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Obstetric MR Imaging¹

¹ Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215 (D.L., R.R.E.), and the Department of Radiology, Children's Hospital, Boston (P.D.B.). Received September 8, 1998; revision requested October 22; revision received November 23; accepted January 6, 1999. Studies of the fetal central nervous system were supported by National Institutes of Health grant NS 37945. Address reprint requests to D.L.

Abstract

The surge in the development of fast magnetic resonance (MR) techniques has revolutionized our ability to image the pregnant patient and the fetus. Fast MR imaging techniques provide excellent resolution for imaging the maternal and fetal anatomies without the need for sedation. This article addresses the use of fast MR imaging techniques in the evaluation of the pregnant patient for adnexal masses, pelvimetry, hydroureteronephrosis of pregnancy, and placenta accreta. In addition, fetal anomalies for which MR imaging has proved useful, such as ventriculomegaly, arachnoid cysts, and abdominal masses, are described.

Index terms: Fetus, abnormalities, 856.874, 856.8765, 856.877, 856.878 • Magnetic resonance (MR), pulse sequences, 858.121411, 858.121412, 858.121416 • Placenta, abnormalities, 857.824 • Pregnancy, complications, 857.824, 858.8223, 858.8231, 858.8262 • Pregnancy, MR, 858.121411, 858.121412, 858.121416

MR imaging is a valuable complement to ultrasonography (US) when additional information is needed to make treatment decisions during pregnancy (1–13). Traditionally, pelvic MR imaging has been used during pregnancy to evaluate the maternal anatomy and abnormalities, such as adnexal masses, that require characterization beyond that available with US. Although adnexal structures can be identified with conventional MR imaging techniques, the fetal anatomy typically cannot be adequately assessed with conventional sequences without degradation of image quality caused by fetal motion during the relatively long acquisition times (4,14,15).

The need for alternative imaging in fetuses with difficult US-based diagnoses has long been recognized. Although computed tomography (CT) has been used in a few cases (16,17), it is of limited use in obstetric patients because of the ionizing radiation. Because MR imaging provides superior contrast resolution compared with CT, does not require ionizing radiation, and permits imaging in more than one plane, it has recently been used in fetal-maternal imaging.

The potential of MR for imaging the fetus was described in 1983 by Smith et al (13). In the following decade, the potential for MR imaging in the evaluation of placenta previa and the maternal and fetal anatomies was described in numerous reports (18-20). The two major limitations of MR imaging in these studies were the long acquisition times, which were difficult for pregnant patients to tolerate, and the need for sedation to limit fetal motion. The surge in the development of fast MR imaging techniques has revolutionized our ability to image the pregnant patient and the fetus (20–22). In addition, short-bore MR imaging systems that can help to alleviate claustrophobia because the patient's head lies outside the magnet are now available. This article addresses the use of fast MR imaging techniques in the evaluation of the pregnant patient and the fetus.

FAST MR IMAGING TECHNIQUES

We have used a 1.5-T superconducting system (Vision; Siemens, Erlangen, Germany) with a four-element phased-array surface coil for MR imaging in our patients. The minimum gradient rise time is 600 µsec (for a 25-mT peak gradient amplitude). The whole body specific absorption rate is less than 3.0 watts per kilogram of body weight in all examinations. Patients are positioned supine and feet first in the magnet to minimize claustrophobia. Images are acquired in the axial, coronal, and sagittal planes of the fetus or orthogonal to the maternal pelvis, depending on the indication for the examination.

T2-weighted MR Imaging
The half-Fourier single-shot rapid acquisition with relaxation enhancement (RARE) technique is used to obtain T2-weighted images. This is a turbo spin-echo technique in which the echo train length is approximately one-half as long as that of a typical RARE sequence. The missing data are artificially created with a computer to shorten the acquisition time. A typical sequence involves a 60-msec echo time, echo train length of 72, one acquisition, 4-mm section thickness, 24 x 24-cm field of view, and 192 x 256 acquisition matrix. A 130° refocusing pulse is used to minimize the amount of the radio-frequency power deposition.

The acquisition time per image is 430 msec. A 1-second delay between image acquisitions minimizes the specific absorption rate. Thus, the imaging time for 13 sections acquired with a single sequence is 17 seconds. The T2-weighted RARE sequences provide excellent contrast resolution of the fetal tissues (21–25). When using RARE imaging for evaluation of the fetus, it may be desirable to acquire contiguous sections, because the fetus may move between sequences. However, for a better signal-to-noise ratio in fetuses where motion is less of an issue, the acquisitions can be interleaved with an intersection gap that is equal to that of the section thickness to minimize the inadvertent radio-frequency excitation of the adjacent sections.

Because the RARE sequence method is a single-section acquisition technique, it limits artifacts related to maternal and fetal motion (21,25), because only the section in which the motion occurs is affected. This generally results in nondepiction of a portion of the fetal anatomy, but it may lead to repeated depiction of a part of the fetus (25). For example, if the hand moves in plane with the sequence, it may be depicted more than once during the imaging.

Because fetal motion generally occurs throughout the examination, when we perform imaging for evaluation of the fetal anatomy, each acquisition serves as the scout for the subsequent acquisition.

T1-weighted MR Imaging
T1-weighted MR imaging of the fetus is achieved by using a fast low-angle shot (FLASH) technique. The FLASH sequence is a gradient-echo sequence that involves a spoiler gradient to disperse residual transverse magnetization. A typical sequence is used during a maternal breath-hold with 126/4 (repetition time msec/echo time msec), an 80° flip angle, a 24 x 32-cm field of view, a 96 x 256 matrix, a 5-mm section thickness, and one signal acquisition, for an acquisition time of 12 seconds. The images are slightly degraded because of motion that occurs during the breath hold. T1-weighted images are useful for confirming hemorrhage or fat.

SAFETY ISSUES

There are no known biological risks associated with MR imaging. To our knowledge, no delayed sequelae from undergoing or performing MR examinations have been encountered, and it is expected that the potential risk of delayed sequelae is extremely small or nonexistent. The MR imaging procedure is not believed to be hazardous to the fetus (26–30). In a survey of female MR workers, no substantial increase in adverse pregnancy outcomes was found (29). According to the Safety Committee of the Society for Magnetic Resonance Imaging (31), MR procedures are indicated for use in pregnant women when other nonionizing diagnostic imaging methods are inadequate or when the examination will provide important information that would otherwise require exposure to ionizing radiation (eg, x-ray CT). It is required that pregnant patients be informed that, although to date there is no indication that the use of clinical MR imaging procedures during pregnancy produces deleterious effects, according to the U.S. Food and Drug Administration (30), the safety of MR imaging procedures during pregnancy has not been definitively proved.

Also, it is well known that dividing cells, as in the case of the developing embryo during the first trimester, are susceptible to injury from a variety of physical agents. Because of limited data, we avoid use of MR imaging in patients in the first trimester of pregnancy whenever feasible.

At our institution, for studies performed for clinical indications, such as evaluation of an adnexal mass, pregnant patients must sign an informed consent form before undergoing MR imaging. For studies performed for research indications, institutional review board approval is obtained, and written informed consent is obtained from the patients.

Gadolinium-based contrast material is not recommended for use in pregnant patients. Gadolinium-based contrast material has been shown to cross the placenta and appear within the fetal bladder moments after intravenous administration (32). From the fetal bladder, the contrast material is excreted into the amniotic fluid, where it is then swallowed and potentially reabsorbed from the gastrointestinal tract. Because of this reabsorption, the half-life of gadolinium-based contrast material in the fetal circulation is not known (32).

Gadopentetate dimeglumine has been shown to slightly retard development in rats when it is given in doses 2.5 times those administered to humans (Magnevist product information; Berlex Laboratories, Wayne, NJ). It is considered to be a pregnancy category C drug, which means that it should be given only when the potential benefit outweighs the risk, because animal studies have revealed adverse effects, but no controlled studies have been performed in humans (Magnevist product information).

MATERNAL IMAGING

Adnexal Masses
It is well established that MR imaging is useful in the pregnant patient for the evaluation of adnexal masses that cannot be fully characterized by using US. In studies by Weinreb et al (5) and Kier et al (2), in seven (44%) of 16 patients and in eight (47%) of 17 patients, respectively, MR imaging provided additional information beyond that provided by US. In these studies, the examinations were performed with conventional spin-echo and T1-weighted techniques. The total examination time in the study by Weinreb et al (5) was as long as 2 hours. As mentioned previously, such long examination times are difficult for pregnant patients to tolerate, and fetal motion degrades images of extrauterine structures. Fast MR imaging techniques provide exquisite characterization of masses (Fig 1) and should be considered when designing a protocol for these examinations.

View larger version (146K): [in this window] [in a new window]   Figure 1a. Ovarian edema in a 28-year-old woman with twins (initially quadruplets) at 14 weeks gestation who had undergone ovulation induction and had an enlarging solid left ovarian mass at US. (a) Sagittal turbo spin-echo MR image (5,139/90, two acquisitions, 140 x 256 matrix, echo train length of seven, 5-mm section thickness, 26 x 35-cm field of view, acquisition time of 3 minutes 3 seconds) of the gravid uterus (U) and enlarged (14 x 9 x 8 cm) left ovary with small peripheral follicles (arrows). The follicles have low signal intensity, which is likely due to hemorrhage. The enlargement of the left ovary was histopathologically shown to be caused by massive ovarian edema. Because of the long imaging time, fetal structures could not be identified. (b) Half-Fourier RARE MR image (single shot/60; field of view, 26 x 35 cm; matrix, 192 x 256; echo train length, 72; one signal acquired; section thickness, 5 mm) in the coronal plane demonstrates the gravid uterus (U) with the body of one fetus (arrowhead), hyperstimulated right ovary (open arrow), and edematous left ovary (O). (c) Coronal RARE MR image obtained slightly more anteriorly shows fetuses (one of which is not alive) in three gestational sacs.

View larger version (159K): [in this window] [in a new window]   Figure 1b. Ovarian edema in a 28-year-old woman with twins (initially quadruplets) at 14 weeks gestation who had undergone ovulation induction and had an enlarging solid left ovarian mass at US. (a) Sagittal turbo spin-echo MR image (5,139/90, two acquisitions, 140 x 256 matrix, echo train length of seven, 5-mm section thickness, 26 x 35-cm field of view, acquisition time of 3 minutes 3 seconds) of the gravid uterus (U) and enlarged (14 x 9 x 8 cm) left ovary with small peripheral follicles (arrows). The follicles have low signal intensity, which is likely due to hemorrhage. The enlargement of the left ovary was histopathologically shown to be caused by massive ovarian edema. Because of the long imaging time, fetal structures could not be identified. (b) Half-Fourier RARE MR image (single shot/60; field of view, 26 x 35 cm; matrix, 192 x 256; echo train length, 72; one signal acquired; section thickness, 5 mm) in the coronal plane demonstrates the gravid uterus (U) with the body of one fetus (arrowhead), hyperstimulated right ovary (open arrow), and edematous left ovary (O). (c) Coronal RARE MR image obtained slightly more anteriorly shows fetuses (one of which is not alive) in three gestational sacs.

View larger version (171K): [in this window] [in a new window]   Figure 1c. Ovarian edema in a 28-year-old woman with twins (initially quadruplets) at 14 weeks gestation who had undergone ovulation induction and had an enlarging solid left ovarian mass at US. (a) Sagittal turbo spin-echo MR image (5,139/90, two acquisitions, 140 x 256 matrix, echo train length of seven, 5-mm section thickness, 26 x 35-cm field of view, acquisition time of 3 minutes 3 seconds) of the gravid uterus (U) and enlarged (14 x 9 x 8 cm) left ovary with small peripheral follicles (arrows). The follicles have low signal intensity, which is likely due to hemorrhage. The enlargement of the left ovary was histopathologically shown to be caused by massive ovarian edema. Because of the long imaging time, fetal structures could not be identified. (b) Half-Fourier RARE MR image (single shot/60; field of view, 26 x 35 cm; matrix, 192 x 256; echo train length, 72; one signal acquired; section thickness, 5 mm) in the coronal plane demonstrates the gravid uterus (U) with the body of one fetus (arrowhead), hyperstimulated right ovary (open arrow), and edematous left ovary (O). (c) Coronal RARE MR image obtained slightly more anteriorly shows fetuses (one of which is not alive) in three gestational sacs.

By using acceptable diameters of greater than 11.0 cm for the anteroposterior pelvic inlet, greater than 9.5 cm for the transverse midpelvic (interspinal) distance, and greater than 11.0 cm for the pelvic outlet, van Loon et al (31) showed that although the use of MR pelvimetry of the fetus in breech presentation at term did not reduce the overall cesarean section rate, it allowed better selection of the delivery route, with a significantly lower emergency cesarean section rate (P = .005).

Placental Evaluation
In patients with suspected placenta previa, a sagittal MR imaging sequence oriented in the plane of the cervix is used to assess the placental margin (3,4,7). Given the widespread use of transvaginal and translabial US, suspected placenta previa is unlikely to be a common indication for MR examination. However, the placental edge is easily identified with fast MR imaging techniques (Fig 2). Unusual placental abnormalities, such as succenturiate lobe, are readily assessed with MR imaging (14).

View larger version (164K): [in this window] [in a new window]   Figure 2. Partial placenta previa in a woman in 29th week of pregnancy. Sagittal RARE MR image (single shot/60) of the cervix shows placental tip (arrow) partially covering the internal os of the cervix.

View larger version (169K): [in this window] [in a new window]   Figure 3. Placenta accreta diagnosed with MR imaging in a patient who previously underwent posterior myomectomy. Coronal RARE MR image (single shot/60) shows an absent myometrial-placental interface in a posterolateral location (arrowhead) surrounded by a normal myometrial-placental interface (arrows). This region was not well identified with US. A fibroid (F) is present in the right lateral aspect of the uterus. (Reprinted, with permission, from reference 36.)

Hydronephrosis
One very promising application of MR imaging in the evaluation of the maternal anatomy is urography. Physiologic hydronephrosis is the most common cause of dilatation of the urinary tract in pregnancy. However, renal colic also can occur secondary to stones in 0.03%–0.24% of pregnant patients (38–40). In physiologic hydroureteronephrosis, the dilatation extends to the sacral promontory, with tapering of the lower part of the dilated ureter. In obstruction due to stones, an intrinsic ureteral mass is present. US is the standard screening method for evaluating the pregnant patient with renal colic. Urography with intravenously administered contrast material is performed occasionally. With the advent of MR urography, it is no longer necessary to expose the pregnant patient to the radiation and contrast material risks of intravenous urography. Roy et al (41) showed that in 15 cases of hydroureteronephrosis of pregnancy, MR urography enabled the correct identification of 10 cases of physiologic dilatation, four cases of stones, and one case of ureteropelvic junction obstruction.

Miscellaneous Maternal Conditions
MR imaging has also been useful in the diagnosis of abdominal pregnancy (42–44), pelvic vein thrombosis during pregnancy (45), and small-bowel obstruction complicating pregnancy (Fig 4).

View larger version (153K): [in this window] [in a new window]   Figure 4a. Distal small-bowel obstruction in a pregnant woman who previously underwent abdominal surgery and had adhesions. (a) Coronal and (b) axial RARE MR images (single shot/60) show a dilated small bowel (sb), nondilated small-bowel loop (curved arrow) in the right lower region, and nondilated colon (straight arrows).

View larger version (110K): [in this window] [in a new window]   Figure 4b. Distal small-bowel obstruction in a pregnant woman who previously underwent abdominal surgery and had adhesions. (a) Coronal and (b) axial RARE MR images (single shot/60) show a dilated small bowel (sb), nondilated small-bowel loop (curved arrow) in the right lower region, and nondilated colon (straight arrows).

Early studies on the use of MR imaging in the evaluation of fetal morphology were hindered by fetal motion. Recommendations were made to limit imaging to late pregnancy or to cases of oligohydramnios, because there is less fetal motion in these conditions (6,46). Garden et al (47) described gradient-echo images for the evaluation of the fetal anatomy. Images were obtained in 3–14 seconds; this reduced the artifact from fetal motion, but the images were compromised by the low signal-to-noise ratio. Other investigators (4,10,14,48–51) have used benzodiazepines to sedate the pregnant patient, or curarization by direct fetal injection to decrease fetal motion. Subsequently, echo-planar imaging was advocated for obstetric MR examinations, because its short (100-msec) imaging time makes fetal paralysis unnecessary (28,52). However, echo-planar imaging has several disadvantages, including susceptibility artifacts, chemical shift artifacts, and limited availability due to special hardware requirements (53). Despite these limitations, fetal MR imaging has been suggested as an important technique to evaluate anomalous fetuses (8,9,13).

In a study to assess single-shot RARE MR imaging, we reviewed images from 54 fetuses and identified 47 organs and anatomic regions in each fetus (25). We found that the anatomy was well depicted in fetuses older than 20 weeks gestation, regardless of the indication for the examination (ie, whether the images were obtained orthogonal to the maternal or fetal anatomy). However, the image quality was poorer in the fetuses younger than 20 weeks gestation. This was usually related to the small size of the organ or region being evaluated and less frequently related to fetal motion. In these early second-trimester fetuses, obtaining images in orthogonal planes improved the depiction of the central nervous system (CNS) anatomy. Maternal motion did not reduce the image quality in any case. For ease of image interpretation, we attempt to align our acquisition planes orthogonal to the fetal anatomy. However, if the fetus moves and the planes are slightly nonorthogonal, the acquisitions are not repeated.

Fetal CNS Anomalies
One area where MR imaging has proved to be especially beneficial is in the evaluation of the fetal CNS. US evaluation of the fetal CNS is limited because of the nonspecific appearance of some anomalies; technical factors that reduce resolution of the side of the brain near the transducer; ossification, which obscures the depiction of posterior fossa structures; and subtle parenchymal abnormalities, which frequently cannot be depicted with US (54). Multiplanar views can be difficult to obtain with US owing to the fetal position or advanced gestational age. MR imaging can be used to obtain multiplanar views. In addition, MR imaging allows direct visualization of the brain parenchyma and thus enables detailed evaluation of the CNS anatomy in a manner that is not possible with US (55). Many case and series reports (8,56–63) detail the potential of MR imaging to improve on the US diagnosis of CNS anomalies.

In an ongoing study, we have evaluated fetal CNS abnormalities with US and MR imaging (55,64). To date, 60 fetal MR examinations have been performed in 56 fetuses for ultrasonographically suspected CNS abnormalities. In all cases, MR imaging demonstrated the region of the US abnormality. In 28 (50%) of 56 cases, the information provided by MR imaging was the type that warrants changes in patient counseling or management.

We have found that MR imaging is helpful in clarifying US findings to assist patients in the decision of whether to continue a pregnancy (Fig 5). MR imaging has facilitated counseling in patients with nonspecific US findings (eg, a large cisterna magna, for which an amniocentesis would be performed for inferior vermian agenesis but not for the normal variant of mega cisterna magna). In addition, fetal MR imaging without sedation has the potential to decrease the need for postnatal imaging studies that require neonatal sedation (Fig 6).

View larger version (156K): [in this window] [in a new window]   Figure 5a. Posterior encephalocele in a fetus at 20 weeks gestation. (a) Axial US scan shows a posterior encephalocele. A small amount of tissue (arrow) is seen within the sac. (b) On the axial RARE MR image (single shot/60), the structures of the posterior fossa appear to be normal. A portion of the encephalocele (arrowheads) is seen posteriorly. The information that the majority of the brain appeared to be normal was helpful to the patient in deciding to continue the pregnancy. Postnatally, a meningocele sac was excised, and the neural content returned to the posterior fossa. At the time this article was written, the baby was doing well at 1 year of age.

View larger version (160K): [in this window] [in a new window]   Figure 5b. Posterior encephalocele in a fetus at 20 weeks gestation. (a) Axial US scan shows a posterior encephalocele. A small amount of tissue (arrow) is seen within the sac. (b) On the axial RARE MR image (single shot/60), the structures of the posterior fossa appear to be normal. A portion of the encephalocele (arrowheads) is seen posteriorly. The information that the majority of the brain appeared to be normal was helpful to the patient in deciding to continue the pregnancy. Postnatally, a meningocele sac was excised, and the neural content returned to the posterior fossa. At the time this article was written, the baby was doing well at 1 year of age.

View larger version (163K): [in this window] [in a new window]   Figure 6a. Dandy-Walker malformation in a fetus at 34 weeks gestation. (a) Axial and (b) sagittal RARE MR images (single shot/60) show a Dandy-Walker malformation (M), with a large posterior fossa and key-hole deformity (arrows in a). The additional information provided by MR imaging, beyond that available with US, was that the corpus callosum (arrows in b) appeared to be normal and the aqueduct (not shown) was not grossly enlarged. It is hoped that, in the future, performing MR examinations in the late third trimester will obviate MR imaging immediately after birth, which requires sedation of the neonate.

View larger version (147K): [in this window] [in a new window]   Figure 6b. Dandy-Walker malformation in a fetus at 34 weeks gestation. (a) Axial and (b) sagittal RARE MR images (single shot/60) show a Dandy-Walker malformation (M), with a large posterior fossa and key-hole deformity (arrows in a). The additional information provided by MR imaging, beyond that available with US, was that the corpus callosum (arrows in b) appeared to be normal and the aqueduct (not shown) was not grossly enlarged. It is hoped that, in the future, performing MR examinations in the late third trimester will obviate MR imaging immediately after birth, which requires sedation of the neonate.

View larger version (136K): [in this window] [in a new window]   Figure 7a. Severe ventriculomegaly in a fetus at 30 weeks gestation. (a) Oblique axial US scan shows a 34-week head size and massively enlarged lateral ventricles. It is difficult to assess the cortical mantle. The posterior fossa appears to be small, and there is a dilated fourth ventricle (arrow), but the cerebellum is difficult to visualize ultrasonographically. (b) Sagittal and (c) coronal RARE MR images (single shot/60) demonstrate a definite rim of smooth cortical tissue (arrowheads in b), malformed brain stem (arrow in b), and Dandy-Walker malformation (M). These findings make hydranencephaly unlikely and suggest a cerebrocerebellar malformation, such as Walker-Warburg syndrome, with aqueductal stenosis. This type of information is important in counseling patients prior to delivery.

View larger version (136K): [in this window] [in a new window]   Figure 7c. Severe ventriculomegaly in a fetus at 30 weeks gestation. (a) Oblique axial US scan shows a 34-week head size and massively enlarged lateral ventricles. It is difficult to assess the cortical mantle. The posterior fossa appears to be small, and there is a dilated fourth ventricle (arrow), but the cerebellum is difficult to visualize ultrasonographically. (b) Sagittal and (c) coronal RARE MR images (single shot/60) demonstrate a definite rim of smooth cortical tissue (arrowheads in b), malformed brain stem (arrow in b), and Dandy-Walker malformation (M). These findings make hydranencephaly unlikely and suggest a cerebrocerebellar malformation, such as Walker-Warburg syndrome, with aqueductal stenosis. This type of information is important in counseling patients prior to delivery.

View larger version (140K): [in this window] [in a new window]   Figure 7b. Severe ventriculomegaly in a fetus at 30 weeks gestation. (a) Oblique axial US scan shows a 34-week head size and massively enlarged lateral ventricles. It is difficult to assess the cortical mantle. The posterior fossa appears to be small, and there is a dilated fourth ventricle (arrow), but the cerebellum is difficult to visualize ultrasonographically. (b) Sagittal and (c) coronal RARE MR images (single shot/60) demonstrate a definite rim of smooth cortical tissue (arrowheads in b), malformed brain stem (arrow in b), and Dandy-Walker malformation (M). These findings make hydranencephaly unlikely and suggest a cerebrocerebellar malformation, such as Walker-Warburg syndrome, with aqueductal stenosis. This type of information is important in counseling patients prior to delivery.

View larger version (179K): [in this window] [in a new window]   Figure 8. Agenesis of the corpus callosum in a fetus at 34 weeks gestation. Coronal RARE MR image (single shot/60) shows the characteristic vertical orientation of the frontal horns (arrows) in agenesis of the corpus callosum. Arrowheads point to the region where the corpus callosum should be depicted. The US scan demonstrated only asymmetric mild ventriculomegaly. (Reprinted, with permission, from reference 55.)

View larger version (148K): [in this window] [in a new window]   Figure 9a. Arachnoid cyst in a fetus at 27 weeks gestation. (a–c) C = extraaxial collection. (a) Axial US scan demonstrates an extraaxial collection, but it is difficult to assess the involvement of the ventricular system. (b) Coronal and (c) sagittal RARE MR images (single shot/60) clearly demonstrate the extraaxial nature of this extensive arachnoid cyst with mass effect on the surrounding structures.

View larger version (164K): [in this window] [in a new window]   Figure 9b. Arachnoid cyst in a fetus at 27 weeks gestation. (a–c) C = extraaxial collection. (a) Axial US scan demonstrates an extraaxial collection, but it is difficult to assess the involvement of the ventricular system. (b) Coronal and (c) sagittal RARE MR images (single shot/60) clearly demonstrate the extraaxial nature of this extensive arachnoid cyst with mass effect on the surrounding structures.

View larger version (163K): [in this window] [in a new window]   Figure 9c. Arachnoid cyst in a fetus at 27 weeks gestation. (a–c) C = extraaxial collection. (a) Axial US scan demonstrates an extraaxial collection, but it is difficult to assess the involvement of the ventricular system. (b) Coronal and (c) sagittal RARE MR images (single shot/60) clearly demonstrate the extraaxial nature of this extensive arachnoid cyst with mass effect on the surrounding structures.

We have also evaluated fetal cerebral cortical development in relation to gestational age compared with the normal development based on findings in anatomic specimens (69). We found that cortical development at MR imaging follows a predictable course and lags slightly compared with that described in anatomic specimens. Cortical development is often further delayed in fetuses with mild ventriculomegaly or other CNS abnormalities. The course of the appearance of normal cortical landmarks is important in screening fetuses for suspected migrational disorders such as lissencephaly. In addition, if the cortical development is markedly delayed compared with the gestational age and the fetus has an otherwise normal-appearing brain, the neonate may benefit from early intervention postnatally.

Non-CNS Fetal Anomalies
The results of a study by Hubbard et al (70) showed that in two of three fetuses with congenital diaphragmatic hernia, MR imaging demonstrated the location of the liver within the chest, whereas its location was not demonstrated with US. Such information is critical for fetal interventional surgery.

MR imaging is contributive in defining fetal abdominal masses. In a case from our laboratory, fetal US demonstrated an echogenic abdominal mass that was suggestive of a meconium pseudocyst. The MR image showed that the mass was more likely to be a gastric duplication cyst, which was confirmed with postnatal imaging (Fig 10). We have demonstrated other anomalies at MR imaging, including omphalocele, gastroschisis, and cloacal malformation (Fig 11). Further studies are needed to assess how additional information from MR imaging affects outcome.

View larger version (162K): [in this window] [in a new window]   Figure 10a. Gastric duplication cyst in a fetus at 28 weeks gestation. (a) Oblique coronal US scan shows an echogenic abdominal mass (calipers), which is suggestive of a meconium pseudocyst. (b–d) m = mass, s = stomach. (b) Sagittal T2-weighted RARE MR image (single shot/60) shows the mass impressing on the stomach. The mass is of slightly lower signal intensity than the stomach on this image. (c) Axial RARE MR image (single shot/60) shows findings similar to those in b. (d) Axial T1-weighted image (126/4, 80° flip angle, 24 x 32-cm field of view, 96 x 256 matrix, 5-mm section thickness, one signal acquired) shows that the mass has slightly lower signal intensity than that of the adjacent stomach. The appearance of the mass suggests a duplication cyst, which was confirmed postnatally.

View larger version (148K): [in this window] [in a new window]   Figure 10b. Gastric duplication cyst in a fetus at 28 weeks gestation. (a) Oblique coronal US scan shows an echogenic abdominal mass (calipers), which is suggestive of a meconium pseudocyst. (b–d) m = mass, s = stomach. (b) Sagittal T2-weighted RARE MR image (single shot/60) shows the mass impressing on the stomach. The mass is of slightly lower signal intensity than the stomach on this image. (c) Axial RARE MR image (single shot/60) shows findings similar to those in b. (d) Axial T1-weighted image (126/4, 80° flip angle, 24 x 32-cm field of view, 96 x 256 matrix, 5-mm section thickness, one signal acquired) shows that the mass has slightly lower signal intensity than that of the adjacent stomach. The appearance of the mass suggests a duplication cyst, which was confirmed postnatally.

View larger version (104K): [in this window] [in a new window]   Figure 10c. Gastric duplication cyst in a fetus at 28 weeks gestation. (a) Oblique coronal US scan shows an echogenic abdominal mass (calipers), which is suggestive of a meconium pseudocyst. (b–d) m = mass, s = stomach. (b) Sagittal T2-weighted RARE MR image (single shot/60) shows the mass impressing on the stomach. The mass is of slightly lower signal intensity than the stomach on this image. (c) Axial RARE MR image (single shot/60) shows findings similar to those in b. (d) Axial T1-weighted image (126/4, 80° flip angle, 24 x 32-cm field of view, 96 x 256 matrix, 5-mm section thickness, one signal acquired) shows that the mass has slightly lower signal intensity than that of the adjacent stomach. The appearance of the mass suggests a duplication cyst, which was confirmed postnatally.

View larger version (126K): [in this window] [in a new window]   Figure 10d. Gastric duplication cyst in a fetus at 28 weeks gestation. (a) Oblique coronal US scan shows an echogenic abdominal mass (calipers), which is suggestive of a meconium pseudocyst. (b–d) m = mass, s = stomach. (b) Sagittal T2-weighted RARE MR image (single shot/60) shows the mass impressing on the stomach. The mass is of slightly lower signal intensity than the stomach on this image. (c) Axial RARE MR image (single shot/60) shows findings similar to those in b. (d) Axial T1-weighted image (126/4, 80° flip angle, 24 x 32-cm field of view, 96 x 256 matrix, 5-mm section thickness, one signal acquired) shows that the mass has slightly lower signal intensity than that of the adjacent stomach. The appearance of the mass suggests a duplication cyst, which was confirmed postnatally.

View larger version (135K): [in this window] [in a new window]   Figure 11a. Fetal cloacal exstrophy. (a,b) Fetus at 20 weeks gestation. (a) Sagittal US scan shows a low anterior abdominal wall defect (solid arrows). A fluid collection (open arrow) is seen in the pelvis, which is suggestive of a deformed bladder; however, the bladder should not be seen in a case of exstrophy. The kidneys are not visible. (b) Axial RARE MR image (single shot/60) shows a pelvic kidney (arrowheads in b and c) in the region of the fluid collection seen in a. The spinal cord (arrow in b and c) is seen at this level, which is consistent with a tethered cord. (c) Axial RARE MR image (single shot/60) of the fetus at 38 weeks gestation again shows the pelvic kidney and tethered cord. These diagnoses were made prospectively after discussions with specialists in pediatric surgery and pediatric radiology. One of the benefits of fetal MR imaging is that specialists who are not accustomed to obstetric US can view MR images, with which they may be more familiar.

View larger version (169K): [in this window] [in a new window]   Figure 11b. Fetal cloacal exstrophy. (a,b) Fetus at 20 weeks gestation. (a) Sagittal US scan shows a low anterior abdominal wall defect (solid arrows). A fluid collection (open arrow) is seen in the pelvis, which is suggestive of a deformed bladder; however, the bladder should not be seen in a case of exstrophy. The kidneys are not visible. (b) Axial RARE MR image (single shot/60) shows a pelvic kidney (arrowheads in b and c) in the region of the fluid collection seen in a. The spinal cord (arrow in b and c) is seen at this level, which is consistent with a tethered cord. (c) Axial RARE MR image (single shot/60) of the fetus at 38 weeks gestation again shows the pelvic kidney and tethered cord. These diagnoses were made prospectively after discussions with specialists in pediatric surgery and pediatric radiology. One of the benefits of fetal MR imaging is that specialists who are not accustomed to obstetric US can view MR images, with which they may be more familiar.

View larger version (154K): [in this window] [in a new window]   Figure 11c. Fetal cloacal exstrophy. (a,b) Fetus at 20 weeks gestation. (a) Sagittal US scan shows a low anterior abdominal wall defect (solid arrows). A fluid collection (open arrow) is seen in the pelvis, which is suggestive of a deformed bladder; however, the bladder should not be seen in a case of exstrophy. The kidneys are not visible. (b) Axial RARE MR image (single shot/60) shows a pelvic kidney (arrowheads in b and c) in the region of the fluid collection seen in a. The spinal cord (arrow in b and c) is seen at this level, which is consistent with a tethered cord. (c) Axial RARE MR image (single shot/60) of the fetus at 38 weeks gestation again shows the pelvic kidney and tethered cord. These diagnoses were made prospectively after discussions with specialists in pediatric surgery and pediatric radiology. One of the benefits of fetal MR imaging is that specialists who are not accustomed to obstetric US can view MR images, with which they may be more familiar.

Potential Pitfalls in Fetal Evaluation with Fast MR Imaging
As in all patients, there are absolute contraindications to MR imaging in pregnant patients (eg, a ferromagnetic cerebral aneurysm clip), and some patients are too claustrophobic to undergo the examination. Use of short-bore magnets should make claustrophobia less of a concern. However, an additional problem is that pregnant patients may have difficulty lying on their backs, especially in the third trimester.

Although artifacts from fetal motion are minimized by using fast imaging techniques, if the fetus moves continuously during an acquisition, reduced image quality is inevitable. The technique that we use currently is not a real-time method; however, real-time fast MR imaging acquisition methods with spiral or gradient-echo pulse sequences are being developed. In some cases when MR imaging is performed for fetal indications, fetal movement that occurs after the scout acquisitions, or between imaging sequences, makes it difficult to obtain images in specific planes. This is especially true in the evaluation of the distal extremities. When motion is not a problem, sequential images enable assessment of the hands and feet. However, even small amounts of motion limit the depiction of the extremities in their entirety. When the entire fetus moves, the region of interest may not be in the imaging plane.

Because of signal-to-noise limitations, small fetal structures may be difficult to identify and evaluate. Thin structures surrounded by fluid can be difficult to identify owing to partial volume averaging over the thickness of the section. Examples include the membranous sac of a neural tube defect, the wall of an arachnoid cyst, and the forming corpus callosum in the second trimester.

CONCLUSION

US continues to be the screening modality of choice for the evaluation of the maternal pelvis and the fetus because of its relatively low cost and real-time capability. However, there are many cases in which MR imaging has proved to be useful as an adjunct to US. These include evaluation of adnexal masses, hydroureteronephrosis of pregnancy, and fetal ventriculomegaly. As our experience with fast MR imaging techniques increases, we will continue to identify patients in whom MR imaging contributes to evaluation. In conclusion, fast MR imaging techniques provide excellent resolution for imaging the maternal and fetal anatomies without the need for maternal or fetal sedation. This exciting area of research has great potential to benefit fetal and maternal care.

Footnotes

Abbreviations: CNS = central nervous system FLASH = fast low-angle shot RARE = rapid acquisition with relaxation enhancement

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