Fetal magnetic resonance imaging and three-dimensional ultrasound in clinical practice: General aspects

Ultrasonography is used routinely during pregnancy to screen and diagnose fetal anomalies. Two-dimensional ultrasound is usually adequate in women at low risk for malformations. When technical factors limit optimal evaluation or a malformation is suspected, further imaging with three-dimensional ultrasound and magnetic resonance imaging is becoming increasingly common. Three-dimensional ultrasound allows the manipulation of data acquired from two-dimensional ultrasound to recreate an infinite number of views, thereby enhancing the ability to evaluate the fetal anatomy. When three-dimensional ultrasound is either unavailable or inadequate, fetal magnetic resonance imaging permits detailed evaluation of the suspected anomaly and assesses the presence of associated anomalies. In this chapter, we review the techniques, advantages, limitations, and clinical applications of these two fetal imaging modalities.     

Fetal magnetic resonance imaging and three-dimensional ultrasound in clinical practice: Applications in prenatal diagnosis

Three-dimensional ultrasound and magnetic resonance imaging are powerful imaging techniques that are used increasingly in evaluating fetal anatomy. In this chapter, we review the main applications of these imaging modalities in current practice and present an overview of the malformations that may benefit from assessment with three-dimensional ultrasound and magnetic resonance imaging.     

Fetal lung imaging

Exponential improvements in imaging techniques over the last ten years, through patients' and physicians' wishes for less invasive fetal work-up, now allow us to better explore and understand fetal lung physiology during pregnancy. Diagnostic and prognostic consequences at stake are huge, especially for fetuses at risk of pulmonary hypoplasia. We will decline in three parts (normal lung, malformative lung and pulmonary hypoplasia), through a review of the literature and at the light of our experience, the potentialities and limitations of all imaging modalities (Ultrasound, Doppler, 3D, MRI). Then, we will dwell on future leads and the need for large-scale collaborative studies.     

Fetal digestive tract imaging

The exploration of the fetal digestive tract is not systematized and the ends (thoracic oesophagus and anorectal area) are often excluded from the examination excluded from screening. Only the foetal digestive pathology of liquid expression (digestive obstruction, digestive duplication) is easily detected. The most frequent digestive pathologies (oesophagus atresia, anorectal malformation) are still very often a neonatal discovery. With echography or MRI, the antenatal analysis of the digestive tract is optimized by the acquisition of coronal (thoracic oesophagus, small intestine, and colon) and sagittal median cuts (rectal reference mark, sacral concavity). The interpretation of the echosignal of the contents of the digestive tract (liquid or meconial) takes into account the transition periods responsible for the physiological modifications in charge of the modification of images (24-26 WA and 29-30 WA).     

Fetal magnetic resonance imaging

PURPOSE OF REVIEW: Fetal magnetic resonance imaging is becoming more used in the evaluation of complex fetal abnormalities. Rapid advances in the technology and application of fetal magnetic resonance imaging necessitate a review of this subject. RECENT FINDINGS: Diffusion-weighted imaging, magnetic resonance spectroscopy and functional magnetic resonance imaging may allow assessment of fetal brain even before anatomical abnormalities are demonstrated. We discuss the uses of fetal magnetic resonance imaging in better assessment of pulmonary hypoplasia, congenital diaphragmatic hernia and renal anomalies. SUMMARY: The additional information from fetal magnetic resonance imaging, beyond that obtained by ultrasound, is invaluable in prenatal counseling, delivery planning and planning for pre- or postnatal intervention. As intrauterine and neonatal surgery evolve, so will the utilization of fetal magnetic resonance imaging.     

Fetal vertebral structure detected by three-dimensional ultrasound

The spinal cord is an important part of the central nervous system as well as the brain. The vertebral structure develops remarkably early during embryonal and fetal periods. As the cranium develops from early pregnancy with ossification and gradual closure of the fontanelles and sutures, the vertebral bony structure is also formed with ossification and gradual closure of the neural arches during the fetal period. Open spina bifida, which results from a failure of neural arch closure, has been demonstrated by ultrasound and magnetic resonance imaging in utero. Postnatal neurological deficit is greatly dependent on the lesion level of spina bifida and the condition of the exposed spinal cord. Therefore, accurate detection of the lesion level and spinal cord appearance before birth has an important role in predicting neurological prognosis. Because it is hard to demonstrate fetal bony structure including vertebral arches and ribs on a single image by two-dimensional (2D) ultrasound, precise diagnosis of spinal lesion is difficult for examiners. Several reports have been published on two-dimensional detection of the spinal lesion level. After introduction of three-dimensional (3D) ultrasound technology in the field of fetal neuroimaging diagnosis, however, more accurate, objective and comprehensive information has been added on fetal vertebra/spine development and spine lesion such as open spina bifida even in the first and early second trimesters. Multiplanar image analysis and rendering of bony structure by 3D ultrasound technology have contributed greatly to the precise demonstration of normal/abnormal vertebra and spinal cord. Exact and objective diagnosis of spina bifida during pregnancy leads to the prediction of postnatal neurological deficit and to proper management.     

Fetal renal volume assessment by three-dimensional ultrasonography

OBJECTIVE: This study was undertaken to measure fetal renal volume by means of three-dimensional ultrasonography and to use those data to establish the accurate constant and formula for fetal renal volume assessment with two-dimensional ultrasonography. STUDY DESIGN: Singleton fetuses between 15 and 40 weeks' gestation were included. The volumes of both fetal kidneys were measured with three-dimensional ultrasonography. Three fetal renal diameters (largest anteroposterior, transverse, and longitudinal diameters) were measured, and the constant of the fetal renal volume formula was calculated. Linear regression curves were made for each kidney from the fetal renal volume, the three diameters, and the constant. RESULTS: A total of 112 fetuses were included. The following fetal renal volume formula was established: Fetal renal volume = Constant (R) x Largest anteroposterior diameter (x) x Largest transverse diameter (y) x Largest longitudinal diameter (z). The volumes, the three diameters, and the constants appeared different between the right and left kidneys but not statistically significantly so. CONCLUSION: Three-dimensional ultrasonography is useful in assessing fetal renal volume. The fetal renal volume formula makes possible accurate measurements of fetal renal volume by two-dimensional ultrasonography.     

Fetal magnetic resonance imaging.

Fast magnetic resonance imaging (MRI) has revolutionized our ability to image the fetus. Using fast scanning techniques, individual images are obtained in 300-400 ms, allowing for imaging of the fetus without sedation. MRI is most useful for evaluation of the anomalous fetal central nervous system, for further characterization of complex anomalies not fully elucidated by ultrasound and for evaluation of patients desiring fetal surgery. This review describes the history of fetal MRI, discusses current applications and mentions developments on the horizon.     

Fetal evaluation by real-time imaging.

The application of real-time imaging to the study of fetal morphology and function is an exciting advance. Rapid accurate study of fetal dynamics is possible with this apparently safe, noninvasive technique. Animal studies have revealed the general physiology of fetal breathing movements, which are now under extensive investigation with real-time imaging in the human. There is evidence for circadian rhythm of human fetal breathing movements as well as a relationship to maternal substrate loading. The presence of fetal breathing movements of the rapid irregular type may be a reassuring sign in the presence of other indicators of fetal compromise. Using real-time B-scan imaging, we have found long periods of apnea in pregnancies with normal perinatal outcome. As suggested in the animal studies, changes in fetal breathing patterns prior to fetal death may be more complex than originally thought. The techique of real-time B-scan imaging with precise definition of apnea, general movements, and fetal breathing will provide valuable information on which to base our evaluation of the clinical usefulness of fetal breathing movement studies in the management of pregnancy.     

Fetal imaging in the skeletal dysplasias

Second-trimester diagnosis of fetal skeletal dysplasia is becoming increasingly common. By careful examination of the fetus for skeletal mineralization, shape and size of long bones and cranium, and abnormalities of other organ systems, a definitive diagnosis can often be achieved by ultrasound. Fetal radiography is very helpful in giving more information about bone shape and mineralization as well as in confirming the diagnosis. Subsequent obstetric management should be dictated by the natural history of the disease, the parents' wishes, and the gestational age at diagnosis. Because many of these disorders have a high recurrence risk, genetic counseling and follow-up are an important part of the management of these patients.     

Fetal microphthalmia diagnosed by magnetic resonance imaging

We report a case of fetal microphthalmia diagnosed midtrimester by ultrasound and fetal MRI. Included is a comparison of MRI measurements of normal fetuses at similar gestational age and a review of the literature.     

Fetal curarization for prenatal magnetic resonance imaging

Fetal magnetic resonance (MR) imaging was performed at 33 weeks of gestation for investigation of a posterior fossa abnormality found at ultrasound screening. Fetal movements were abolished by vecuronium injected under ultrasound guidance into the umbilical vein. MR images showed atrophy of the left cerebellar lobe with cisternal dilatation. These were confirmed postnatally by CT scan.     

Fetal magnetic resonance imaging: a review

PURPOSE OF REVIEW: To describe advances in magnetic resonance technology and the current indications and advantages of magnetic resonance imaging that have led to increased utilization in fetal medicine. RECENT FINDINGS: The article covers the most common uses of magnetic resonance imaging in fetal medicine. The advantages of magnetic resonance imaging for the diagnosis of fetal malformations are described, in particular the advantages of magnetic resonance imaging in central nervous system malformations not optimally diagnosed by ultrasound are described. These cases include malformations of migration, malformations of development, such as agenesis of the corpus callosum, and destructive lesions. Noncentral nervous system lesions include chest abnormalities, abdominal wall defects, gastrointestinal and genitourinary abnormalities and fetal neoplasms. Abnormalities of placentation and other maternal factors affecting pregnancy are shown. SUMMARY: Recent studies have shown that magnetic resonance imaging can add significantly to the prenatal diagnosis and management of congenital abnormalities. In addition, placental abnormalities have been diagnosed with greater accuracy.     

Fetal MR imaging in urogenital system anomalies

Fetal magnetic resonance imaging (MRI) is an important adjunct to ultrasound in evaluation of fetal urogenital system. While, ultrasound remains the primary modality, MRI helps in more complicated cases or where ultrasound is limited due to technical factors such as poor acoustic window. The goal of this article is to review the common fetal urogenital anomalies and their imaging manifestations on ultrasound and MRI, and discuss how MRI adds diagnostic value.     

Fetal brain magnetic resonance imaging and long-term neurodevelopmental impairment

Objective

To determine the incidence of fetal brain injury by fetal brain magnetic resonance imaging (MRI) in pregnancies complicated with preterm labor (PL), preterm premature rupture of the membranes (PPROM), and intrauterine growth restriction (IUGR), and to compare fetal brain MRI with prenatal surveillance methods, and with immediate and long-term neurodevelopmental outcome.

Methods

Between February 2007 and January 2009, high-risk pregnancies were analyzed by MRI at 1.5 Tesla after 24 weeks of gestation at the Clinical Hospital Center Zagreb, Croatia. Long-term outcome was defined as neurodevelopmental outcome at 24 months.

Results

Among 70 pregnancies analyzed, 40.0% had abnormal fetal brain MRI. The highest incidence occurred in the PL group. There was no correlation between abnormal MRI and fetal surveillance methods (ultrasound, Doppler blood flow analysis, cardiotocography, biophysical profile) or immediate neonatal outcome (1-minute Apgar score, umbilical cord pH). Via MRI, fetal brain injury would have been diagnosed for 45.7% of fetuses with a long-term neurodevelopmental handicap. Binary logistic regression showed that, as compared with other surveillance methods, fetal brain MRI was the best predictor of long-term neurodevelopmental disability.

Conclusion

PL, IUGR, and PPROM were associated with an early intrauterine CNS insult that was not accurately detected by existing prenatal testing options.     

Fetal weight prediction by thigh volume measurement with three-dimensional ultrasonography

OBJECTIVE: To evaluate the usefulness and accuracy of a simple method of predicting fetal weight by measuring fetal thigh volume with three-dimensional ultrasonography. METHODS: In 84 pregnant women, fetuses without structural or chromosomal anomalies were studied prospectively and cross-sectionally. Biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL) were measured by two-dimensional ultrasound. Fetal thigh volume was measured by three-dimensional ultrasound, using three cross-sectional images of femur, from proximal, middle, and distal parts of femur diaphysis. Infants were delivered within 48 hours after ultrasound examinations. RESULTS: Modified thigh volume measurements using three cross-sectional images of femur by three-dimensional ultrasound were correlated strongly with birth weight (R(2) = 0.921, P <.001). Using linear and polynomial regression, we calculated a new best-fit formula: Birth weight (g) = 165.32 + 28.78 x modified thigh volume (mL). The mean and standard deviation of the residual were 121.8 and 110.4, respectively, in three-dimensional formulas, which were significantly smaller than those of two-dimensional formulas. CONCLUSION: Thigh volume measurement using three cross-sectional images of femur by three-dimensional ultrasound was simple, and there was better accuracy with this method than with two-dimensional ultrasound methods for predicting fetal weight during the third trimester of pregnancy.     

Fetal adrenal gland volume and preterm birth: a prospective third-trimester screening evaluation

Objective: Given the importance of the fetal adrenal gland in producing hormones critical to labor, we sought to evaluate whether sonographic three-dimensional measurements of the adrenal gland are a useful screening tool for spontaneous preterm birth (SPTB).

Method: We prospectively screened 128 non-anomalous singletons from 24 to 36 weeks' gestation with volumetric measurements of the fetal adrenal gland at their indicated antenatal sonogram. Labor and delivery outcomes were assessed and compared with respect to adrenal volume.

Results: When corrected for estimated fetal weight, the 11 women (9%) who delivered following SPTB had smaller adrenals than those who did not, 0.33?cm³/kg compared with 0.57?cm³/kg, respectively (p?=?0.006). There was no difference in volumes between those who delivered by SPTB within 7 days or greater than 14 days from measurement (0.34?cm³/kg versus 0.33?cm³/kg, p?=?0.79). Among women at increased risk of SPTB, those with SPTB had smaller adrenals than those who did not: 0.32?cm³/kg versus 0.53?cm³/kg, p?=?0.06.

Conclusion: We found fetal adrenal glands significantly smaller for those delivering preterm. Given the prior literature and our asymptomatic population, our data support multiple pathways leading to SPTB.     

Magnetic resonance imaging of the adrenal gland in women with late-onset adrenal hyperplasia

To determine the presence of structural abnormalities of the adrenal in late-onset adrenal hyperplasia, four consecutive patients were studied by MRI before beginning glucocorticoid replacement therapy. Three women were diagnosed as 21-hydroxylase deficient late-onset adrenal hyperplasia by a 17-OHP level greater than 1,000 ng/dL 30 minutes after acute adrenal stimulation, and one patient was diagnosed as 11-hydroxylase deficient late-onset adrenal hyperplasia when her 11-deoxycortisol level was threefold the upper 95th percentile of normal. Two patients with 21-hydroxylase deficient late-onset adrenal hyperplasia had normal adrenal glands on MRI. Another 21-hydroxylase deficient late-onset adrenal hyperplasia patient was noted to have a 2.5 x 3.3-cm left adrenal nodule, which had been documented some 4 years earlier on CT scan and had not changed in size during that interval. This patient was 40 years of age when the diagnosis of late-onset adrenal hyperplasia was established. The patient with 11-hydroxylase deficient late-onset adrenal hyperplasia demonstrated a diffuse enlargement of the left adrenal gland consistent with hyperplasia, with no focal lesions. In conclusion, although patients with late-onset adrenal hyperplasia may often demonstrate nodular or diffuse adrenocortical hyperplasia on MRI, not all patients with endocrinologically evident disease demonstrate such abnormalities, consistent with a lesser degree of ACTH stimulation compared with women with classical congenital adrenal hyperplasia.