(Radiographics. 1999;19:S215-S227.)
© RSNA, 1999
Single-Shot Fast Spin-Echo MR Imaging of the Fetus: A Pictorial Essay1
Bonnie J. Huppert, MD,
Kathy R. Brandt, MD,
Kirk D. Ramin, MD and
Bernard F. King, MD
1 From the Departments of Diagnostic Radiology (B.J.H., K.R.B., B.F.K.) and Obstetrics and Gynecology/Maternal and Fetal Medicine (K.D.R.), Mayo Clinic, 200 First St SW, Rochester, MN 55905. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received February 9, 1999; revision requested May 20 and received June 29; accepted June 29. Address reprint requests to B.J.H.
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Abstract
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Ultrasonography (US) is the modality of choice for prenatal screening, but occasionally additional imaging information is needed. Magnetic resonance (MR) imaging is an attractive alternative but until recently has been limited by motion artifact. Single-shot fast spin-echo MR imaging was used to depict normal and abnormal anatomy in 26 fetuses. Thirteen studies were performed for maternal indications and 13 were performed to evaluate fetal abnormalities identified or suspected at US. Three of the fetal abnormalities involved the central nervous system (CNS) and 10 involved other anatomic sites. Results were correlated with findings at postnatal clinical examination, imaging, and pathologic analysis. MR imaging demonstrated normal fetal anatomy without substantial motion artifact. CNS structures were well visualized as early as 18–20 weeks gestation, as were most other normal anatomic structures except the heart. MR imaging also allowed characterization of a variety of abnormalities of the CNS (Arnold-Chiari malformation, Walker-Warburg syndrome, amniotic band syndrome) as well as of other structures (renal agenesis, multicystic dysplastic kidney, abdominal masses, severe limb–body wall defect, clubfoot with arthrogryposis, diaphragmatic hernia). US findings were confirmed in most cases, and additional information about the precise diagnosis or the severity or location of the anomaly often helped guide clinical management. Single-shot fast spin-echo MR imaging of the fetus is a useful adjunct to US in difficult diagnostic situations.
Index Terms: Fetus, abnormalities, 85.1893, 85.874, 85.8754, 85.8775, 85.8779 • Fetus, central nervous system, 85.874 • Fetus, MR, 85.121416 • Magnetic resonance (MR), rapid imaging, 85.121416 • Pregnancy, MR, 85.121416
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INTRODUCTION
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Ultrasonography (US), the modality of choice for prenatal screening, provides cost-effective, real-time fetal images. However, diagnostic difficulties occasionally arise and additional imaging information is needed. This can occur when results of US are suboptimal owing to maternal body habitus, oligohydramnios, prominent uterine fibroids, or complex fetal anomalies. Magnetic resonance (MR) imaging is an attractive alternative because, like US, it lacks ionizing radiation. To our knowledge, there have been no reports of reproducible adverse effects on maternal or fetal health due to clinical MR imaging (1–3). Most studies involve laboratory animals or cell cultures; few published series include long-term follow-up in humans. Although no deleterious effects from fetal MR imaging have been shown, imaging during the first trimester (when organogenesis occurs) is generally being avoided until more human imaging data are available. Intravenous contrast agents such as gadopentetate dimeglumine that are used in MR imaging are known to cross the placenta. However, because data concerning their safety are limited, these agents are generally not used during pregnancy.
MR imaging of the fetus is not new. Early attempts at fetal MR imaging involved the use of conventional T1- and T2-weighted spin-echo sequences, but fetal motion resulted in severe image degradation. Suppression of fetal motion was attempted with fetal curarization and maternal sedation. However, these methods were less than desirable because of their invasive and potentially hazardous nature. With the evolution of faster techniques such as fast spin-echo and echo-planar imaging, fetal imaging capability improved but remained inadequate for clinical use. Fast fetal imaging with half-Fourier techniques, which was first described in 1996, has virtually eliminated motion artifact (4,5). In this article, we discuss and illustrate our experience with single-shot fast spin-echo MR imaging in the evaluation of normal and abnormal fetal anatomy.
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MATERIALS AND METHODS
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Between December 1996 and May 1998, 26 women carrying fetuses 18–33 weeks in gestational age underwent single-shot fast spin-echo MR imaging. Informed consent was obtained in all cases. Thirteen studies were performed for maternal indications. In these cases, no fetal abnormalities were identified at US or MR imaging. The remaining 13 examinations were performed to further evaluate fetal abnormalities that were unclear at prenatal US performed at our institution. Detailed US findings were not known prior to interpretation of MR imaging results; however, the anatomic area of concern was specified. In each patient, MR imaging was performed within 2 weeks of US. Three MR imaging examinations were performed to further evaluate anomalies of the central nervous system (CNS) and 10 were performed to evaluate other anomalies. Pathologic conditions were confirmed with postnatal imaging, autopsy, surgical pathologic examination, or clinical follow-up.
All fetal MR imaging was performed with single-shot fast spin-echo sequences on a 1.5-T system (Signa; GE Medical Systems, Milwaukee, Wis) with a phased-array torso coil. Data were acquired sequentially during a single radio-frequency excitation period. Acquisition time was less than 1 second per section. T2-weighted MR images were obtained with a repetition time equal to the length of a maternal breath hold (20,000–30,000 msec) and an effective echo time of 80–110 msec. Images were obtained in fetal sagittal, coronal, and axial planes. Images from the previous series were used as scout images for the next series to minimize orientation problems associated with shifts in fetal position.
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RESULTS
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Normal Fetal Anatomy
Central Nervous System.—Single-shot fast spin-echo MR imaging demonstrated the normal anatomy of the fetal CNS without motion artifact. CNS structures were exceptionally well visualized as early as 18–20 weeks gestation, including three layers of the cerebrum, the cerebellum, the midbrain, and cerebrospinal fluid spaces (Fig 1). At the gestational ages studied, maturational changes in the fetal CNS were also apparent (Figs 2, 3).
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Figure 1a. Normal CNS findings in a 20-week-old fetus. (a) Axial T2-weighted MR image of the head obtained early in gestational development shows a smooth cortical surface. The cerebrospinal fluid-filled lateral ventricles have high signal intensity. Three layers of the cerebrum are identified including the hypointense cortical gray matter (curved arrow), the hyperintense unmyelinated white matter, and the lower-signal-intensity germinal matrix (straight arrow). (b) Axial T2-weighted MR image shows the midbrain (straight arrow), developing cerebellum (arrowhead), and optic chiasm (curved arrow). (c) Axial T2-weighted MR image depicts the cerebellar hemispheres (arrowhead) and the orbits.
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Figure 1b. Normal CNS findings in a 20-week-old fetus. (a) Axial T2-weighted MR image of the head obtained early in gestational development shows a smooth cortical surface. The cerebrospinal fluid-filled lateral ventricles have high signal intensity. Three layers of the cerebrum are identified including the hypointense cortical gray matter (curved arrow), the hyperintense unmyelinated white matter, and the lower-signal-intensity germinal matrix (straight arrow). (b) Axial T2-weighted MR image shows the midbrain (straight arrow), developing cerebellum (arrowhead), and optic chiasm (curved arrow). (c) Axial T2-weighted MR image depicts the cerebellar hemispheres (arrowhead) and the orbits.
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Figure 1c. Normal CNS findings in a 20-week-old fetus. (a) Axial T2-weighted MR image of the head obtained early in gestational development shows a smooth cortical surface. The cerebrospinal fluid-filled lateral ventricles have high signal intensity. Three layers of the cerebrum are identified including the hypointense cortical gray matter (curved arrow), the hyperintense unmyelinated white matter, and the lower-signal-intensity germinal matrix (straight arrow). (b) Axial T2-weighted MR image shows the midbrain (straight arrow), developing cerebellum (arrowhead), and optic chiasm (curved arrow). (c) Axial T2-weighted MR image depicts the cerebellar hemispheres (arrowhead) and the orbits.
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Figures 2, 3. (2) Normal CNS findings in a 29-week-old fetus. (a) Axial MR image shows developing cortical sulci and gyri, the posterior horns of the lateral ventricles (arrowhead), and the cavum septum pellucidum (arrow). With fetal maturation, the brain loses its smooth, lissencephalic contour and normal anatomy is more clearly defined. (b) Axial MR image obtained inferior to a shows the cerebellar hemispheres, midbrain (arrowhead), and optic chiasm (arrow). (3) Normal CNS findings in a 32-week-old fetus. Sagittal MR image demonstrates further development of the major cortical sulci and gyri. The cerebellum (arrowhead) and brain stem (arrow) are also visible. The midbrain, pons, medulla, and upper spinal cord are seen as separate structures.
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Figures 2, 3. (2) Normal CNS findings in a 29-week-old fetus. (a) Axial MR image shows developing cortical sulci and gyri, the posterior horns of the lateral ventricles (arrowhead), and the cavum septum pellucidum (arrow). With fetal maturation, the brain loses its smooth, lissencephalic contour and normal anatomy is more clearly defined. (b) Axial MR image obtained inferior to a shows the cerebellar hemispheres, midbrain (arrowhead), and optic chiasm (arrow). (3) Normal CNS findings in a 32-week-old fetus. Sagittal MR image demonstrates further development of the major cortical sulci and gyri. The cerebellum (arrowhead) and brain stem (arrow) are also visible. The midbrain, pons, medulla, and upper spinal cord are seen as separate structures.
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Figures 2, 3. (2) Normal CNS findings in a 29-week-old fetus. (a) Axial MR image shows developing cortical sulci and gyri, the posterior horns of the lateral ventricles (arrowhead), and the cavum septum pellucidum (arrow). With fetal maturation, the brain loses its smooth, lissencephalic contour and normal anatomy is more clearly defined. (b) Axial MR image obtained inferior to a shows the cerebellar hemispheres, midbrain (arrowhead), and optic chiasm (arrow). (3) Normal CNS findings in a 32-week-old fetus. Sagittal MR image demonstrates further development of the major cortical sulci and gyri. The cerebellum (arrowhead) and brain stem (arrow) are also visible. The midbrain, pons, medulla, and upper spinal cord are seen as separate structures.
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Non-CNS Anatomy.—Most normal non–CNS fetal anatomy was also well visualized with this technique (Figs 4–6), due in large part to the presence of high-signal-intensity amniotic fluid surrounding the fetus and fluid in the fetal lungs, gastrointestinal tract, gallbladder, kidneys, and bladder. Fluid was often seen within the oropharynx, and both orbits were easily identified. Intraabdominal detail improved as gestational age increased. However, cardiac imaging was limited in all cases owing to inherent motion artifact.
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Figures 4, 5. (4) Normal non-CNS findings in a 27-week-old fetus. (a) Sagittal MR image shows various anatomic features highlighted by high-signal-intensity amniotic fluid, including the facial profile (straight solid arrow), the oropharynx (arrowhead), the foot (open arrow), and loops of the three-vessel umbilical cord (curved arrow). P = placenta. (b) Sagittal MR image shows the urine-filled renal collecting system and bladder (white arrow) with high signal intensity. The male genitalia (black arrow) are seen inferior to the bladder. Arrowhead indicates the kidneys. (5) Normal non-CNS findings in a 29-week-old fetus. (a) Coronal MR image shows the kidneys (arrowheads) adjacent to the spine. (b) Coronal MR image obtained anterior to a shows the liver with low signal intensity and the gallbladder filled with fluid (arrowhead). The stomach (black arrow) and the urinary bladder (white arrow) are also visible. The diaphragm is clearly seen separating the thorax from the abdomen.
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Figures 4, 5. (4) Normal non-CNS findings in a 27-week-old fetus. (a) Sagittal MR image shows various anatomic features highlighted by high-signal-intensity amniotic fluid, including the facial profile (straight solid arrow), the oropharynx (arrowhead), the foot (open arrow), and loops of the three-vessel umbilical cord (curved arrow). P = placenta. (b) Sagittal MR image shows the urine-filled renal collecting system and bladder (white arrow) with high signal intensity. The male genitalia (black arrow) are seen inferior to the bladder. Arrowhead indicates the kidneys. (5) Normal non-CNS findings in a 29-week-old fetus. (a) Coronal MR image shows the kidneys (arrowheads) adjacent to the spine. (b) Coronal MR image obtained anterior to a shows the liver with low signal intensity and the gallbladder filled with fluid (arrowhead). The stomach (black arrow) and the urinary bladder (white arrow) are also visible. The diaphragm is clearly seen separating the thorax from the abdomen.
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Figures 4, 5. (4) Normal non-CNS findings in a 27-week-old fetus. (a) Sagittal MR image shows various anatomic features highlighted by high-signal-intensity amniotic fluid, including the facial profile (straight solid arrow), the oropharynx (arrowhead), the foot (open arrow), and loops of the three-vessel umbilical cord (curved arrow). P = placenta. (b) Sagittal MR image shows the urine-filled renal collecting system and bladder (white arrow) with high signal intensity. The male genitalia (black arrow) are seen inferior to the bladder. Arrowhead indicates the kidneys. (5) Normal non-CNS findings in a 29-week-old fetus. (a) Coronal MR image shows the kidneys (arrowheads) adjacent to the spine. (b) Coronal MR image obtained anterior to a shows the liver with low signal intensity and the gallbladder filled with fluid (arrowhead). The stomach (black arrow) and the urinary bladder (white arrow) are also visible. The diaphragm is clearly seen separating the thorax from the abdomen.
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Figures 4, 5. (4) Normal non-CNS findings in a 27-week-old fetus. (a) Sagittal MR image shows various anatomic features highlighted by high-signal-intensity amniotic fluid, including the facial profile (straight solid arrow), the oropharynx (arrowhead), the foot (open arrow), and loops of the three-vessel umbilical cord (curved arrow). P = placenta. (b) Sagittal MR image shows the urine-filled renal collecting system and bladder (white arrow) with high signal intensity. The male genitalia (black arrow) are seen inferior to the bladder. Arrowhead indicates the kidneys. (5) Normal non-CNS findings in a 29-week-old fetus. (a) Coronal MR image shows the kidneys (arrowheads) adjacent to the spine. (b) Coronal MR image obtained anterior to a shows the liver with low signal intensity and the gallbladder filled with fluid (arrowhead). The stomach (black arrow) and the urinary bladder (white arrow) are also visible. The diaphragm is clearly seen separating the thorax from the abdomen.
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Figure 6a. Normal non-CNS findings in a 29-week-old fetus. (a) Axial MR image of the thorax demonstrates the main pulmonary vessels (black arrow) and shows the heart with relatively low signal intensity (arrowhead). The interventricular cardiac septum is identified (white arrow), but other intracardiac details are obscured by motion artifact. The normal lung has high signal intensity owing to its fluid content. (b) Axial MR image shows the low-signal-intensity aorta and inferior vena cava (straight arrow), fluid-filled bowel loops (curved arrow), and the kidneys (arrowheads). As the fetus develops, the lower-signal-intensity renal cortex can be distinguished from the higher-signal-intensity medulla and the urine-filled renal collecting system.
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Figure 6b. Normal non-CNS findings in a 29-week-old fetus. (a) Axial MR image of the thorax demonstrates the main pulmonary vessels (black arrow) and shows the heart with relatively low signal intensity (arrowhead). The interventricular cardiac septum is identified (white arrow), but other intracardiac details are obscured by motion artifact. The normal lung has high signal intensity owing to its fluid content. (b) Axial MR image shows the low-signal-intensity aorta and inferior vena cava (straight arrow), fluid-filled bowel loops (curved arrow), and the kidneys (arrowheads). As the fetus develops, the lower-signal-intensity renal cortex can be distinguished from the higher-signal-intensity medulla and the urine-filled renal collecting system.
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Fetal Abnormalities
Central Nervous System.—Three MR imaging studies were performed to further evaluate CNS abnormalities that had been detected or suspected at US. In all three cases, MR imaging helped confirm suspected US findings and often provided additional information that assisted in clinical management. In two cases, US was difficult owing to maternal body habitus. In one of these cases, US findings were suggestive of a lumbar myelomeningocele, and MR imaging depicted unsuspected hydrocephalus and changes consistent with Arnold-Chiari malformation (Figs 7, 8). In the second case, US depicted microcephaly and a possible cyst in the posterior fossa, and MR imaging helped characterize this combination of complex CNS anomalies (Walker-Warburg syndrome), especially the degree of cerebral and cerebellar hypoplasia (Fig 9). In the third case, US findings were suggestive of but not classic for anencephaly. MR imaging helped confirm the severity of the CNS deformity, allowing the parents to proceed more confidently with termination of the pregnancy (Fig 10). An autopsy showed amniotic band syndrome.
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Figure 7a. Arnold-Chiari malformation in a 20-week-old fetus. (a) Axial MR image shows neural elements extending through the posterior elemental defect in the spine (arrowhead). (b) Axial MR image shows a fluid-filled myelomeningocele extending into the amniotic fluid (arrowhead).
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Figure 7b. Arnold-Chiari malformation in a 20-week-old fetus. (a) Axial MR image shows neural elements extending through the posterior elemental defect in the spine (arrowhead). (b) Axial MR image shows a fluid-filled myelomeningocele extending into the amniotic fluid (arrowhead).
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Figure 8a. Arnold-Chiari malformation in the same fetus as in Figure 7. (a) On a coronal MR image, the lateral ventricles are dilated with a "bat-wing" appearance. (b) Sagittal MR image demonstrates hydrocephalus, a small posterior fossa, and a low-lying cerebellum (white arrowhead). More caudally, there is a posterior elemental defect in the low lumbar spine with a myelomeningocele (black arrowhead).
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Figure 8b. Arnold-Chiari malformation in the same fetus as in Figure 7. (a) On a coronal MR image, the lateral ventricles are dilated with a "bat-wing" appearance. (b) Sagittal MR image demonstrates hydrocephalus, a small posterior fossa, and a low-lying cerebellum (white arrowhead). More caudally, there is a posterior elemental defect in the low lumbar spine with a myelomeningocele (black arrowhead).
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Figures 9, 10. (9) Walker-Warburg syndrome in a 27-week-old fetus. (a) Axial MR image shows prominent lateral ventricles with only a thin rim of smooth cerebral tissue (arrow) and no differentiation between gray and white matter. (b) Coronal MR image shows prominent lateral ventricles with a "dangling choroid plexus" appearance (arrow). (c) Sagittal MR image shows a prominent, fluid-filled posterior fossa with a paucity of cerebellar tissue (arrowhead). The brain stem appears abnormally small. Autopsy helped confirm complex anomalies of the CNS, including microcephaly; thinned, lissencephalic cerebral hemispheres; cerebellar hypoplasia; and partial absence of the corpus callosum (Walker-Warburg syndrome). (10) Amniotic band syndrome in a 21-week-old fetus. Sagittal midline MR image shows an abnormally truncated fetal head and absence of the normal calvaria with only amorphous soft tissue present. No cerebral hemispheres (arrowhead), cerebellum, or facial features including the orbits (arrow) are evident.
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Figures 9, 10. (9) Walker-Warburg syndrome in a 27-week-old fetus. (a) Axial MR image shows prominent lateral ventricles with only a thin rim of smooth cerebral tissue (arrow) and no differentiation between gray and white matter. (b) Coronal MR image shows prominent lateral ventricles with a "dangling choroid plexus" appearance (arrow). (c) Sagittal MR image shows a prominent, fluid-filled posterior fossa with a paucity of cerebellar tissue (arrowhead). The brain stem appears abnormally small. Autopsy helped confirm complex anomalies of the CNS, including microcephaly; thinned, lissencephalic cerebral hemispheres; cerebellar hypoplasia; and partial absence of the corpus callosum (Walker-Warburg syndrome). (10) Amniotic band syndrome in a 21-week-old fetus. Sagittal midline MR image shows an abnormally truncated fetal head and absence of the normal calvaria with only amorphous soft tissue present. No cerebral hemispheres (arrowhead), cerebellum, or facial features including the orbits (arrow) are evident.
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Figures 9, 10. (9) Walker-Warburg syndrome in a 27-week-old fetus. (a) Axial MR image shows prominent lateral ventricles with only a thin rim of smooth cerebral tissue (arrow) and no differentiation between gray and white matter. (b) Coronal MR image shows prominent lateral ventricles with a "dangling choroid plexus" appearance (arrow). (c) Sagittal MR image shows a prominent, fluid-filled posterior fossa with a paucity of cerebellar tissue (arrowhead). The brain stem appears abnormally small. Autopsy helped confirm complex anomalies of the CNS, including microcephaly; thinned, lissencephalic cerebral hemispheres; cerebellar hypoplasia; and partial absence of the corpus callosum (Walker-Warburg syndrome). (10) Amniotic band syndrome in a 21-week-old fetus. Sagittal midline MR image shows an abnormally truncated fetal head and absence of the normal calvaria with only amorphous soft tissue present. No cerebral hemispheres (arrowhead), cerebellum, or facial features including the orbits (arrow) are evident.
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Figures 9, 10. (9) Walker-Warburg syndrome in a 27-week-old fetus. (a) Axial MR image shows prominent lateral ventricles with only a thin rim of smooth cerebral tissue (arrow) and no differentiation between gray and white matter. (b) Coronal MR image shows prominent lateral ventricles with a "dangling choroid plexus" appearance (arrow). (c) Sagittal MR image shows a prominent, fluid-filled posterior fossa with a paucity of cerebellar tissue (arrowhead). The brain stem appears abnormally small. Autopsy helped confirm complex anomalies of the CNS, including microcephaly; thinned, lissencephalic cerebral hemispheres; cerebellar hypoplasia; and partial absence of the corpus callosum (Walker-Warburg syndrome). (10) Amniotic band syndrome in a 21-week-old fetus. Sagittal midline MR image shows an abnormally truncated fetal head and absence of the normal calvaria with only amorphous soft tissue present. No cerebral hemispheres (arrowhead), cerebellum, or facial features including the orbits (arrow) are evident.
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Non-CNS Anatomy.—In 10 fetuses, non–CNS abnormalities were initially detected or suspected at US. In each case, the clinician desired more information than that provided by US. These non–CNS abnormalities included four genitourinary anomalies (bilateral renal agenesis [n = 2] [Fig 11], unilateral renal agenesis, multicystic dysplastic kidney [Fig 12]), two abdominal masses (infradiaphragmatic extralobar pulmonary sequestration [Fig 13], ovarian torsion with changes suggestive of dermoid tumor), one severe limb–body wall defect (Fig 14), one goiter, one case of clubfoot and arthrogryposis (Fig 15), and one right-sided diaphragmatic hernia (Fig 16). In all but one case, MR imaging helped confirm the abnormality, often providing additional information about the precise diagnosis or the severity or location of the anomaly. MR imaging did not help confirm the goiter, which was seen at US in an 18-week-old fetus. After the institution of maternal medical treatment, the goiter was not identifiable at birth. The right-sided diaphragmatic hernia was not appreciated at US. After MR imaging clearly showed liver herniation into the right side of the chest, the mother was transferred to another institution for delivery, where extracorporeal membrane oxygenation was available.
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Figures 11, 12. (11) Bilateral renal agenesis in a 26-week-old fetus. (a) Sagittal midline MR image demonstrates nearly complete absence of amniotic fluid with crowding of the fetus. The thoracic cavity is small and dominated by the heart (arrowhead). Typical high-signal-intensity lung tissue is not seen. The bladder is not apparent. Image quality is poor owing to the lack of surrounding amniotic fluid and fluid within the viscera. (b) Axial MR image shows the renal fossae to be devoid of normal-appearing renal structures (arrowheads) and filled with loops of small bowel. (12) Multicystic dysplastic kidney in a 23-week-old fetus. (a) Sagittal MR image shows a large, multicystic structure in the left renal fossa (arrowhead). The cysts vary in size and are not connected, findings that are consistent with multicystic dysplastic kidney. The stomach is seen just inferior to the diaphragm (arrow). (b) Sagittal MR image shows a normal right kidney (arrowhead).
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Figures 11, 12. (11) Bilateral renal agenesis in a 26-week-old fetus. (a) Sagittal midline MR image demonstrates nearly complete absence of amniotic fluid with crowding of the fetus. The thoracic cavity is small and dominated by the heart (arrowhead). Typical high-signal-intensity lung tissue is not seen. The bladder is not apparent. Image quality is poor owing to the lack of surrounding amniotic fluid and fluid within the viscera. (b) Axial MR image shows the renal fossae to be devoid of normal-appearing renal structures (arrowheads) and filled with loops of small bowel. (12) Multicystic dysplastic kidney in a 23-week-old fetus. (a) Sagittal MR image shows a large, multicystic structure in the left renal fossa (arrowhead). The cysts vary in size and are not connected, findings that are consistent with multicystic dysplastic kidney. The stomach is seen just inferior to the diaphragm (arrow). (b) Sagittal MR image shows a normal right kidney (arrowhead).
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Figures 11, 12. (11) Bilateral renal agenesis in a 26-week-old fetus. (a) Sagittal midline MR image demonstrates nearly complete absence of amniotic fluid with crowding of the fetus. The thoracic cavity is small and dominated by the heart (arrowhead). Typical high-signal-intensity lung tissue is not seen. The bladder is not apparent. Image quality is poor owing to the lack of surrounding amniotic fluid and fluid within the viscera. (b) Axial MR image shows the renal fossae to be devoid of normal-appearing renal structures (arrowheads) and filled with loops of small bowel. (12) Multicystic dysplastic kidney in a 23-week-old fetus. (a) Sagittal MR image shows a large, multicystic structure in the left renal fossa (arrowhead). The cysts vary in size and are not connected, findings that are consistent with multicystic dysplastic kidney. The stomach is seen just inferior to the diaphragm (arrow). (b) Sagittal MR image shows a normal right kidney (arrowhead).
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Figures 11, 12. (11) Bilateral renal agenesis in a 26-week-old fetus. (a) Sagittal midline MR image demonstrates nearly complete absence of amniotic fluid with crowding of the fetus. The thoracic cavity is small and dominated by the heart (arrowhead). Typical high-signal-intensity lung tissue is not seen. The bladder is not apparent. Image quality is poor owing to the lack of surrounding amniotic fluid and fluid within the viscera. (b) Axial MR image shows the renal fossae to be devoid of normal-appearing renal structures (arrowheads) and filled with loops of small bowel. (12) Multicystic dysplastic kidney in a 23-week-old fetus. (a) Sagittal MR image shows a large, multicystic structure in the left renal fossa (arrowhead). The cysts vary in size and are not connected, findings that are consistent with multicystic dysplastic kidney. The stomach is seen just inferior to the diaphragm (arrow). (b) Sagittal MR image shows a normal right kidney (arrowhead).
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Figure 13a. Infradiaphragmatic extralobar bronchopulmonary sequestration in a 28-week-old fetus. (a) Coronal MR image shows a well-circumscribed, high-signal-intensity infradiaphragmatic mass (arrow) superior to and separate from the left kidney (arrowhead). (b) On an axial MR image, the mass (thick arrow) is seen medial to the stomach (thin arrow) and abutting the left side of the aorta (small arrowhead). The right kidney appears normal (large arrowhead).
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Figure 13b. Infradiaphragmatic extralobar bronchopulmonary sequestration in a 28-week-old fetus. (a) Coronal MR image shows a well-circumscribed, high-signal-intensity infradiaphragmatic mass (arrow) superior to and separate from the left kidney (arrowhead). (b) On an axial MR image, the mass (thick arrow) is seen medial to the stomach (thin arrow) and abutting the left side of the aorta (small arrowhead). The right kidney appears normal (large arrowhead).
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Figure 14. Limb-body wall defect in an 18-week-old fetus. Sagittal MR image shows a large abdominal wall defect, with the liver, stomach, and bowel spilling into the amniotic fluid (arrowhead). The thorax and lungs appear hypoplastic and hypointense (arrow). A large myelomeningocele is seen protruding posteriorly. The normal pelvic girdle and the lower extremities are not visualized.
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Figures 15, 16. (15) Clubfoot with arthrogryposis in a 21-week-old fetus. (a) Sagittal MR image shows inversion of the foot (arrow), a finding that is suggestive of clubfoot. Five toes are partially visualized (arrowhead). The hips are flexed and the knees extended. (b) Coronal MR image shows the wrists in a flexed position (arrows). (16) Diaphragmatic hernia in a 33-week-old fetus. (a) Coronal MR image shows an intact left hemidiaphragm, a normal-appearing left lung (arrowhead), and a large right-sided diaphragmatic hernia. The liver, the gallbladder, and bowel loops are seen filling the right side of the thorax (arrow). Only a small portion of the right lung is visible. (b) Sagittal MR image shows the liver, the gallbladder, and bowel loops (straight arrow) herniating through the right side of the diaphragm into the thorax and compressing the right lung (curved arrow).
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Figures 15, 16. (15) Clubfoot with arthrogryposis in a 21-week-old fetus. (a) Sagittal MR image shows inversion of the foot (arrow), a finding that is suggestive of clubfoot. Five toes are partially visualized (arrowhead). The hips are flexed and the knees extended. (b) Coronal MR image shows the wrists in a flexed position (arrows). (16) Diaphragmatic hernia in a 33-week-old fetus. (a) Coronal MR image shows an intact left hemidiaphragm, a normal-appearing left lung (arrowhead), and a large right-sided diaphragmatic hernia. The liver, the gallbladder, and bowel loops are seen filling the right side of the thorax (arrow). Only a small portion of the right lung is visible. (b) Sagittal MR image shows the liver, the gallbladder, and bowel loops (straight arrow) herniating through the right side of the diaphragm into the thorax and compressing the right lung (curved arrow).
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View larger version (157K):
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[Download PPT slide]
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Figures 15, 16. (15) Clubfoot with arthrogryposis in a 21-week-old fetus. (a) Sagittal MR image shows inversion of the foot (arrow), a finding that is suggestive of clubfoot. Five toes are partially visualized (arrowhead). The hips are flexed and the knees extended. (b) Coronal MR image shows the wrists in a flexed position (arrows). (16) Diaphragmatic hernia in a 33-week-old fetus. (a) Coronal MR image shows an intact left hemidiaphragm, a normal-appearing left lung (arrowhead), and a large right-sided diaphragmatic hernia. The liver, the gallbladder, and bowel loops are seen filling the right side of the thorax (arrow). Only a small portion of the right lung is visible. (b) Sagittal MR image shows the liver, the gallbladder, and bowel loops (straight arrow) herniating through the right side of the diaphragm into the thorax and compressing the right lung (curved arrow).
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View larger version (151K):
[in this window]
[in a new window]
[Download PPT slide]
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Figures 15, 16. (15) Clubfoot with arthrogryposis in a 21-week-old fetus. (a) Sagittal MR image shows inversion of the foot (arrow), a finding that is suggestive of clubfoot. Five toes are partially visualized (arrowhead). The hips are flexed and the knees extended. (b) Coronal MR image shows the wrists in a flexed position (arrows). (16) Diaphragmatic hernia in a 33-week-old fetus. (a) Coronal MR image shows an intact left hemidiaphragm, a normal-appearing left lung (arrowhead), and a large right-sided diaphragmatic hernia. The liver, the gallbladder, and bowel loops are seen filling the right side of the thorax (arrow). Only a small portion of the right lung is visible. (b) Sagittal MR image shows the liver, the gallbladder, and bowel loops (straight arrow) herniating through the right side of the diaphragm into the thorax and compressing the right lung (curved arrow).
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DISCUSSION
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Our experience demonstrates the ability of single-shot fast spin-echo MR imaging to depict normal and abnormal fetal anatomy without substantial motion artifact. CNS anatomy, including the posterior fossa, is particularly well delineated with this technique. The ability to successfully image abnormalities of the fetal CNS with this technique has been reported in several series (6–10). We also found single-shot fast spin-echo MR imaging to be a useful adjunct to US in the depiction of these abnormalities. In our series, the additional information provided by MR imaging was used to guide clinical management.
To date, few studies have described the use of half-Fourier MR imaging techniques in the evaluation of non–CNS fetal abnormalities. Our experience supports the use of this technique as a means of accurately depicting non-CNS anomalies as well as pathologic conditions of the CNS. For instance, the detection of diaphragmatic hernias can be challenging with US; with coronal and sagittal MR imaging, the relationship of the diaphragm to adjacent structures is well demonstrated (Fig 16). In our small series, MR imaging was also useful for evaluating genitourinary anomalies. The high signal intensity of urine within the collecting system provides signal contrast that helps define anatomy. As a fetus matures, the lower-signal-intensity renal cortex can be seen as separate from the renal medulla. In our study, MR imaging allowed distinction between a multicystic dysplastic kidney and hydronephrosis suspected at US in one case and depicted renal agenesis in three other cases. When present, the kidneys have a well-defined contour that is useful in determining the organ of origin of an abdominal mass. MR imaging clearly showed that the two abdominal masses in our study—an infradiaphragmatic extralobar bronchopulmonary sequestration and a torsive ovary—were not of renal origin. The capacity for multiplanar imaging aids in further characterizing the location and organ of origin of fetal abdominal masses. MR imaging may help determine the presence and extent of pulmonary hypoplasia, as was demonstrated in the two cases of bilateral renal agenesis and the case of limb–body wall defect in our study. In the aforementioned cases, hypoplastic lung tissue had abnormally low signal intensity and the thorax was small and dominated by the heart (Figs 11, 14).
Single-shot fast spin-echo MR imaging has some limitations. Imaging time is still not fast enough to eliminate cardiac motion, which obscures intracardiac detail. Imaging of the extremities is compromised by the inability to predictably visualize an entire extremity on one image or two consecutively obtained images. Whereas evaluation of fetal extremity movement is possible with US, it is not possible with MR imaging. This limitation was evident in the case of arthrogryposis in our study. The fixed position of multiple joints and the clubfeet were difficult to appreciate without the advantage of real-time imaging. As with US, image quality and detail in cases of oligohydramnios are degraded owing to the lack of surrounding amniotic fluid and fluid within the viscera, which normally provide signal contrast. Characterization of fetal masses with single-shot fast spin-echo MR imaging alone is limited because this technique produces only T2-weighted images. Intravenously administered gadopentetate dimeglumine, which is often used to characterize masses, is relatively contraindicated in pregnancy and was not used in our series. Finally, even with this fast technique, fetal motion may still cause image degradation. However, unlike conventional spin-echo or fast spin-echo imaging, only the images acquired at the time of fetal movement are affected. We did not find fetal motion to be a substantial problem in any of our cases.
As we gain experience with single-shot fast spin-echo fetal MR imaging, we increasingly appreciate the ability to depict normal and abnormal anatomy without motion artifact and become aware of the limitations of this technique. Fetal MR imaging may be most useful in situations in which further characterization of fetal anomalies detected at US will help determine the clinical course. These situations might include cases that require difficult decisions regarding pregnancy termination, assessment of fetal CNS and non–CNS anomalies, and planning for postnatal care. The decision to proceed with fetal MR imaging should be made on a case-by-case basis in close consultation with the referring obstetrician. Some authors have used fetal MR imaging prior to in utero surgical procedures including repair of congenital diaphragmatic herniation (11–13). The ability to accurately detect liver herniation into the thorax is important for prognosis and surgical planning. Other investigators have attempted to assess pulmonary maturation and lung volume to help evaluate growth and determine postnatal viability (14). In some instances, fetal MR imaging in the late third trimester may be of sufficient quality to replace or at least delay immediate postnatal MR imaging, thereby eliminating the need for neonatal sedation.
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CONCLUSIONS
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At the gestational ages studied, normal fetal anatomy (except that of the heart) was well demonstrated with single-shot fast spin-echo MR imaging. This technique complemented US in the characterization of suspected CNS and non-CNS fetal anomalies, thereby helping to guide pregnancy management and postnatal care. In difficult diagnostic situations, single-shot fast spin-echo MR imaging of the fetus should be considered a useful adjunct to prenatal US.
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Footnotes
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Abbreviation: CNS = central nervous system
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References
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Abstract
INTRODUCTION
