Assessment of artifacts in simultaneous PET and MR imaging

The authors have assessed the possibility of artifacts that can arise from performing simultaneous PET and MR imaging using a small prototype MR compatible PET scanner (McPET). The scanner consists of a single ring of 72, 2×2×5 mm³, LSO (lutetium oxyorthosilicate) crystals read out by 3 Phillips XP1722 multi-channel photomultiplier tubes (MC-PMT´s). The crystals are connected to the MC-PMT pixels using 4 meter long, 2 mm diameter double clad optical fibers, thus allowing the PET electronics to reside outside the main magnetic field. Experiments have been performed to critically examine any susceptibility artifacts or electromagnetic interference artifacts in the MR images caused by the PET system. The MR images were acquired by using a Siemens Vision 1.5 T clinical scanner. In addition, artifacts in the PET images caused by the operation of the PET scanner inside the MR system were investigated. Biological tissue (fruit) and a T2-weighted spin echo sequence (TR=2000 ms, TE=99 ms) were used to examine susceptibility artifacts due to McPET´s components (LSO, optical fibers) in the MR field of view. A uniform disk shaped phantom was imaged using a T1-weighted spin echo sequence (TR=280 ms, TE=12 ms) with and without the PET electronics switched on to determine electromagnetic interference effects. A range of MR pulse sequences were studied for any possible artifacts they might introduce in either the PET or MR images at different field strengths. The results demonstrate that it is possible to achieve artifact free PET and MR images simultaneously using the authors´ McPET scanner with several different conventional MR scanners