Short TE and Low Flip Angle VIBE Sequence for Lung Evaluation in PET-MRI

PURPOSE Low-dose CT in standard PET/CT offers a better morphological lung characterization when compared with Dixon sequences in PET/MRI. Our aim is to improve diagnostic accuracy for lung lesions detection with PET/MRI avoiding a significant increase in the acquisition time, adding a short TE (Echo Time) and low FA (Flip Angle) T1-weighted sequence (gradient-echo volumetric interpolated breath-hold examination [VIBE]) to the standard acquisition protocol. METHOD AND MATERIALS We enrolled 21 oncological patients (11 M and 10 F) who underwent both thoracic CT scan and 3T PET-MRI (Siemens Biograph mMR) including lung dedicated short TE and low FA VIBE (inspiration, 3 mm slice thickness, axial plane). Time interval between the two examinations was < 2 weeks. VIBE sequences were retrospectively and independently evaluated by two radiologists for the detection of pulmonary nodules, parenchymal consolidations and dense bands; to assess inter-observer agreement Cohen's kappa coefficient (κ) was calculated. Results were then compared with CT scans findings, considered as the gold standard. Sensitivity and specificity were calculated. RESULTS VIBE sensitivity in lung nodules evaluation was 80% (8/10 patients) for nodules >5 mm and 57,1 % (4/7 patients) for nodules <= 5 mm, compared to CT imaging. In both cases specificity was 100%. All dense bands and parenchymal consolidation were found both with CT scan and VIBE. Inter-observer agreement was 95 % for nodules >5 mm (κ =0.90, p<0.001), 85% for nodules <= 5 mm (κ=0.44 p<0.005), 90% for dense bands (κ =0.56, p<0.001) and 100% for parenchymal consolidations. CONCLUSION VIBE showed high sensitivity in the evaluation of lung nodules > 5 mm; sensitivity was less satisfactory for smaller nodules. This sequence obtained also a very good inter-observer agreement, resulting a very reproducible imaging technique in pulmonary lesion investigation. Considering also the short acquisition time (15-18 s), it seems be reasonable to integrate PET/MRI protocols with a short TE and low FA VIBE, improving its diagnostic performance in lung evaluation. CLINICAL RELEVANCE/APPLICATION Short TE and low FA VIBE sequence can improve PET-MRI diagnostic performance in pulmonary lesions detection, without a significant increase in the acquisition time.