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Objective:
The goal of this study is to describe the technique of using three-dimensional (3D) slice-to-volume (SVR) magnetic resonance imaging (MRI) to visualize the fetal optic pathway (FOP) and create a nomogram of FOP measurements of normal fetuses ranging from 24 to 38 weeks gestational age. The FOP measurements include optic nerve width in the pre-chiasmatic segment bilaterally, optic chiasm width, and optic tract width bilaterally.

Materials and Methods:
In this single-site retrospective study, all fetal MRI scans done at Phoenix Children’s Hospital between 01/01/2020 and 08/01/2022 were reviewed, and those with sufficient image quality for 3D SVR image reconstruction were selected. A medical student reader first examined the unprocessed fetal brain MRI, either from balanced turbo-field-echo sequences or T2-weighted single-shot fast spin echo (T2 SSFSE) sequences, and attempted FOP measurements where feasible. 3D SVR reconstructions of the fetal brain images were then performed using a minimum of six T2 SSFSE imaging sequences. The reader then examined the reconstructed images and took FOP measurements. All FOP measurements were reviewed by two pediatric neuroradiologists with more than 8 years of reading experience. Nomograms were generated by using linear regression fit and Pearson correlation coefficient (r) to estimate the relationship between FOP measurements of normal fetuses and gestational age.

Results:
Seventy fetal MRI scans were selected for this study. Out of the 70 cases, the FOP was visualized in 9 cases in unprocessed fetal MRIs compared to 55 cases in 3D SVR images. Among the 55 3D SVR cases, pre-chiasmatic optic nerve width was successfully measured bilaterally in 53 cases, optic chiasm width in all 56 cases, and bilateral optic tract width in 30 cases. A linear regression fit estimated the relationship between optic chiasm width in normal fetuses and gestational age as OCW = 0.11 × GA+2.0 (R^2 = 0.30), where OCW is optic chiasm width (mm) and GA is gestational age (weeks). Similarly, the relationship between pre-chiasmatic optic nerve width and gestational age was estimated as PC = 0.04 × GA +0.24 (R^2 = 0.34), where PC is the pre-chiasmatic optic nerve width (mm). Further results are pending.

Conclusion:
Abnormal development of the FOP can lead to severe visual defects in adult life. Early detection of FOP defects is therefore crucial for improving patient outcomes. Prior studies have utilized transabdominal and transvaginal ultrasonography (US) to perform FOP measurements in normal fetuses during gestation. However, US-based techniques were often limited by drawbacks such as limited fetal presentation and low reproducibility. Similarly, conventional MRI sequences do not allow consistent visualization of the FOP due to low resolution (i.e., large slice thickness). 3D SVR MRI is a novel technique that allows precise delineation and measurement of the FOP. Although further research is required to establish the reliability of 3D SVR MRI, our preliminary results nevertheless demonstrate the promises and utility of this technique.