diffuse reflectors may also be used. Diffuse reflectors include polymer films (such as Teflon, porous Teflon, polyethylene, and the like), inorganic materials such as substantially white crystal layers, inorganic material doped polymers, and the like.  
      Reflectors may also be provided at the edges of the crystal and/or windows. The edge of the crystal will typically reflect scintillation light. However, additional reflector(s) can be provided.  
      Generally, the groove spacing will be much greater than the wavelength of scintillation light, so as not to provide a photonic band gap (PBG) effect or other interference effect. However, PBG regions or other reflective films can be provided at the edges or radiation receiving face of the crystal, if required. This can be in addition to any grooves or other light guides provided.  
      Absorbers may also be provided to reduce unwanted stray reflection from any surface, for example around the apertures of the sensors.  
      Sensors and Circuitry  
      Sensors which may be used include photomultiplier tubes CPMT), solid state sensors such as avalanche photodiodes, or other light sensitive devices. The sensors can be provided in a linear or two-dimensional array. Positional information of scintillation events can be determined from sensor signals.  
      After grooves are cut in a crystal, there may be a sensitivity reduction, due to the reduced local volume of crystal, requiring an attenuation correction. A linearity correction can also be provided to correct for any warping or spatial distortion of the image.  
      Amplifiers may be provided to improve the signal-to-noise ratio of the sensor array. The incident radiation may have phase information that can be used by lock-in amplifiers, an optical modulator through which scintillation light passes, or similar to reduce signal noise. Analysis circuitry can include a computer, algorithms to correct for edge effects and the like, noise reducing circuitry, and the like.  
      An optical coupling material, such as a liquid, gel, grease, polymer, or the like, can be provided between components so as to reduce scattering from refractive index discontinuities. For example, an optical coupling material can be provided between sensors and a window (or crystal, if no window is used) to cut down light losses.  
      Applications  
      Applications include gamma ray cameras, and other radiation detectors and imaging devices, such as nuclear medical devices. Applications include positron emission tomography (PET), single photon emission computed tomography (SPECT), combined PET/SPECT, x-ray imaging, UV imaging, cosmic ray detection, and other imaging and detection applications.  
      The improved radiation detection efficiency, compared with provision of uniform grooves across an entire light emitting face, is advantageous for all applications, particularly where sensitivity is an issue (such as combined PET/SPECT devices).  
      The methods and apparatus described herein may also be adapted to other applications, such as reducing edge effects in other materials, such as lenses, fluorescent materials, light emitting materials, light guiding materials (such as light pipes) and the like. Other examples are discussed in more detail below.  
      General Removal of Edge Effects from Optical Elements  
      Approaches described herein can be used generally to remove edge effects from optical elements. For example, a lens may be provided with one or more grooves around the periphery of one or both surfaces of the lens. Grooves (the term is used generally to refer to any light guide) may also be provided around the peripheral edges of corneal implants, spectacle lenses, and other lenses and/or lens arrays. For example, in a spectacle application, one or more grooves could be partially or completely covered by a frame element.  
      A lasing material may be provided with one or more grooves in the peripheral region of the light emitting face of the laser material. This may be used to reduce stray light emerging from the laser material.  
      Grooves may also be provided in the peripheral regions of other optical components and systems, such as along the peripheral region of waveguides, or other components of integrated optical systems. For example, they may help reduce edge effects within a waveguide.  
      Hence, an example improved radiation detector includes a scintillator, the scintillator having a light-emitting face, a radiation receiving face, and a periphery between the light-emitting face and the radiation receiving face, the periphery including an edge having an edge thickness. The scintillator emits scintillation light from the light emitting face in response to radiation incident on the radiation receiving face. The scintillator has a peripheral region proximate to the edge, the scintillator including one or more light guides formed only within the peripheral region. This is in contrast to other designs where light guides are formed uniformly across the surface of the scintillator. The peripheral region can be a region within approximately three to eight times the edge thickness from the edge, or within a distance approximately equal to a sensor spacing or sensor diameter if an array of sensors is used, or within half a sensor diameter (or sensor spacing). The area of the peripheral region can be less than the area of a non-peripheral region (such as a central region) not proximate to the edge. The light guide provides an internal reflection or redirection of scintillation light within the scintillator, before the scintillation emerges from the light emitting face.  
      If the radiation detector comprises a scintillator, a window, and an array of sensors, each sensor in optical communication with the light emitting face of the scintillator through the window, the light sensors having a light sensor diameter, the scintillator or the window can be provided with one or more grooves formed in one or both faces thereof, the one or more grooves being formed within a distance less than a light sensor diameter from an edge thereof.  
      The invention is not restricted to the illustrative examples described above. Examples are not intended as limitations on the scope of the invention. Methods, apparatus, compositions, and the like described herein are exemplary and not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art. The scope of the invention is defined by the scope of the claims. Subheadings in the specification are provided for convenience only. Examples, alternatives, and the like should be sought within the entire specification.  
      Patents or publications mentioned in this specification are incorporated herein by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. In particular, U.S. Prov. Pat. App. Ser. No. 60/523,765 filed Nov. 20, 2003, is incorporated herein in its entirety. Additional information concerning imaging systems can be found in Applicant&#39;s issued patents U.S. Pat. Nos. 6,525,320; 6.525,321; and 6,504,157; and Pub. App. No. 2003/0136912, the entire content of all of which are incorporated herein by reference.