Subarachnoid Space Measurements in the Second Trimester Using MR Imaging

The subarachnoid space is an important component of the developing intracranial compartment. As fetal brain MRI is becoming more commonplace for early CNS disease diagnosis, it is imperative to determine age-based standards for normal subarachnoid space depth. We aim to provide 2D reference measurements of the supratentorial subarachnoid spaces in apparently healthy mid and later second-trimester fetuses.

In this cross-sectional study, we included all singleton fetal brain MRIs between gestational weeks 19 and 27 without visible pathologies. The frontal, insular, and inferior temporal subarachnoid space widths were measured from the inner calvaria to the brain surface bilaterally. Intraclass coefficients (ICCs) and Bland-Altman plots were utilized to evaluate agreement between 2 raters. Left- and right-side measurements were compared by using Wilcoxon tests. Quade tests were used to compare measurements between boys and girls. Generalized additive modeling for location, scale, and shape was used to create centile curves.

A total of 159 cases were included. ICC was highest (0.943) for the coronal plane insula width and lowest (0.667) for the coronal plane frontal width. Neither left-right (P > .573) nor male-female (P > .102) measurements were significantly different when considering age as a confounder; therefore, a single chart was created for each measurement.

Subarachnoid space depth adapts to the growing calvaria during the mid- to late second trimester. This study provides normal reference ranges for future clinical and research purposes.