Development of an angled Si-PM-based detector unit for positron emission mammography (PEM) system

摘要

Positron emission mammography (PEM) systems have higher sensitivity than clinical whole body PET systems because they have a smaller ring diameter. However, the spatial resolution of PEM systems is not high enough to detect early stage breast cancer. To solve this problem, we developed a silicon photo-multiplier (Si-PM) based detector unit for the development of a PEM system. Since a Si-PM's channel is small, Si-PM can resolve small scintillator pixels to improve the spatial resolution. Also Si-PM based detectors have inherently high timing resolution and are able to reduce the random coincidence events by reducing the time window. We used 1.5 × 1.9 × 15 mm LGSO scintillation pixels and arranged them in an 8 × 24 matrix to form scintillator blocks. Four scintillator blocks were optically coupled to Si-PM arrays with an angled light guide to form a detector unit. Since the light guide has angles of 5.625°, we can arrange 64 scintillator blocks in a nearly circular shape (a regular 64-sided polygon) using 16 detector units. We clearly resolved the pixels of the scintillator blocks in a 2-dimensional position histogram where the averages of the peak-to-valley ratios (P/Vs) were 3.7 + 0.3 and 5.7 + 0.8 in the transverse and axial directions, respectively. The average energy resolution was 14.2 + 2.1% full-width at half-maximum (FWHM). By including the temperature dependent gain control electronics, the photo-peak channel shifts were controlled within ± 1.5% with the temperature from 23 ℃ to 28 ℃. With these results, in addition to the potential high timing performance of Si-PM based detectors, our developed detector unit is promising for the development of a high-resolution PEM system.