MicroSPECT is one of the main functional imaging techniques used in the preclinical setting. Even though high-resolution images can be obtained with currently available systems, their sensitivity is often quite low due to the use of multi-pinhole collimation. This results in long acquisition times and hampers dynamic imaging. However, it has already been shown that this limited sensitivity can be overcome using high-resolution detectors. In this article, we therefore investigated the use of a digital photon counter (DPC) in combination with a 2 mm thick monolithic LYSO crystal for SPECT imaging. These light sensors contain arrays of avalanche photodiodes whose signals are directly digitised. The DPCs have the advantage that they are very compact, have a high intrinsic resolution, are MR compatible and allow disabling cells with a high dark count rate. In order to investigate the influence of the temperature dependent dark count rate on the detector performance, we compared it at 3 °C and 18 °C. At 3 °C, we observed an energy resolution of 28.8% and an intrinsic spatial resolution of 0.48 mm. Furthermore, the count rate at 10% loss is 60 kcps. Next, we looked at the event loss at 18 °C caused by the higher dark count rate and found a 5% loss compared to the 3 °C measurements. At this higher temperature the energy resolution becomes 29.2% and the intrinsic spatial resolution decreases to 0.52 mm. Due to the 5% count loss, the count rate at 10% loss increases to 63 kcps. A small degradation of the detector performance is thus observed at 18 °C.These results show the usefulness of this detector for SPECT imaging together with its excellent intrinsic spatial resolution. A drawback of the detector is its low, spatially varying energy resolution. Even though the detection efficiency and intrinsic spatial resolution are better at 3 °C, results are still acceptable at 18 °C.

Dual Energy Computed Tomography (DECT) allows performing simple material separation or differentiation based on the energy dependence of the attenuation coefficient by acquiring an image at high and low energy. Up to date, many applications of DECT exist in clinical practice, but the translation of this method to preclinical systems has not been investigated thoroughly. The aim of this study is to validate the feasibility of bone - iodine separation using DECT on a standard FLEX Triumph-II system. Compared to clinical CT, microCT images are inherently prone to a higher amount of noise. A possible application may be found in vascular studies where scanning an animal at two different energies allows to separate the different iodine filled vascular structures from the surrounding non-vascular structures like bone. Using a density calibration phantom and different iodine concentrations, the optimal energy combination for preclinical DECT was determined. It was found that a 50-90 kVp energy pair was suited best to perform bone - iodine decomposition, without requiring additional filtration for the high-and low-energy scan. Next, a three material decomposition algorithm for bone, iodine and soft tissue was validated using the density calibration phantom. As this test yielded good results, a mouse study was performed to validate if similar results could be obtained in-vivo and ex-vivo. Based on the obtained results, we found that material separation using the standard FLEX Triumph-II system was possible both on phantoms and in-vivo. However, the in-vivo results still showed some iodine contributions in the bone image, possibly due to the simplicity of the decomposition algorithm. In the future, more complex, raw data based algorithms may be required together with the use of additional filtration and more advanced iterative reconstruction algorithms to achieve better spectral separation

Silicon photomultipliers (SiPM) are a promising alternative light sensor for the classic photomultiplier tubes currently used as single photon emission computed tomography (SPECT) detectors due to their compact dimensions and smaller pixelization. Furthermore, these SiPMs are also known to be MRhard which brings on new possibilities for simultaneous SPECT-MR imaging. In contrast to other research on SiPMs which is mainly focussed on PET applications, we will focus on SPECT imaging. The detector used in this work, consists of a dSiPM (DPC-3200-22-44, Philips Digital Photon Counting) optically coupled to a monolithic LYSO scintillator of 2 mm thick. Using a collimated 57 Co source, a filter was developed to remove the dark count events from the spectrum and a comparison was made between the detection efficiency of the DPC-based detector and a PSPMTbased detector. Furthermore, the intrinsic spatial resolution was determined using both a resolution collimator and beam source measurements. The same beam source measurements were also used to calculate the energy resolution across the detector. Finally, the count rate performance of the DPC was investigated by measuring a decaying 99m Tc source in front of the detector. Measurements showed that, despite the presence of dark count events in the spectrum, the DPCs have approximately the same detection efficiency as our gold standard, the PSPMT. Furthermore, we demonstrate that the detector has an intrinsic spatial resolution of 0.486 mm and an energy resolution of 22.3 %. This work demonstrates the usefulness of the DPCs as SPECT detectors.978-1-4799-0534-8/13/$31.00 ©2013 IEEE