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 (see Fig. 6.3 ) 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 Figs. 6.4 and 6.5 . At the level of the iliac crest the paired psoas muscles lie on either side of the spine (see Fig. 6.4 ). 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 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 region is an anatomical area located posteriorly in the pelvic girdle. The muscles of the gluteal region move the lower limb at the hip joint and can be broadly divided into a superficial group that abducts and extends the femur and deep group that laterally rotates the femur (see Fig. 6.6 ).

Hip muscles: see Figs. 6.6 and 6.7 .

The superficial abductors and extenders comprise the glutei and tensor fascia lata (see Figs. 6.6 and 6.7 ).

The 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. It arises from the posterior ilium, sacrum and coccyx, and inserts into the gluteal tuberosity of the femur. The gluteus medius and minimus are more anteriorly placed, the gluteus minimus being the smallest and the most deeply placed. Both originate from the gluteal surface of the ilium and insert into the greater trochanter; the gluteus medius inserts into the lateral part and the gluteus minimus inserts into the anterior part. The tensor fascia late is a small superficial muscle originating for the anterior superior iliac spine and inserts onto the lateral condyle of the tibia.

The deep lateral rotators comprise the piriformis, obturator internus, gemellus superior and inferior and quadratus femoris (see Figs. 6.6 and 6.7 ). These are smaller muscles that mainly rotate the femur laterally. They also stabilize and help to hold the femoral head in the acetabulum. The piriformis is the most superior and arises from the anterior surface of the sacrum. It travels inferolaterally through the greater sciatic foramen, inserting into the greater trochanter. The obturator internus forms the lateral wall of the pelvic cavity. It originates from the pubis and ischium around the rim of the obturator foramen as well as from the inner surface of the obturator membrane. It passes through the lesser sciatic foramen, inserting into the greater trochanter. The superior gemellus arises from the ischial spine and the inferior gemellus arises from the ischial tuberosity. The gemelli are two small triangular muscles and insert into the greater trochanter. The quadratus femoris is a flat, square muscle and is the deepest of the gluteal muscles, located below the obturator and gemelli muscles. It arises from the lateral side of the ischial tuberosity and inserts into the intertrochanteric crest of the femur.

Plain Films (see Fig. 6.2 )

Bony landmarks may be identified on plain radiograph (see Fig. 6.2 ). The sacral promontory and superior part of the pubic bone define the pelvic inlet and the iliopectineal line , running between and separating 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.

Cross-Sectional Imaging

The muscles of the pelvis may be seen on computed tomography (CT) and MRIs (see Figs. 6.6 and 6.7 ).

Dynamic Proctography

Indirect imaging of the rectal floor is possible with defecating proctography, where 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 ‘clenching’ or ‘Kegel’ manoeuvre to elevate the pelvic floor and during straining/evacuation to evaluate pelvic floor movement and the dynamics of rectal evacuation.

Magnetic Resonance Imaging (see Figs. 6.10, 6.11 )

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

T2-weighted axial magnetic resonance image through the female perineum.

• 1.
  

  Anterior wall of vagina

  
  
• 2.
  

  Posterior wall of vagina

  
  
• 3.
  

  External urethral sphincter

  
  
• 4.
  

  Anal canal

  
  
• 5.
  

  External anal sphincter

  
  
• 6.
  

  Puborectalis

  
  
• 7.
  

  Obturator internus

  
  
• 8.
  

  Obturator externus

  
  
• 9.
  

  Pectineus

  
  
• 10.
  

  Gluteus maximus

  
  
• 11.
  

  External iliac vessels

  
  
• 12.
  

  Ischioanal fossa

  
  
• 13.
  

  Pubic symphysis

  
  
• 14.
  

  Coccyx

  
  
• 15.
  

  Ischial tuberosity

Blood Supply ( Figure 5.24 , Figure 5.25 )

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.

Mesorectal Fascia (see Fig. 6.13 )

The mesorectal fascia is a continuation of the fascia of the sigmoid colon at the rectosigmoid junction and surrounds the rectum and perirectal fat, from the origin of the rectum to its lower end at the levator ani. The mesorectal fascia is continuous anteriorly with the rectovesical fascia (Denonvilliers fascia), separating rectum from bladder, seminal vesicles and prostate. Posteriorly it is fused with the presacral fascia (Waldeyer fascia). The mesorectum is perirectal fat surrounding the rectum and carries the vessels and nerves that supply it. It is delineated by the mesorectal fascia.

Radiology of the Anal Canal (see Figs. 6.13, 6.14 )

MRI, with its excellent soft-tissue resolution, is the imaging modality of choice for anal and perianal abnormalities.

The internal and external sphincters can be identified with the intersphincteric space between them. Images are obtained in the coronal plane and at right angles to the anal canal.

Endoanal ultrasound ( Fig. 6.14B ) is used to assess the integrity of the sphincters, mainly in the investigation of obstetric injuries. The internal and external sphincters, transverse perineal muscles and puborectalis can be evaluated longitudinally and axially at upper mid- and lower anal canal levels. The test can be combined with endoanal physiology using a pressure-sensitive catheter to assess pressures at rest and during squeezing and bearing-down manoeuvres. In the upper (proximal) anal canal, the U-shaped fibres of the puborectalis muscle may be identified sweeping posteriorly. At mid-anal canal level, where the internal anal sphincter (IAS) and external anal sphincter (EAS) overlap, the following layers can be identified from inside out: the echogenic mucosa, the intermediate signal subepithelial layer (submucosa), the hypoechoic internal anal sphincter, a thin higher signal longitudinal muscle layer outside the IAS, the intersphincteric space outside the longitudinal muscle and the striated EAS outermost. At mid-canal level, the EAS forms a complete ring. At distal anal canal level, the IAS has terminated and only the EAS is seen. At this level the EAS does not form a complete ring.

Blood Supply ( Figs. 6.15, 6.16 )

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 communication between the systemic and portal systems.

Internal iliac artery and branches.

Aortogram: external and internal iliac branches.

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  Abdominal aorta

  
  
• 2.
  

  Lumbar artery

  
  
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  Right common iliac artery

  
  
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  Left common iliac artery

  
  
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  Left external iliac artery

  
  
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  Left internal iliac artery

  
  
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  Inferior mesenteric artery

  
  
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  Median sacral artery

  
  
• 9.
  

  Posterior trunk of right internal iliac artery

  
  
• 10.
  

  Iliolumbar artery (branch of 9)

  
  
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  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

Lymph Drainage

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

Plain Films (see Fig. 5.2 )

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.

CT Colonography

The cleansed colon is insufflated with gas (CO ₂ ) to achieve gaseous distension of the entire colon. Ingestion of a cup of contrast such as gastrografin the day before the scan coats residual stool and differentiates it from polyps (faecal tagging). 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.

Computed Tomography

The rectum and perirectal tissues are readily assessed by CT. Detail is considerably enhanced by administration of intravenous contrast. The rectal wall, perirectal fat and fascia and pararectal fat may be identified. Images may be reconstructed in multiple planes.

Contrast Enema (see Fig. 5.23 )

The best detail is achieved by using a double-contrast barium technique. A small amount of thick barium is used to coat the mucosa, followed by sufficient air or gas to distend the bowel. The patient is rotated to allow the barium and gas to reach and coat all regions of the colon from the rectum to the tip of the caecum. Various projections are required to open up the overlapping loops satisfactorily, especially sigmoid colon, splenic and hepatic flexures. This ensures adequate views of all the large bowel. The valves of Houston are the transverse mucosal folds or indentations of the rectum seen on an anterior view and are usually less than 5 mm thick. The appendix may be seen as a long, blind-ending structure in the right iliac fossa if it fills (50%–70%). Above the appendix, an indentation from the ileocaecal valve may be identified on the medial wall of the right colon. This demarcates the transition from caecum to ascending colon. The longitudinal columns of Morgagni are best identified after evacuation of barium, when the rectum is not distended. These normally measure 3 mm in width. 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 contrast after removal of the enema catheter; it makes an acute, posteriorly directed angle with the rectum (see also Chapter 5 ).

Magnetic Resonance Imaging ( Fig. 6.13 )

MRI is highly useful for assessing stage and evaluating the mesorectal fat in rectal tumours, both for operative planning and to assess response to chemoradiation. The anal sphincter can also be evaluated for pathology such as perianal inflammatory conditions leading to fistulae and sinus tracts. It is important to image parallel to and at right angles to the plane of the structure being imaged. The rectum or anal canal are first identified on sagittal images. Coronal and axial images of the rectum and anal canal can then be acquired perpendicular and parallel to the structure being imaged. High-resolution T2 images are best for anatomy detail.

Dynamic Imaging ( Fig. 6.17 )

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.

Static T2-weighted sagittal image during a dynamic pelvic magnetic resonance image. There is contrast in the rectum and vagina (sterile gel). The cervix and posterior vaginal fornix are well delineated by gel.

Orange line: Pelvic inlet – line drawn from superior pubic bone to sacral promontory.

Green line: Pubococcygeal line – from inferior pubis to last sacrococcygeal joint, representing levator/coccygeus complex.

Yellow line: H line (hiatus) – anteroposterior dimension of levator hiatus, inferior pubis to puborectalis impression at anorectal junction.

Blue line: Anorectal angle – line drawn parallel to posterior wall of rectum intersects with line drawn parallel to posterior wall of anal canal. This angle should be 90 degrees or less. This angle opens during passage of contrast.

Branches of the Anterior Trunk

These are as follows:

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  Umbilical artery : Before birth, this carries blood from the internal iliac artery to the placenta. It is obliterated after birth to become the medial umbilical ligament and usually gives rise to one or several superior vesical arteries .

  
  
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  Obturator artery: Passes anteriorly through the obturator foramen.

  
  
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  Inferior vesical artery (male) or uterine artery (female): Supplies the 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 the uterus, cervix and fallopian tubes, with branches to the ovary and vagina.

  
  
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  Middle rectal artery : To inferior rectum with branches to the seminal vesicles, prostate or vagina.

  
  
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  Inferior gluteal artery : Passes through the greater sciatic foramen to supply muscles of the buttock and thigh.

  
  
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  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 the obturator internus) in the lateral wall of this fossa, exiting the pelvis medial to the ischial tuberosity to form the dorsal and deep arteries of the penis or clitoris.

Branches of the Posterior Trunk

These are as follows:

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  Iliolumbar artery : This ascends in front of the sacroiliac joint. It supplies the psoas and iliacus muscles and gives a branch to the cauda equina.

  
  
• ■
  

  Lateral sacral arteries : Usually two. These pass through the anterior sacral foramina and exit through the posterior sacral foramina. They supply the contents of the 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.

Branches of the Iliac Artery

Both branches arise just above the inguinal ligament and are as follows:

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  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.

Conventional Angiography

In conventional angiography contrast medium is injected 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.

CT Angiography

CT following the administration of intravenous contrast gives excellent cross-sectional information about the vessels in the pelvis and their relationship to surrounding structures. Acquisition of images is timed to optimize the contrast opacification of the vessels being imaged. The arteries have a round, even calibre whereas the veins are usually larger and more oval-shaped in the supine position. Reconstructions in coronal, sagittal and various oblique planes may be performed on the raw data to display the vessels optimally.

Magnetic Resonance Imaging

MRI of the iliac vessels can be performed without contrast, although contrast images are diagnostically superior. The vessels may be imaged in any plane.

Venography

The veins are identified on cross-sectional contrast imaging. Direct contrast opacification of the internal iliac veins would require intraosseous injection of contrast into the bodies of the pubic bones.

Ultrasound

Ultrasound 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.

Lymphangiography

Direct lymphangiography can be performed using an oily contrast agent (Lipiodol) which is imaged with fluoroscopy. Lymphangiography is the only radiological method available for direct demonstration of the lymphatics, and the technique has been adapted to allow visualization of the lymphatics on MRI, for example in the study of lymphedema. Lymphatics in the foot can be identified by injecting a blue dye into the web spaces of the toes. A ‘blue’ lymphatic vessel is cannulated with a very fine cannula and lipiodol contrast injected for direct fluoroscopic imaging. A gadolinium agent may be injected for MR lymphography. The external iliac, obturator, common iliac and para-aortic nodes can be demonstrated. The internal iliac nodes are not visualized using this method as lymph from the lower limb does not drain to the internal iliac nodes (see also lymphography in Chapter 5 ).

Cross-Sectional Imaging

Lymph nodes are visible on ultrasound, CT and MRI, especially when over 5 mm. Lymph nodes enhance after contrast, aiding their visibility. In a normal node, the cortex should be smooth and uniform, and is usually less than 3 mm. Although a lymph node can be over 2 cm in length, its short-axis measurement should not be more than 1 cm in the abdomen and pelvis. A normal lymph node typically has a fatty hilum and a uniform cortical thickness. Measurements on cross-sectional imaging are always made on the short axis.

In MRI, sequences that suppress fat signal allow easier identification of nodes on conventional MRI. In a further refinement, injected super magnetic iron oxide particles (USPIO) accumulate in the normal reticuloendothelial structure of the lymph nodes, causing them to appear dark (due to iron) on MRI.