Imaging the fetus: when does MRI really help?

Indications for magnetic resonance imaging of the fetal central nervous system: recommendations from the European Society of Paediatric Radiology Fetal Task Force

Fetal central nervous system MRI is a well-established method to complement a high-quality fetal ultrasound and to clarify sonographically detected abnormalities in complex pregnancies. However, there is still worldwide heterogeneity and confusion regarding the indications of fetal central nervous system MRI, which has roots in differences among countries regarding the performance of ultrasound examinations and legislation on pregnancy termination. The purpose of this article is to clarify the indications for fetal central nervous system MRI by focusing on the ultrasound findings that guide further investigation with MRI and highlight the strengths and the weaknesses of each modality on imaging the fetal central nervous system.

Survey about the current use of fetal MRI in Spain

In June 2019 in Seville, at the first course in fetal MRI, endorsed by the Spanish Society of Medical Radiology (SERAM) and the Spanish Society of Pediatric Radiology (SERPE), the Spanish fetal MRI group was founded. To establish this group, a questionnaire was designed for radiologists dedicated to prenatal imaging in Spain and disseminated to the SERAM's members. The questions were related to the type of hospital, to MRI studies (magnetic field, gestational age, use of sedation, number of studies per year, proportion of fetal neuroimaging studies), and to teaching and research about fetal MRI. A total of 41 responses were received from radiologists in 25 provinces (88% working in public hospitals). Very few radiologists in Spain perform prenatal ultrasonography (7%) or prenatal CT. MRI is done in the second trimester (34%) or in the third trimester (44%). In 95% of centers, fetal brain MRI studies predominate. In 41% of the centers, studies can be done on 3 T MRI scanners. Maternal sedation is used in 17% of centers. The number of fetal MRI studies per year varies widely, being much higher in Barcelona and Madrid than in the rest of Spain.

European overview of current practice of fetal imaging by pediatric radiologists: a new task force is launched

A new task force dedicated to prenatal imaging was created in 2018 by the European Society of Pediatric Radiology. In order to establish a network of European prenatal imaging practice, we sent a questionnaire to radiologists practicing prenatal imaging in Europe. The questions were related to the type of institution, the local legislation for termination of pregnancy, the type of imaging modality and the following items regarding magnetic resonance imaging (MRI): magnetic field, gestational age at which it is performed, use of maternal sedation, number of examinations per year, proportion of cerebral versus body indications and proportion of repeated examinations. We collected responses from 20 European countries, 52 cities and 67 institutions (82% public). In most countries, the upper gestational age limit for termination of pregnancy is 24 weeks of gestation. In some countries, it is earlier and in other countries, there is no limit. Very few radiologists practice fetal ultrasonography and computed tomography. In some countries, fetal MRI is mainly performed before 24 weeks of pregnancy, while in others, it is mainly performed in the third trimester. Neurological indications are by far predominant and 30% of the institutions have access to a 3-tesla (T) unit for fetal MRI. Maternal sedation is rarely used. The number of scans per year is highly variable with an average of 140, which is not necessarily correlated to the size of the population.

PVR: Patch-to-Volume Reconstruction for Large Area Motion Correction of Fetal MRI

In this paper, we present a novel method for the correction of motion artifacts that are present in fetal magnetic resonance imaging (MRI) scans of the whole uterus. Contrary to current slice-to-volume registration (SVR) methods, requiring an inflexible anatomical enclosure of a single investigated organ, the proposed patch-to-volume reconstruction (PVR) approach is able to reconstruct a large field of view of non-rigidly deforming structures. It relaxes rigid motion assumptions by introducing a specific amount of redundant information that is exploited with parallelized patchwise optimization, super-resolution, and automatic outlier rejection. We further describe and provide an efficient parallel implementation of PVR allowing its execution within reasonable time on commercially available graphics processing units, enabling its use in the clinical practice. We evaluate PVR's computational overhead compared with standard methods and observe improved reconstruction accuracy in the presence of affine motion artifacts compared with conventional SVR in synthetic experiments. Furthermore, we have evaluated our method qualitatively and quantitatively on real fetal MRI data subject to maternal breathing and sudden fetal movements. We evaluate peak-signal-to-noise ratio, structural similarity index, and cross correlation with respect to the originally acquired data and provide a method for visual inspection of reconstruction uncertainty. We further evaluate the distance error for selected anatomical landmarks in the fetal head, as well as calculating the mean and maximum displacements resulting from automatic non-rigid registration to a motion-free ground truth image. These experiments demonstrate a successful application of PVR motion compensation to the whole fetal body, uterus, and placenta.

Toward the automatic quantification of in utero brain development in 3D structural MRI: A review

Investigating the human brain in utero is important for researchers and clinicians seeking to understand early neurodevelopmental processes. With the advent of fast magnetic resonance imaging (MRI) techniques and the development of motion correction algorithms to obtain high-quality 3D images of the fetal brain, it is now possible to gain more insight into the ongoing maturational processes in the brain. In this article, we present a review of the major building blocks of the pipeline toward performing quantitative analysis of in vivo MRI of the developing brain and its potential applications in clinical settings. The review focuses on T1- and T2-weighted modalities, and covers state of the art methodologies involved in each step of the pipeline, in particular, 3D volume reconstruction, spatio-temporal modeling of the developing brain, segmentation, quantification techniques, and clinical applications. Hum Brain Mapp 38:2772-2787, 2017. © 2017 Wiley Periodicals, Inc.

MR Imaging of the Pituitary Gland and Postsphenoid Ossification in Fetal Specimens

BACKGROUND AND PURPOSE

A thorough knowledge of fetal growth and development is key to understanding both the normal and abnormal fetal MR imaging findings. We investigated the size and signal intensity of the normal pituitary gland and the intrasphenoidal ossification around the Rathke pouch in formalin-fixed fetuses on MR imaging.

MATERIALS AND METHODS

Thirty-two fetuses with undamaged brains were included in this study (mean age, 19.93 weeks; age range, 12-31 weeks). Visual inspection of the pituitary and ossification around the Rathke pouch in the sphenoid bone or the postsphenoid ossification was conducted. The extent of pituitary and postsphenoid ossification, pituitary/pons signal ratio, and postsphenoidal ossification/sphenoid bone signal ratio was compared according to gestational age.

RESULTS

The pituitary gland was identified as a hyperintense intrasellar structure in all cases, and postsphenoid ossification was identified as an intrasphenoidal hyperintense area in 27 of the 32 cases (84%). The mean pituitary/pons signal ratio was 1.13 ± 0.18 and correlated weakly with gestational age (R(2) = 0.243), while the mean postsphenoid ossification/sphenoid bone signal ratio was 2.14 ± 0.56 and did not show any increase with gestational age (R(2) = 0.05). No apparent change in the size of pituitary hyperintensity was seen with gestational age (R(2) = 0.001). Postsphenoid ossification showed an increase in size with gestational age (R(2) = 0.307).

CONCLUSIONS

The fetal pituitary gland was hyperintense on T1-weighted images and the pituitary/pons ratio and extent of postsphenoid ossification correlated weakly with gestational age.

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.

Multi-atlas multi-shape segmentation of fetal brain MRI for volumetric and morphometric analysis of ventriculomegaly

The recent development of motion robust super-resolution fetal brain MRI holds out the potential for dramatic new advances in volumetric and morphometric analysis. Volumetric analysis based on volumetric and morphometric biomarkers of the developing fetal brain must include segmentation. Automatic segmentation of fetal brain MRI is challenging, however, due to the highly variable size and shape of the developing brain; possible structural abnormalities; and the relatively poor resolution of fetal MRI scans. To overcome these limitations, we present a novel, constrained, multi-atlas, multi-shape automatic segmentation method that specifically addresses the challenge of segmenting multiple structures with similar intensity values in subjects with strong anatomic variability. Accordingly, we have applied this method to shape segmentation of normal, dilated, or fused lateral ventricles for quantitative analysis of ventriculomegaly (VM), which is a pivotal finding in the earliest stages of fetal brain development, and warrants further investigation. Utilizing these innovative techniques, we introduce novel volumetric and morphometric biomarkers of VM comparing these values to those that are generated by standard methods of VM analysis, i.e., by measuring the ventricular atrial diameter (AD) on manually selected sections of 2D ultrasound or 2D MRI. To this end, we studied 25 normal and abnormal fetuses in the gestation age (GA) range of 19 to 39 weeks (mean=28.26, stdev=6.56). This heterogeneous dataset was essentially used to 1) validate our segmentation method for normal and abnormal ventricles; and 2) show that the proposed biomarkers may provide improved detection of VM as compared to the AD measurement.

Penile biometry on prenatal magnetic resonance imaging

OBJECTIVE

In view of the implementation of magnetic resonance imaging (MRI) as an adjunct to ultrasonography in prenatal diagnosis, this study sought to demonstrate normal penile growth on prenatal MRI.

METHODS

This was a retrospective study of MRI of 194 male fetuses (18-34 weeks' gestation) with normal anatomy or minor abnormalities. On sagittal T2-weighted MRI sequences, we measured penile length from the glans tip to the scrotal edge (outer length) and from the glans tip to the symphyseal border (total length). Descriptive statistics, as well as correlation and regression analysis, were used to evaluate penile length in relation to gestation. T-tests were calculated to compare mean outer/total length on MRI with published ultrasound data.

RESULTS

Mean length values, including 95% CIs and percentiles, were defined. Penile length as a function of gestational age was expressed by the following regression equations: outer mean length = - 5.514 + 0.622 × gestational age in weeks; total mean length = - 8.865 + 1.312× gestational age in weeks. The correlation coefficients, r = 0.532 and r = 0.751, respectively, were statistically significant (P < 0.001). Comparison of outer penile length on MRI with published ultrasound penile length data showed no significant differences, while total penile length on MRI was significantly greater than ultrasound penile length (P < 0.001).

CONCLUSION

Our MRI results provide a reference range of fetal penile length, which, in addition to ultrasonography, may be helpful in the identification of genital anomalies. Outer penile length on MRI is equivalent to penile length measured on ultrasound, whereas total length is significantly greater.

Abnormalities of the upper extremities on fetal magnetic resonance imaging

OBJECTIVE

In view of the increasing use of fetal magnetic resonance imaging (MRI) as an adjunct to prenatal ultrasonography, we sought to demonstrate the visualization of upper extremity abnormalities and associated defects on MRI, with regard to fetal outcomes and compared with ultrasound imaging.

METHODS

This retrospective study included 29 fetuses with upper extremity abnormalities visualized with fetal MRI following suspicious ultrasound findings and confirmed by postnatal assessment or autopsy. On a 1.5-Tesla unit, dedicated sequences were applied to image the extremities. Central nervous system (CNS) and extra-CNS anomalies were assessed to define extremity abnormalities as isolated or as complex, with associated defects. Fetal outcome was identified from medical records. MRI and ultrasound findings, when available, were compared.

RESULTS

Isolated upper extremity abnormalities were found in three (10.3%) fetuses. In 26 (89.7%) fetuses complex abnormalities, including postural extremity disorders (21/26) and structural extremity abnormalities (15/26), were demonstrated. Associated defects involved: face (15/26); musculoskeletal system (14/26); thorax and cardio/pulmonary system (12/26); lower extremities (12/26); brain and skull (10/26); and abdomen (8/26). Of the 29 cases, 18 (62.1%) pregnancies were delivered and 11 (37.9%) were terminated. MRI and US findings were compared in 27/29 cases: the diagnosis was concordant in 14 (51.9%) of these cases, and additional findings were made on MRI in 13/27 (48.1%) cases.

CONCLUSIONS

Visualization of upper extremity abnormalities on fetal MRI enables differentiation between isolated defects and complex ones, which may be related to poor fetal prognosis. MRI generally confirms the ultrasound diagnosis, and may provide additional findings in certain cases.

Fetal brain volumetry through MRI volumetric reconstruction and segmentation

PURPOSE

Fetal MRI volumetry is a useful technique but it is limited by a dependency upon motion-free scans, tedious manual segmentation, and spatial inaccuracy due to thick-slice scans. An image processing pipeline that addresses these limitations was developed and tested.

MATERIALS AND METHODS

The principal sequences acquired in fetal MRI clinical practice are multiple orthogonal single-shot fast spin echo scans. State-of-the-art image processing techniques were used for inter-slice motion correction and super-resolution reconstruction of high-resolution volumetric images from these scans. The reconstructed volume images were processed with intensity non-uniformity correction and the fetal brain extracted by using supervised automated segmentation.

RESULTS

Reconstruction, segmentation and volumetry of the fetal brains for a cohort of twenty-five clinically acquired fetal MRI scans was done. Performance metrics for volume reconstruction, segmentation and volumetry were determined by comparing to manual tracings in five randomly chosen cases. Finally, analysis of the fetal brain and parenchymal volumes was performed based on the gestational age of the fetuses.

CONCLUSION

The image processing pipeline developed in this study enables volume rendering and accurate fetal brain volumetry by addressing the limitations of current volumetry techniques, which include dependency on motion-free scans, manual segmentation, and inaccurate thick-slice interpolation.

The skeleton and musculature on foetal MRI

BACKGROUND: Magnetic resonance imaging (MRI) is used as an adjunct to ultrasound in prenatal imaging, the latter being the standard technique in obstetrical medicine. METHODS: Initial results demonstrate the ability to visualise the foetal skeleton and muscles on MRI, and highlight the potentially useful applications for foetal MRI, which has significantly profited from innovations in sequence technology. Echoplanar imaging, thick-slab T2-weighted (w) imaging, and dynamic sequences are techniques complementary to classical T2-w imaging. RESULTS: Recent study data indicate that foetal MRI may be useful in the imaging of spinal dysraphism and in differentiating between isolated and complex skeletal deformities with associated congenital malformations, which might have an impact on pre- and postnatal management. CONCLUSION: More research and technical refinement will be necessary to investigate normal human skeletal development and to identify MR imaging characteristics of skeletal abnormalities.

Abnormalities of the foetal cerebral cortex

Prenatal ultrasound has concentrated on readily visible cerebral structures including head size, shape, ventricles, CSP (cavum septi pellucidi), cerebellar size/vermian presence and cisterna magna. However, apart from these easily visible structures it is important to evaluate the brain itself. Patients who initially appear to have mild isolated findings such as borderline ventriculomegaly in fact can have many more subtle findings that significantly alter prognosis and management that can be detected on detailed examination of the brain. There has been rapid evolution in imaging these foetuses, especially with neurosonography and MRI, and a revolution in understanding the underlying genetic and biochemical mechanisms. There is increasing emphasis to detect cortical abnormalities as early as possible. This article reviews development of the cerebral cortex, the classification, aetiologies and clinical manifestations of cortical disorders, normal and abnormal appearances at ultrasound and MRI, and approaches to investigation.