A diffusion-weighted template for gestational age-related apparent diffusion coefficient values in the developing fetal brain

Diffusion tensor imaging of fetal brain: principles, potential and limitations of promising technique

Human brain development is a complex process that begins in the third week of gestation. During early development, the fetal brain undergoes dynamic morphological changes. These changes result from events such as neurogenesis, neuronal migration, synapse formation, axonal growth and myelination. Disruption of any of these processes is thought to be responsible for a wide array of different pathologies. Recent advances in magnetic resonance imaging, especially diffusion-weighted imaging and diffusion tensor imaging (DTI), have enabled characterization and evaluation of brain development in utero. In this review, aimed at practitioners involved in fetal medicine and high-risk pregnancies, we provide a comprehensive overview of fetal DTI studies focusing on characterization of early normal brain development as well as evaluation of brain pathology in utero. We also discuss the reliability and limitations of fetal brain DTI. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.

Postmortem Diffusion-Weighted Magnetic Resonance Imaging of the Brain in Perinatal Death: An Animal Control Study to Detect the Influence of Postmortem Interval

OBJECTIVES

Diffusion-weighted imaging may be useful as part of a postmortem magnetic resonance imaging protocol. However, apart from the effect of temperature on apparent diffusion coefficient (ADC), normal postmortem ADC changes can influence the interpretation. Therefore, this study was conducted to evaluate the correlation between normal ADC changes and postmortem intervals (PMIs) and develop a reference standard for postmortem changes after temperature correction.

MATERIALS AND METHODS

Six premature lambs were scanned at different PMIs. ADC values were measured at different parenchymal locations. Correlation and linear regression between ADC values and PMI were analyzed for all locations, both uncorrected and corrected for temperature.

RESULTS

All locations showed a significant negative correlation between the PMI and ADC value, with (R2 = 0.581-0.837, P < 0.001) and without (R2 = 0.183-0.555, P < 0.001-0.018) temperature correction.

CONCLUSIONS

The postmortem interval is negatively correlated with ADC values in the brain. A correlation coefficient for the PMI can be calculated after temperature correction to predict ADC changes. However, further research is required to evaluate its clinical application in humans.

Anatomical and diffusion-weighted imaging of brain abnormalities in third-trimester fetuses with cytomegalovirus infection

OBJECTIVES

In this study of cytomegalovirus (CMV)-infected fetuses with first-trimester seroconversion, we aimed to evaluate the detection of brain abnormalities using magnetic resonance imaging (MRI) and neurosonography (NSG) in the third trimester, and compare the grading systems of the two modalities. We also evaluated the feasibility of routine use of diffusion-weighted imaging (DWI) fetal MRI and compared the regional apparent diffusion coefficient (ADC) values between CMV-infected fetuses and presumed normal, non-infected fetuses in the third trimester.

METHODS

This was a retrospective review of MRI and NSG scans in fetuses with confirmed first-trimester CMV infection performed between September 2015 and August 2019. Brain abnormalities were recorded and graded using fetal MRI and NSG grading systems to compare the two modalities. To investigate feasibility of DWI, a four-point rating scale (poor, suboptimal, good, excellent) was applied to assess the quality of the images. Quantitative assessment was performed by placing a freehand drawn region of interest in the white matter of the frontal, parietal, temporal and occipital lobes and the basal ganglia, pons and cerebellum to calculate ADC values. Regional ADC measurements were obtained similarly in a control group of fetuses with negative maternal CMV serology in the first trimester, normal brain findings on fetal MRI and normal genetic testing.

RESULTS

Fifty-three MRI examinations of 46 fetuses with confirmed first-trimester CMV infection were included. NSG detected 24 of 27 temporal cysts seen on MRI scans, with a sensitivity of 78% and an accuracy of 83%. NSG did not detect abnormal gyration visible on two (4%) MRI scans. Periventricular calcifications were detected on two MRI scans compared with 10 NSG scans. While lenticulostriate vasculopathy was detected on 11 (21%) NSG scans, no fetus demonstrated this finding on MRI. MRI grading correlated significantly with NSG grading of brain abnormalities (P < 0.0001). Eight (15%) of the DWI scans in the CMV cohort were excluded from further analysis because of insufficient quality. The ADC values of CMV-infected fetuses were significantly increased in the frontal (both sides, P < 0.0001), temporal (both sides, P < 0.0001), parietal (left side, P = 0.0378 and right side, P = 0.0014) and occipital (left side, P = 0.0002 and right side, P < 0.0001) lobes and decreased in the pons (P = 0.0085) when compared with non-infected fetuses. The ADC values in the basal ganglia and the cerebellum were not significantly different in CMV-infected fetuses compared with normal controls (all P > 0.05). Temporal and frontal ADC values were higher in CMV-infected fetuses with more severe brain abnormalities compared to fetuses with mild abnormalities.

CONCLUSIONS

Ultrasound and MRI are complementary during the third trimester in the assessment of brain abnormalities in CMV-infected fetuses, with a significant correlation between the grading systems of the two modalities. On DWI in the third trimester, the ADC values in several brain regions are abnormal in CMV-infected fetuses compared with normal controls. Furthermore, they seem to correlate in the temporal area and, to a lesser extent, frontal area with the severity of brain abnormalities associated with CMV infection. Larger prospective studies are needed for further investigation of the microscopic nature of diffusion abnormalities and correlation of different imaging findings with postnatal outcome. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.

Early imaging predictors of fetal cerebral ischemic injury in monochorionic twin pregnancy complicated by spontaneous single intrauterine death

OBJECTIVE

Monochorionic twin pregnancies are at increased risk of single intrauterine death (sIUD) and subsequent brain injury in the surviving twin owing to shared placentation. We assessed the association between middle cerebral artery peak systolic velocity (MCA-PSV) and cerebral injury on magnetic resonance imaging (MRI) and examined the association between cerebral findings on diffusion-weighted imaging (DWI) and those on T2-weighted imaging following spontaneous sIUD.

METHODS

This was a retrospective cohort study of monochorionic pregnancies complicated by spontaneous sIUD followed at a tertiary center between January 2008 and January 2020. Pregnancies with sIUD following laser treatment, those with selective feticide, double IUD occurring on the same day or sIUD before 14 weeks' gestation were excluded, as were cases in which MCA-PSV was not measured or DWI-MRI was not performed. The ability of MCA-PSV Doppler to predict subsequent cerebral injury on MRI was assessed, and DWI findings were analyzed and compared with those on susceptibility-weighted imaging (SWI) and T2-weighted MRI to determine its diagnostic accuracy.

RESULTS

We assessed 64 monochorionic pregnancies complicated by spontaneous sIUD. Of these, 47 (73.4%) pregnancies underwent fetal brain MRI and met the inclusion criteria. Sixteen (34.0%) of these fetuses demonstrated cerebral injury on MRI. The median interval between the diagnosis of sIUD and MRI examination was 5 days. Fetuses with increased MCA-PSV > 1.5 multiples of the median (MoM) following sIUD were significantly more likely to demonstrate cerebral injury on MRI than were those with normal MCA-PSV (68.8% vs 38.7%; P = 0.05). The sensitivity and specificity of MCA-PSV > 1.5 MoM for predicting cerebral injury on MRI were 68.8% (95% CI, 41.3-88.9%) and 61.3% (95% CI, 42.2-78.2%), respectively. Patterns of early cerebral injury on T2-weighted and SWI-MRI included acute or subacute tissue swelling (n = 6), parenchymal atrophy (n = 7), loss of cortical ribbon (n = 1) and hemorrhage (n = 8). Early MRI within approximately 2 weeks after the diagnosis of sIUD demonstrated abnormal DWI along with coexisting SWI and T2-weighted sequelae in 56.3% (9/16) of cases. When DWI was normal and a second MRI examination was performed later (n = 7), there were no ischemic changes evident on T2-weighted imaging.

CONCLUSIONS

Increased MCA-PSV is associated with, but predicts poorly, cerebral injury after sIUD. Early MRI with DWI within approximately 2 weeks after the diagnosis of sIUD is valuable in identifying any cerebral injury. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Imaging diagnosis and legal implications of brain injury in survivors following single intrauterine fetal demise from monochorionic twins - a review of the literature

Brain injury of the surviving twin from monochorionic pregnancies following intrauterine fetal demise during the second and third trimesters is a rare but severe complication. Monochorionicity and gestational age at the time of stillbirth seem to be decisive factors in terms of long-term neurologic outcome prediction for the survivor. Magnetic resonance imaging (MRI), diffusion weighted imaging (DWI) in particular, seem to bring the earliest and most accurate diagnosis. Ultrasound detection of brain damage is possible in later stages of fetal brain injury. It is essential to provide early diagnosis and multidisciplinary counsel to the parents to ensure informed decision making. For couples who choose to terminate pregnancy legislation related to late abortion might lead to further distress. Our paper aims to stress the importance of MRI DWI in the evaluation of surviving twins following single intrauterine fetal demise in monochorionic pregnancies and the delicate context of the medical professionals and parents facing this clinical situation, sometimes complicated by legal constraints.

Brain fetal magnetic resonance imaging to evaluate maturation of normal white matter during the third trimester of pregnancy

BACKGROUND

Quantitative magnetic resonance imaging (MRI) could improve the estimation of fetal brain maturation and the interpretation of white matter signal intensity in pathological conditions.

OBJECTIVE

To investigate T2-based and diffusion-weighted imaging (DWI) measurements for the evaluation of fetal brain maturation during the last trimester of pregnancy.

MATERIALS AND METHODS

One hundred sixty-eight fetal brain MRIs were retrospectively analyzed (age range: 28-37 weeks of gestation) after ensuring that none of the children developed psychomotor or cognitive impairment (median follow-up: 4.7 years). Bilateral regions of interest were drawn on the frontal, occipital, parietal and temporal lobes from T2-W imaging and DWI, when available, to evaluate signal intensity and apparent diffusion coefficient (ADC) values. Ratios were calculated with two references (pons or thalamus and cerebrospinal fluid) to standardize signal intensities. Reproducibility was evaluated with intraclass correlation coefficients (ICCs) and Bland-Altman plots. Correlations with gestational age were evaluated with univariate and multivariate linear regressions.

RESULTS

T2 measurements were achieved in all cases, and DWI was available in 37 cases. Measurements and ratios were reproducible in eight localizations (i.e. intra- and interobserver ICCs >0.5): frontal T2/thalamus, parietal T2/thalamus, occipital T2/pons, parietal ADC/thalamus, occipital ADC/pons, temporal ADC/pons, occipital ADC and temporal ADC. The frontal T2/thalamus and parietal T2/thalamus correlated with gestational age (P<0.0001 and P=0.014, respectively). In the multivariate modeling, frontal T2/thalamus remained an independent predictor of the gestational age (P<0.0001).

CONCLUSION

The frontal T2/thalamus ratio emerged as a potential additional biomarker of fetal brain maturation during the last trimester of pregnancy.

Voxelwise and Regional Brain Apparent Diffusion Coefficient Changes on MRI from Birth to 6 Years of Age

Background A framework for understanding rapid diffusion changes from 0 to 6 years of age is important in the detection of neurodevelopmental disorders. Purpose To quantify patterns of normal apparent diffusion coefficient (ADC) development from 0 to 6 years of age. Materials and Methods Previously constructed age-specific ADC atlases from 201 healthy full-term children (108 male; age range, 0-6 years) with MRI scans acquired from 2006 to 2013 at one large academic hospital were analyzed to quantify four patterns: ADC trajectory, rate of ADC change, age of ADC maturation, and hemispheric asymmetries of maturation ages. Patterns were quantified in whole-brain, segmented regional, and voxelwise levels by fitting a two-term exponential model. Hemispheric asymmetries in ADC maturation ages were assessed using t tests with Bonferroni correction. Results The posterior limb of the internal capsule (mean ADC: left hemisphere, 1.18 ×10³μm²/sec; right hemisphere, 1.17 ×10³μm²/sec), anterior limb of the internal capsule (left, 1.11 ×10³μm²/sec; right, 1.09 ×10³μm²/sec), vermis (1.26 ×10³μm²/sec), thalami (left, 1.17 ×10³μm²/sec; right, 1.15 ×10³μm²/sec), and basal ganglia (left, 1.26 ×10³μm²/sec; right, 1.23 ×10³μm²/sec) demonstrate low initial ADC values, indicating an earlier prenatal time course of development. ADC maturation was completed between 1.3 and 2.4 years of age, depending on the region. The vermis and left thalamus matured earliest (1.3 years). The frontolateral gray matter matured latest (right, 2.3 years; left, 2.4 years). ADC maturation occurred earlier in the left hemisphere (P < .001) in several regions, including the frontal (mean ± standard deviation) (left, 2.16 years ± 0.29; right, 2.19 years ± 0.31), temporal (left, 1.93 years ± 0.22; right, 1.99 years ± 0.22), and parietal (left, 1.92 years ± 0.30; right, 2.03 years ± 0.28) white matter. Maturation occurred earlier in the right hemisphere (P < .001) in several regions, including the thalami (left, 1.63 years ± 0.32; right, 1.45 years ± 0.33), basal ganglia (left, 1.79 years ± 0.31; right, 1.70 years ± 0.37), and hippocampi (left, 1.93 years ± 0.34; right, 1.78 years ± 0.33). Conclusion Normative apparent diffusion coefficient developmental patterns on diffusion-weighted MRI scans were quantified in children aged 0 to 6 years. This work provides knowledge about early brain development and may guide the detection of abnormal patterns of maturation. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Rollins in this issue.

Prognostic value of diffusion-weighted magnetic resonance imaging of brain in fetal growth restriction: results of prospective multicenter study

OBJECTIVE

To measure prospectively apparent diffusion coefficient (ADC) values between 28 and 32 weeks of gestation in different cerebral territories of fetuses with estimated fetal weight (EFW) ≤ 5^th centile, and analyze their association with adverse perinatal outcome.

METHODS

This was a prospective study involving six tertiary-level perinatal centers. In the period 22 November 2016 to 11 September 2017, we included singleton, small-for-gestational-age (SGA) fetuses with EFW ≤ 5^th percentile, between 28 and 32 weeks of gestation, regardless of the umbilical artery Doppler and maternal uterine artery Doppler findings. A fetal magnetic resonance imaging (MRI) examination with diffusion-weighted sequences (DWI) was performed within 14 days following inclusion and before 32 weeks. ADC values were calculated in the frontal and occipital white matter, basal ganglia and cerebellar hemispheres. An ultrasound examination was performed within 1 week prior to the MRI examination. The primary outcome was a composite measure of adverse perinatal outcome, defined as any of the following: perinatal death; admission to neonatal intensive care unit with mechanical ventilation > 48 h; necrotizing enterocolitis; Grade III-IV intraventricular hemorrhage; periventricular leukomalacia. A univariate comparison of median ADC values in all cerebral territories between fetuses with and those without adverse perinatal outcome was performed. The association between ADC values and adverse perinatal outcome was then analyzed using multilevel logistic regression models to adjust for other common prognostic factors for growth-restricted fetuses.

RESULTS

MRI was performed in 64 patients, of whom five were excluded owing to fetal movement artifacts on DWI and two were excluded for termination of pregnancy with no link to fetal growth restriction (FGR). One intrauterine death occurred secondary to severe FGR. Among the 56 liveborn neonates, delivered at a mean ± SD gestational age of 33.6 ± 3.0 weeks, with a mean birth weight of 1441 ± 566 g, four neonatal deaths occurred. In addition, two neonates required prolonged mechanical ventilation, one of whom also developed necrotizing enterocolitis. Overall, therefore, seven out of 57 (12.3%) cases had an adverse perinatal outcome (95% CI, 3.8-20.8%). The ADC values in the frontal region were significantly lower in the group with adverse perinatal outcome vs those in the group with favorable outcome (mean values of both hemispheres, 1.68 vs 1.78 × 10^-3  mm² /s; P = 0.04). No significant difference in ADC values was observed between the two groups in any other cerebral territory. A cut-off value of 1.70 × 10^-3  mm² /s was associated with a sensitivity of 57% (95% CI, 18-90%), a specificity of 78% (95% CI, 63-88%), a positive predictive value of 27% (95% CI, 8-55%) and a negative predictive value of 93% (95% CI, 80-98%) for the prediction of adverse perinatal outcome. A mean frontal ADC value < 1.70 × 10^-3  mm² /s was not associated significantly with an increased risk of adverse perinatal outcome, either in the univariate analysis (P = 0.07), or when adjusting for gestational age at MRI and fetal sex (odds ratio (OR), 6.06 (95% CI, 0.9-37.1), P = 0.051) or for umbilical artery Doppler (OR, 6.08 (95% CI, 0.89-41.44)).

CONCLUSION

This first prospective, multicenter, cohort study using DWI in the setting of SGA found lower ADC values in the frontal white-matter territory in fetuses with, compared with those without, adverse perinatal outcome. To determine the prognostic value of these changes, further standardized evaluation of the neurodevelopment of children born with growth restriction is required. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

Apparent diffusion coefficient of different areas of brain in foetuses with intrauterine growth restriction

Purpose

This study aimed to compare the apparent diffusion coefficient (ADC) values of different brain areas between two groups of intrauterine growth restricted (IUGR) foetuses and control cases.

Material and methods

A total of 38 foetuses with IUGR and 18 normal control foetuses with similar gestational age were compared using a 3T magnetic resonance scanner. IUGR cases included 23 foetuses with clinical severity signs (group A) and 15 foetuses without clinical severity signs (group B). ADC values were measured in different brain regions and compared among groups. Foetuses with structural brain abnormalities were excluded from the study.

Results

All foetuses had normal foetal structural brain anatomy. Head circumference (HC) < 5% was more common in IUGR group A compared to IUGR group B (56.5% vs. 13.3%, p < 0.0001). In comparison to the normal group, the ADC values in IUGR foetuses were significantly lower in cerebellar hemispheres (CH) (1.239 vs. 1.280.5 × 10^-3 mm²/s, p = 0.045), thalami (1.205 vs. 1.285 × 10^-3 mm²/s, p = 0.031) and caudate nucleus (CN) (1.319 vs. 1.394 × 10^-3 mm²/s, p = 0.04). However, there were no significant differences in ADC values between IUGR subtypes. Among all brain regions, pons had the lowest ADC values.

Conclusions

ADC values of thalami, CN, and CH were significantly lower in IUGR than control foetuses, while there was no significant difference among IUGR groups. Further studies are needed to evaluate the prognostic value of ADC changes in IUGR foetuses.

The Influence of Various Cerebral and Extracerebral Pathologies on Apparent Diffusion Coefficient Values in the Fetal Brain

BACKGROUND AND PURPOSE

The changing MRI signal accompanying brain maturation in fetal brains can be quantified on apparent diffusion coefficient (ADC) maps. Deviations from the natural course of ADC values may reflect structural pathology. The purpose of this study was to determine the influence of fetal pathologies on the ADC values in different regions of the fetal brain and their evolution with increasing gestational age.

METHODS

This was a retrospective study of 291 fetuses evaluated between the 14th and the 40th week of gestation using diffusion‐weighted imaging (DWI). Fetuses with normal MRI findings but sonographically suspected pathology or fetuses with abnormalities not affecting the brain were analyzed in the control group and compared to fetuses suffering from different pathologies like hydrocephalus/ventriculomegaly, brain malformations, infections, ischemia/hemorrhage, diaphragmatic hernias, and congenital heart disease. Pairwise ADC measurements in each side of the white matter (WM) of the frontal, parietal, and occipital lobes, in the basal ganglia and the cerebellum, as well as a single measurement in the pons were performed and were plotted against gestational age.

RESULTS

In the control group, brain maturation followed a defined gradient, resulting in lower ADC values in the most mature regions. Each disorder group experienced abnormal patterns of evolution of the ADC values over time deviating from the expected course.

CONCLUSIONS

The ADC values in different regions of the fetal brain and their evolution with increasing gestational age are influenced by pathologies compromising the cerebral maturation.

Fetal brain development at 25-39 weeks gestational age: A preliminary study using intravoxel incoherent motion diffusion-weighted imaging

BACKGROUND

The fetal brain developmental changes of water diffusivity and perfusion has not been extensively explored.

PURPOSE/HYPOTHESIS

To evaluate the fetal brain developmental changes of water diffusivity and perfusion using intravoxel incoherent motion diffusion-weighted imaging (IVIM-DWI).

STUDY TYPE

Prospective.

POPULATION

Seventy-nine normal singleton fetuses were scanned without sedation of healthy pregnant women.

FIELD STRENGTH/SEQUENCE

5 T MRI/T₁ /₂ -weighted image and IVIM-DWI.

ASSESSMENT

Pure diffusion coefficient (D), pseudodiffusion coefficient (D*), and perfusion fraction (f) values were calculated in the frontal (FWM), temporal (TWM), parietal (PWM), and occipital white matter (OWM) as well as cerebellar hemisphere (CH), basal ganglia region (BGR), thalamus (TH), and pons using an IVIM model.

STATISTICAL TESTS

One-way analysis of variable (ANOVA) followed by Bonferroni post-hoc multiple comparison was employed to reveal the difference of IVIM parameters among the investigated brain regions. The linear and the nonlinear polynomial regression analyses were utilized to reveal the correlation between gestational age (GA) and IVIM parameters.

RESULTS

There were significant differences in both D (F(7,623) = 96.64, P = 0.000) and f values (F(7,623) = 2.361, P = 0.0219), but not D* values among the varied brain regions. D values from TWM (r²  = 0.1402, P = 0.0002), PWM (r²  = 0.2245, P = 0.0002), OWM (r²  = 0.2519, P = 0.0002), CH (r²  = 0.2245, P = 0.0002), BGR (r²  = 0.3393, P = 0.0001), TH (r²  = 0.1259, P = 0.0001), and D* value from pons (r²  = 0.2206, P = 0.0002) were significantly correlated with GA using linear regression analysis. Quadratic regression analysis led to results similar to those using the linear regression model.

DATA CONCLUSION

IVIM-DWI parameters may indicate fetal brain developmental alterations but the conclusion is far from reached due to the not as high-powered correlation between IVIM parameters and GA.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;50:899-909.

Direct comparison between apparent diffusion coefficient and macromolecular proton fraction as quantitative biomarkers of the human fetal brain maturation

BACKGROUND

Apparent diffusion coefficient (ADC) is known as a quantitative biomarker of prenatal brain maturation. Fast macromolecular proton fraction (MPF) mapping is an emerging method for quantitative assessment of myelination that was recently adapted to fetal MRI.

PURPOSE

To compare the capability of ADC and MPF to quantify the normal fetal brain development.

STUDY TYPE

Prospective.

POPULATION

Forty-two human fetuses in utero (gestational age [GA] = 27.7 ± 6.0, range 20-38 weeks).

FIELD STRENGTH/SEQUENCE

1.5 T; diffusion-weighted single-shot echo-planar spin-echo with five b-values for ADC mapping; spoiled multishot echo-planar gradient-echo with T₁ , proton density, and magnetization transfer contrast weightings for single-point MPF mapping.

ASSESSMENT

Two operators measured ADC and MPF in the medulla, pons, cerebellum, thalamus, and frontal, occipital, and temporal cerebral white matter (WM).

STATISTICAL TESTS

Mixed repeated-measures analysis of variance (ANOVA) with the factors of pregnancy trimester and brain structure; Pearson correlation coefficient (r); Hotelling-Williams test to compare strengths of correlations.

RESULTS

From the 2^nd to 3^rd trimester, ADC significantly decreased in the thalamus and cerebellum (P < 0.005). MPF significantly increased in the medulla, pons, thalamus, and cerebellum (P < 0.005). Cerebral WM had significantly higher ADC and lower MPF compared with the medulla and pons in both trimesters. MPF (r range 0.83, 0.89, P < 0.001) and ADC (r range -0.43, -0.75, P ≤ 0.004) significantly correlated with GA and each other (r range -0.32, -0.60, P ≤ 0.04) in the medulla, pons, thalamus, and cerebellum. No significant correlations or distinctions between regions and trimesters were observed for cerebral WM (P range 0.1-0.75). Correlations with GA were significantly stronger for MPF compared with ADC in the medulla, pons, and cerebellum (Hotelling-Williams test, P < 0.003) and similar in the thalamus. Structure-averaged MPF and ADC values strongly correlated (r = 0.95, P < 0.001).

DATA CONCLUSION

MPF and ADC demonstrated qualitatively similar but quantitatively different spatiotemporal patterns. MPF appeared more sensitive to changes in the brain structures with prenatal onset of myelination.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;50:52-61.

Current state of MRI of the fetal brain in utero

In this article we provide an overview of fetal brain development, describe the range of more common fetal neuropathology, and discuss the relevance of in utero MR (iuMR). Although ultrasonography remains the mainstay of fetal brain imaging, iuMR imaging is both feasible and safe, but presents several challenges. We discuss those challenges, the techniques employed to overcome them, and new approaches that may extend the clinical applicability of fetal iuMR. Level of Evidence: Technical Efficacy Stage. J. Magn. Reson. Imaging 2019;49:632-646.

Diffusion-weighted magnetic resonance imaging of the fetal brain in intrauterine growth restriction

OBJECTIVE

Diffusion-weighted magnetic resonance imaging (DWI) is a sensitive method for assessing brain maturation and detecting brain lesions, providing apparent diffusion coefficient (ADC) values as a measure of water diffusion. Abnormal ADC values are seen in ischemic brain lesions, such as those associated with acute or chronic hypoxia. The aim of this study was to assess whether ADC values in the fetal brain were different in fetuses with severe intrauterine growth restriction (IUGR) compared with normal controls.

METHODS

Brain magnetic resonance imaging (MRI) with single-shot axial DWI (b = 0 and b = 700 s/mm² ) was performed in 30 fetuses with severe IUGR (estimated fetal weight < 3^rd centile with absent or reversed umbilical artery Doppler flow) and in 24 normal controls of similar gestational age. Brain morphology and biometry were analyzed. ADC values were measured in frontal and occipital white matter, centrum semiovale, thalami, cerebellar hemisphere and pons. Frontal-occipital and frontal-cerebellar ADC ratios were calculated, and values were compared between IUGR fetuses and controls.

RESULTS

There was no difference in gestational age at MRI between IUGR and control fetuses (IUGR, 30.2 ± 1.6 weeks vs controls, 30.7 ± 1.4 weeks). Fetal brain morphology and signals were normal in all fetuses. Brain dimensions (supratentorial ± infratentorial) were decreased (Z-score, < -2) in 20 (66.7%) IUGR fetuses. Compared with controls, IUGR fetuses had significantly lower ADC values in frontal white matter (1.97 ± 0.23 vs 2.17 ± 0.22 × 10^-3 mm² /s; P < 0.0001), thalami (1.04 ± 0.15 vs 1.13 ± 0.10 ×10^-3 mm² /s; P = 0.0002), centrum semiovale (1.86 ± 0.22 vs 1.97 ± 0.23 ×10^-3 mm² /s; P = 0.01) and pons (0.85 ± 0.19 vs 0.94 ± 0.12 ×10^-3 mm² /s; P = 0.043). IUGR fetuses had a lower frontal-occipital ADC ratio than did normal fetuses (1.00 ± 0.11 vs 1.08 ± 0.05; P = 0.003).

CONCLUSIONS

ADC values in IUGR fetuses were significantly lower than in normal controls in the frontal white matter, thalami, centrum semiovale and pons, suggesting abnormal maturation in these regions. However, the prognostic value of these ADC changes is still unknown. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

Brain Development in Fetuses of Mothers with Diabetes: A Case-Control MR Imaging Study

BACKGROUND AND PURPOSE

Offspring exposed to maternal diabetes are at increased risk of neurocognitive impairment, but its origins are unknown. With MR imaging, we investigated the feasibility of comprehensive assessment of brain metabolism (¹H-MRS), microstructure (DWI), and macrostructure (structural MRI) in third-trimester fetuses in women with diabetes and determined normal ranges for the MR imaging parameters measured.

MATERIALS AND METHODS

Women with singleton pregnancies with diabetes (n = 26) and healthy controls (n = 26) were recruited prospectively for MR imaging studies between 34 and 38 weeks' gestation.

RESULTS

Data suitable for postprocessing were obtained from 79%, 71%, and 46% of women for ¹H-MRS, DWI, and structural MRI, respectively. There was no difference in the NAA/Cho and NAA/Cr ratios (mean [SD]) in the fetal brain in women with diabetes compared with controls (1.74 [0.79] versus 1.79 [0.64], P = .81; and 0.78 [0.28] versus 0.94 [0.36], P = .12, respectively), but the Cho/Cr ratio was marginally lower (0.46 [0.11] versus 0.53 [0.10], P = .04). There was no difference in mean [SD] anterior white, posterior white, and deep gray matter ADC between patients and controls (1.16 [0.12] versus 1.16 [0.08], P = .96; 1.54 [0.16] versus 1.59 [0.20], P = .56; and 1.49 [0.23] versus 1.52 [0.23], P = .89, respectively) or volume of the cerebrum (243.0 mL [22.7 mL] versus 253.8 mL [31.6 mL], P = .38).

CONCLUSIONS

Acquiring multimodal MR imaging of the fetal brain at 3T from pregnant women with diabetes is feasible. Further study of fetal brain metabolism in maternal diabetes is warranted.

Region-specific reductions in brain apparent diffusion coefficient in cytomegalovirus-infected fetuses

OBJECTIVE

To evaluate the effects of cytomegalovirus (CMV) infection on apparent diffusion coefficient (ADC) values of the fetal brain in utero.

METHODS

In this retrospective analysis we compared 58 fetal head magnetic resonance imaging (fhMRI) scans of PCR-verified CMV-infected fetuses, obtained in 2008-2012, with those of a normal control group of 36 gestational age (GA)-matched uninfected fetuses scanned between 2006 and 2012. Estimated GA at infection ranged from 1 to 32 weeks, and fhMRI was performed at 24 to 38 weeks. The frontal, parietal, temporal and occipital lobes (mainly white matter), basal ganglia, thalamus, pons and cerebellum were analyzed by assessing ADC values. Two pregnancies were terminated and postmortem confirmation was available in these cases.

RESULTS

ADC values of CMV-infected fetuses correlated significantly and negatively with GA in all brain regions except the basal ganglia. The cerebellum had the greatest reduction (r = -0.52, P < 0.0001). Maternal age correlated positively with ADC in the frontal lobe (P < 0.05). GA at infection and overt pathological changes did not affect ADC significantly. Compared with non-infected fetuses, ADC values of affected fetuses were significantly reduced in the frontal (P < 0.0001), parietal (P < 0.0001), occipital (P = 0.0005) and temporal (P = 0.001) lobes and thalamus (P = 0.006).

CONCLUSION

CMV infection of the fetal brain results in a highly significant, region-dependent reduction of ADC values in the frontal, parietal, occipital and temporal lobes and thalamus, probably reflecting hypercellularity and inclusion bodies in damaged areas. Further studies are needed to determine if reduction in ADC values may serve as a prognostic factor in CMV-infected fetuses. Copyright © 2014 ISUOG. Published by John Wiley & Sons Ltd.

Surface reconstructions of foetal brain abnormalities using ultrafast steady state 3D acquisitions

MRI of the foetal brain in utero is performed in routine clinical practice using sequences that produce two-dimensional (2D) images. Recent developments in image post-processing have allowed the construction of three-dimensional (3D) volume data sets from 2D images acquired in different anatomical planes, but these have limitations due to the unpredictable nature of foetal movement. These limitations have been overcome by development of several different advanced computer techniques, which require specialist knowledge, software, and processing methods, which are rarely available in routine clinical settings. Our aim was to develop a technique that can be used in routine clinical situations without the need for custom-developed or expensive software by utilizing MRI sequences that can produce a 3D data set in "ultrafast" timescales. The 3D dataset, combined with versatile image post-processing and visualization techniques, has resulted in the production of high-resolution images of foetal brain surfaces in utero. The aim of this paper is to demonstrate our methods and early results by way of a pictorial review illustrating a range of developmental brain disease in utero.

Multi-tiered analysis of brain injury in neonates with congenital heart disease

Early brain injury occurs in newborns with congenital heart disease (CHD) placing them at risk for impaired neurodevelopmental outcomes. Predictors for preoperative brain injury have not been well described in CHD newborns. This study aimed to analyze, retrospectively, brain magnetic resonance imaging (MRI) in a heterogeneous group of newborns who had CHD surgery during the first month of life using a detailed qualitative CHD MRI Injury Score, quantitative imaging assessments (regional apparent diffusion coefficient [ADC] values and brain volumes), and clinical characteristics. Seventy-three newborns who had CHD surgery at 8 ± 5 (mean ± SD) days of life and preoperative brain MRI were included; 38 also had postoperative MRI. Thirty-four (34 of 73, 47 %) had at least one type of preoperative brain injury, and 28 of 38 (74 %) had postoperative brain injury. The 5-min APGAR score was negatively associated with preoperative injury, but there was no difference between CHD types. Infants with intraparenchymal hemorrhage, deep gray matter injury, and/or watershed infarcts had the highest CHD MRI Injury Scores. ADC values and brain volumes were not different in infants with different CHD types or in those with and without brain injury. In a mixed group of CHD newborns, brain injury was found preoperatively on MRI in almost 50 %, and there were no significant baseline characteristic differences to predict this early brain injury except 5-min APGAR score. We conclude that all infants, regardless of CHD type, who require early surgery should be evaluated with MRI because they are all at high risk for brain injury.

Development and validation of a semiquantitative brain maturation score on fetal MR images: initial results

PURPOSE

To develop a valid, reliable, and simple-to-use semiquantitative visual scale of fetal brain maturation for use in clinical fetal MR imaging assessment and interpretation.

MATERIALS AND METHODS

This is a retrospective assessment of data from a previous study that was prospective, institutional review board approved, and HIPAA compliant. Forty-eight normal pregnancies with a gestational age (GA) of 25 to 35 weeks were studied. A fetal total maturation score (fTMS) was developed by utilizing six subscores that evaluated cortical sulcation, myelination, and the germinal matrix and provided a single combined numerical value to be evaluated as a marker of brain maturity. The fTMS was correlated with GA and segmented brain volume. A regression model that associated GA based on the visual fTMS scoring was determined. The model was validated with a leave-one-out cross validation procedure.

RESULTS

Mean GA was 29.3 weeks ± 2.9 (standard deviation) (range, 25.2-35.3 weeks) and mean fTMS was 8.6 ± 3.7 (range, 4-16). The intraclass correlation coefficient between the three readers (independent and blinded) was 0.948 (P < .001). The correlations were as follows: GA and brain volume, r = 0.964 (P < .001); fTMS and brain volume, r = 0.970 (P < .001); and GA and fTMS, r = 0.975 (P < .001). A regression model to calculate GA based on fTMS yielded the following equation: calculated GA (weeks) = 22.86 + 0.748 fTMS (P < .001; adjusted R(2) = 0.946). The standard error of the model for calculation of fetal GA from the visual fTMS scale was ± 4.8 days.

CONCLUSION

If validated further, the fTMS scale might be used to assess morphologic brain maturity of fetuses between 25 and 35 weeks GA on a single-case basis in a clinical setting.

The brain in the belly: what and how of fetal neuroimaging?

This work reviews magnetic resonance imaging in the developing human brain. It focuses on fetal brain imaged in vivo and in utero with complementary sections on abnormalities seen in clinical settings, and on potential of diffusion tensor imaging and of proton magnetic resonance spectroscopy. The main purposes are to illustrate the normal fetal developing brain and its abnormalities commonly encountered in utero, and to emphasize the potential role of adjunct techniques such as diffusion imaging and spectroscopy that may help elucidate fetal brain maturation and its abnormalities.

Magnetic resonance diffusion-weighted imaging: reproducibility of regional apparent diffusion coefficients for the normal fetal brain

OBJECTIVE

To evaluate the reproducibility of regional apparent diffusion coefficient (ADC) measurements of the normal fetal brain in the second and third trimesters of pregnancy.

METHODS

Fifty normal singleton fetuses from healthy pregnant women between 19 and 37 weeks' gestation were studied without sedation. Single-shot diffusion-weighted images of the fetal brain were obtained using a 1.5-Tesla magnetic resonance scanner and a six-channel body array coil. ADC maps were created using 0 and 1000 b-values along three orthogonal directions. Two examiners independently measured ADC values in the cerebellar hemispheres (CH), pons, thalamus, basal ganglia (BG), centrum semiovale (CSO), and frontal (FWM), parietal (PWM), temporal (TWM) and occipital (OWM) white matter. Correlation between ADC values and menstrual age was assessed by linear regression analysis. The bias and agreement of ADC measurements were determined using Bland-Altman plots.

RESULTS

ADC values either remained constant (BG, FWM, PWM, TWM, OWM, CSO) or decreased (CH, pons, thalamus) with advancing menstrual age. Mean intraobserver bias for ADC measurements was not significantly different from zero. Small interobserver differences in mean ADC measurements (i.e. a small mean bias) were detected for CH (1.26 ± 0.20 vs 1.20 ± 0.18 μm(2) /ms, P = 0.006), PWM (1.37 ± 0.29 vs 1.33 ± 0.26 μm(2) /ms, P = 0.02) and CSO (1.36 ± 0.29 vs 1.33 ± 0.28 μm(2) /ms, P < 0.0001). Measurement agreement was acceptable.

CONCLUSIONS

ADC measurements in normal unsedated fetuses in the second and third trimesters are reproducible except for small differences for PWM, CH and CSO between examiners.

Identification of fetal precentral gyrus on diffusion weighted MRI

To investigate the association of the diffusion-weighted MR imaging characteristics of fetal preCG and gestational age. Forty-four fetuses with normal brain MRI findings were included in the study. Gestational ages ranged from 18 to 36 weeks (mean 25.2 weeks). All exams were performed with a 1.5-T scanner using a body array coil during free maternal breathing without sedation. Precentral gyrus was defined as the hyperintense strip anterior to the central sulcus, on the superior section of axial brain images at the level of superior frontal cortex. The presence of preCG hyperintensity was noted as observed/subtle/not observed at different b values (500, 1000 s/mm(2)) and on apparent diffusion coefficient (ADC) maps and compared to the imaging characteristics of the superior frontal cortex. Precentral gyrus was first detected at 25 weeks as a hyperintense strip on DWI and hypointense strip on ADC maps. Display of preCG b 1000 s/mm(2) images were better than b 500 s/mm(2). Between 25 and 27 weeks, in 40% of fetuses preCG was observed on one hemisphere, and it was evident bilaterally in 60% of cases. Starting from the 28th week, preCG was observed on both hemispheres in 100% of cases. Diffusion weighted imaging helps better understanding of the evolution of fetal preCG. The hyperintense preCG strip starts to appear at 25 weeks, and when interpreting fetal DWI after 28 weeks this may be a sign to be sought for in all fetuses and an indicator for normal development.

Contributions of diffusion-weighted imaging to fetal and neonatal imaging

Diffusion-weighted imaging has contributed to our understanding of fetal and neonatal brain development. This article will review the role of diffusion-weighted imaging and diffusion tensor imaging in fetuses and neonates, focusing primarily on normal brain developmental processes and then on pathologic processes that can be characterized using diffusion tensor imaging.

[Diffusion-weighted MR imaging of the normal fetal brain: marker of fetal brain maturation]

PURPOSE

To determine the reliability and variations of apparent diffusion coefficient (ADC) values in normal fetuses. Materials and methods. Retrospective study (2007-2008) on 22 normal fetal MR examinations, performed between 30 and 34 of gestation, using a routine protocol (T1W and T2W images in 3 planes, b=1,000 diffusion-weighted imaging) without sedation. ADC values were measured by placing 3 adjacent regions of interest (ROI) including a centrally located ROI over the right frontal and occipital white matter (6 ROI).

STATISTICAL ANALYSIS

reproducibility of adjacent ADC values (intraclass correlation coefficient: ICC) and comparison between frontal and occipital ADC values (Wilcoxon).

RESULTS

The mean ADC value was 1.78 mm(2)/s for the frontal white matter (+ or - 0.10) and 1.66 mm(2)/s for the occipital white matter (+ or - 0.12) with excellent reproducibility (ICC=0.91 in the frontal lobe) and good reproducibility for adjacent measurements (ICC=0.7). A linear inverse correlation existed between ADC values and gestational age in the occipital lobes, and a significant fronto-occipital gradient existed after 32 weeks of gestational age.

CONCLUSION

ADC value measurements are reliable and inversely correlated with gestational age due to fetal brain maturation. The existence of a fronto-occipital gradient after 32 weeks of gestational age could be a marker of normal maturation used in clinical practice.

Diffusion tensor imaging of the cortical plate and subplate in very-low-birth-weight infants

BACKGROUND

Many intervention studies in preterm infants aim to improve neurodevelopmental outcome, but short-term proxy outcome measurements are lacking. Cortical plate and subplate development could be such a marker.

OBJECTIVE

Our aim was to provide normal DTI reference values for the cortical plate and subplate of preterm infants.

MATERIALS AND METHODS

As part of an ongoing study we analysed diffusion tensor imaging (DTI) images of 19 preterm infants without evidence of injury on conventional MRI, with normal outcome (Bayley-II assessed at age 2), and scanned in the first 4 days of life. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values in the frontal and temporal subplate and cortical plate were measured in single and multiple voxel regions of interest (ROI) placed on predefined regions.

RESULTS

Using single-voxel ROIs, statistically significant inverse correlation was found between gestational age (GA) and FA of the frontal (r = -0.5938, P = 0.0058) and temporal (r = -0.4912, P = 0.0327) cortical plate. ADC values had a significant positive correlation with GA in the frontal (r = 0.5427, P = 0.0164) and temporal (r = 0.5540, P = 0.0138) subplate.

CONCLUSION

Diffusion tensor imaging allows in vivo exploration of the evolving cortical plate and subplate. We provide FA and ADC values of the subplate and cortical plate in very-low-birth-weight (VLBW) infants with normal developmental outcome that can be used as reference values.

Non-invasive fetal lung assessment using diffusion-weighted imaging

OBJECTIVES

The main goal was to develop a reproducible method for estimating the diffusion of water in human fetal lung tissue using diffusion-weighted imaging (DWI). A secondary objective was to determine the relationship of the apparent diffusion coefficients (ADCs) in the fetal lung to menstrual age and total lung volume.

METHODS

Normal pregnant volunteers were scanned on a 1.5-Tesla (T) magnetic resonance imaging (MRI) system. The MRI system was equipped with 40-mT/m gradients (slew rate 200 T/m/s, rise time 0.2 ms). A six-channel body array coil was used for signal reception. Single-shot DWI utilized TE/TR 125/3400 ms, slice thickness 4 mm, field of view 280 mm x 280 mm, interslice gap 0.8 mm and a matrix of 128 x 128. The voxel size was 2.5 mm x 2.5 mm x 4.0 mm. Two b-values (0 and 1000) were chosen along three orthogonal directions. ADC maps were created using assigned b-values. Simple linear regression was performed with Pearson correlation coefficient. Interexaminer and intraexaminer bias, and 95% limits of agreement (LOA) were determined using Bland-Altman plots.

RESULTS

Forty-seven scans were performed at a mean +/- SD of 29.2 +/- 4.5 weeks. The median coefficient of variation for ADC was 5.6% (interquartile range, 4.0-8.1%). No differences in ADC values were found between right and left lungs. Normally distributed ADC measurements were not significantly correlated with either total lung volume (r(2) = 0.0001, P = 0.94) or menstrual age (r(2) = 0.003, P = 0.70). The mean ADC value was 1.75 (95% CI, 1.63-1.86). Mean +/- SD intraexaminer bias was -0.15 +/- 2.3 (95% LOA, -4.7 to + 4.4) and interexaminer bias was 2.2 +/- 3.5 (95% LOA, -4.7 to + 9.1).

CONCLUSIONS

Our findings suggest that ADC measurements of the fetal lung are reproducible between blinded examiners and are independent of menstrual age, as well as lung volume.

Normative apparent diffusion coefficient values in the developing fetal brain

BACKGROUND AND PURPOSE

Previous studies of diffusion-weighted imaging (DWI) in fetuses are limited. Because of the need for normative data for comparison with young fetuses and preterm neonates with suspected brain abnormalities, we studied apparent diffusion coefficient (ADC) values in a population of singleton, nonsedated, healthy fetuses.

MATERIALS AND METHODS

DWI was performed in 28 singleton nonsedated fetuses with normal or questionably abnormal results on sonography and normal fetal MR imaging results; 10 fetuses also had a second fetal MR imaging, which included DWI. ADC values in the periatrial white matter (WM), frontal WM, thalamus, basal ganglia, cerebellum, and pons were plotted against gestational age and analyzed with linear regression. We compared mean ADC in different regions using the Tukey Honestly Significant Difference test. We also compared rates of decline in ADC with increasing gestational age across different areas by using the t test with multiple comparisons correction. Neurodevelopmental outcome was assessed.

RESULTS

Median gestational age was 24.28 weeks (range, 21-33.43 weeks). Results of all fetal MR imaging examinations were normal, including 1 fetus with a normal variant of a cavum velum interpositum. ADC values were highest in the frontal and periatrial WM and lowest in the thalamus and pons. ADC declined with increasing gestational age in periatrial WM (P = .0003), thalamus (P < .0001), basal ganglia (P = .0035), cerebellum (P < .0001), and pons (P = .024). Frontal WM ADC did not significantly change with gestational age. ADC declined fastest in the cerebellum, followed by the thalamus.

CONCLUSIONS

Regional differences in nonsedated fetal ADC values and their evolution with gestational age likely reflect differences in brain maturation and are similar to published data in premature neonates.

Contribution of fetal cerebral MRI for diagnosis of structural anomalies

More than 20 years after its introduction, magnetic resonance imaging (MRI) is now considered a useful complementary tool in the imaging work-up of fetal brain structural anomalies, but its real contribution in neuro-fetal imaging is still controversial. Our purpose is to present an overview of the most valuable indications of fetal cerebral MRI in current practice as guided by dedicated ultrasound analysis. On the basis of a review of the literature and our personal experience, we underline herein the real complementarities between these two techniques in different clinical or imaging settings and show how MRI adds significant information compared with ultrasound, especially in the late second and third trimesters. We assess the interest of using MRI from a technical point of view (complete and extensive anatomical analysis, analysis of developmental and pathological processes) and from a practical point of view in different imaging and clinical settings.