Performance of a 511 keV Gamma-Ray Imager Using a LYSO (Ce) Crystal Array With Wavelength Shifter

Positron Emission Tomography (PET) is a powerful medical tool for the early diagnosis of cancer but it is still too expensive for widespread use. This is mainly due to the cost of the many crystals and photosensors used. In building a PET imager, crystal read-out using wavelength shifter fibers (WLS) permits the use of many small crystals with a reduced number of photomultipliers, thus promising good spatial resolution at lower cost. However, the main drawback of this method has been low efficiency of photon collection. Improvements in design of our readout system over the last several years have resulted in a factor of more than ten in efficiency. As a result, from the point of view of detection efficiency, this method can now be considered as applicable to the construction of a real device. A pad composed of 16 16 LYSO (Ce) crystals has been read out by using a 2 16 Kuraray Y-11 WLS and Hamamatsu H6568ModIII photomultipliers, in a coincidence measurement using a 22Na β
⁺
source. The size of each crystal is 1 mm 1 mm 20 mm. The attenuation of the number of photons compared with the direct readout is about 1/47, and this attenuation factor seems not to vary from event to event. This results in a detection efficiency almost equal to that of the block detector with the same thickness. The matching of the refractive indices of the crystal and of the thin plastic sheet, inserted between the crystals and WLS, removes the crosstalk between the crystals, which results in a very good spatial resolution close to the intrinsic resolution of 0.33 mm RMS. The timing fluctuation of the detector is such that if a pair of such detectors are used in an actual PET, and if a time window of 15 ns is applied, the event loss due to the fluctuation of the timing in the coincidence will be about 1.6%. A full size animal PET with 20 cm internal diameter built with this arrangement will need only 54 of such photomultipliers.