Abstract:
A method for determining depth-of-interaction correction in a PET system includes modifying crystal and readout configuration to improve depth-dependent arrival profile of scintillation photons, creating a photon dispersion model within a scintillator crystal, measuring photon arrival profile of individual gamma ray event, deriving an estimated depth-of-interaction, and deriving a gamma ray event time based on a time stamp corrected with the estimated depth-of-interaction. The method can include modeling dispersion at different depths of interaction within the scintillator crystal; providing a reflector layer to delay back-reflected photons; providing two respective readouts for the same gamma ray event from two respective pixels optically coupled by a backside reflector or modified crystal configuration, calculating a time difference of the photon arrival at the two pixels, and estimating the depth-of-interaction by applying a statistical weighting. A non-transitory computer readable medium including program code to instruct a processor to determine depth-of-interaction correction is also disclosed.

Description:
BACKGROUND 
       [0001]    For time-of-flight (TOF) positron emission tomography (PET) imaging detectors with good timing resolution are required for improving image quality. One of the effects degrading the timing resolution is the statistical fluctuation of the number of scintillation photons and their transportation inside the crystal. 
         [0002]    A depth-of-interaction (DOI) correction is important to achieve sub-250 ps timing resolution in TOF-PET detectors utilizing currently available fast scintillators such as LSO (Lutetium Oxy-orthosilicate) or LYSO (Lutetium Yttrium Oxy-orthosilicate) and fast photosensors. Due to optical photon dispersion (different path lengths) in the scintillator crystal, timing resolution deteriorates with increasing crystal length; at the same time, a long (about 20-30 mm) crystal is needed for good detection efficiency (high stopping power). 
         [0003]    Traditionally the DOI correction could be implemented by using a dual-ended readout, and/or using a multiple-layer scintillator. However, the timing resolution is significantly worse for both of these approaches due to lower light collection efficiency. Additionally, the cost associated with a complex electronic logic and detector package (additional scintillator and/or photodetector) has mostly precluded their commercialization for next-generation (&lt;250 ps) clinical PET scanners. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIGS. 1A-1C  graphically depict the relationship between photon arrival profile and interaction position; 
           [0005]      FIGS. 2A-2B  depict pixel structures in accordance with some embodiments; 
           [0006]      FIG. 3  depicts an alternative pixel structure in accordance with some embodiments; and 
           [0007]      FIG. 4  depicts a process for determining depth of interaction and event timing of a gamma ray in accordance with embodiments. 
       
    
    
     DESCRIPTION 
       [0008]    In accordance with embodiments, systems and methods measure the arrival profile of detected photons in the first 0.1-0.5 nanoseconds (ns) of a radiation event to derive the event timing. Since the temporal profile of light (scintillation photons) collection in the first 0.1-0.5 ns depends on the gamma ray interaction position and scintillator size, the interaction depth can be estimated based on the measured variation of signal collection (photon arrival) time. For sub-250 ps TOF-PET detectors, the discriminator threshold needs to be set very low to achieve the best timing resolution, while the signal crossing the threshold depends on the light (1-10 photoelectrons) collection in the first 0.1-0.3 ns. One of the effects degrading the timing resolution in long crystals is the depth dependent variation of scintillation photon propagation at the different depths. This introduces additional timing pickoff dispersion when a leading edge discriminator (LED) method is used for timing measurement. 
         [0009]      FIGS. 1A-1C  depict graphically the relationship between photon arrival profile and interaction position. As depicted in  FIGS. 1A-1C , when a gamma ray interaction occurs in the crystal, about half of the scintillation photons will be directed toward the photodetector (such as SiPM) while the other half will travel a longer distance only to reach the photodetector after being reflected at the top of the crystal. There is a correlation between the initial slope of the photon arrival profile and interaction position (depth). Even accounting for the travel time of the gamma-ray in the scintillator crystal, the transportation delay is about 30 ps/cm for scintillation photons directly detected. Additionally, highly delayed scintillation photons (e.g., back-reflected photons) generally would lead to a secondary peak in the arrival time histogram. These reflected photons arrive too late at the SiPM to contribute to the initial determination of the time-stamp—that is, the moment when the discriminator triggers on the first few (e.g., 1-10) photoelectrons. 
         [0010]    In accordance with embodiments, the DOI-dependent initial rising edge of the signal offers the capabilities to quantitatively determine the depth of interaction information and also correct the time variation coming from photon propagation. For a conventional TOF PET detector the capability of DOI determination based on the measurement of arrival time of first photons is severely limited by the relatively slow photosensor(s). 
         [0011]    Embodying systems and methods implement an algorithm on next-generation photosensors with the capabilities of time arrival measurements of individual photoelectrons. In accordance with embodiments, two approaches for the arrival time profile estimation can be implemented to reduce the variability in the time estimate by basing the time stamp on characterizing scintillation photon spreading in the first 0.1-0.5 ns. 
         [0012]    Embodying DOI correction methods and systems can be applied on digital silicon photomultiplier (dSiPM), multiple-digital silicon photomultiplier (MD-SiPM), etc. For example, a time-to-digital converter (TDC) can be incorporated into each microcell or group of microcells, and the arrival time of the first scintillation photons could be measured independently. Complete time information, in particular the time distribution of photons collected in first 0.5 ns, can be utilized to estimate the depth of interaction. The depth of gamma interaction in the scintillator and corrections of the interaction time measurement could be performed accurately instead of the statistical approach that utilizes assumptions of Poisson arrival statistics. 
         [0013]      FIG. 2A  depicts pixel structure  210  with reflector  212  at the top of the pixel structure in accordance with some embodiments. Pixel structure  210  includes adjacent pixels Pixel  1 , Pixel  2  of a pixel array. As can be understood, the pixel array is not limited to two pixels, nor is the pixel array one-dimensional. For purposes of this discussion, two adjacent pixels of one linear array dimension are depicted. 
         [0014]    By sharing scintillation light from individual gamma ray event between adjacent pixels where the scintillator pixels are connected with a back side reflector, embodying methods and systems can determine the difference of scintillation photon transportation inside the crystals. For example, a gamma ray event can occur at depth H 1  of Pixel  1 , and a second gamma ray event at depth H 2 . For clarity, the signal at Pixel  1  generated by the gamma ray event at depth H 1  will arrive long before the signal at Pixel  2  for the same event. For gamma ray event at depth H 2 , the time difference between signals generated at Pixel  1  and Pixel  2  will be significantly smaller. A number of scintillation photons can reflect off the reflector at the top of the pixel, and then be detected at Pixel  2 . For each gamma ray event, the time delay ΔT between the rising edge of the signal at Pixel  1  and the rising edge of the signal at Pixel  2  can be measured and used to accurately calculate the depth of H 1 , H 2  respectively.  FIG. 2A  depicts an embodiment with a reflector along the top of the pixel array structure and gap  218  separating scintillation crystals  216 A,  216 B for the respective pixels. 
         [0015]      FIG. 2B  depicts pixel structure  220  with modified-shaped crystal surfaces  222 ,  224  at the top of the pixel structure in accordance with some embodiments. Pixel structure  220  includes gap  228  between adjacent pixels of the pixel array. In this configuration, the modified-shape crystal acts as a prism to deflect scintillation photons into an adjacent pixel. The gap  228  does not extend all the way to the top of shaped crystal  222 ,  224 . Only photons reflected by the shaped crystal surfaces cross to the adjacent crystal. Gap  228  prevents other photons from crossing. 
         [0016]      FIG. 3  depicts pixel structure  300  with reflector layer  310  at the top of the pixel in accordance with some embodiments. In this configuration, from the gamma ray event detected at Pixel  1 , a number of scintillation photons are reflected off reflector  310  back to the same pixel (Pixel  1 ) and then detected. As can be seen, without the reflector layer  310  a time delay ΔT 1  occurs for the gamma ray event at depth H 2 . With the reflector introducing an extra time delay, the time delay ΔT 2  becomes greater than time delay ΔT 1  due to more photon reflection in the reflector layer  310 . This increase in the arrival time difference of direct and back-photons makes measurement of rising edge of photon arrival profile more accurate. 
         [0017]    Embodying methods estimate the depth of interaction of individual gamma ray events based on the initial photon arrival profile. The timing-stamp can then be corrected when using a leading edge discriminator. Both photon dispersion and Poisson statistics (including timing jitter of SiPM device) need to be taken into consideration. This approach can be implemented on digital silicon photomultiplier (dSiPM), multiple-digital silicon photomultiplier (MD-SiPM), etc. However, embodiments are not so limited and the disclosed approach can be implemented on other design schemes. 
         [0018]    There can be two peaks in the arrival time profile of scintillation photons: (1) directly emitted to the SiPM; or (2) emitted to the other end of the crystal and reflected at the crystal surface (top). To strengthen the position dependence of scintillation photon dispersion, the specific-design reflector or modified crystal configuration have been proposed to be applied at the top surface of the crystal. Both the reflector and modified-shaped crystal disclosed above maximize the time delay of photon propagation from the interaction point to the SiPM between direct emission and top reflection. When a certain number of scintillation photons get reflected into a neighboring pixel, and their arrival time is detected separately in both pixels, it is possible to get a more accurate estimate of the timing and depth of interaction of the event by considering both the arrival time(s) and the amplitude(s) of the two pixel signals. An estimation of the depth of interaction can be based on these factors, which in turns allows for a correction of the time stamp. The depth of interaction information can further be incorporated into the reconstruction algorithm, reducing the parallax error commonly encountered in high resolution PET scanners for lines of response that occur when the radiation is incident at an angle to the long axis of the scintillator. 
         [0019]    Embodying systems and methods can measure the arrival time profile of scintillation photons in the first 0.1-0.5 ns of a gamma ray event, where a statistical weighted model is used to determine the depth information to derive the event timing.  FIG. 4  depicts process  400  for determining depth of interaction and timing of a gamma ray event in accordance with embodiments. A photon dispersion model is created, step  405 . This dispersion model can model photon dispersion at different depths of interaction with the scintillator crystal. The model can be based on the combination of experimental measurements by using a gamma source or an ultra-fast laser and also comprehensive simulations of a detector response function. For example, a detector could be scanned by a gamma-ray source or an ultra-fast laser along the length of the scintillator crystal to determine the scintillation photon dispersion model at different depths. 
         [0020]    The first 0.1-0.5 ns arrival profile of scintillation photons generated by an individual gamma ray event is measured, step  410 , during a PET scan. 
         [0021]    A determination is made as to whether the readout event is from a single pixel or from two pixels, step  415 . In accordance with implementations, there can be at least four crystal pixel configurations: (1) a single readout from a standard single crystal; (2) a single readout from a crystal pixel with a reflector layer located at its top ( FIG. 3 ); (3) a two pixel readout where there is a reflector added between adjacent pixels ( FIG. 2A ), and (4) a two pixel readout where there is modified-shaped surfaces on the top of crystal ( FIG. 2B ). With the reflector layer located at the top of the scintillation crystal (configuration  2 ), the arrival time of back-reflected photons would be substantially delayed. This delay associated with the photon being reflected from the top surface reflector provides an improved opportunity to derive the depth-dependent arrival profile. In another implementation, by sharing scintillation photons of individual gamma ray event between adjacent pixels that include a back side reflector (configuration  3 ) or modified crystal top surface (configuration  4 ), the difference of photon transportation inside the crystal can be determined. 
         [0022]    For single pixel readout (whether a standard single crystal or a crystal with a reflector layer), the depth of interaction is estimated, step  420 , by applying a statistical weighting based on the dispersion model (simulated and/or measured) and the measured arrival profile of scintillation photons for each gamma ray event. 
         [0023]    For a two pixel readout (configuration  3  and  4 ), first an arrival time difference of scintillation photons at the two pixels is calculated, step  425 . For the two pixel configuration, process  400  then continues to step  420  where the depth of interaction is estimated. The timing information T′=function(T, depth) is derived, step  430 , based on the original triggering time corrected with the estimated depth information. 
         [0024]    In accordance with some embodiments, a computer program application stored in non-volatile memory or computer-readable medium (e.g., register memory, processor cache, RAM, ROM, hard drive, flash memory, CD ROM, magnetic media, etc.) may include code or executable instructions that when executed may instruct and/or cause a controller or processor of the PET system to perform methods discussed herein such as a method for determining the depth-of-interaction correction in a PET imaging system, as described above. 
         [0025]    The computer-readable medium may be a non-transitory computer-readable media including all forms and types of memory and all computer-readable media except for a transitory, propagating signal. In one implementation, the non-volatile memory or computer-readable medium may be external memory. 
         [0026]    Although specific hardware and methods have been described herein, note that any number of other configurations may be provided in accordance with embodiments of the invention. Thus, while there have been shown, described, and pointed out fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.