Letter to the editor re: molecular imaging in oncology: the acceptance of PET/CT and the emergence of MR/PET imaging

Recently, Schiepers et al. [1] have illuminated the emerging field of MR/PET imaging and its expected great potential. We are in agreement with the authors that a lower radiation exposure, a reliable image registration, constant physiological conditions, multifunctional imaging possibilities and a higher patient through-put are the major advantages of simultaneous MR/PET imaging. To illustrate simultaneous MR/PET imaging in the field of oncology we have examined a 70-year old patient with histologically confirmed diffuse large B-cell lymphoma in the left tibia. Before therapy onset with R-CHOP a baseline whole-body [F]-FDG-PET/CT (Biograph 16; Siemens Healthcare, Germany) acquiring 9 bed positions (3 min/bed) (Fig. 1a-c) and a subsequent (approx. 2 h after tracer injection) MR/PET (30 min acquisition time) (Fig. 2a-c) of the tibia were performed. The MR/PET system consists of an MRI-compatible PET system (BrainPET; Siemens Healthcare, USA) based on avalanche photo diodes (APDs) and LSO scintillation crystals that slip-fits into a modified 3 T whole-body MRI system (Magnetom Tim Trio; Siemens Healthcare, Germany). The sensitivity of the MR/PET is 7.4%, approximately three times higher than the PET/CT. Additionally, the spatial resolution in the center of the FOV is <2.5 mm compared to 4.5 mm in the PET/CT. For the BrainPET acquisition, the attenuation correction was calculated using tissue specific thresholds on T1weighted MR images. According to MRI (Fig. 2a), both medullary (abnormal signal intensity on all sequences as well as increased focal contrast enhancement) and extramedullary manifestations of lymphoma could be clearly diagnosed in the resulting images. On low-dose CT, the soft tissue mass was only vaguely visualized (Fig. 1a). While the clinical [F]-FDGPET disclosed only extramedullary lymphoma (Fig. 1b), [F]-FDG-BrainPET also revealed intramedullary manifestations (Fig. 2b) probably due to a longer FDG uptake and consecutively higher SNR. Three months later a therapy response evaluation was performed with the same protocol revealing resolution of the extramedullary lymphoma on T1-weighted post-gadolinium sequence, but residual contrast enhancement in the adjacent bone marrow (Fig. 4a). A. Sauter :N. Schwenzer : C. Pfannenberg : C. Claussen : M. Horger Department of Diagnostic and Interventional Radiology, University Hospital of Tuebingen, Tuebingen, Germany