MRI of Fetal and Maternal Diseases in Pregnancy 1st ed.

4. How to Read and to Report a Fetal MRI Examination

Sahar N. Saleem 

(1)

Department of Radiology, Kasr Al Ainy Faculty of Medicine, Cairo University, Cairo, Egypt

Sahar N. Saleem

Email: saharsaleem1@gmail.com

Email: saharsaleem1@ymail.com

Keywords

FetalMRIInterpretationReportCounseling

4.1 Introduction

Fetal MRI is an interactive scanning of the moving fetus owed to the use of ultrafast MRI sequences [1]. T2 single-shot fast spin echo (SSFSE) is the standard sequence; balanced steady-state free precession (SSFP) demonstrates well the fetal anatomy and cardiovascular system, and T1-weighted gradient echo sequence is primarily used to show hemorrhage, calcification, fat, and meconium [23]. Advanced MRI techniques, such as diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI), and magnetic resonance spectroscopy (MRS), have been increasingly used lately [4]. The use of fetal MRI is not designed to replace ultrasound as the obstetric diagnostic tool of choice, but rather to act as an adjunct in certain indications [5]. The aim of fetal MRI is to help to reach an accurate and complete prenatal diagnosis [6].

4.2 Who Is Qualified to Do and Interpret Fetal MRI?

Fetal MRI should only be carried out in a center containing an up-to-date MRI scanner (the standard is 1.5 T machine), in presence of an experienced radiologist in fetal MRI, in addition to clinical expertise in fetal medicine for extensive prenatal counseling [6].

Fetal MRI scanning and image interpretation should be preferably carried out by the same radiologist who must have adequate recognized MRI training including appropriate protocol selection and modification if necessary. Knowing the developing fetal MRI anatomy, recognizing pathology, and understanding the short- and long-term consequences of congenital abnormalities are also essential requirements for proper interpretation of fetal MRI. Health services should carefully address the credential scope of practice and audit before providing fetal MRI service. The ideal setting for fetal MRI is thus a tertiary referral medical center with standardized referral and scanning protocols, established patients counseling, and regular review and evaluation of outcomes [78].

4.3 What Do You Need to Know Before Interpreting Fetal MRI?

Fetal MRI should never be carried out in a vacuum. Referral clinical data and up-to-date ultrasound information are necessary for planning the MRI technique and establishing the focus of image interpretation [5].

4.3.1 Before Doing Fetal MRI Examination

Make sure you know the following:

1.

2.

3.

4.3.2 At the Time of Fetal MRI Examination

The radiologist should monitor fetal MRI scan and tailor the MRI examination for evaluation of the abnormalities that were detected or suspected by prenatal ultrasonography. Fetal MRI usually focuses on the organ of referral query. However, as prognosis of fetal anomaly is related to the presence of additional abnormalities, fetal MRI is used in some centers to scan all fetal organ systems [5].

The patient usually requests a provisional report following the fetal MRI scan. However, it is prudent not to discuss the findings at this early stage till you take your time in interpreting the images and finalizing a detailed report.

4.4 How to Interpret Fetal MR Images?

We include our tips for successful interpretation of fetal MRI.

4.4.1 Examine Every Fetal MR Image Even the Scout

Each image of the fetal MRI may add a piece of information that may help in confirming or excluding fetal abnormalities. Start with examining the scout MR images along the maternal planes (coronal, axial, and sagittal) (Fig. 4.1). These large field of view images allow overview of the fetus and the surrounding maternal structures. Large field of view images help in determination of the number of gestations, position of the fetus, and the right and left side of the fetus in relation to the mother which is important for detection of situs abnormalities [5].

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Fig. 4.1

Selected images of SSFP survey scan of a fetus at 24 weeks of gestation. (a) Coronal image through the pregnant women shows the full size of the gravid uterus and the surrounding maternal structures; it also provides a midline sagittal section through a single fetus in a breach position. (b) Sagittal image shows the position of the cervix and maternal structures such as lumbosacral spine and yields an axial slice through the fetal abdomen showing a fluid-filled stomach, and (c) axial image provides an axial slice through the fetal head and shows the anterior position of the placenta

4.4.2 Recognize the Appearances of the Fetal Structures in the Different MRI Sequences for Tissue Characterization

Recognition of the appearances of several fetal structures on both T2- and T1-weighted sequences helps in their tissue characterization.

T2-weighted (SSFSE) sequence is the standard sequence in fetal MRI and provides most of information. T1-weighted (gradient echo) sequence has lower resolution and provides little information over T2 weighted. However, the following structures can be recognized by their high signal intensity appearance on T1-weighted images: blood, fat, meconium, thyroid, and liver (Fig. 4.2) [5].

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Fig. 4.2

Coronal T1-weighted gradient echo of a fetus at 30 weeks of gestation. The liver (short arrow) and meconium (long arrow) appear as high signal intensity on T1-weighted images. Note that the clear fluid within the stomach appears as low signal intensity (arrowhead)

CSF and clear fluid appear as high signal intensity on T2-weighted images and low on T1-weighted images. The liver appears low signal intensity on T2-weighted images and high on T1-weighted images. The spleen shows homogeneous low signal intensity on T2-weighted images [57].

Balanced steady-state free precession (SSFP) is the preferred sequence for visualization of the cardiovascular structures. The fetal heart and vessels appear as high signal intensity structures (bright blood) on SSFP, while they are less depicted on T2 SSFSE (dark blood) (Fig. 4.3) [2].

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Fig. 4.3

Axial MR images of fetal thorax at 24 weeks of gestation using T2-weighted SSFSE (a) and SSFP (b). The fetal heart and vessels appear as low signal (dark blood) on SSFSE and high signal (bright blood) on SSFP. Clear fluid appears as high signal intensity in both SSFSE and SSFP sequences

4.4.3 Focus on the Organ of the Referred Query

Fetal MRI is usually carried to confirm or exclude inconclusive sonographic findings. The most common referral queries for fetal MRI are neurological such as ventriculomegaly, dysgenetic corpus callosum, cortical malformations, posterior fossa abnormalities, or to determine the level of the defect in myelomeningocele before in utero surgery [16]. The next common query for fetal MRI interpretation is whether a mass in the chest is a congenital diaphragmatic hernia or pulmonary [7]. Less commonly are queries about the possibility of airway obstruction caused by neck or thoracic masses, organ system origin of a complex abdominal mass, or presence of fetal abnormalities in cases of oligohydramnios [78]. Recently, MRI is requested for assessment of fetal complex malformations of the cardiovascular system [9].

4.4.4 Full Fetal Anatomic Survey: When Do You Need It?

Fetal MRI is usually required to assess a certain anatomic region of inconclusive or occult ultrasound diagnosis. However, a complete anatomic survey of the fetus can be required especially in cases with suspected complex anomalies. The large field of view of MR images facilitates examination of fetuses with large or complex anomalies and enables visualization of the lesion within the context of the entire fetal body (Fig. 4.4).

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Fig. 4.4

The large field of view of MRI visualizes large anomaly (sacrococcygeal teratoma) within the context of the entire fetal body. Sagittal T2-weighted SSFP image of a fetus at 30 weeks of gestation shows the extra-fetal (short arrow) and intra-fetal (long arrow) components of a large sacrococcygeal teratoma

4.4.5 Follow a Scheme for Image Interpretation of Fetal Organ Systems

Following a systematic order for evaluation of the fetal structures in MR images makes it possible to diagnose accurately and less likely to miss findings [5].

The following is a suggestion of organ system-oriented approach for interpretation of fetal MRI (Table 4.1).

Table 4.1

System-oriented approach for interpretation of fetal MRI

Fetal system

Structure/feature

MRI planes and sequences

MRI appearance of normal fetal anatomy

Related pathology

CNS

Parenchyma

Axial T2-WI

Before 26 W: 3 layers (germinal matrix and cortex are low-signal, and intermediate layer in between)

After 26 W: 2 layers (germinal matrix disappears leaving cortex and white matter)

Cortical malformation, hemorrhage, gliosis

Myelination

T1-WI

Myelin appears as high signal

Impaired

Cortical sulci

Axial, sagittal, and coronal T2-WI

Smooth brain surface early in gestation then timely progressive appearance of sulci and gyri

Malformation of cerebral cortical development, Lissencephaly, polymicrogyria, schizencephaly, SEH

Ventricular system

Axial T2-WI

Symmetrical lateral ventricles; normally atrial diameter is less than 10 mm

Ventriculomegaly

Corpus callosum

Sagittal T2-WI

By 15 W, all parts are formed and appears as low signal C-shaped structure

Agenesis (partial, complete), and hypogenesis

Posterior fossa

Axial, sagittal and coronal T2-WI

Convex anterior border of pons; TCD increases progressively during gestation; inferior vermis develops by 17-18; normal anteroposterior diameter of cisterna magna is less than 10 mm

Cerebellar hypoplasia, vermian hypoplasia, pontocerebellar hypoplasia, JSRD, DWM, Chiari malformation

Spine

Axial, sagittal, and coronal T2-WI

The entire spine and vertebral elements could be visualized

NTD, sacrococcygeal tumors

Face

Face

Axial, coronal, and sagittal T2-WI

Fetal profile: interorbital space, eyes, lens, external ear, teeth buds, mandible, palate

Craniofacial anomalies: anophthalmia, micrognathia, cleft palate

Neck

Neck

Axial, coronal, and sagittal T2-WI

Airways: high signal

Teratoma, hamartoma, lymphangioma

Chest

Lungs

Axial, coronal, and Sagittal T2-WI

Initial intermediate signal intensity on T2-WI; signal intensity and volume increases with advanced gestion

Pulmonary hypoplasia, CCAM, sequestration, CDH

Airways

Axial, coronal T1-WI and T2-WI

Fluid of high signal intensity on T2-WI and low on T1-WI

Congenital pulmonary airway malformation; CHAOS

Cardiovascular

Heart

Axial, coronal, sagittal. SSFP (bright blood); T2 SSFSE (dark blood)

Viscero-atrial situs, cardiac position and axis, chambers, inflow veins, outflow vessels, ventriculo-arterial concordance, side of aortic arch, cardiac masses, and pericardial effusion

Situs inversus totalis, ectopia cordis, CHD, rhabdomyoma, pericardial effusion

Abdomen and pelvis

Liver

Axial, coronal T1-WI and T2-WI

High signal on T1-WI and low on T2-WI

Hepatic cyst, choledochal cyst

spleen

Axial, coronal T2-WI

Detectable by 20 W, moderate low signal on T1-WI and high signal on T2-WI

Polysplenia, splenomegaly

GIT

Axial, coronal, Sagittal T1-WI and T2-WI

Esophagus, stomach, and proximal intestine contain fluid of low signal on T1-WI and high signal on T2-WI

Distal intestine after 20 W contain meconium: high signal on T1-WI and low signal on T2-WI

Bowel atresia, gastroschisis, and omphalocele

Genitourinary tract

Axial, coronal, sagittal T2-WI

Renal cortex is hypointense to medulla on T2-WI. Urinary bladder is filled with fluid of high signal on T2-WI and low on T1-WI

Renal cystic disease, renal tumors, obstructive uropathy, ovarian cysts, hypospadia/epispadia

Musculoskeletal

Bones

EPI, thick-slab T2-WI, dynamic SSFP

EPI shows bone (low signal) and epiphyseal cartilage (high signal); thick-slab T2-WI, overview 3D impression; dynamic SSFP, movements

Skeletal dysplasia, deformities (club foot, arthrogryposis)

Pregnancy-related structures

 

Axial, coronal, and sagittal T1-WI and T2-WI

Placenta, umbilical cord, amniotic fluid

Placental insertion abnormalities, placental tumors, oligohydramnios, polyhdramnios

AbbreviationsACC agenesis of corpus callosum, AP anteroposterior, CCAM congenital cystic adenomatoid malformation, CDH congenital diaphragmatic hernia, CHAOS: congenital high airway obstruction sequence, CHDcongenital heart disease, CNS central nervous system, DWM Dandy-Walker malformaiton, EPI echoplanar imaging, GIT gastrointestinal tract, JS Joubert syndrome, NTD neural tube defect, SEH subependymal heterotopias, SSFPsteady-state free precession, TCD transverse cerebellar diameter, W week of gestation, WI weighted images, 3D three-dimensional

4.4.5.1 CNS

The initial MRI appearance of the fetal brain on T2-weighted images is characterized by smooth surface, large ventricles, and multilayered cerebral parenchyma composed from inside outward of hypointense germinal matrix, intermediate layer, and hypointense cortex [510]. As the brain matures with advancement of gestation, the following changes occur simultaneously: timely progressive appearance of sulci and gyri, the multilayered cerebral parenchyma changes to cortex and white matter; myelination, and reduction of size of ventricles and CSF spaces (Fig. 4.5) [10].

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Fig. 4.5

MRI interpretation of normal fetal brain maturation. (a) Axial T2-weighted image of the brain at 19 weeks of gestation. The brain has a smooth surface and multilayered parenchyma: an inner low signal intensity germinal matrix (short arrow), an intermediate layer, and an outer low signal intensity developing cortex (long arrow). The ventricles and the subarachnoid spaces are prominent. (b) Axial T2-weighted image of the brain at 35 weeks of gestation shows normal maturation in the form of appearance of the cortical sulci, disappearance of the germinal zone leaving two layers (cortex and white matte), and reduction of the size of the ventricles and the subarachnoid space

Interpretation of fetal MR images of the brain should include cortical sulci and gyri, cerebral parenchyma and myelination, ventricles and CSF spaces, corpus callosum, and posterior fossa.

Cortical Sulci and Gyri

We include here a suggested scheme for detecting maturation of cerebral sulcation in T2-weighted fetal MR images (Fig. 4.6).

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Fig. 4.6

Applying a scheme for detecting maturation of cerebral sulcation in T2-weighted MR images in a fetus at 30 weeks of gestation. (a) Axial MR image shows sylvian fissure (arrowhead); (b) midline sagittal image shows parieto-occipital fissure (short black arrow), calcarine (long white arrow), cingular (short white arrow), and marginal sulcus (long black arrow). Appearance of the marginal sulcus indicates a gestational age of 27 weeks or more; (c) sagittal image off the midline shows development of the central (black arrowhead), precentral (white arrow), and postcentral (black arrow) sulci. The appearance of postcentral sulcus indicates a gestational age of 28 weeks or older; (d) anterior coronal image shows superior (short arrow) and inferior (long arrow) frontal sulci indicating a gestational age of 29 weeks or older. (e) Coronal plane at the third ventricle shows appearance of the superior temporal sulcus (arrow) but not the inferior which starts to appear at 33 weeks. The gestational age-related cortical development is likely more than 29 weeks and less than 33 weeks of gestation; this correlated well with the referred gestational age of (30 weeks)

Start with axial MR images. Look for the sylvian fissure which normally begins to appear at 16 weeks of gestation; however, note that the cortex does not undergo infolding and opercular formation until 34 weeks of gestation.

Then examine sagittal MR images. Start with the midline sagittal image search for the parieto-occipital fissure (appears at 22 weeks), the calcarine (appears at 24 weeks), the cingular sulcus (appears at 24 weeks), and the marginal (appears at 27 weeks). Then examine the sagittal images just lateral to the midline. Search for the central sulcus (appears starting from 26 weeks), the precentral (appears at 27 weeks), and the postcentral sulcus (appears at 28 weeks).

Finally, examine the coronal images. In the anterior coronal images, search for the superior and inferior frontal sulci (both appear starting from 29 weeks of gestation). In the coronal plane at the level of the third ventricle, search for the superior temporal sulcus (appears at 27 weeks) and the inferior temporal sulcus (appears at 33 weeks) [11]. Note that by 34 weeks of gestation, most all primary and some secondary sulci are visible on fetal MRI [12].

The degree of sulcation on fetal MRI indicates the gestational age-related cortical development [10]. This age is to be correlated with the referred gestational age (based on ultrasound examination done early in pregnancy); discordance may alert possible abnormality. Cortical malformations are identified in fetal MRI by noting alteration of the normal sulcation pattern: too many sulci (polymicrogyria), too few sulci (lissencephaly), and abnormally deep or abnormally located sulci (schizencephaly) [1510]. However, for diagnosing sulcation abnormality using fetal MRI, the examination should be undertaken late in pregnancy at least after 30 weeks of gestation [5]. Subependymal heterotopias can be detected as nodules along the ventricular walls. Heterotopias can be differentiated from subependymal tubers based on their MRI signal appearance. Heterotopias are isointense relative to the germinal matrix, while tubers are hyperintense on T1-weighted images and hypointense on T2-weighted images [59].

Cerebral Parenchyma and Myelination

Detection of altered signal of the fetal cerebral parenchyma on T1- and T2-weighted MR images indicates pathology. Hemorrhage can appear as high signal intensity on T1-weighted images and low signal intensity on T2-weighted images. Gliosis and white matter edema appear as hypointense on T1-weighted images and hyperintense on T2-weighted images [11]. Myelination can be detected normally on T1-weighted images as high signal intensity in the tegmentum (22 weeks of gestation), middle cerebellar peduncle (28 weeks of gestation), and in the posterior limb of the internal capsule (30 weeks of gestation) [13].

Ventricles and CSF Spaces

Fetal ventriculomegaly is diagnosed when the atrial width measures more than 10 mm. Ventriculomegaly is a heterogeneous disease that can be caused by several etiologies including ischemia, infection, and impaired CSF circulation or can be part of a brain syndrome [11].

Cerebral atrophy and porencephaly can be seen in ventriculomegaly caused by tissue loss due to ischemia or infection.

The shape of the dilated lateral ventricles can help in indicating the underlying cause. Dilated occipital horn of the lateral ventricle (colpocephaly) is associated with agenesis of corpus callosum (ACC), pointed posterior horns are seen in Chiari II malformations, and widened posterior horns combined with a widened V-shaped third ventricle are seen in aqueduct stenosis [1314].

The prognosis of fetal ventriculomegaly is worse in presence of additional abnormalities; the role of MRI is thus to search for associated anomalies such as cortical malformations, agenesis of corpus callosum, and posterior fossa anomalies [57].

Corpus Callosum and Midline Structures

The midline sagittal T2-weighted image enables direct visualization of the corpus callosum which is normally seen as hypointense C-shaped structure at the superior margin of the cavum septum pellucidum. Anomalies of the corpus callosum such as hypoplasia or complete or partial agenesis can be diagnosed using the midline sagittal T2-weighted image. Axial and coronal MR images can provide indirect signs of complete agenesis of corpus callosum (ACC): straight parallel lateral ventricles, colpocephaly, and high-riding third ventricle [11113].

Posterior Fossa

Fetal MRI enables morphological and biometrical analysis of the posterior fossa structures. The midline sagittal MR image shows the position of the tentorium cerebelli, assesses the global volume of the posterior fossa, shows the contour of the brainstem (normal anterior pontine flexure), and identifies the cerebellar vermis (Fig. 4.7). The cerebellar hemispheres are best assessed on axial and coronal images where measurement of the transverse cerebellar diameter (TCD) can be achieved. Marked reduction of TCD is noted in cases with cerebellar hypoplasia. Partial agenesis of the vermis is always inferior owed to the cranio-caudal development of this structure [11] (Fig. 4.8).

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Fig. 4.7

Normal midline sagittal MRI anatomy of fetal brain at 29 weeks of gestation. The position of tentorium cerebelli (arrowhead) and size of the posterior fossa are normal. The fourth ventricle has a normal size and separates between the brainstem anteriorly and the cerebellar (vermis) posteriorly. Note the normal contour of the brainstem with its anterior pontine flexure (short arrow). The corpus callosum appears as low signal intensity C-shaped structure (long arrow) above the cavum septum pellucidum

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Fig. 4.8

Fetal MRI diagnosis of inferior vermian agenesis. Midline sagittal MRI of a fetal brain at the 24th week of gestation shows development of the superior vermis and absence of the inferior vermis with a consequent dilatation of the cisterna magna (arrow). Diagnosis of inferior vermian agenesis can be suggested only after the 20th week of gestation to avoid misdiagnosis caused by physiological nondevelopment of the inferior vermis at earlier age. Note that dilatation of the cisterna magna points to abnormal posterior fossa and necessitates detailed examination of the fetal brain

Malformation can affect multiple posterior fossa structures. In pontocerebellar hypoplasia, the hypoplastic cerebellum is associated with flat pons due to absent anterior pontine flexure. Molar tooth sign (MTS) is a sign that results from a combination of midbrain, vermian, and superior cerebellar peduncle abnormalities. MTS is pathognomonic for Joubert syndrome and related cerebellar disorders (JSRD) and can be detected on axial MR images as early as 17th–18th weeks of gestation [1516].

Cisterna magna is an important landmark in the posterior fossa. Abnormal anteroposterior diameter of the cisterna magna (normally less than 10 mm) is associated with multiple posterior fossa anomalies and should prompt a detailed examination of the fetal brain (Fig. 4.9) [17]. Enlarged cisterna magna is the hallmark for diagnosing Dandy-Walker continuum, a spectrum of anomalies that include mega cisterna magna (normal fourth ventricle, vermis, and tentorium), Blake’s pouch cyst (cystic dilatation of the fourth ventricle, normal vermis, and normal tentorium), vermian hypoplasia (cystic dilatation of the fourth ventricle, hypoplastic vermis, and normal tentorium), and Dandy-Walker malformation (DWM) (cystic dilatation of the fourth ventricle, hypoplastic vermis, and superiorly displaced tentorium). Obliterated cisterna magna is the consistent abnormality in Arnold-Chiari malformation. Chiari II is characterized with small posterior fossa, herniation of the inferior vermis and fourth ventricle, cerebellar hypoplasia, and usually supratentorial ventriculomegaly [117].

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Fig. 4.9

Complex fetal brain malformation. Midline sagittal T2-weighted MRI of a fetal brain at 21 weeks of gestation shows cystic enlargement of the posterior fossa (arrow), hypoplastic cerebellum, deformed brainstem with absence of the anterior pontine flexure, agenesis of corpus callosum, and supratentorial ventriculomegaly. Multiple abnormalities of the fetal brain structures can coexist, so keep looking

Obliteration of the cisterna magna should suggest a spinal defect as myelomeningocele is always associated with Chiari II malformation (Fig. 4.10) [13].

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Fig. 4.10

Chiari II malformation. Sagittal T2-weighted MR image of a fetus at 25 gestational weeks shows the characteristic findings of Chiari II malformation (long arrow: small posterior fossa, obliterated cisterna magna, and dilated supratentorial ventricles). Full assessment of the brain condition requires attention to the associated spinal defect (myelomeningocele) (short arrow)

4.4.5.2 Spine

The entire length of the fetal spine should be examined in multiple planes on MRI [15]. Neural tube defects (NTD) result from failure of normal closure of the neural tube early in gestation and include: anencephaly, cephalocele, spina bifida, and iniencephaly. Cephaloceles are diagnosed when contents protrude through a skull defect; similarly, a vertebral defect in spina bifida results in exposure of the contents of the neural canal. Fetal MRI identifies the site and extent of the bony defect as well as the size and tissue characterization of the protruding sacs [17]. For fetal MRI interpretation of spinal masses, e.g., sacrococcygeal teratoma, T1- and T2-weighted sequences in multiple planes are used to describe the location, extent, contents, and the effect of the mass on the adjacent fetal structures (see Fig. 4.4) [5].

4.4.5.3 Face

Fetal MR images in multiple planes can help assessment of the profile of the face, interorbital space, eyes, lens, external ear, teeth buds, mandible, and palate. Interpretation of fetal MRI is important for assessment of complex craniofacial malformations (Fig. 4.11) [18].

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Fig. 4.11

Abnormal profile of the face may point to a complex craniofacial malformation. Midline sagittal T2-weighted MR image of a fetus at 29 weeks of gestation (a) shows abnormal profile in the form of flat midface and absent nose (black arrow). This warrant detailed assessment of the brain which shows dilated cisterna magna (long white arrow), hypoplastic cerebellum, anterior agenesis of corpus callosum (short white arrow), and abnormal high signal intensities in the frontal cerebral parenchyma (arrowhead). Axial T2-weighted image of the brain of the same fetus (b) shows non-cleavage of the cerebrum anteriorly (arrow) while the posterior areas of the cerebrum are cleaved; findings are impressive of semilobar holoprosencephaly

4.4.5.4 Neck and Chest

The fetal airway is fluid filled and appears as high signal intensity on T2-weighted images. MRI can help in assessment of the relationship of the fetal neck masses to the airways and mediastinum. The most common fetal neck masses are teratoma, hemangioma, and goiter anteriorly and cystic hygroma posterolaterally. Thyroid has characteristic signal pattern (high signal intensity on T1-weighted images and moderate signal on T2-weighted images) that can differentiate it from other fetal neck masses [5]. Fetal MRI can assess the infiltration of cystic hygroma into fetal tissues and differentiate between it and posterior encephalocele by confirming intactness of the skull (Fig. 4.12) [17].

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Fig. 4.12

The MRI appearance of fetal neck mass can help in reaching a correct diagnosis. Coronal T2-weighted fetal MRI of a fetus at 20 weeks of gestation shows a large mass at the right lateral aspect of the neck (black arrow) that extends to the thoracic inlet (white arrow). The mass is cystic with no evidence of solid components or septa. This MRI appearance is suggestive of infiltrative cystic hygroma rather than teratoma or goiter

The fetal lungs can be seen between 17 and 23 weeks of gestation with intermediate signal intensity on T2-weighted images. With advancement of gestation, the signal intensity and volume of the lungs increase. Fetal pulmonary hypoplasia is characterized by marked reduction of signal intensity and lung volume [19].

The most common fetal pulmonary masses are congenital cystic adenomatoid malformation (CCAM), bronchopulmonary sequestration, and congenital diaphragmatic hernia. Sagittal and coronal MR images can show the distinction between the thoracic and abdominal structures and can thus differentiate between congenital diaphragmatic hernia and pulmonary masses. Fetal MRI sequences can also characterize the herniating organs in CDH [5].

On T2-weighted images, fetal CCAM type I and CCAM type II appear as large and moderate-sized high signal intensity cysts, respectively. Microcystic CCAM type III shows homogeneous moderately high signal intensity on T2-weighted images that could be indistinguishable from bronchopulmonary sequestration in absence of a visible feeding artery from the aorta to suggest sequestration (Fig. 4.13) [19].

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Fig. 4.13

MRI helps to reach a diagnosis of a fetal chest mass. (a) Axial T2-weighted MR image of the chest of a fetus at 22 weeks of gestation shows a homogeneous moderately hyperintense lesion in the left hemithorax (black arrow). The mass expands the left hemithorax and displaces the heart and mediastinum to the right (white arrow). (b) Left sagittal T2-weighted MR image of the torso of the same fetus as in (a) shows the distinction between the mass (arrow) and abdominal structures (arrowhead) and thus excludes congenital diaphragmatic hernia and suggests the pulmonary origin of the mass. Absence of a visible feeding artery from the aorta suggests the mass to be congenital cystic adenomatoid malformation (CCAM type III) rather than sequestration

4.4.5.5 Cardiovascular

The cardiovascular system appears as flow void (dark blood) on T2-weighted SSFSE; this appearance contrasts with the hyperintense surrounding structures. Balanced steady-state free precession (SSFP) sequence is superior in visualization of the heart and vessels as hyperintense structures (bright blood).

Recently, MRI of the fetal heart along body and cardiac planes has been introduced which shows potential value in detecting congenital heart diseases (CHD), cardiac tumors, and vascular malformations [29]. A modified segmental approach can be used to analyze MRI of the fetal heart that includes the following: viscero-atrial situs, cardiac position and axis, cardiac chambers, ventricular looping, inflow veins, outflow vessels, ventriculo-arterial concordance, side of the aortic arch, cardiac masses, and pericardial effusion [2].

Viscero-atrial situs: Examine MR images along maternal coronal or axial planes to determine the fetal situs. In cases of situs inversus totalis, cardiac apex and stomach are on the right side of the fetal body [9].

Cardiac position and axis: The axial MRI normally shows most of the heart at the left side of the thorax with its apex directed to the left (see Fig. 4.3b) [2]. Cardiac axis in normal MRI of the fetal heart measures 37.25 ± 7.15° which is comparable with sonographic norms. Abnormal cardiac axis increases the risk of abnormal heart or intrathoracic pathology cardiac displacement or rarely placement of the heart outside the chest (ectopia cordis) can be diagnosed using different fetal MRI planes [9].

Cardiac chambers: Four-chamber view allows assessment of the number of cardiac chambers (normally four), comparing the sizes of both atria and both ventricles (normally comparable in sizes) and assessment of the intactness of the interventricular septum (Fig. 4.14). Imbalanced cardiac chambers occur in cases of hypoplastic right heart, hypoplastic left heart, and Ebstein’s anomaly. On SSFP, the interventricular septum appears as a thin low signal intensity structure in between the right and left ventricles. Defects in the interventricular septum can be detected in fetal MRI as an isolated anomaly or part of a complex malformation.

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Fig. 4.14

Abnormal four-chamber SSFP MR image of a fetus at 25 weeks of gestation points to a complex cardiovascular malformation. The four-chamber views show a persistent left superior vena cava (arrowhead) draining in a dilated right coronary sinus into a dilated right atrium (arrow)

Inflow vessels: Coronal and sagittal fetal MR images can show normal superior and inferior venae cavae draining to the right atrium and detect anomalies such as persistent left superior vena cava (Fig. 4.15). The four-chamber view can show normal pulmonary veins (at least one vein) draining to the left atrium and detect anomalous pulmonary venous drainage [9].

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Fig. 4.15

Coronal SSFP MR image of the heart of a normal fetus at 27 weeks of gestation. The image shows the left ventricular outflow tract (black arrow), as well as the superior vena cava (white arrow) and the inferior vena cava (white arrowhead) draining to the right atrium

Outflow vessels: In fetal MRI, right outflow tract (pulmonary) is detected on axial, sagittal, and short axis views; left outflow tract (aorta) can be detected in axial, coronal, and short axis views. Right and left outflow tracts are normally equal in size and cross at their origin [2]. Fetal MRI can detect abnormalities in outflow vessels such as Fallot’s tetralogy, coarctation of aorta, and transposition of great arteries [9].

Ventriculo-arterial concordance: Fetal MRI can show the morphological right ventricle through identification of the moderator band. In normal ventriculo-arterial concordance, the pulmonary artery arises from a morphological right ventricle and bifurcates at its distal end, while the aorta arises for the left ventricle and can be traced to a regular arch that gives three neck vessels.

Side of the aortic arch: The aortic arch is usually on the left; right-sided aortic arch can be detected in MRI [2].

Cardiac masses and pericardial effusion: Fetal MRI can identify well cardiac tumors (rare, mainly rhabdomyomas) and pericardial fluid [9].

4.4.5.6 Abdomen and Pelvis

Normal fetal liver parenchyma appears as high signal on T1-weighted images and low signal on T2-weighted images (Fig. 4.16). The normal gallbladder appears as a fluid-filled sac starting from 18 weeks of gestation; however, the normal biliary ducts cannot be detected. Fetal MRI can aid in diagnosing cystic lesions related to the liver (Fig. 4.17). Choledochal cyst communicates with the biliary system, unlike congenital hepatic cyst which does not [2021].

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Fig. 4.16

Sagittal oblique SSFP MR image of the body of a normal fetus at 27 weeks of gestation. The image shows the abdominal organs: stomach (black arrow) and small bowel (black arrowhead) filled with high signal intensity fluid. The liver (white arrow) has low signal intensity appearance. In the thorax, the normal pulmonary artery (white arrowhead) is seen originating from the right ventricle of the heart

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Fig. 4.17

Hepatic cyst in a fetus at 32 weeks of gestation: value of T1- and T2-weighted sequences in tissue characterization. Sagittal oblique MR images of the fetal abdomen using T2-weighted sequences (a) and T1-weighted (b) sequences show a large cyst (arrowheads) located under the surface of the liver (long arrows). The cyst contains clear fluid and appears high signal on T2-weighted images and low signal on T1-weighted images. Note that the distal bowel contains meconium that emits high signal on T1-weighted images and low signal on T2-weighted images (short arrows)

The spleen can be normally detected on T2-weighted images by 20 weeks of gestation as low signal intensity; it shows low signal intensity on T1-weighted images [520]. Fetal MRI can detect abnormalities related to spleen such as polysplenia [9].

The esophagus, stomach, and proximal bowel are normally fluid filled and appear as low signal on T1-weighted images and high signal on T2-weighted images. After 20 weeks of gestation, the distal bowel contains meconium that emits high signal on T1-weighted images and low signal on T2-weighted images (see Fig. 4.2) [20]. Recognition of meconium in the colon on T1-weighted images can aid in the diagnosis of complex fetal anomalies such as gastroschisis and omphalocele [5].

The fetal renal cortex is hypointense to medulla on T2-weighted MR images. Renal cortex/medulla signal intensity ratio progressively increases with gestational age reaching its maximum at term [2223]. The fetal urinary bladder is filled with fluid of high signal intensity on T2-weighted images and low signal on T1-weighted images. Fetal MRI can show morphological features of several urinary diseases such as obstructive uropathy, renal tumors, and polycystic kidneys [23]. Dysplastic kidney shows increased signal intensity on T2-weighted images. Fetal MRI can also reveal fetal genital diseases such as ovarian cysts, as well as hypospadia and epispadia [22].

4.4.5.7 Musculoskeletal System

Fetal musculoskeletal can be assessed by several MRI sequences. In echoplanar imaging (EPI), the fetal bones appear of low signal intensity, while the epiphyseal cartilages have high signal intensity. EPI can be useful to obtain an overview of skeletal development. Thick-slab T2-weighted images can give an overview of the fetus with 3D impression which helps in identification of complex musculoskeletal abnormalities. Dynamic SSFP sequence helps in detection of abnormal fetal movements [18].

4.4.5.8 Pregnancy-Related Structures

Evaluation of the placenta, umbilical cord, and amniotic sac is part of interpretation of fetal MRI. The MRI appearance of the placenta changes during gestation. At 19–33 weeks of gestation, the placenta looks homogeneous on T2-weighted images. With advancement of gestation, the placenta shows increase in the number of its lobules and T2 signal intensity. During placental aging, it shows a heterogeneous map-like appearance [24]. MRI can detect placental insertion abnormalities and placental tumors and can guide better management. Amniotic fluid abnormality, oligohydramnios, or polyhydramnios may give the first hint of an underlying fetal abnormality. MRI assesses well pregnancies complicated by oligohydramnios which can limit ultrasound examination [25].

4.4.6 Know the Normal Developing Fetal Anatomy and Its MRI Appearance to Avoid Pitfalls in Diagnosis

In young fetuses, it may be difficult to identify with clarity the fetal anatomy because of the small size of the structures and fetal movements. With advancing gestation, the size of the fetal structures increase and the fetal motion decreases which will enhance the conspicuity of the fetal anatomy [1578].

The morphology and MRI signal appearance of the fetal anatomy change with advancement of gestation. So for proper interpretation of fetal MRI, be familiar with the normal developing fetal anatomy and its corresponding MRI appearance in relation to the age of gestation [5]. By this, you can avoid misdiagnosis of normal developing structures as pathology. Examples include the following:

·               The normal smooth brain surface early in gestation resembles lissencephaly; follow-up after 28 weeks of gestation is recommended to avoid misdiagnosis (see Fig. 4.5).

·               The physiologic prominence of the lateral ventricles early in gestation could be misdiagnosed as ventriculomegaly, but atrial diameter measurement is normal (See Fig. 4.5a).

·               The inferior vermis develops at 17–18 weeks of gestation; so it is prudent to wait after the 20th week of gestation to rule out diagnosis of inferior vermian agenesis in fetal MRI (see Fig. 4.8) [11].

·               Since the fetal pelvis is very small, the filled urinary bladder may occupy considerable portions of the abdomen in fetuses older than 30 weeks of gestation and should not be misdiagnosed as urinary obstruction.

4.4.7 Look Carefully at the Entirety of the Field of View of Each Image for Associated Abnormalities

If you find a fetal abnormality, look carefully at the entirety of the field of view for associated abnormalities. The additional findings may help in distinguishing possible genetic syndromes from sporadic disorders [5]. Accurate examination of the different fetal body systems enables determination of the extent of the syndrome and any associated abnormalities. The additional findings provided by MRI can be helpful in understanding the severity of the abnormality and determining the prognosis of the pregnancy (Fig. 4.18) [2627].

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Fig. 4.18

Meckel-Gruber syndrome. Sagittal SSFP MR image of a fetus at 24 weeks of gestation shows enlarged kidney with increased signal intensity of the renal parenchyma which is produced by cystic changes (arrow). Both kidneys are affected (one only is shown). A small posterior cephalocele is identified (arrowhead). The identification of this brain anomaly in presence of renal cystic disease suggested the diagnosis of Meckel-Gruber syndrome. The posterior cephalocele was missed on ultrasound due to the associated oligohydramnios

4.4.8 Fetal Biometry

Fetal biometry measurements using ultrasound have been the standard for assessment of fetal development and gestational age. However, several studies recently established similar standards for fetal MRI [2836].

MRI enables accurate delineation and measurements of fetal structures [10]. The literature includes MRI reference biometric data of the different fetal structures such as brain [1028], eye [29], lungs [3031], colon [32], liver, and other structures [3334]. A detailed list of references for MRI fetal biometry can be found at the Web page of the Society of Pediatric Radiology (SPR) available at http://www.pedrad.org/Education/ParentsPatients/FetalReferences.aspx.

Important fetal measurements are the biparietal and occipto-frontal diameters of skull and brain measured at the greatest diameter of skull and brain, respectively. MRI enables estimation of the actual size of the brain and eliminates measurement errors linked with widening of peri-cerebral space [35]. MRI measurements of the fetal brain also showed excellent agreement when compared with those measured by US [28].

Studies also demonstrated an excellent correlation between MRI and ultrasound measurements for several fetal biometrics at different gestational ages [2836]. Discordance between the fetal measurement and the established norm at the same referred gestational age usually suggests pathology. However, before indicating a biometry as abnormal, it is prudent to reassess the accuracy of the referred gestational age. In this case, communicate with the referral physician for confirmation of the gestational age. Request to know the estimated gestational age in early pregnancy ultrasound (<24 weeks) as it enables accurate assessment of the gestational age [37].

4.5 Writing a Fetal MRI Report

Before reporting fetal MRI, communicate with the related personnel who know best the history, findings of previous imaging, and dealing with counseling. Lack of communication may create conflict in data and mistrust.

4.5.1 Suggested Headings of Fetal MRI Report

The following headings are suggested for a fetal MRI report (Fig. 4.19).

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Fig. 4.19

A suggested template for fetal MRI report

·               History

Include in your report a brief history that includes the gestational age and findings of an up-to-date ultrasonography.

·               Referral indication

Write clearly the referral query and indication that justify doing fetal MRI.

·               MRI Technique

Write a brief description of the fetal MRI technique that you used including the MRI machine, MRI sequences, and the fetal anatomy part(s) included in the examination.

·               Comment

·                                   Start your report with describing in details the fetal MRI findings that are directly related to the referral query.

·                                   Include then all of the pathological findings or possible abnormalities that you may have found during fetal MR image interpretation.

·                                   Include the normal findings and measurements that you may have found during your systematic evaluation of the examined fetal organ(s) and pregnancy structures.

·               Conclusion

Conclude your report by answering the referral query. Correlate the fetal MRI findings with previous US examination(s). Include a diagnosis or a short list of differential diagnosis. Consider in the diagnosis if the detected fetal findings are random cluster of abnormalities or can be diagnosed as a known syndrome. This section of the report may also include information regarding prognosis.

4.5.2 Abnormal Fetal MRI: How to Deliver the Bad News?

In the condition that fetal MRI detects or confirms fetal structural anomaly, who should deliver the bad news to the pregnant woman and how?

In a survey of 76 women who received the news of an abnormal prenatal US imaging, the women valued most “immediate, clear information with different options explained, enough time to ask questions, information regarding follow up, privacy and the sympathy of the person giving the bad news”[38]. The pregnant woman should receive information regarding the abnormal fetal MRI findings in a clear sympathetic fashion and in a private environment. Counseling starts as soon as the pregnant woman knows about her fetal anomaly [39].

4.5.3 Role of Fetal MRI Report in Prenatal Counseling

A successful health care necessitates collaborative care and interactions among clinicians engaged in the care of the patient. Prenatal counseling is a process of communicating medical aspects about a fetal abnormality to help the family to decide between prenatal therapies, postnatal therapies, or the interruption of the pregnancy [26].

The evolving role of the radiologist is that of an active and value-generating contributor to the care team around the patient. In fact, radiologists are increasingly perceived as physician consultants. Communication of the radiologist with the referring physician would provide a much more complete patient history that enables a more focused approach to do and report fetal MRI. The accurate diagnosis of fetal abnormalities is of paramount importance [1]. Fetal MRI is a powerful tool to diagnose fetal disorders as it can provide valuable information that helps distinguish possible genetic syndromes from likely sporadic disorders [15]. Through accurate demonstration of the extent of the syndrome and any associated abnormalities, fetal MRI can play a role in determination pregnancy prognosis and management [2740]. The additional findings provided by MRI are helpful to the expectant parents and their clinicians in understanding the severity of the abnormality, reinforcing the feeling that they were able to come to an informed decision regarding pregnancy continuation or termination. For expectant parents who choose termination of pregnancy because of fetal malformation, fetal MRI gives insight into its diagnosis, cause, and recurrence risk [2627].

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