Abstract:
A scintillation detector for PET imaging devices includes a wavelength shifting material for shifting the emission wavelength of a scintillator toward the peak wavelength sensitivity of a photodetector array coupled to receive light from the scintillator. Preferably the scintillator is an LSO scintillator and the photodetector array is a silicon-based array such APDs or SiPMs.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of nuclear medical imaging, such as Positron Emission Tomography (PET). In particular, the present invention relates to improvements in light collection efficiency of PET detectors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Medical imaging is one of the most useful diagnostic tools available in modern medicine. Medical imaging allows medical personnel to non-intrusively look into a living body in order to detect and assess many types of injuries, diseases, conditions, etc. Medical imaging allows doctors and technicians to more easily and correctly make a diagnosis, decide on a treatment, prescribe medication, perform surgery or other treatments, etc. 
         [0003]    In traditional PET imaging, a patient is injected with a radioactive substance with a short decay time. As the substance undergoes positron emission decay, it emits positrons which, when they collide with electrons in the patient&#39;s tissue, emit two high energy (e.g. 511 keV), simultaneous gamma rays at substantially opposite directions. These rays emerge from the patient&#39;s body and eventually reach a pair of scintillators positioned around the patient. There are often a ring of scintillators surrounding the patient. When the gamma rays interact with (i.e., are absorbed by) a scintillator, a number of light photons are emitted from the scintillation material. The light is usually transmitted through a lightguide to a photodetector. The light detected by the photodetector is then converted to an electrical signal, which is processed by computational circuitry of the apparatus to determine the spatial location and energy of the light signal. 
         [0004]    In PET as well as SPECT it is important to match the scintillator emission wavelength to the optimal wavelength quantum efficiency (QE) of the photodetector. For example, a typical photomultiplier tube (PMT) used in PET applications has a peak wavelength sensitivity at 420 nm while a typical scintillator used in PET (LSO) emits at 420 nm. Therefore, PMTs and LSO are very well matched in terms of wavelength matching. LSO is a very good scintillator for PET because of its high density, high light output, and non-hygroscopic characteristics, but it is not well matched for use with other types of photodetectors, such as avalanche photodiodes (APDs), silicon photomultipliers (SiPMs), or other solid-state based photodetectors. These silicon photodetectors usually have a peak wavelength sensitivity at ≧500 nm. The QE of some devices, such as SiPMs, may increase 2-3 times from 420 nm to &gt;500 nm. It is difficult to make a scintillator material with good PET properties and to make it emit at a certain desired wavelength. As such, there remains a need in the art to match the emission wavelength of PET scintillators to the peak wavelength sensitivity of solid-state photodetectors such as silicon-based photodetectors, to increase the quantum efficiency of such photodetectors. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention solves the existing need in the art by providing a PET detector system having a wavelength shifting device that shifts the emission wavelength of a scintillator optimized for PET detection towards a peak sensitivity wavelength of a solid-state detector. Wavelength shifting materials absorb light of a first wavelength, such as a low wavelength, higher energy lightwave and re-emit the light at a second wavelength, such as a higher wavelength, lower energy lightwave. 
         [0006]    According to a first embodiment, a block detector is provided. The detector is comprised of a scintillator array, which is coupled to a wavelength shifting lightguide. The wavelength shifting lightguide is further coupled to a plurality of photodetectors. 
         [0007]    In a second embodiment, a scintillator array is coupled to a lightguide. The entry surface of the lightguide is coated with a wavelength shifting coating. Finally an array of photodetectors is coupled to the wavelength shifting lightguide. 
         [0008]    In a third embodiment, a scintillator array is coupled to a lightguide. The exit surface of the lightguide is coated with a wavelength shifting coating. Finally a photodetector is coupled to the wavelength shifting lightguide. 
         [0009]    According to another aspect of the invention, a PET scanner is provided. The PET scanner includes a number of scintillators with a wavelength shifting material coupled to each scintillator. The PET scanner also includes a number of photodetectors coupled to the wavelength shifting materials, a processor for receiving data from the photodetectors, and software running on the processor for analyzing the data from the photodetectors and for creating and outputting an image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention will now be described in greater detail in the following by way of example only and with reference to the attached drawings, in which: 
           [0011]      FIG. 1  is a depiction of a conventional block detector. 
           [0012]      FIG. 2  is a depiction of a block detector where the normal lightguide is replaced by a wavelength shifting lightguide, in accordance with an embodiment of the present invention. 
           [0013]      FIG. 3  is a depiction of a block detector where the entry surface of the lightguide has a wavelength shifting coating, in accordance with another embodiment of the present invention. 
           [0014]      FIG. 4  is a depiction of a block detector where the exit surface of the lightguide has a wavelength shifting coating, in accordance with yet another embodiment of the present invention. 
           [0015]      FIG. 5  is a graph showing the absorption and emission wavelengths of an example wavelength shifting material. 
           [0016]      FIG. 6  is a schematic of a PET scanner using a wavelength shifting material. 
           [0017]      FIG. 7  is a graph showing a transmission spectrum of a wavelength shifting material in accordance with the invention. 
           [0018]      FIG. 8  is a graph comparing transmission spectrum results for a SiPM with and without a wavelength shifting material in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    As required, disclosures herein provide detailed embodiments of the present invention; however, the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, there is no intent that specific structural and functional details should be limiting, but rather the intention is that they provide a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0020]      FIG. 1  depicts a typical block detector  100  used in medical imaging. The block detector  100  has a scintillator array  110 . When a gamma photon is absorbed in the scintillator array  110 , it emits light photons. The light photons travel out of the scintillator array  110  and into a lightguide  120  where the light photons are guided to photodetectors  130 . Often, lightguide  120  is made of optical fibers and is used to transport the light to the photodetectors  130  that are located at a distance from the scintillator array  110 . 
         [0021]      FIG. 2  depicts an embodiment of a block detector  200  in accordance with an embodiment of the present invention. Block detector  200  includes a scintillator array  210 . The light emitted from scintillator array  210  in response to gamma-ray interaction is absorbed by a wavelength shifting lightguide  220 . Wavelength shifting lightguide  220  absorbs the light and re-emits it at a different wavelength, shifted towards the peak wavelength sensitivity of photodetectors  230 . Wavelength shifting lightguide  220  may be a lightguide doped with a wavelength shifting chemical, a dye, a plastic, or any other wavelength shifting material. A typical known wavelength shifting material known as BC-482A (polyvinyltoluene) made by Saint Gobain, has an absorption peak at 420 nm and an emission peak at 494 nm. The shifted-wavelength light re-emitted from wavelength shifting lightguide  220  may then be detected by the photodetectors  230 . The scintillator could be made of any appropriate scintillation material, such as LSO, crystal material or other type of material. 
         [0022]      FIG. 3  depicts a second embodiment of the invention in the form of block detector  300 . Block detector  300  includes a scintillator array  310 . The light emitted from scintillator array  310 , in response to interaction with a gamma ray, may pass through a wavelength shifting coating  340  on the entry surface of an optical lightguide  320 . As the light passes through wavelength shifting coating  340 , the wavelength increases as the energy of the light photons is partially dissipated in the wavelength shifting coating. The coating may be a wavelength shifting chemical, a dye, a plastic, or any other wavelength shifting material. The light is then transmitted through lightguide  320  to photodetectors  330 . 
         [0023]      FIG. 4  depicts a third embodiment of the invention in the form of block detector  400 . Block detector  400  includes a scintillator array  410 . The light emitted from scintillator array  410 , in response to interaction with a gamma ray, is transmitted through optical lightguide  420 . As the light exits lightguide  420 , it passes through a wavelength shifting coating  440  on the exit surface of lightguide  420 . As the light passes through wavelength shifting coating  440 , the wavelength increases. The coating may be a wavelength shifting chemical, a dye, a plastic, or any other wavelength shifting material. The light then reaches photodetectors  430 . 
         [0024]      FIG. 5  is a graph showing the ranges of an absorption spectrum  510  and an emission spectrum  520  of a typical wavelength shifting material BC-482A made by Saint Gobain. Such wavelength shifting material may have an absorption peak at 420 nm and an emission peak at 494 nm. If such a material were used in place of an optical lightguide it would increase the light collection of APDs from 70% to over 80%; with SiPMs, it may increase the number of photons collected by a factor of 4. There may be some attenuation losses due to using a wavelength shifting material, but the light collection gains would be much larger than the losses. 
         [0025]      FIG. 6  is a diagram of a PET scanning system  600  using a wavelength shifting material in the block detector in accordance with another aspect of the invention. PET scanning system  600  consists of a number of block detectors  620 . The block detectors may be arranged in a ring configuration. The ring of block detectors  620  forms a space large enough for an adult human body to pass through. Each block detector may consist of a scintillator array, a wavelength shifting material and a photodetector. The ring of block detectors  620  may be connected to a processor  630 . Processor  630  is capable of analyzing the data received from the ring of block detectors  620 , reconstructing an image from the acquired data, and outputting tomographic images of the object or patient scanned. PET scanning system  600  may further include a table or other support structure  610  capable of holding the object or patient to be scanned. The table or other support structure  610  may be adapted to pass through the bore formed by the ring of block detectors  620 . 
         [0026]      FIG. 7  shows transmission spectra for a wavelength shifting material commercially available and manufactured by Eljen Technologies, versus a blank. The material is 0.25 mm thick and was applied to a scintillation crystal as described above. The graph illustrates that the material effectively absorbs all light on the order of 420 nm and shifts it to the range of approximately 500 nm, while the blank transmission has a relatively flat spectrum. 
         [0027]      FIG. 8  shows comparative results for a SiPM with a wavelength shifting material according to the invention, versus no wavelength shifting material. Using the wavelength shifting material resulted in an increase of ˜18% in light collection as well as an improvement in energy resolution. 
         [0028]    The invention having been thus described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be covered within the scope of the following claims.