Chapter 48: MRI OF THE FEMALE PELVIS: PROBLEM SOLVING SONOGRAPHIC UNCERTAINTIES

The technologic advances in magnetic resonance imaging (MRI) in providing faster imaging techniques have further expanded the already important role of MRI for genitourinary and gynecologic imaging. Not only tissue characterization and tumor staging, but also pelvic floor relaxation, urinary incontinence, and fetal imaging using MRI have become mainstream in modern practice.

Ultrasound (US) is cheaper, faster, and more easily accessible than MRI. However, US is hampered by a 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.

MRI has superior contrast resolution, volumetric and multiplanar imaging capability, flow-sensitive sequences, and larger field of view than US. The increased cost of MRI and the lack of proficiency of some radiologists to implement it for body imaging have been impediments to wider usage.

MRI is used as a problem solving tool after equivocal US (Figure 48-1). Although pelvic sonography is the study of choice for the initial evaluation of most pelvic conditions, some conditions are best evaluated by MRI. These include adenomyosis, large 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 (3D US) has recently gained acceptance in clinical practice (see also Chapter 47). Three-dimensional US offers volumetric imaging with reconstruction in any imaging plane, thereby nearly eliminating operator dependence. A more accurate and reproducible volume data set enables accurate tumor volume measurements that can be used to judge 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 3D US in pelvic imaging have been most appreciated in defining fetal anomalies, congenital uterine anomalies, and with 3D sonohysterography.³

Four-dimensional ultrasound (4D US), which is 3D US 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 material kinetics.^4,5

There are many advantages to pelvic MRI. It 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 contrast-enhanced scans.

This chapter emphasizes those pelvic disorders in which MRI can be used for problem solving. The exact utility of these diagnostic modalities may differ from institution to institution, depending on the expertise and experience of the diagnostic imagers.

The pulse sequences selected for pelvic MRI depend upon the purpose of the scan. New, faster scan techniques have enabled short T1-weighted gradient echo and breath held triplanar T2-weighted scans (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 probably benign lesions. However, these fast scans have decreased resolution compared to longer high-resolution echo train techniques and gadolinium-enhanced fat-suppressed T1-weighted gradient echo scans with thinner slices performed in long and short axes of the uterus for tumor staging in the uterus and cervix.^6,7 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 infertility. Initial evaluation of patients with uterine malformations typically includes both transabdominal sonography (TAS) and transvaginal sonography (TVS). Sonohysterography may be used to evaluate the endometrial canal. 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%.^8,9

Müllerian anomalies have been classified by the American Society for Reproductive Medicine into 3 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 7 classes of anomalies (Table 48-1).

The most common anomaly is the septate uterus (Figure 48-2). It is important to differentiate septate from bicornuate uteri because the treatment differs. Bicornuate uteri may be differentiated from septate uteri by careful evaluation of the uterine fundus (Figure 48-3). Bicornuate uteri typically have a significant notch (>1 cm) in the fundus between the 2 uterine horns, whereas septate uteri have normal or <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. Magnetic resonance imaging 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 2 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 a ectopic pregnancy in the rudimentary horn with potentially fatal consequences (Figure 48-4).^8,9

The uterus didelphys is a complete duplication of the uterus and cervix with nonfusion of the 2 müllerian ducts. The patients are usually asymptomatic unless there is an obstructing (transverse) vaginal septum, which is less common than a longitudinal septum. MRI shows 2 separate uterine bodies and cervices often with a longitudinal vaginal septum (Figure 48-5).

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%).^{8,9, and 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 female offspring, about 1% developed clear cell adenocarcinoma. Many exposed females 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.^{8,9, and 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 48-6).

Ultrasound 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. Magnetic resonance imaging 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. There is variable enhancement in leiomyomas. 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 or laparotomy. 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

Magnetic resonance imaging is the preferred study for preoperative evaluation for triage and follow-up of patients getting angioembolization of the uterine arteries for treatment of leiomyomas. The size, signal characteristics, and morphology of the fibroids can be best seen with MRI and the uterine arterial anatomy. The assessment of perfusion, necrosis or degeneration, and concomitant adenomyosis are shown to best advantage by MRI. Response to treatment is also monitored with MRI (Figure 48-7).^12,13

Bright signal on T1-weighted images from hemorrhagic necrosis within a fibroid or 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 endometrial gland 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.^{18,19,20, and 21}

On MRI, the uterus is enlarged, with thickening (>11 mm) of the junctional zone seen on T2-weighted images. Cystic foci of bright fluid signal or 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 48-8). Uterine involvement with adenomyosis can be diffuse or focal. There may be coexistent endometriosis.^{19,20, and 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. Magnetic resonance imaging has been a more reliable diagnostic tool, although more expensive than sonography. The MRI features of lenticular or circumferential thickening of the junctional zone that may show areas of high signal intensity associated with hemorrhage or cysts should indicate the diagnosis. Adenomyosis may enhance, as may endometriosis.

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.^{22,23,24, and 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 48-9).

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

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

Magnetic resonance imaging is an excellent means for detection of macroscopic endometrial implants as areas of high signal intensity on T1-weighted scans. The concomitant T2-weighted images often reveal the typical darkening of signal within endometriomas, referred to as T2 shading (Figure 48-10). 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 re-bleed. This finding 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 3 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 fairly 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.

Magnetic resonance with fat-suppression techniques (chemical shift imaging) can be definitive in the diagnosis of dermoid cysts. The fatty contents can be readily distinguished from hemorrhage within endometriomas or hemorrhagic cysts because the fat in dermoid cysts becomes dark on T1-weighted fat-suppressed images, whereas blood remains bright in signal (Figure 48-11). If there is chemical shift artifact within the lesion in the frequency-encoding direction at a fat-fluid interface, this is pathognomonic of a dermoid. (Figure 48-12).^28,29

Color Doppler sonography can be used to assess the presence of torsion in patients with dermoid cysts. In general, the absence of intraovarian venous flow associated with high-impedance arterial flow within an enlarged ovary is highly indicative of torsion. Further discussion of this topic is covered in the chapter on pelvic pain. Occasionally, the torsed vascular pedicle can be recognized on MRI. Most dermoids are asymptomatic. Because of the potential complications, such as torsion and rupture, patients are often referred for surgical resection. This must be weighed against the patient's desire to preserve fertility. Malignant degeneration is rare (occurring in 1% to 2 % of cases).^{29,30, and 31}

Ovarian Fibromas

Ovarian fibromas are ovarian stromal tumors that are made up of spindle cells and collagen. These lesions account for 4% of all ovarian neoplasms, but should be recognized by virtue of their unique radiographic features so as not to be confused with malignancy. Usually seen in women in the fifth decade, they are usually unilateral and asymptomatic.³² Up to 10% may calcify and 10% may be bilateral.

These lesions have a very distinctive sonographic appearance: hypoechoic and hyperattenuating.³³ On MRI, they typically have low signal intensity on both T1- and T2-weighted images (Figure 48-13).³²

The differential diagnosis includes a broad ligament or pedunculated uterine fibroid that can be identified by virtue of its connection to the uterus with bridging vessels. Brenner tumors are benign transitional cell tumors that frequently calcify, but which are identical to fibromas on MRI and US due to their fibrous stroma. Cystic degeneration in a fibroma usually renders it indistinguishable from other more worrisome cystic epithelial neoplasms.³⁴

If necrotic or complicated, or with other elements such as thecal cells, fibromas can be cystic and complex (i.e., indistinguishable from malignant masses). The fibrothecomas are often hormonally active, producing unopposed estrogen, which increases the risk of endometrial hyperplasia and endometrial carcinoma.^35,36

In Meigs syndrome, a benign ovarian fibroma is associated with pleural effusions and ascites. With Gorlin syndrome (an autosomal dominant disorder), there is an association with bilateral ovarian fibromas, multiple basal cell carcinomas of the skin, and odontogenic cysts, and skeletal abnormalities.³⁷ Only 1% of these lesions are thought to undergo malignant degeneration.

Fallopian Tube Abnormalities

US is the study of choice for initial and primary evaluation of adnexal masses, including those resulting from enlargement of the fallopian tubes. The most common presentation of an abnormal fallopian tube is that of enlargement of the tube from hydrosalpinx. If pelvic inflammatory disease is the source, then the tubes may contain infected fluid or frank pus in the acute phase. However, in clinical practice, the workup of tubo-ovarian abscess will usually include computed tomography (CT) in order to screen for other inflammatory or gastrointestinal pathology, and to plan for image-guided percutaneous drainage if required for cases refractory to medical therapy. The role of MRI is limited, but recognition of an enlarged fallopian tube is useful in the event that one is encountered unexpectedly. The simple hydrosalpinx contains fluid signal on all image sequences within a tubular serpiginous-shaped adnexal structure, whereas an acutely inflamed tube will have wall thickening and surrounding fatty stranding with pelvic fluid (Figure 48-14). The ovary is usually contiguous but can be distinguished from the tube. With frank pus, there can be air in the dilated tube, which can also show aberrant signal characteristics due to hyperproteinaceous content.³⁸

Magnetic resonance imaging is excellent for preoperative staging of gynecologic neoplasms. Not only lymphadenopathy, but also local tumor extension is well seen by MRI. Magnetic resonance imaging is the study of choice for preoperative staging of cervical cancer. Bowel, bladder, and pelvic sidewall involvement can be shown to advantage using MRI, enabling identification of tumor invasion and penetration.

Cervical Cancer

Although the mortality rates have declined over the last several decades due to screening and early detection, cervical carcinoma is still the third most common gynecologic malignancy in the United States. Worldwide, cervical cancer is the fifth most common cause of cancer deaths.^39,40

Most cervical carcinomas are squamous cell carcinomas and occur at the squamocolumnar junction. They can be infiltrative or polypoid. On MRI, cervical cancer typically appears solid with a signal intensity brighter than myometrium that stands out in sharp contrast to the low-signal junctional zone. The International Federation of Gynecology and Obstetrics (FIGO) staging system is used to clinically stage cervical carcinoma, but it has limitations. FIGO does not account for deep lymphadenopathy, assessment of tumor volume, or local extension beyond the fibrous stroma or into the uterus—all findings that can be demonstrated to advantage with MRI.⁴¹

Imaging is performed in the long and short axes of the uterus and cervix to show the tumor extent to best advantage. If the tumor extends beyond the fibrous stroma outside the cervix into the parametrium (stage IIB or greater), the lesion is usually not considered amenable to curative surgery (Figure 48-15).⁴² Such patients almost always undergo chemoradiation.⁴³ Other findings such as invasion of surrounding organs (bladder or rectum), adenopathy, and invasion of the pelvic sidewall (obturator internus, pyriformis, or levator muscles) are well demonstrated on MRI. Magnetic resonance imaging for radiation treatment planning, tumor volumetry, and monitoring response to therapy helps to guide the duration of radiation therapy and to permit addition of adjunctive therapy. In addition, if patients have received a prior maximal dose of radiation, then they may be candidates for pelvic exenteration, unless there is invasion of the pelvic sidewall. After radiation therapy, MRI has been used for surveillance to search for recurrence.⁴⁴

Endometrial Cancer

Endometrial carcinoma is the most common gynecologic malignancy and the fourth most common cancer in females.⁴⁰ It usually presents with postmenopausal bleeding resulting in the observation that most endometrial cancers are found at an early stage—an important point because the survival rates are greater than 95% for localized disease.⁴⁰

Most endometrial cancers are adenocarcinomas. The patient's prognosis depends on the histologic grading, the presence of myometrial invasion (which carries a poorer prognosis), local extension (involving the cervix, vaginal canal, and tissues outside the uterus), and the presence of lymph nodes (which implies extrauterine spread). MRI has been a valuable addition to preoperative evaluation of patients with endometrial carcinoma in order to identify the primary site of the lesion as corpus or cervix, show deep myometrial invasion, invasion of the cervical fibrous stroma, assess risk of adenopathy requiring lymphadenectomy, and to guide subspecialty referrals.⁴⁵

On MRI, endometrial carcinoma usually is isointense to myometrium on T1-weighted scans and bright on T2-weighted scans. After contrast administration, myometrial invasion is best identified on imaging performed after a 2- to 3-minute delay.^46,47

FIGO staging of endometrial carcinoma is based only on the surgical and histologic findings and does not incorporate imaging findings. Surgical staging is performed for endometrial carcinoma with total abdominal hysterectomy and bilateral salpingo-oophorectomy (TAHBSO), and lymph node dissection usually performed for patients with stage I disease confined to the endometrium (Figure 48-16). If there is an extremely high-grade tumor, preoperative radiation may be added. Pretreatment MRI is valuable because contrast-enhanced MRI can show myometrial invasion, which is associated with a greater risk of adenopathy, worse tumor grade, extension of tumor to the cervix, and a worse prognosis.⁴¹ With dynamic enhanced MRI with a fast gradient echo technique, the inner muscle layer in patients can be detected, even those who are postmenopausal with poor visualization of the junctional zone on T2-weighted images.⁴⁸

Otherwise, it can be difficult or impossible to find myometrial invasion. If occult adenopathy or deep myometrial invasion by endometrial carcinoma is discovered preoperatively by MRI, then the patient may require more extensive nodal dissection and surgical exploration. Although the treatment of choice may still be radical surgery with hysterectomy and oophorectomy, the use of preoperative MRI is a useful predictor of more extensive disease that may require consultation from trained gynecologic oncologists.

Patients with stage II endometrial carcinoma usually undergo surgery and postoperative external beam radiation. If MRI shows a bulky lesion, it may be irradiated before surgery. Instead of administering preoperative intracavitary radiation, surgery is sometimes performed to remove as much of the tumor as possible, and the entire pelvis is irradiated afterward.

With stage III disease, which is unexpected preoperatively, surgeons will try to remove as much of the tumor as they possibly can with a TAHBSO and a node dissection, and then perform radiation postoperatively. If stage III tumor is known ahead of time, usually only radiation is administered. With stage IV disease, radiation and other adjuvant therapies are usually employed, because most patients do not do any better with radical surgery than with palliative therapy. Diagnosis of stage IV disease by preoperative MRI avoids unnecessary surgery (Figure 48-17).

Ovarian Cancer

Although it is the second most common gynecologic cancer after endometrial cancer with an estimated 22,400 new cases in the United States in 2007, ovarian cancer is the most common cause of cancer death from the female reproductive system, with 15,280 deaths from ovarian cancer in 2007.⁴⁰ This is because ovarian cancer is usually discovered late in its course since there is no good screening mechanism and symptoms are nonspecific. Ovarian cancer is most commonly of epithelial origin. Seventy percent of women will have advanced disease at the time of diagnosis. The 5-year survival rate for advanced disease is 15% to 20%, whereas for early disease (stage I) the 5-year survival is as high as 90%.⁴⁹ Although cancer antigen 125 (CA-125) can be a used as a tumor marker, especially in postmenopausal women, there is no documented cost-effective, accurate screening method to detect ovarian cancer at the time of this writing.

The use of MRI for evaluation of ovarian cancer is controversial. Magnetic resonance imaging does not have a place in screening patients because it is too expensive, but it may be helpful for lesion characterization when US is inconclusive, such as with identification of dermoids or endometriomas.²⁸ However, MRI can be used for preoperative staging. With ovarian cancer, the diagnosis is established histologically and surgical treatment is given at the same time. Regardless of initial clinical staging, almost all patients undergo surgery or surgery with adjuvant therapy. The exceptions are advanced stage III and stage IV tumors. Moreover, understaging at surgery can occur in up to a third or more of cases in some series.

Magnetic resonance imaging can be used to stage ovarian cancer and determine resectability, although spiral CT is more widely used.⁵⁰ Unfortunately, the implants from ovarian cancer may be so small that cross-sectional imaging has been of limited value for detection of tiny implants, which are only apparent from laparotomy and tissue sampling. These include lesions in the peritoneum and lesions in the omentum (Figure 48-18). Gadolinium enhancement optimizes the contrast of the lesion in relation to surrounding tissue. Fast scanning with gadolinium using fat-suppression techniques (eg, spoiled gradient echo sequence or fast multiplanar spoiled-grass scans) is helpful in cases of abdominal disease to cut down on motion artifact from bowel peristalsis and to better identify enhancing tumor on a background of dark fat (Figure 48-19). Tumor implants are shown to best advantage even in the omentum or peritoneum.

The features suggestive of ovarian malignancy that can be recognized on MRI or CT are complex lesions greater than 4 cm in size or greater than 3-mm wall thickness, septations, vegetations and frond-like projections, solid elements, necrosis, omental and peritoneal implants, adenopathy, and ascites.⁵¹

The argument for cross-sectional imaging is that one can predict the extent of disease from preoperative cross-sectional imaging. This assists in planning surgery and identifying nonresectable disease. In addition, MRI and CT permit referral to gynecologic oncologists for optimal treatment.⁵²

Staging with the FIGO system is surgical and includes total abdominal hysterectomy, bilateral salpingo-oophorectomy, omentectomy, peritoneal washings and biopsies, and pelvic and para-aortic lymph node dissection.

Chemotherapy is probably preferable to initial surgical cytoreduction in patients with nonresectable disease. Computed tomography has been used for this purpose for a very long time, is widely available, and is well accepted by radiologists and referring physicians. Shortened scan times with fast imaging and enhanced techniques on MRI afford better evaluation of extent of disease.

Second-look laparotomy has fallen out of favor because surgical cytoreduction tends not to be any better than chemotherapy, particularly if residual disease exceeds 1.5 to 2 cm in size. MRI and CT have both been reliable in finding lesions of 2 cm or larger. Patients with lesions of this size are the very ones in whom surveillance can make a difference. MRI and CT have both been used to scan patients with a history of ovarian cancer, but CT is probably more commonly used because it is better understood and accepted by both radiologists and clinicians. MRI may have some advantages due to its superior soft tissue contrast, especially with use of enhanced fast scans with fat-suppression techniques to demonstrate smaller lesions on serosal surfaces, which are difficult to detect. Whether MRI or CT is used, cross-sectional imaging can play an important role in lesion characterization, preoperative staging, and follow-up, often directing appropriate therapy for recurrence in ovarian cancer. Molecular imaging with positron emission computed tomography using F-18 fluorodeoxyglucose positron emission tomography (FDG-PET/CT) (Figure 48-20).

Recurrent Gynecologic Malignancy

Magnetic resonance imaging has been a valuable tool for patients who have been treated for malignancy, for surveillance for recurrence and for assessing response to treatments such as chemo- and radiation therapy. In an extent of disease workup, it is important to evaluate the pelvic sidewall for invasion. Pelvic sidewall invasion has a poor prognosis. The pelvic sidewall is comprised of 3 major muscle groups: the obturator internis, the piriformis, and the levator ani muscles. Magnetic resonance has a very high, nearly 100%, negative predictive value for absence of disease in the pelvic sidewall. If there is abnormal signal in the pelvic sidewall, there may be contiguity with tumor from direct extension or gross invasion; however, there are some false-positive cases (radiation changes, infection, etc). Overall, accuracy is in the 80% to 90% and higher range. Thus, MRI is extremely useful to evaluate patients who are candidates for radical surgery for cure with pelvic exenteration. Pelvic sidewall invasion generally precludes an attempt at radical surgery in most cases and is a critically important MR finding.

The use of MRI with contrast enhancement in patients with endometrial cancer permits specific assessment of myometrial invasion. Tumors with deep myometrial invasion even for stage I disease have a worse prognosis and higher incidence of adenopathy. Magnetic resonance imaging can identify deep myometrial invasion reliably. Limitations may occur in patients with fibroids or other abnormalities of the uterine myometrium.

Magnetic resonance imaging may also be useful in evaluating patients with other endometrial and myometrial disorders, such as patients on tamoxifen treatment. Irregularities in the endometrial and myometrial interface can be assessed, particularly in patients with submucous fibroids, polyps, or other processes such as adenomyosis that may distort this interface (Figure 48-21).

Magnetic resonance imaging can be used to evaluate patients with distorted pelvic anatomy. This may occur in patients with fibroids displacing the ovaries and adnexa outside the field of view of sonography, or in those who have had prior surgery. The most common site of recurrence for all gynecologic cancer is in the vaginal apex (see Figure 48-19). Consequently, the vaginal apex and vaginal canal must be carefully inspected in all patients who are undergoing surveillance. It is very important to know the normal postoperative pelvic anatomy to recognize disruption from residual or recurrent tumor.

The multiplanar imaging of MRI with a larger field of view, which is not limited by bowel gas or by body fat, combined with excellent soft tissue contrast permits more complete delineation of pelvic structures that may be distorted or displaced.

Magnetic resonance imaging affords a detailed delineation of the pelvic structures involved in patients with pelvic stress urinary incontinence or prolapse of the bladder, uterus, or rectum. MRI has emerged as a noninvasive study of choice for evaluation of pelvic floor relaxation and organ prolapse that aids in planning surgical repair. Dynamic fast T2-weighted sagittal MR imaging at rest and with the patient straining with Valsalva have enabled pelvic floor and organ prolapse evaluation noninvasively, with greater patient comfort and without radiation exposure compared to the traditional methods, including voiding cystourethrography, defecography, and cystocolpoproctography (Figure 48-22).^{53,54,55,56, and 57}

MRI is the noninvasive study of choice for visualization of the female urethra and periurethral tissues. Radiographic techniques such as voiding cystourethrography and double-balloon catheter urethrography will only demonstrate the urethral lumen and not the surrounding tissues. MRI can be performed with endovaginal coils to image the urethra to best advantage, or with endoluminal or endorectal coils. The bright signal contents of urethral and periurethral lesions such as diverticula are well demonstrated due to the superb soft tissue contrast and bright signal on MRI. MRI may demonstrate the communication of a diverticulum with the urethra, or complications such as stone formation, superinfection, or neoplasm (Figure 48-23).^58,59

High Field Systems

High field MRI systems are now commercially available. Use of 3T in body imaging is still controversial. Acceptance of 3T body MR imaging has been slow, requiring pulse sequence optimization; reduction of specific absorption rates, which interferes with rapid sequences during breath holding; requirements for greater shimming; and increased magnetic susceptibility artifacts. 3T offers advantages of greater signal to noise, which can be appreciated using 3D gradient echo and fast spin echo sequences, which provide better resolution with little penalty at 3T compared to 1.5 T (Figure 48-24).^60,61 The theoretical advantages of 3T over 1.5 T have yet to be validated with evidence-based investigations.

Parallel Imaging

Parallel imaging, whether using k-space techniques or with sensitivity/spatial sensitivity encoding methods, offer speed or reduced artifacts, respectively. Dynamic imaging is improved with parallel imaging methods. Parallel imaging reduces T2* effects, specific absorption rate, and noise at the expense of ghosting and aliasing artifacts (Figure 48-25).^{62,63, and 64}

New MRI Contrast Materials

New MRI contrast materials under development include blood pool agents, iron oxide agents for lymph node assessment such as USPIO (ultrasmall superparamagnetic iron oxide), molecular-imaging-targeted agents using antibodies or proteins, and magnetic nanoprobes that seek specific proteins or DNA.^65,66

Diffusion-Weighted MR Imaging

Diffusion-weighted MR imaging (DWI) uses the motion of water molecules to reveal properties of tissues. The ADC, or apparent diffusion coefficient, may have some utility in differentiation of normal tissue from tumor. However, DWI is very sensitive to artifacts from motion of respiration, bowel peristalsis, or vascular pulsation (Figures 48-26 and 48-27).^{67,68, and 69}

MR Spectroscopy

MR spectroscopy can show resonant peaks of metabolites that may show tissue signatures. Thus far, spectroscopy has had limited applications in imaging the female pelvis, though much work has been done with prostate cancer imaging.⁷⁰ Spectroscopy of invasive cervical cancer has been reported.⁷¹

This chapter discussed and illustrated some conditions in which MRI can be used as a problem-solving modality with equivocal sonographic examinations. Because of the superb soft tissue contrast, multiplanar capabilities, and lack of ionizing radiation, when used selectively, pelvic MRI can improve diagnostic specificity and sensitivity.