Reliability of MR Imaging-Based Posterior Fossa and Brain Stem Measurements in Open Spinal Dysraphism in the Era of Fetal Surgery

Abstract

Background and purpose: Fetal MR imaging is part of the comprehensive prenatal assessment of fetuses with open spinal dysraphism. We aimed to assess the reliability of brain stem and posterior fossa measurements; use the reliable measurements to characterize fetuses with open spinal dysraphism versus what can be observed in healthy age-matched controls; and document changes in those within 1 week after prenatal repair.

Materials and methods: Retrospective evaluation of 349 MR imaging examinations took place, including 274 in controls and 52 in fetuses with open spinal dysraphism, of whom 23 underwent prenatal repair and had additional early postoperative MR images. We evaluated measurements of the brain stem and the posterior fossa and the ventricular width in all populations for their reliability and differences between the groups.

Results: The transverse cerebellar diameter, cerebellar herniation level, clivus-supraocciput angle, transverse diameter of the posterior fossa, posterior fossa area, and ventricular width showed an acceptable intra- and interobserver reliability (intraclass correlation coefficient > 0.5). In fetuses with open spinal dysraphism, these measurements were significantly different from those of healthy fetuses (all with P < .0001). Furthermore, they also changed significantly (P value range = .01 to < .0001) within 1 week after the fetal operation with an evolution toward normal, most evident for the clivus-supraocciput angle (65.9 ± 12.5°; 76.6 ± 10.9; P < .0001) and cerebellar herniation level (-9.9 ± 4.2 mm; -0.7 ± 5.2; P < .0001).

Conclusions: In fetuses with open spinal dysraphism, brain stem measurements varied substantially between observers. However, measurements characterizing the posterior fossa could be reliably assessed and were significantly different from normal. Following a fetal operation, these deviations from normal values changed significantly within 1 week.

Fig 1.

T2-weighted imaging (HASTE) in the coronal (A) plane shows the biparietal diameter (white line in A) and the ventricular width (black line in A). In the sagittal plane (B–D), the mamillopontine distance (white line with circles at both ends in B) is demonstrated along with the foramen magnum diameter (dotted white line in B) and the cerebellar herniation level (white line perpendicular to the dotted line in B). The midsagittal posterior fossa area is shown in C (dotted free form), and the clivus-supraocciput angle is demonstrated in D with a line according to the clivus (white line) and the occiput (dotted line).

Fig 2.

The individual observations in the control population (black circles) with the mean (full line) and 95% confidence interval (dashed lines) compared with the preoperative fetuses with open spinal dysraphism (white triangles) for the posterior fossa area, ventricular width, transverse diameter of the posterior fossa, transverse cerebellar diameter, cerebellar herniation level, and clivus-supraocciput angle.

Fig 3.

Boxplot demonstrating the minimum, first quartile, median, third quartile, and maximum of the observed over expected ratio in preoperative fetuses with open spinal dysraphism and postoperative fetuses with OSD at 1 week for the posterior fossa area, ventricular width, transverse diameter of the posterior fossa, transverse cerebellar diameter, cerebellar herniation level, and clivus-supraocciput angle.