Characteristics of compact detectors based on pixellated NaI(Tl) crystal arrays

Results are presented on the preliminary evaluation of the pixellated NaI(Tl) crystal arrays for use in high-resolution small field-of-view gamma cameras. A prototype detector was developed using the pixellated NaI(Tl) arrays attached to a 5" diameter position-sensitive photomultiplier tube (PSPMT). Two 5.3 cm square pixellated NaI(Tl) crystal arrays from Saint-Gobain with pixel sizes of 1×1×6 mm and 2×2×6 mm were tested. A conventional charge division readout using a resistive chain was used with the PSPMT. The detector was tested using a uniform flood source and a ∼0.8 mm diameter collimated Tc-99m source. The collimated source was scanned over both crystal arrays. The performance characteristics including spatial resolution, energy resolution and the ability to resolve the pixellated crystal elements were determined. The response of the pixellated detector was also measured. With the conventional resistive-chain readout the individual pixels were well resolved for the 2 mm pixellated crystal array but not for the 1 mm pixellated crystal array from the images of a uniform flood source. An average FWHM of 1.4 mm was obtained with a 0.8 mm diameter pencil-beam over the active crystal area for both crystal arrays. The energy resolutions from the individual pixels were 10.8% and 10.0% at 140-keV photon energy for the 1 mm and 2 mm pixellated crystals, respectively. Our study indicates that using conventional resistive-chain readout, a compact detector comprised of a pixellated NaI(Tl) crystal array and a 5" diameter PSPMT at best can achieve a spatial resolution of ∼1.1 mm. We conclude that the use of pixellated crystal arrays with ∼1.5 mm pixel element is likely to be an optimal choice for the combination of this PSPMT with resistive chain readout technology for development of high resolution small field-of-view gamma cameras.