Effects of MR coils on PET artifacts and quantification for multimodality PET/CT/MR scans

Objectives The goal of this work is to investigate the effects of MRI surface coils on attenuation-corrected PET emission data. A method for registering MR-PET-CT scans is to transport the subject on a stereotacticly registered table between the scanners. A challenge to this approach is the attenuation correction (AC) in the presence of MR coils, particularly for CT-based AC (CTAC), as MR coils contain materials difficult to image and measure AC coefficients using CT. Also, combined MR/PET scanners may not visualize MR coils for attenuation correction purposes.

Methods PET/CT images were acquired of uniform and anthropomorphic phantoms. We performed four scans: 1) MR coil present in the PET data, but not the CTAC; 2) full coils present in both PET and CT; 3) only bottom half of a split body coil present in PET and CT, the top was removed without moving the object; 4) no coil present in either scan.

Results Artifacts were seen in PET images when the coil is ignored in AC. Including the coil in the CTAC scan eliminated these artifacts. Quantitative accuracy was significantly affected: the activity was underestimated by 19% when the CTAC did not include the head coil, which agreed with simulation results. Activity was overestimated by 25% when the CTAC included the head coil in the measurements. Metal components in full coils caused artifacts in CT and CTAC images. These artifacts were significantly reduced when half of the coil was removed for the CT and PET scans.

Conclusions The presence of full MR coils during PET or PET/CT scanning can cause artifacts and quantification errors. Alternative CT techniques that mitigate metal-artifacts should be used to improve AC accuracy. When possible, removing segments of the coil prior to the PET/CT exam is recommended. Further, MR coils could be redesigned to reduce artifacts by rearranging placement of the most attenuating materials