PET/MRI accurately registers significantly more than PET/CT

T1-weighted gradient-echo with radial stack-of-stars trajectory and T2-weighted PET/MR images have more accurate spatial registration than that of PET/CT images, according to a study published in the November issue of the American Journal of Roentenology.

Combined PET/CT is the current modality of choice for diagnosing cancer and evaluating treatment responses. While the imaging method provides higher spatial resolution and 3D views, the registration accuracy is vulnerable to errors due to mechanical misalignment, voluntary patient motion, and cardiac and respiratory motion between consecutive acquisitions, according to the study’s background information.

A newer modality, PET/MRI, adds superior anatomic resolution and soft-tissue contrast while lowering patient radiation exposure. However, “MRI motion artifacts due to cardiac, respiratory, and involuntary patient motion have been described. This may lead to the same technical limitations in registration in PET/MRI as in PET/CT,” wrote Rajan Rakheja, MD, of the NYU Langone Medical Center, and colleagues.

Rakheja and colleagues thus designed a study to compare PET/CT’s accuracy of special registration with that of PET/MRI in patients with FDG-avid, well-circumscribed metastatic lesions.

Thirteen patients with known metastatic lesions underwent FDG PET/CT followed by PET/MRI with a hybrid whole-body system. Tumor analysis was performed for spherical or oval FDG-avid tumors that were clearly visible with both CT and MRI.

The spatial coordinates of the estimated centers of the lesions were determined for PET/CT (PET and CT independently), PET/MRI (PET, T1-weighted gradient-echo sequence with radial stack-of-stars-trajectory, T2-weighted sequence), and the b0 images of an echo-planar imaging (EPI) diffusion-weighted imaging (DWI) acquisition. All MRI sequences were performed in the axial plane with free breathing.

The researchers measured the distance between the isocenter of the lesion on PET images and on the images obtained with the anatomic modalities. Misregistration was then calculated and compared between PET/CT and PET/MRI.

Nineteen lesions were evaluated in the study. The average total of misregistration for PET/CT images was 4.13 mm. For PET/MR images, lesion misregistration between PET and T1-weighted gradient-echo images had a shift of 2.41 mm. The misregistration between PET and b0 DW images was 5.97 mm. The paired Student t test found significant differences between the varying image types.

“The EPI-based b0 DWI datasets had significant misregistration compared with the PET/CT datasets, especially in the thorax,” wrote the study’s authors. “Radiologists reading PET/MR images obtained with EPI-based sequences should be aware of the potential for misregistration due to inherent spatial distortion associated with this type of MRI acquisition.”