A Quantitative Evaluation of Joint Activity and Attenuation Reconstruction in TOF-PET/MR Brain Imaging.

Time of flight (TOF) positron emission tomography (PET) data provide an effective means for attenuation correction (AC) when no (or incomplete/inaccurate) attenuation information is available. Since magnetic resonance (MR) scanners provide little information on photon attenuation of different tissue types, AC in hybrid PET/MR scanners has always been challenging. In this contribution, we aim at validating the activity reconstructions of the maximum likelihood activity and attenuation reconstruction (MLAA) algorithm on a patient brain dataset. We present a quantitative comparison of the joint reconstructions with the current clinical gold-standard maximum likelihood expectation maximization (MLEM) reconstruction using computed tomography (CT) based AC in PET/CT as well as the current state-of-the-art in PET/MR, i.e. zero time echo (ZTE) based AC. Methods: The TOF-PET emission data were initially used in a preprocessing stage to estimate crystal maps of efficiencies, TOF-offsets and TOF-resolutions. Applying these additional corrections during reconstructions, atlas-based, ZTE-based and MLAA attenuation correction techniques were analyzed. In our initial study, we used the CT-based estimate of the expected scatter, and later used the ZTE-based and MLAA attenuation estimates to compute the expected scatter contribution of the data during reconstructions. In all reconstructions, a maximum likelihood (ML) scaling of the single scatter simulation estimate to the emission data was used for scatter correction. The reconstruction results were analyzed in 86 segmented regions of interest (ROIs) of the Hammers' atlas. Results: Our quantitative analysis shows that in practice a tracer activity difference of + 0.5% (±2.1%) and + 0.1% (±2.3%) could be expected for the state-of-the-art ZTE-based and MLAA AC methods, respectively in PET/MR compared to the clinical gold-standard in PET/CT. Conclusion: Joint activity and attenuation estimation methods can provide an effective solution to the challenging AC problem for brain studies in hybrid TOF-PET/MR scanners. With an accurate TOF-based (offsets and resolutions) calibration, and similar to the results of the state-of-the-art method in PET/MR, regional errors of joint TOF-PET reconstructions are within a few percent.