Optimization of fetal weight estimates using MRI: comparison of acquisitions

The purpose of this study was to determine whether differences are seen in calculation of fetal weight using 5-mm sagittal, 3-mm coronal, and 8-mm axial MRI acquisitions compared with term birth weight and contemporaneous sonography.

Fetal volume measurements were obtained from MRI acquisitions as follows: 5-mm sagittal (2 acquisitions), 3-mm coronal (2 acquisitions), and 8-mm axial (1 acquisition). A 90-sec single-shot fast spin-echo sequence was used. MRI and sonographic studies for fetal weight estimates were performed within 3 hr of term delivery. MRI calculation was based on the equation 0.12 + 1.031 x fetal volume (fetal area x slice thickness) (mL) = MRI fetal weight (kg). The sonographic fetal weight estimate was calculated using the Hadlock formula. MRI and sonographic calculations were compared with birth weight. Concordance coefficient analysis was performed.

Thirty-five retrospective fetal calculations were performed. Concordance coefficients, gram weight means and standard deviations (mean +/- SD) between birth weight and MRI acquisitions were as follows: 8-mm axial, 0.91 (3,554 +/- 431 g); 3-mm coronal, 0.84 (3,752 +/- 578 g); and 5-mm sagittal, 0.83 (3,685 +/- 567 g), compared with 0.78 (3,518 +/- 332 g) for sonography. The MRI axial concordance coefficient was significantly different from that of the sonographic estimates (p = 0.05). MRI axial concordance coefficient was not statistically different from that of the MRI coronal concordance coefficient (p = 0.22) or the MRI sagittal concordance coefficient (p = 0.19).

Calculated weights from a 90-sec single-shot fast spin-echo sequence MR acquisition with 8-mm-thick slices in the axial plane at term are better than sonographic estimates.