DOI: 10.1148/rg.245045036
RadioGraphics 2004;24:1301-1316
© RSNA, 2004
MR Imaging of Maternal Diseases of the Abdomen and Pelvis during Pregnancy and the Immediate Postpartum Period1
John R. Leyendecker, MD,
Vladislav Gorengaut, MD and
Jeffrey J. Brown, MD
1 From the Department of Radiology, Wake Forest University Baptist Medical Center, Medical Center Blvd, Winston-Salem, NC 27157 (J.R.L.); and the Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, Mo (V.G., J.J.B.). Presented as an education exhibit at the 2003 RSNA scientific assembly. Received March 12, 2004; revision requested April 1 and received May 11; accepted May 12. All authors have no financial relationships to disclose. Address correspondence to J.R.L. (e-mail: jleyende@wfubmc.edu).
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Abstract
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Magnetic resonance (MR) imaging provides multiplanar large field-of-view images of the body with excellent soft-tissue contrast and without ionizing radiation. As a result, MR imaging is increasingly being used to image the maternal abdomen and pelvis during and immediately after pregnancy. Results of rapid T1- and T2-weighted imaging are often diagnostic, and blood vessels, ductal structures, and the urinary tract can frequently be visualized without intravenous administration of contrast material. Until more conclusive safety data become available, MR imaging should be reserved for cases in which results of ultrasonography are inconclusive and patient care depends on further imaging. In the setting of acute abdomen during pregnancy, MR imaging allows identification of areas of inflammation, abscess formation, hemorrhage, and bowel obstruction. MR imaging also helps determine the organ of origin, extent, and composition of maternal neoplasms and is useful in evaluation of müllerian duct anomalies and abnormalities of placental formation, position, and implantation. Many postpartum complications such as retained products of conception and uterine dehiscence may be diagnosed with MR imaging when results of other modalities are indeterminate.
© RSNA, 2004
Index Terms: Placenta, abnormalities, 857.824 • Pregnancy, abnormalities, 85.82 • Pregnancy, complications, 85.82 • Pregnancy, MR, 85.1214
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Introduction
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It is not uncommon for a patient to require diagnostic imaging during the course of her pregnancy for reasons unrelated to the pregnancy or for maternal disorders that may jeopardize the fetus. Magnetic resonance (MR) imaging may be beneficial in this setting, providing useful diagnostic information about the mother with minimal risk to the fetus. In many cases, MR imaging provides definitive results that are critical to patient care.
In this article, we discuss the use of MR imaging for maternal diseases of the abdomen and pelvis during pregnancy and the postpartum period. Specific topics discussed are the rationale for maternal imaging with MR, fetal safety, MR imaging techniques, and applications and examples of maternal MR imaging.
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Rationale for Maternal Imaging with MR
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Because it is safe, relatively inexpensive, and readily available, ultrasonography (US) remains the imaging modality of choice for the initial evaluation of many clinically suspected maternal abnormalities of the abdomen and pelvis. However, US is operator dependent and may be limited by patient body habitus or bowel gas when seeking to evaluate deeper structures. When US results are nondiagnostic or inadequate, many centers rely on computed tomography (CT) for further evaluation of suspected abnormalities. Unfortunately, CT performed to evaluate abdominal or pelvic symptoms often includes the fetus in the field of view, resulting in an estimated fetal dose between 12.5 and 35 mSv for a typical study, with higher doses possible depending on section thickness and equipment manufacturer (1). In addition, CT often requires the use of intravenous contrast agents to provide satisfactory soft-tissue image contrast, particularly when imaging solid organs.
MR imaging is capable of providing large field-of-view images of maternal abnormalities in any plane with excellent soft-tissue contrast. Images obtained with MR do not expose the mother or fetus to ionizing radiation and are often diagnostic in the absence of intravenous contrast material. In addition to standard T1- and T2-weighted images, MR can also provide dedicated images of the pancreatic and biliary ducts, blood vessels, and genitourinary tract without requiring intravenous contrast material.
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Fetal Safety
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Until more extensive clinical experience suggests otherwise, pregnant patients should undergo MR imaging only if the required information cannot be obtained via other nonionizing means such as US, the information is likely to alter patient care, and the examination cannot wait until after completion of the pregnancy (2). While preliminary evidence suggests that MR imaging is safe in the setting of pregnancy, several specific concerns regarding fetal safety deserve additional comment (3,4).
Tissue Heating
The radiofrequency radiation used in MR imaging is nonionizing. However, radiofrequency pulses result in energy deposition and can potentially result in tissue heating. The amount of energy deposited in a patient as a result of an MR imaging examination is referred to as the specific absorption rate (SAR), with units of watts per kilogram. The SAR for MR imaging units is regulated in the United States by the Food and Drug Administration, and separate regulated limits do not currently exist for pregnant patients.
The SAR increases with the static magnetic field strength, flip angle, and number and spacing of radiofrequency pulses. Therefore, single-shot echo train spin-echo sequences are associated with a relatively high SAR, while gradient-echo sequences, which do not depend on radiofrequency refocusing, are associated with a relatively low SAR. Despite the potential of long echo trains to cause fetal heating, the use of single-shot echo train spin-echo sequences is commonplace in fetal imaging and unlikely to result in significant temperature changes (5).
First-Trimester Imaging
There is no scientific evidence in humans to suggest that the risk to the fetus from a routine MR imaging examination is significantly increased during the first trimester. However, because this is the period of active organogenesis and the absolute safety of MR imaging during this period is difficult to establish, MR imaging is to be avoided unless the potential benefits outweigh the theoretical risks.
Intravenous Contrast Material
Gadolinium agents administered intravenously cross the placenta and may enter the fetal circulation to be excreted into amniotic fluid from the fetal bladder (6). The effects of gadolinium-based agents on fetal well-being are not fully understood, and adequate controlled trials of commercially available gadolinium chelates have not been performed in pregnant patients. Therefore, gadolinium compounds should be used in the setting of pregnancy only when a compelling clinical indication exists and the potential benefit to the patient outweighs the potential risk to the fetus. Informed consent may be prudent in this setting. Fortunately, we have found that the use of intravenous gadolinium agents is often not critical to patient care in the setting of pregnancy.
Gadolinium agents are excreted in low concentrations in human breast milk (7,8). However, the need to interrupt breast-feeding after gadolinium agent administration is somewhat controversial (9,10). There is currently no evidence to suggest that the minute amount of gadolinium excreted in breast milk that is absorbed by the fetal gastrointestinal tract is harmful to the fetus. While some centers continue to recommend that the mother interrupt breast-feeding for 24–48 hours after gadolinium agent administration pending additional safety data, other facilities have abandoned this policy.
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MR Imaging Techniques
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Challenges
MR imaging of the abdomen and pelvis in the setting of pregnancy presents several unique challenges. In addition to maternal physiologic motion such as peristalsis and breathing, one must also contend with fetal motion, which may contribute to image degradation. The effects of maternal peristalsis and fetal motion may be reduced by having the patient fast for approximately 4 hours prior to the study, provided the additional delay will not adversely affect the patient’s clinical status. The patient’s ability to suspend respiration and remain in the supine position for an extended period is likely to be limited during the third trimester. Finally, as the uterus enlarges, normal structures within the abdomen and pelvis become compressed and displaced.
Often, pregnant patients can be imaged in the supine position. However, imaging in the left lateral decubitus position may be necessary during the latter stages of pregnancy to avoid impairing venous return from the pelvis and lower extremities. Left lateral decubitus positioning is preferred over right, because it more adequately prevents compression of the inferior vena cava by the gravid uterus. Impaired venous return decreases cardiac output and may result in dizziness or syncope. An attempt should be made to use a surface coil to improve the signal-to-noise ratio and allow improved resolution. To reduce maternal fatigue and discomfort, imaging times should be as short as possible without compromising diagnostic information.
Sequences
In most cases, single-shot echo train spin-echo sequences provide motion-free T2-weighted images and are useful for identifying anatomy, fluid-filled structures and fluid collections, and bowel abnormalities (Fig 1). However, images tend to exhibit blurring, and flow artifacts within fluid are pronounced. The SAR is relatively high with this sequence. We perform the majority of our T1-weighted imaging using spoiled gradient-echo sequences. These sequences allow breath-hold imaging and minimize SAR. When higher-resolution T1- or T2-weighted imaging is required, we rely on echo train spin-echo imaging or a hybrid gradient-echo and spin-echo technique (GRASE). GRASE imaging combines aspects of echo train spin-echo imaging with gradient refocusing, resulting in both time savings and reduced SAR over echo train spin-echo techniques. Steady-state free-precession sequences are also useful for maternal imaging when available, because they provide rapid, exquisitely sharp images demonstrating bright fluid as well as vascular structures (Fig 1). These sequences are resistant to flow artifacts and are associated with a relatively low SAR. However, images are more prone to artifacts related to magnetic field heterogeneity.
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Figure 1a. Single-shot echo train spin-echo and steady-state gradient-echo imaging in a pregnant woman with clinical evidence of small-bowel obstruction. Arrow = fetus. (a) Coronal half-Fourier rapid acquisition with relaxation enhancement (RARE) image of the maternal abdomen shows fluid-filled loops of small intestine. (b) Coronal true fast imaging with steady-state precession (FISP) image clearly shows the dilated fluid-filled small intestine. The patient was admitted and treated conservatively.
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Figure 1b. Single-shot echo train spin-echo and steady-state gradient-echo imaging in a pregnant woman with clinical evidence of small-bowel obstruction. Arrow = fetus. (a) Coronal half-Fourier rapid acquisition with relaxation enhancement (RARE) image of the maternal abdomen shows fluid-filled loops of small intestine. (b) Coronal true fast imaging with steady-state precession (FISP) image clearly shows the dilated fluid-filled small intestine. The patient was admitted and treated conservatively.
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Fat Suppression
MR imaging is sensitive and specific for detection of both macroscopic amounts of fat as well as intracytoplasmic lipid within abdominal and pelvic tumors. Fat suppression is also useful for distinguishing between fat and hemorrhage on T1-weighted images and improves sensitivity for detection of fluid and edema on T2-weighted images. For suppression of macroscopic amounts of fat (as in most cases of renal angiomyolipoma, adrenal myelolipoma, or ovarian cystic teratoma), frequency-selective fat saturation or water excitation techniques work well. For detection of intracytoplasmic lipid (as in cases of pregnancy-related hepatic steatosis or adrenal adenoma), in- and opposed-phase gradient-echo imaging is recommended.
MR Angiography
MR angiography allows noninvasive assessment of maternal arterial and venous structures that may be difficult to evaluate sonographically during pregnancy. High-quality MR angiograms may be created without intravenous contrast material by using an inflow technique (eg, time-of-flight), phase-contrast angiography, or a steady-state free-precession sequence (Fig 2).
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Figure 2. Renal MR angiography without intravenous contrast material for evaluation of hypertension at 31 weeks gestation. Coronal three-dimensional phase-contrast MR angiogram obtained without intravenous contrast material shows the main renal arteries and multiple branches, thus allowing exclusion of renal artery stenosis. The acquisition time was 4 minutes 9 seconds. The patient gave birth prematurely 2 weeks later, and the hypertension resolved afterward.
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MR Cholangiopancreatography
As a result of alterations in hepatobiliary function during pregnancy, pregnant women are at increased risk for biliary stone disease. Before the advent of MR cholangiopancreatography, the diagnosis of choledocholithiasis was difficult to establish noninvasively during pregnancy. With MR cholangiopancreatography, common bile duct stones can be rapidly diagnosed with sensitivity exceeding 90% without significant maternal or fetal risk (Fig 3) (11–14). MR cholangiopancreatography exploits the long T2 relaxation time of fluid to create high-quality images of the biliary tract by using heavily T2-weighted echo train spin-echo sequences. MR cholangiopancreatography helps limit subsequent endoscopic retrograde cholangiopancreatography to therapeutic interventions by excluding patients without choledocholithiasis. In addition, MR cholangiopancreatography may be used to guide stone extraction and reduce the need for fluoroscopy during stone removal (15,16).
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Figure 3. Choledocholithiasis in a 20-year-old woman with elevated liver enzyme levels and right upper quadrant pain at 34 weeks gestation. US failed to demonstrate the distal common bile duct. Coronal thick-slab MR cholangiopancreatogram shows an obstructing stone in the common bile duct (arrow). The patient subsequently underwent endoscopic stone removal. Note the hydronephrosis of pregnancy involving the right kidney (arrowhead).
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MR Urography
While US is safe and accurate for establishing the presence of hydronephrosis during pregnancy, difficulty may be encountered when attempting to diagnose the level and cause of obstruction. Excretory MR urography involves intravenous administration of gadolinium agents and has been shown to correlate well with renal scintigraphy in the setting of pregnancy while revealing physiologic hydronephrosis, which results from compression of the ureter between the gravid uterus and iliopsoas muscle (17). Static MR urography techniques are virtually identical to the heavily T2-weighted sequences employed for MR cholangiopancreatography and accurately depict the level and, in many cases, the cause of ureteral obstruction without requiring intravenous contrast material (18,19).
Physiologic hydronephrosis of pregnancy is the most commonly encountered cause of ureteral dilatation in pregnancy and may be present in over one-half of women by the third trimester (Fig 4). The prevalence of ureteral calculi during pregnancy ranges from 0.03% to 0.4%, depending on the population, and it is important to distinguish between physiologic hydronephrosis and stone disease. Hydronephrosis of pregnancy is characterized by smooth tapering of the ureter at the level of the sacral promontory (18). The presence of a filling defect within the ureter or an unusual site of obstruction (eg, ureteropelvic junction or ureterovesical junction) suggests that the obstruction is not physiologic. Perinephric or periureteral edema or fluid may be present in the setting of obstructive calculi.
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Figure 4a. Hydronephrosis of pregnancy in a 25-year-old woman during the third trimester. (a) Coronal static thick-slab MR urogram shows severe right hydronephrosis (arrowhead). (b) Sagittal half-Fourier RARE image obtained through the right kidney shows tapering of the ureter (arrow) proximal to the site of compression by the uterus.
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Figure 4b. Hydronephrosis of pregnancy in a 25-year-old woman during the third trimester. (a) Coronal static thick-slab MR urogram shows severe right hydronephrosis (arrowhead). (b) Sagittal half-Fourier RARE image obtained through the right kidney shows tapering of the ureter (arrow) proximal to the site of compression by the uterus.
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Imaging Protocols
The majority of our examinations performed for maternal indications include multiplanar T2-weighted single-shot echo train spin-echo (half-Fourier rapid acquisition with relaxation enhancement [RARE], single-shot turbo spin-echo, or single-shot fast spin-echo) imaging in addition to axial T1-weighted gradient-echo imaging with and without fat suppression. However, because of variability in such factors as clinical indication, gestational age, and placental position, it is difficult to advocate a single detailed comprehensive protocol for maternal imaging in the setting of pregnancy. Instead, protocols should be modified to answer the specific clinical question in the shortest amount of time and with the least amount of maternal discomfort.
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Applications and Examples of Maternal MR Imaging
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Maternal Acute Abdomen
Differentiation of obstetric from nonobstetric causes of acute abdomen during pregnancy is critical to minimize maternal-fetal morbidity and mortality. However, the existence of pregnancy may complicate the clinical evaluation of nonspecific symptoms and delay diagnosis (20). Appendicitis is the most common maternal cause of acute abdomen during pregnancy. While MR imaging has been shown to be accurate for the diagnosis of acute appendicitis (21,22), the MR imaging diagnosis of acute appendicitis has not been thoroughly evaluated in the setting of pregnancy. A variety of obstetric and nonobstetric disorders other than appendicitis may manifest as acute abdomen during pregnancy (23). MR imaging may be particularly beneficial in demonstrating the site of transition in bowel obstruction and identifying areas of inflammation, abscess formation, or hemorrhage within the abdomen and pelvis (Figs 5–9).
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Figure 5a. Small-bowel obstruction in a young woman with a history of abdominal surgery who presented to the emergency department during the third trimester with diffuse abdominal pain. (a) Axial half-Fourier RARE image shows a dilated small intestine (arrows), although the transition site could not be localized with certainty. (b) Coronal half-Fourier RARE image shows the site of obstruction in the jejunum (arrow). An obstructing adhesion was found at surgery.
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Figure 5b. Small-bowel obstruction in a young woman with a history of abdominal surgery who presented to the emergency department during the third trimester with diffuse abdominal pain. (a) Axial half-Fourier RARE image shows a dilated small intestine (arrows), although the transition site could not be localized with certainty. (b) Coronal half-Fourier RARE image shows the site of obstruction in the jejunum (arrow). An obstructing adhesion was found at surgery.
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Figure 6a. Crohn disease manifesting as abdominal pain during the late second trimester. (a) Coronal half-Fourier RARE image shows an abnormally thickened and narrowed left colon (arrows). (b) Coronal half-Fourier RARE image shows the distal extent of the colonic narrowing (arrow) and dilated proximal intestine (arrowhead).
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Figure 6b. Crohn disease manifesting as abdominal pain during the late second trimester. (a) Coronal half-Fourier RARE image shows an abnormally thickened and narrowed left colon (arrows). (b) Coronal half-Fourier RARE image shows the distal extent of the colonic narrowing (arrow) and dilated proximal intestine (arrowhead).
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Figure 7. Retroperitoneal abscess in an obese woman with a history of resected colon cancer 2.5 years earlier who presented to the emergency department at 30 weeks gestation with right flank pain. US was performed at another institution; the results were interpreted as showing a solid right renal mass. Coronal single-shot turbo spin-echo image shows that the abnormality actually consists of fluid (arrow), which displaces the kidney (arrowhead). The diagnosis of an abscess was confirmed at drainage, although the cause was not definitively ascertained.
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Figure 8a. Hemorrhagic renal mass manifesting as acute right flank pain during the second trimester. US showed an echogenic mass, but the origin of the mass (adrenal vs renal) and its composition (fat vs hemorrhage) could not be determined. (a) Coronal T1-weighted gradient-echo image shows a heterogeneous mass with areas of high signal intensity (arrow) originating from the upper pole of the right kidney. (b) Coronal T1-weighted gradient-echo image obtained with fat suppression shows persistent bright areas (arrows), which represent hemorrhage, and a central dark area (arrowhead), which represents fat within the lesion. This case demonstrates the usefulness of multiplanar imaging and the ability of MR imaging to allow differentiation between fat and hemorrhage. The patient was treated conservatively and successfully gave birth but was lost to follow-up for 2 years. When she returned for evaluation of a subsequent pregnancy, repeat MR imaging showed no interval growth of the mass (which was interpreted as an angiomyolipoma) and complete resolution of the hemorrhage.
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Figure 8b. Hemorrhagic renal mass manifesting as acute right flank pain during the second trimester. US showed an echogenic mass, but the origin of the mass (adrenal vs renal) and its composition (fat vs hemorrhage) could not be determined. (a) Coronal T1-weighted gradient-echo image shows a heterogeneous mass with areas of high signal intensity (arrow) originating from the upper pole of the right kidney. (b) Coronal T1-weighted gradient-echo image obtained with fat suppression shows persistent bright areas (arrows), which represent hemorrhage, and a central dark area (arrowhead), which represents fat within the lesion. This case demonstrates the usefulness of multiplanar imaging and the ability of MR imaging to allow differentiation between fat and hemorrhage. The patient was treated conservatively and successfully gave birth but was lost to follow-up for 2 years. When she returned for evaluation of a subsequent pregnancy, repeat MR imaging showed no interval growth of the mass (which was interpreted as an angiomyolipoma) and complete resolution of the hemorrhage.
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Figure 9a. Pyelonephritis in a young woman with hypercoagulable state, pulmonary embolism, and right flank pain during the second trimester. Renal vein thrombosis was suspected at clinical evaluation. (a) Axial fat-suppressed T2-weighted gradient- and spin-echo image shows an enlarged, edematous right kidney with striated signal intensity (arrow). The atrophic left kidney (arrowhead) resulted from a prior ischemic event. (b) Axial targeted maximum intensity projection image from time-of-flight MR angiography shows a patent renal vein (arrow). A urine culture yielded Escherichia coli.
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Figure 9b. Pyelonephritis in a young woman with hypercoagulable state, pulmonary embolism, and right flank pain during the second trimester. Renal vein thrombosis was suspected at clinical evaluation. (a) Axial fat-suppressed T2-weighted gradient- and spin-echo image shows an enlarged, edematous right kidney with striated signal intensity (arrow). The atrophic left kidney (arrowhead) resulted from a prior ischemic event. (b) Axial targeted maximum intensity projection image from time-of-flight MR angiography shows a patent renal vein (arrow). A urine culture yielded Escherichia coli.
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A syndrome of hemolysis, elevated liver enzyme levels, and thrombocytopenia may develop during pregnancy. Given the acronym HELLP (hemolysis, elevated liver enzyme levels, low platelet count) by Weinstein (24) in 1982, this disorder typically occurs in the setting of severe preeclampsia or eclampsia but may manifest during the postpartum period. The most common serious intraabdominal complications include liver hemorrhage, infarction, or rupture. Histopathologic analysis of the liver in HELLP syndrome demonstrates fibrin deposits, periportal hemorrhage, and hepatocellular necrosis (25). MR imaging of the liver may demonstrate subcapsular or intrahepatic hematoma and areas of infarction (Fig 10).
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Figure 10a. HELLP syndrome in a 24-year-old woman who recently gave birth. (a) Axial T1-weighted gradient-echo image obtained through the liver shows a large subcapsular fluid collection containing blood products (arrow), a finding consistent with a hematoma. (b) Coronal gadolinium-enhanced T1-weighted gradient-echo image shows the hematoma (arrow) as well as poorly perfused areas of developing hepatic necrosis (arrowhead). Splenic and intrathoracic hemorrhage also developed, but the patient eventually recovered.
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Figure 10b. HELLP syndrome in a 24-year-old woman who recently gave birth. (a) Axial T1-weighted gradient-echo image obtained through the liver shows a large subcapsular fluid collection containing blood products (arrow), a finding consistent with a hematoma. (b) Coronal gadolinium-enhanced T1-weighted gradient-echo image shows the hematoma (arrow) as well as poorly perfused areas of developing hepatic necrosis (arrowhead). Splenic and intrathoracic hemorrhage also developed, but the patient eventually recovered.
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Maternal Neoplasms
MR imaging may be useful for the noninvasive evaluation of benign and malignant maternal neoplasms of the abdomen and pelvis (Figs 11 –13) (26,27). Because of the inherently high tissue contrast achievable with MR imaging, intravenous contrast material is often unnecessary to determine the origin and extent of abdominopelvic tumors. The use of multiple imaging planes aids in establishing the organ of origin of large tumors, and application of fat suppression is useful for tissue characterization.
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Figure 11. Gastric adenocarcinoma in a young woman who was referred for MR imaging during the third trimester for evaluation of a gastric mass. Extent of the mass could not be ascertained with endoscopy. Coronal half-Fourier RARE image shows a large tumor (arrow) extending into the gastric lumen. Serosal involvement was found at surgery, although no uterine involvement was detected with imaging or at surgery. Arrowhead = fetal bladder.
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Figure 12a. Mature ovarian cystic teratoma at 9 weeks gestation. Routine obstetric US demonstrated a large right adnexal mass. (a) Axial T1-weighted image shows a heterogeneous right adnexal mass with predominantly high signal intensity (arrow). A normal right ovary was not identified with any sequence. (b) Axial T1-weighted image obtained with fat suppression shows suppression of the high-signal-intensity areas within the mass (arrow), confirming the diagnosis of teratoma. We do not routinely perform MR imaging during the first trimester. In this case, the referring obstetrician thought the clinical indication was sufficiently compelling, and consent was obtained from the patient. The mass was surgically removed during pregnancy.
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Figure 12b. Mature ovarian cystic teratoma at 9 weeks gestation. Routine obstetric US demonstrated a large right adnexal mass. (a) Axial T1-weighted image shows a heterogeneous right adnexal mass with predominantly high signal intensity (arrow). A normal right ovary was not identified with any sequence. (b) Axial T1-weighted image obtained with fat suppression shows suppression of the high-signal-intensity areas within the mass (arrow), confirming the diagnosis of teratoma. We do not routinely perform MR imaging during the first trimester. In this case, the referring obstetrician thought the clinical indication was sufficiently compelling, and consent was obtained from the patient. The mass was surgically removed during pregnancy.
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Figure 13. Leiomyosarcoma at 26 weeks gestation. US showed a left-sided pelvic mass, but the origin and extent of the mass could not be determined. Coronal fat-suppressed T2-weighted image shows a bilobed high-signal-intensity mass (arrows), which appears to be contiguous to the uterus. On the basis of the MR imaging findings, the diagnosis of leiomyoma was suggested; however, a myxoid leiomyosarcoma was found at surgery 2 days later.
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Uterine Abnormalities
Acquired or congenital abnormalities of the uterus may affect fetal well-being or complicate delivery. MR imaging is useful for detecting uterine abnormalities and delineating uterine anatomy.
Uterine leiomyomas increase the likelihood of complications during pregnancy, labor, and delivery and increase the cesarean section rate (28). Complications of leiomyomas during pregnancy include pain, bleeding, spontaneous abortion, placental abruption, fetal malposition, and mechanical obstruction of the cervix (Fig 14). Most leiomyomas appear as low-signal-intensity uterine masses on T2-weighted images and are nearly isointense to the uterus on T1-weighted images. However, leiomyomas may have a highly variable appearance resulting from hyaline, cystic, myxoid, and red degeneration or an unusual histologic composition (29). MR imaging allows accurate determination of the location and size of uterine leiomyomas, although exophytic or pedunculated masses may be confused with solid adnexal tumors. In the case of an exophytic leiomyoma, demonstration of curvilinear signal voids extending between the uterus and the mass ("bridging vascular sign") may aid in diagnosis (30).
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Figure 14. Obstructing leiomyoma in a 27-year-old woman with an elevated level of ß-human chorionic gonadotropin and uncertain gestational dates. US did not allow adequate visualization of the uterine contents or pelvic structures. Sagittal T2-weighted image shows a large leiomyoma in the lower uterus (arrow). The tumor occupies most of the pelvis, creating a potential impediment to delivery.
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Uterine anomalies are relatively prevalent in women of reproductive age. While patients with müllerian duct anomalies may become pregnant and carry to term, some uterine anomalies, particularly septate uterus, are associated with an increased prevalence of pregnancy loss or complication. In the case of pregnancy occurring within one horn of a bicornuate uterus, the nongravid uterine horn can be readily identified as a separate structure from the gravid horn. Occasionally, a distended noncommunicating rudimentary horn is present, and pregnancy may rarely occur in such a rudimentary horn (31). Because renal anomalies may be present in patients with uterine anomalies, a set of coronal images through the retroperitoneum should be included routinely in this setting. In the case of uterus didelphys associated with obstructed hemivagina and unilateral renal agenesis, the renal agenesis typically occurs on the same side as the vaginal septum (Fig 15) (32–34).
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Figure 15a. Uterus didelphys with pyocolpos at 20 weeks gestation. The patient was referred for MR imaging because of purulent discharge from the vagina and uncertain uterine anatomy at US. (a) Sagittal T2-weighted image shows a gravid uterus containing a fetus in addition to a second uterine horn (straight arrow). Fluid fills the obstructed vagina of the nongravid uterus (arrowhead). The fluid collection communicates with the patent vagina via a small fistula (curved arrow), which was confirmed in the operating room. (b) Coronal half-Fourier RARE image shows that the nongravid uterine horn is on the left side (arrow). (c) Coronal half-Fourier RARE image obtained through the abdomen shows absence of the left kidney (arrow). The renal agenesis is on the same side as the obstructed vagina and nongravid uterine horn.
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Figure 15b. Uterus didelphys with pyocolpos at 20 weeks gestation. The patient was referred for MR imaging because of purulent discharge from the vagina and uncertain uterine anatomy at US. (a) Sagittal T2-weighted image shows a gravid uterus containing a fetus in addition to a second uterine horn (straight arrow). Fluid fills the obstructed vagina of the nongravid uterus (arrowhead). The fluid collection communicates with the patent vagina via a small fistula (curved arrow), which was confirmed in the operating room. (b) Coronal half-Fourier RARE image shows that the nongravid uterine horn is on the left side (arrow). (c) Coronal half-Fourier RARE image obtained through the abdomen shows absence of the left kidney (arrow). The renal agenesis is on the same side as the obstructed vagina and nongravid uterine horn.
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Figure 15c. Uterus didelphys with pyocolpos at 20 weeks gestation. The patient was referred for MR imaging because of purulent discharge from the vagina and uncertain uterine anatomy at US. (a) Sagittal T2-weighted image shows a gravid uterus containing a fetus in addition to a second uterine horn (straight arrow). Fluid fills the obstructed vagina of the nongravid uterus (arrowhead). The fluid collection communicates with the patent vagina via a small fistula (curved arrow), which was confirmed in the operating room. (b) Coronal half-Fourier RARE image shows that the nongravid uterine horn is on the left side (arrow). (c) Coronal half-Fourier RARE image obtained through the abdomen shows absence of the left kidney (arrow). The renal agenesis is on the same side as the obstructed vagina and nongravid uterine horn.
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Placental Abnormalities
Pregnancy may be complicated by abnormalities of placental formation, position, or implantation. Abnormal placental formation results in a spectrum of gestational trophoblastic diseases including complete hydatidiform mole, partial mole, invasive mole, choriocarcinoma, and placental site trophoblastic tumor. All of these entities may be demonstrated with MR imaging, although usually US is sufficient to suggest the diagnosis and MR imaging is reserved for difficult cases (35–37). Complete molar pregnancy is characterized pathologically by hydropic swelling of chorionic villi and variable epithelial hyperplasia. On T2-weighted images, the endometrial cavity appears expanded with heterogeneous high-signal-intensity material (Fig 16). A normal gestational sac and embryo are absent, although cystic spaces may be present.
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Figure 16a. Hydatidiform mole in a 22-year-old woman who presented at 10 weeks gestation with an elevated level of ß-human chorionic gonadotropin and no visible gestational sac at US. (a, b) T2-weighted images obtained in the coronal (a) and axial (b) planes relative to the uterus show heterogeneous but predominantly high-signal-intensity material filling the uterine cavity (arrow). No gestational sac is present. (c) Axial unenhanced fat-suppressed T1-weighted image shows that the uterine contents have increased signal intensity (arrow). A complete molar pregnancy was confirmed at pathologic analysis.
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Figure 16b. Hydatidiform mole in a 22-year-old woman who presented at 10 weeks gestation with an elevated level of ß-human chorionic gonadotropin and no visible gestational sac at US. (a, b) T2-weighted images obtained in the coronal (a) and axial (b) planes relative to the uterus show heterogeneous but predominantly high-signal-intensity material filling the uterine cavity (arrow). No gestational sac is present. (c) Axial unenhanced fat-suppressed T1-weighted image shows that the uterine contents have increased signal intensity (arrow). A complete molar pregnancy was confirmed at pathologic analysis.
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Figure 16c. Hydatidiform mole in a 22-year-old woman who presented at 10 weeks gestation with an elevated level of ß-human chorionic gonadotropin and no visible gestational sac at US. (a, b) T2-weighted images obtained in the coronal (a) and axial (b) planes relative to the uterus show heterogeneous but predominantly high-signal-intensity material filling the uterine cavity (arrow). No gestational sac is present. (c) Axial unenhanced fat-suppressed T1-weighted image shows that the uterine contents have increased signal intensity (arrow). A complete molar pregnancy was confirmed at pathologic analysis.
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Invasive mole is characterized by growth of trophoblastic elements into the myometrium, although tumor penetration may extend into parametrial tissues (38). Evidence of disruption of the junctional zone on T2-weighted images or tumor extension into the myometrium implies invasive mole. Invasive mole may be associated with dilated vessels within the tumor, myometrium, and parametrium and enhances intensely following intravenous administration of gadolinium agents (36). Choriocarcinoma lacks a villous pattern, may be locally invasive, and frequently metastasizes (38). MR imaging typically demonstrates necrosis, hemorrhage, and enhancing solid components.
The term placenta previa refers to abnormally low implantation of the placenta at or over the internal cervical os that precludes vaginal delivery. When the internal os is completely covered, the condition is referred to as complete previa. US is adequate for diagnosing the vast majority of cases of placenta previa. However, sagittal T2-weighted MR images accurately and noninvasively depict the position of the placenta with respect to the internal os (Fig 17). Placenta previa is associated with an increased risk of placenta accreta.
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Figure 17. Placenta previa manifesting as vaginal bleeding at 30 weeks gestation. Sagittal T2-weighted image shows the placenta completely covering the internal cervical os (arrow), an appearance consistent with complete central previa.
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Placenta accreta, increta, and percreta constitute a spectrum of abnormal placental attachments that can have a catastrophic impact if unsuspected at the time of delivery. Placental implantation on the uterine wall is referred to as placenta accreta (the most common variant). Deep myometrial invasion by the placenta characterizes placenta increta, while placental extension through the serosa constitutes placenta percreta. These entities have an association with prior cesarean section, uterine surgery or instrumentation, multiparity, and placenta previa (39). While placenta accreta can occur in other locations within the uterus, the lower uterine segment is most commonly involved. US is often helpful in cases of placenta accreta with an anterior placenta, but MR imaging may be more beneficial in the setting of a posterior placenta (40).
In the case of placenta accreta, MR imaging may demonstrate focal thinning and indistinctness of the myometrium. When placental tissue is seen completely interrupting or traversing the normally low-signal-intensity uterine wall, the diagnosis of placenta percreta is suggested (41). Bladder involvement by placenta percreta is suggested by interruption or indistinctness of the normally low-signal-intensity bladder wall (Fig 18). While MR imaging shows promise for improving the diagnosis of placental abnormalities, the prospective diagnosis and localization of placenta accreta remain a challenge (42). Dynamic contrast material–enhanced MR imaging allows clear differentiation between the intensely enhancing placenta and weakly enhancing myometrium and may be of benefit in the diagnosis of placental abnormalities (43,44). However, further research in this area is needed before recommending the routine use of gadolinium-based contrast agents in this setting.
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Figure 18a. Placenta percreta in a 31-year-old woman with a history of two cesarean sections. At 28 weeks gestation, the patient was referred for evaluation of a placenta that appeared abnormal at US. Coronal T2-weighted images (a obtained posterior to b) show disruption of the normally continuous low-signal-intensity bladder dome (arrow), an appearance consistent with placenta percreta. The obstetrician was notified before cesarean section about the likelihood of placenta percreta, although the exact longitudinal extent of invasion was uncertain before surgery. During surgery, placental invasion of the midline bladder was confirmed and resulted in extensive cystotomy.
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Figure 18b. Placenta percreta in a 31-year-old woman with a history of two cesarean sections. At 28 weeks gestation, the patient was referred for evaluation of a placenta that appeared abnormal at US. Coronal T2-weighted images (a obtained posterior to b) show disruption of the normally continuous low-signal-intensity bladder dome (arrow), an appearance consistent with placenta percreta. The obstetrician was notified before cesarean section about the likelihood of placenta percreta, although the exact longitudinal extent of invasion was uncertain before surgery. During surgery, placental invasion of the midline bladder was confirmed and resulted in extensive cystotomy.
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Postpartum Disorders
Retained products of conception may complicate a terminated pregnancy (most common) or term vaginal or cesarean delivery. Retained products of conception typically appear as variably enhancing soft-tissue masses within the uterine cavity on gadolinium-enhanced MR images (Fig 19) (45). The uterine contents have a heterogeneous appearance on T2-weighted images. The imaging appearance may overlap with that of gestational trophoblastic disease. The serum ß–human chorionic gonadotropin level may be only mildly elevated or normal with retained products of conception, in contrast to gestational trophoblastic disease, which usually results in persistently elevated levels of the hormone. Because of the vascular nature of retained products of conception, this entity may also be confused with uterine arteriovenous malformation (Fig 19) (46).
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Figure 19a. Retained products of conception in a postpartum woman with vaginal bleeding who was referred for evaluation of a possible uterine arteriovenous malformation. (a) Coronal gadolinium-enhanced T1-weighted image obtained at another institution shows enhancing tissue within the endometrial cavity (arrow). (b) Coronal arterial phase gadolinium-enhanced MR angiogram from the same study shows increased vascularity of the uterus (arrow). This finding was thought to represent a uterine arteriovenous malformation. Subsequent dilation and curettage revealed retained products of conception, and MR imaging was repeated after 6 weeks. (c) Repeat coronal gadolinium-enhanced MR angiogram shows normal pelvic vascularity. (d) Repeat sagittal T2-weighted image shows a normal endometrium (arrow).
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Figure 19b. Retained products of conception in a postpartum woman with vaginal bleeding who was referred for evaluation of a possible uterine arteriovenous malformation. (a) Coronal gadolinium-enhanced T1-weighted image obtained at another institution shows enhancing tissue within the endometrial cavity (arrow). (b) Coronal arterial phase gadolinium-enhanced MR angiogram from the same study shows increased vascularity of the uterus (arrow). This finding was thought to represent a uterine arteriovenous malformation. Subsequent dilation and curettage revealed retained products of conception, and MR imaging was repeated after 6 weeks. (c) Repeat coronal gadolinium-enhanced MR angiogram shows normal pelvic vascularity. (d) Repeat sagittal T2-weighted image shows a normal endometrium (arrow).
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Figure 19c. Retained products of conception in a postpartum woman with vaginal bleeding who was referred for evaluation of a possible uterine arteriovenous malformation. (a) Coronal gadolinium-enhanced T1-weighted image obtained at another institution shows enhancing tissue within the endometrial cavity (arrow). (b) Coronal arterial phase gadolinium-enhanced MR angiogram from the same study shows increased vascularity of the uterus (arrow). This finding was thought to represent a uterine arteriovenous malformation. Subsequent dilation and curettage revealed retained products of conception, and MR imaging was repeated after 6 weeks. (c) Repeat coronal gadolinium-enhanced MR angiogram shows normal pelvic vascularity. (d) Repeat sagittal T2-weighted image shows a normal endometrium (arrow).
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Figure 19d. Retained products of conception in a postpartum woman with vaginal bleeding who was referred for evaluation of a possible uterine arteriovenous malformation. (a) Coronal gadolinium-enhanced T1-weighted image obtained at another institution shows enhancing tissue within the endometrial cavity (arrow). (b) Coronal arterial phase gadolinium-enhanced MR angiogram from the same study shows increased vascularity of the uterus (arrow). This finding was thought to represent a uterine arteriovenous malformation. Subsequent dilation and curettage revealed retained products of conception, and MR imaging was repeated after 6 weeks. (c) Repeat coronal gadolinium-enhanced MR angiogram shows normal pelvic vascularity. (d) Repeat sagittal T2-weighted image shows a normal endometrium (arrow).
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Uterine dehiscence may occur at the incision site of a cesarean section, resulting in a transmural gap in the anterior uterus that may be demonstrated with MR imaging (Fig 20). Dehiscence may be complicated by the presence of hematoma or abscess, complicating the appearance at imaging. To best demonstrate the myometrial gap, an imaging plane perpendicular to the incision is recommended (47). During the early postpartum period, the cesarean section incision will normally demonstrate increased signal intensity on T1- and T2-weighted images, mimicking near-complete dehiscence, and an area of increased signal intensity is often present between the bladder and uterus, representing bladder flap hematoma (48,49).
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Figure 20a. Uterine dehiscence in a 24-year-old woman with vaginal bleeding and pelvic pain approximately 1 month after cesarean section. US findings were suggestive of a blood clot anterior to the uterus, but a myometrial defect could not be identified. (a) Sagittal T2-weighted image shows a large, anterior, transmural myometrial defect (arrows). No normal myometrial coverage is visible, and an adjacent complex fluid collection is present. (b) Axial fat-suppressed T1-weighted image shows that the fluid anterior to the uterus has high signal intensity (arrow), a finding consistent with blood products. At surgery, transmural myometrial necrosis was present in addition to retained placental tissue, which was suspicious for placenta accreta.
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Figure 20b. Uterine dehiscence in a 24-year-old woman with vaginal bleeding and pelvic pain approximately 1 month after cesarean section. US findings were suggestive of a blood clot anterior to the uterus, but a myometrial defect could not be identified. (a) Sagittal T2-weighted image shows a large, anterior, transmural myometrial defect (arrows). No normal myometrial coverage is visible, and an adjacent complex fluid collection is present. (b) Axial fat-suppressed T1-weighted image shows that the fluid anterior to the uterus has high signal intensity (arrow), a finding consistent with blood products. At surgery, transmural myometrial necrosis was present in addition to retained placental tissue, which was suspicious for placenta accreta.
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Septic puerperal ovarian vein thrombosis manifests in the postpartum period with nonspecific symptoms of pain and fever. Most cases occur on the right and may be confused clinically with appendicitis, pyelonephritis, or other inflammatory disorders. While many cases of ovarian vein thrombosis are evident with US or contrast-enhanced CT, MR venography has been shown to be extremely sensitive and specific for detection of thrombosis and may be helpful when results of other modalities are nondiagnostic or inconclusive (50,51). Time-of-flight or phase-contrast MR imaging may demonstrate thrombus within the ovarian vein, but flow artifacts may be confused with thrombus at nonenhanced MR imaging. As a result, gadolinium-enhanced MR venography may be preferable in the postpartum period.
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Conclusions
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MR imaging is useful in the assessment of maternal diseases of the abdomen and pelvis during pregnancy and the postpartum period. MR imaging avoids exposure of the mother and fetus to ionizing radiation and often does not require intravenous contrast material in the setting of pregnancy. Newer sequences make this a potentially time-efficient modality that should be considered when US has nondiagnostic results or is not feasible.
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Footnotes
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Abbreviations: RARE = rapid acquisition with relaxation enhancement,
SAR = specific absorption rate
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References
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- El-Khoury GY, Madsen MT, Blake ME, Yankowitz J. A new pregnancy policy for a new era. AJR Am J Roentgenol 2003; 181:335-340.[
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Abstract
Introduction
