Optimization of fetal brain MRI at 0.55 T with slice-to-volume reconstruction

Fetal brain MRI is extremely challenging due to motion and relies almost exclusively on 2D single-shot imaging with multi-stack acquisition. 3D slice-to-volume reconstruction (SVR) has been used to improve through-plane resolution and conspicuity and compensate for the fetal motion. Lower field strengths such as 0.55 T are emerging as an important tool for fetal evaluation due to improved safety and patient comfort, reduced cost, and potential for better access. This study aims to determine the impact of 0.55 T fetal brain MRI acquisition parameters on fetal brain SVR reconstruction performance and determine optimal settings.

We recruited nine healthy pregnant women (ten fetuses). Fetal brain imaging was performed at 0.55 T using T2-weighted half-Fourier acquisition single-shot turbo spin echo (HASTE) sequence in three orthogonal orientations. We acquired scans for four echo time (TE) values (98,140,181, and 272 ms) with 12 stacks (4 in each major orientation, fetal-axial, fetal-sagittal, and fetal-coronal). SVR was performed for 2 to 12 stacks for four the TE values; the results were evaluated qualitatively and quantitatively by a pediatric radiologist.

With increasing TE, WM-GM contrast improves, and WM and GM SNR efficiency decreases, as expected. As the number of stacks increases, the SNR, structural similarity, and normalized error of SVR reconstruction monotonically improve, as expected. A TE of 140 ms, coupled with six stacks, provided radiologist preferred contrast to noise, with the shortest possible scan time.

SVR is applicable to fetal brain MRI at 0.55 T and benefits from a longer TE and larger number of stacks than is typically used at 1.5 T or 3 T.