Myelin characterization of fetal brain with mono-point estimated T1-maps

In vivo T2 measurements of the fetal brain using single-shot fast spin echo sequences

PURPOSE

We propose a quantitative framework for motion-corrected T2 fetal brain measurements in vivo and validate the single-shot fast spin echo (SS-FSE) sequence to perform these measurements.

METHODS

Stacks of two-dimensional SS-FSE slices are acquired with different echo times (TE) and motion-corrected with slice-to-volume reconstruction (SVR). The quantitative T2 maps are obtained by a fit to a dictionary of simulated signals. The sequence is selected using simulated experiments on a numerical phantom and validated on a physical phantom scanned on a 1.5T system. In vivo quantitative T2 maps are obtained for five fetuses with gestational ages (GA) 21-35 weeks on the same 1.5T system.

RESULTS

The simulated experiments suggested that a TE of 400 ms combined with the clinically utilized TEs of 80 and 180 ms were most suitable for T2 measurements in the fetal brain. The validation on the physical phantom confirmed that the SS-FSE T2 measurements match the gold standard multi-echo spin echo measurements. We measured average T2s of around 200 and 280 ms in the fetal brain grey and white matter, respectively. This was slightly higher than fetal T2* and the neonatal T2 obtained from previous studies.

CONCLUSION

The motion-corrected SS-FSE acquisitions with varying TEs offer a promising practical framework for quantitative T2 measurements of the moving fetus.

Preliminary Study on Quantitative Assessment of the Fetal Brain Using MOLLI T1 Mapping Sequence

BACKGROUND

Prenatal quantitative evaluation of myelin is important. However, few techniques are suitable for the quantitative evaluation of fetal myelination.

PURPOSE

To optimize a modified Look-Locker inversion recovery (MOLLI) T1 mapping sequence for fetal brain development study.

STUDY TYPE

Prospective observational preliminary cohort study.

POPULATION

A total of 71 women with normal fetuses divided into mid-pregnancy (gestational age 24-28 weeks, N = 25) and late pregnancy (gestational age > 28 weeks, N = 46) groups.

FIELD STRENGTH/SEQUENCE

A 3 T/MOLLI sequence.

ASSESSMENT

T1 values were measured in pedunculus cerebri, basal ganglia, thalamus, posterior limb of the internal capsule, temporal white matter, occipital white matter, frontal white matter, and parietal white matter by two radiologists (11 and 16 years of experience, respectively).

STATISTICAL TESTS

The Kruskal-Wallis test was used for reginal comparison. For each region of interest (ROI), differences in T1 values between the mid and late pregnancy groups were assessed by the Mann Whitney U test. Pearson correlation coefficients (r) were used to evaluate the correlations between T1 values and gestational age for each ROI. Intraobserver and interobserver agreement was determined by the intraclass correlation coefficient (ICC). A P value <0.05 was considered statistically significant.

RESULTS

Interobserver and intraobserver agreements of T1 were good for all ROIs (all ICCs > 0.700). There were significant differences in T1 values between lobal white matter and deep regions, respectively. Significant T1 values differences were found between middle and late pregnancy groups in pedunculus cerebri, basal ganglion, thalamus, posterior limb of the internal capsule, temporal, and occipital white matter. The T1 values showed significantly negative correlations with gestational weeks in pedunculus cerebri (r = -0.80), basal ganglion (r = -0.60), thalamus (r = -0.68), and posterior limb of the internal capsule (r = -0.77).

DATA CONCLUSION

The T1 values of fetal brain may be assessed using the MOLLI sequence and may reflect the myelination.

EVIDENCE LEVEL

3 TECHNICAL EFFICACY: Stage 2.

Fetal brain tissue characterization at 1.5 T using STrategically Acquired Gradient Echo (STAGE) imaging

OBJECTIVES

To estimate human fetal brain MRI tissue properties including apparent T₁ (T_1app) and apparent proton density (PD_app) by using a rapid multi-contrast acquisition protocol called STrategically Acquired Gradient Echo (STAGE) imaging.

METHODS

STAGE data were collected using two flip angles (15° and 60°, with a TR = 600 ms) for 30 pregnant women at 1.5 T (15 healthy controls: gestational age (GA) range 19 + 1/7 weeks to 34 + 5/7 weeks; 11 abnormal subjects with ventriculomegaly: GA range 21 + 5/7 weeks to 31 + 5/7 weeks; 4 subjects with other abnormalities). Both T_1app and PD_app maps of the fetal brain were calculated from the STAGE data. A region-of-interest-based approach was used to measure T_1app and PD_app in the subplate/intermediate zone (SP/IZ), cortical plate (CP), and cerebrospinal fluid (CSF) in the fetal brain.

RESULTS

The ratios of T_1appSP/IZ/T_1appCP and PD_appSP/IZ/PD_appCP were larger than unity while T_1appSP/IZ/T_1appCSF and PD_appSP/IZ/PD_appCSF were both less than unity.

CONCLUSIONS

STAGE imaging provides a potential practical approach to estimate multi-parametric properties of the human fetal brain.

KEY POINTS

• STAGE is feasible in measuring fetal brain tissue properties. • Water content in cortical plate and subplate/intermediate zone approaches that of cerebrospinal fluid in early gestational ages.

3D in utero quantification of T2* relaxation times in human fetal brain tissues for age optimized structural and functional MRI

PURPOSE

Maximization of the blood oxygen level-dependent (BOLD) functional MRI (fMRI) contrast requires the echo time of the MR sequence to match the T2* value of the tissue of interest, which is expected to be higher in the fetal brain compared with the brain of a child or an adult.

METHODS

T2* values of the cortical plate/cortical gray matter tissue in utero in healthy fetuses from mid-gestation onward (20-36 gestational weeks) were measured using 3D T2* maps calculated from 2D dual-echo T2*-weighted data corrected for between-slice motion and reconstructed in 1.0 mm³ isotropic resolution from a sequence of multiple time points, together with 1.0 mm³ isotropic resolution T2-weighted structural data.

RESULTS

Mean T2* relaxation times of the cortical tissue were about twice as high as those reported previously in adults. In a supporting experiment applying single seed analysis, default mode and auditory networks appeared better localized and less noisy while using an echo time of 100 ms versus 43 ms. The results of the previous study reporting a trend for T2* values to decrease with fetal age were reproduced and extended to include cortical tissues and subjects in earlier gestation (20-26 gestational weeks).

CONCLUSION

The first measurement of T2* values in fetal cortical tissues suggested the appropriate echo time range for fetal BOLD fMRI protocol optimization to be 130-190 ms. Magn Reson Med 78:909-916, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

T2* relaxometry of fetal brain at 1.5 Tesla using a motion tolerant method

PURPOSE

The aim of this study was to determine T2* values for the fetal brain in utero and to compare them with previously reported values in preterm and term neonates. Knowledge of T2* may be useful for assessing brain development, brain abnormalities, and for optimizing functional imaging studies.

METHODS

Maternal respiration and unpredictable fetal motion mean that conventional multishot acquisition techniques used in adult T2* relaxometry studies are not practical. Single shot multiecho echo planar imaging was used as a rapid method for measuring fetal T2* by effectively freezing intra-slice motion.

RESULTS

T2* determined from a sample of 24 subjects correlated negatively with gestational age with mean values of 220 ms (±45) for frontal white matter, 159 ms (±32) for thalamic gray matter, and 236 ms (±45) for occipital white matter.

CONCLUSION

Fetal T2* values are higher than those previously reported for preterm neonates and decline with a consistent trend across gestational age. The data suggest that longer than usual echo times or direct T2* measurement should be considered when performing fetal fMRI to reach optimal BOLD sensitivity.

Fetal magnetic resonance imaging at 3.0 T

Magnetic resonance imaging (MRI) has been used to image the in utero fetus for the past 3 decades. Although not as commonplace as other patient-oriented MRI, it is a growing field and demonstrating a role in the clinical care of the fetus. Indeed, the body of literature involving fetal MRI exceeds 3000 published articles. Indeed, there is interest in accessing even the healthy fetus with MRI to further understand the development of humans during the fetal stage. On the horizon is fetal imaging using 3.0-T clinical systems. Although a clear path is not necessarily determined, experiments, theoretical calculations, advances in pulse sequence design, new hardware, and experience from imaging at 1.5 T help define the path.