Effects of age-related breathing characteristics on the performance of four-dimensional magnetic resonance imaging reconstructed by prospective gating for radiation therapy planning

Background and purpose Four-dimensional magnetic resonance imaging (4D MRI) has advanced recently by incorporating prospective gating, but its performance on pediatric populations has not been investigated. This study aimed to determine the age-related performance of prospective gating, as compared with retrospective sorting. Materials and methods Prospectively gated 4D MRI scans were acquired on a motion phantom driven by real respiratory waveforms obtained from 23 pediatric and young adult patients (aged 5–24 years). The correlations between patient-specific breathing characteristics and the performance of 4D MRI were comparatively evaluated against retrospective sorting for the same scan time. For six patients who underwent both 4D MRI and 4D CT, the internal target volumes (ITVs) determined by the two modalities were compared. Results Longer scan time and greater sorting error were most highly correlated (P < 0.001) with breathing irregularity and extent of diaphragm motion, but age was not a strong covariate because of interindividual variation. Prospective gating was more accurate than retrospective sorting except for those patients with severe breathing irregularity (peak-to-peak coefficient of variation >30%). The ITVs of 4D MRI and 4D CT were comparable (Dice similarity: >90%) unless the breathing characteristics changed between the two imaging sessions. Conclusions For most patients analyzed in this study, prospective gating provided more accurate 4D MRI (95th percentile of deviation: <1.5 mm) than did retrospective sorting within a clinically feasible scan time (median: 5.9 min). The 4D MRI tended to take longer and to give larger sorting errors with deeper and irregular breathers.