T2* relaxometry of fetal brain structures using low-field (0.55T) MRI

Abstract

Purpose: Human brain development during gestation is complex, as both structure and function are rapidly forming. Structural imaging methods using MRI are well developed to explore these changes, but functional imaging tools are lacking. Low-field MRI is a promising modality to bridge this gap. The longer intrinsic T₂* values at low field strengths increase the dynamic range and enable the quantification of individual brain regions with low T₂* values, such as deep gray matter. This study investigates regional brain T₂* quantification throughout the second half of gestation on low-field 0.55T MRI.

Methods: Dynamic multi-echo gradient-echo sequences were acquired in 135 cases at 0.55 T between 20 and 40 weeks' gestation. Automatic high-resolution reconstruction and segmentation tools were developed, resulting in T₂* values of seven individual anatomical brain structures for each subject. These regional brain T₂* values were analyzed throughout gestation.

Results: All regional fetal brain T₂* values decreased throughout gestation (p < 0.01). Each anatomical brain structure had varying ranges and decay rates, with the cerebellum and white matter displaying the highest (nonfluid structure) values, with the maximum values between 350 and 400 ms at about 20 weeks. The brainstem and deep gray matter had the lowest range of T₂* values, reaching values of 250 ms early in gestation. The matched volumetric assessment of the different structures demonstrated expected growth, matching current literature.

Conclusion: Low-field MRI allows for a detailed, regional T₂* analysis of the fetal brain, with more inclusive norms to be developed due to its wider bore.

Keywords: T2* relaxometry; fetal brain development; low‐field MRI.

FIGURE 1

Image processing pipeline. Multi‐echo gradient‐echo echo‐planar images (MEGE‐EPI) of the whole uterus were acquired, and the fetal brain was automatically masked. Multichannel SVRTK reconstruction was then performed, and the reconstruction was re‐oriented to standard space, followed by automatic segmentation using BOUNTI and T₂* map calculation.

FIGURE 2

Study population flow chart.

FIGURE 3

(A) Distribution of gestational age at the time of MRI scan. (B) Gestational age at birth. (C) Distribution of maternal body mass index (BMI). (D) Maternal age at the time of the MRI scan.

FIGURE 4

The first‐, second‐, and third‐echo brain reconstructions of fetuses with ventriculomegaly (VM), along with the T₂* maps calculated from the three echoes and the automatically generated label maps. GA, gestational age.

FIGURE 5

Fetal brain regional T₂* maps for a subject with ventriculomegaly and agenesis of the corpus callosum (left column), Trisomy 21 (T21; middle column), and pre‐eclampsia (right column). The T₂‐weighted reconstruction (top row), third‐echo reconstruction (middle row), and calculated T₂* map (bottom row) are displayed for each subject. ACC, agenesis of the corpus callosum; PE, pre‐eclampsia; VM, ventriculomegaly.

FIGURE 6

Fetal brain regional T₂* maps for a subject with a high maternal body mass index (BMI = 49.8 kg/m², 32.1 weeks' gestational age). Low‐field MRI is ideal for patients with a high BMI due to the larger bore width (80 cm, rather than the standard 60 cm), allowing for a more comfortable scanning experience.

FIGURE 7

Mean T₂* values for each anatomical brain structure across the gestational age (GA) range (external cerebrospinal fluid [eCSF; p < 0.01], gray matter [p < 0.01], white matter [p < 0.01], ventricles [p < 0.01], brainstem [p < 0.01], cerebellum [p < 0.01], deep gray matter [p < 0.01]). The fitting is performed only on the control fetuses. Mean T₂* decreases in each region throughout gestation and decreases at a higher rate later in gestation for most regions except the brainstem, which appears to decrease linearly throughout gestation. Blue, control fetuses; green, ventriculomegaly; yellow, preterm birth; purple, hypertension (chronic, postnatal, pregnancy‐induced); orange, gestational diabetes; red, all other pathologies.

FIGURE 8

Mean volumes for each anatomical brain structure (external cerebrospinal fluid [eCSF; p < 0.01], gray matter [p < 0.01], white matter [p < 0.01], ventricles [p = 0.02], brainstem [p < 0.01], cerebellum [p < 0.01], deep gray matter [p < 0.01]) across the gestational age range. The fitting is performed only on the control fetuses. Blue, control fetuses; green, ventriculomegaly; yellow, preterm birth; purple, hypertension (chronic, postnatal, pregnancy‐induced); orange, gestational diabetes; red, all other pathologies.