SCINTILLATOR STURUCTURE Comparison of columnar and pixelated scintillators for small field of view hybrid gamma camera imaging 1 Space Research Centre, University of Leicester, Leicester, LE1 7RH, UK 2 Imaging Clinical Group, Sandwell and West Birmingham NHS Hospital Trust, West Bromwich B71 4HJ, UK 3 Radiological and Imaging Sciences, Medical Physics and Clinical Engineering, Medical School, University of Nottingham and Nottingham University Hospitals NHS Trust, NG7 2UH, UK L.K. Jambi1, J.E. Lees1, S.L. Bugby1, M.S. Alqahtani1, B.S. Bhatia1,2, W.R. McKnight1, N.S. Dawood1 , A.H. Ng3 and A.C. Perkins3 INTRODUCTION In intraoperative medical imaging, the development of small field of view (SFOV) hybrid gamma cameras is considered an expanding area of research. A new innovation combining both gamma and optical imaging in a co-aligned configuration, the Hybrid Gamma Camera (HGC), could provide intraoperative imaging with superior spatial resolution and may enhance the physical localisation of radiopharmaceutical uptake during critical surgical procedures such as in head and neck sentinel node biopsies [1]. The aim of this study was to compare the performance characteristics of the HGC with either a thallium doped caesium iodide (CsI:TI) columnar scintillator or a pixelated gadolinium oxysulfide (GOS) ceramic scintillator installed. SCINTILLATOR STURUCTURE DISCUSSION In this study, the characterisation of the HGC has been evaluated and compared. The results in table 1 indicate that the HGC has significantly better spatial resolution when using CsI:TI columnar scintillator. However, the HGC is more sensitive when using pixelated GOS scintillator but with a poorer spatial resolution and count rate capability. CONCLUSION The choice of material and its structure are key parameters when deciding which scintillator to use for particular imaging applications. For the current application of the HGC where spatial resolution is a priority CsI:TI has significant advantages over the evaluated GOS scintillator. MATERIALS AND METHODS The hybrid gamma camera (HGC) consists of an electron multiplying CCD coupled to a 1500µm thick scintillator (in this study both CsI:TI and GOS scintillators were used). A fused gamma optical image is achieved using a tungsten pinhole collimator co-aligned with a mirror and optical camera. A performance characterisation of the HGC has been evaluated using a columnar CsI:TI and a pixelated gadolinium oxysulfide GOS ceramic scintillator. Characteristics investigated include spatial resolution (both intrinsic and extrinsic), intrinsic uniformity, sensitivity (both intrinsic and extrinsic) and count rate capability [2]. Figure 2. Scanning electron microscope images of: GOS scintillator (top) each pixel is 400µm x 400µm, with an individual pixels separated by a 40µm wide boundary material coated with TiO2 and CsI:TI scintillator (bottom) tightly packed columns with approximately 1µm diameter. RESULTS The performance characteristics of the HGC fitted with each scintillator are summarised in table 1 Parameters CsI:TI GOS Intrinsic spatial resolution FWHM 230 ± 25 µm 1090 ± 200 µm (no scattering material) 1.8 ± 0.027 mm 1.97 ± 0.34 mm (including scattering material) 2.02 ± 0.09 mm 2.13 ± 0.46 mm Intrinsic uniformity Co-efficient of variation 20 ± 15 % 17 ± 9 % Intrinsic sensitivity At 140.5 keV 40 ± 3 % 54 ± 4 % At 50 mm 6.6 ± 0.5 cps/MBq 18.5 ± 0.3 cps/MBq 3.3 ± 0.5 cps/MBq 8.1 ± 0.3 cps/MBq Count rate capability Maximum recorded count rate 35700 ± 200 cps 3170 ± 30 cps Extrinsic spatial resolution Extrinsic sensitivity Figure 1. Schematic of Hybrid Gamma Camera HGC. Table 1. Performance characteristics for HGC with either 1500 µm thick CsI:TI or 1500 µm thick GOS scintillator installed. Reference [1] J E Lees, S L Bugby, B S Bhatia, L K Jambi, M S Alqahtani, W R McKnight, A H Ng and A C Perkins, A small field of view camera for hybrid gamma and optical imaging, Journal of Instrumentation 9 (2014) C12020 [2] S L Bugby, L K Jambi and J E Lees, A comparison of CsI:TI and GOS in a scintillator-CCD detector for nuclear medicine imaging , Journal of Instrumentation 11(2016) P09009 Contact: Lj97@le.ac.uk