3D R 2 * mapping of the placenta during early gestation using free‐breathing multiecho stack‐of‐radial MRI at 3T

Background Multiecho gradient‐echo Cartesian MRI characterizes placental oxygenation by quantifyingR 2 * . Previous research was performed at 1.5T using breath‐held 2D imaging during later gestational age (GA). Purpose To evaluate the accuracy and repeatability of a free‐breathing (FB) 3D multiecho gradient‐echo stack‐of‐radial technique (radial) for placentalR 2 *mapping at 3T and report placentalR 2 *during early GA. Study Type Prospective. Population Thirty subjects with normal pregnancies and three subjects with ischemic placental disease (IPD) were scanned twice: between 14–18 and 19–23 weeks GA. Field Strength 3T. Sequence FB radial. Assessment Linear correlation (concordance coefficient, ρ c ) and Bland–Altman analyses (mean difference, MD) were performed to evaluate radialR 2 *mapping accuracy compared to Cartesian in a phantom. RadialR 2 *mapping repeatability was characterized using the coefficient of repeatability (CR) between back‐to‐back scans. The mean and spatial coefficient of variation (CV) ofR 2 *was determined for all subjects, and separately for anterior and posterior placentas, at each GA range. Statistical Tests ρ c was tested for significance. Differences in meanR 2 *and CV were tested using Wilcoxon Signed‐Rank and Rank‐Sum tests. P < 0.05 was considered significant. Z‐scores for the IPD subjects were determined. Results FB radial demonstrated accurate (ρ c ≥0.996; P < 0.001; |MD|<0.2s −1 ) and repeatable (CR<4s −1 )R 2 *mapping in a phantom, and repeatable (CR≤4.6s −1 )R 2 *mapping in normal subjects. At 3T, placentalR 2 *mean ± standard deviation was 12.9s −1  ± 2.7s −1 for 14–18 and 13.2s −1  ± 1.9s −1 for 19–23 weeks GA. The CV was significantly greater ( P  = 0.043) at 14–18 (0.63 ± 0.12) than 19–23 (0.58 ± 0.13) weeks GA. At 19–23 weeks, the CV was significantly lower ( P < 0.001) for anterior (0.49 ± 0.08) than posterior (0.67 ± 0.11) placentas. One IPD subject had a lower meanR 2 *than normal subjects at both GA ranges (Z<–2). Data Conclusion FB radial provides accurate and repeatable 3DR 2 *mapping for the entire placenta at 3T during early GA. Level of Evidence : 2 Technical Efficacy : Stage 1 J. Magn. Reson. Imaging 2019;49:291–303.