Facial Anonymization and Privacy Concerns in Total-Body PET/CT.

Total-body PET/CT images can be rendered to produce images of a subject's face and body. In response to privacy and identifiability concerns when sharing data, we have developed and validated a workflow that obscures (defaces) a subject's face in 3-dimensional volumetric data. Methods: To validate our method, we measured facial identifiability before and after defacing images from 30 healthy subjects who were imaged with both [18F]FDG PET and CT at either 3 or 6 time points. Briefly, facial embeddings were calculated using Google's FaceNet, and an analysis of clustering was used to estimate identifiability. Results: Faces rendered from CT images were correctly matched to CT scans at other time points at a rate of 93%, which decreased to 6% after defacing. Faces rendered from PET images were correctly matched to PET images at other time points at a maximum rate of 64% and to CT images at a maximum rate of 50%, both of which decreased to 7% after defacing. We further demonstrated that defaced CT images can be used for attenuation correction during PET reconstruction, introducing a maximum bias of -3.3% in regions of the cerebral cortex nearest the face. Conclusion: We believe that the proposed method provides a baseline of anonymity and discretion when sharing image data online or between institutions and will help to facilitate collaboration and future regulatory compliance.

Study of Cerenkov Light Emission in the Semiconductors TlBr and TlCl for TOF-PET.

Thallium bromide (TlBr) and thallium chloride (TlCl) are semiconductor materials with high transparency to visible light, high index of refraction, and high detection efficiency for gamma rays and annihilation photons. This manuscript reports on measurements of the light intensity and timing response of Cerenkov light emitted in one 3 mm × 3 mm × 5 mm slab of each of these materials operated in coincidence with a lutetium fine silicate (LFS) crystal with dimensions of 3 mm × 3 mm × 20 mm. A 22Na radioactive source was used. The measured average number of detected photons per event was 1.5 photons for TlBr and 2.8 photons for TlCl when these materials were coupled to a silicon photomultiplier. Simulation predicts these results with an overestimation of 12%. The best coincidence time resolution (CTR) for events in TlBr and TlCl were 329 ± 9 ps and 316 ± 9 ps, respectively, when events with 4 photons and >7 photons were selected. Simulation showed the CTR degraded from 120 ps to 405 ps in TlCl, and from 160 ps to 700 ps in TlBr when the first or second Cerenkov photon were selected. Results of this work show TlCl has a stronger Cerenkov light emission compared to TlBr and a greater potential to obtain the best timing measurements. Results also stress the importance of improving detection efficiency and transport of light to capture the first Cerenkov photon in timing measurements.

Optimization of a depth of interaction encoding PET block detector for a PET/MRI insert.

Preclinical positron emission tomography, combined with magnetic resonance imaging (PET/MRI), is increasingly used as a tool to simultaneously characterize functional processes in vivo. Many emerging preclinical applications, however, are limited by PET detection sensitivity, especially when generating short imaging frames for quantitative studies. One such application is dynamic multifunctional imaging, which probes multiple aspects of a biological process, using relationships between the datasets to quantify interactions. These studies have limited accuracy due to the relatively low sensitivity of modern preclinical PET/MRI systems. The goal of this project is to develop a preclinical PET/MRI insert with detection sensitivity above 15% (250-750 keV) to improve quantitation in dynamic PET imaging. To achieve this sensitivity, we have developed a detector module incorporating a 2 cm thick crystal block, which will be arranged into a system with 8 cm axial FOV, targeting mice and rats. To maintain homogenous spatial resolution, the detector will incorporate dual-ended depth-of-interaction (DOI) encoding with silicon photomultiplier (SiPM) based photodetector arrays. The specific aim of this work is to identify a detector configuration with adequate performance for the proposed system. We have optimized the SiPM array geometry and tested two crystal array materials with pitch ranging from 0.8 to 1.2 mm and various surface treatments and reflectors. From these configurations, we have identified the best balance between crystal separation, energy resolution, and DOI resolution. The final detector module uses two rectangular SiPM arrays with 5 × 6 and 5 × 4 elements. The photodetector arrays are coupled to a 19 × 19 array of 1 mm pitch LYSO crystals with polished surfaces and a diffuse reflector. The prototype design has 14.3% ± 2.9% energy resolution, 3.57 ± 0.88 mm DOI resolution, and resolves all elements in the crystal array, giving it sufficient performance to serve as the basis for the proposed high sensitivity PET/MRI insert.