The pelvis

6 The pelvis



Chapter Contents


The bony pelvis, muscles and ligaments 218

The pelvic floor 221

The sigmoid colon, rectum and anal canal 222

Blood vessels, lymphatics and nerves of the pelvis 226

The lower urinary tract 230

The male urethra 232

The female urethra 233

The male reproductive organs 233

The female reproductive tract 239

Cross-sectional anatomy 247


The bony pelvis, muscles and ligaments (Figs 6.1–6.7)


The pelvis (Fig. 6.1A, B) is a bony ring consisting of paired innominate bones, the sacrum and coccyx. The innominate bones articulate with each other anteriorly and with the sacrum posteriorly. Each innominate bone is composed of three parts, which fuse at the acetabulum.


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Figure 6.1 • Bony pelvis: (A) anterior view of pelvis; (B) inner surface of hemipelvis.


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Figure 6.2 • Pelvic muscles: (A) coronal section; (B) viewed from inside the pelvis.


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Figure 6.3 • AP radiograph of male pelvis.



1 Spinous process of L5 vertebra

2 Transverse process of L5 vertebra

3 Spinous process of first sacral segment

4 Lateral sacral mass

5 Ilium

6 Sacroiliac joint

7 Sacral foramen of S2 vertebra

8 Spinous process of S3 vertebra

9 Coccyx: first segment of three

10 Iliac crest

11 Anterior superior iliac spine

12 Anterior inferior iliac spine

13 Pelvic brim

14 Ischial spine

15 Body of ischium

16 Ischial tuberosity

17 Ramus of ischium

18 Inferior pubic ramus

19 Body of pubic bone

20 Pubic symphysis

21 Superior pubic ramus

22 Acetabulum

23 Head of femur

24 Fovea

25 Neck of femur

26 Greater trochanter of femur

27 Lesser trochanter of femur

28 Intertrochanteric femur

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Figure 6.4 • CT scan of pelvis: axial section through male bladder. Intravenous contrast has been given. The full bladder displaces small bowel loops superiorly.



1 Subcutaneous fat

2 Rectus abdominis muscle

3 Iliacus muscle

4 Psoas muscle and tendon

5 External iliac vein

6 External iliac artery

7 Inferior epigastric vessels

8 Sartorius muscle

9 Tensor fasciae latae muscle

10 Gluteus minimus muscle

11 Gluteus medius muscle

12 Gluteus maximus muscle

13 Piriformis muscle

14 Obturator internus muscle

15 Acetabulum

16 Lower sacral segment: S4 vertebra

17 Bladder: full of layering urine and contrast

18 Perivesical fat

19 Seminal vesicle

20 Rectum containing air, faeces and contrast medium

21 Wall of rectum

22 Perirectal (mesorectal) fat

23 Perirectal fascia (known as Denonvillier’s fascia anteriorly)

24 Pararectal fat

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Figure 6.5 • MRI pelvis – coronal section through male pelvis: (A) anterior section; (B) posterior section.


(A) and (B)



1 Ischium

2 Transverse perineal muscle

3 Obturator internus

4 Levator ani

5 Attachment of levator to tendinous arch of fascia overlying obturator muscle

6 Body of uterus

7 Endometrial cavity

8 Peritoneum reflected to pelvic side wall

9 Uterine vessels entering broad ligament (deep to fascia)

10 Internal iliac vessels

11 Sigmoid colon

12 Levator ani contributing to external anal sphincter

13 Sciatic nerve

14 Ischiorectal fossa

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Figure 6.6 • Axial anatomy of the pelvis: (A) CT section through male perineum; (B) MR section through female perineum; (C) Axial diagram of male perineum.



1 Pectineus muscle

2 Obturator externus muscle

3 Iliacus muscle

4 Obturator internus muscle

5 Pubic bone

6 Ischial tuberosity

7 Gluteus maximus muscle

8 Levator ani

9 Anus

10 External anal sphincter

11 Vagina

12 Urethra surrounded by external urethral sphincter

13 Sciatic nerve and inferior gluteal vessels

14 Ischiorectal fossa

The ilium is a flat curved bone and bears the iliac crest superiorly. The anterior and posterior superior iliac spines are on either end of the iliac crest, with the anterior and posterior inferior iliac spines below them. The inner surface of the bone is smooth and has a sharp crest at its base– the arcuate line – running from the sacroiliac joint to the iliopectineal eminence. This line extends anteriorly to the pubic tubercle as the anatomic iliopectineal line.



Radiology pearl


The arcuate and anatomic iliopectineal lines are known together as the radiological iliopectineal line, visible on a plain radiograph, and demarcating the division between the true and false pelvis.


The pubic bone consists of a body, and inferior and superior rami. The body of the pubic bone articulates with its fellow at the symphysis pubis. It bears the iliopectineal eminence on its superolateral aspect and the pubic tubercle on its superomedial aspect. The articular surfaces of the symphysis pubis are covered in hyaline cartilage with a fibrocartilaginous disc between them. The pubic bone is strengthened on all sides by dense ligaments.


The ischium is composed of a body and an inferior ramus, which joins the inferior pubic ramus. The body bears the ischial tuberosity inferiorly and a spine posteriorly. The ischial spine defines the greater and lesser sciatic notches above and below.


The obturator foramen is bounded by the body and rami of the pubic bone and the body and ramus of the ischial bone.




The sacrum


Five fused vertebrae comprise this triangular bone, which is curved posteriorly. The anterior part of its upper end is termed the sacral promontory. It articulates with the lumbar spine superiorly and with the coccyx inferiorly. Anteriorly the sacrum has four pairs of sacral foramina, which transmit nerves from the sacral canal. Lateral to these are the lateral masses or alae of the sacrum. The sacrum also bears four pairs of posterior sacral foramina and the canal ends posteriorly in the sacral hiatus – a midline opening that transmits the fifth sacral nerves.



The coccyx


This is composed of three to five fused vertebrae. The first segment is often separate. It articulates at an acute angle with the sacrum.



The sacroiliac joints


The sacroiliac joints are covered with cartilage. The anterior aspect of the joint is lined with synovium. The joint surface is flat and uneven, and this irregularity helps lock the sacrum into the iliac bones. Ligaments support the front and back of the joint. There are dense interosseous sacroiliac ligaments which further lock the sacrum to the iliac bones, limiting movement in all planes.


The sacrospinous ligament runs from the ischial spine to the sides of the sacrum and coccyx. It defines the inferior limit of the greater sciatic foramen.


The sacrotuberous ligament runs from the ischial tuberosity to the sides of the sacrum and coccyx. It defines the posterior limit of the lesser sciatic foramen.


The iliolumbar ligament runs from the transverse process of L5 to the posterior part of the iliac crest, further stabilizing the joint.


The pelvic muscles are shown in Figure 6.2. At the level of the iliac crest the paired psoas muscles lie on either side of the spine. They descend anteriorly, fusing with the iliacus muscle, which arises from the inner surface of the ilium. The fused iliopsoas muscle passes anteriorly under the inguinal ligament to insert into the lesser trochanter of the femur.


The piriformis muscles pass obliquely from the anterior aspect of the sacrum exiting the pelvis through the greater sciatic foramen behind the acetabulum to insert into the greater trochanter of the femur.


The obturator membrane closes the obturator foramen. The obturator internus muscle arises from the inner surface of the membrane as well as the bony rim of the obturator foramen. Its fibres converge towards the lesser sciatic foramen as it hooks around the posterior part of the ischium between its tuberosity and spine. It passes through the lesser sciatic foramen to insert into the greater trochanter of the femur.


The aponeurosis of the abdominal wall muscles inserts into the superior surface of the pubic bone. A thickening of the aponeurosis is the inguinal ligament, which runs from the pubic tubercle to the anterior superior iliac spine. All the muscles of the anterior, lateral and posterior abdominal walls insert, to some degree, into the iliac crest, inguinal ligament and pubic bone.


The gluteal muscles arise from the external surface of the iliac bone and the iliac crest and insert into the upper femur. Gluteus maximus is the largest, the most superficial and the most posterior gluteal muscle, covering the posterior part of the ilium and the sacroiliac joints. Gluteus medius and minimus are more anteriorly placed, gluteus minimus being the smallest and the most deeply placed.



Radiology of the pelvic ring



Plain films


The bony landmarks may be identified on the plain radiograph (Fig. 6.3). The sacral promontory and superior part of the pubic bone define the pelvic inlet and the iliopectineal line running between separates the true pelvis below from the false pelvis above. The sacroiliac joints are not optimally seen on the frontal view owing to their obliquity. Special views may be performed so that the X-ray beam passes through the joint to demonstrate it clearly.



Anomalous lumbosacral anatomy


Some variations of the lower lumbar spine and sacrum occur. The first sacral segment may be partially or completely separate – so-called lumbarization of the sacrum. Similarly, the lowest lumbar vertebra may be partially or completely fused to the sacrum – known as sacralization of the lumbar spine. The posterior elements of the lower lumbar vertebra or the sacral vertebrae may not be fused in some people, with no apparent sequelae.



Differences between male and female pelvis



• The muscle attachments are more prominent in the male.

• The pelvic inlet is heart-shaped in the male and oval in the female.

• The angle between the inferior pubic rami is narrow in the male and wide in the female.


Cross-sectional imaging


The muscles of the pelvis may be seen on CT and MR images (Figs 6.4–6.6).



The pelvic floor (Figs 6.5–6.7; see also Figs 6.17, 6.21–6.25, 6.27, 6.28)


A sling of muscles closes the floor of the pelvis. The urethra and rectum and the vagina in the female pierce the pelvic floor. The floor is composed of two muscular layers, the levator ani/coccygeus complex and the perineum.


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Figure 6.7 • Male pelvis: (A) sagittal section showing pelvic floor and urethra; (B) coronal section showing the pelvic floor.


The levator ani muscle is the principal support of the pelvic floor. It provides muscular support for the pelvic organs and reinforces the urethral and rectal sphincters. Levator ani arises in a line from the posterior aspect of the superior ramus of pubis to the ischial spine. Between these bony points, the muscle arises from the fascia covering the obturator internus muscle on the inner wall of the ilium in an arc known as the tendinous arch or white line (Fig. 6.5A). Its fibres sweep posteriorly, inserting into the perineal body (a fibromuscular node behind the urethra in males or the urethra and vagina in females), the anococcygeal body (a fibromuscular node between the anus and coccyx) and the lowest two segments of the coccyx. The midline raphe of the levator ani anterior to the coccyx is also known as the levator plate. The fibres of levator ani sling around the prostate gland or vagina and rectum, blending with the external anal sphincter (Fig. 6.5B).


The components of levator ani are named according to their attachments: pubococcygeus is the main part of levator ani. It arises from the inner surface of the body of pubis and tendinous arch running posteriorly to the sacrum and coccyx. Puborectalis is the thickest, most medial aspect of the muscular sling, arising from the inner surface of the pubic bone and forming a sling behind the anorectal junction. Iliococcygeus is the posterior part of the muscle and runs from the posterior tendinous arch and the ischial spine to coccyx.


The coccygeus muscle is in the same tissue plane as levator ani. It arises from the ischial spine and sacrotuberous ligament and inserts into the side of the coccyx and lower sacrum. It aids levator ani in supporting the pelvic organs.


The perineum is the diamond-shaped space between the pubis, the ischial tuberosities and the coccyx. It is divided into two compartments by the transverse perineal muscles, which arise from the ischial tuberosity and run medially to insert into the perineal body.


The anterior compartment is the anterior urogenital triangle. The anterior urogenital triangle contains a tough sheet of fascia – the perineal membrane – which is pierced by the urethra and in females by the vagina as well. The external urethral sphincter is reinforced by this layer. Its inferior surface gives attachment to the bulb and crura of the penis or clitoris (bulbocavernosus and ischiocavernosus).


The posterior compartment is the anal triangle. It contains the anus and its sphincters, with the ischiorectal fossa on either side. The ischiorectal fossa is the space below and lateral to the posterior part of levator ani and medial to the inner wall of the pelvis (Fig. 6.5). It is bounded posteriorly by sacrotuberous ligaments and the gluteus maximus muscle, laterally by the fascia of the obturator internus muscle and anteriorly by the perineal body. It contains mainly fat.


It is of importance in pathological conditions of the rectum. The anococcygeal body extends from the anus to the coccyx posteriorly. It receives fibres from the anal sphincter and levator ani muscles.



Radiology of the pelvic floor


Imaging of the pelvic floor is mostly required for women with excessive pelvic floor laxity leading to problems with urinary and bowel continence and rectal evacuation.



Dynamic proctography


Indirect imaging of the rectal floor is possible with defecating proctography, when the rectum, bladder and vagina are outlined with contrast and the subject is imaged in the lateral plane. Images are obtained at rest, during a ‘squeezing up’ manoeuvre to elevate the pelvic floor and during straining/evacuation to evaluate pelvic floor movement and dynamics of rectal evacuation.



Magnetic resonance imaging (Figs 6.5, 6.27, 6.28)


The muscles of the pelvic floor can be directly imaged by MRI. Pelvic floor movement can be assessed dynamically (see section on dynamic MRI, p. 246).



The sigmoid colon, rectum and anal canal (Figs 6.8, 6.9; see Fig. 5.19)



The sigmoid colon


The sigmoid colon is variable in length (12–75 cm, average 40 cm). It is covered by a double layer of peritoneum, supported on its own mesentery which is attached to the posterior and left lateral pelvic wall. It has the same sacculated pattern as the rest of the colon in young people. In older subjects it may appear featureless. The redundant sigmoid lies on the other pelvic structures.


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Figure 6.8 • Rectum: (A) anterior view and blood supply; (B) lateral view showing peritoneal reflections.


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Figure 6.9 • Anatomy of anal canal.



Blood supply (Figs 5.8, 5.22, 5.23)


The arterial supply is from the inferior mesenteric artery via the sigmoid arteries, which join in the arterial cascade of the large bowel. Venous drainage is to the portal system via the inferior mesenteric vein.



Lymph drainage


This is via the blood supply to preaortic nodes around the origin of the inferior mesenteric artery.



The rectum


This is about 12 cm long. It commences anterior to S3 and ends in the anal canal 2–3 cm in front of the tip of the coccyx. It forms an anteroposterior curve in the hollow of the sacrum. The rectum has no sacculations or mesentery. Its upper third is covered by peritoneum on its front and sides; its middle third has peritoneum on its anterior surface only, and its lower third has no peritoneum. The lower part of the rectum is dilated into the rectal ampulla. In the resting state the mucosa of the rectum has three to four longitudinal mucosal folds, known as the columns of Morgagni. The taenia coli fuse in the rectum. Some longitudinal shortening of the rectum occurs, giving the rectum a slight S shape. This creates three lateral mucosal folds – left, right and left from above downward. These are known as the valves of Houston.


Posteriorly are the sacrum and coccyx. Anteriorly, loops of small bowel and sigmoid colon lie in the peritoneal cul-de-sac between the upper two-thirds of the rectum and the bladder or uterus. The lower third is related to the vagina in the female and to the seminal vesicles, prostate and bladder base in the male.


The rectum is surrounded by perirectal fat. Fascia known as the perirectal fascia surrounds the perirectal fat, and lateral to the perirectal fascia is the pararectal fat. The perirectal fascia separates the seminal vesicles and prostate from the rectum anteriorly.



The anal canal (Fig. 6.9)


This is directed posteriorly almost at right-angles to the rectum. It is a narrow, muscular canal. It has an internal sphincter of involuntary muscle and an outer external sphincter of voluntary muscle, which blends with levator ani. The internal (smooth muscle, involuntary) sphincter occupies the upper two-thirds of the anal canal. The external (striated, voluntary) sphincter occupies the lower two-thirds. Thus, the sphincters overlap in the middle third, with the internal deep to the external. The junction of the rectum and anal canal is at the pelvic floor where the puborectal sling encircles it, causing its anterior angulation.


Anteriorly, the perineal body separates the anus from the vagina in the female and the bulb of the urethra in the male. Posteriorly, the anococcygeal body is between it and the coccyx, and laterally is the ischiorectal fossa.


The anal canal is lined by mucous membrane in its upper two-thirds and by skin in its lower third. The mucosa of the anal canal has several vertical folds.



Radiology pearl


The mucocutaneous junction is known as the dentate line or Hilton’s white line. This defines a division between arterial supply and venous and lymphatic drainage.



Blood supply (Figs 5.22, 5.23)


The superior, middle and inferior rectal (haemorrhoidal) arteries form a rich submucous plexus supplying the rectum and anal canal. The superior rectal artery is a branch of the inferior mesenteric artery. The middle and inferior arteries arise from the internal iliac artery. The plexus drains via a superior rectal vein to the inferior mesenteric vein and via middle and inferior veins to the internal iliac vein. This represents a communication between systemic and portal systems.



Lymph drainage


The rectum and upper anal canal drain to pararectal nodes, and from here to preaortic nodes around the inferior mesenteric artery and to internal iliac nodes. The lower anal canal drains to superficial inguinal nodes.



Radiology of the sigmoid and rectum



Plain films (see Fig. 5.1)


The sigmoid colon and rectum may be identified on plain radiographs outlined by air, faeces or both. The sigmoid colon has a characteristic S-shaped curve as it joins the descending colon to the rectum, which lies anterior to the sacrum.



Barium enema (see Fig. 5.19)


This is probably still the main radiological examination used to assess these structures, though CT is gaining in prevalence. The best detail is attained by the double-contrast technique, that is, the use of a small amount of barium to coat the mucosa and sufficient air or gas to distend the bowel. The valves of Houston are the lateral mucosal folds of the rectum and are usually less than 5 mm thick. The longitudinal columns of Morgagni are best identified after evacuation of barium, when the rectum is not distended. These measure 3 mm in width. On the lateral view, the postrectal space is measured between the posterior wall of the rectum and the anterior part of the sacrum at the level of S4. This measurement should not exceed 1 cm. The posterior impression of the pubococcygeal fibres of levator ani may be identified at the lower limit of the rectum. The anal canal may be identified if outlined with barium paste; it makes an acute, posteriorly directed angle with the rectum.



Computed tomography


The rectum and perirectal tissues are readily assessed by CT. Visualization of the rectum may be improved by the administration of dilute rectal contrast or air. The rectal wall, perirectal fat and fascia and pararectal fat may be identified. The latest generation of CT scanners allows acquisition of data that can be reconstructed in any plane without loss of resolution.



CT colonography


The cleansed colon is insufflated with room air to achieve gaseous distension of the entire colon. This technique allows examination of the mucosa for polyps through a number of viewing techniques, including surface rendered imaging, in which the interface between mucosa and air in the lumen is electronically rendered into a surface image of the lumen. Computerized programs allow the operator to virtually travel through the colon, observing the mucosa for irregularities.



Magnetic resonance imaging


MRI is also used to image the rectum. Coronal and sagittal images are used to assess tumour spread and to evaluate the mesorectal fat (see Figs 6.5, 6.17) in rectal tumours.



Dynamic imaging (Fig. 6.28)


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 (Figs 6.10, 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.


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Figure 6.10 • Internal iliac artery and branches.


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Figure 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


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 medial umbilical ligament and usually gives rise to one or several superior vesical arteries.

• Obturator artery – passes anteriorly through the obturator foramen.

• 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 – passes through the greater sciatic foramen and curves around the ischial spine or sacrospinous ligament to re-enter the pelvis through the lesser sciatic foramen thence running anteriorly in the lateral aspect of the ischiorectal fossa. It passes through the pudendal canal (a fascial sheath derived from obturator internus) in the lateral wall of this fossa, exiting the pelvis 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 sacroiliac 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.

• 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.


Radiology pearls (Fig. 6.13)



• 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 and can also be seen on CT (Fig. 6.13).

• The lateral umbilical ligaments are the folds of peritoneum covering the inferior epigastric arteries as they run from the external iliac arteries to enter the posterior aspect of rectus sheath muscle. See below for median umbilical ligament.


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 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.

• 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 (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 with its fellow to the right of and slightly posterior to the aortic bifurcation.



Radiology pearl


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 (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 with 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 of the iliac vessels can be performed without contrast, though contrast images are diagnostically superior. 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.

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Dec 19, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on The pelvis

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