Evaluation of Median Root Prior for Robust In-Beam PET Reconstruction

Abstract

Dose delivery verification in proton beam radiotherapy is used to ensure the delivery of the dose to the correct location. A positron emission tomography (PET) scanner can be used to detect the secondary radiation during the treatment, so-called in-beam PET. This is a challenging application for PET due to the low counts and limited angular coverage. We propose a maximum a posteriori (MAP) reconstruction with median root prior (MRP) for the reconstruction of in-beam PET data. The proposed method was compared against MAP with total variation (TV) prior and maximum likelihood expectation maximization (MLEM), which have previously been used for this application. The effects of different ring configurations and time-of-flight information were tested with simulations of a geometrical phantom and a realistic patient treatment plan. The results indicate that both MAP methods produced sharper edges than MLEM, allowing more accurate edge localization in the reconstructed images. Even for the partial ring configurations, no elongation was observed with MAP methods. MAP-MRP successfully reduced the noise, whereas MAP-TV resulted in checkerboard artifacts. MAP-MRP was also more stable against the selection of the reconstruction parameters. In conclusion, MAP-MRP offers a simple and robust alternative for the reconstruction of in-beam PET data. Index Terms-List-mode (LM) positron emission tomography (PET), partial ring scanner, particle beam radiotherapy, proton beam radiotherapy, time-of-flight (TOF). I. INTRODUCTIONT HE BEAMS used in particle beam radiotherapy have a well-defined, finite penetration depth with high dose deposition close to the end of the beam's trajectory, the so-called Bragg peak. This enables treatments in which less healthy tissue is irradiated as compared to irradiation with photons, leading to a reduction in irradiation-induced complications. However, as a result of the Bragg peak, large dose deposition errors can occur if the actual treatment situation is different from the situation assumed during Manuscript