The technologic advances in magnetic resonance imaging (MRI) that provide faster imaging techniques have further expanded the already important role of MRI for genitourinary and gynecologic imaging. Pelvic MRI is an accurate modality for evaluating congenital uterine anomalies, staging most gynecologic malignancies, evaluating pelvic pain (including appendicitis in pregnant patients), problem solving in ectopic pregnancies, and fetal imaging. Additionally, dynamic/cine MRI can be helpful to evaluate pelvic floor relaxation. Ultrasonography (US) is cheaper, faster, and more easily accessible than MRI. However, US is hampered by a relatively small field of view, obscured findings due to overlying bowel gas or fatty tissue, and its operator dependency. US can be limited for visualization of deep nodes and for imaging obese patients.

Although MRI remains far more expensive than US, it has many advantages, including superior contrast resolution, volumetric and multiplanar imaging capability, flow-sensitive sequences, and a larger field of view than US. MRI is frequently used as a problem-solving tool after an equivocal US (Figure 50-1). Although pelvic sonography is the study of choice for the initial evaluation of most pelvic conditions, some conditions are best evaluated by MRI. These conditions include: adenomyosis, large pelvic masses, dermoids, some endometrial disorders, and staging (but not screening) of pelvic malignancies. The interested reader is referred to specific texts that cover pelvic applications of MRI.^1,2

Three-dimensional ultrasound (3DUS) has been more extensively utilized in clinical practice (see also Chapter 49). 3DUS offers volumetric imaging with reconstruction in any imaging plane, thereby minimizing operator dependence. A more accurate and reproducible volume data set enables precise tumor volume measurements that can be used to assess response to therapy and the need for intervention with recurrence, or to guide biopsies. Although the larger data sets require additional training and workstations with computer hardware and software, the advantages of 3DUS in pelvic imaging have been most appreciated in defining fetal anomalies, congenital uterine anomalies, and with 3D sonohysterography.³

Four-dimensional ultrasound (4DUS), which is 3DUS in real time, has been combined with power Doppler to depict tumor vascularity and to improve tissue characterization of lesions such as ovarian carcinoma. Four-dimensional imaging of vascular networks can show tumor neovascularity and permits evaluation of tissue permeability and contrast enhancement kinetics.^4,5

When US and CT are inconclusive in making a diagnosis, MRI is best used as a problem-solving modality to evaluate the female pelvis. MRI is less operator dependent than two-dimensional sonography and affords direct delineation of pelvic disorders in multiple scan planes. By using both T1- and T2-weighted pulse sequences, some tissue-specific patterns can be observed. The use of gadolinium as a contrast agent may afford better delineation of tissue planes and contrast resolution, as well as selection of viable tumor sites for biopsy or tissue sampling at surgery. However, the additional cost mandates selective use of MRI contrast-enhanced scans. Due to excellent soft tissue contrast, MRI even without contrast provides useful staging information for gynecologic malignancy, evaluation of benign pelvic masses, and assessment of pelvic anatomy.

This chapter emphasizes those pelvic disorders in which MRI can be used for problem solving. The exact utility of these diagnostic modalities may differ between medical centers depending on the expertise and experience of the diagnostic imaging physicians.

The pulse sequences selected for pelvic MRI depend upon the indication for examination (Table 50-1). New, faster scan techniques have enabled short T1-weighted, gradient echo, and breath-held triplanar T2-weighted sequences (such as half-Fourier acquisition single-shot turbo spin-echo [HASTE] or single-shot fast spin-echo [SSFSE]) that are satisfactory for patients with large lesions or who are getting tissue characterization of lesions that are likely to be benign. However, these fast scans may suffer from artifacts (eg, edge blurring and decreased contrast to noise) due to magnetization transfer as compared to longer high-resolution multi-shot spin-echo T2-weighted techniques.

Postgadolinium-enhanced T1 fat-suppressed 3D spoiled gradient echo techniques (eg, VIBE, LAVA, THRIVE) with thinner slices are helpful for the evaluation of leiomyomas and for tumor staging in the uterus and cervix.^6,7 ECHO train spin-echo (FSE, TSE) 3D T2-weighted techniques have been developed that acquire isotopic data sets that can be reformatted in multiple planes. Use of fat suppression is important to facilitate differentiation of fat and blood products in a lesion.^1,2

Müllerian Anomalies

Müllerian anomalies have an approximately a 1% incidence in the general population and a 3% incidence in infertility patients. Patients with congenital uterine malformations may present with pelvic pain and/or history of subfertility. Initial evaluation of patients with uterine malformations typically includes both transabdominal sonography (TAS) and transvaginal sonography (TVS). Sonohysterography and 3DUS may be used to evaluate the endometrium and its lumen. Accurate identification of the uterine morphology is critical in triaging of patients to the proper treatment. The accuracy of MRI for this purpose approaches 100%, and TVS (especially with 3D reconstruction) has also proven to be a very accurate method.^8,9

Müllerian anomalies have been classified by the American Society for Reproductive Medicine into three categories: (1) agenesis or hypoplasia, (2) failure of fusion of the paired Müllerian ducts, and (3) failure of resorption of tissue between fused ducts. There are seven classes of anomalies (Table 50-2).

The most common uterine fusion anomaly is the septate uterus (Figure 50-2). It is important to differentiate septate from bicornuate uteri because treatment options differ. Bicornuate uteri may be differentiated from septate uteri by detailed evaluation of the uterine fundus (Figure 50-3). Bicornuate uteri typically have a significant notch (>1 cm) in the fundus between the two uterine horns, whereas septate uteri have normal or less than 1 cm fundal indentation. The thickness of a uterine septum may be evaluated using sonohysterography or hysterosalpingography. However, MRI can reveal not only the outer fundal contour, but also the composition of the septum as fibrous, muscular, or both, as well as the full extent of the septum. The distinction between septate and bicornuate uteri is critically important in that a septate uterus can be treated with hysteroscopic septectomy, whereas a bicornuate uterus cannot. MRI is also useful in evaluation of the uterus, cervix, and vagina, and to search for associated anomalies, most commonly renal agenesis or renal malformations.^9,10

Incomplete development of one of two Müllerian ducts results in the unicornuate uterus. Another important distinction is between the unicornuate uterus with and without a rudimentary horn that has endometrial lining and is obstructed. If the rudimentary horn with lining is obstructed (noncommunicating), surgery is indicated to relieve pain that occurs with onset of menstruation and to prevent implantation of an ectopic pregnancy in the rudimentary horn with potentially fatal consequences (Figure 50-4).^8,9

The uterus didelphys is a complete duplication of the uterus and cervix with nonfusion of the two Müllerian ducts. Patients are usually asymptomatic unless there is an obstructing (transverse) vaginal septum, which is less common than a longitudinal septum. MRI shows two separate uterine bodies and cervices often with a longitudinal vaginal septum (Figure 50-5).

Congenital absence of the uterus, cervix, and vagina, also known as the Mayer-Rokitansky-Küster-Hauser syndrome, has a high incidence of renal anomalies (up to 40%).^8-10 Diethylstilbestrol was given to about 3 million women to treat abortions, preeclampsia, diabetes, and preterm labor from 1948 to 1971, exposing 1 million female offspring of these women. Of these exposed female offspring, about 1% developed clear cell adenocarcinoma. Many exposed females also had genitourinary anomalies including uterine hypoplasia and T-shaped uteri. MRI is a very accurate alternative to sonography of uterine malformations and may be used to search for commonly associated anomalies such as renal agenesis or renal malformations.^8-10

Leiomyomas

Leiomyomas (fibroids), benign smooth muscle and connective tissue tumors of the uterus, are the most common pelvic disorder in women, seen in up to 40% of female patients. They often present with vaginal bleeding or heavy menses, and can be associated with pressure, pain, and secondary symptoms from mass effect on the bowel or the bladder, and even infertility. Most fibroids are intramural, but the submucosal ones may present with bleeding from protrusion into the endometrial canal. Although uncommon, cervical fibroids are especially problematic in pregnancy when they may grow under the influence of increased circulating estrogen. Subserosal lesions can be pedunculated and may simulate an adnexal mass or may undergo painful torsion (Figure 50-6).

TVS remains the study of choice in the diagnostic evaluation of uterine leiomyomas, which are generally detected as uterine enlargement or pelvic mass on physical examination (Chapter 33). MRI should be reserved for cases in which sonographic findings are equivocal or if precise anatomic localization of the leiomyoma is required.

MRI can improve mapping of fibroids for patients undergoing myomectomy to preserve fertility or other directed therapies such as ablation or embolization, and it is especially useful in cases with multiple fibroids or with a large uterus. Most leiomyomas are of decreased signal intensity on all image sequences due to their fibrous tissue contents. They are usually spherical in shape and sharply demarcated from the myometrium, unlike adenomyosis. If there is degeneration, bright signal or hemorrhagic foci may be found. The enhancement pattern of leiomyomas is variable, but tends to be heterogeneous and less than that of the myometrium. MRI is the more sensitive, but also the more expensive, imaging modality for the diagnosis of fibroids.

Surgical treatments of fibroids are not only hysterectomy and laparotomy, but also laparoscopy. Laparoscopy is used for resection of pedunculated lesions that are subserosal, and transvaginal resection for submucosal lesions in the endometrial lining. Other alternatives to traditional surgical methods include: growth-hormone-related agonists (especially for women who are perimenopausal), selective progesterone receptor modulators, endometrial ablation, uterine artery embolization, and myolysis. Myolysis has been accomplished using thermal ablation (eg, cryoprobes), radiofrequency ablation, or MR-guided focused US (eg, thermocoagulation).¹¹

MRI is the preferred study for preoperative evaluation and treatment of leiomyomas. MRI can also be particularly helpful in pre-embolization assessment for mapping the location of leiomyomas, assessing for pedunculated lesions, and evaluating uterine arterial supply. The size, signal characteristics, and morphology of the fibroids can be best seen with MRI. The assessment of perfusion, necrosis or degeneration, and concomitant adenomyosis are better seen with MRI. Response to treatment is also monitored with MRI (Figure 50-7).^12,13

Bright signal on T1-weighted images from hemorrhagic necrosis within a fibroid or from a large dominant subserosal fibroid predicts a poor response, whereas bright signal on T2-weighted scans and/or enhancement after intravenous gadolinium predicts a response to treatment.^13,14 Most likely, these findings reflect a better response with vascular lesions than with some necrotic ones. MRI can demonstrate successful reduction in the size of the treated uterus, or persistent enhancement and regrowth of fibroids after embolization. The advantages of uterine artery embolization (UAE) are control of bulk symptoms in up to 90% of patients, shrinkage of both individual fibroid and uterine volume by at least 50%, and relatively good patient tolerance of the procedure. Subsequent pregnancy has been reported.¹⁵

Adenomyosis

Adenomyosis results from the presence of ectopic endometrial glands implanted within the myometrium. This disorder is extremely common, seen in up to 40% of hysterectomy specimens, although it is not uniformly symptomatic.¹⁶

Patients may present with signs and symptoms similar to fibroids including pain, bleeding, menorrhagia, and pressure symptoms.¹⁷ In its early stages, adenomyosis may be difficult to detect on sonography or MRI. However, once the disease becomes more advanced, irregular, diffuse areas of enlargement and disruption of myometrial interfaces can be observed on sonography. The uterus appears globular and heterogeneous with cystic foci.^18-21

On MRI, the uterus is enlarged, with thickening (>12 mm) of the junctional zone seen on T2-weighted images. Cystic foci of bright fluid signal or foci of hemorrhage are common within adenomyosis on both T1- and T2-weighted scans. Typically, the lesion is lenticular in shape with thickening of the junctional zone, whereas most fibroids are spherical and distinct from the junctional zone (Figure 50-8). Uterine involvement with adenomyosis is typically a diffuse process; however, focal areas of involvement (adenoymyomas) can also be seen. There may be coexistent endometriosis.^19-21

On color Doppler sonography, fibroids typically demonstrate peripheral flow due to their displacement of larger radial vessels. Adenomyosis may demonstrate the lack of a definite mass effect in the myometrium and a dispersed vascular pattern. In general, MRI should be used for equivocal cases on sonography or in cases clinically suspected to have adenomyosis to confirm its presence. MRI has been a more reliable diagnostic tool, although it is more expensive than sonography. The MRI features of lenticular or circumferential thickening of the junctional zone (>12 mm) that may show small foci of high T2 signal intensity (associated with hemorrhage or cysts) should indicate the diagnosis. Adenomyosis usually appears slightly hypointense to myometrium on postcontrast images.

Adenomyosis is a relatively common cause of pelvic pain. It is important to differentiate adenomyosis from fibroids because some intramural and submucosal fibroids may be resected with symptomatic relief, whereas adenomyosis can be definitively treated by hysterectomy. Because there are different choices for treatment of fibroids, differentiating fibroids from adenomyosis is important. Recently, uterine artery embolization has been used to treat adenomyosis with promising results. This was incidentally discovered because adenomyosis may coexist with fibroids.^22-25

MRI has an important role after equivocal US or for further workup of complex adnexal masses. The power of MRI lies in the ability to definitively characterize lesions as benign, sparing patients unnecessary additional workup, intervention, and/or surgery. For example, broad ligament fibroids simulating an ovarian mass, endometriosis, and dermoid cysts can be specifically diagnosed on MRI, avoiding further unnecessary workup. MRI can be used to locate and evaluate a high-riding ovary or one that is obscured by overlying bowel gas. This is especially important in women with a clinically palpable mass when the ipsilateral ovary cannot be found on US (Figure 50-9).

Endometriosis

Endometriosis is ectopic glandular tissue and stroma that is hormonally sensitive. The ectopic endometrial tissue usually produces microscopic implants on the serosa of major pelvic organs, uterosacral ligaments, and surrounding structures. Endometriosis can cause severe cyclic pelvic pain. In the chronic state, areas of fibrosis may surround even small implants. The gold standard for diagnosis is laparoscopy,²⁶ but with microscopic and tiny implants, even laparoscopy may not find lesions that are inaccessible due to adhesions. For this reason, MRI may be useful in cases when even laparoscopy cannot visualize endometriosis lesions deep within the pelvis.²⁷

In some cases, sonography shows a hemorrhagic mass with diffuse low-level internal echoes, referred to as “ground glass” texture, arising from organized clot associated with an endometrioma. There is enhanced through-transmission. However, sonography may show a nonspecific cystic ovarian or adnexal mass.

MRI is an excellent means for detection of macroscopic endometrial implants as areas of high-signal intensity on T1-weighted scans. Fat-suppressed sequences not only increase the conspicuity of endometrial implants, but also help distinguish hemorrhagic deposits from fat. The concomitant T2-weighted images often reveal the typical darkening of signal within endometriomas, referred to as T2 shading (Figure 50-10). This relatively specific MRI finding for the diagnosis of endometriosis results from the breakdown of hemoglobin into intracellular methemoglobin in the early stages of clot lysis. This finding is commonly seen in endometriomas because they bleed and rebleed; however, it is uncommon in hemorrhagic cysts. Endometriosis may enhance after intravenous gadolinium administration.

Dermoid Cysts

Dermoid cysts, also known as mature cystic teratomas, are the most common ovarian neoplasms. These benign lesions are made up of variable amounts of all three germ layers including ectodermal, endodermal, and mesodermal tissue. They may contain skin, epithelial cells, fat, sebum, and calcifications. The Rokitansky nodule is a “plug” or solid nodule in the wall or in a septation of the dermoid containing hair, sebaceous glands, fat, and calcifications, or even bone. These lesions can be bilateral in about 15% of cases.²⁸ On imaging, the most important and characteristic feature is the fatty contents. All MR image sequences typically show macroscopic fat signal in these lesions.

As a general rule, ultrasound is accurate in the diagnosis of most dermoid cysts. However, dermoids may demonstrate a variety of sonographic features, ranging from densely echogenic solid complex masses with shadowing to complex cystic and solid masses containing irregular borders and/or areas of dense shadowing from calcifications. Because dermoid cysts are usually pedunculated from the ovary, their locations may be posterior to the bowel or in unusual locations that may not be accessible on TAS or TVS. In fact, some may be confused for bowel due to their increased echogenicity and shadowing, whereas others appear as discrete echogenic masses within the ovary. When ultrasound is inconclusive, MR imaging has a definite role.