Imaging of the Female Pelvis

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What is the first-line imaging modality of the female pelvis and why?

Common indications for pelvic imaging in a female patient include pelvic pain, pelvic masses, and abnormal bleeding. Ultrasonography (US) is the imaging modality of choice for evaluating the female pelvis because it offers excellent visualization of the pelvic organs. It is also fast, inexpensive, and portable and requires no intravenous or oral contrast material. If US results are inconclusive, magnetic resonance imaging (MRI) may be obtained for further evaluation. Computed tomography (CT) is generally reserved for emergent settings including trauma, hemodynamic instability, or acute lower abdominal or pelvic symptomatology when the differential diagnosis is broad, or for evaluation of patients with gynecologic malignancy. Positron emission tomography (PET) in combination with CT is most often performed for evaluation of suspected or known pelvic malignancy.

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How are US, CT, and MRI of the female pelvis performed?

US is performed using a transducer probe that sends and receives the ultrasound waves. Typically, images of the pelvis are initially obtained using a transabdominal approach (i.e., the transducer is placed over the abdomen and a conductive gel is applied) with a fluid-filled bladder. This is routinely followed by a transvaginal approach (i.e., a condom-covered sterilized transducer is placed in the vagina and covered in sterile conductive gel), preferably with an empty bladder. Transvaginal US typically demonstrates better delineation of the endometrium and adnexal regions, but these techniques are complementary. Images of the pelvic organs are obtained in the longitudinal and transverse planes. 2D and 3D imaging can be obtained, but 3D US requires specialized 3D transducers. Sonohysterography may be performed which combines hysterography with US. This is a slightly more invasive procedure because saline is infused into the endometrial canal. It is reserved for select patients with an abnormal endometrium. Transperineal and transrectal US are reserved for patients who are not candidates for transvaginal US and for assessment of pelvic organ prolapse. Although US is operator dependent and requires physical examination of the patient, it allows real-time imaging and is free of ionizing radiation. It takes at least 15 to 30 minutes to complete an examination, depending on its complexity.

MRI utilizes strong magnetic fields to produce images, is similarly free of ionizing radiation, and is operator independent. For optimal image acquisition, a phased array coil is placed around the patient to optimize signal in the area of interest and to speed up image acquisition time. Patients must be screened prior to entering the MRI for any metallic devices. It can take up to 30 to 40 minutes to complete the examination, but with improvements in the technology, scan time continues to improve. 2D images may be acquired in any plane, and 3D imaging can also be performed. Multiple imaging sequences are obtained with different contrast weightings such as T1-weighted and T2-weighted images. Endovaginal and endorectal coils are occasionally used for urethral and perineal imaging. Gadolinium-based intravenous contrast material may be used if indicated. An antiperistalsis agent may sometimes be administered to decrease motion artifact from bowel peristalsis. Oral contrast material is sometimes administered for dedicated bowel imaging.

CT is a technique that utilizes ionizing radiation. It is fast and nonoperator dependent. Scanning is performed in a single breath hold. Images are acquired in the transverse plane but can be reconstructed in coronal and sagittal planes, which are particularly helpful for evaluation of female pelvic anatomy and pathology. Iodinated intravenous contrast material and oral contrast material are often used for CT imaging.

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Which imaging modality best demonstrates the anatomy of the uterus?

MRI has superior soft tissue contrast compared to US or CT and best demonstrates uterine anatomy.

The uterus consists of three zones with differential T2-weighted signal intensity on MRI: endometrium, inner myometrium (also called the junctional zone), and outer myometrium. The glandular endometrium has high T2-weighted signal intensity, the inner myometrium has low T2-weighted signal intensity due to presence of dense smooth muscle, and the outer myometrium has intermediate-slightly high T2-weighted signal intensity that contains more blood and less dense layers of smooth muscle ( Figure 38-1 ). Sometimes, a very high T2-weighted signal intensity endometrial canal containing fluid is also seen.

The cervix similarly demonstrates three distinct zones: a high T2-weighted signal intensity layer of endocervical glands (along with lower signal intensity longitudinal mucosal ridges [plicae palmitae]) that is contiguous with the endometrium, a low T2-weighted signal intensity inner cervical stroma that is contiguous with the junctional zone, and an intermediate-slightly high T2-weighted signal intensity outer cervical stroma that is contiguous with the outer myometrium. A very high T2-weighted signal intensity endocervical canal containing fluid may sometimes also be seen.

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What are the more common congenital uterine anomalies and their clinical significance?

Müllerian duct anomalies can cause abnormalities in uterine development that may be associated with infertility, miscarriage, and endometriosis. Accurate classification of müllerian duct anomalies is critical because treatment options vary depending on the type of underlying abnormality. MRI is the study of choice for delineation of uterine anomalies, while US and hysterosalpingography are often used for initial evaluation.

The classification of anomalies relates to the embryology of uterine development. Two paired müllerian ducts develop into the anatomic structures of the female reproductive tract including the fallopian tubes, uterus, cervix, and upper two thirds of the vagina. The ovaries and lower third of the vagina are derived from different origins.

There are three phases of development, and disruption of these processes can lead to anatomic variation and uterine anomalies. Failure of development during the organogenesis phase where one or both müllerian ducts do not fully develop can result in uterine agenesis, uterine hypoplasia, or a unicornuate uterus ( Figure 38-2, A-B ). The fusion phase refers to both lateral and vertical fusion of the paired müllerian ducts. Failure of lateral fusion can lead to bicornuate or didelphys uterus, and vertical fusion failure can result in incomplete development of the vagina. When the müllerian ducts fuse, a central septum is created which subsequently must then be resorbed to form a single uterine cavity. If not, this can lead to a septate uterus ( Figure 38-2, C-D ). Septate uterus is the most common type of congenital uterine anomaly and is associated with the highest risk of complications such as recurrent spontaneous abortions and premature delivery.

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What is the normal thickness of the endometrium in a premenopausal patient?

In premenopausal women, the appearance of the endometrium changes depending on the phase of the menstrual cycle. On US, during menstruation the endometrium is identified as a thin, echogenic line measuring 1 to 4 mm, sometimes with blood products or sloughed endometrial lining seen in the uterine cavity. During the proliferative phase, the endometrium is thicker, with a trilaminar appearance typically measuring 5 to 7 mm, composed of a thin echogenic central line, a hypoechoic functional layer, and a surrounding outer echogenic basal layer ( Figure 38-3 ). During the secretory phase, the endometrium reaches its maximum thickness (up to about 16 mm) and is homogenously echogenic.

On MRI, the endometrium is typically homogenous, is high in signal intensity on T2-weighted images and low in signal intensity on T1-weighted images, and can vary in thickness depending on the phase of the menstrual cycle. On CT, the measurement of endometrial thickness should be performed on sagittal reconstructed images and can be accurate and reproducible compared with that obtained from transvaginal US.

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What is the normal thickness of the endometrium in postmenopausal patients, and what is the significance of thickened endometrium in these patients?

The endometrium in postmenopausal women should appear as a thin, echogenic layer measuring less than 4 to 5 mm. Endometrial thickness greater than 5 mm with or without surface irregularity and endometrial heterogeneity in the setting of postmenopausal bleeding may be related to benign etiologies such as endometrial hyperplasia, submucosal leiomyomas (i.e., fibroids), or endometrial polyps. However, further workup should be pursued to exclude endometrial cancer, because US findings are often nonspecific.

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What are the most common causes of vaginal bleeding in a postmenopausal woman?

In postmenopausal women, atrophy of the endometrium is the most common etiology of vaginal bleeding. Other common causes include endometrial polyps, endometrial hyperplasia, and endometrial cancer. Less common etiologies include uterine leiomyomas, adenomyosis, hormone replacement therapy, and cervical cancer. Approximately 12% of women with postmenopausal bleeding have endometrial cancer. Endometrial cancer is one of the most common gynecologic malignancies, and the risk of development increases with age.

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In a postmenopausal patient with vaginal bleeding, what is the role of imaging?

Endometrial polyps, endometrial hyperplasia, submucosal fibroids, and endometrial cancer can all present with vaginal bleeding. Although vaginal bleeding is most often due to a benign etiology, imaging can be performed to exclude an underlying anatomic cause and potentially guide tissue sampling if needed. Transvaginal US is the first line of imaging because it can visualize the endometrium and assess endometrial thickness. Endometrial thickness greater than 4 mm on US has a sensitivity of 98% for detection of endometrial cancer and requires further workup, whereas endometrial thickness of 4 mm or less has a negative predictive value of 99%. Sonohysterography and MRI may be performed for further evaluation as needed. Tissue sampling of the endometrium may be warranted using dilation and curettage (D&C) or hysteroscopy to enable visualization of the endometrium and directed biopsies. A diagnostic feature favoring endometrial cancer rather than endometrial hyperplasia or polyp is the presence of myometrial invasion or metastatic disease.

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What is adenomyosis, and what are its imaging characteristics?

Adenomyosis is the presence of ectopic endometrial tissue that develops and grows within the muscular wall of the uterus (i.e., the myometrium). It typically affects women of reproductive age, and symptoms may include severe menstrual cramps, bloating, dysmenorrhea, and menorrhagia. Uterine adenomyosis can be focal, diffuse, or even masslike (i.e., an adenomyoma) and mimic leiomyomas. Transvaginal US is typically used as the first line of imaging. US features of adenomyosis include heterogeneous, most commonly hypoechoic, myometrium with striations and an ill-defined endomyometrial junction. The uterus may appear asymmetric with globular thickening of a wall in focal adenomyosis ( Figure 38-4, A-B ). MRI can be very helpful to confirm this diagnosis. On MRI, adenomyosis is identified as thickening of the junctional zone (≥12 mm), often with presence of subcentimeter foci of T2-weighted (and sometimes T1-weighted) hyperintensity representing ectopic endometrial glands. Focal adenomyosis typically has indistinct margins and does not exert much, if any, mass effect upon adjacent structures.

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