Characterizing T1 in the fetal brain and placenta over gestational age at 0.55T

Abstract

Purpose: T1 mapping and T1-weighted contrasts have a complimentary but currently under utilized role in fetal MRI. Emerging clinical low field scanners are ideally suited for fetal T1 mapping. The advantages are lower T1 values which results in higher efficiency and reduced field inhomogeneities resulting in a decreased requirement for specialist tools. In addition the increased bore size associated with low field scanners provides improved patient comfort and accessibility. This study aims to demonstrate the feasibility of fetal brain T1 mapping at 0.55T.

Methods: An efficient slice-shuffling inversion-recovery echo-planar imaging (EPI)-based T1-mapping and postprocessing was demonstrated for the fetal brain at 0.55T in a cohort of 38 fetal MRI scans. Robustness analysis was performed and placental measurements were taken for validation.

Results: High-quality T1 maps allowing the investigation of subregions in the brain were obtained and significant correlation with gestational age was demonstrated for fetal brain T1 maps ( ) as well as regions-of-interest in the deep gray matter and white matter.

Conclusions: Efficient, quantitative T1 mapping in the fetal brain was demonstrated on a clinical 0.55T MRI scanner, providing foundations for both future research and clinical applications including low-field specific T1-weighted acquisitions.

Keywords: fetal MRI; low cost; low‐field; relaxometry.

Figure 1. Processing pipeline from acquisition to T1 map.

Figure 2. Pulse Diagram for the T1 sequences employed.

(A) Inversion-recovery slice-shuffled single-shot Gradient Echo EPI with a nonselective adiabatic pulse. (B) The signal recovery curve illustrated over subsequent excitations.

Figure 3

Example at 32 + 6 weeks GA showing in (A) the contrasts obtained for an axial slice over different inversion times (TIs) (left to right and top to bottom) as well as in (B) the coronal and sagittal views of the first dynamic each slice at a different TI (top) and same views with the reordered slices, resulting in all slices at same TI (bottom).

Figure 4. T1 fetal brain results for a subject at 32 + 6 weeks GA.

(A) Individual data points across inversion time and their corresponding T1 fits for hemispheric white matter (blue) and deep grey matter (orange), without and with the denoising step. Selected voxels are shown in a colored box next to the plots. (B) Axial view of the whole image T1, inversion efficiency and proton density maps, from left to right.

Figure 5

Obtained high-resolution brain T1 maps in axial, coronal and sagittal orientation from (A) six control fetuses sorted by gestational age and (B) four clinical examples with ventriculomegaly, mid-line brain cyst, unilateral ventriculomegaly and enlarged cisterna magna from left to right and top to bottom are shown. The same scaling is used for all cases as indicated in (A). Most cases were acquired axial to the fetal brain, these acquired in tilted orientations were rotated to true radiological planes. Blue arrow points at white matter, green arrow to deep grey matter and red arrow to ventricles.

Figure 6

(A) Axial, (B) sagittal, and (C) coronal views from subsequent mid-brain slices in a brain T1 map from a healthy fetus images at 35 + 6 weeks gestational age using the high-resolution protocol. (A) Axial slices from ventral to dorsal; (B) Sagittal slices from left to right; (C) coronal from rostral to caudal.

Figure 7

Mean quantitative T1 results for (A) the whole brain and the selected brain regions (B,C) over gestational age. (B,C) also illustrate these selected regions in the upper right corner with a white outline. The blue dots denote the values from the control participants, red dots from clinical cases with fetal brain-related pathologies, the black lines are the regression line and dotted lines are the confidence interval for control participants. Finally, (D) A Bland–Altman plot for mean T1 for the fetal brain before and after the denoising step is given.