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Normal Anatomy of the Fetus at MR Imaging¹

¹ From the Department of Radiology, University of Texas Medical School, Houston. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received February 8, 1999; revision requested May 20 and received June 14; accepted June 14. Address reprint requests to A.K., Department of Radiology, Lyndon B. Johnson General Hospital, 5656 Kelley St, Houston, TX 77026.

	Abstract

Top Abstract INTRODUCTION APPLICATIONS TECHNIQUE NORMAL FETAL ANATOMY SAFETY CONCLUSIONS References

 
Owing to recent advances in magnetic resonance (MR) imaging, the role of obstetric MR imaging has increased in cases in which the results of ultrasonography are equivocal. Fast MR imaging sequences, such as T2-weighted fast spin-echo (SE), half-Fourier single-shot fast SE, 0.5-signal-acquired single-shot fast SE, and echo-planar imaging, have virtually eliminated the need for fetal premedication, with a concomitant improvement in image resolution and diminished blurring. Artifacts related to maternal respiratory motion and fetal motion no longer limit the anatomic detail that can be demonstrated with MR imaging. With such advances in obstetric MR imaging, knowledge of normal fetal anatomy at MR imaging is essential to detect disease in utero. MR imaging can demonstrate fetal anatomy in detail, especially the brain, thorax, abdomen, pelvis, and vasculature. Major developmental structures of the fetus, particularly the cranial nervous system, naso- and oropharynx, lungs, and major abdominal viscera, can be adequately evaluated with targeted fast MR imaging as early as the beginning of the second trimester. However, MR imaging of the heart remains limited. Fetal MR imaging during the first trimester remains controversial secondary to biosafety issues and is limited due to diminutive fetal size.

Index Terms: Fetus, central nervous system, 856.92 • Fetus, MR, 856.121416 • Magnetic resonance (MR), rapid imaging, 856.121416 • Magnetic resonance (MR), safety • Pregnancy, MR, 856.121416

	INTRODUCTION

Top Abstract INTRODUCTION APPLICATIONS TECHNIQUE NORMAL FETAL ANATOMY SAFETY CONCLUSIONS References

 
Under certain circumstances, when the results of fetal ultrasonography (US) are indeterminate, magnetic resonance (MR) imaging is a useful adjuvant in evaluation of disease (1,2). Initial attempts at MR imaging of the fetus were limited due to extensive fetal motion, which produced significant imaging artifacts. Pharmacologic intervention in the form of oral administration of diazepam to the mother or administration of pancuronium bromide into the umbilical vein was required to reduce the effects of fetal motion (3). However, pharmacologic intervention introduces a risk to the mother as well as the fetus. Fast MR imaging sequences performed during suspension of maternal breathing virtually eliminate the need for fetal sedation and significantly decrease artifacts created by maternal respiratory motion and fetal movement. Although the length of the MR imaging examination has decreased, the quality of the images obtained has been preserved if not relatively improved. With these recent improvements, greater anatomic detail of the fetus can be demonstrated in utero.

With the advances in obstetric MR imaging, knowledge of normal fetal anatomy at MR imaging is essential to detect disease in utero. In this article, the applications and technique of fetal MR imaging are described. Normal fetal anatomy at MR imaging is then presented; the illustrations show the high signal-to-noise ratio and improved overall image quality of the half-Fourier single-shot fast spin-echo (SE) and 0.5-signal-acquired single-shot fast SE sequences even relative to the breath-hold fast SE and echo-planar imaging sequences (1,2). Finally, the safety of fetal MR imaging is reviewed.

	APPLICATIONS

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MR imaging serves as a complementary examination in evaluation of the second- and third-trimester fetus, especially when US evaluation is limited (4,5) (Table). In cases of oligohydramnios or breech presentation, US is often limited due to lack of an adequate acoustic window. However, MR imaging can clearly demonstrate the fetal anatomy in detail in both instances. Also, third-trimester evaluation of the fetus can be compromised due to ossification of the calvaria, which limits visualization of the posterior fossa at US (3). Intracranial pathologic conditions such as posterior fossa malformations (Chiari II syndrome, cerebellar aplasia or hypoplasia) are often suggested at US. The US findings can be confirmed with MR imaging, and in certain cases, such as neuronal migrational abnormalities, MR imaging can provide a diagnosis (6). The ability to evaluate the fetus in multiple planes within a larger field of view is clearly an advantage of MR imaging. T1-weighted MR imaging can demonstrate acute and chronic hemorrhage of the germinal matrix as well as ischemic changes. Although US is effective in detection of corpus callosum abnormalities, many of the cranial nervous system anomalies associated with agenesis of the corpus callosum can be missed with US. Because agenesis of the corpus callosum has such a variable prognosis, finding the associated cranial nervous system anomalies at MR imaging can aid in pre- and postnatal medical management (7).

In-depth, multiplanar characterization of placental abnormalities such as placenta accreta, placenta previa, abruptio placentae, and gestational trophoblastic disease can guide medical and surgical management (11,12).

Preliminary studies have shown that certain high-risk pregnancies may also benefit from obstetric MR imaging (13). Birth weight can be accurately predicted and the diagnosis of intrauterine growth retardation or macrosomia can be made with confidence with a combination of fetal weight estimates and liver volume measurements (13,14).

	TECHNIQUE

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Patients are imaged in the supine position, when possible, with a variety of breath-hold pulse sequences. To alleviate patient discomfort, the MR imaging examination during the late third trimester can be performed with the patient in an oblique or decubitus position. The images in this article were selected from a variety of cases in which pregnancy was complicated or the US findings were equivocal in the second or third trimester. Review of the previous US scan before MR imaging is important so that a targeted MR imaging evaluation can be performed. The multiplanar sequences used include T2-weighted fast SE, half-Fourier single-shot fast SE, 0.5-signal-acquired single-shot fast SE, flow-sensitive gradient-echo (GRE), and echo-planar imaging. After coronal, axial, and sagittal images are obtained as localizers, targeted oblique images can be acquired. Relocalization may be necessary if the fetus changes presentation or lie during the examination.

The T2-weighted fast SE, 0.5-signal-acquired single-shot fast SE, flow-sensitive GRE, and echo-planar images were obtained with a 1.5-T imager (Signa; GE Medical Systems, Milwaukee, Wis) and a phased-array body coil. The half-Fourier single-shot fast SE images were obtained with a 1.5-T superconductive system (Magnetom Vision; Siemens, Erlangen, Germany). These examinations followed US evaluation and were performed in the second and third trimesters (fetal gestational age varied from 22 weeks to 38 weeks). Attempts at imaging the fetus in the first trimester have been limited due to small fetal size and fetal motion (15). The total examination time varied from approximately 15 to approximately 30 minutes depending on the indication for the examination. In each case, informed consent was obtained from the mother.

T1-weighted images can be obtained with conventional SE and breath-hold spoiled GRE sequences. Although these sequences are slower and more susceptible to motion artifact than half-Fourier or 0.5-signal-acquired single-shot fast SE imaging, they are particularly useful for demonstrating general anatomic relationships and vasculature. T2-weighted images can be obtained with the fast SE, half-Fourier or 0.5-signal-acquired single-shot fast SE, and echo-planar imaging sequences. The majority of the detailed fetal MR imaging evaluation is best performed with T2-weighted sequences.

Single-Shot Fast SE Imaging
The half-Fourier and the 0.5-signal-acquired single-shot fast SE sequences allow high-resolution T2-weighted imaging that is relatively motion resistant with breath-holding techniques. A single 90° radio-frequency pulse is used to obtain a series of echoes. The half-Fourier and 0.5-signal-acquired algorithms collect the sequential phase encoding from a single excitation pulse to produce an image. This single radio-frequency pulse design also allows short echo spaces, thus providing the means to perform section acquisition in less than 1 second.

Fast SE Imaging
The breath-hold fast SE technique uses a longer repetition time to produce T2-weighted images. Relative to the half-Fourier and 0.5-signal-acquired single-shot techniques, the number of phase-encoding steps that can be obtained is greater, thus increasing the data acquisition time. Unfortunately, the longer acquisition time increases susceptibility to motion artifact. In addition, interecho spacing is increased with fast SE sequences, thus resulting in increased blurring artifact on T2-weighted images.

Echo-planar Imaging
Echo-planar imaging is related to fast GRE imaging. The echo-planar technique uses one short repetition time to acquire all phase-encoding steps. Although images can be obtained in a very short acquisition time (130 msec), the overall soft-tissue resolution is inferior to that of half-Fourier or 0.5-signal-acquired single-shot fast SE imaging. Echo-planar imaging is limited by considerable magnetic susceptibility artifact.

	NORMAL FETAL ANATOMY

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Head and Neck
The majority of obstetric MR imaging involves evaluation of the fetal brain. Evaluation of the fetus is limited in the first trimester and into the early second trimester mainly due to small fetal size (15). However, at approximately 25 weeks gestation, anatomic detail of the head and neck is apparent. Beginning in the early second trimester, the structures that evolve from the telencephalon (cerebral hemispheres), mesencephalon (midbrain including thalamus), and rhombencephalon (pons, cerebellum) are clearly seen (although the figures show slightly older fetuses [Figs 1–4]). Certain cortical sulci and gyri in addition to gray-white matter junctions can be seen as early as the second trimester (Fig 1). Although the early stages of gyral development resemble lissencephaly, at 22–25 weeks gestation certain sulci (prerolandic, middle temporal, postrolandic, superior frontal, and lateral occipital sulci) and gyri (pre- and postrolandic, middle temporal, superior and inferior frontal, and superior and inferior occipital gyri; superior and inferior parietal lobules) show a more characteristic cortical pattern (16). However, their normal development relative to gestational age can vary by up to 3 weeks (3). The formation of gyral patterns continues well after the neonatal period.

View larger version (179K): [in this window] [in a new window] [Download PPT slide]   Figure 1a.   Fetus at 28 weeks gestation. (a, b) Coronal 0.5-signal-acquired single-shot fast SE images of the brain (repetition time msec/echo time msec = 16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show the cerebral hemisphere (a), thalamus (b), medulla oblongata (c), temporal lobe (d), cerebral peduncle (curved arrow), third ventricle (straight white arrow), and gray-white matter junction (black arrows). (c, d) Coronal 0.5-signal-acquired single-shot fast SE images of the face (16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show that the orbits have moved to a more medial position, which is reflective of normal embryologic development. Both frontal lobes of the cerebral hemispheres are present. Other structures seen on these images include the tongue (a) within the oral cavity, fluid within the maxillary sinus (white arrow), the nasal conchae and nasal meatus (straight black arrow), and the flow void associated with the superior sagittal sinus (curved arrow).

View larger version (183K): [in this window] [in a new window] [Download PPT slide]   Figure 1b.   Fetus at 28 weeks gestation. (a, b) Coronal 0.5-signal-acquired single-shot fast SE images of the brain (repetition time msec/echo time msec = 16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show the cerebral hemisphere (a), thalamus (b), medulla oblongata (c), temporal lobe (d), cerebral peduncle (curved arrow), third ventricle (straight white arrow), and gray-white matter junction (black arrows). (c, d) Coronal 0.5-signal-acquired single-shot fast SE images of the face (16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show that the orbits have moved to a more medial position, which is reflective of normal embryologic development. Both frontal lobes of the cerebral hemispheres are present. Other structures seen on these images include the tongue (a) within the oral cavity, fluid within the maxillary sinus (white arrow), the nasal conchae and nasal meatus (straight black arrow), and the flow void associated with the superior sagittal sinus (curved arrow).

View larger version (171K): [in this window] [in a new window] [Download PPT slide]   Figure 1c.   Fetus at 28 weeks gestation. (a, b) Coronal 0.5-signal-acquired single-shot fast SE images of the brain (repetition time msec/echo time msec = 16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show the cerebral hemisphere (a), thalamus (b), medulla oblongata (c), temporal lobe (d), cerebral peduncle (curved arrow), third ventricle (straight white arrow), and gray-white matter junction (black arrows). (c, d) Coronal 0.5-signal-acquired single-shot fast SE images of the face (16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show that the orbits have moved to a more medial position, which is reflective of normal embryologic development. Both frontal lobes of the cerebral hemispheres are present. Other structures seen on these images include the tongue (a) within the oral cavity, fluid within the maxillary sinus (white arrow), the nasal conchae and nasal meatus (straight black arrow), and the flow void associated with the superior sagittal sinus (curved arrow).

View larger version (178K): [in this window] [in a new window] [Download PPT slide]   Figure 1d.   Fetus at 28 weeks gestation. (a, b) Coronal 0.5-signal-acquired single-shot fast SE images of the brain (repetition time msec/echo time msec = 16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show the cerebral hemisphere (a), thalamus (b), medulla oblongata (c), temporal lobe (d), cerebral peduncle (curved arrow), third ventricle (straight white arrow), and gray-white matter junction (black arrows). (c, d) Coronal 0.5-signal-acquired single-shot fast SE images of the face (16,692/99.7, 24 x 24-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 150 matrix, 12 sections, total acquisition time of 17 seconds) show that the orbits have moved to a more medial position, which is reflective of normal embryologic development. Both frontal lobes of the cerebral hemispheres are present. Other structures seen on these images include the tongue (a) within the oral cavity, fluid within the maxillary sinus (white arrow), the nasal conchae and nasal meatus (straight black arrow), and the flow void associated with the superior sagittal sinus (curved arrow).

View larger version (138K): [in this window] [in a new window] [Download PPT slide]   Figure 2.   Fetus at 30 weeks gestation. Axial half-Fourier single-shot fast SE image through the brain (4,300/60, 30 x 35-cm field of view, 7.5-mm section thickness with 5.0-mm spacing, 192 x 256 matrix, 13 sections, total acquisition time of 19 seconds) shows the lateral ventricles (a), cerebral peduncles (arrowhead), interpeduncular cistern, and cerebral aqueduct (arrow). (Courtesy of Deborah Levine, MD, and Robert R. Edelman, MD, Beth Israel Deaconess Medical Center, Boston, Mass.)

View larger version (149K): [in this window] [in a new window] [Download PPT slide]   Figures 3, 4.   Third-trimester fetal anatomy. (3) Sagittal half-Fourier single-shot fast SE image of a fetus (4,300/60, 30 x 35-cm field of view, 192 x 256 matrix, total acquisition time of 19 seconds) shows the hyperintense lung (a) and the low-signal-intensity heart (b). A kidney is seen posteriorly (c) just below the level of the diaphragm. Additional structures seen on this image include the pons (d), oropharynx (straight white arrow), esophagus (arrowhead), fourth ventricle (black arrow), and umbilical vein (curved arrow). (4) Sagittal half-Fourier single-shot fast SE image of a fetus (4,300/60, 30 x 35-cm field of view, 7.5-mm section thickness with 5.0-mm spacing, 192 x 256 matrix, 13 sections, total acquisition time of 19 seconds) shows the lateral ventricle (a), diencephalon (b), cerebellum (c), umbilical cord (straight arrows), umbilical cord insertion (curved arrow), and teeth (arrowheads). (Figs 3 and 4 courtesy of Deborah Levine, MD, and Robert R. Edelman, MD, Beth Israel Deaconess Medical Center, Boston, Mass.)

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figures 3, 4.   Third-trimester fetal anatomy. (3) Sagittal half-Fourier single-shot fast SE image of a fetus (4,300/60, 30 x 35-cm field of view, 192 x 256 matrix, total acquisition time of 19 seconds) shows the hyperintense lung (a) and the low-signal-intensity heart (b). A kidney is seen posteriorly (c) just below the level of the diaphragm. Additional structures seen on this image include the pons (d), oropharynx (straight white arrow), esophagus (arrowhead), fourth ventricle (black arrow), and umbilical vein (curved arrow). (4) Sagittal half-Fourier single-shot fast SE image of a fetus (4,300/60, 30 x 35-cm field of view, 7.5-mm section thickness with 5.0-mm spacing, 192 x 256 matrix, 13 sections, total acquisition time of 19 seconds) shows the lateral ventricle (a), diencephalon (b), cerebellum (c), umbilical cord (straight arrows), umbilical cord insertion (curved arrow), and teeth (arrowheads). (Figs 3 and 4 courtesy of Deborah Levine, MD, and Robert R. Edelman, MD, Beth Israel Deaconess Medical Center, Boston, Mass.)

Studies have suggested that normal anatomic features of fetal development (eg, cerebellar development) at MR imaging can lag behind correlated pathologic findings at autopsy by approximately 5 weeks. In the future, an MR imaging template of normal fetal development, adjusted for any discrepancy between the MR imaging findings and known stages of development, would potentially be valuable for evaluating gross fetal anatomy when the estimated gestational age is provided (19).

Although subtle anatomic detail of the neck is not distinctly visualized, the oropharynx and nasopharynx are discernible (Figs 3, 5). Evaluation of the spinal column is important in the prenatal examination to detect such abnormalities as neural tube defects and paraspinal tumors. The entire length of the cord can be studied in the sagittal (Fig 6) and axial planes.

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 5a.   Fetus at 26 weeks gestation. Coronal 0.5-signal-acquired single-shot fast SE image of a fetus (sagittal section through the maternal pelvis) (12,595/97.6, 36 x 36-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 160 matrix, nine sections, total acquisition time of 13 seconds) (a) and sagittal 0.5-signal-acquired single-shot fast SE image of the fetus (coronal section through the maternal pelvis) (13,117/100, 28 x 28-cm field of view, 4.0-mm section thickness with 1.5-mm spacing, 256 x 192 matrix, eight sections, total acquisition time of 13 seconds) (b) show the facial anatomy. Structures seen include the palate (straight arrow), tongue (arrowhead in a), oropharynx (curved arrow), and nasopharynx (arrowhead in b).

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 5b.   Fetus at 26 weeks gestation. Coronal 0.5-signal-acquired single-shot fast SE image of a fetus (sagittal section through the maternal pelvis) (12,595/97.6, 36 x 36-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 160 matrix, nine sections, total acquisition time of 13 seconds) (a) and sagittal 0.5-signal-acquired single-shot fast SE image of the fetus (coronal section through the maternal pelvis) (13,117/100, 28 x 28-cm field of view, 4.0-mm section thickness with 1.5-mm spacing, 256 x 192 matrix, eight sections, total acquisition time of 13 seconds) (b) show the facial anatomy. Structures seen include the palate (straight arrow), tongue (arrowhead in a), oropharynx (curved arrow), and nasopharynx (arrowhead in b).

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 6.   Fetus at 26 weeks gestation. Sagittal image of a fetus shows the entire spinal column. The sacral portion of the spine demonstrates a mild degree of blurring; however, no abnormalities were seen on other images from the same series (not shown). Hyperintense fluid-filled bowel loops (b) and the intermediate-signal-intensity heart (h) are shown. The low-signal-intensity soft-tissue structure in the retroperitoneum is the collecting system of the kidney (c).

View larger version (187K): [in this window] [in a new window] [Download PPT slide]   Figure 7.   Fetus at 38 weeks gestation. Coronal heavily T2-weighted fast SE image of the thorax and upper abdomen (11,000/198, 32 x 24-cm field of view, 4.0-mm section thickness with 1.0-mm spacing, 512 x 256 matrix, 14 sections, total acquisition time of 45 seconds) shows the lung (L) and tracheobronchial tree (white arrows), which appear hyperintense because they are filled with amniotic fluid. The pulmonary vasculature (black arrow) and heart (h) are hypointense. Just inferior to the left side of the diaphragm is the stomach (s), which is hyperintense.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 8a.   Fetus at 22 weeks gestation. (a) Oblique fast SE image of a fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the trachea (arrow), the carina (white arrowhead), and the low-signal-intensity aortic arch and descending aorta (black arrowhead). (b) Oblique fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the right and left brachiocephalic veins (straight arrows) joining the superior vena cava (arrowhead). Two chambers of the heart are shown as low-signal-intensity structures. The inferior vena cava (curved arrow) is shown joining the right atrium. (c) Sagittal fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the aortic root (arrowhead) with a portion of the descending aorta. Also seen is the posterior hemidiaphragm (arrow). (d) Coronal fast SE image of the fetus (5,714/84, 40 x 30-cm field of view, 3.0-mm section thickness with 2.0-mm spacing, 10 sections, total acquisition time of 80 seconds) shows the main pulmonary trunk (arrowhead) with the left pulmonary artery. Also seen are the liver (a) and stomach (b) just below the diaphragm.

View larger version (152K): [in this window] [in a new window] [Download PPT slide]   Figure 8b.   Fetus at 22 weeks gestation. (a) Oblique fast SE image of a fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the trachea (arrow), the carina (white arrowhead), and the low-signal-intensity aortic arch and descending aorta (black arrowhead). (b) Oblique fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the right and left brachiocephalic veins (straight arrows) joining the superior vena cava (arrowhead). Two chambers of the heart are shown as low-signal-intensity structures. The inferior vena cava (curved arrow) is shown joining the right atrium. (c) Sagittal fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the aortic root (arrowhead) with a portion of the descending aorta. Also seen is the posterior hemidiaphragm (arrow). (d) Coronal fast SE image of the fetus (5,714/84, 40 x 30-cm field of view, 3.0-mm section thickness with 2.0-mm spacing, 10 sections, total acquisition time of 80 seconds) shows the main pulmonary trunk (arrowhead) with the left pulmonary artery. Also seen are the liver (a) and stomach (b) just below the diaphragm.

View larger version (147K): [in this window] [in a new window] [Download PPT slide]   Figure 8c.   Fetus at 22 weeks gestation. (a) Oblique fast SE image of a fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the trachea (arrow), the carina (white arrowhead), and the low-signal-intensity aortic arch and descending aorta (black arrowhead). (b) Oblique fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the right and left brachiocephalic veins (straight arrows) joining the superior vena cava (arrowhead). Two chambers of the heart are shown as low-signal-intensity structures. The inferior vena cava (curved arrow) is shown joining the right atrium. (c) Sagittal fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the aortic root (arrowhead) with a portion of the descending aorta. Also seen is the posterior hemidiaphragm (arrow). (d) Coronal fast SE image of the fetus (5,714/84, 40 x 30-cm field of view, 3.0-mm section thickness with 2.0-mm spacing, 10 sections, total acquisition time of 80 seconds) shows the main pulmonary trunk (arrowhead) with the left pulmonary artery. Also seen are the liver (a) and stomach (b) just below the diaphragm.

View larger version (160K): [in this window] [in a new window] [Download PPT slide]   Figure 8d.   Fetus at 22 weeks gestation. (a) Oblique fast SE image of a fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the trachea (arrow), the carina (white arrowhead), and the low-signal-intensity aortic arch and descending aorta (black arrowhead). (b) Oblique fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the right and left brachiocephalic veins (straight arrows) joining the superior vena cava (arrowhead). Two chambers of the heart are shown as low-signal-intensity structures. The inferior vena cava (curved arrow) is shown joining the right atrium. (c) Sagittal fast SE image of the fetus (5,714/91, 40 x 30-cm field of view, 4.0-mm section thickness with 2.0-mm spacing, 256 x 128 matrix, 10 sections, total acquisition time of 80 seconds) shows the aortic root (arrowhead) with a portion of the descending aorta. Also seen is the posterior hemidiaphragm (arrow). (d) Coronal fast SE image of the fetus (5,714/84, 40 x 30-cm field of view, 3.0-mm section thickness with 2.0-mm spacing, 10 sections, total acquisition time of 80 seconds) shows the main pulmonary trunk (arrowhead) with the left pulmonary artery. Also seen are the liver (a) and stomach (b) just below the diaphragm.

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 9.   Fetus at 28 weeks gestation. Sagittal 0.5-signal-acquired single-shot fast SE image of a fetus (22,458/64.1, 46 x 32-cm field of view, 7.0-mm section thickness with 3.0-mm spacing, 256 x 192 matrix, 12 sections, total acquisition time of 22 seconds) shows the thoracic descending aorta (long arrow), thymus (arrowhead), heart (short arrow), and liver (a).

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 10.   Fetus at 38 weeks gestation. Axial echo-planar image through the lung bases (1,699/70, 36 x 27-cm field of view, 6.0-mm section thickness with 1.5-mm spacing, 256 x 128 matrix, 15 sections, total acquisition time of 29 seconds) shows low-signal-intensity structures such as the pulmonary vasculature (arrowhead), the descending aorta (black arrow), and two chambers of the heart (a) separated by the interventricular septum (white arrow).

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 11.   Third-trimester fetal anatomy. Axial 0.5-signal-acquired single-shot fast SE image through the lungs (57,738/99.3, 38 x 38-cm field of view, 5.0-mm section thickness with 1.0-mm spacing, 512 x 512 matrix) shows low-signal-intensity structures including the heart (h) and descending aorta (arrowhead).

On fast SE images, the renal collecting systems, intestine, and bladder appear hyperintense (Fig 12). On 0.5-signal-acquired single-shot fast SE images of the retroperitoneum, the kidneys appear isointense and the renal collecting systems appear hyperintense (Fig 13). On flow-sensitive GRE images, these structures appear hypointense. However, structures such as the hepatic veins, inferior vena cava, and descending aorta appear hyperintense with GRE sequences (Fig 14). Techniques such as fast SE and GRE imaging allow adequate evaluation of abdominal and pelvic structures, but the overall spatial resolution and soft-tissue characterization are inferior to those achieved with half-Fourier or 0.5-signal-acquired single-shot fast SE imaging.

View larger version (163K): [in this window] [in a new window] [Download PPT slide]   Figure 12a.   Fetus at 38 weeks gestation. (a) Axial flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the fluid-filled proximal renal collecting systems (arrows) and bladder (bl) as hypointense structures. (b) Axial fast SE image through the abdomen (11,000/198, 40 x 30-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 512 x 256 matrix, 20 sections, total acquisition time of 45 seconds) shows the fluid-filled dilated bladder (bl), dilated renal collecting systems (c), and bowel loops (bo) as hyperintense structures. Both postnatal US and cystography showed the collecting systems and bladder to be normal. (c) Coronal fast SE image of the fetus (11,000/198, 32 x 24-cm field of view, 4.0-mm section thickness with 1.0-mm spacing, 512 x 256 matrix, 14 sections, total acquisition time of 45 seconds) shows bowel loops (bo) and the bladder (bl).

View larger version (182K): [in this window] [in a new window] [Download PPT slide]   Figure 12b.   Fetus at 38 weeks gestation. (a) Axial flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the fluid-filled proximal renal collecting systems (arrows) and bladder (bl) as hypointense structures. (b) Axial fast SE image through the abdomen (11,000/198, 40 x 30-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 512 x 256 matrix, 20 sections, total acquisition time of 45 seconds) shows the fluid-filled dilated bladder (bl), dilated renal collecting systems (c), and bowel loops (bo) as hyperintense structures. Both postnatal US and cystography showed the collecting systems and bladder to be normal. (c) Coronal fast SE image of the fetus (11,000/198, 32 x 24-cm field of view, 4.0-mm section thickness with 1.0-mm spacing, 512 x 256 matrix, 14 sections, total acquisition time of 45 seconds) shows bowel loops (bo) and the bladder (bl).

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 12c.   Fetus at 38 weeks gestation. (a) Axial flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the fluid-filled proximal renal collecting systems (arrows) and bladder (bl) as hypointense structures. (b) Axial fast SE image through the abdomen (11,000/198, 40 x 30-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 512 x 256 matrix, 20 sections, total acquisition time of 45 seconds) shows the fluid-filled dilated bladder (bl), dilated renal collecting systems (c), and bowel loops (bo) as hyperintense structures. Both postnatal US and cystography showed the collecting systems and bladder to be normal. (c) Coronal fast SE image of the fetus (11,000/198, 32 x 24-cm field of view, 4.0-mm section thickness with 1.0-mm spacing, 512 x 256 matrix, 14 sections, total acquisition time of 45 seconds) shows bowel loops (bo) and the bladder (bl).

View larger version (151K): [in this window] [in a new window] [Download PPT slide]   Figure 13.   Fetus at 28 weeks gestation. Axial 0.5-signal-acquired single-shot fast SE image through the abdomen (57,738/99.3, 38 x 38-cm field of view, 5.0-mm section thickness with 1.0-mm spacing, 512 x 512 matrix) shows the kidneys. The normal renal collecting systems appear as high-signal-intensity structures (arrows).

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 14a.   Fetus at 38 weeks gestation. (a) Axial T1-weighted flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the liver parenchyma (L) with the hyperintense hepatic vasculature (arrowhead). The hyperintense inferior vena cava (black arrow) and descending aorta (white arrow) are also evident. The relatively low-signal-intensity structure posterior to the left hepatic lobe is the stomach (S). (b) Axial T1-weighted flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the hepatic veins (black arrow) approaching the inferior vena cava. The subtle hyperintense vascular structure just lateral to the aorta is the azygos vein (white arrow).

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 14b.   Fetus at 38 weeks gestation. (a) Axial T1-weighted flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the liver parenchyma (L) with the hyperintense hepatic vasculature (arrowhead). The hyperintense inferior vena cava (black arrow) and descending aorta (white arrow) are also evident. The relatively low-signal-intensity structure posterior to the left hepatic lobe is the stomach (S). (b) Axial T1-weighted flow-sensitive GRE image through the abdomen (167/4.7, 60° flip angle, 34 x 34-cm field of view, 6.0-mm section thickness with 3.0-mm spacing, 256 x 128 matrix, 26 sections, total acquisition time of 25 seconds) shows the hepatic veins (black arrow) approaching the inferior vena cava. The subtle hyperintense vascular structure just lateral to the aorta is the azygos vein (white arrow).

View larger version (167K): [in this window] [in a new window] [Download PPT slide]   Figure 15.   Fetus at 28 weeks gestation. Sagittal 0.5-signal-acquired single-shot fast SE image through the maternal pelvis (29,336/64.1, 48 x 48-cm field of view, 7.0-mm section thickness with 3.0-mm spacing, 256 x 192 matrix, 18 sections, total acquisition time of 29 seconds) shows an intrauterine gestation with a high, posterior placenta (arrow). The fetus is in a longitudinal vertex position. The three-vessel umbilical cord is wrapped around the fetus' neck (arrowhead).

View larger version (134K): [in this window] [in a new window] [Download PPT slide]   Figure 16a.   Fetus at 26 weeks gestation. Sagittal 0.5-signal-acquired single-shot fast SE image of a fetus (coronal section through the maternal pelvis) (36,070/63.8, 46 x 46-cm field of view, 7.0-mm section thickness with 3.0-mm spacing, 256 x 192 matrix, 22 sections, total acquisition time of 36 seconds) (a) and coronal 0.5-signal-acquired single-shot fast SE image of the fetus (sagittal section through the maternal pelvis) (12,595/97.6, 36 x 36-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 160 matrix, nine sections, total acquisition time of 13 seconds) (b) show the fetus to be in a longitudinal breech position with a right anterolateral placenta (arrow).

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 16b.   Fetus at 26 weeks gestation. Sagittal 0.5-signal-acquired single-shot fast SE image of a fetus (coronal section through the maternal pelvis) (36,070/63.8, 46 x 46-cm field of view, 7.0-mm section thickness with 3.0-mm spacing, 256 x 192 matrix, 22 sections, total acquisition time of 36 seconds) (a) and coronal 0.5-signal-acquired single-shot fast SE image of the fetus (sagittal section through the maternal pelvis) (12,595/97.6, 36 x 36-cm field of view, 5.0-mm section thickness with 2.5-mm spacing, 256 x 160 matrix, nine sections, total acquisition time of 13 seconds) (b) show the fetus to be in a longitudinal breech position with a right anterolateral placenta (arrow).

	SAFETY

Top Abstract INTRODUCTION APPLICATIONS TECHNIQUE NORMAL FETAL ANATOMY SAFETY CONCLUSIONS References

 
Although MR imaging is a noninvasive diagnostic examination that does not involve ionizing radiation, the comprehensive effects of MR imaging on the developing fetus are currently unknown. The U.S. Food and Drug Administration has not approved MR imaging as a primary modality for imaging the fetus, mainly because the safety of such imaging has yet to be established. Conversely, significant attempts have been made to study the safety of US performed during pregnancy and its effects on the developing fetus. In bioeffects studies, no significant adverse effects of diagnostic US have been noted; however, it is always prudent to apply the ALARA (as low as reasonably achievable) principle. Furthermore, no conclusive data suggesting that US is safer than MR imaging are available, to our knowledge (21). The uncertainty and confusion surrounding MR imaging performed during pregnancy are largely due to the lack of studies focusing on the effects on the human embryo or fetus. Although the majority of studies performed in nonhuman embryos or fetuses have shown no teratogenic effects when a magnetic field strength similar to that used clinically (1.5 T) is applied, concern and skepticism are mainly derived from a few early studies. Heinrichs et al (22) showed a significant decrease in the crown-rump length of mice exposed during midgestation. Tyndall and Sulik (23) showed an increased rate of eye malformations in an exposed mouse strain (10% rate of spontaneous eye malformations). However, to our knowledge, no study has shown adverse effects on exposed human embryos or fetuses, particularly at clinical exposure strengths. In a study in which normal human fetuses were exposed to 1.5-T echo-planar imaging, no significant decrease in intrauterine fetal growth or neonatal birth weight was demonstrated (24). In addition, no significant deleterious effects attributable to MR imaging were demonstrated in a 3-year follow-up study (24,25).

The International Non-Ionizing Radiation Committee of the International Radiation Protection Association takes the following position (26):

There is no firm evidence that mammalian embryos are sensitive to the magnetic fields encountered in magnetic resonance systems. However, pending the accumulation of more data regarding MR in pregnancy, it is recommended that elective examination of pregnant women should be postponed until after the first trimester. Because ultrasound is the modality of choice for fetal and uterine examination during pregnancy, an MR examination should be limited to cases in which unique diagnostic information can be obtained.

Similar recommendations have been made by the Safety Committee of the Society for Magnetic Resonance Imaging (27) and the National Radiological Protection Board of Great Britain (28).

Currently, use of gadolinium-based, intravenously administered MR imaging contrast agents is not recommended during pregnancy because it is known that gadolinium chelates cross the placental barrier (29). Any delay in clearance of the gadolinium complex from the fetus can potentially increase the risk of exposure to free gadolinium ions and produce deleterious effects in utero (21). Nevertheless, the overall risk of fetal injury from a single dose of gadolinium is low (29). In certain cases in which specific indications may potentially affect patient care and outcome, contrast material has been administered during pregnancy (20).

	CONCLUSIONS

Top Abstract INTRODUCTION APPLICATIONS TECHNIQUE NORMAL FETAL ANATOMY SAFETY CONCLUSIONS References

 
Although US remains the examination of choice for evaluating the fetus, obstetric MR imaging serves as a useful adjuvant study. With the development of faster MR imaging sequences, greater anatomic detail of the fetus is visualized. Thus, a clear understanding of MR imaging techniques and fetal anatomy is crucial for optimal examination and image interpretation.

	Footnotes

 
Abbreviations: GRE = gradient echo SE = spin echo

	References

Top Abstract INTRODUCTION APPLICATIONS TECHNIQUE NORMAL FETAL ANATOMY SAFETY CONCLUSIONS References

 

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