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ANATOMY FOR DIAGNOSTIC IMAGING
Dynamic imaging The rectum can be evaluated dynamically during straining or evacuation with dynamic MRI or barium defecography. During defecation, the rectum and anus descend with the pelvic floor and the acute anorectal angle increases (straightens out) as contrast material is evacuated. During contraction of the pelvis the rectum and anus ascend, the anorectal angle narrows and a posterior impression is seen at the lower end of the rectum owing to the action of puborectalis. BLOOD VESSELS, LYMPHATICS AND NERVES OF THE PELVIS Overview of the arteries and veins The aorta bifurcates at the level of L4 slightly to the left of midline. At the level of the iliac crest the common iliac arteries are slightly anterior to the common iliac veins. Both vessels are located on the medial border of the psoas muscle as this passes anteriorly into the pelvis. The vessels pass behind the distal ureters. At the level of the pelvic inlet the internal iliac vessels run medially and posteriorly towards the sciatic notch, and the external iliac vessels continue down on the medial aspect of the iliopsoas muscle, passing under the inguinal ligament to enter the thigh. Internal iliac artery (Fig. 6.10; see Fig. 6.11) This artery arises in front of the sacroiliac joint at the level of L5/S1 or the pelvic inlet. It descends to the sciatic foramen and divides into an anterior trunk, which continues down towards the ischial spine, and a posterior trunk which
Fig. 6.10
Internal iliac artery and branches.
passes back towards the foramen. Anterior to the internal iliac artery are the distal ureters, and in the female the ovary and fallopian tube. The internal iliac vein is posterior and the external iliac vein and psoas muscle are lateral. Peritoneum separates its medial aspect from loops of bowel. Branches of the anterior trunk These are as follows: • Umbilical artery - before birth, this carries blood from the internal iliac artery to the placenta. It is obliterated after birth to become lateral umbilical ligament and usually gives rise to one or several superior vesical arteries. • Obturator artery - passes anteriorly through the obturator foramen and gives branches to muscle and bone. • Inferior vesical artery (male) or uterine artery (female) - supplies seminal vesicles, prostate and fundus of the bladder in the male. In the female, the uterine artery runs medially in the broad ligament to supply uterus, cervix and fallopian tubes, with branches to the ovary and vagina. • Middle rectal artery - to inferior rectum with branches to the seminal vesicles, prostate or vagina. • Inferior gluteal artery - passes through greater sciatic foramen to supply muscles of the buttock and thigh. • Internal pudendal artery - curves around the ischial spine or sacrospinous ligament to pass through lesser sciatic foramen into the ischio-anal fossa. It passes through the pudendal canal in the lateral wall of this fossa, exiting medial to the ischial tuberosity to form dorsal and deep arteries of penis or clitoris.
