(Radiographics. 2002;22:563-580.)
© RSNA, 2002
Fast MR Imaging in Obstetrics1
Masako Nagayama, MD,
Yuji Watanabe, MD,
Akira Okumura, MD,
Yoshiki Amoh, MD,
Satoru Nakashita, MD and
Yoshihiro Dodo, MD
1 From the Department of Radiology, Kurashiki Central Hospital, 1-1-1 Miwa, Kurashiki, Okayama 710-8602, Japan. Presented as an education exhibit at the 2000 RSNA scientific assembly. Received March 15, 2001; revision requested April 4 and received November 2; accepted November 7. Address correspondence to M.N. (e-mail: md5210@kchnet.or.jp).
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Abstract
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Ultrasonography (US) is the initial imaging modality of choice for evaluation of patients in obstetrics. However, the results of US are not always sufficient. Magnetic resonance (MR) imaging, which uses no ionizing radiation, may be an ideal method for further evaluation. Although MR imaging is not recommended during the first trimester and use of contrast material is not recommended in pregnant patients, fast MR imaging is useful in various obstetric settings and can provide more specific information with excellent tissue contrast and multiplanar views. In pregnant patients with acute conditions, various diseases (eg, red degeneration of a uterine leiomyoma) may be diagnosed. MR imaging allows characterization of pelvic masses discovered during pregnancy and diagnosis of postpartum complications (eg, abscess, hematoma, ovarian vein thrombosis). In pregnant patients with hydronephrosis, MR urography can demonstrate the site of obstruction and the cause (eg, a ureteral stone). MR pelvimetry may be beneficial in cases of breech presentation. Contrast material–enhanced dynamic MR imaging allows one to evaluate the vascularity of a placental polyp, detect the viable component of a gestational trophoblastic tumor, and diagnose a uterine arteriovenous malformation. MR imaging enables diagnosis of rare forms of ectopic pregnancy and early diagnosis of ectopic pregnancy.
© RSNA, 2002
Index Terms: Placenta, abnormalities, 857.824 • Pregnancy, abnormalities, 856.82 • Pregnancy, complications, 856.82 • Pregnancy, MR, 856.1214, 856.121412, 856.121415
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
- Discuss the safety of MR imaging and use of contrast material during pregnancy and lactation.
- Recognize the clinical features and MR imaging features of maternal abnormalities in obstetrics.
- Discuss the usefulness of fast MR imaging in obstetrics.
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Introduction
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Ultrasonography (US) is the initial imaging modality of choice for assessment of patients in obstetrics because it is safe and inexpensive and can be easily performed at the bedside. Various diseases associated with pregnancy can be diagnosed with US (1). However, it is highly operator dependent and factors such as intervening bowel gas, the gravid uterus, and a patient’s obesity may interfere with the examination. In addition, its specificity in tissue characterization is limited. Computed tomography (CT) may be of limited use in obstetric patients because of ionizing radiation (2).
Magnetic resonance (MR) imaging could be an ideal modality for evaluation of the female pelvis and fetus because it uses no ionizing radiation. Furthermore, MR imaging has the advantages of excellent tissue contrast and multiplanar capability (2–4).
The recent development of fast MR imaging techniques has made the data acquisition time short enough to examine the fetus and patients with acute conditions without sedation (2–4). MR hydrographic techniques enable MR urography in a short imaging time, which can depict the urinary tract noninvasively without use of contrast material and ionizing radiation. Fast field echo techniques have increased the time resolution of contrast material–enhanced dynamic MR imaging, which allows evaluation of lesion vascularity and vascular structures (3,5).
In a variety of clinical settings in obstetrics, fast MR imaging may be considered a valuable complement to US when US findings are inconclusive and additional information is needed to make proper management decisions. In this article, we illustrate the applications and usefulness of fast MR imaging in cases of various maternal abnormalities (Table 1) and describe the clinical features of each disease. Specific topics discussed are the safety of MR imaging and contrast material, fast MR imaging techniques, abnormal pregnancy, postpartum complications, planning delivery, and maternal abnormalities during pregnancy.
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Safety of MR Imaging and Contrast Material
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MR Imaging during Pregnancy
Although to date there is no conclusive evidence that use of clinical MR imaging produces deleterious effects on human embryos or fetuses, the safety of MR imaging procedures during pregnancy has not been definitively proved (2,4). In a few studies, prolonged exposure to electromagnetic radiation has been linked with some deleterious effects on embryogenesis, chromosomal structure, or fetal development (4,6,7). On the other hand, many more studies have failed to demonstrate any measurable harmful effect of electromagnetic fields on animal or human embryos (4,6). The risk of exposing the developing fetus to any radiologic diagnostic imaging technique that uses ionizing radiation is probably greater than the theoretical risk of MR imaging (6).
Because of the potential and unknown effects, MR imaging should be performed only when results of US are inadequate for diagnosis or when MR imaging is expected to provide important information for proper treatment of the fetus or mother (2,4,6). The MR imaging examination should be designed to obtain sufficient information with the least number of imaging sequences. MR imaging is not recommended during the first trimester because the developing embryo is susceptible to injury from various physical agents (2,4). Pregnant patients should be informed of these issues (2,6).
Use of Contrast Material during Pregnancy
Gadolinium-based contrast material has been shown to cross the placenta after intravenous administration and appear within the fetal bladder (2,4,6,8). Then, it is excreted into the amniotic fluid, swallowed by the fetus, and reabsorbed from the gastrointestinal tract. The half-life of the drug in the fetal circulation and the effect of this drug on the developing human fetus are unknown (2,6,8). In animal studies, growth retardation has been reported after administration of a high dose of the drug (Magnevist product information; Berlex Laboratories, Wayne, NJ). The safety of intravenous administration of the drug in pregnant patients has not been widely tested and established (2,6,8,9). Therefore, use of the drug is generally not recommended in pregnant patients (2,6,8).
Use of Contrast Material during Lactation
A very small amount of gadolinium-based contrast material is excreted into breast milk following intravenous injection. After 24 hours, a tiny fraction of the administered dose can be detected in the breast milk (6,10). Therefore, a precautionary 24-hour suspension of breast-feeding is generally recommended following administration of the drug (6).
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Fast MR Imaging Techniques
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We perform MR imaging with a 1.5-T unit (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands). Our standard pulse sequences for fast MR imaging of the female pelvis are summarized in Table 2 (3,5). A body coil or a body phased-array coil is used.
Nonenhanced MR Imaging
After acquisition of localizing images, we usually obtain nonenhanced T1-weighted, fat-suppressed T1-weighted, fat-suppressed T2-weighted, and heavily T2-weighted images using turbo spin-echo sequences or gradient and spin-echo sequences. For fat suppression, a chemically selective suppression technique is used. The total examination time for nonenhanced MR imaging is about 15 minutes.
A combination of T1-weighted images and fat-suppressed T1-weighted images is used to confirm the presence of fat components and allow a specific diagnosis of cystic teratoma (3,11). Fat-suppressed T1-weighted images clearly show hyperintense lesions such as endometriomas surrounded by fat (3,11). Bloody ascites is easy to recognize as slightly hyperintense areas relative to urine in the bladder. The addition of fat suppression to T2-weighted imaging also improves the detection and conspicuity of hyperintense lesions such as ascites and cysts surrounded by fat. Inflammatory infiltration or edema is depicted as ill-defined slightly hyperintense areas (3). On heavily T2-weighted images, ascites and urine are markedly hyperintense, whereas fat is moderately hyperintense (3). Hematoma has a characteristic appearance that depends on the age of the blood.
Contrast-enhanced MR Imaging
Contrast-enhanced MR imaging is performed selectively in patients suspected of having uterine arteriovenous malformation (AVM), placental polyp, gestational trophoblastic tumor, abscess, and so on (3). Pregnant patients do not undergo injection of contrast material. Immediately after nonenhanced imaging, we decide whether contrast-enhanced MR imaging is needed.
Two methods of contrast-enhanced MR imaging are used (3). One is dynamic MR imaging with bolus injection of contrast material, which allows creation of MR angiograms by postprocessing. The other is contrast-enhanced fat-suppressed T1-weighted imaging. Usually, we perform both methods to evaluate lesion vascularity. When an inflammatory lesion is suspected, contrast-enhanced fat-suppressed T1-weighted images are obtained. The total examination time is about 30 minutes when both of these contrast-enhanced imaging methods are used.
Contrast-enhanced dynamic MR imaging is performed in the sagittal or coronal plane by using a three-dimensional fast-field-echo sequence (3,5). The images are obtained before and after rapid intravenous injection of a 0.1 mmol/kg dose of gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany). After injection of the contrast material, five imaging data sets are consecutively obtained. The data set obtained before contrast material injection is subtracted from each of the five data sets obtained after contrast material injection, section by section. Dynamic subtraction MR angiograms are created by compressing subtracted images of each phase with maximum intensity projection postprocessing (3,5).
Contrast-enhanced dynamic MR imaging enables tracking of the dynamics of a contrast material bolus, which allows one to obtain information about lesion vascularity and the anatomic structures of related vessels (5,12). Image subtraction effectively suppresses the signal of the background, including fat and hematomas, and facilitates identification of enhanced areas and determination of the degree of enhancement (3). Both source images and subtracted images are observed in detail on a workstation. Dynamic subtraction MR angiograms can provide an overview of the vessels and demonstrate vascular anatomy, vascular occlusion, and abnormal vessels (3,5,12). Bleeding points in hemorrhagic lesions with persistence of bleeding may be seen as extravasation of contrast material (3). MR venography can be performed when an arterial phase data set is subtracted from a venous phase data set (5,12).
Contrast-enhanced fat-suppressed T1-weighted imaging is performed by using turbo spin-echo sequences or gradient and spin-echo sequences with a chemically selective fat suppression technique. Fat suppression can improve determination of the extent of enhancement of lesions (13). Areas with delayed enhancement such as an abscess wall, inflammatory infiltration, and vascular pooling are clearly demonstrated (3).
MR Urography
MR urography is performed by using extremely heavily T2-weighted sequences such as single-shot turbo spin echo or half-Fourier acquisition single-shot turbo spin echo (14,15). The images can depict the urinary tract without use of contrast material. In this method, static or slowly flowing fluids are seen as high signal intensity against a background of very low signal intensity (14).
There are two variations of this imaging method: the single-section method and the multisection method (15). The single-section method can demonstrate the urinary tract by means of projection images. It is performed with a thick single section in a short imaging time and does not require any postprocessing. The multisection method is performed with multiple contiguous or overlapping thin-section images and requires maximum intensity projection postprocessing to obtain pyelographic images (14,15).
MR urography is noninvasive, does not require ionizing radiation, and does not depend on renal function, in contrast to conventional excretory urography (15). Therefore, it is considered suitable for evaluation of pregnant patients with hydronephrosis (2,15). The dilated ureter and the site of obstruction are readily demonstrated. The thin-section source images of multisection MR urography are useful for evaluating the details of the obstructed site and can reveal a ureteral stone as a filling defect (14,15).
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Abnormal Pregnancy
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Ectopic Pregnancy
Ectopic pregnancy is a well-known acute condition associated with pregnancy. Massive hemorrhage due to rupture leads to symptoms of shock. If the start of appropriate therapy is delayed, this condition may be fatal. The ß–human chorionic gonadotropin (hCG) level is useful for detection of early ectopic pregnancies. Transvaginal US is the initial imaging modality of choice for establishing gestational location and allows one to make the diagnosis in combination with the ß-hCG level (16–18).
The role of MR imaging in diagnosis of ectopic pregnancy has not been defined. Some authors have stated that MR imaging is not usually essential for diagnosis (16,18). However, others have stated that MR imaging is useful for specific diagnosis of rare or complicated forms of ectopic pregnancy (19), such as abdominal pregnancy (20), interstitial pregnancy (18), myometrial pregnancy (21), and cervical pregnancy (22), when the results of US are insufficient or inconclusive.
The MR imaging findings of ectopic pregnancy include hematosalpinx (17), a hemorrhagic mass (16,17,19) (Fig 1), a gestational sac (17,18), a heterogeneous mass (18), bloody ascites (16,17), and tubal dilatation and wall enhancement (17). Hematoma has a characteristic appearance that depends on the age of the blood. The gestational sac has low signal intensity on T1-weighted images and high signal intensity on T2-weighted images with or without fetoplacental tissue (17,18).
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Figure 1a. Right tubal pregnancy in a 27-year-old woman with a history of repeated artificial pregnancy. Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a pelvic mass (short arrows) adjacent to the uterine body; the mass has intermediate signal intensity on the T1-weighted image (a) and high signal intensity on the T2-weighted image (b). Note the left tubal hematoma (arrowhead) and the dead fertilized ovum in the uterine cavity (long arrow), which is due to the repeated artificial pregnancies.
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Figure 1b. Right tubal pregnancy in a 27-year-old woman with a history of repeated artificial pregnancy. Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a pelvic mass (short arrows) adjacent to the uterine body; the mass has intermediate signal intensity on the T1-weighted image (a) and high signal intensity on the T2-weighted image (b). Note the left tubal hematoma (arrowhead) and the dead fertilized ovum in the uterine cavity (long arrow), which is due to the repeated artificial pregnancies.
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Abdominal pregnancy is rare. Early, accurate diagnosis is essential to avoid serious complications such as severe bleeding, infection, and peritonitis. At US, when there is no interposing uterine wall between the fetal head and maternal urinary bladder, a specific diagnosis of an abdominal pregnancy can be made (20). However, interpretation of US images may be difficult. MR imaging can demonstrate the uterine wall, placenta, amniotic fluid, and fetus with excellent contrast and global multiplanar views and shows the extrauterine fetus and placenta (20).
Myometrial pregnancy in a previous cesarean section scar (Fig 2) is dangerous because of the risk of uterine rupture and hemorrhage (21). On sagittal MR images, the gestational sac is seen in the anterior part of the isthmic portion, bulging at the serosal surface. The myometrium between the sac and bladder is very thin in contrast with that in cervical pregnancy (21).
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Figure 2a. Myometrial pregnancy in a cesarean section scar in a 26-year-old woman. The patient was treated conservatively with direct injection of methotrexate. (a) Sagittal fat-suppressed T2-weighted MR image obtained 3 days after injection of methotrexate shows a heterogeneous hyperintense mass (arrows) of the deep myometrium in the lower, anterior part of the uterus. Markedly hyperintense areas in the mass are suggestive of a degenerated gestational sac. Note that the myometrial wall between the sac and the bladder is very thin (arrowheads). (b) Sagittal fat-suppressed T2-weighted MR image obtained 3 months after injection of methotrexate shows a decrease in the size of the mass (arrow).
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Figure 2b. Myometrial pregnancy in a cesarean section scar in a 26-year-old woman. The patient was treated conservatively with direct injection of methotrexate. (a) Sagittal fat-suppressed T2-weighted MR image obtained 3 days after injection of methotrexate shows a heterogeneous hyperintense mass (arrows) of the deep myometrium in the lower, anterior part of the uterus. Markedly hyperintense areas in the mass are suggestive of a degenerated gestational sac. Note that the myometrial wall between the sac and the bladder is very thin (arrowheads). (b) Sagittal fat-suppressed T2-weighted MR image obtained 3 months after injection of methotrexate shows a decrease in the size of the mass (arrow).
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Combined pregnancy, which is the simultaneous existence of intrauterine and extrauterine pregnancies, has increased in incidence with the recent development and spread of infertility treatment (23). Ovarian hyperstimulation also increases the risk for ovarian cancer (24). MR imaging may be helpful in detecting this unexpected combined pregnancy and differentiating between extrauterine pregnancy and ovarian cancer (24).
MR imaging may also be helpful in differentiating an intrauterine pregnancy in an anomalous uterus, such as a bicornuate uterus, from an interstitial pregnancy or abdominal pregnancy.
Accurate and early diagnosis of ectopic pregnancy allows one to avoid the risk of massive hemorrhage and an emergency hysterectomy; it also allows use of more conservative treatment such as local injection of methotrexate to maintain fertility (17,21) (Fig 2). Kataoka et al (17) reported that MR imaging (including contrast-enhanced imaging) allowed early diagnosis of tubal pregnancy in patients who were clinically stable and minimally symptomatic and enabled determination of the usefulness of early conservative therapy with methotrexate.
Gestational trophoblastic tumor must be included in the differential diagnosis of ectopic pregnancy, such as intramural pregnancy and interstitial pregnancy (18,19). The level of ß-hCG may be helpful in such differentiation.
Placenta Accreta
Placenta accreta is a condition in which the placental villi attach directly to the uterine myometrium without intervening decidual membrane (25). The spectrum of this entity is subclassified according to the degree of villous invasion: placenta accreta (myometrial invasion), placenta increta (deep myometrial invasion), and placenta percreta (invasion through the uterine serosa with potential invasion of adjacent organs) (25–28). The predisposing factors for placenta accreta are repeated cesarean section, other surgical procedures such as myomectomy, dilation and curettage, multiparity, placenta previa, miscarriage, and so on (2,25–28). When placenta accreta is present, it is usually difficult or impossible to completely extract the placenta at the time of delivery. Hysterectomy is frequently necessary because of uncontrollable bleeding (2,26,27). In addition, placenta accreta can result in retained placenta, which may cause postpartum bleeding and infection (26,27).
US (including Doppler US) is useful for diagnosis of anterior placenta accreta. However, in cases of fundal or posterior placenta accreta, it may be difficult to make the diagnosis at US because the location is far from the transducer (2,26,28). In placenta percreta, US may also be limited in detection and evaluation of the degree of extrauterine extent. MR imaging is helpful as a complement to US in such cases (26).
MR imaging shows the placenta in a gravid uterus as moderately hyperintense on T2-weighted images and allows differentiation between the placenta and the hypointense myometrium. Images obtained perpendicular to the boundary plane between the placenta and myometrium may be most useful for evaluation of the depth of placental invasion into the myometrium. Placenta accreta and increta are suggested by thinning, irregularity, or focal disruption of the subjacent myometrium (26). However, minimal placental adhesion such as in placenta accreta may be difficult to identify (26). Placenta percreta is suggested by extension of the placenta transmurally through the myometrium. When irregularity or disruption of the normal bladder wall is seen, invasion of the bladder may be present. The findings of placenta percreta may be similar to those of invasive gestational trophoblastic tumor (26, 29). When the zonal structure of the uterus is intact at MR imaging and US, placenta percreta may be more likely (29). The level of ß-hCG may be helpful in the differentiation (26).
Placenta Previa
Placenta previa is abnormally low implantation of the placenta so that it covers all or part of the internal cervical os. Accurate prenatal diagnosis is crucial because placenta previa precludes delivery through the vagina and may cause fatal bleeding; most cases require delivery by cesarean section. Placenta accreta occurs in 5% of patients with placenta previa (2,26,28).
In evaluation of placenta previa, transvaginal US is useful and MR imaging is not always indicated. A sagittal T2-weighted image oriented in the plane of the cervix can allow assessment of the relationship between the placental edge and the internal cervical os (2).
Gestational Trophoblastic Disease
Gestational trophoblastic disease is characterized by abnormal proliferation of pregnancy-associated trophoblastic tissue with malignant potential (30). It includes hydatidiform mole and gestational trophoblastic tumors (invasive mole and choriocarcinoma) (30). The ß-hCG level is a useful serologic marker for diagnosis, assessment of response to treatment, and surveillance for recurrence (30–32). Assessment of local extension and distant metastasis (including to the lung, brain, and liver) is required for optimal therapy. US is the initial examination of choice for diagnosis. However, myometrial invasion and extension into the parametrium can be difficult to detect with US alone (30).
Relative to US and CT, MR imaging may demonstrate the tumor, myometrial invasion, and extension into the parametrium clearly withexcellent soft-tissue contrast (30) (Figs 3, 4). Hydatidiform mole usually appears as a heterogeneous markedly hyperintense mass on T2-weighted images (30) that distends the endometrial cavity. On contrast-enhanced T1-weighted images, characteristic numerous cystic areas are clearly seen in the mass (Fig 3). The normal myometrium remains and surrounds the mass (30). Invasive gestational trophoblastic tumor has a heterogeneous pattern of high and low signal intensity on T2-weighted images (31). The tumor distorts uterine zonal structures, and the bound-aries between the tumor and the myometrium are irregular and indistinct (31). The tumor is usually extremely hypervascular, and signal voids representing dilated vessels are often prominent within the tumor, adjacent myometrium, and parametrium (31,32) (Figs 4, 5). Although necrosis, hemorrhage, and cystic areas within the tumor may not be distinguished from the viable part (31,32), contrast-enhanced dynamic MR imaging may be useful for detection of myometrial viable tumor as an area of focal early enhancement (32) (Fig 5).
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Figure 3a. Hydatidiform mole in a 29-year-old woman. (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a molar mass (long arrows) in the distended endometrial cavity with attachment to the anterior wall of the uterine body (arrowheads). The mass has intermediate signal intensity on the T1-weighted image (a) and heterogeneous marked high signal intensity on the T2-weighted image (b). (c, d) Axial (c) and coronal (d) contrast-enhanced fat-suppressed T1-weighted MR images show numerous small cystic areas within the mass (long solid arrows in c). The normal myometrium surrounds the mass. A hematoma (short solid arrows) within the mass has intermediate signal intensity with peripheral markedly high signal intensity on the T1-weighted images (a, c) and high signal intensity on the T2-weighted image (b). Bloody fluid in the endometrial cavity has markedly high and intermediate signal intensity on the T1-weighted images (a, c) and intermediate and low signal intensity on the T2-weighted image (b). Note the distortion of the anterior wall of the uterine body due to a contraction (open arrow in c).
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Figure 3b. Hydatidiform mole in a 29-year-old woman. (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a molar mass (long arrows) in the distended endometrial cavity with attachment to the anterior wall of the uterine body (arrowheads). The mass has intermediate signal intensity on the T1-weighted image (a) and heterogeneous marked high signal intensity on the T2-weighted image (b). (c, d) Axial (c) and coronal (d) contrast-enhanced fat-suppressed T1-weighted MR images show numerous small cystic areas within the mass (long solid arrows in c). The normal myometrium surrounds the mass. A hematoma (short solid arrows) within the mass has intermediate signal intensity with peripheral markedly high signal intensity on the T1-weighted images (a, c) and high signal intensity on the T2-weighted image (b). Bloody fluid in the endometrial cavity has markedly high and intermediate signal intensity on the T1-weighted images (a, c) and intermediate and low signal intensity on the T2-weighted image (b). Note the distortion of the anterior wall of the uterine body due to a contraction (open arrow in c).
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Figure 3c. Hydatidiform mole in a 29-year-old woman. (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a molar mass (long arrows) in the distended endometrial cavity with attachment to the anterior wall of the uterine body (arrowheads). The mass has intermediate signal intensity on the T1-weighted image (a) and heterogeneous marked high signal intensity on the T2-weighted image (b). (c, d) Axial (c) and coronal (d) contrast-enhanced fat-suppressed T1-weighted MR images show numerous small cystic areas within the mass (long solid arrows in c). The normal myometrium surrounds the mass. A hematoma (short solid arrows) within the mass has intermediate signal intensity with peripheral markedly high signal intensity on the T1-weighted images (a, c) and high signal intensity on the T2-weighted image (b). Bloody fluid in the endometrial cavity has markedly high and intermediate signal intensity on the T1-weighted images (a, c) and intermediate and low signal intensity on the T2-weighted image (b). Note the distortion of the anterior wall of the uterine body due to a contraction (open arrow in c).
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Figure 3d. Hydatidiform mole in a 29-year-old woman. (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a molar mass (long arrows) in the distended endometrial cavity with attachment to the anterior wall of the uterine body (arrowheads). The mass has intermediate signal intensity on the T1-weighted image (a) and heterogeneous marked high signal intensity on the T2-weighted image (b). (c, d) Axial (c) and coronal (d) contrast-enhanced fat-suppressed T1-weighted MR images show numerous small cystic areas within the mass (long solid arrows in c). The normal myometrium surrounds the mass. A hematoma (short solid arrows) within the mass has intermediate signal intensity with peripheral markedly high signal intensity on the T1-weighted images (a, c) and high signal intensity on the T2-weighted image (b). Bloody fluid in the endometrial cavity has markedly high and intermediate signal intensity on the T1-weighted images (a, c) and intermediate and low signal intensity on the T2-weighted image (b). Note the distortion of the anterior wall of the uterine body due to a contraction (open arrow in c).
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Figure 4a. Invasive mole in a 37-year-old woman. (a) Axial fat-suppressed T2-weighted MR image shows a large mass with marked high signal intensity (long arrows) that distends the endometrial cavity. The mass invades the myometrium along the right and fundal wall of the uterus and extends into the right side of the parametrium (short arrows). (b) Axial contrast-enhanced fat-suppressed T1-weighted MR image shows that the mass (long arrows) consists of mixed small cystic areas and intensely enhanced areas and invades the myometrium and parametrium on the right side of the uterus (short arrows). Serpentine signal voids (arrowheads), which are suggestive of dilated vessels, are seen within the adjacent myometrium and parametrium.
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Figure 4b. Invasive mole in a 37-year-old woman. (a) Axial fat-suppressed T2-weighted MR image shows a large mass with marked high signal intensity (long arrows) that distends the endometrial cavity. The mass invades the myometrium along the right and fundal wall of the uterus and extends into the right side of the parametrium (short arrows). (b) Axial contrast-enhanced fat-suppressed T1-weighted MR image shows that the mass (long arrows) consists of mixed small cystic areas and intensely enhanced areas and invades the myometrium and parametrium on the right side of the uterus (short arrows). Serpentine signal voids (arrowheads), which are suggestive of dilated vessels, are seen within the adjacent myometrium and parametrium.
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Figure 5a. Invasive mole in a 46-year-old woman 8 days after dilation and curettage. (a) Sagittal fat-suppressed T2-weighted MR image shows an ill-defined mass with mixed signal intensity (long arrows) within the myometrium. Multiple small signal voids suggestive of enlarged vessels are seen (arrowheads). The zonal anatomy is partially obscured in this area. Diffuse low signal intensity is seen along the posterior wall (short arrows). (b) Sagittal contrast-enhanced dynamic MR image (early phase) shows marked enhancement (arrows) along the fundal wall of the myometrium. This enhanced area probably indicates viable tumor. Areas with no enhancement (+) are suggestive of necrosis or hemorrhage. Hysterectomy was performed, and an invasive mole deep within the myometrium was found at histologic examination. The diffusely hypointense area on the T2-weighted image (short arrows in a) probably correlates with adenomyosis, which was seen in the specimen.
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Figure 5b. Invasive mole in a 46-year-old woman 8 days after dilation and curettage. (a) Sagittal fat-suppressed T2-weighted MR image shows an ill-defined mass with mixed signal intensity (long arrows) within the myometrium. Multiple small signal voids suggestive of enlarged vessels are seen (arrowheads). The zonal anatomy is partially obscured in this area. Diffuse low signal intensity is seen along the posterior wall (short arrows). (b) Sagittal contrast-enhanced dynamic MR image (early phase) shows marked enhancement (arrows) along the fundal wall of the myometrium. This enhanced area probably indicates viable tumor. Areas with no enhancement (+) are suggestive of necrosis or hemorrhage. Hysterectomy was performed, and an invasive mole deep within the myometrium was found at histologic examination. The diffusely hypointense area on the T2-weighted image (short arrows in a) probably correlates with adenomyosis, which was seen in the specimen.
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An arteriovenous shunt may occur in gestational trophoblastic tumor (31,33). The Doppler US and MR imaging features of gestational trophoblastic tumor may be similar to those of uterine AVM (33). Placental polyp is also included in the differential diagnosis (34). The hCG level, which is generally extremely high in choriocarcinoma, is helpful for differential diagnosis (34).
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Postpartum Complications
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Pelvic Hematoma
Postpartum hemorrhage results from laceration of the perineum, vagina, cervix, or uterus during vaginal delivery. It may cause sudden, life-threatening hypotension and remains a significant cause of maternal morbidity and mortality (35,36). Early diagnosis can be difficult, and the degree of hemorrhage may be clinically underestimated until shock develops because patients may present with nonspecific symptoms such as pelvic pain or fullness (36). Hematomas may occur in the potential pelvic extraperitoneal spaces, including the perivaginal space, pericervical space, presacral space, and broad ligament space, and may extend superiorly to contiguous abdominal extraperitoneal spaces (35,36).
US allows early detection of pelvic hematoma in patients with suspected postpartum hemorrhage (35). Hematoma is seen as a solid or complex mass at US (37); these findings may be difficult to differentiate from those of other pathologic conditions such as abscess and endometrioma (37,38).
MR imaging can demonstrate a hematoma as a mass with characteristic signal intensities, which depend on the age of the blood (35,36), and can provide information about its size, anatomic location, and extent with a global multiplanar view, which is helpful in making decisions about intervention and selecting a procedure (35,36). The presence of air (which is seen as a signal void on MR images) in the hematoma suggests an infected hematoma (37).
Uterine Dehiscence and Bladder Flap Hematoma
After cesarean section, a bladder flap hematoma can form as a result of bleeding and dehiscence at the incision site. A small bladder flap hematoma may be seen normally (39), but a hematoma larger than 5 cm is uncommon (39,40) and has been reported with uterine dehiscence (40). US can depict bladder flap hematomas, but the findings may be difficult to distinguish from those of an abscess or infected hematoma (37). US has had little success in demonstrating the myometrial defect that indicates uterine dehiscence (41).
Sagittal MR images can clearly depict a transverse uterine incision (Fig 6). In a patient with a longitudinal incision, the transmural discontinuity is demonstrated better on axial images than on sagittal images (40). The normal post–cesarean section incision site shows continuity of underlying endometrial and serosal layers and signal intensity consistent with a hematoma within the myometrium (40). On the other hand, disruption of the endometrial or serosal layer is seen in uterine dehiscence (40).
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Figure 6a. Uterine dehiscence and chronic bladder flap hematoma complicated by gynecologic disseminated intravascular coagulation in a 26-year-old woman 2 months after cesarean section (transverse uterine incision). (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a hyperintense fluid-filled cystic mass (hematoma) with a thick hypointense rim (arrows) at the vesicouterine fossa. (c) Sagittal fat-suppressed T2-weighted MR image shows disruption of the endometrial layer (arrowheads) and a hyperintense area with peripheral low signal intensity (short arrows) within the myometrium at the incision site. This is the characteristic appearance of uterine dehiscence and intramural hematoma. The bladder flap hematoma (long arrows) is located near the incision site.
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Figure 6b. Uterine dehiscence and chronic bladder flap hematoma complicated by gynecologic disseminated intravascular coagulation in a 26-year-old woman 2 months after cesarean section (transverse uterine incision). (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a hyperintense fluid-filled cystic mass (hematoma) with a thick hypointense rim (arrows) at the vesicouterine fossa. (c) Sagittal fat-suppressed T2-weighted MR image shows disruption of the endometrial layer (arrowheads) and a hyperintense area with peripheral low signal intensity (short arrows) within the myometrium at the incision site. This is the characteristic appearance of uterine dehiscence and intramural hematoma. The bladder flap hematoma (long arrows) is located near the incision site.
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Figure 6c. Uterine dehiscence and chronic bladder flap hematoma complicated by gynecologic disseminated intravascular coagulation in a 26-year-old woman 2 months after cesarean section (transverse uterine incision). (a, b) Axial fat-suppressed T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a hyperintense fluid-filled cystic mass (hematoma) with a thick hypointense rim (arrows) at the vesicouterine fossa. (c) Sagittal fat-suppressed T2-weighted MR image shows disruption of the endometrial layer (arrowheads) and a hyperintense area with peripheral low signal intensity (short arrows) within the myometrium at the incision site. This is the characteristic appearance of uterine dehiscence and intramural hematoma. The bladder flap hematoma (long arrows) is located near the incision site.
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A bladder flap hematoma is seen between the posterior bladder wall and the anterior wall of the lower uterine segment. Gas within the hematoma strongly suggests an infected hematoma (37).
Pelvic Abscess
Formation of a pelvic abscess can occur after delivery, most commonly after cesarean section (42). US may not allow differentiation of an abscess from an endometrioma, hematoma, ovarian tumor, or pyosalpinx (37,38), and the source of the abscess (eg, appendicitis, Crohn disease) may be unclear.
MR imaging may provide more specific findings than US. An abscess is depicted as an ill-defined mass with thick, irregular walls that contains fluid. The fluid has low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images (38). The thick wall and surrounding soft-tissue inflammatory changes are intensely enhanced on contrast-enhanced fat-suppressed T1-weighted images (Fig 7) (3,13). A most specific finding of abscess is the presence of gas, which is seen as signal voids on MR images, but this finding is unusual (13). Layering by debris or clots is seen as an underlying hypointense area on T2-weighted images (3, 13,38). The multiplanar sections of MR imaging are helpful in distinguishing an abscess from fluid-filled bowel loops and pyosalpinx (38).
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Figure 7a. Abscess in the pouch of Douglas in a 25-year-old woman 2 weeks after delivery. Axial fat-suppressed T2-weighted (a) and contrast-enhanced fat-suppressed T1-weighted (b) MR images show a fluid-filled cystic mass (arrows) with a thick, irregular wall in the pouch of Douglas. The fluid content has low signal intensity on the T1-weighted image (b) and high signal intensity on the T2-weighted image (a). The abscess wall and surrounding parametrium are intensely enhanced on the contrast-enhanced fat-suppressed T1-weighted image (b).
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Figure 7b. Abscess in the pouch of Douglas in a 25-year-old woman 2 weeks after delivery. Axial fat-suppressed T2-weighted (a) and contrast-enhanced fat-suppressed T1-weighted (b) MR images show a fluid-filled cystic mass (arrows) with a thick, irregular wall in the pouch of Douglas. The fluid content has low signal intensity on the T1-weighted image (b) and high signal intensity on the T2-weighted image (a). The abscess wall and surrounding parametrium are intensely enhanced on the contrast-enhanced fat-suppressed T1-weighted image (b).
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MR imaging can also provide information about the anatomic location, size, and extent of an abscess in a multiplanar presentation, which may facilitate decisions about intervention and the choice of appropriate routes for intervention.
Puerperal Septic Ovarian Vein Thrombosis
Puerperal ovarian vein thrombosis is an uncommon but important disease that may result in a fatal condition such as septic pulmonary emboli (43,44). Appropriate treatment consists of a combination of anticoagulants and antibiotics (44). The symptoms are fever and lower quadrant pain, which are nonspecific and make clinical diagnosis difficult (44). Ovarian vein thrombosis must be differentiated from diseases such as appendicitis and ovarian torsion, which may require surgical treatment (43). In 80%–90% of cases, ovarian vein thrombosis occurs in the right ovarian vein (44). The role of US is often limited (43). Helical CT with bolus injection of iodinated contrast material and conventional venography allow evaluation of the ovarian veins, but these methods require ionizing radiation.
MR imaging is useful for investigating the causes of puerperal fever, such as an abscess or infected hematoma. However, confident diagnosis of venous thrombosis is considered difficult because signal intensities within the vein can be confused with the flow signals of a nonthrombosed ovarian vein. Nonenhanced MR venography performed with the phase-contrast or time-of-flight technique can depict veins with flowing blood and occlusion sites (44,45). This method is useful for diagnosis, especially in prepartum patients, in whom use of contrast material is not recommended. However, this method may be degraded by flow artifacts and saturation effect (5,45). On the other hand, contrast-enhanced MR angiography based on T1 shortening can provide better and more reliable visualization of vascular systems (5,12). This method can demonstrate venous systems selectively by using a subtraction technique. Coronal source images are useful for evaluating the extent of a thrombus (5,12). Therefore, we recommend this technique in postpartum patients with suspected vascular disease.
Retained Placenta and Placental Polyp
Retained placenta may cause complications such as bleeding and infection (27). The incidence of retained placenta increases in a patient with placenta accreta. A placental polyp, which develops from retained placenta, is an intrauterine polypoid or pedunculated mass of placental or chorionic tissue that is retained for an indefinite period after delivery or abortion (34,46,47) and usually contains viable tissue at histologic analysis (34). It may cause massive bleeding in the puerperal period or months after the last delivery or abortion (34,47).
MR imaging is useful for detecting the retained placenta and placental polyp, identifying the site of implantation, and evaluating vascularity (34,47). Placental polyps are isointense on T1-weighted images and hyperintense on T2-weighted images (34) and may be accompanied by remarkable flow voids and prominent enhancement (34,47). Contrast-enhanced dynamic MR imaging allows evaluation of vascularity and the depth of myometrial invasion and differentiation of viable tissue with an abundant blood supply from tissue with little or no blood supply, such as degeneration, necrosis, or clot (34,47). The information about vascularity may be helpful for selection of treatment. When the lesion is hypervascular, methotrexate therapy, hysterectomy, or transarterial embolization may be selected (3,34). If it is totally avascular due to necrosis or hyaline degeneration, dilation and curettage can be performed without the risk of bleeding (Fig 8).
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Figure 8a. Retained placenta with hyaline degeneration and necrosis in a 24-year-old woman with persistent genital bleeding 2 months after delivery. (a, b) Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a retained placenta (arrows), which has intermediate signal intensity with a hyperintense rim on the T1-weighted image (a) and mixed low and high signal intensity on the T2-weighted image (b). (c) Sagittal contrast-enhanced dynamic MR image (late phase) obtained with subtraction shows no enhancement of the retained placenta (arrows). Curettage was performed, and histologic examination demonstrated a retained placenta with necrosis and hyaline degeneration. Arrowheads = dilation device.
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Figure 8b. Retained placenta with hyaline degeneration and necrosis in a 24-year-old woman with persistent genital bleeding 2 months after delivery. (a, b) Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a retained placenta (arrows), which has intermediate signal intensity with a hyperintense rim on the T1-weighted image (a) and mixed low and high signal intensity on the T2-weighted image (b). (c) Sagittal contrast-enhanced dynamic MR image (late phase) obtained with subtraction shows no enhancement of the retained placenta (arrows). Curettage was performed, and histologic examination demonstrated a retained placenta with necrosis and hyaline degeneration. Arrowheads = dilation device.
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Figure 8c. Retained placenta with hyaline degeneration and necrosis in a 24-year-old woman with persistent genital bleeding 2 months after delivery. (a, b) Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a retained placenta (arrows), which has intermediate signal intensity with a hyperintense rim on the T1-weighted image (a) and mixed low and high signal intensity on the T2-weighted image (b). (c) Sagittal contrast-enhanced dynamic MR image (late phase) obtained with subtraction shows no enhancement of the retained placenta (arrows). Curettage was performed, and histologic examination demonstrated a retained placenta with necrosis and hyaline degeneration. Arrowheads = dilation device.
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Choriocarcinoma may have MR imaging findings similar to those of placental polyp. Differential diagnosis based on only MR imaging findings may be difficult without the aid of the hCG level (34). The features of placental polyp may mimic those of uterine AVM (34). When a postpartum intrauterine mass is avascular, MR imaging does not always allow differentiation of a blood clot from a retained placenta with necrosis.
Uterine AVM
Uterine AVMs cause life-threatening massive genital bleeding in young women (33,48). Uterine AVM is also known by various synonyms, such as uterine arteriovenous fistula and uterine cirsoid aneurysm (48). Traditionally, uterine AVMs have been classified as congenital or acquired. More commonly, uterine AVMs are an acquired disease associated with obstetric disorders and procedures, such as multiple pregnancies, spontaneous abortion, dilation and curettage, therapeutic abortion, and cesarean section (33,48).
Although gray-scale US findings of uterine AVMs are nonspecific, Doppler US reveals characteristic intense color signals in the myometrium (33). However, Doppler US is limited in demonstrating the precise extent of an AVM within the pelvis (33). Angiography was once a standard method for diagnosis of uterine AVMs. However, this method is invasive and uses ionizing radiation.
MR imaging allows one to confirm the diagnosis of uterine AVM noninvasively (3,33,48). Multiple serpentine flow-related signal voids are seen in the uterine wall, endometrial cavity, and parametrium on T1- and T2-weighted images. Contrast-enhanced dynamic MR angiography can depict complex serpentine abnormal vessels that enhance as intensely as normal vessels, show early venous return, and demonstrate the extent of the malformation (3,33). Uterine AVMs can be treated with transcatheter embolization. Contrast-enhanced dynamic MR angiography may be useful for planning transarterial embolization and for follow-up after the procedure (3).
Arteriovenous shunting within a uterine mass can be detected in a variety of benign or malignant gynecologic conditions, such as normal intrauterine pregnancy, miscarriage, ectopic pregnancy, malignant gestational trophoblastic disease, and sarcoma (33). As mentioned earlier, the MR imaging features of AVM and placental polyp may be similar to those of trophoblastic disease (33,34).
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Planning Delivery
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Radiographic pelvimetry may be beneficial in patients with a fetus in breech presentation who desire a trial of vaginal delivery (2,49). However, this technique involves the disadvantage of fetal exposure to ionizing radiation. MR imaging can provide information about maternal bony structures, soft tissue, and the fetal position in multiple planes without ionizing radiation (2,49). Van Loon et al (49) reported that measurements obtained with MR pelvimetry were as reliable as those obtained with radiographic pelvimetry and that MR pelvimetry could be used for safe management of breech presentation.
Large leiomyomas located in the lower uterine segment or cervix and placenta previa may preclude delivery through the vagina. Sagittal T2-weighted images can depict the internal cervical os, the leiomyoma or placenta previa, the fetus, and the relationship between them (Fig 9) (2) and may contribute to planning of the delivery (see the "Placenta Previa" section).
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Figure 9. Large leiomyoma in the lower uterine segment in a 28-year-old woman. Sagittal fat-suppressed T2-weighted MR image shows a large leiomyoma (long arrows) in the anterocaudal part of the uterus, protruding into the endometrial cavity. Delivery through the vagina was expected to be difficult, and a cesarean section was performed. Note the gravid uterus (short arrows) and placenta (+).
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Maternal Abnormalities during Pregnancy
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Pelvic Masses
When a pelvic mass is incidentally discovered during pregnancy, precise characterization is essential to determine the treatment of both the mother and the fetus (50). Although US is useful for evaluating a pelvic mass, the US appearance is not always conclusive (50). CT is not desirable as a further examination method in obstetric patients due to ionizing radiation.
MR imaging can be a suitable further examination method when the results of US are insufficient (2,11,50). For example, when US shows an endometrioma as a solid tumor due to its echogenicity, MR imaging may allow characterization of the mass as an endometrioma (50). At US, the fat component of a cystic teratoma may be seen as a solid area, suggestive of malignancy. The combination of signal intensities on T1- and fat-suppressed T1-weighted images or recognition of chemical shift artifact allows one to confirm a fat component and confidently diagnose a mature cystic teratoma (11,50) (Fig 10).
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Figure 10a. Mature teratoma at 6 weeks gestation in a 21-year-old woman. (a, b) Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a large mass (arrows) to the left of and posterior to the uterus (+). The mass has predominantly high signal intensity on the T1-weighted image (a) and heterogeneous marked high signal intensity on the fat-suppressed T2-weighted image (b) with irregular internal structures. In the anterior part of the mass, small areas (arrowheads) are markedly hyperintense on the T1-weighted image (a) and hypointense on the fat-suppressed T2-weighted image (b). (c) Axial fat-suppressed T1-weighted MR image shows that these small areas (arrowheads) are hypointense, thus indicating a fat component. A definitive diagnosis of a mature teratoma could be made. Arrows = mass, + = uterus.
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Figure 10b. Mature teratoma at 6 weeks gestation in a 21-year-old woman. (a, b) Axial T1-weighted (a) and fat-suppressed T2-weighted (b) MR images show a large mass (arrows) to the left of and posterior to the uterus (+). The mass has predominantly high signal intensity on | |
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Abstract
LEARNING OBJECTIVES FOR TEST...
