Patent Publication Number: US-2023135695-A1

Title: Targeted Collimation of Detectors Using Rear Collimators

Description:
CROSS-REFERENCE 
     The present application relies on U.S. Patent Provisional Application No. 63/263,527, titled “Targeted Collimation of Detectors Using Associated Collimators”, and filed on Nov. 4, 2021, for priority, which is herein incorporated by reference in its entirety. 
    
    
     FIELD 
     The present specification relates generally to X-ray scanning systems and, more specifically, to the positioning and orientation of individual collimators or each collimator of a plurality of collimators with reference to an associated detector element of a plurality of detector elements. 
     BACKGROUND 
     To produce a radiographic image, an X-ray radiation source for producing an X-ray beam is required in addition to an array of detectors that are sensitive to the X-ray beam produced. In transmission scanning systems, an object required to be imaged passes between the radiation source and the array of detectors and attenuates the X-ray beam, producing a contrast in the signals observed on the detector array. At all points in this simple model, scattered X-rays occur, which are undesirable for this form of transmission imaging. The scattered X-rays result in background noise which can distort the true X-ray signal on the detector array and reduce the contrast required for imaging. Scatter reduction is achieved by using collimation, which can be implemented in various ways to occlude parts of the X-ray beam at the radiation source or shield parts of the detection system to prevent scattered photons from reaching the detectors. 
     As shown in  FIG.  1 A , for mechanical ease, large scale scanning systems often have a plurality of detectors that are arranged in an upside-down L-shape, where there is a vertical arrangement of detectors  105  and a horizontal arrangement of detectors  110 . An X-ray source  102  is positioned and collimated such that it can produce a fan shaped beam  112  which is incident on the vertical detectors  105  and horizontal detectors  110 . Within the vertical and horizontal detector arrangements, each individual detector or a plurality of individual detectors  115 , as shown in  FIG.  1 B , is positioned such that it directly points towards the X-ray source  102 . The detectors  115  may form a staggered or saw-tooth arrangement within the confines of a detector housing such as, for example, a vertical detector housing  120 . 
     However, the aforementioned vertical detector arrangement  105  and horizontal detector arrangement  110  is fraught with scatter occurring between each individual detector unit  115  and the detector housing  120 , as illustrated in  FIG.  1 C , where the possible scatter paths taken by scattered photons  125  of an incident X-ray beam  112  are shown. Here, the underside of each detector unit  115  is exposed to the scattered photons  125  which causes an increase in signal in the unit  115  that can distort the radiographic image. In areas of high attenuation and due to the inverted L-shaped detector arrangement formed by vertical detector array  105  and horizontal detector array  110 , scatter causes visible artifacts in the radiographic image that affect the X-ray inspection system&#39;s ability to penetrate dense areas of target object under inspection, such as cargo. A large level of scatter is observed in the bottom detector units  115  of detector arrangements  105 / 110 . The level of scatter can be more than twice as much as observed in the other individual detector units  115 , resulting in visible artifacts in the scan images. Scatter emanating from below the sensitive detector units appears to be further scattered by detector housing  120  into first individual detector units  115  of each detector arrangement  105 / 110  that subsequently acts as a shield for the remainder of the detectors  115 . Although algorithms/filters can help clean up these images to some degree, reducing the amount of scattered X rays is the most effective way to increase the image quality. 
     Therefore, there is a need for a targeted collimation of each detector in an array of detectors to prevent scattered X-ray photons from entering the detectors. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods, which are meant to be exemplary and illustrative, and not limiting in scope. The present application discloses numerous embodiments. 
     The present specification discloses a detector array adapted to detect radiation emitted from a radiation source, comprising: a plurality of detector elements enclosed in a housing, wherein the housing has at least a first wall and a second opposing wall positioned a predefined distance away from the first wall, wherein the first wall faces the radiation source, wherein a front side of each of the plurality of detector elements substantially faces the first wall, and wherein a rear side of each of the plurality of detector elements substantially faces the second opposing wall; and a plurality of collimators, wherein each of the plurality of collimators is positioned behind one of the plurality of detector elements such that said one of the plurality of detector elements is positioned between said each of the plurality of collimators and the first wall and such that said each of the plurality of collimators extends from the rear side of said one of the plurality of detector elements to a position proximate the second side of the housing. 
     Optionally, each of the plurality of collimators extends from the rear side of said one of the plurality of detector elements to physically touch the second side of the housing. 
     Optionally, each of the plurality of detector elements has a length, wherein each of the plurality of collimators is positioned at a point that is at half of the length of the corresponding one of the plurality of detector elements. 
     Optionally, each collimator of the plurality of detector elements is oriented perpendicular to the rear side of the corresponding one of the plurality of detector elements. 
     Optionally, each of the plurality of collimators has a width, wherein the radiation emitted from the radiation source is an X-ray beam having a thickness, wherein said width is in a range of 10% to 100% of said thickness of the X-ray beam. 
     Optionally, each of the plurality of collimators has a width in a range of 2 mm to 5 mm. 
     The present specification also discloses a detection system for scanning an object, comprising: an X-ray source configured to generate an X-ray beam that is projected onto the object; a detector array adapted to receive the X-ray beam that is transmitted through the object, wherein the detector array comprises a plurality of detector elements enclosed in a housing, wherein the housing has at least a first side and a second opposing side positioned a first distance away from the first side, wherein each of the plurality of detector elements is positioned between the first side and the second opposing side such that a front side of each detector is positioned to receive the X-ray beam at a perpendicular angle relative to a center of the front side; and a plurality of collimators, wherein each of the plurality of collimators is positioned behind one of the plurality of detector elements and wherein each of the plurality of collimators is oriented perpendicular to a rear side of the corresponding one of the plurality of detector elements. 
     Optionally, the plurality of detector elements is arranged in a saw-tooth arrangement. 
     Optionally, each of the plurality of detector elements is tilted substantially towards the X-ray source for limiting parallax detection of the X-ray beam. 
     Optionally, each of the plurality of detector elements is tilted substantially towards the X-ray source such that a central axis of each of the plurality of detector elements is directly aligned with at the X-ray source. 
     Optionally, each of the plurality of detector elements is separated from the second opposing side by a second distance. Optionally, each of the plurality of collimators has a length that is equal to 50% to 99% of said second distance. Optionally, each of the plurality of collimators has a length that is substantially equal to the second distance. Optionally, each of the plurality of collimators has a length that is equal to 80% to 100% of said second distance. 
     Optionally, each of the plurality of detector elements has a length, and each of the plurality of collimators is positioned at a point that is in a range of 40% to 60% of the length of the corresponding one of the plurality of detector elements. 
     Optionally, each of the plurality of detector elements has a length, and each of the plurality of collimators is positioned at a point that is in a range of 48% to 52% of the length of the corresponding one of the plurality of detector elements. 
     Optionally, each of the plurality of collimators is positioned at in a range of 10 mm to 0 mm from a center of the rear side of the corresponding one of the plurality of detector elements. 
     Optionally, the X-ray beam is a fan beam having a first width, wherein each of the plurality of collimators has a second width adapted to cover 50% to 100% of the first width. 
     Optionally, each of the plurality of collimators has a thickness of 3 mm. 
     Optionally, each of the plurality of collimators has a thickness of in a range of 1.5 mm to 4.5 mm. 
     The present specification also discloses a detector for sensing radiation arriving from an object being scanned by a radiation beam, wherein the radiation beam is generated by an X-ray radiation source, the detector comprising: a plurality of detector elements enclosed in a housing, wherein a first side of the housing faces the radiation source and a second side of the housing is opposite to the first side, wherein each of the plurality of detector elements has a third side and a fourth side opposite to the third side, and wherein a first axis normal to the third side points substantially towards the radiation source; and a plurality of collimators, wherein each of the plurality of collimators has an associated detector element of the plurality of detector elements, wherein each of the plurality of collimators has a length that covers all or a substantial part of a distance between the fourth side of the associated detector element and the second side of the housing. 
     Optionally, each of the plurality of collimators has a surface facing the fourth side of the associated detector element, wherein a second normal to the surface also points substantially towards the radiation source. Optionally, the first and second normal are substantially parallel. Optionally, the first and second normal are substantially coaxial. 
     Optionally, the associated detector element has a length, and each of the plurality of collimators is positioned at a point which is at half of the length of the associated detector element. 
     Optionally, each of the plurality of collimators has a thickness of about 3 mm. 
     The present specification also discloses a detector arrangement comprising: a plurality of detector elements enclosed in a housing, wherein the housing has a first side, a second side, a third side and a fourth side, wherein the first side faces a radiation source and the second side opposes the first side, and wherein each of the plurality of detector elements points substantially towards the radiation source; and a plurality of collimators, wherein each of the plurality of collimators is positioned behind an associated detector element of the plurality of detector elements such that it extends from a rear side of the associated detector element to either the second side of the housing or a point proximate to the second side of the housing. 
     Optionally, the associated detector element has a length, wherein each of the plurality of collimators is positioned at a point which is positioned at half of the length of the associated detector element. 
     Optionally, each of the plurality of detector elements is defined by an enclosure having a fifth side facing the radiation source and a sixth side opposite to the fifth side, and wherein each of the plurality of collimators is oriented perpendicular to the sixth side of the associated detector element. 
     Optionally, each of the plurality of collimators has a width in a range of 10% to 100% of thickness of an X-ray beam for detection by the detector arrangement. 
     Optionally, each of the plurality of collimators has a thickness in a range of 2 mm to 5 mm. 
     The present specification also discloses a system for scanning an object, comprising: a radiation source for generating an X-ray beam that is projected onto the object; a detector arrangement for receiving the X-ray beam that is transmitted through the object, wherein the detector arrangement includes a plurality of detector elements enclosed in a housing, wherein a first side of the housing faces the radiation source and a second side of the housing is opposite to the first side, wherein each of the plurality of detector elements has a third side and a fourth side opposite to the third side, and wherein a first axis normal to the third side points substantially towards the radiation source; and a plurality of collimators, wherein each of the plurality of collimators is positioned behind an associated detector element of the plurality of detector elements, wherein each of the plurality of collimators has a surface facing the fourth side of the associated detector element, and wherein a second axis normal to the surface also points substantially towards the radiation source. 
     Optionally, the plurality of detector elements is arranged in a saw-tooth arrangement. 
     Optionally, the first axis and the second axis are substantially parallel. 
     Optionally, the first axis and the second axis are substantially coaxial. 
     Optionally, each of the plurality of collimators has a length that covers all or a substantial part of a distance between the fourth side of the associated detector element and the second side of the housing. 
     Optionally, the associated detector element has a length, and wherein each of the plurality of collimators is positioned at a point which is at half of the length of the associated detector element. 
     Optionally, each of the plurality of collimators is oriented perpendicular to the fourth side of the associated detector element. 
     Optionally, the X-ray beam is a fan beam, and wherein each of the plurality of collimators has a width adapted to cover 50% to 100% of a width of the fan beam. 
     Optionally, each of the plurality of collimators has a thickness of about 3 mm. 
     The present specification discloses a detector for sensing radiation arriving from an object being scanned by a radiation beam, wherein the radiation beam is generated by an X-ray radiation source, the detector comprising: a plurality of detector elements enclosed in a housing, wherein a first side of the housing faces the radiation source and a second side of the housing is opposite to the first side, wherein each of the plurality of detector elements has a third side and a fourth side opposite to the third side, and wherein a first axis normal to the third side points substantially towards the radiation source; and a plurality of collimators, wherein each of the plurality of collimators has an associated detector element of the plurality of detector elements, wherein each of the plurality of collimators has a length that covers all or a substantial part of a distance between the fourth side of the associated detector element and the second side of the housing. 
     Optionally, each of the plurality of collimators has a surface facing the fourth side of the associated detector element, and wherein a second normal to the surface also points substantially towards the radiation source. 
     Optionally, the first and second normal are substantially parallel. 
     Optionally, the first and second normal are substantially coaxial. 
     Optionally, the associated detector element has a length, and wherein each of the plurality of collimators is positioned at a point which is at half of the length of the associated detector element. 
     Optionally, each of the plurality of collimators has a thickness of about 3 mm. 
     The aforementioned and other embodiments of the present specification shall be described in greater depth in the drawings and detailed description provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate various embodiments of systems, methods, and embodiments of various other aspects of the disclosure. Any person with ordinary skills in the art will appreciate that the illustrated element boundaries (e.g. boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. It may be that in some examples one element may be designed as multiple elements or that multiple elements may be designed as one element. In some examples, an element shown as an internal component of one element may be implemented as an external component in another and vice versa. Furthermore, elements may not be drawn to scale. Non-limiting and non-exhaustive descriptions are described with reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating principles. 
         FIG.  1 A  illustrates an X-ray scanning system with a vertical detector arrangement and a horizontal detector arrangement, forming an upside-down or inverted “L” shaped detector array; 
         FIG.  1 B  illustrates a saw-tooth arrangement of a plurality of detectors housed in a vertical detector arrangement; 
         FIG.  1 C  illustrates a plurality of scatter paths due to an X-ray beam incident on an arrangement of individual detectors within a detector array, as is seen in the conventional detectors described in  FIG.  1 A  and  FIG.  1 B ; 
         FIG.  2 A  shows a first side view of a portion of a detector arrangement, in accordance with some embodiments of the present specification; 
         FIG.  2 B  shows a second side view of a portion of the detector arrangement of  FIG.  2 A , in accordance with some embodiments of the present specification; 
         FIG.  2 C  is a first top-down view of a portion of the detector arrangement of  FIG.  2 A , in accordance with some embodiments of the present specification; 
         FIG.  2 D  is a second top-down view of a portion of the detector arrangement of  FIG.  2 A , in accordance with some embodiments of the present specification; 
         FIG.  2 E  is an X-ray beam-view of a portion of the detector arrangement of  FIG.  2 A , in accordance with some embodiments of the present specification; and 
         FIG.  2 F  is a side elevation view of a detector arrangement as shown in  FIG.  2 A  with collimators in accordance with some embodiments of the present specification. 
     
    
    
     DETAILED DESCRIPTION 
     The present specification is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention. 
     In the description and claims of the application, each of the words “comprise”, “include”, “have”, “contain”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. Thus, they are intended to be equivalent in meaning and be open-ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It should be noted herein that any feature or component described in association with a specific embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise. 
     It must also be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context dictates otherwise. Although any systems and methods similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present disclosure, the preferred, systems and methods are now described. 
     In various embodiments, the system includes at least one processor capable of processing programmatic instructions, has a memory capable of storing programmatic instructions, and employs software comprised of a plurality of programmatic instructions for performing the processes described herein. 
     In various embodiments, a computing device may be employed to receive and process data signals and image data and may include an input/output controller, at least one communication interface and a system memory. The system memory includes at least one random access memory (RAM) and at least one read-only memory (ROM). These elements are in communication with a central processing unit (CPU) to enable operation of the computing device. In various embodiments, the computing device may be a conventional standalone computer or alternatively, the functions of the computing device may be distributed across a network of multiple computer systems and architectures. In some embodiments, execution of a plurality of sequences of programmatic instructions or code, which are stored in one or more non-volatile memories, enable or cause the CPU of the computing device to perform or enable various functions, processes and algorithms, such as, for example, performing image reconstruction for display on a screen. In alternate embodiments, hard-wired circuitry may be used in place of, or in combination with, software instructions for implementation of the processes of systems and methods described in this application. Thus, the systems and methods described are not limited to any specific combination of hardware and software. 
       FIGS.  2 A and  2 B  represent a first side view and a second side view, respectively, of a portion of a detector arrangement  200 , in accordance with some embodiments of the present specification.  FIGS.  2 C and  2 D  represent a first top-down view and a second top-down view, respectively, of a portion of a detector arrangement  200 , in accordance with some embodiments of the present specification.  FIG.  2 E  represents an X-ray beam-view of a portion of a detector arrangement  200 , in accordance with some embodiments of the present specification.  FIG.  2 F  is a side elevation view of detector arrangement  200  with collimators, in accordance with some embodiments of the present specification. Reference will now be made to  FIGS.  2 A- 2 F , which will be described in detail simultaneously. 
     As shown in  FIG.  2 A , in some embodiments, an X-ray radiation source  202  is configured to project an X-ray beam  215 , such as a fan beam, onto an object  203  being scanned so that the X-ray beam  215  is transmitted through the object  203  and impinges upon the detector arrangement  200 . Alternatively, the detector arrangement  200  may be positioned on the same side of the object  203  as the radiation source  202 , so that the detector arrangement  200  receives X-ray beam, which has been backscattered by the object  203 . 
     In some embodiments, the detector arrangement  200  includes at least one array of a plurality of detector elements or units  205  positioned within a detector housing or enclosure having a first wall/side  210 , a second wall/side  212 , a third wall/side (not shown in the FIGS.), and a fourth wall/side (not shown in the FIGS.). It should be appreciated that while  FIG.  2 A  illustrates a vertical configuration of the detector arrangement  200 , in other embodiments the detector arrangement  200  may be configured horizontally with reference to the radiation source  202 . In some embodiments, a plurality of detector arrangements  200  may be used in either one of or both vertical and horizontal configurations.  FIGS.  2 A and  2 C  show both a first or front side  210  of the housing that faces the X-ray radiation source  202  and a second or rear side  212  opposite to the first or front side  210 . 
     In some embodiments, the plurality of detector elements or units  205  are solid-state detectors formed from an array of scintillation crystals with associated photosensitive devices. A scintillation crystal absorbs the X-rays  215  transmitted through the object  203  and converts the absorbed radiation into photons of visible light (optical signal). This light is transmitted through the bulk of the scintillation material to a photosensitive device. The photosensitive device converts the input optical signal to an electrical current for subsequent integration by an electronic circuit. The integrated signal can then by converted to digital data for transfer to at least one processor which is able to render the data (onto a display or monitor) to a human readable form such as a scan image. In some embodiments, scintillation materials have high density and high atomic number and may be selected from a set of materials such as, but not limited to, CsI, NaI, BGO, CdWO4, LSO, GSO, LYSO and LaBr3. In some embodiments, the photosensitive device is a semiconductor device such as a photodiode, an avalanche photodiode or a silicon photomultiplier or it may be a vacuum tube device such as a photomultiplier tube or a more hybrid device such as a micro-channel plate with photodiode readout. 
     Referring now to  FIGS.  2 A through  2 F , simultaneously, each detector element or unit  205  has a length l d , a width w d  and a thickness t d . The size of each detector unit  205  may vary in different embodiments based on the image quality of the detection system, the source energy, and the detector material. Each detector element or unit  205  has a first side or front side  205   a  facing the X-ray radiation source  202  (or, in other words, the first side or front side  205   a  receives the X-ray beam  215 ) and a second side or rear side  205   b  opposite to the first side or front side  205   a.  In some embodiments, each detector element or unit  205  is tilted substantially towards the source  202  of X-ray radiation in order to limit parallax detection of the radiation beam  215 . In embodiments, detector units  205  are tilted so that a central axis of each unit  205  is directly pointing at X-ray source  202 . Therefore, a surface of a front side  205   a  of each unit  205  is perpendicular to radiation beam  215  and is positioned to receive radiation beam  215  at the center of the surface of front side  205   a.  As a result, each detector unit  205  has a different tilt angle, where the angle is dependent on the distance of each unit  205  from the X-ray source  202 . The tilt angles of detector units  205  and the position of each unit  205  relative to other units  205 : a) may be offset as long as no detector unit  205  overlaps any other detector unit  205  and, hence, occludes the x-ray beam, and b) is based on detector housing constraints. Thus, a first normal  220  to the first, front side  205   a  of each detector element or unit  205  points towards or is tilted towards the X-ray radiation source  202 . In some embodiments, this is achieved by using a “saw-tooth” arrangement of the plurality of detector elements or units  205 , which is illustrated schematically in  FIG.  2 A . 
     In accordance with aspects of the present specification, each of the plurality of detector elements or units  205  is associated with a collimator  225 . Each collimator  225  has a length l c  (along the direction of the X-ray beam  215 ), a width w c , and a thickness t c . For each detector element or unit  205 , the associated collimator  225  is positioned relative to the corresponding detector element or unit  205  in order to achieve the following: 1) a first condition of minimizing the scatter of the X-ray beam  215  (occurring due to the X-ray beam  215  impinging upon the second or rear side  212  of the detector housing), and b) a second condition of minimizing the amount of scatter produced by the collimator itself. 
     It should be noted herein that the detector elements  205  are positioned so that they point directly at the X-ray source  202 . Stated differently, the centerline of the detectors  205  is angled so that the middle of each of the detector elements point directly at the X-ray source  202 . Thus, as described below, each associated collimator  225  is positioned along the centerline at an angle of 90 degrees to the surface of rear side  205   b  of detector unit  205 , so that it is “pointing at” or “inline” with the X-ray fan beam  215 . In doing so, the X-rays are incident on as little of the rear collimator  225  as possible, minimizing any scatter from collimator  225  back into the detector units  205 . If the collimator angle is not 90 degrees, then the X-ray will have a greater surface area to scatter from. Similarly, if collimator  225  is positioned away from the centerline (or midpoint), then it will not be in-line with the X-ray paths (as shown in  FIG.  1 C ) and will have a greater surface area to cause scatter. If angles of the detector elements  205  are changed, for example, if detectors elements  205  are aligned to the X-ray source  202  at the top edge or bottom edge of front side  205   a  of detector units  205 , then each corresponding collimator  225  will also have to be re-aligned or re-positioned such that it satisfies the minimal surface area for scatter. 
     In order to achieve or enable the first and second conditions stated above, each collimator  225  may be characterized by at least one of the following specifications. As a first characteristic, in some embodiments, each collimator  225  may be positioned directly behind the corresponding detector element or unit  205 . Thus, each collimator  225  is positioned behind the second or rear side  205   b  of the corresponding detector element or unit  205 . In some embodiments, each collimator  225  is positioned in a range of 10 mm to 0 mm, and any numerical increment therein, from a center of the rear side of the corresponding detector element or unit  205 . 
     As a second characteristic, each collimator  225  may be positioned half-way (or at a mid-point) down the length la of the corresponding detector element or unit  205 . That is, the collimator  225  is positioned at a point that is at half of the length of the corresponding detector element or unit  205 . Stated differently, the collimator  225  is positioned at the center of the second or rear side  205   b  of the corresponding detector element or unit  205 , such that each collimator  225  is directly in line with X-ray beam  215 . In some embodiments, each collimator  225  is positioned at a point that is in a range of 40% to 60%, and any numerical increment therein, of the length of the corresponding detector element or unit  205 . In some embodiments, each collimator  225  is positioned at a point that is in a range of 48% to 52%, and any numerical increment therein, of the length of the corresponding detector element or unit  205 . 
     As a third characteristic, each collimator  225  may have a second normal  230  that is substantially parallel to the first normal  220 , wherein the second normal  230  is perpendicular to the surface of rear side  205   b  of corresponding detector unit  205 . In some embodiments, the first and second normal  220 ,  230  are substantially co-axial. Thus, the collimator  225  is oriented perpendicular (90°) to the second or rear side  205   b  of a corresponding detector element or unit  205  as a result of which the second normal  230  is also tilted (or points back) substantially towards the X-ray radiation source  202 . 
     As a fourth characteristic, the collimator  225  may have a length l c  (see  FIG.  2 F ) that extends a substantial portion of a distance ‘D’ from the second or rear side  205   b  of the corresponding detector element or unit  205  towards the second or rear side  212  of the detector housing. In some embodiments, the collimator  225  extends from the second or rear side  205   b  of the corresponding detector element or unit  205  to a point proximate the second or rear side  212  of the detector housing. In some embodiments, the length l c  is equal to the distance ‘D’. In some embodiments, the length l c  is at least ⅔ rd  of the distance ‘D’. In some embodiments, the length l c  is equal to 50% to 99%, and any numerical increment therein, of the distance ‘D’. In some embodiments, the length l c  is equal to 80% to 100%, and any numerical increment therein, of the distance ‘D’. In embodiments, a maximum length of D (D max ) is as long as physically possible within the detector housing, and a minimum length of D (D min ) is the length of offset of detector unit  205  above the corresponding collimator  225 . Thus, the collimator  225  covers all or a substantial part of the length from the rear of the corresponding detector element or unit  205  to the rear of the detector housing  212 . It should be appreciated that the length l c  of each collimator  225  will be different and varies owing to a different distance ‘D’ of each detector element or unit  205  from the second or rear side  212  of the detector housing. In embodiments, the collimator  225  should be of a sufficient length to cover the full air gap between the rear of the detector element or unit  205  and the rear of the detector housing  212  (or any “rear object” that may be positioned behind the detector, such as a beam stop). This design allows for any scatter that is generated to be fully covered by the collimators  225 . 
     As a fifth characteristic, each collimator  225  may have a width w c  that is sufficient to cover a substantial width of the X-ray fan beam  215 . In embodiments, w c  covers an amount ranging within full width of beam  215  to at least 50% of beam  215 , thereby ensuring that collimators  225  cover the full width half maximum region and, therefore, most of the beam intensity. The X-ray fan beam  215  is created via collimation at the X-ray source  202 . From a top-down view, the fan beam  215  appears thin at the source of collimation (on the order of 3 mm to 5 mm). The beam  215 , however, is divergent and becomes wider as the distance from the source  202  increases. The distance of the detector elements  205  from the source  202  is system dependent and thus, the X-ray beam  215  width varies with the system. It should be appreciated that the width w d  of each detector element or unit  205  is substantially small compared to the width of the X-ray fan beam  215 . In some embodiments, the width w d  is about 100 mm. In some embodiments, the width w d  ranges from 85 mm to 110 mm. Thus, in embodiments, the width of the rear collimator  225  is designed such that it covers the full-width-half maximum of the beam  215  at the point where the beam  215  reaches the detector elements  205 . 
     As a sixth characteristic, each collimator  225  may have a thickness t c  that enables the collimators  225  to be sufficiently thin such that when viewed from the point of view of the incoming X-rays  215  there is very little collimator surface for scatter to occur while at the same time having the thickness t c  that enables the collimator  225  to be thick enough to stop photons from being scattered from the second or rear side  212  of the detector housing. In some embodiments, the thickness t c  is about 3 mm. In some embodiments, the thickness t c  ranges from 2 mm to 5 mm depending on the material. In some embodiments, the thickness t c  is in a range of 1.5 mm to 4.5 mm and any numerical increment therein. 
     In various embodiments, the material used to fabricate collimators  225  is dependent on the energy of the X-ray source  202 . High-energy sources will require the use of materials with high atomic numbers such as, for example, Tungsten or Lead. Lower energy sources may use steel, copper, or aluminum, for example. 
     The above examples are merely illustrative of the many applications of the system of present specification. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.