Fetal MR imaging

Ultrasonography is the primary prenatal screening modality used in the evaluation of the fetus and the maternal pelvis. However, fetal MR imaging plays a complementary role to prenatal ultrasound in the evaluation of the fetus with suspected abnormalities. MR imaging's role includes confirming or excluding possible lesions, defining their full extent, aiding in their characterization, and demonstrating other associated abnormalities. As newer techniques such as diffusion imaging, MR spectroscopy, and functional studies are used more widely, it is hoped that additional information will be made available by this modality to physicians evaluating and taking care of fetuses.     

Fetal magnetic resonance imaging

Ultrafast magnetic resonance imaging (MRI) sequences have changed the use of MRI to evaluate fetal abnormalities. Currently, the best application is the evaluation of suspected brain abnormalities found on ultrasound. MRI differentiates the various types of fetal ventriculomegaly. Superior posterior fossa visualization allows differentiation of Dandy-Walker malformation from a large cisterna magna. Anomalies of the corpus callosum can be seen. MRI also is valuable in the evaluation of fetal giant neck masses for planning delivery of the baby and surgery for life-threatening airway obstruction. In the chest, MRI differentiates masses such as diaphragmatic hernia, cystic adenomatoid malformation, and sequestration, and it aids in planning fetal surgery because MRI directly visualizes the position of the lung, liver, and bowel.     

Fetal brain MR imaging

In the past 10 years, improvements in MR imaging and faster imaging techniques have dramatically increased the use of in-utero fetal brain MR imaging. Challenging abnormalities now can be diagnosed prenatally through a careful analysis of morphology and signal changes. Using illustrations of normal brain development as a starting point, this article illustrates and discusses major brain malformations and specific morphologic changes, destructive lesions, and isolated ventriculomegalies, and the advantages and disadvantages of T1 and T2 sequences are provided.     

Fetal diffusion imaging: pearls and solutions

ABSTRACT: Recently, diffusion-weighted (DWI) magnetic resonance imaging of the fetus has evolved from a basic research application to an important diagnostic imaging tool in fetal magnetic resonance imaging. Although technically challenging and still plagued with several sources of artifacts, DWI can add clinically important information, which cannot be provided by any other prenatal imaging modality. Its potential to noninvasively probe tissue structures on the basis of Brownian molecular motion enables the detection of early changes associated with acute fetal diseases, as well as structural alterations of functionally diverse compartments of different fetal organs. In this article, the current clinical applications of fetal brain and body DWI are outlined, as well as its current limitations.     

胎儿磁共振——磁共振检查的新领域

随着磁共振成像技术的改进,MRI现在可应用于胎儿诊断。MRI提供的胎儿结构信息的深度,意味着它不仅仅是超声有用的辅助诊断手段,史能清晰地显示胎儿结构,避免超声的局限性。目前,MRI在胎儿中枢神经系统异常中的诊断最有价值,在中枢神经系统,在脑室扩大、胼胝体发育不全和后颅窝畸形的诊断中最有优势,此外,在胎儿胸部和其他系统畸形诊断中逐渐显示优势。MRI还能准确测量胎儿器官体积和重量,可以评估胎儿生长受限。一些MRI新技术如弥散加权成像、磁共振波谱和功能成像也逐渐应用于胎儿,可以提高我们对胎儿在子宫内代谢和发育信息的认识。本文将详细介绍胎儿MRI的安全性、技术及各系统临床应用。     

Fetal brain imaging: a comparison between magnetic resonance imaging and dedicated neurosonography.

OBJECTIVES: To evaluate whether fetal brain magnetic resonance imaging (MRI) adds useful clinical information to that obtained by dedicated fetal neurosonography using a combined transabdominal and transvaginal approach in fetuses with suspected brain anomalies. METHODS: In the 2-year period between January 2000 and January 2002, 42 fetuses underwent neurosonographic and MRI examinations of the brain. The referral indications were: asymmetric ventriculomegaly (13), ventriculomegaly (7), periventricular cysts (2), suspected midline findings (7), agenesis of the corpus callosum (3), infratentorial pathology (3), cytomegalovirus (CMV) infection (2) and miscellaneous indications (5). RESULTS: Neurosonography and MRI produced similar diagnoses in 29 fetuses: normal examination (10), isolated asymmetric ventriculomegaly (11), isolated ventriculomegaly (3), periventricular cysts (2), agenesis of the corpus callosum (1), pericallosal lipoma (1) and cerebellar hemorrhage (1). The neurosonographic diagnoses were more accurate in seven patients: hemimegalencephaly, pericallosal lipoma, signs of CMV infection, brain anomalies associated with agenesis of the corpus callosum and three fetuses with a normal ultrasound scan in which MRI suggested a parenchymal abnormality. MRI provided a more accurate diagnosis in three patients: a third ventricular dilatation was ruled out, normal ventricles in a fetus with an ultrasonographic finding of asymmetric ventricles, and diagnosis of progression of asymmetric ventriculomegaly. In three patients the identified pathologies were differently interpreted, each examination provided another aspect of the anomaly or a definitive diagnosis was not possible. CONCLUSIONS: Our study demonstrated that dedicated neurosonography is equal to MRI in the diagnosis of fetal brain anomalies. In most of the cases MRI confirmed the ultrasonographic diagnosis; in a minority of cases each modality provided additional/different information. The major role of MRI was in reassurance of the parents regarding the presence or absence of brain anomalies.     

Fetal magnetic resonance imaging as a complement to fetal ultrasonography

Ultrasonography is the screening method of choice for the evaluation of the fetus. It is safe, inexpensive, and easily performed. However, it is operator dependent, and evaluation may be limited because of fetal position, maternal obesity, overlying bone, and/or oligohydramnios. Magnetic resonance imaging is an alternative modality that uses no ionizing radiation, has excellent tissue contrast and a large field of view, is not limited by obesity or overlying bone, and can image the fetus in multiple planes, no matter the fetal lie. Faster scanning techniques allow studies to be performed without sedation in the second and third trimester with minimal motion artifact.     

Fetal and placental volumetric and functional analysis using echo-planar imaging

Recent and past work using echo-planar imaging (EPI) in pregnancy has allowed important anatomic and physiological information to be obtained, giving advantages over conventional radiological methods such as ultrasound. EPI is a quick, convenient method of measuring organ volumes. The volumetric estimates throughout gestation correlate well with known fetal weight at these gestations. Relaxation time measurements also can be made in the placenta and lungs. By combining the changes in relaxation and volume with gestation in the future, it may be possible to develop an "index of maturity." This could be used to accurately reflect lung maturation. T1 and T2 parameters in the placenta decreased with gestational age and with abnormal placentation. EPI can be used to assess perfusion in the placenta and flow in the uterine arteries because of its rapid acquisition times. These techniques have been applied to assess perfusion within the fetal brain.     

Fetal magnetic resonance imaging in the prenatal diagnosis of cerebellar hemorrhage.

We report the case of a fetus with a sonographic mid-gestation diagnosis of hyperechogenic cerebellum suspected to be of hemorrhagic origin on fetal brain magnetic resonance imaging (MRI). No etiological factors for fetal hemorrhage were found other than a maternal heterozygocity for factor V Leiden. Following termination of the pregnancy, autopsy confirmed the prenatal diagnosis of massive cerebellar hemorrhage without underlying vascular anomaly. As an additional tool to ultrasonography, fetal brain MRI can affirm the hemorrhagic origin of hyperechogenic cerebellar lesions, especially by showing a high signal on T1-weighted images.     

Fetal and placental power Doppler imaging in normal and high-risk pregnancy.

OBJECTIVE: Computer analysis of organ power Doppler (PD) imaging has recently become available. The aim of the study was to evaluate gestational trends in placenta-, fetal lung-, liver- and kidney-blood flow in normal pregnancies and relate it to signals in high-risk pregnancies. METHODS: PD signals were recorded in normal singleton pregnancies between 26 and 42 weeks of gestation. Signals were also recorded in 63 high-risk pregnancies. Fixed preinstalled PD system installations for each organ were used during examinations. Images from PD scan were recorded on video tape and off-line analysed by computer. Mean flow signal intensity was calculated for each organ. Umbilical and uterine artery Doppler velocimetry were also recorded in high-risk pregnancies. RESULTS: PD signals from the four organs indicated increasing organ blood flow until approximately 34 weeks of gestation, from where the organ signals seemed to decrease. High-risk pregnancies seemed to have lower PD signal intensity, which was more pronounced in cases with signs of placental vascular resistance. CONCLUSION: The results suggest that a decrease in fetal organ blood flow might indicate a centralisation of fetal circulation in normal pregnancy at term, as a physiological response to a decrease in placental perfusion. In the high-risk pregnancies the placental and fetal organ blood flow seem to be even further reduced, suggesting a more intense centralisation of circulation.     

Fetal lung volume: three-dimensional ultrasonography compared with magnetic resonance imaging.

OBJECTIVES: An accurate and reliable method for measuring fetal lung volumes would be helpful in predicting the outcome in cases with suspected impaired lung growth. Recent studies show that it is possible to obtain fetal lung volume estimations with magnetic resonance imaging (MRI) and three-dimensional (3D) ultrasonography. The purpose of this study was to assess the agreement of lung volumes measured with 3D ultrasonography and MRI in uncomplicated pregnancies. METHODS: This was a prospective study in which MRI and 3D ultrasonography examinations were conducted on the same day to measure the fetal lung volumes of 10 women with uncomplicated pregnancies. Intraclass correlation was used to evaluate the agreement between fetal lung volume measurements obtained by MRI and 3D ultrasonography. A proportionate Bland-Altman plot was constructed. RESULTS: The intraclass correlation coefficient between MRI and 3D ultrasonography measurements for the right lung was 0.92 (95% CI 0.71-0.98) and for the left lung was 0.95 (95% CI 0.82-0.99). The proportionate limits of agreement between the methods were for the right lung -32.57% to 20.03% and for the left lung -21.26% to 17.13%. CONCLUSIONS: There is good agreement between lung volumes measured by MRI and those measured by 3D ultrasonography.     

Fetal echocardiography and color Doppler flow imaging: the Rotterdam experience.

The adjunctive role of Doppler color flow mapping in the evaluation of cardiac structures and function was studied in 440 normal fetuses between 17 and 22 weeks of gestation (median, 20 weeks) and in 73 fetuses with suspected congenital heart disease between 16 and 38 weeks of gestation (median, 28 weeks). Flow through atrioventricular and arterial valves was generally easy to identify and identification was successful in approximately 90% of the normal fetuses. Flow in the pulmonary veins and through the foramen ovale was visualized in approximately 60% of normal cases. In the group with suspected congenital heart disease, Doppler color flow imaging provided additional information on both cardiac structure and function in 34 fetuses, on function alone in 13 fetuses and on structure alone in 20 fetuses. No additional information was collected in six fetuses.     

Fetal magnetic resonance imaging in the antenatal diagnosis and management of hydrocolpos.

Hydrocolpos should be considered systematically when an abdominopelvic cystic mass is diagnosed in a female fetus. Because the prognosis and neonatal management of isolated hydrocolpos with spontaneous resolution differs greatly from that of hydrocolpos associated with a cloacal malformation, it is important to ascertain prenatally whether there are associated anomalies. We report the prenatal characteristics of three fetuses with hydrocolpos; in two cases there was spontaneous resolution and one infant was born with digestive tract atresia. The principal ultrasound findings were an oblong anechoic pelvic mass with or without a sagittal septum, located behind a normal bladder. On magnetic resonance imaging (MRI), the cervical imprint on the vagina confirmed the diagnosis of hydrocolpos and helped to diagnose cloacal malformation by demonstrating the absence of meconium beside the bladder on T1 sequences. Our cases show that MRI is useful for differentiating isolated hydrocolpos from hydrocolpos associated with cloacal malformation.