Prompt Gamma Correction for Ga-68 PSMA PET studies

Ga-68 Prostate Specific Membrane Antigen also known as PSMA is currently used in prostate cancer PET imaging. The resulting images show high uptakes in kidney and bladder which could produce a photopenic artifact (halo) and potentially mask tumor lesions or bone metastasis at the level of kidney or bladder. The measured contrasts between these organs and background could be as high as 200:1 and 50:1 for kidney and bladder respectively. The correct quantification in these areas requires precise scatter correction which needs to account for the effect of prompt gamma. Ga-68 has a prompt gamma at 1077 keV with a branching ratio of 3.2%. An unscattered prompt gamma ray of 1077 keV in the object has a small probability to be detected. An object scattered prompt gamma has a higher detection probability. When the contrast is low, more accurate quantification can be achieved. On the contrary, when the contrast is very high, halo artifact can be observed around high uptake organs. The purpose of this work is to evaluate the effect of Ga-68 prompt gamma in clinical PSMA studies. The halo artifact around kidney and bladder is strongly reduced by applying a Prompt Gamma Correction. Selected studies were performed on a Siemens mCT and acquired by Technische Universität München, Germany.