AbstractsBiology & Animal Science

A prototype modular detector design for high resolution positron emission tomography imaging

by Nan Zhang

Institution: McGill University
Department: Department of Biomedical Engineering
Degree: PhD
Year: 2002
Keywords: Engineering, Biomedical.; Health Sciences, Radiology.
Record ID: 1729397
Full text PDF: http://digitool.library.mcgill.ca/thesisfile84452.pdf


Current challenges facing us in developing dedicated positron emission tomography (PET) systems for metabolic breast mammography (PEM) and small animal (ANIPET) are to achieve high spatial resolution (less than 2 mm) and high efficiency. It is also crucial to extend the sensitive areas of PEM detectors to their periphery in order to overcome the difficulty in imaging near a patient's chest wall. This limitation of the periphery dead region was revealed in the clinical trials of our previously developed PEM-I system. In the new study, we developed prototype detectors by using position-sensitive photomultiplier tubes (PS-PMTS) and pixelated bismuth germanate (BGO) crystals with depth encoding scheme to detect and localize gamma rays. We used the following methods in crystal processing: (1) Crystal cutting – each crystal block was cut by diamond saw into small elements of 2.1 mm x 2.1 mm (2.2 mm pitch) on two opposite faces. The elements on one face of the block offset by half the crystal pitch from those on the opposite face in both X and Y dimensions. The depths of two layers were 11.5 mm and 6.5 mm, respectively. The middle solid space between the two layers was 2 mm. (2) Crystal polishing and encapsulating – The very roughly cut surfaces were chemically polished by acid etching method and the cut slots were encapsulated with an epoxy-compound mixture. (3) Crystal separating – the pseudo discrete crystal blocks were cut along the middle solid space into two discrete segments. (4) Crystal coupling – the two crystal segments were glued together and optically coupled to the PS-PMTs window. We also developed front-end electronic circuits including high-voltage dividers, anode resistor chains, position readout circuits, and last-dynode timing circuits. Methods for combining four PS-PMTs with simple X+, X-, Y+, Y- outputs have been developed to further simplify the position recording. The detectors were constructed in the structure of arrays (two in the system) – modules (four in each array) – units (four in each module). The basic unit of one crystal and one PS-PMT was formed as field replaceable unit. The acquired list-mode data were analyzed with MATLAB and C. Different methods to generate distortion look-up-table were examined and evaluated. Our new prototype detectors have spatial resolutions of 1.8 mm (vs. 2.8 mm in PEM-I), timing resolution of 10.3 ns (vs. 12 ns in PEM-I), and a field-of-view of 88 mm x 88 mm (vs. 64 mm x 56 mm in PEM-I). Our analysis shows that the design improves the spatial resolution, enhances the detector field-of-view, and significantly reduces the peripheral dead regions.