JNM: PET/CT meets its match in PET/MR
The findings of the study could form the foundation for further research designed to investigate the added value of PET/MR over PET/CT, according to study authors Alexander Drzezga, MD, of the Technical University of Munich, and colleagues.
“This new technology bears the potential to repeat the success of PET/CT, particularly for oncologic indications, which are better addressed with MRI than with CT,” they wrote. “With regard to soft-tissue contrast, CT is known to be clearly inferior to MRI.”
The authors explained that alternative methods for combining PET and MRI data in the past—fusing data retrospectively using dedicated software registration algorithms or using separate, but adjacent, PET and MRI scanners—suffer from problems stemming from patient position and motion. However, replacing conventional photomultipliers with avalanche photodiodes, which are unaffected by strong magnetic fields, has allowed the integration of PET and MRI technology within a single machine.
One such scanner was installed at the Technical University of Munich in November 2010. To determine whether integrated whole-body PET/MR is feasible in a clinical setting, Drzezga and colleagues took 32 patients with different oncologic diagnoses and conducted a single-injection, dual-imaging protocol consisting of both a PET/CT and PET/MR scan. PET/CT scans were performed 86 minutes after injection of 401 MBq of 18F-FDG at two minutes per bed position. This acquisition was followed by a PET/MR scan 140 minutes after injection at four minutes per bed position.
PET images were reconstructed iteratively, with attenuation, scatter correction and regional allocation of PET findings performed using low-dose CT data for PET/CT. For PET/MR, the researchers used an MRI-based attenuation map generated on the basis of a two-point Dixon MRI sequence allowing the estimation of the distribution of different tissue types throughout the body.
The authors reported that PET/MR acquisition times using this protocol were short, clocking in at 20 minutes or fewer. Results showed no significant differences in lesion detection as 20 lesion-positive patients were identified with both modalities. A total of 80 suspicious lesions were detected with PET/CT, and 78 were detected with PET/MR, but this difference was not statistically significant. Quantitative comparisons of standardized uptake values (SUVs) demonstrated significant decreases from PET/CT data to PET/MR data. However, correlation analysis of SUVs in lesions and background tissue had high Spearman correlation coefficients of 0.93 and 0.92, respectively.
“These encouraging results indicate that relative proportions of tracer uptake in lesions and in the background are preserved in PET/MR, as compared with PET/CT, despite different technologies and different approaches for attenuation correction. This implies that the PET/MR scanner is suitable for quantitative evaluation (e.g., of a therapy response) in longitudinal studies but that care should be taken in comparing SUVs between PET/MR and other scanner types,” wrote the authors.
Drzezga and colleagues suggested that future studies of the added value of PET/MR compared with PET/CT should include various diagnostic MRI sequences for specific indications.