Cross-sectional anatomy

inner zone of myometrium is of low signal intensity and is continuous w i th 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 w i th 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 w i th 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 w i th 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 w i th 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, w i th 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.

Computed tomography (see Fig. 6.29b) 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 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 w i th 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 w i th 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.

Midsacral level - male or female (Fig. 6.27a and b) This level is above the bladder. The sigmoid colon may be seen close to its junction w i th 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, w i th the piriformis muscle arising from its anterior surface. The piriformis muscle passes anteriorly, inferiorly and laterally to insert into the greater tuberosity

Fig. 6.26 Hysterosalpingogram.

1. Body of uterus 2. Fundus of uterus 3. Uterine cornu 4. Isthmus of fallopian tube 5. Ampulla of fallopian tube 6. Peritoneal spill of contrast

Fig. 6.27 (a, b) Cross-section of male/female pelvis: mid-sacral level.

Fig. 6.28 (a, b) Cross-section of male pelvis: lower sacral level.

of the femur. The sciatic nerve lies on the piriformis. The gluteus muscles are posterior; gluteus maximus is the largest, most posterior and most superficial. Gluteus minimus is the smallest, most anterior and deepest gluteal muscle. The psoas and iliacus muscles are relatively anterior at this level, w i th the external iliac vessels lying on them, the artery being slightly anterior and lateral to the vein.

The ureters are lateral on the side wall of the pelvis and are extraperitoneal in location. At first they lie anterior to the bifurcation of the iliac artery. At a lower level they are more anteriorly located. In the male, the vasa deferentia pass over the ureters and may be identified by CT or MRI, running posteriorly from the inguinal canal around the side of the bladder to join w i th the ducts of the seminal vesicles at a lower level. Lower sacral level - male (Fig. 6.28a and b)

The bladder has a rather square shape when it contains urine. It extends anteriorly to the anterior abdominal wall. Posteriorly are the seminal vesicles. The rectum is just behind the seminal vesicles, separated from them by the perirectal fascia, and is surrounded by perirectal fat. Outside the perirectal fascia is the pararectal fat. This is continuous w i th the fat lining the pelvis above the levator ani muscle. The obturator internus muscle is deep to the lateral pelvic wall. It can be seen hooking around the posterior part of the ischium to insert into the greater trochanter of the femur. The ureters insert into the posterolateral aspect of the bladder. They are not seen at the level of the seminal vesicles as they pass over them.

Fig. 6.29 (a, b) CT section of female pelvis showing uterus and ovaries.

1. Endometrial cavity 2. Uterus 3. Left ovary 4. Right ovary 5. Bladder 6. External iliac vein 7. External iliac artery 8. Inferior epigastric vessels 9. Round ligament 10. Piriformis muscle 11. Branches of internal iliac vessels 12. Sigmoid colon 13. Inferior gluteal vessels 14. Gluteus maximus 15. Gluteus medius 16. Gluteus minimus

Lower sacral level - female (Fig. 6.29a and b) The uterus is seen closely applied to the posterior surface of the bladder. The position of the ovaries is variable and they may be found lateral to the uterus or behind it. The ureters are close to the posterolateral aspect of the bladder at this level. They hook medially in the broad ligament of the uterus (which cannot be identified as a separate structure), passing under the uterine artery and above the lateral vaginal fornix before entering the bladder.

Section through the perineum - male (see Fig. 6.6a) The anterior compartment (urogenital triangle) is immediately deep to the lower part of the symphysis pubis. It consists of the prostate gland and prostatic urethra. Immediately behind is the anorectal junction in the posterior (anal) triangle. The ischiorectal fossa is seen on either side of and behind the anal canal (see earlier discussion for boundaries.) Note how far anterior the upper part of the anal canal is. The puborectalis fibres of levator ani surround the external anal sphincter. Laterally are the obturator internus muscle and the ischial tuberosity. The sacrotuberous ligament can usually be identified running laterally from the coccyx and sacrum to the ischial tuberosity. Posteriorly are the gluteus maximus muscle and the coccyx.

Section through the perineum - female (see Fig. 6.6b) The posterior compartment is the same as that for the male. The anterior compartment has the urethra just behind the pubis, w i th the vagina immediately behind. These structures are seen more clearly w i th MRI than w i th CT.