Initial experiences of performing fetal fMRI

Reliability and Feasibility of Low-Field-Strength Fetal MRI at 0.55 T during Pregnancy

Background The benefits of using low-field-strength fetal MRI to evaluate antenatal development include reduced image artifacts, increased comfort, larger bore size, and potentially reduced costs, but studies about fetal low-field-strength MRI are lacking. Purpose To evaluate the reliability and feasibility of low-field-strength fetal MRI to assess anatomic and functional measures in pregnant participants using a commercially available 0.55-T MRI scanner and a comprehensive 20-minute protocol. Materials and Methods This prospective study was performed at a large teaching hospital (St Thomas' Hospital; London, England) from May to November 2022 in healthy pregnant participants and participants with pregnancy-related abnormalities using a commercially available 0.55-T MRI scanner. A 20-minute protocol was acquired including anatomic T2-weighted fast-spin-echo, quantitative T2*, and diffusion sequences. Key measures like biparietal diameter, transcerebellar diameter, lung volume, and cervical length were evaluated by two radiologists and an MRI-experienced obstetrician. Functional organ-specific mean values were given. Comparison was performed with existing published values and higher-field MRI using linear regression, interobserver correlation, and Bland-Altman plots. Results A total of 79 fetal MRI examinations were performed (mean gestational age, 29.4 weeks ± 5.5 [SD] [age range, 17.6-39.3 weeks]; maternal age, 34.4 years ± 5.3 [age range, 18.4-45.5 years]) in 47 healthy pregnant participants (control participants) and in 32 participants with pregnancy-related abnormalities. The key anatomic two-dimensional measures for the 47 healthy participants agreed with large cross-sectional 1.5-T and 3-T control studies. The interobserver correlations for the biparietal diameter in the first 40 consecutive scans were 0.96 (95% CI: 0.7, 0.99; P = .002) for abnormalities and 0.93 (95% CI: 0.86, 0.97; P < .001) for control participants. Functional features, including placental and brain T2* and placental apparent diffusion coefficient values, strongly correlated with gestational age (mean placental T2* in the control participants: 5.2 msec of decay per week; R2 = 0.66; mean T2* at 30 weeks, 176.6 msec; P < .001). Conclusion The 20-minute low-field-strength fetal MRI examination protocol was capable of producing reliable structural and functional measures of the fetus and placenta in pregnancy. Clinical trial registration no. REC 21/LO/0742 © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Gowland in this issue.

Fetal and neonatal neuroimaging

Magnetic resonance imaging (MRI) can provide detail of the soft tissues of the fetal and neonatal brain that cannot be obtained by any other imaging modality. Conventional T1 and T2 weighted sequences provide anatomic detail of the normally developing brain and can demonstrate lesions, including those associated with preterm birth, hypoxic ischemic encephalopathy, perinatal arterial stroke, infections, and congenital malformations. Specialized imaging techniques can be used to assess cerebral vasculature (magnetic resonance angiography and venography), cerebral metabolism (magnetic resonance spectroscopy), cerebral perfusion (arterial spin labeling), and function (functional MRI). A wealth of quantitative tools, most of which were originally developed for the adult brain, can be applied to study the developing brain in utero and postnatally including measures of tissue microstructure obtained from diffusion MRI, morphometric studies to measure whole brain and regional tissue volumes, and automated approaches to study cortical folding. In this chapter, we aim to describe different imaging approaches for the fetal and neonatal brain, and to discuss their use in a range of clinical applications.

Detecting default mode networks in utero by integrated 4D fMRI reconstruction and analysis

Recently, there has been considerable interest, especially for in utero imaging, in the detection of functional connectivity in subjects whose motion cannot be controlled while in the MRI scanner. These cases require two advances over current studies: (1) multiecho acquisitions and (2) post processing and reconstruction that can deal with significant between slice motion during multislice protocols to allow for the ability to detect temporal correlations introduced by spatial scattering of slices into account. This article focuses on the estimation of a spatially and temporally regular time series from motion scattered slices of multiecho fMRI datasets using a full four-dimensional (4D) iterative image reconstruction framework. The framework which includes quantitative MRI methods for artifact correction is evaluated using adult studies with and without motion to both refine parameter settings and evaluate the analysis pipeline. ICA analysis is then applied to the 4D image reconstruction of both adult and in utero fetal studies where resting state activity is perturbed by motion. Results indicate quantitative improvements in reconstruction quality when compared to the conventional 3D reconstruction approach (using simulated adult data) and demonstrate the ability to detect the default mode network in moving adults and fetuses with single-subject and group analysis. Hum Brain Mapp 37:4158-4178, 2016. © 2016 Wiley Periodicals, Inc.

Inner-volume echo volumar imaging (IVEVI) for robust fetal brain imaging

PURPOSE

Fetal functional MRI studies using conventional 2-dimensional single-shot echo-planar imaging sequences may require discarding a large data fraction as a result of fetal and maternal motion. Increasing the temporal resolution using echo volumar imaging (EVI) could provide an effective alternative strategy. Echo volumar imaging was combined with inner volume (IV) imaging (IVEVI) to locally excite the fetal brain and acquire full 3-dimensional images, fast enough to freeze most fetal head motion.

METHODS

IVEVI was implemented by modifying a standard multi-echo echo-planar imaging sequence. A spin echo with orthogonal excitation and refocusing ensured localized excitation. To introduce T2* weighting and to save time, the k-space center was shifted relative to the spin echo. Both single and multi-shot variants were tested. Acoustic noise was controlled by adjusting the amplitude and switching frequency of the readout gradient. Image-based shimming was used to minimize B₀ inhomogeneities within the fetal brain.

RESULTS

The sequence was first validated in an adult. Eight fetuses were scanned using single-shot IVEVI at a 3.5 × 3.5 × 5.0 mm³ resolution with a readout duration of 383 ms. Multishot IVEVI showed reduced geometric distortions along the second phase-encode direction.

CONCLUSIONS

Fetal EVI remains challenging. Although effective echo times comparable to the T2* values of fetal cortical gray matter at 3 T could be achieved, controlling acoustic noise required longer readouts, leading to substantial distortions in single-shot images. Although multishot variants enabled us to reduce susceptibility-induced geometric distortions, sensitivity to motion was increased. Future studies should therefore focus on improvements to multishot variants. Magn Reson Med 80:279-285, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Robust preprocessing for stimulus-based functional MRI of the moving fetus

Fetal motion manifests as signal degradation and image artifact in the acquired time series of blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) studies. We present a robust preprocessing pipeline to specifically address fetal and placental motion-induced artifacts in stimulus-based fMRI with slowly cycled block design in the living fetus. In the proposed pipeline, motion correction is optimized to the experimental paradigm, and it is performed separately in each phase as well as in each region of interest (ROI), recognizing that each phase and organ experiences different types of motion. To obtain the averaged BOLD signals for each ROI, both misaligned volumes and noisy voxels are automatically detected and excluded, and the missing data are then imputed by statistical estimation based on local polynomial smoothing. Our experimental results demonstrate that the proposed pipeline was effective in mitigating the motion-induced artifacts in stimulus-based fMRI data of the fetal brain and placenta.

MRI evaluation and safety in the developing brain

Magnetic resonance imaging (MRI) evaluation of the developing brain has dramatically increased over the last decade. Faster acquisitions and the development of advanced MRI sequences, such as magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), perfusion imaging, functional MR imaging (fMRI), and susceptibility-weighted imaging (SWI), as well as the use of higher magnetic field strengths has made MRI an invaluable tool for detailed evaluation of the developing brain. This article will provide an overview of the use and challenges associated with 1.5-T and 3-T static magnetic fields for evaluation of the developing brain. This review will also summarize the advantages, clinical challenges, and safety concerns specifically related to MRI in the fetus and newborn, including the implications of increased magnetic field strength, logistics related to transporting and monitoring of neonates during scanning, and sedation considerations, and a discussion of current technologies such as MRI conditional neonatal incubators and dedicated small-foot print neonatal intensive care unit (NICU) scanners.

MR imaging of the fetal brain at 1.5T and 3.0T field strengths: comparing specific absorption rate (SAR) and image quality

OBJECTIVES

Our two objectives were to evaluate the feasibility of fetal brain magnetic resonance imaging (MRI) using a fast spin echo sequence at 3.0T field strength with low radio frequency (rf) energy deposition (as measured by specific absorption rate: SAR) and to compare image quality, tissue contrast and conspicuity between 1.5T and 3.0T MRI.

METHODS

T2 weighted images of the fetal brain at 1.5T were compared to similar data obtained in the same fetus using a modified sequence at 3.0T. Quantitative whole-body SAR and normalized image signal to noise ratio (SNR), a nominal scoring scheme based evaluation of diagnostic image quality, and tissue contrast and conspicuity for specific anatomical structures in the brain were compared between 1.5T and 3.0T.

RESULTS

Twelve pregnant women underwent both 1.5T and 3.0T MRI examinations. The image SNR was significantly higher (P=0.03) and whole-body SAR was significantly lower (P<0.0001) for images obtained at 3.0T compared to 1.5T. All cases at both field strengths were scored as having diagnostic image quality. Images from 3.0T MRI (compared to 1.5T) were equal (57%; 21/37) or superior (35%; 13/37) for tissue contrast and equal (61%; 20/33) or superior (33%, 11/33) for conspicuity.

CONCLUSIONS

It is possible to obtain fetal brain images with higher resolution and better SNR at 3.0T with simultaneous reduction in SAR compared to 1.5T. Images of the fetal brain obtained at 3.0T demonstrated superior tissue contrast and conspicuity compared to 1.5T.

Disrupted developmental organization of the structural connectome in fetuses with corpus callosum agenesis

Agenesis of the corpus callosum is a model disease for disrupted connectivity of the human brain, in which the pathological formation of interhemispheric fibers results in subtle to severe cognitive deficits. Postnatal studies suggest that the characteristic abnormal pathways in this pathology are compensatory structures that emerge via neural plasticity. We challenge this hypothesis and assume a globally different network organization of the structural interconnections already in the fetal acallosal brain. Twenty fetuses with isolated corpus callosum agenesis with or without associated malformations were enrolled and fiber connectivity among 90 brain regions was assessed using in utero diffusion tensor imaging and streamline tractography. Macroscopic scale connectomes were compared to 20 gestational age-matched normally developing fetuses with multiple granularity of network analysis. Gradually increasing connectivity strength and tract diffusion anisotropy during gestation were dominant in antero-posteriorly running paramedian and antero-laterally running aberrant pathways, and in short-range connections in the temporoparietal regions. In fetuses with associated abnormalities, more diffuse reduction of cortico-cortical and cortico-subcortical connectivity was observed than in cases with isolated callosal agenesis. The global organization of anatomical networks consisted of less segregated nodes in acallosal brains, and hubs of dense connectivity, such as the thalamus and cingulate cortex, showed reduced network centrality. Acallosal fetal brains show a globally altered connectivity network structure compared to normals. Besides the previously described Probst and sigmoid bundles, we revealed a prenatally differently organized macroconnectome, dominated by increased connectivity. These findings provide evidence that abnormal pathways are already present during at early stages of fetal brain development in the majority of cerebral white matter.

Fetal functional imaging portrays heterogeneous development of emerging human brain networks

The functional connectivity architecture of the adult human brain enables complex cognitive processes, and exhibits a remarkably complex structure shared across individuals. We are only beginning to understand its heterogeneous structure, ranging from a strongly hierarchical organization in sensorimotor areas to widely distributed networks in areas such as the parieto-frontal cortex. Our study relied on the functional magnetic resonance imaging (fMRI) data of 32 fetuses with no detectable morphological abnormalities. After adapting functional magnetic resonance acquisition, motion correction, and nuisance signal reduction procedures of resting-state functional data analysis to fetuses, we extracted neural activity information for major cortical and subcortical structures. Resting fMRI networks were observed for increasing regional functional connectivity from 21st to 38th gestational weeks (GWs) with a network-based statistical inference approach. The overall connectivity network, short range, and interhemispheric connections showed sigmoid expansion curve peaking at the 26-29 GW. In contrast, long-range connections exhibited linear increase with no periods of peaking development. Region-specific increase of functional signal synchrony followed a sequence of occipital (peak: 24.8 GW), temporal (peak: 26 GW), frontal (peak: 26.4 GW), and parietal expansion (peak: 27.5 GW). We successfully adapted functional neuroimaging and image post-processing approaches to correlate macroscopical scale activations in the fetal brain with gestational age. This in vivo study reflects the fact that the mid-fetal period hosts events that cause the architecture of the brain circuitry to mature, which presumably manifests in increasing strength of intra- and interhemispheric functional macro connectivity.

Resting State fMRI in the moving fetus: a robust framework for motion, bias field and spin history correction

There is growing interest in exploring fetal functional brain development, particularly with Resting State fMRI. However, during a typical fMRI acquisition, the womb moves due to maternal respiration and the fetus may perform large-scale and unpredictable movements. Conventional fMRI processing pipelines, which assume that brain movements are infrequent or at least small, are not suitable. Previous published studies have tackled this problem by adopting conventional methods and discarding as much as 40% or more of the acquired data. In this work, we developed and tested a processing framework for fetal Resting State fMRI, capable of correcting gross motion. The method comprises bias field and spin history corrections in the scanner frame of reference, combined with slice to volume registration and scattered data interpolation to place all data into a consistent anatomical space. The aim is to recover an ordered set of samples suitable for further analysis using standard tools such as Group Independent Component Analysis (Group ICA). We have tested the approach using simulations and in vivo data acquired at 1.5 T. After full motion correction, Group ICA performed on a population of 8 fetuses extracted 20 networks, 6 of which were identified as matching those previously observed in preterm babies.

Functional connectivity MRI in infants: exploration of the functional organization of the developing brain

Advanced neuroimaging techniques have been increasingly applied to the study of preterm and term infants in an effort to further define the functional cerebral architecture of the developing brain. Despite improved understanding of the complex relationship between structure and function obtained through these investigations, significant questions remain regarding the nature, location, and timing of the maturational changes which occur during early development. Functional connectivity magnetic resonance imaging (fcMRI) utilizes spontaneous, low frequency (< 0.1 Hz), coherent fluctuations in blood oxygen level dependent (BOLD) signal to identify networks of functional cerebral connections. Due to the intrinsic characteristics of its image acquisition and analysis, fcMRI offers a novel neuroimaging approach well suited to investigation of infants. Recently, this methodology has been successfully applied to examine neonatal populations, defining normative patterns of large-scale neural network development in the maturing brain. The resting-state networks (RSNs) identified in these studies reflect the evolving cerebral structural architecture, presumably driven by varied genetic and environmental influences. Principal features of these investigations and their role in characterization of the tenets of neural network development during this critical developmental period are highlighted in this review. Despite these successes, optimal methods for fcMRI data acquisition and analysis for this population have not yet been defined. Further, appropriate schemes for interpretation and translation of fcMRI results remain unknown, a matter of increasing importance as functional neuroimaging findings are progressively applied in the clinical arena. Notwithstanding these concerns, fcMRI provides insight into the earliest forms of cerebral connectivity and therefore holds great promise for future neurodevelopmental investigations.

Fetal MEG evoked response latency from beamformer with random field theory

Analysis of fetal magnetoencephalographic brain recordings is restricted by low signal to noise ratio (SNR) and non-stationarity of the sources. Beamformer techniques have been applied to improve SNR of fetal evoked responses. However, until now the effect of non-stationarity was not taken into account in detail, because the detection of evoked responses is in most cases determined by averaging a large number of trials. We applied a windowing technique to improve the stationarity of the data by using short time segments recorded during a flash-evoked study. In addition, we implemented a random field theory approach for more stringent control of false-positives in the statistical parametric map of the search volume for the beamformer. The search volume was based on detailed individual fetal/maternal biometrics from ultrasound scans and fetal heart localization. Average power over a sliding window within the averaged evoked response against a randomized average background power was used as the test z-statistic. The significance threshold was set at 10% over all members of a contiguous cluster of voxels. There was at least one significant response for 62% of fetal and 95% of newborn recordings with gestational age (GA) between 28 and 45 weeks from 29 subjects. We found that the latency was either substantially unchanged or decreased with increasing GA for most subjects, with a nominal rate of about -11 ms/week. These findings support the anticipated neurophysiological development, provide validation for the beamformer model search as a methodology, and may lead to a clinical test for fetal cognitive development.

The emerging role of functional MRI for evaluating fetal brain activity

Although functional magnetic resonance imaging is a technique that is widely used in adult populations, its use within a fetal environment has been extremely limited. Problems associated with movement and technical scanning issues have limited its effectiveness in providing reliable and spatially accurate details of fetal brain activity. However, initial research has indicated that it is a viable tool for assessing functional maturation in the fetus, and recent advances in echo-planar imaging sequences on the abdomen at 3-T provide the potential for more reliable activation detection and higher resolution spatial information. If the technique can be further developed such that a similar reliability in activity patterns is observed as in conventional functional MRI, then fetal functional MRI could offer a useful contribution at a clinical level as well as at a research one in the assessment of brain development and maturation.

Advancing fetal brain MRI: targets for the future

Fetal MRI is becoming an increasingly powerful imaging tool for studying brain development in vivo. Until recently, the application of advanced magnetic resonance imaging techniques was limited by motion in the nonsedated fetus. Extensive research efforts currently underway are focusing on the development of dedicated magnetic resonance imaging sequences and sophisticated postprocessing techniques that are revolutionizing our ability to study the healthy and compromised fetus. The ongoing refinement of these magnetic resonance imaging techniques will undoubtedly lead to the development of cornerstone biomarkers that will provide healthcare caregivers with vital, and currently lacking, information upon which to counsel parents effectively, and base rational decisions regarding the timing and type of novel medical and surgical interventions currently on the horizon.

Blood oxygenation level dependent functional magnetic resonance imaging: current and potential uses in obstetrics and gynaecology

Blood-oxygenation-level-dependent functional magnetic resonance imaging is a noninvasive technique that has become increasingly popular in the neurosciences. It measures the proportion of oxygenated haemoglobin in specific areas of the brain, mirroring blood flow and therefore function. Here we review how the findings from functional studies impact on areas of gynaecological practice as diverse as chronic pain, continence, and premenstrual dysphoric disorder. Finally we review some of the more novel applications of the technique, such as imaging of pelvic floor function and the effects of hypoxia on the fetus.

MR imaging methods for assessing fetal brain development

Fetal magnetic resonance imaging provides an ideal tool for investigating growth and development of the brain in vivo. Current imaging methods have been hampered by fetal motion but recent advances in image acquisition can produce high signal to noise, high resolution 3-dimensional datasets suitable for objective quantification by state of the art post acquisition computer programs. Continuing development of imaging techniques will allow a unique insight into the developing brain, more specifically process of cell migration, axonal pathway formation, and cortical maturation. Accurate quantification of these developmental processes in the normal fetus will allow us to identify subtle deviations from normal during the second and third trimester of pregnancy either in the compromised fetus or in infants born prematurely.

Fetal cortical activation to sound at 33 weeks of gestation: a functional MRI study

Hearing already functions before birth, but little is known about the neural basis of fetal life experiences. Recent imaging studies have validated the use of functional magnetic resonance imaging (fMRI) in pregnant women at 38-weeks of gestation. The aim of the present study was to examine fetal brain activation to sound, using fMRI at the beginning of the third trimester of pregnancy. 6 pregnant women between 28- and 34-weeks of gestation were scanned using a magnetic strength of 1.5 T, with an auditory stimulus applied to their abdomen. 3 fetuses with a gestational age of 33 weeks, showed significant activation to sound in the left temporal lobe, measured using a new data-driven approach (Independent Component Analysis for fMRI time series). Only 2 of these fetuses showed left temporal activation, when the standard voxel-wise analysis method was used (p=0.007; p=0.001). Moreover, motion parameters added as predictors of the General Linear Model confirmed that motion cannot account for the signal variance in the fetal temporal cortex (p=0.01). Comparison between the statistical maps obtained from MRI scans of the fetuses with those obtained from adults, made it possible to confirm our hypothesis, that there is brain activation in the primary auditory cortex in response to sound. Measurement of the fetal hemodynamic response revealed an average fMRI signal change of +3.5%. This study shows that it is possible to use fMRI to detect early fetal brain function, but also confirms that sound processing occurs beyond the reflexive sub-cortical level, at the beginning of the third trimester of pregnancy.

Functional MRI of the newborn

In order to provide accurate prognosis and developmental intervention to newborns, new methods of assessing cerebral functions are needed. The non-invasive technique of functional magnetic resonance imaging (fMRI) can be considered as the leading technique for functional exploration of the infant's brain. Several studies have previously applied fMRI in both healthy and diseased newborns with different sensory and cognitive tasks. In this chapter, the methodological issues that are proper to the use of fMRI in the newborn are detailed. In addition, an overview of the major findings of previous fMRI studies is provided, with a focus on notable differences from those in adult subjects. More specifically, the functional responses and the localization of cortical activations in healthy and diseased newborns are discussed. We expect a rapid expansion of this field and the establishment of fMRI as a valid clinical diagnostic tool in the newborn.

Fetal magnetoencephalography

The assessment of the neurological integrity of the human fetus in utero is a technically challenging problem. New brain imaging devices can substantially improve our capabilities to describe functional brain activity in the fetus. It has been well established by fetal behavioral studies and investigations in preterm and term newborns that the brain is functional in utero. The major effort required to perform effective neurological functional investigations is designation of an integrated approach to neurological assessment and the generation of normative data. Currently, it is possible to record evoked brain activity elicited by auditory and visual stimulation from the fetus. In addition, there is growing evidence that spontaneous brain activity can be recorded in the fetus. This paper explores the current status of the brain-imaging field for fetal investigations and currently available data.

New advances in fetal MR neuroimaging

MR is now routinely and widely used in fetal neuroimaging and has proven to be valuable in the detection of many cerebral lesions, either genetically determined or acquired in utero. However, its efficiency has certain limits in the detection of diffuse white-matter abnormalities, the evaluation of fibre development and the demonstration of metabolic disorders. Moreover, conventional fetal MR imaging provides only a morphological approach to the fetal brain. New techniques such as diffusion-weighted imaging, diffusion tensor imaging, proton MR spectroscopy and functional MR imaging are developing. The majority of these are not used routinely. The principles, aims, technical problems and possible applications of these techniques for imaging the fetus are discussed.

Methods of fetal MR: beyond T2-weighted imaging

The present work reviews the basic methods of performing fetal magnetic resonance imaging (MRI). Since fetal MRI differs in many respects from a postnatal study, several factors have to be taken into account to achieve satisfying image quality. Image quality depends on adequate positioning of the pregnant woman in the magnet, use of appropriate coils and the selection of sequences. Ultrafast T2-weighted sequences are regarded as the mainstay of fetal MR-imaging. However, additional sequences, such as T1-weighted images, diffusion-weighted images, echoplanar imaging may provide further information, especially in extra- central-nervous system regions of the fetal body.