Abstract
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Objectives: We have designed and built a prototype novel imaging system which can bring simultaneous and near real-time PET/Ultrasound imaging to the patient bedside to support point-of-care (PoC) molecular imaging applications. This study validates the feasibility and functionality of the system and evaluates its imaging strategies for different applications.
Methods: The system consists of two PET detectors and an ultrasound (US) transducer. One detector is mounted to a rotation stage on an optical table. The 2nd detector and an ultrasound transducer are mounted to a robotic arm. Robotic arm can position the PET detector and US transducer at arbitrary locations to simultaneously collect both PET coincidence events and US signals. Each PET detector consists of a multi-anode PMT (Hamamatsu H8500) coupled to 48× 48 LYSO crystals of 1.0× 1.0× 10.0 mm3 each. Custom front-end electronics were designed and fabricated to multiplex the 64 anode signals to 4 position-encoded signals. These multiplexed outputs are used to compute energy and position of each event while the fast dynode signal is used to get the timing. Coincidence events were acquired from 28 different sampling angles when a cylindrical phantom with 9 tumors of different sizes was placed between the two PET detectors. The 28 sets of data were grouped into different combinations and fed into a GPU-based near real-time 3D reconstruction engine which supports arbitrary geometry to evaluate the functionality of the system with or without Time-of-Flight(ToF) and to explore imaging strategies for different applications. In a combined PET and US imaging experiment, we inserted a tubing into a gelatin phantom and filled it with Cu-64 solution to mimic a line source. Results: The coincidence resolving time (CRT) between the two PET detectors is around 740 ps(FWHM). Cylinder phantom images show that with limited number of sampling angles and events, the compact system can still offer useful PET images. Improvement in timing resolution results in fewer artifacts caused by the incomplete and non-uniform sampling. Detectors with good TOF performance allows one to reduce the number of sampling angles and still maintain reasonable image quality. Combined PET/US image of a gelatin phantom shows simultaneous imaging capability of the system. Conclusions: The compact PET/US imaging system with its fast imaging reconstruction capability may be useful for PoC imaging applications such as imaging of theranostic interventions. Additional imaging studies will be conducted and presented.