Design for a high-resolution small-animal SPECT system using pixellated Si(Li) detectors for in vivo 125I imaging

We propose a design for a high-resolution single-photon emission computed tomography (SPECT) system for in vivo ¹²⁵I imaging in small animal using pixellated lithium-drifted silicon [Si(Li)] detectors. The proposed detectors are expected to have high interaction probability (>90%), good energy resolution [<15% full-width at half-maximum (FWHM)], and good intrinsic spatial resolution (∼1 mm FWHM). The SPECT system will consist of a dual head detector geometry with the distance between the detectors ranging 30-50 mm to minimize the imaging distance between the mouse and the detectors. The detectors, each with an active area of 64 × 40 mm [64 × 40 array of 1 mm² pixels and a 6 mm thick Si(Li) detector], will be mounted on a rotating gantry with an axial field-of-view of 64 mm. The detector signals will be read out by custom application-specific integrated circuits (ASICs). Using a high-resolution parallel-hole collimator, the expected spatial resolution is 1.6 mm FWHM at an imaging distance of 20 mm, and sensitivity is 6.7 cps/μCi. ¹²⁵I is a readily available radioisotope with a long half-life of 59.4 days and it is commonly used to label biological compounds in molecular biology. Conventional gamma cameras are not optimized to detect the low emission energies (27 to 35 keV) of ¹²⁵I. However, Si(Li) detector provides an ideal solution for detecting the low-energy emissions of ¹²⁵I. In addition to presenting the design of the system, this paper presents a feasibility study of using Si(Li) detectors to detect the emissions of ¹²⁵I.