Reproducibility of MR-based Attenuation Maps in PET/MRI and the Impact on PET Quantification in Lung Cancer

AIM: Quantitative positron emission tomography/magnetic resonance imaging (PET/MRI) is dependent upon reliable and reproducible MR-based attenuation correction (MR-AC). In this study we evaluated the quality of current vendor-provided thoracic MR-AC maps, and further investigated the reproducibility of these in terms of their impact on fluorodeoxyglucose (FDG)-PET quantification in patients with non-small cell lung cancer (NSCLC). MATERIAL AND METHODS: Eleven patients with inoperable NSCLC underwent two to five thoracic PET/MRI scan-rescan examinations within 22 days. FDG-PET data were acquired along with two Dixon MR-AC maps for each examination. Two PET images (PETA and PETB) were reconstructed using identical PET emission data but with MR-AC from these intra-subject repeated attenuation maps. A total of 90 MR-AC maps were evaluated visually for quality and occurrence of categorized artifacts by two PET/MRI experienced physicians. Each tumor was outlined by a volume of interest (VOI; 40% isocontour of maximum) on PETA, which was then projected onto the corresponding PETB. Mean standardized uptake value (SUVmean) and maximum (SUVmax) were assessed from the PET images. Within-examination coefficients of variation and Bland-Altman analyses were conducted for the assessment of SUV variations between PETA and PETB. RESULTS: Image artifacts were observed in 86% of the MR-AC maps and 30% of the MR-AC maps were subjectively expected to affect the tumor SUV. SUVmean and SUVmax resulted in coefficients of variation of 5.6% and 6.6%, respectively, and scan-rescan SUV variations were within ±20% in 95% of the cases. Substantial SUV variations were mainly seen for scan-rescan examinations affected by respiratory motion. CONCLUSION: Artifacts occur frequently in standard thoracic MR-AC maps affecting the reproducibility of PET/MRI. This, in combination with other well-known sources of error associated with PET/MRI examinations, leads to inconsistent SUV measurements in serial studies and follow-up scenarios, which may affect the reliability of therapy response assessment. A thorough visual inspection of the thoracic MR-AC map and Dixon images from which it is derived remains crucial for the detection of MR-AC artifacts that may influence the reliability of SUV.