Developing freehand PET - status and challenges

Objectives The use of positron emitting radiotracers for whole-body PET has proven to be a valuable tool for cancer diagnosis and staging. There is however no established method to transfer this information to an interventional setting to provide live radioguidance, such as to identify cancerous tissue during surgery. Indeed, while diagnostic systems are limited by their most common gantry-based design as well as their long acquisition time, the surgeon mostly relies on cognitive fusion of preinterventional image information with the surgical reality. We suggest the development of a dedicated system, where a light-weight handheld PET detector is combined with an external plate for the live acquisition of PET images. The design of the handheld detector, which should be brought as close as possible to the region of interest, also allows an endoscopic application and consequent increase in sensitivity. The work presented in this submission demonstrates the feasibility of freehand PET (fhPET) with a detector prototype and a first phantom study.

Methods This pilot system is based on matrices of 4x4 LYSO:Ce crystals with an individual size of 3.5x3.5x15 cubic mm coupled to arrays of 4x4 SiPMs. The plate consists in 256 matrices grouped by 4 and read out by 64-channel STiC-ASICs, of which 16 are hosted by each of the 4 front-end boards. The plate is held by a mechanical arm that allows its placement and fixation. The endoscope is made out of 2 matrices read out by a TOFPET ASIC on a front-end board. Each frontend board contains the FPGAs that concentrate event data and transmit it via an HDMI cable to a PCIe data acquisition card, where further event handling and coincidence creation are handled. The position of the plate and endoscope are tracked by an optical tracking system. An iterative MLEM algorithm reconstructs the image. We conduct 3 experiments with a phantom made out of four Eppendorf tubes filled with 5 MBq of 18F-FDG each. We acquire data by rotating the detectors by 360, 180 and 90 degrees around the phantom to study the influence of full versus partial angular information. The acquisition time was fixed to 100s. The energy spectra are recorded. The number of iterations in the reconstruction is varied.

Results Energy peaks are clearly identifiable for both detectors. While image quality decreases with reduced angular coverage, all images clearly showed four hotspots. 5 iterations are sufficient to provide an image.

Conclusions This submission presents first results acquired on phantoms with a freehand PET system, giving a first hint at image quality that can be achieved. Few iterations are sufficient to identify all sources, allowing for a reduction of the total reconstruction time in view of live interventional fhPET. Further developments and experiments are ongoing towards including excellent time-of-flight performance such as to provide a system that can distinguish smallest radioactive hotspots in a noisy environment.