UCLA

Background

Multiecho gradient-echo Cartesian MRI characterizes placental oxygenation by quantifying R2* . 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 placental R2* mapping at 3T and report placental R2* 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 radial R2* mapping accuracy compared to Cartesian in a phantom. Radial R2* mapping repeatability was characterized using the coefficient of repeatability (CR) between back-to-back scans. The mean and spatial coefficient of variation (CV) of R2* 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 mean R2* 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 ) R2* mapping in a phantom, and repeatable (CR≤4.6s^-1 ) R2* mapping in normal subjects. At 3T, placental R2* 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 mean R2* than normal subjects at both GA ranges (Z<-2).

Data conclusion

FB radial provides accurate and repeatable 3D R2* 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.