Genital Tract: CT, MR, and Radiographic Imaging

CHAPTER Forty-nine GENITAL TRACT-CT, MR, AND RADIOGRAPHIC IMAGING

FEMALE GENITAL TRACT

The primary modality for imaging of the female genital tract is US using transabdominal, transvaginal, and Doppler techniques. Sonography of the genital tract is reviewed in Chapter Fifty-one. MR and CT are used to stage and follow-up pelvic malignancies and to supplement US by providing additional characterization of lesions. MR, because of its excellent capacity to differentiate tissue types, is particularly useful in making an imaging diagnosis of pelvic disease. Diffusion-weighted MR has potential to aid in the discrimination between benign and malignant lesions and to provide improved detection of peritoneal metastases and tumor recurrence. MDCT with isotropic voxel acquisition allows for multiplanar reformatted images of high quality to improve recognition of anatomic variants and complex pathology. In addition, many uterine and adnexal lesions may be discovered incidentally by pelvic CT or MR performed for other reasons. Hysterosalpingography is combined with US, CT, and MR to diagnose congenital anomalies of the female genital tract and mechanical causes of infertility. The hysterosalpingography is performed by cannulating the cervix and injecting a contrast agent into the cavity of the uterus and fallopian tubes. Free communication of these lumina with the peritoneal cavity is evidenced by free spill of the contrast agent into the peritoneal cavity outlining loops of bowel. Sonohysterography is an alternative to hysterosalpingography. Isotonic saline is injected into the uterine cavity while the uterus is examined sonographically. Virtual hysterosalpingography is an emerging MDCT technique that offers the potential of high-resolution images depicting both the internal and external surfaces of the uterus and fallopian tubes.

Anatomy

The uterus is a pear-shaped muscular organ located between the bladder and rectum. The anterior and posterior surfaces of the uterus are covered by peritoneum, the folds of which extend laterally to the pelvic sidewalls forming the broad ligament. Peritoneum reflecting off the uterus and the bladder forms a shallow anterior vesicouterine pouch. A "bare area" of extraperitoneal space is present between the lower uterus and bladder. This is an important area for direct spread of tumor from one organ to the other. Posteriorly the peritoneum reflects onto the rectum and forms a deep recto-uterine pouch or cul-de-sac. The peritoneum completely covers the uterus and the posterior vaginal fornix. Only the thin wall of the vagina separates the vaginal cavity from the cul-de-sac, allowing transvaginal access to the intraperitoneal space for US-guided culdocentesis or biopsy. The uterus, cervix, and upper one-third of the vagina are derived from the Müllerian ducts, while the lower two-thirds of the vagina arise from the urogenital sinus. Parametrium refers to the connective tissue adjacent to the uterus between the folds of the broad ligament and adjacent to the vagina. Uterine vessels and lymphatics pass through the parametrium. The broad ligament covers the fallopian tubes hanging over them like a sheet folded on a clothesline enveloping the vessels of the parametrium. The broad ligament is well outlined when fluid is present in the pelvic peritoneal cavity. The fundus of the uterus is that portion that extends cephalad from the origin of the fallopian tubes. The body extends from the fallopian tubes to the isthmus, a slight constriction that marks the location of the internal cervical os. The cervix is cylindrical in shape and three to four centimeters in length. Its lower portion, including the external os, protrudes into the vagina and is surrounded by the vaginal fornices. The ureters pass two centimeters lateral to the supravaginal portion of the cervix. The vagina is a muscular tube that is a flattened oval shape on cross-sectional images. The urethra is a prominent tubular structure that courses in the anterior wall of the vagina.

Ovaries vary in size and appearance depending on the woman's age, hormonal status, and stage of the menstrual cycle. The adult ovary is oval with maximal dimensions of five by three by two centimeters. Abnormalities of size are best determined by calculating ovarian volume using the formula (length x width x thickness x zero point five two). Maximum ovarian volume is nine cubic centimeters before menarche, twenty-two cubic centimeters in menstruating women, and six cubic centimeters in postmenopausal women. The location of the ovaries is variable in different patients and even in the same patient at different times depending on degree of bladder filling and the presence and size of other structures in the pelvis. The typical location is lateral, superior, or posterior to the uterine fundus, or in the cul-de-sac. When the uterus is retroverted the ovaries are anterior or lateral to the uterus. The pelvic ureters form an important anatomic landmark that assist in the recognition of the origin of pelvic masses. The ovaries are anterior to the ureters, so an ovarian mass will displace the ureter posteriorly or posterolaterally. Iliac lymph nodes are lateral to ureters, so adenopathy will displace the ureters medially or anteromedially.

Normal MR Anatomy. The internal anatomy of the uterus is depicted best on T two WI. On T two WI, the endometrium appears as a high signal intensity central stripe surrounded by the low signal intensity junctional zone myometrium. The endometrium may normally be up to fourteen millimeters in thickness in women of menstrual age. The bulk of the myometrium is intermediate signal intensity. The low signal intensity of inner junctional zone of the myometrium on T two WI is due to lower water content. On T one WI, the entire uterus is low in signal intensity and the internal anatomy of the uterus is poorly demonstrated. With gadolinium enhancement, uterine zonal anatomy becomes evident on T one WI. The cervix is largely composed of collagenous tissues that are low in signal intensity on both T one WI and T two WI, providing a dark background for visualization of hyperintense cervical carcinomas. The endocervical epithelium and mucus are homogeneous high signal on T two WI. High-resolution MR using surface or intravaginal coils shows two zones in the cervical fibromuscular stroma, a darker inner zone contiguous with the uterine junctional zone and an intermediate signal outer zone distinctly darker than the myometrium. Vaginal anatomy is also best seen on T two WI, which shows the muscular vaginal wall as low in signal with the epithelium and mucus as high in signal. Aqueous vaginal gel may be inserted for MR scanning to distend the vagina and optimize evaluation of the vagina and cervix. The normal ovaries of fertile women are easily identified by the bright signal of the follicles on T two WI. The follicles are low or intermediate in signal on T one WI. The cortex of the ovary in the premenopausal woman is darker in the signal than the medulla on T two WI. The postmenopausal ovary is more difficult to identify because of the absence of follicles and the cortex and medulla being nearly equal in signal on both T one WI and T two WI.

MR is sensitive to physiologic changes that affect the uterus and ovary during the menstrual cycle. Signal intensity of the myometrium is highest during late proliferative and early secretory phases and is lowest during menstruation and early proliferative phase. Low-intensity myometrial lesions such as leiomyomas and adenomyomas are best demonstrated when the myometrium has the highest signal intensity in mid-menstrual cycle. The ovaries vary in size and appearance during the menstrual cycle and are largest with a dominant follicle just prior to ovulation.

Normal CT Anatomy. Because the position of the uterus is so variable on axial plane CT the outline of the uterus often appears lobulated or bulbous solely because of position. The uterus is uniform in soft tissue attenuation and its internal anatomy is not well demonstrated by unenhanced CT. Because the myometrium is highly vascular, the uterus enhances more than most other pelvic organs. Fluid in the uterine cavity is usually low density. The ovaries are easily mistaken for unopacified bowel loops in the pelvis. Ovarian follicles are recognized by their fluid attenuation. The vagina is seen in cross-section as a flattened ellipse of soft tissue density between the bladder and rectum. Normal fallopian tubes are usually not evident on CT. Multiplanar reformatted MDCT images are of great value in interpretation of complex pelvic anatomy and pathology.

Hysterosalpingography is primarily used for the evaluation of infertility to demonstrate the morphology and patency of the uterine canal and fallopian tubes. Contrast injected into the uterine cavity outlines the endocervical canal, uterine cavity, and lumen of the fallopian tubes with free spill of contrast into the peritoneal cavity in the normal patient. The uterine cavity is sharply defined and triangular in shape with normal mild concavity in the fundal region. The size of the cavity varies with parity. The endocervical canal is cylindrical in shape, three to four centimeters in length, and one to three centimeters in width. Folds in the endocervical mucosa form a normal serrated appearance. The normal fallopian tubes are ten to twelve centimeters in length extending from the cornua of the uterus. The lumen is thread-like, one to two millimeters, until it reaches the ampulla where it expands to five to ten millimeters and rugal folds become visible. Patency of the tubes is confirmed by dispersal of contrast within the peritoneal cavity outlining loops of bowel.