Implementations of maximum-likelihood position estimation in a four-PMT scintillation detector

Maximum-likelihood (ML) position estimation in small scintillation detectors often involves the use of a look-up table (LUT) to map event characterization vectors to the associated position estimates. As the number of possible characterization vectors determines the size of the LUT, the number of these vectors needs to be manageable. This implies that a critical component of an implementation of ML position estimation is the mapping of detector output signals to characterization vectors. The use of different photomultiplier tube (PMT) output mappings in ML estimation in a small camera with four round PMTs and a single 5 × 100 × 100 mm NaI(Tl) crystal was investigated. The employed mappings of the four detector signals resulted in 20 (5 bits/PMT), 22 (8 bits each for Anger x and y; 6 bits for the sum signal), 24 (6 bits/PMT), and 28 (7 bits/PMT) bit event characterizations. Using present implementations of ML estimation, significant improvement in image quality results if a 24-bit characterization is used rather than 20 or 22. Improvement in image quality is not as marked when 28-bit characterization is used instead of 24, but measurable improvement is obtained