Abstract:
A system and method is provided for determining depth of interaction (DOI) information. The system and method includes a detector configured to generate DOI information as a result of radiation emitted from a radiation source. The system and method further includes a plurality of scintillator pixels forming a block, wherein the plurality of scintillator pixels have a first portion and a second portion. A first medium distributed in an alternating pattern of coupling and separation between each of the scintillator pixels in a first portion or second portion of the block is also provided. A plurality of sensors for detecting scintillation events across the plurality of scintillators based on the alternating pattern of coupling and separation between each of the scintillator pixels, wherein DOI information is provided by a position profile of the block, and an image processor for generating a 3 dimensional image from the DOI information are also included.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM FOR PRIORITY 
     This application is a non-provisional of, and claims priority under 35 U.S.C. §119(e) from, provisional application Ser. No. 61/300,146, filed Feb. 1, 2010, entitled “PET Detector With Depth-Of-Interaction Resolution”, the entire disclosure of which is incorporated herein by reference into the present application. 
    
    
     TECHNICAL FIELD 
     In general, the invention relates to nuclear medical imaging. More particularly, the invention relates to gamma radiation detectors used, for example, in positron emission tomography (PET) scanning. 
     BACKGROUND 
     In PET imaging, positrons are emitted from a radiopharmaceutically doped organ or tissue mass of interest. The positrons combine with electrons and are annihilated and, in general, two gamma photons which travel in diametrically opposite directions are generated upon that annihilation. Opposing crystal detectors, which each scintillate upon being struck by a gamma photon, are used to detect the emitted gamma photons. By identifying the location of each of two essentially simultaneous gamma interactions as evidenced by two essentially simultaneous scintillation events, a line in space along which the two gamma photons have traveled (a “line of response,” or “LOR”) can be determined. The LORs associated with many million gamma interactions with the detectors are calculated and “composited” to generate an image of the organ or tissue mass of interest, as is known in the art. 
     In some of the earlier PET systems, the gamma detectors could be used only to determine the location of gamma interaction with the detector in two dimensions, which gave rise to parallax errors. More particularly, a conventional two-dimensional measurement of the spatial location of a detected gamma ray absorption event in the scintillating crystal is limited to a two-dimensional point in the X,Y plane of the crystal. However, because the number of scintillation photons reaching each detector element (e.g., either a PMT or a photodiode) is dependent on the solid angle subtended by the area of that detector element to the point of the gamma ray absorption within the crystal, the amount of scintillation photons received by each detector is also a function of the depth of interaction (DOI) of the incident gamma ray within the crystal, i.e., along the Z axis of the crystal. 
     The DOI is an important parameter when applied to imaging detector geometries where the directions from which incident gamma rays impinge upon the crystal are not all substantially normal to the crystal surface. If incident gamma rays intersect the crystal from directions not normal to the crystal, the unknown depth of interaction of those gamma rays within the crystal will result in an additional uncertainty in the measured position of the interaction because of the parallax effect, if only a two dimensional (i.e., X,Y) spatial location is calculated for such an absorption event. A detailed explanation of the importance of and the problems associated with the DOI is provided in “Maximum Likelihood Positioning in the Scintillation Camera Using Depth of Interaction,” D. Gagnon et al., IEEE Transactions on Medical Imaging, Vol. 12, No. 1, March 1993, pp. 101-107. 
     Thus, parallax errors could be reduced by using depth of interaction (DOI) information to increase the spatial resolution of the system, i.e., to provide the location of gamma interaction in three dimensions in space. In this regard, some research brain PET scanners are able to provide DOI information using so-called “phoswich” (for “phosphorescence sandwich”) detectors, constructed as axially stacked scintillators, using a pulse shape discrimination method to minimize parallax error as disclosed in U.S. Pat. No. 6,288,399 to Andreaco et al. 
     The articles J. Vaquero et al., “A depth-encoding PET detector module with improved spatial sampling”. 1998 Conf. Rec. IEEE NSS and MC Conf. M6-29 and F. Cayouette et al., “Monte Carlo simulation using Detect 2000 of a multilayered scintillation block and fit to experimental data”. IEEE Trans. Nucl. Sci. 50, 339-343, 2003 describe another solution for measuring DOI. Their approach describes the use of two layers of crystal arrays that are offset in the x and y direction. This allows the identification of depth by assigning the events to one of the layers with the superimposed, overall crystal map. This concept is often used in combination with multi-anode PMTs, because a large number of channels is required to achieve the necessary position resolution 
     US Patent Application No. 2007/0090298 by Shao and the article W. Moses and S. Derenzo, “Design studies for a PET detector module using a PIN photodiode to measure depth of interaction”. IEEE Trans. Nucl. Sci. 41, 1441-1445, 1994 describe still another approach for measuring DOI. In their approach, detectors with dual-ended readout of the scintillator array are used. This is particularly advantageous in combination with thin photosensors such as PIN photodiodes, avalanche photodiodes (APDs) or silicon photomultipliers (SiPMs). Those thin photosensors can be coupled to the scintillator even on the side facing the incident radiation, without leading to high absorption losses and without using much space. DOI information is then obtained by analysing the ratio of signals read out at either end of the scintillator. 
     The use of multiple, stacked detector modules, each consisting of a scintillator block read out by a number of photosensors is described in G. Llosa, “Experimental results and applications of FBK-irst SiPM pixels and matrices by the DASIPM collaboration”, NDIP08 Conference Talk. This is approach is used in combination with thin photosensors such as SiPMs, that do not occupy much space in between the scintillator layers. 
     Lewellen et al. have proposed an approach in “DMice—a depth-of-interaction detector design for PET scanners”, 2004 Conf. Rec. IEEE NSS and MIC, and in U.S. Patent Application No. 2009/0224164, to obtain DOI information by measuring the degree of light sharing for pairs of crystals with a systematically varied optical coupling along the common interface. Detector blocks are then assembled from a multitude of such pairs. Their read-out relies on a one-on-one coupling scheme, where each of the crystals in a pair is co-registered with the anode pad of a multi-anode PMT or a SiPM read-out channel. The result is an encoding of the DOI information in the one-dimensional light sharing profile within each crystal pair. 
     However, each of these approaches have deficiencies such as high cost, difficulty of implementation, need for additional equipment, and the like. 
     There remains a need in the art, however, for further improvement in the light collection efficiency and spatial resolution of such a DOI scintillation detector. 
     SUMMARY 
     A system and method is provided for determining depth of interaction (DOI) information. The system and method includes a detector configured to generate DOI information as a result of radiation emitted from a radiation source. The system and method further includes a plurality of scintillator pixels forming a block, wherein the plurality of scintillator pixels have a first portion and a second portion. A first medium distributed in an alternating pattern of coupling and separation between each of the scintillator pixels in a first portion or second portion of the block is also provided. A plurality of sensors for detecting scintillation events across the plurality of scintillators based on the alternating pattern of coupling and separation between each of the scintillator pixels, wherein DOI information is provided by a position profile of the block, and an image processor for generating a 3 dimensional image from the DOI information are also included. 
     Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a diagram of a detection system utilizing a positron emission tomography (PET) scintillation block for generating depth-of-interaction (DOI) information, according to various embodiments; 
         FIG. 2A  is a diagram depicting a DOI PET block detector, based on an exemplary array of 4×4 scintillator crystals and 2×2 optical sensors in accordance with an embodiment of the present invention; 
         FIG. 2B  is a diagram depicting a cross sectional view along the xz plane of the z-position dependent optical coupling structures between crystals in accordance with an embodiment of the present invention; 
         FIG. 3A  is a diagram of a DOI block having couplers, separators and a light guide in accordance with an embodiment of the present invention. 
         FIG. 3B  is a diagram of a DOI block having separators and a light guide in accordance with an embodiment of the present invention; 
         FIG. 3C  is a diagram of a DOI block having couplers and a light guide in accordance with an embodiment of the present invention; 
         FIG. 3D  is a diagram of a DOI block having couplers, separators and a light guide all in an inverted position in accordance with an embodiment of the present invention; 
         FIG. 3E  is a diagram of a DOI block having 6×6 crystals coupled to 2×2 sensors, with alternating separator structures in accordance with an embodiment of the present invention; 
         FIG. 3F  is a diagram of a DOI block having 8×8 crystals coupled to 4×4 sensors, with alternating separators in accordance with an embodiment of the present invention; 
         FIG. 3G  is a diagram of a DOI block having 8×8 crystals coupled to 3×3 sensors, with separators located at the bottom of the block in accordance with an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating a DOI resolving PET block detector, manufactured by laser structuring of a monolithic scintillator crystal in accordance with an embodiment of the present invention; The thickness of the structures between the pixels indicates the degree of optical separation (thicker line means higher scatter probability). 
         FIG. 5A  is a graph depicting simulated position results for the block detector described in  FIGS. 2A and 2B  in accordance with an embodiment of the present invention; 
         FIG. 5B  is a graph depicting mean values for position results in each of three depth bins depicted in  FIG. 5C  in accordance with an embodiment of the present invention; 
         FIG. 5C  is a diagram depicting three depth bins used in the graphs of  FIGS. 5A and 5B  in accordance with an embodiment of the present invention; and 
         FIG. 6  is a diagram of a computing hardware that can be used to implement various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring exemplary embodiments. 
       FIG. 1  is a diagram of a detection system utilizing a PET scintillation block for generating depth-of-interaction (DOI) information, according to various embodiments. As shown, a detection system  100  includes a detector  101  to observe events stemming from a radiation source  103  emitting radiation (e.g., gamma rays) from a subject (patient)  105 . The detector  101  outputs data to a data processor  107 , which includes a data acquisition module  109  and an image processor  111 . The data acquisition module  109  uses spatial coordinate signals to produce input to the image processor  111 . The image processor  111  can then produce, for example, an image of tissues in the patient  105 . The image can then be displayed on a display unit  113 . 
     The system  100  provides a high spatial resolution PET detector with DOI capability. The DOI information is obtained from the details of the position profile. This is preferably achieved by systematically varying the optical coupling between scintillator pixels as a function of vertical position within the pixels e.g., using an alternating pattern of coupling and separating the pixel interfaces at the bottom and/or top of crystal block. The three-dimensional light sharing over the whole crystal block is influenced by the optical coupling configuration in the vicinity of the interaction site, and the DOI information becomes encoded in the position profile using various embodiments. 
       FIG. 2A  is a diagram depicting a DOI PET block detector  101 , based on an exemplary array of 4×4 scintillator pixels  204  comprising crystal pixel  204   1  to  204   16 , 2×2 optical sensors  206  coupled to the scintillator pixel array  204  by sensor mount  208  via coupling film  214  in accordance with an embodiment of the present invention. The plurality of scintillator pixels is selected from the group consisting of a LSO crystal, a GSO crystal, a LuAP crystal, a LYSO crystal, a BGO crystal, LFS crystal, a NAI crystal, CSI crystal, and a LGSO crystal. 
     Optical sensors  206  are selected from the group consisting of a solid state detector, a photomultiplier tube, a SiPM, a pin diode, a CCD, and an avalanche photodiode. 
     U.S. Pat. No. 7,019,297 to Aykac which is incorporated herein by reference and is commonly assigned discloses exemplary optical coupler and separator materials that can be used. However, the invention is not limited to those materials. 
     Incident radiation  202  impacts the scintillator crystals  204 . In accordance with various embodiments of the invention optical couplers, optical separators, air gaps, thin films and the like distributed in positions throughout the scintillator crystals  204  are used to control light distribution throughout the crystal block. 
     It should be appreciated by those skilled in the art that although a 4×4 scintillator pixel array is disclosed with a 2×2 optical sensor, the invention is not limited to this configuration. Other configurations of arrays and sensors can be used without departing from the scope of the present invention. 
       FIG. 2B  is a diagram depicting a cross sectional view along the xz plane of the z-position dependent optical coupling structures between crystals in accordance with an embodiment of the present invention. Specifically, DOI block  200  depicts incident radiation  202 , scintillator pixel  204   1 ,  204   2 ,  204   3  and  204   4 , sensor mount  208 , optical coupler  210 , air gap  209 , optical separator  212  and coupling film  214 . The scintillator pixels are discrete crystals, which are assembled into a two-dimensional array and coupled via air gaps, optical separator foils and optical coupling foils, as shown. It should be appreciated by those skilled in the art that the invention is not limited to foils, the couplers and separators can comprise films, powders, paints, paper, plastics, metals and combinations of the materials. 
     In this example, the alternating use of partial optical separators at the top or upper portion  216  of the block and bottom or lower portion  218  of the block of the crystal-crystal interfaces cause a spread of the position peaks. Near the top or upper portion  216 , the positions of events within the corner  204   1 ,  204   4 ,  204   13  and  204   16  and edge pixels  204   2 ,  204   3 ,  204   5 ,  204   8 ,  204   9 ,  204   12 ,  204   14 , and  204   15  are moved inwards, because of light sharing across the air gap  209 . Near the bottom or lower portion  218  of the block, the partial separators  212  lead to a more outward position of edge pixels  204   2 ,  204   3 ,  204   5 ,  204   8 ,  204   9 ,  204   12 ,  204   14 , and  204   15  and corner pixels  204   1 ,  204   4 ,  204   13  and  204   16  because the light sharing is reduced by the optical separator  212 , and many photons will hit the sensor area directly, without being reflected and scattered across the whole block. 
     For the four inner pixels, the effect works in a similar, but opposite way: Near the top or upper portion  216  of the block, light sharing towards the inner pixels  204   6 ,  204   7 ,  204   10 , and  204   11  of the block is hindered by the separator  212 , while coupling toward the corner is possible. Near the bottom or lower portion  218  of the block, a spread toward the corner is limited by the separator  212  due to reflection, whereas in this example a spread toward the other detector half is even further enhanced by an additional optical coupling film via optical coupler  210  between the crystals. Due to these conditions, events at the lower portion  218  of the block are pulled towards the center of the position profile, while events interacting near the upper portion  216  of the block are pulled further out of the position profile. 
     The sample block shown in  FIGS. 2A, 2B , and simulated in  FIGS. 5A, 5B and 5C  has no additional light guide between the scintillator crystals  204  and the sensors  206  except for thin coupling film  214 . In this case, the optical coupling between the crystals occurs mostly via the air gaps  209  and/or the optical coupling film used for optical coupler  210  at the central interfaces of the block. It is also possible to include additional light guiding objects to further tune the light sharing between crystals and to improve the resolution between crystals and for different interaction depths. Such additional light guides are likely to become more relevant if larger crystal arrays are used, for example, in combination with more optical sensor elements. It should be appreciated by those skilled in the art that coupling film  214  can be eliminated and a compression fit used without departing from the scope of the present invention. 
     The basic concept of the invention is to use partial or graded optical elements between the crystals in order to vary the light distribution over the whole block as a function of interaction height. Another embodiment to  FIGS. 2A and 2B  may also be achieved by the use of optical separators alone or by the use of optical couplers alone with air gaps used for the alternating pattern, for example, optical separator, then air gap and so on, or optical coupler then air gap and so on. Preferably, the arrangement is optical separator then optical coupler and so on. Similarly, it is possible to use optical coupling or separator elements only near the bottom or only near the top of the block. 
     A preferable arrangement for providing DOI resolution over the whole block is to use an alternating pattern of separation near the bottom/coupling near the top, then coupling near the bottom/separation near the top and so on. For a good DOI resolution of the corner pixels, it seems advantageous to start the series with separation near the bottom for the first interface from the block edge. But an inverted structure starting with separation near the top at the first interface is also possible in principle. A range of different DOI block detector examples is shown in  FIG. 3 . 
       FIG. 3  comprises various diagrams illustrating different examples of DOI resolving block detectors with different partial light sharing elements and different crystal and sensor numbers. For example,  FIG. 3A  is a diagram of a DOI block having couplers  210 , separators  212  and a light guide  214 ;  FIG. 3B  is a diagram of a DOI block having separators  212  and a light guide  214 ;  FIG. 3C  is a diagram of a DOI block having couplers  210  and a light guide  214 ;  FIG. 3D  is a diagram of a DOI block having couplers  210 , separators  212  and a light guide  214  all in an inverted position;  FIG. 3E  is a diagram of a DOI block having 6×6 crystals coupled to 2×2 sensors, with alternating separator structures. For example upper portion begins with an air gap  309  then separator  312  and so on in an alternating fashion. The lower portion of the block begins with separator  312  then air gap  309  then so on in an alternating fashion.  FIG. 3F  is a diagram of a DOI block having 8×8 crystals coupled to 4×4 sensors, with alternating separators. For example the arrangement is similar to  FIG. 3E  except there are more pixels and sensors.  FIG. 3G  is a diagram of a DOI block having 8×8 crystals coupled to 3×3 sensors, with separators  312  located only at the bottom of the block in alternating fashion with air gap  309 . The upper portion of the block uses only air gaps between the pixels in accordance with an embodiment of the present invention. There are no separators  312  used in the upper portion of the block. 
     Other embodiments of the invention can use more than one optical separator and/or more than one optical coupling material in order to provide even more different degrees of light sharing for different depth zones. For example, different types of coupling or separation materials can be used to add greater control to scintillations across the block. 
       FIG. 4  is a diagram illustrating a DOI block detector  400 , manufactured by laser structuring of a monolithic scintillator crystal in accordance with an embodiment of the present invention. The thickness of the structures  407  between the pixels  404  indicates the degree of optical separation e.g., thicker line indicates higher scatter probability. Specifically, the DOI block detector is based on a monolithic scintillator crystal with optical structures  407  introduced afterwards. Such scintillator patterning may be achieved, for example, by laser structuring or by partial cutting of grooves between the pixels and possibly filling the grooves with optical separators, optical coupling media or coatings. 
     The method of optical patterning by laser structuring eliminates the need for elaborate mechanical processes and also the need for additional optical materials. Another advantage of this method is that it provides a straightforward way for a continuous change in optical separation between pixels as a function of height. This is possible because a different size and/or different density of scatter centers can be created, for example, by varying the laser power along the z direction. For a general description of the laser structuring method for PET detectors see patent application US2004/0262526A1, which is incorporated herein by reference. 
     It should be noted that the same effect of a graded, continuously varying scatter probability may also be achieved by a graded reflective coating, e.g. a modified version of the VM2000 reflective film by 3M, where the reflectivity varies with the z position. In this case, the structures  407  are graded wherein one end of the separator is thicker than the opposing end and represents varying reflectivity wherein a thicker line indicates higher reflectivity of such a graded reflective film, assembled between discrete scintillator pixels  404 . Thus, structure  407  can comprise a graded orifice and can be considered an optical separator on one portion of the block and considered an optical coupler for an opposing vertical portion of the block. The adjacent orifice can be flipped to create an alternating couple/separator pattern. 
     The depth-encoding ability of aspects of the invention was verified by the use of optical simulations with the ray-tracing software ZEMAX. LSO was used as the scintillator material, with the optical parameters disclosed in U.S. Published Application No. 2009/0224164 to Lewellen et al., the contents of which are incorporated by reference. 
     VM2000 film by 3M was used as reflective material between the pixels (where indicated) and as a reflective wrapping around the whole block. As an optical coupling film the same acrylic carrier material was defined within the crystal gap, but without the reflective coating. The simulated crystal size was 2.5×2.5×20 mm 3 , and four optical sensors with 3×3 mm 2  active area and 50% quantum efficiency were defined for light detection. The position coordinates x and y were determined from the signals on the four light sensors a, b, c, d by the equations:
 
 x =( b+d )/( a+b+c+d );  y =( a+b )/( a+b+c+d ).
 
     Simulated position results for 30 random events in each crystal are shown in  FIG. 5 .  FIG. 5A  is a graph depicting simulated position results for the block detector described in  FIGS. 2A and 2B  in accordance with an embodiment of the present invention;  FIG. 5B  is a graph depicting mean values for position results in each of three depth bins depicted in  FIG. 5C  in accordance with an embodiment of the present invention; and  FIG. 5C  is a diagram depicting three depth bins used in the graphs of  FIGS. 5A and 5B  in accordance with an embodiment of the present invention. The shapes of the data points indicate whether the interaction occurred in the top 6 mm of the crystal (circle), the next 6 mm near the middle (square) or the bottom 8 mm (triangle). 
     As seen in  FIGS. 5A and 5B , the position peaks are spread out by the combined effects of the partial reflectors and partial optical couplers. As the shapes show, this spread is characteristic for the depth of interaction. All 16 pixels show a depth-dependent elongation, with the strongest (diagonal) spread observed for the four pixels near the center of the block e.g. scintillator pixel  204   6 ,  204   7 ,  204   10 ,  204   11 . But even for the corner pixels, there is a reasonable resolution between top and bottom events. Utilizing these details of the position profile in the reconstruction leads to an improved overall spatial resolution by the reduction of parallax errors. The DOI resolution can be further improved with better photon statistics, e.g. due to a higher light output, higher quantum efficiency or higher sensor area fill factor than in this example. 
       FIG. 5C  depicts three depths but the invention will work using two depths. Three depths were used for illustrative purposes. Upper portion  216  and lower portion  218  were illustrated as being substantially equal in size. This is preferable. However, the ratio can be 70/30 in either direction and not depart from the scope of the present invention. The vertical height of the separators are substantially the same with each other within the block. The vertical height of the optical couplers are also substantially the same with each other within the block. 
       FIG. 6  illustrates a computing hardware  600  upon which an embodiment according to various exemplary embodiments can be implemented. For example, the processes described herein can be implemented using the computer system  600 . The computer system  600  includes a bus  601  or other communication mechanism for communicating information and a processor  603  coupled to the bus  601  for processing information. The computer system  600  also includes main memory  605 , such as a random access memory (RAM) or other dynamic storage device, coupled to the bus  601  for storing information and instructions to be executed by the processor  603 . Main memory  605  can also be used for storing temporary variables or other intermediate information during execution of instructions by the processor  603 . The computer system  600  may further include a read only memory (ROM)  607  or other static storage device coupled to the bus  601  for storing static information and instructions for the processor  603 . A storage device  609 , such as a magnetic disk or optical disk, is coupled to the bus  601  for persistently storing information and instructions. 
     The computer system  600  may be coupled via the bus  601  to a display  611 , such as a cathode ray tube (CRT), liquid crystal display, active matrix display, or plasma display, for displaying information to a computer user. An input device  613 , such as a keyboard including alphanumeric and other keys, is coupled to the bus  601  for communicating information and command selections to the processor  603 . Another type of user input device is a cursor control  615 , such as a mouse, a trackball, or cursor direction keys, for communicating direction information and command selections to the processor  603  and for controlling cursor movement on the display  611 . 
     According to one embodiment contemplated herein, the processes described are performed by the computer system  600 , in response to the processor  603  executing an arrangement of instructions contained in main memory  605 . Such instructions can be read into main memory  605  from another computer-readable medium, such as the storage device  609 . Execution of the arrangement of instructions contained in main memory  605  causes the processor  603  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the instructions contained in main memory  605 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the certain embodiments. Thus, the exemplary embodiments are not limited to any specific combination of hardware circuitry and software. 
     The computer system  600  also includes a communication interface  617  coupled to bus  601 . The communication interface  617  provides a two-way data communication coupling to a network link  619  connected to a local network  621 . For example, the communication interface  617  may be a digital subscriber line (DSL) card or modem, an integrated services digital network (ISDN) card, a cable modem, a telephone modem, or any other communication interface to provide a data communication connection to a corresponding type of communication line. As another example, communication interface  617  may be a local area network (LAN) card (e.g. for Ethernet, SONET or an Asynchronous Transfer Model (ATM) network) to provide a data communication connection to a compatible LAN. Wireless links can also be implemented. In any such implementation, communication interface  617  sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information. Further, the communication interface  617  can include peripheral interface devices, such as a Universal Serial Bus (USB) interface, a PCMCIA (Personal Computer Memory Card International Association) interface, etc. Although a single communication interface  617  is depicted in  FIG. 6 , multiple communication interfaces can also be employed. 
     The network link  619  typically provides data communication through one or more networks to other data devices. For example, the network link  619  may provide a connection through local network  621  to a host computer  623 , which has connectivity to a network  625  (e.g. a wide area network (WAN) or the global packet data communication network now commonly referred to as the “Internet”) or to data equipment operated by a service provider. The local network  621  and the network  625  both use electrical, electromagnetic, or optical signals to convey information and instructions. The signals through the various networks and the signals on the network link  619  and through the communication interface  617 , which communicate digital data with the computer system  600 , are exemplary forms of carrier waves bearing the information and instructions. 
     The computer system  600  can send messages and receive data, including program code, through the network(s), the network link  619 , and the communication interface  617 . In the Internet example, a server (not shown) might transmit requested code belonging to an application program for implementing an exemplary embodiment through the network  625 , the local network  621  and the communication interface  617 . The processor  603  may execute the transmitted code while being received and/or store the code in the storage device  609 , or other non-volatile storage for later execution. In this manner, the computer system  600  may obtain application code in the form of a carrier wave. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to the processor  603  for execution. Such a medium may take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as the storage device  609 . Volatile media include dynamic memory, such as main memory  605 . Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise the bus  601 . Transmission media can also take the form of acoustic, optical, or electromagnetic waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, CDRW, DVD, any other optical medium, punch cards, paper tape, optical mark sheets, any other physical medium with patterns of holes or other optically recognizable indicia, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read. 
     Various forms of computer-readable media may be involved in providing instructions to a processor for execution. For example, the instructions for carrying out various exemplary embodiments may initially be borne on a magnetic disk of a remote computer. In such a scenario, the remote computer loads the instructions into main memory and sends the instructions over a telephone line using a modem. A modem of a local computer system receives the data on the telephone line and uses an infrared transmitter to convert the data to an infrared signal and transmit the infrared signal to a portable computing device, such as a personal digital assistant (PDA) or a laptop. An infrared detector on the portable computing device receives the information and instructions borne by the infrared signal and places the data on a bus. The bus conveys the data to main memory, from which a processor retrieves and executes the instructions. The instructions received by main memory can optionally be stored on storage device either before or after execution by processor. 
     The data and imaging processes described herein may be implemented via software, hardware (e.g., general processor, Digital Signal Processing (DSP) chip, an Application Specific Integrated Circuit (ASIC), Field Programmable Gate Arrays (FPGAs), etc.), firmware or a combination thereof. Such exemplary hardware for performing the described functions is detailed below. 
     The embodiments of the present invention can achieve several technical effects, including not requiring differential pulse shape discrimination, not requiring the use of materials with different emission decay times. Therefore the fastest and best scintillator material can be used for the whole block. This advantage is particularly relevant for making a DOI resolving time-of-flight PET detectors. The number of scintillator pixels is not higher than what is necessary for the required spatial resolution. For example, the number of scintillation pixels is not increased to provide DOI information. The blocks are relatively easy to manufacture, since there are no additional requirements for the scintillator material, in particular no additional layering of the scintillator itself along the z direction. Optical multiplexing can be used, for example, the number of read-out channels can be lower than the crystal number. Embodiments of the present invention are not restricted to the use of any particular sensor type. The embodiments work with PMTs as well as silicon based photosensors such as APDs and SiPMs. 
     In terms of medical imaging an advantage for PET imaging is that the DOI information will help to provide better spatial resolution in the reconstructed images, especially for off-center positions. The use of DOI information can also help to reduce the PET ring diameters to the size needed to accommodate patients and therefore reduce overall system cost. 
     The present disclosure enjoys industrial applicability in medical imaging, but other applications and uses can be found such as oil exploration, optical data storage, lasers, and homeland security. 
     In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.