The mini-PET system employed in this study incorporates two blocks of BOO detector crystals. Each detector module contains a 25 x 54 mm block of BOO which is segmented into a 6 x 8 array of crystal elements, 3.5 mm ( transaxially ) by 6.25 mm ( axially ) and 30 mm ( deep ). The crystals are coupled to two Hamamatsu R1548 dual photomultiplier tubes ( PMTs ). The segmented BOO detector block is currently being employed in a number of the commercially available positron tomographs. While the above mentioned block detector has significant advantages over discrete detectors, better energy resolution, sensitivity and axial resolution, it also has disadvantages such as: inter-detector scattering, and variations in sensitivity and energy and spatial resolution across the face of the detector block. In this study the variation in the efficiency, energy resolution, position of the full energy photopeaks, spatial resolution and the amount of inter-crystal scatter fraction for the individual detector crystals across the face of the detector block were investigated. The factors contributing to these variations were identified and suggestions for reducing their effects were made. For example, the inter-detector scattering and the light sharing scheme, employed in the detector block for the identification of the crystal of interaction, were found to be the cause of mispositioning the events. Two novel techniques which showed several advantages over the techniques, employed by other groups, for evaluating the amount and distribution of inter-crystal scattered events were also established. The amount of inter-crystal scatter fraction for one of the central detector crystals, utilising both of the above mentioned techniques, was obtained and found to be almost 36%. The spatial distribution of such scattered events had a FWHM of 1.4 mm which its convolution with the so called ideal LSF of the crystal deteriorated its FWHM by 0.5 mm. It was shown that the energy distribution of the majority of the inter-detector scattering events was around the 511 ke V peak and these scattering events were detected within the full energy photopeak of the true events. Hence it is not possible to reject or discard these events by means of setting a higher energy threshold. This fact, in conjunction with the contribution of these events in worsening the spatial resolution of the PET systems utilizing crystals of small dimensions, demands special consideration to be given to the problem of inter-crystal scattering when further reducing the size of the crystals. Therefore, it is recommended that prior to incorporating detector blocks, which are cut into crystals of small dimensions, in PET systems, the amount and the distribution of the scattered events and their contribution in deteriorating the spatial resolution should be investigated by employing the techniques established in this work.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:308401 |
| Date | January 1994 |
| Creators | Sarkar, Saeed |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/770241/ |
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