Measuring the fetal brain using 3D volume in utero MRI

Ultrafast in utero magnetic resonance imaging (iuMRI)can be used to measure the volume of the intracranial compartment of fetuses. This exam is performed when ultrasound suggests the presence of fetal structural brain abnormalities. Measurements of normal fetuses at gestational ages of 19 through 36 weeks have been published as a freely accessible reference in the July 2019 issue of European Radiology.

Radiologists from the University of Sheffield in the United Kingdom reviewed 200 fetuses whose ante-natal ultrasound exams showed no brain or somatic abnormalities and whose parents had no history of developmental abnormalities. Subjects were enrolled prospectively and had an iuMRI performed on the same 1.5T whole body scanner. At least six fetuses were imaged for every gestational week age (with up to 20 fetuses at week 29).

Standard clinical iuMRI protocol was used to acquire two-dimensional (2D) images in all three orthogonal planes and to acquire three-dimensional (3D) data sets in the axial plan using a balanced steady-state imaging sequence. This sequence of 18 to 22 seconds permitted acquisition of the entire fetal brain during maternal suspended respiration. All brain scans were reported as normal by an experienced pediatric neuroradiologist.

MRI researcher, radiographer, and lead author Deborah A. Jarvis took linear measurements of skull sizes on 2D images reconstructed to the formal orthogonal planes from the 3D datasets. She also measured the bi-parietal diameter in the axial plane and occipito-frontal diameter from the outer table to outer table of the skull. The authors manually segmented the 3D volume imaging to delineate ventricular volume, brain parenchymal volume, extra-axial volume, and total intra-cranial volume. They determined that the linear skull measurements they made on in utero MRI closely correlated with published data from ultrasonography.

The authors hope their freely-accessible measurements will provide reference data in situations where iuMRI scans are ordered to diagnose suspected brain abnormalities.

“Linear measurements of the skull may be useful up to a point, but it seems intuitively correct that accurate measurement of the intracranial contents will improve diagnosis of fetal neuro-pathologies,” wrote the authors. “Ultrafast 3D MR imaging permits the volume of intracranial compartments to be measured after post-processing. Defining normality is the first stage in qualifying abnormalities.”

The authors did not confirm neurodevelopment outcomes in this study, but they intend to do so in future research. This will also allow comparisons of male and female fetal brain measurements.

REFERENCE

1. Jarvis DA, Finney CR, Griffiths PD. Normative volume measurements of the fetal intra-cranial compartments using 3D volume in utero MR imaging. Eur Radiol. 2019;29(7):3488-3495.