Front-illuminated versus back-illuminated photon-counting CCD-based gamma camera: important consequences for spatial resolution and energy resolution

Abstract

Charge-coupled devices (CCDs) coupled to scintillation crystals can be used for high-resolution imaging with x-rays and gamma rays. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by a real-time image analysis of the scintillation light flashes ('photon-counting mode'). The electron-multiplying CCD (EMCCD) is well suited for fast read out, since even at high frame rates it has extremely low read-out noise. Back-illuminated (BI) EMCCDs have much higher quantum efficiency than front-illuminated (FI) EMCCDs. Here we compare the spatial and energy resolution of gamma cameras based on FI and BI EMCCDs. The CCDs are coupled to a 1000 microm thick columnar CsI(Tl) crystal for the purpose of Tc-99m and I-125 imaging. Intrinsic spatial resolutions of 44 microm for I-125 and 49 microm for Tc-99m were obtained when using a BI EMCCD, which is an improvement by a factor of about 1.2-2 over the FI EMCCD. Furthermore, in the energy spectrum of the BI EMCCD, the I-125 signal could be clearly separated from the background noise, which was not the case for the FI EMCCD. The energy resolution of a BI EMCCD for Tc-99m was estimated to be approximately 36 keV, full width at half maximum, at 141 keV. The excellent results for the BI EMCCD encouraged us to investigate the cooling requirements for our setup. We have found that for the BI EMCCD, the spatial and energy resolution, as well as image noise, remained stable over a range of temperatures from -50 degrees C to -15 degrees C. This is a significant advantage over the FI EMCCD, which suffered from loss of spatial and especially energy resolution at temperatures as low as -40 degrees C. We conclude that the use of BI EMCCDs may significantly improve the imaging capabilities and the cost efficiency of CCD-based high-resolution gamma cameras.

Imaging and spectroscopy using a scintillator-coupled EMCCD

“…In previous studies of this nature it is often the case that the peak signal (or sum of 3 × 3 pixels) of a photon flash is assumed to be proportional to the energy of the incident X-ray [1,2,10]. Ignoring the depth of interaction effects (discussed later) this would be the case for unity gain.…”

Micro-machined retro-reflector for improving light yield in ultra-high resolution gamma cameras

“…Here we only summarize its characteristics; a comprehensive description of this gamma camera is provided elsewhere , Heemskerk et al 2007. Gamma photons are captured in the scintillation crystal and the light flashes that are generated are detected by a back-illuminated EMCCD.…”

“…It has been shown that very high spatial resolutions (below 100 μm) can be obtained with a detector consisting of CsI(Tl) micro-columnar scintillation crystals read-out by a CCD operating at high frame rates (e.g. Beekman and De Vree 2005, Teo et al 2006, Meng 2006, Heemskerk et al 2007, Soesbe et al 2007, Westra et al 2009. However, due to the low capture efficiency of CsI(Tl) and the limited thickness of available micro-columnar crystals, these crystals are not well suited for small-animal imaging.…”

An enhanced high-resolution EMCCD-based gamma camera using SiPM side detection

“…De Vree et al (2005), Miller et al (2006), Nagarkar et al (2006), Heemskerk et al (2007), Meng and Fu (2008), Westra et al (2009)). To improve the applicability for medical imaging, the application of continuous (or monolithic) crystals can improve the sensitivity for incoming 99m Tc gamma photons (140 keV) at some cost in spatial and energy resolution (Korevaar et al 2009a, Heemskerk 2010.…”