Role of Magnetic Resonance Imaging in the Evaluation of Gynecologic Disorders

Uterus

The uterus typically measures from 6 to 9 cm in length in women of reproductive age; the cervix accounts for approximately 30% to 50% of the total uterine length in this age group, but is proportionally larger before the menarche. Uterine zonal anatomy is best visualized using T2-weighted images, typically acquired in the axial and sagittal planes, and may be described by considering two separate regions, the cervix and the uterine body.

The cervical anatomy is divided into three zones on T2-weighted sequences: the endocervical mucosa centrally, which demonstrates high signal intensity as a result of the presence of mucus within glands; the cervical stroma, which demonstrates low signal intensity owing to the presence of fibrous connective tissue; and the peripherally located smooth muscle, which demonstrates intermediate signal intensity ( Fig. 36-1 ). Varying amounts of mucus may be seen within the endocervical canal, which has a very high signal intensity similar to that of fluid.

The zonal anatomy of the body of the uterus also consists of three distinct regions on T2-weighted sequences: centrally, the endometrium demonstrates very high signal intensity; subjacent to the endometrium, the junctional zone, which represents the innermost layer of the myometrium, demonstrates low signal intensity; and the outer myometrium, which demonstrates intermediate signal intensity ( Fig. 36-2 ). At its inferior aspect, the junctional zone is continuous with the low signal intensity fibrous cervical stroma.

Identification of the junctional zone is extremely important when evaluating the uterine magnetic resonance zonal anatomy. Studies have shown that the appearance of the zonal anatomy on MRI is influenced by female hormones. Normally, the junctional zone should not measure more than 11 mm in thickness in women of reproductive age. In postmenopausal women, women taking oral contraceptives, or in girls before menarche, the zonal anatomy is frequently indistinct and the junctional zone may be very thin.

The MRI appearance of the endometrium is also hormonally influenced and varies with the menstrual cycle. The endometrium becomes progressively thicker between the early follicular (proliferative) phase and ovulation, increasing from 1 to 3 mm in thickness to an average of 8 to 9 mm. The endometrium should not measure more than 15 mm in thickness in women of menstrual age, and typically it measures 11 mm or less in postmenopausal women who are not on hormonal replacement therapy. The endometrium is highest in signal intensity during the luteal (secretory) phase. It is imperative to recognize that the evaluation of endometrium thickness using T2-weighted images may result in overestimation because fluid within the endometrial cavity is also high in signal intensity on T2-weighted sequences ( Fig. 36-3 ).

Unlike the uterine corpus, the cervical zonal anatomy does not demonstrate significant changes during the menstrual cycle. In addition, because the cervix is composed largely of fibrous elastic tissue, enhancement of the cervix following the administration of intravenous gadolinium differs from enhancement of the uterine corpus. The uterine corpus is a very vascular structure when compared to the cervix and enhances earlier and more avidly than the cervix ( Fig. 36-4 ). This is important because this difference in enhancement pattern may give the appearance of a pseudomass and lead to a misdiagnosis of cervical cancer.

Ovaries

The normal ovaries demonstrate homogeneous intermediate T1-weighted signal intensity; however, on T2-weighted sequences, an outer high signal intensity cortex and a hypointense central medulla will be observed. The ovaries measure approximately 2 × 2 × 3 cm; however, their size fluctuates according to hormonal status, largely because of follicle production and enlargement during the menstrual cycle. The ovaries are most readily identified in women of reproductive age by the presence of such follicles, which appear as small simple cysts of high signal intensity on T2-weighted sequences and low signal intensity on T1-weighted sequences ( Fig. 36-5 ). Primordial follicles are usually smaller, measuring up to 9 mm in diameter, whereas stimulated graafian follicles may reach up to 3 to 5 cm. Other features of these physiologic cysts seen on T2-weighted sequences include imperceptible or thin walls (less than 3 mm) and absence of mural nodules (see Fig. 36-5 ). The number of ovarian follicles decreases following menopause in women who are not taking hormonal replacement therapy. However, simple ovarian cysts are a common incidental finding in postmenopausal women, seen in 14% of women over 55 years of age, and may resolve or remain stable. Ovarian stroma enhances after intravenous contrast agent administration and the use of fat-suppressed sequences helps to identify the ovaries when follicles are not visible. The normal fallopian tubes are not visualized on MRI.

Müllerian Duct Anomalies

The müllerian ducts are paired embryologic structures that normally fuse between the 6th and 11th weeks of gestation, forming the uterus, fallopian tubes, cervix, and most of the upper vagina. However, in 0.1% to 10.0% of the general population, either fusion fails to occur or the ducts do not develop normally, resulting in one of the multiple types of anomalies. Although müllerian duct anomalies may be asymptomatic depending upon the exact type, they may be associated with primary amenorrhea, infertility, obstetric complications, and endometriosis. It is estimated that as many as 25% of women who present with infertility and miscarriage have müllerian duct anomalies. In addition, 30% to 50% of these women have concurrent renal anomalies, which include renal agenesis, ectopia, fusion, malrotation, and duplication. Patients suspected of having müllerian duct anomalies are often initially investigated with transvaginal sonography, because it is widely available, safe, and of relatively low cost. Although MRI is considered the reference standard for the evaluation of müllerian duct anomalies with a diagnostic accuracy of almost 100%, it is frequently reserved for indeterminate cases or cases in which further information is needed. T2-weighted sequences are the most useful because identification of the zonal anatomy is necessary for complete evaluation and analysis. In addition, the study must include an oblique coronal and axial plane to ensure adequate visualization of the uterine fundal contour.

Although there are many different classifications of müllerian duct anomalies, the American Society for Reproductive Medicine classification is perhaps the most widely used and the best known ( Fig. 36-6 ).

Uterus Didelphys

Uterus didelphys arises through complete failure of müllerian duct fusion in utero and is characterized by the presence of two separate uteri and cervices with a vertical septum in the upper vagina. Very rarely, two completely separated vaginas and vaginal orifices may be present. Patients are usually asymptomatic, but hematometrocolpos may occur in the presence of an obstructing transverse hemivaginal septum. Clinical complications are similar to those associated with a unicornuate uterus. MRI may be used to readily diagnose uterus didelphys by demonstration of two widely separated uterine horns and two cervices ( Fig. 36-7 ). The zonal anatomy is preserved. Vaginal septa may not always be identified with MRI, and physical examination may be necessary for confirmation and differentiation from a bicornuate bicollis uterus.

Bicornuate Uterus

This anomaly is the result of partial failure of fusion of the müllerian ducts. It is characterized by an increased intercornual distance, reportedly measuring wider than 4 cm; a concave external contour of the fundus, measuring more than 1 cm in depth; and a fibrous-muscular septum bisecting the endometrial cavity. The septum may be incomplete, partially dividing the uterine cavity (bicornuate unicollis [ Fig. 36-8 ]), or complete, extending all the way to the external cervical os (bicornuate bicollis). The vagina develops normally. Images obtained in the oblique coronal plane, along the long axis of the uterus, are useful in assessing the fundal contour. The dividing septum often exhibits signal characteristics similar to myometrium, but may be of low signal intensity inferiorly when fibrous tissue is present.

Septate Uterus

Septate uterus is the most common müllerian duct anomaly and is thought to result from failure of resorption of a fibrous septum in later stages of development. The result of such failure of resorption is a characteristically small endometrial cavity with a dividing septum, which may be partial or may extend to the external cervical os. Reproductive complications are greatest in this class of müllerian duct anomaly and are attributed to difficulties of implantation. Identification of a convex, flat, or minimally indented fundal contour without a fundal cleft on MRI obtained in the coronal plane ( Fig. 36-9 ) is decisive in distinguishing a septate from a bicornuate uterus. Other diagnostic MRI features include an intercornual distance of less than 4 cm (i.e., not increased) and a thin, dividing, often fibrous septum typically of low T1- and T2-weighted signal intensity, although the septum may have a signal intensity similar to that of myometrium if myometrial smooth muscle is the primary component of the septum. The septum may be partial or complete.

Vaginal Anomalies

The lower third of the vagina originates from the urogenital sinus rather than the müllerian ducts and is developmentally distinct. Absence of the distal or lower third of the vagina or the presence of a transverse membrane (mimicking an imperforate hymen) results from failure of urogenital sinus development. In both eventualities, the vaginal obstruction causes the vagina and uterus to distend with fluid, referred to as hematometrocolpos when blood products are identified. The upper vagina demonstrates very thin walls, whereas the uterus may have either thin or thick myometrium ( Fig. 36-10 ). Hematometrocolpos is characterized as a heterogeneous collection with high T1- and intermediate or low T2-weighted signal intensity.

Ovarian Anomalies

The ovaries arise from primitive germ cells that migrate to the embryonic gonadal ridge and subsequently descend into the pelvis. Congenital anomalies of the ovaries are typically associated with chromosomal disorders and include gonadal dysgenesis, commonly occurring with Turner syndrome (45,X karyotype) and Turner syndrome mosaicism (45,X/46XX karyotype). The ovaries of patients with gonadal dysgenesis are absent or seen as fibrous streaks. Fat-suppressed T2-weighted and gadolinium-enhanced T1-weighted sequences are most helpful in the identification of the ovaries.

Definition and Epidemiology

Leiomyomas are benign tumors arising from uterine smooth muscle cells and occur in more than 80% of African-American women and in approximately 70% of Caucasian women by age 50 years. African-American women are diagnosed with leiomyomas at earlier ages, are more likely to be symptomatic, and are more likely to undergo myomectomies and hysterectomies compared to Caucasian women. Leiomyomas have a significant impact on public health, accounting for at least one third of the 600,000 hysterectomies performed each year in the United States.

Clinical Aspects

Most patients with leiomyomas are asymptomatic. Clinical symptoms associated with leiomyomas include dysfunctional uterine bleeding, pelvic pressure, and pelvic pain. In addition, there is an association with impaired reproductive function, which may be related to impaired implantation, tubal obstruction, and an increased rate of miscarriage as well as preterm labor. Approximately 90% of leiomyomas occur in the uterus, whereas 5% to 10% are cervical in location. Leiomyomas can enlarge during pregnancy, and cervical leiomyomas may cause dystocia. A minority of leiomyomas occur in the round and broad ligaments ( Fig. 36-11 ) or adnexa.

Location of the leiomyoma often predicates the symptoms. Uterine leiomyomas are classified as submucosal, intramural, or subserosal ( Fig. 36-12 ). Approximately 5% of leiomyomas are submucosal in location ( Fig. 36-13 ). Submucosal leiomyomas typically present with menometrorrhagia. In addition, a submucosal leiomyoma is more likely to be associated with infertility, dysfunctional uterine bleeding, dysmenorrhea, and anemia. Submucosal leiomyomas are often effectively treated with hysteroscopic resection, depending upon size and intracavitary extent of the mass. Submucosal leiomyomas are distinguished from large intramural leiomyomas based on identification of the lesion's epicenter within the endometrial cavity rather than within the myometrium, and less than 180-degree encirclement by surrounding myometrium.

Subserosal leiomyomas account for approximately 10% to 20% of uterine leiomyomas, are located just under the uterine serosa, and may be pedunculated or sessile ( Figs. 36-13 and 36-14 ). Symptoms are associated with large size owing to mass effect or torsion of a pedunculated lesion. The bridging vascular sign (vessels originating from the uterus and feeding a pelvic mass) is seen in approximately 77% of exophytic leiomyomas and is helpful in differentiating these tumors from solid adnexal masses. Conversely, an ovarian vascular pedicle (identified by the presence of a gonadal vein) may be helpful in establishing that a mass is ovarian in origin. A pedicle is seen in 92% of ovarian masses and 13% of pedunculated subserosal leiomyomas.

Pathophysiology

The pathophysiology of leiomyomas is unknown; however, genetic predisposition and hormonal status are involved in tumor development and growth. Responsiveness of leiomyomas to hormonal stimulation frequently results in enlargement during pregnancy and involution following menopause. Various types of benign degeneration are common, particularly in large leiomyomas, and include hyaline (most common and present in at least 60% of leiomyomas), cystic, myxoid, fatty, and hemorrhagic degeneration ( Fig. 36-15 ). Imaging-based differentiation between the types of degeneration is usually not possible; however, certain features may favor one or another type. Degeneration within a leiomyoma may make the sonographic diagnosis difficult as a degenerated leiomyoma may have cystic components and, thus, may mimic the sonographic appearance of a complex ovarian mass. In such cases, MRI may be used for diagnosis.

Knowledge of atypical locations and presentations, such as benign metastasizing leiomyomas, intravenous leiomyomas, diffuse leiomyomatosis, and peritoneal disseminated leiomyomatosis, is important. Although these uncommon presentations may suggest a malignant process, they are essentially a variation in the growth pattern of this benign lesion. Uterine leiomyosarcomas have been reported to occur in association with leiomyomas. However, genetic studies suggest that these are two different entities arising from separate pathways. Malignant degeneration of a benign leiomyoma into a leiomyosarcoma is considered by most pathologists to be extremely rare, if it occurs at all. Sometimes, the distinction between a leiomyoma and a leiomyosarcoma cannot be reliably made based on MRI characteristics of the primary lesion; however, rapid growth, irregular borders, evidence of extrauterine extension, lymphadenopathy, and metastases are, to different degrees, suggestive of malignancy ( Fig. 36-16 ).

Treatment

The preferred treatment for leiomyomas depends upon their size and location, as well as clinical presentation. The most common therapeutic options are hysterectomy, myomectomy (transabdominal, laparoscopic, or hysteroscopic), myolysis, and uterine artery embolization (UAE). Hysterectomy is usually performed in women who have completed childbearing and do not wish to preserve the uterus. Myomectomy is often the choice for those patients who wish to preserve the uterus. In these cases, an abdominal approach is used for multiple large leiomyomas; the laparoscopic approach is appropriate for resection of subserosal and pedunculated lesions, whereas the hysteroscopic technique is preferred for patients with submucosal lesions that are greater than 50% intracavitary. UAE is a newer, safe, and effective technique with the potential to become the treatment of choice for symptomatic uterine leiomyomas. Contraindications to UAE include active pelvic infection, renal insufficiency, and contrast agent allergy. The reported principal advantage of UAE in appropriate candidates is lower morbidity compared to more invasive surgical techniques.