The present invention relates generally to imaging devices, and more particularly to a light guide for an array of detectors in an imaging device.
In certain types of imaging devices, such as positron emission tomography (PET) scanners, arrays of detector elements serve the function of detecting radiation emanating from the patient. In a PET scanner, for example, arrays of scintillator crystals detect gamma rays which are generated inside the patient. The gamma rays are produced when a positron emitted from a radiopharmaceutical injected into the patient collides with an electron causing an annihilation event. The scintillator crystals receive the gamma rays and generate photons in response to the gamma rays.
One of the challenges in designing a high resolution PET scanner relates to the space requirements of the electronics associated with the detector crystals, in particular the photomultiplier tubes (PMTs) which are situated behind the detector crystals. The function of the photomultiplier tubes is to receive photons produced by the scintillator crystals and to generate an analog signal with a magnitude representative of the number of photons received. The photomultiplier tubes typically cannot be diminished in size beyond a certain point, so that generally each photomultiplier tube is situated behind a number of smaller detector crystals. For example, a detector module in a PET scanner may comprise a 2×2 array of photomultiplier tubes situated behind a 6×6 array of scintillator crystals. In response to a scintillation event, each PMT produces an analog signal which is representative of the number of photons it has received. The relative magnitudes of the four PMT signals are then used to determine where the scintillation event took place and which crystal detected the event.
In determining the location of the scintillation event, it is generally advantageous to have a high degree of separation of the relative signal levels arising from each of the individual scintillation crystals in the detector array. Various arrangements have been proposed for increasing the spatial resolution of the detector crystals by controlling the light distribution within the detector array. For example the light distribution within the array of detector crystals can be controlled by applying various surface finishes having known light scattering and reflective properties to each crystal. These arrangements generally attempt to control the light distribution such that the proportion of light reaching each photomultiplier tube is relatively consistent and well defined for each event occurring at a particular detector crystal. In this way, the analog signals from the photomultiplier tubes may consistently determine which detector crystal produced the scintillation event.
As the demands for higher resolution in PET scanners continue to increase, one approach to achieving higher resolution is to increase the number of crystals in each detector array without increasing the size of the array. For example, a 6×6 array of detector crystals might be replaced with an 8×8 array. However, an increase in the number of crystals may introduce additional complexities and costs to the surface finishes and optical coupling which may be necessary for acceptable spatial resolution of the scintillation events. An increased number of smaller crystals may also introduce additional challenges with respect to light loss in the corner crystals and the tolerances for mechanical alignment of the array with respect to the photomultiplier tubes. The present invention provides an apparatus and method which can overcome these problems.