Blood vessels, lymphatics and nerves of the pelvis

Dynamic imaging The rectum can be evaluated dynamically during straining or evacuation w i th dynamic MRI or barium defecography. During defecation, the rectum and anus descend w i th 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 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, w i th branches to the ovary and vagina. • Middle rectal artery - to inferior rectum w i th 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.

Branches of the posterior trunk These are as follows:

• Iliolumbar artery - this ascends in front of the SI joint.

It supplies psoas and iliacus muscles and gives a branch to the cauda equina; • Lateral sacral arteries - usually two. These pass through anterior sacral foramina and exit through the posterior sacral foramina. They supply the contents of sacral canal and muscle and skin of the lower back; and • Superior gluteal artery - a continuation of the posterior trunk. This passes through the greater sciatic foramen to supply the muscles of the pelvic wall and gluteal region.

Radiological point of interest The umbilical artery is the first branch of the internal iliac artery in the fetus. (In fact, it is the continuation of the internal iliac artery in the fetus.) In the fetus, the paired umbilical arteries ascend on the inner surface of the anterior abdominal wall to the umbilicus and into the cord, where they coil around a single umbilical vein. After birth the umbilical arteries persist as a fibrous cord called the medial umbilical ligaments. They may be identified if outlined by air on the plain radiograph as in pneumoperitoneum (see also Chapter 5).

External iliac artery (Fig. 6.11) This runs downwards and laterally on the medial border of the psoas muscle to a point midway between the anterior superior iliac spine and the pubic symphysis. At this point it passes under the inguinal ligament to become the common femoral artery. In front of and medial to the vessel, peritoneum separates it from loops of bowel. Its origin may be crossed by the ureter. In the female it is crossed by the

Fig. 6.11 Aortogram: external and internal iliac branches.

1. Abdominal aorta 2. Lumbar artery 3. Right common iliac artery 4. Left common iliac artery 5. Left external iliac artery 6. Left internal iliac artery 7. Inferior mesenteric artery 8. Median sacral artery 9. Posterior trunk of right internal iliac artery 10. Iliolumbar artery (branch of 9) 11. Lateral sacral artery (branch of 9) 12. Superior gluteal artery (branch of 9) 13. Anterior trunk of right internal iliac artery 14. Obturator artery (branch of 13) 15. Vesical artery (branch of 13) 16. Inferior gluteal artery (branch of 13) 17. Deep circumflex iliac artery

ovarian vessels. In the male it is crossed by the testicular vessels and the ductus deferens. The external iliac vein is posterior to its upper part and medial to its lower part.

Branches Both branches arise just above the inguinal ligament and are as follows: • Inferior epigastric artery - this runs superiorly on the posterior surface of anterior abdominal wall to enter the rectus sheath; and • Deep circumflex iliac artery - this ascends laterally to the anterior superior iliac spine behind the inguinal ligament and passes back on the inner surface of the iliac crest, supplying the abdominal wall muscles.

The iliac veins (see Fig. 6.4) The external and internal iliac veins accompany the arteries. At the groin, the veins lie medial to the arteries. As the veins ascend, they become posterior to the arteries. The upper part of the left common iliac vein then passes to the right behind its artery to form the inferior vena cava by joining w i th its fellow to the right of and slightly posterior to the aortic bifurcation. The tributaries of the iliac veins match the arterial branches, except for the gonadal (ovarian and testicular) veins. These drain to the left renal vein on the left and directly to the inferior vena cava on the right.

Radiology of the iliac vessels (see Fig. 6.11)

Angiography The iliac arteries are demonstrated by angiography. In most cases this is performed by injecting contrast medium under high pressure into the distal aorta via a pigtail catheter, which is inserted retrogradely through a common femoral artery. Images may be acquired using plain radiographs or using digital subtraction angiography, where information is acquired on an image intensifier and processed by computer. The vessels may also be demonstrated by CT angiography after intravenous contrast, and by MR angiography performed w i th or without intravenous contrast.

Venography The external iliac veins may be demonstrated by injection of intravenous contrast into leg veins. Contrast opacification of the internal iliac veins requires intraosseous injection of contrast, often into the bodies of the pubic bones. This technique is rarely if ever performed now, as cross-sectional imaging is used to assess the pelvic veins.

Computed tomography CT during the administration of intravenous contrast gives excellent cross-sectional information about the vessels in the pelvis and their relationship to surrounding structures. The arteries have a round, even calibre, whereas the veins are usually larger and more oval-shaped in the supine position.

Magnetic resonance imaging MRI has some advantages over CT. Contrast is not required. The vessels may be imaged along in their own plane, giving an equivalent image to an angiographic study.

Ultrasound This may also identify the common and external iliac vessels, although visualization is somewhat dependent on unpredictable factors such as bowel gas and body habitus of the subject. The vessels are seen as anechoic linear structures. Colour and pulsed wave Doppler techniques demonstrate the direction of blood flow and facilitate identification.

The lymphatics

Lymph drainage runs in lymphatic channels related to the blood vessels. Three chains accompany the external iliac vessels - one (antero)lateral to the artery, one (postero)medial to the vein, and a variable middle chain anteriorly between the vessels. The obturator node is part of the middle chain. These chains drain to the common iliac and para-aortic nodes. The internal iliac lymph vessels and nodes drain to nodes around the common iliac and from here superiorly to para-aortic nodes. Sacral nodes drain to the internal chain.

Radiology of the lymphatics

Lymphangiography Although now performed rarely, lymphangiography is the only radiological method available for direct demonstration of the lymphatics. A lipid-based contrast agent is injected slowly into a lymphatic vessel in either foot. The external iliac, obturator, common iliac and para-aortic nodes are demonstrated. The internal iliac nodes are not visualized using this method.

Cross-sectional imaging CT and MRI may demonstrate the lymph nodes, particularly if enlarged.

Important nerves of the pelvis

The femoral, sciatic and obturator nerves pass through the pelvis to supply the lower limb. They are formed by the union of various branches of the lumbosacral plexus of