Ultrafast MRI of the fetus

Fetal Neuroimaging Update

OBJECTIVES

To review the current imaging techniques available for the evaluation of the fetal brain.

FINDINGS

Ultrasound remains the initial screening modality with routine scanning typically performed at 18-20 weeks gestation. When a central nervous system (CNS) abnormality is noted by ultrasound, MRI is increasingly being used to further clarify findings. Fetal MRI has the unique ability to provide high detailed anatomical information of the entire human fetus with high contrast resolution. This technique has grown due to the development of rapid single shot image acquisition sequences, improvement of motion correction strategies and optimizing shimming techniques.

CONCLUSIONS

The assessment of fetal CNS anomalies continues to improve. Advanced MRI techniques have allowed for further delineation of CNS anomalies and have become a cornerstone in the assessment of fetal brain well-being. Those interpreting fetal studies need to be familiar with the strengths and limitations of each exam and be sensitive to the impact discussing findings can have regarding perinatal care and delivery planning. Collaboration with neurologists, neurosurgeons, geneticists, counselors, and maternal fetal specialists are key in providing the best care to the families we treat.

Measurement of fetal conus distance with 3D ultrasonography as a reliable prenatal diagnosis method for tethered cord syndrome

AIM

The aim of this study is to investigate if three-dimensional (3D) ultrasonography is a reliable diagnosis method for prenatal tethered cord syndrome (TCS) by measuring fetal conus distance (CD) in comparison to two-dimensional (2D) ultrasonography and magnetic resonance imaging (MRI).

METHODS

This retrospective study included 468 normal fetuses as control group and 14 TCS fetuses as tethered group. CD were measured by 2D, 3D ultrasonography and MRI, and the reliability and repeatability of CD measurement was compared between two experienced ultrasound specialists or among the multiple measurements for each specialist.

RESULTS

The results showed that 3D ultrasonography was superior to 2D ultrasonography in the repeatability of measuring CD. The CD were positively correlated to gestational ages in control group (P < 0.05). The CD of TCS group (-1.342 ± 0.124) was significantly shorter than that of normal control group (0.013 ± 0.965) (P < 0.01). There were no significant differences in the effectiveness of locating conus medullaris and measuring CD between 3D ultrasonography and MRI.Conclusion Measurement of fetal CD with 3D ultrasonography can serve as a reliable and cost-effective prenatal diagnosis method for TCS.

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.

Congenital and acquired developmental problems of the upper airway in newborns and infants

AIM

To review the current knowledge on congenital and acquired developmental problems of the upper airway in newborns and infants.

DATA SYNTHESIS

Causes of airway obstruction include problems with the nasal airway (choanal atresia), craniofacial syndromes (Apert syndrome, Crouzon syndrome), problems with facial/tongue anatomy (Pierre-Robin syndrome), the tongue (Down syndrome), or the larynx (laryngomalacia, vocal cord palsy, subglottic stenosis, subglottic hemangioma), along with lower developmental problems (tracheo/bronchomalacia). After establishing a safe airway, a detailed assessment and appropriate management are necessary. Treatment may involve simple observation, conservative management, chest physiotherapy, CPAP ventilation, and surgery, urgently or in a second phase.

CONCLUSION

Upper airway diseases in neonates and infants may be life threatening, or challenging regarding diagnosis and management. There should be a very low threshold for referring these children, after establishing a safe airway, for a specialist opinion and care in a tertiary unit, if local facilities are limited or unavailable.

Magnetic resonance imaging of the normal fetal esophagus

OBJECTIVES

To establish the appearance on magnetic resonance imaging (MRI) of the normal fetal esophagus and to provide biometric data from 20 to 39 gestational weeks.

METHODS

In this retrospective study of 250 fetal MRI examinations (20-39 gestational weeks) the visibility of the upper (E1, cervical to tracheal bifurcation) and lower (E2, tracheal bifurcation to diaphragm) esophageal segments was assessed separately in each of three section planes. Segments were scored as being visible or not, and whether they were fluid-filled was noted. Maximum esophageal diameters were recorded. To study the age dependency of esophageal visualization, fetuses were divided into three age groups: 20-25, 26-31 and 32-39 weeks. In 19 cases, there were dynamic studies and these were analyzed for the duration of fluid-filling.

RESULTS

Segment E1 was visualized only when it was fluid-filled and it was visible in 16.8% of axial, 4% of frontal and 22.4% of sagittal acquisitions. Segment E2 could also be detected in the unfilled state, when it appeared solid; it was seen in 96.4% of axial, 74% of frontal and 39.6% of sagittal acquisitions, being fluid-filled in 62.7-88.9% of these. Depending on the amount of fluid-filling and the gestational age, E2 segments had a variable appearance. Dynamic scans showed E1 segments to be fluid-filled for periods of 0.5-1.5 s and E2 segments for periods of 3-35 s. Both the frequency of visualization and the maximum diameter increased linearly with gestational age.

CONCLUSION

Visualization of the fetal esophagus by MRI depends on section plane, segment and gestational age, with considerable differences between the two segments: while E1 requires fluid-filling related to fetal swallowing, the E2 portion can usually be demonstrated independent of fluid-filling, as a consequence of its topographical relationships. The frequently fluid-filled state of E2 may reflect the immature nature of esophageal neuromuscular activity, and the lack of an anti-reflux mechanism due to the short length of the abdominal esophagus.

Water imaging (hydrography) in the fetus: the value of a heavily T2-weighted sequence

BACKGROUND

Since the development of fast imaging sequences, MR has proved to be a helpful tool in the evaluation of fetal pathology. Because of the high water content of fetal tissues and pathology, hydrography imaging (MR fetography) can provide additional diagnostic information.

OBJECTIVE

To demonstrate the benefit of MR fetography in fetal imaging.

MATERIALS AND METHODS

From 2004 to 2005, 126 fetal MR examinations were performed for evaluation of an abnormality depicted on an antenatal sonogram. Single-shot fast spin-echo MR imaging and MR fetography were performed through the area of fetal pathology. The two studies were retrospectively compared.

RESULTS

The primary diagnosis was not changed with the addition of MR fetography. New findings, particularly in the kidneys and spine, were identified in 9% of the patients. When fetal pathology was of high water content (80% patients), the MR fetography imaging increased diagnostic confidence. In 11% of the patients, those with cardiovascular or low water pathology, the MR fetography was not beneficial.

CONCLUSION

The mainstay of fetal imaging is currently the HASTE and SSFSE sequences. However, MR fetography is an excellent adjunct that highlights fetal pathology by reinforcing the diagnosis, identifying additional findings, and providing high-contrast high-resolution images that are helpful when counseling clinicians and patients.

Dynamic motion analysis of fetuses with central nervous system disorders by cine magnetic resonance imaging using fast imaging employing steady-state acquisition and parallel imaging: a preliminary result

OBJECT

The authors present a novel cine magnetic resonance (MR) imaging, two-dimensional (2D) fast imaging employing steady-state acquisition (FIESTA) technique with parallel imaging. It achieves temporal resolution at less than half a second as well as high spatial resolution cine imaging free of motion artifacts for evaluating the dynamic motion of fetuses in utero. The information obtained is used to predict postnatal outcome.

METHODS

Twenty-five fetuses with anomalies were studied. Ultrasonography demonstrated severe abnormalities in five of the fetuses; the other 20 fetuses constituted a control group. The cine fetal MR imaging demonstrated fetal head, neck, trunk, extremity, and finger as well as swallowing motions. Imaging findings were evaluated and compared in fetuses with major central nervous system (CNS) anomalies in five cases and minor CNS, non-CNS, or no anomalies in 20 cases. Normal motility was observed in the latter group. For fetuses in the former group, those with abnormal motility failed to survive after delivery, whereas those with normal motility survived with functioning preserved. The power deposition of radiofrequency, presented as specific absorption rate (SAR), was calculated. The SAR of FIESTA was approximately 13 times lower than that of conventional MR imaging of fetuses obtained using single-shot fast spin echo sequences.

CONCLUSIONS

The following conclusions are drawn: 1) Fetal motion is no longer a limitation for prenatal imaging after the implementation of parallel imaging with 2D FIESTA, 2) Cine MR imaging illustrates fetal motion in utero with high clinical reliability, 3) For cases involving major CNS anomalies, cine MR imaging provides information on extremity motility in fetuses and serves as a prognostic indicator of postnatal outcome, and 4) The cine MR used to observe fetal activity is technically 2D and conceptually three-dimensional. It provides four-dimensional information for making proper and timely obstetrical and/or postnatal management decisions.

The fetus cannot exercise like an astronaut: gravity loading is necessary for the physiological development during second half of pregnancy

On the basis of published Magnetic Resonance Images and the values of the specific fetal and amniotic fluid weights, apparent weight of the fetus from the 18th week of gestation until term was determined. Up to the 21-22nd gestation week the fetus is in conditions similar to neutral floating, while after the 26th gestation week the apparent weight of the fetus is 60-80% of the actual weight. Decreased effect of the buoyant forces that affect the fetus in human species during the last trimester has a number of implications for the colonization of the solar system. During space flight it is impossible to apply the existing countermeasures against microgravity deconditioning of the muscular and cardiovascular systems to the fetus. Absence of gravitational loading during the last trimester of gestation would cause hypotrophy of the spinal extensors and lower extremities muscles, reduction in the amount of myosin heavy chain type I in the extensor muscles of the trunk and legs, hypoplasy and osteopeny of the vertebras and lower extremities long bones, and hypotrophy of the left ventricle of the heart muscle. Because of decreased capacity of postural and locomotor stability, acquisition of the gross developmental milestones such as sitting, standing and walking could be delayed. In the authors' opinion, only artificial gravity (rotating platform) during space flight will allow physiological development of the human fetus. Independency of offspring's of the guinea pig as regards locomotion and nursing increases probability of successful breeding in microgravity compared with rat offspring's, and make this species a candidate for future experiments under conditions of microgravity and hypergravity. Examining the gestation of this species in different gravities requires first the experimental determination of the amount of buoyant force to which the fetus is exposed in physiological conditions.

Comparison of ultrasound and magnetic resonance imaging in 100 singleton pregnancies with suspected brain abnormalities

OBJECTIVE

To compare the diagnostic accuracy of the current reference standard-ultrasound with in utero magnetic resonance imaging, in a selected group of patients.

DESIGN

Prospective study.

SETTING

Five fetal maternal tertiary referral centres and an academic radiology unit.

SAMPLE

One hundred cases of fetuses with central nervous system abnormalities where there has been diagnostic difficulties on ultrasound. In 48 cases the women were less than 24 weeks of gestation and in 52 cases later in pregnancy.

METHODS

All women were imaged on a 1.5 T clinical system using a single shot fast spin echo technique. The results of antenatal ultrasound and in utero magnetic resonance were compared.

MAIN OUTCOME MEASURES

The definitive diagnosis was made either at autopsy or by postmortem magnetic resonance imaging, in cases that went to termination of pregnancy, or a combination of postnatal imaging and clinical follow up in the others.

RESULTS

In 52 of cases, ultrasound and magnetic resonance gave identical results and in a further 12, magnetic resonance provided extra information that was judged not to have had direct effects on management. In 35 of cases, magnetic resonance either changed the diagnosis (29) or gave extra information that could have altered management (6). In 11 of the 30 cases where magnetic resonance changed the diagnosis, the brain was described as normal on magnetic resonance.

CONCLUSIONS

In utero magnetic resonance imaging is a powerful tool in investigating fetal brain abnormalities. Our results suggest that in selected cases of brain abnormalities, detected by ultrasound, antenatal magnetic resonance may provide additional, clinically useful information that may alter management.