3D VISUALISATION AND VOLUMETRIC ASSESSMENT OF THE FETAL LARGE BOWEL ACROSS GESTATION USING MRI: A PILOT...

Fetal bowel remains challenging to assess antenatally. Ultrasound assessment is limited to its echogenicity and luminal diameter. Although MRI has been used to visualise fetal bowel, uncertainties remain and reference ranges for 2D diameter of large bowel have only recently been reported. Advanced MRI techniques now facilitate motion-corrected 3D analysis of fetal abdominal structures. This study aimed to apply these techniques to fetal large bowel enabling generation of high-resolution datasets for volumetric visualisation and quantification.

All participants underwent fetal MRI on a 3T system, with T2-weighted images acquired in multiple orthogonal planes. Motion-correction pipelines were applied to obtain 3D reconstruction of fetal abdomen, and manual segmentation of five large bowel regions was performed to generate a 3D reconstruction of large bowel facilitating volumetric measurements. To assess reproducibility, intra- and inter-observer variability were performed using intraclass correlation coefficient (ICC). Linear regression was used to assess relationship between large bowel volume and gestation.

Forty datasets from fetuses between 16 and 38 weeks' gestation were analysed. The whole fetal large bowel was identified in 100% of datasets. Intra- and inter-observer agreement were good to excellent. Volume increased linearly with advancing gestational age in all five large bowel segments. Model-derived estimates of total large bowel volume increased from 1266 cm³ at 18 weeks' gestation to 13,550 cm³ at 38 weeks (p<0.001).

This study demonstrates 3D reconstruction and volumetric assessment of fetal large bowel is a feasible and reproducible technique in all large bowel regions. This technique may improve understanding of normal fetal bowel development enabling antenatal identification of pathologies, currently rarely detected antenatally.