Septa design study for volumetric imaging in positron emission tomography

Positron emission tomography (PET) has become an essential imaging tool to localize tumors in diagnostic radiology and to monitor the size of a lesion before and during the treatment in oncology. Currently, commercial PET cameras operate only in two extreme modes: i) no-septa: three-dimensional (3-D) acquisition mode, also called volumetric imaging; and ii) all-septa-in: two-dimensional (2-D) acquisition mode, also called multislice imaging. In this paper, intermediate septa designs in the brain mode have been sought to minimize the scatter and accidental coincidences with acceptable sensitivity loss. To achieve this goal, Monte Carlo simulations were performed to evaluate possible septa designs. Information about septal penetration, absorption and scattering components from simulations guided us to modify the thickness and total number of septa in the new configurations. Count rate performances of three septa configurations were measured on our experimental high resolution PET camera with 3.85-cm axial field-of-view (AFOV); the measurements were compared with the simulation results. No-septa configuration had lower noise equivalent count (NEC) compared the other two configurations because of small AFOV. Monte Carlo simulations were performed to predict the count rate performances of seven septa configurations for 13.1-cm AFOV. In the large AFOV, because of increased number of line of responses, NEC rate for no-septa case peaked rapidly at 0.07 μCi/cc. No-septa configuration is not recommended for high count rate studies. In this case where activity density is 0.3 μCi/cc or more, 10-septa and 15-septa configurations performed better than the other configurations. Intermediate 5-septa designs with variable septum length performed better between 0.07-μCi/cc and 0.3-μCi/cc range which represents the clinical activity level for ¹⁸F-FDG brain studies.