Estimation of MR-coil attenuation in the simultaneous PET/MR BrainPET

Objective: Simultaneous positron emission tomography/magnetic resonance (PET/MR) acquisition with the Siemens BrainPET employs a transmit/receive coil that fits concentrically within the system´s PET insert. The coil is transparent in MR but visible in the PET FOV and occupies space between the patient and PET detectors. This work develops an estimate of the coil position within the BrainPET FOV and provides an estimate of the coil attenuation correction factors. Methods: Ten Na-22 point sources were taped in various positions on the transmit/receive coil. PET scans of the coil were collected on three scanners: BrainPET, GE DST, and Siemens HRRT. Transmission scans were also collected, including CT, Ge-68 rod source data, and Cs-137 point source data. The emission data from the GE DST and HRRT were co-registered with the BrainPET emission data. The transformation matrices created from these registrations where used to transform respective scanner transmission μ-maps to BrainPET space. μ-map images were created and compared by smoothing to the DST resolution and creating joint histograms using the DST 511keV μ-map as the standard. Results: The 511keV and CT μ-maps showed a strong correlation between their attenuation values with the CT values slightly overestimating the 511keV values by a factor of 1.14. Agreement between the 511keV μ-map and Cs-137 μ-map was not as strong, likely due to missing features in the coil´s posterior portion in the Cs-137 μ-map. This occurred because there was inadequate sampling of the coil at the edge of the HRRT FOV. Conclusion: CT provides a good estimate of the BrainPET coil attenuation properties after minor adjustment to match the 511keV μ-map.