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ANATOMY FOR DIAGNOSTIC IMAGING
inner zone of myometrium is of low signal intensity and is continuous with the fibrous stroma of the cervix. This is known as the junctional zone. The hypointense signal from this layer, which is histologically similar to the rest of the myometrium, is thought to result from an increased nuclear cellular ratio in the relatively more packed inner myometrial layer. The outer myometrium is of intermediate signal intensity. The outer serosa is a thin hypointense layer. The cervical canal is also of high signal intensity, continuous with the endometrial canal. The narrow os may be seen, and occasionally the mucosal folds of the cervix, the plica palmatae, may be seen as transverse ridges on sagittal images. Dilated glands adjacent to the cervical canal (nabothian cysts) may be seen as hyperintense foci. Outside the high-signal endocervical canal is a low-intensity fibrous stroma continuous with the junctional zone and the myometrium above. The vaginal canal is of high signal owing to secretions. The mucosa is of low signal similar to the cervix and junctional zone, and the muscular vagina wall slightly higher signal than myometrium. The paravaginal and paracervical venous plexuses can be seen as bright structures on axial and coronal T2 images. The ovaries are isointense with fat on T2-weighted images and the follicles stand out as hyperintense spots in an instantly recognizable pattern. On T1-weighted images the uterus and ovaries are of homogeneously intermediate signal intensity with poor intrinsic contrast, but the ligamentous structures are very well seen, being of low signal intensity compared to the surrounding fat. The round ligaments are seen coursing anteriorly from the upper lateral part of the uterus to the inguinal canal. The uterosacral ligaments can be seen extending back to the sacrum. The peritoneal reflection of the broad ligament and pelvic floor is best appreciated on coronal images, outlined inferiorly by extraperitoneal fat. The muscles of the pelvic floor and sidewall are also very well seen. Imaging in the coronal plane allows evaluation of the lymphatic drainage along the internal and common iliac chains. Dynamic MR imaging of the pelvic floor may be performed, with sagittal midline images acquired at rest and during straining (Valsalva) to assess for abnormal laxity of the pelvic floor muscles and ligaments. A line drawn between the inferior aspect of the pubic bone and the last joint of the coccyx represents the iliococcygeal line. The bladder neck, vaginal fornices and anorectal junction should be above this line, and descend minimally on straining.
enhancement may be seen in the myometrium and endometrium, especially mid-cycle. Non-enhancing fluid may also be seen in the uterine cavity during the secretory phase of the cycle. Enhancing vessels may be seen on either side of the lower uterus. The ovaries may usually be identified as small round structures of soft tissue density, occasionally with small cysts. The broad ligament is not identified, unless abnormal amounts of abdominal free fluid are present to outline it, but the round ligaments can usually be seen, running anteriorly to the inguinal ring. The levator ani complex and muscles of the pelvic side wall can be identified. Hysterosalpingography (Fig. 6.26) This technique outlines the cavity of the uterus and tubes by injection of water-soluble contrast via the cervical canal. The cervical canal is approximately one-third the length of the entire uterine long axis. Longitudinal ridges are seen on the anterior and posterior walls of the cervical canal. In nulliparous women these may have branches running laterally - the plicae palmatae. Cervical glands may be outlined by contrast as outpouchings from the cervical canal. The isthmus is seen as a narrow area above the cervix, and the internal os may sometimes be identified as a constriction of the lumen of the isthmus. The uterine cavity is seen to be triangular on the frontal view. It is usually smooth-walled. The triangular cornua lead to the fallopian tubes, which are 5-6 cm long. The isthmus of the tube is uniformly narrow and opens into the wide ampulla. Contrast spills freely into the peritoneal cavity. The walls of the uterus may demonstrate longitudinal folds. Polypoid filling defects may be seen in the secretory phase in normal women. Filling of endometrial glands may also be seen in normal women in the secretory phase. Vaginography This technique outlines the vagina with contrast. The characteristic rectangular shape of the vagina is demonstrated. It is of the utmost importance to recognize this shape in the case of inadvertent filling of the vagina during a barium examination. CROSS-SECTIONAL ANATOMY (Figs 6.27-6.29) The anatomy described can be identified on both CT and MRI images.
Computed tomography (see Fig. 6.29b)
Midsacral level - male or female (Fig. 6.27a and b)
On CT the uterus is seen as a round structure of soft-tissue density lying on or behind the bladder. Oral contrast helps to differentiate loops of bowel, which lie on and around it. A tampon in the vagina may also aid interpretation, showing as a rounded air-density below the uterus on cross-sectional images. Intravenous contrast improves contrast between the uterus and surrounding structures, and
This level is above the bladder. The sigmoid colon may be seen close to its junction with the rectum. Loops of small bowel lie in the pelvis, on top of the pelvic organs. Pelvic and mesenteric fat separates the various bowel loops. The sacrum is posterior, with the piriformis muscle arising from its anterior surface. The piriformis muscle passes anteriorly, inferiorly and laterally to insert into the greater tuberosity
