Investigating the limits of PET/CT imaging at very low true count rates in Ion-Beam Therapy monitoring

At the Heidelberg Ion-Beam Therapy Center (HIT), post-irradiation PET/CT imaging with a Biograph mCT scanner is used to verify in-vivo the treatment delivery. One main challenge is the extremely low number of true coincidences of 80 · 10³ - 1.3 · 10⁶. This work investigates the performance of the scanner in terms of precision and accuracy in activity quantification and shape recovery at varying counting statistics for different reconstruction methods and settings. To this aim, 30 subsequent PET scans of a cylindrical water phantom with a cylindrical FDG activity insert (emulating a sharp gradient for patient-like range verification) were performed at different statistical conditions, down to 40 · 10³ true counts at 98% random fraction (RF). The data analysis aimed at identifying the best performing reconstruction algorithm with optimal settings. Moreover, it addressed the influence of the high random background induced by the LSO detectors, by analysing different subsets of the data at fixed true numbers but different RFs. Additional simulations confirmed the experimental findings over an even wider range of true counts and RFs than accessible in the measurement and enabled to analyse the impact of scatter coincidences on the reconstructed images. The overall best results in terms of image noise, precise quantification and accurate geometrical recovery could be achieved with a small number (below 50) of MLEM-like iterations using the OSEM reconstruction method with time-of-flight (TOF) and point-spread function (PSF) information. In this condition, the LSO-related random background was found not to affect the above-mentioned image properties. The identified optimal reconstruction scheme applied to typical post-irradiation patient data-sets indeed resulted in improved image quality with respect to the typically used settings at HIT, which were based upon experience in nuclear medicine practice. These findings - an also impact other emerging PET applications at very low count rates.