Detector Performance Characterization for High Sensitivity Single-Photon Imaging

We are developing detectors for an un-collimated scanner for very high sensitivity single-photon imaging. The scanner consists of two large, thin, closely spaced, pixelated scintillation detectors of either thallium-doped sodium iodide (NaI(Tl)), sodium-doped cesium iodide (CsI(Na)), or bismuth germanate (BGO). The scintillator arrays are interchangeable to optimize performance for a wide range of isotope energies and imaging subjects. For each scintillator, flood histograms were obtained utilizing test gamma sources:
¹⁰⁹
Cd,
²⁴¹
Am,
⁵⁷
Co,
¹³³
Ba,
¹³⁷
Cs,
²²
Na, and beta source
³²
Si. Using a 3 mm thick 21 ×41 NaI(Tl) pixelated array ( 2 mm ×2 mm pixels on a 2.2 mm pitch), read out by two adjacent Hamamatasu H8500 multi-channel photomultiplier tubes (MCPMTs), the energy resolution for
⁵⁷
Co was 12.2% over the center of the MCPMTs and 45.3% for pixels over the gap, with flood histogram peak-to-valley ratios > 6.1 for the center of the MCPMTs and > 4.2 for the gap between the two MCPMTs. The NaI(Tl) detector consistently produced peak-to-valley ratios of > 1.6 in the center of the MCPMTs for all radioisotopes studied. CsI(Na) and BGO detectors had flood histograms of similar quality to NaI(Tl) at intermediate to high-energies, allowing for optimal imaging across a wide energy range. The measured sensitivity for point sources placed 10 mm from the detector approached 35% for low-energy isotopes and all scintillator materials, which results in an expected system sensitivity (two opposing detectors) of ~ 70% for these isotopes. The 5 mm thick BGO arrays had the greatest efficiency for high-energy isotopes due to the increased stopping power of this material. The measured spatial resolution for
^99m
Tc for the CsI(Na) detector (1.6 mm pitch) was 7.5 mm FWHM at the aluminum entrance window (1.5 mm from scintillator face), decreasing to 15 mm at a distance of 5 mm from the scintillator face. The NaI(Tl) detector showed similar spatial resolution. This spatial resolution is sufficient for imaging well-localized radiotracer distributions in thin objects, and the very high sensitivity can be used for fast dynamic imaging, or imaging using very low injected doses.