Comparison of scintillators for positron emission mammography (PEM) systems

Positron emission mammography (PEM) has promise as an effective method for the detection of breast lesions. Perhaps the most significant design feature of a PEM system is the choice of scintillator material. In this investigation we compared three scintillators for use in PEM: NaI(Tl), gadolinium oxyorthosilicate (GSO), and lutetium-gadolinium oxyorthosilicate (LGSO). The PEM systems consisted of two 30×30 arrays of pixelated scintillators (3×3×10 mm³ for GSO and LGSO and 3×3×19 mm³ for NaI(Tl)) coupled to arrays of square position-sensitive photomultiplier tubes. The Compton scatter fraction, system energy resolution, spatial resolution, spatial resolution uniformity, and detection sensitivity were compared. Compton scatter fractions for the systems were comparable, between 8% and 9%. The NaI(Tl) system produced the best system energy resolution (18.2%), the GSO system had the worst system energy resolution (28.7%). Spatial resolution for each system was relatively uniform across the face of the detectors, though the magnitude was dependent upon scintillator material. The NaI(Tl) system produced the lowest mean resolution (3.54±0.05 mm for horizontal profiles and 3.51±0.04 mm for vertical profiles), while the LGSO system produced the greatest mean spatial resolution (3.19±0.04 mm for horizontal profiles and 3.20±0.03 mm for vertical profiles). Detection sensitivity varied among the three systems: NaI(Tl)=217.7 c/s/kBq/ml, GSO=383.9 c/s/kBq/ml and LGSO=646.9 c/s/kBq/ml. Imaging of a simulated breast containing various sized spheres demonstrated that the LGSO system produced the greatest detectability for small spheres (as gauged by the contrast-to-noise ratio), while the NaI(Tl) system had the worst detectability. These differences were due mainly to the lower sensitivity of the NaI(Tl) system compared to the LGSO and GSO imagers. This investigation demonstrated the very important connection between scintillator selection and performance of PEM systems.