Overview
A thorough understanding of the peritoneal cavity, its reflections, recesses, and continuity with the retroperitoneum and pelvis is essential for interpreting cross-sectional imaging. Pathological processes - ascites, abscesses, haemoperitoneum, and metastatic seeding - follow predictable paths dictated by peritoneal anatomy and gravity. Similarly, the pelvic floor, perineum, and their fascial compartments determine how pelvic disease spreads and presents on CT, MRI, and ultrasound.
Compartmental Anatomy of the Abdomen and Pelvis
Major Compartments
The abdominopelvic cavity is divided into three principal compartments:
| Compartment | Location | Key Boundaries |
|---|---|---|
| Peritoneal cavity (greater sac + lesser sac) | Intraperitoneal | Peritoneal membrane; divided at foramen of Winslow |
| Retroperitoneum / extraperitoneal space | Behind peritoneum; continuous into pelvis | Transversalis fascia posteriorly, peritoneum anteriorly |
| Perineum | Below pelvic diaphragm | Pelvic outlet superiorly, skin inferiorly |
The Peritoneal Cavity
The peritoneal cavity is a potential space lined by mesothelium. It is divided into:
- Greater peritoneal cavity: the main compartment
- Lesser sac (omental bursa): communicates with the greater sac via the epiploic foramen (of Winslow)
Key Recesses and Spaces (Greater Sac)
| Recess | Location | Clinical Significance |
|---|---|---|
| Right subphrenic space | Between right lobe of liver and diaphragm | Abscess post-surgery; free air collection |
| Left subphrenic space | Between stomach/spleen and diaphragm | Communicates freely with left subhepatic space; separated from right by falciform ligament |
| Morrison's pouch (right hepatorenal fossa) | Between right lobe of liver and right kidney | Most dependent recess supine; first to collect free fluid |
| Left subhepatic space | Below left lobe of liver, above stomach | Adjacent to lesser sac |
| Right paracolic gutter | Lateral to ascending colon | Freely communicates inferiorly to pelvis; main conduit for fluid from upper abdomen to pelvis |
| Left paracolic gutter | Lateral to descending colon | Superior communication to left subphrenic partially restricted by phrenicocolic ligament |
| Pouch of Douglas (recto-uterine pouch) | Between uterus and rectum (females) | Most dependent recess of pelvis; collects ascites, blood, pus, drop metastases; accessible by transvaginal/transrectal drainage |
| Rectovesical pouch | Between bladder and rectum (males) | Male equivalent; extends to level of seminal vesicles |
The right paracolic gutter is the primary conduit for fluid passage between the upper abdomen and pelvis. The left paracolic gutter is partially restricted superiorly by the phrenicocolic ligament, limiting communication with the left subphrenic space.
Fluid Flow Pathways
In the supine patient, free intraperitoneal fluid accumulates preferentially in: 1. Morrison's pouch (right hepatorenal fossa) 2. The pelvic cul-de-sac (pouch of Douglas or rectovesical pouch)
These two regions are connected via the right paracolic gutter, forming the main inferior route for peritoneal fluid dissemination.
Retroperitoneal Compartments
The retroperitoneum is divided by the renal (Gerota's) fascia into three spaces:
| Space | Contents | Boundaries |
|---|---|---|
| Anterior pararenal space | Pancreas, ascending and descending colon, duodenum (2nd-4th parts) | Posterior parietal peritoneum anteriorly; anterior renal fascia posteriorly |
| Perirenal space | Kidneys, adrenals, proximal ureters, perirenal fat | Enclosed within Gerota's fascia |
| Posterior pararenal space | Fat only (properitoneal fat stripe) | Posterior renal fascia anteriorly; transversalis fascia posteriorly |
- The posterior pararenal fat continues laterally into the properitoneal fat stripe, visible on plain abdominal radiographs as a lucent flank line. Obliteration suggests retroperitoneal pathology.
- Medial limits of the posterior pararenal space are the psoas and quadratus lumborum muscles.
- Isolated posterior pararenal collections are rare; the most common cause is spontaneous haemorrhage into the psoas from anticoagulation.
- Disease from the anterior pararenal space (pancreas or colon) may track posteriorly to the kidney by separating the two layers of the posterior renal fascia.
- The extraperitoneal pelvic space is continuous with the retroperitoneal space, so pelvic pathology spreads preferentially into retroperitoneal compartments.
Peritoneal Reflections and Ligaments
| Ligament / Fold | Connects | Contains |
|---|---|---|
| Falciform ligament | Liver to anterior abdominal wall | Ligamentum teres (obliterated umbilical vein), paraumbilical veins |
| Hepatoduodenal ligament | Liver to duodenum | Portal triad: portal vein, hepatic artery, bile duct (free edge of lesser omentum) |
| Gastrosplenic ligament | Stomach to spleen | Short gastric vessels, left gastroepiploic vessels |
| Splenorenal ligament | Spleen to left kidney | Splenic artery/vein, tail of pancreas |
| Broad ligament | Lateral uterus to pelvic sidewall | Uterine tube (mesosalpinx), ovary (mesovarium), uterine vessels, round ligament, ligament of ovary |
| Uterosacral ligaments | Cervix/uterus to sacrum | Sympathetic and parasympathetic fibres; secondary uterine support |
| Transverse cervical (cardinal) ligaments | Cervix/vaginal vault to pelvic sidewall | Primary uterine support; uterine vessels; ureters run nearby |
| Suspensory ligament of ovary (infundibulopelvic ligament) | Superior pole of ovary to lateral pelvic wall | Ovarian artery, vein, lymphatics, nerves |
| Pubocervical ligament | Cervix to pubic bone | Anterior uterine support; deficiency → anterior prolapse |
| Puboprostatic ligament | Prostate to pubic bone (males) | Contains prostatic venous plexus |
Broad Ligament - Three Divisions
| Division | Relation | Contents |
|---|---|---|
| Mesometrium | Lateral pelvic wall to body of uterus (largest part) | Uterine vessels, parametrial fat |
| Mesosalpinx | Superior margin (suspends uterine tube) | Uterine tube |
| Mesovarium | Posterior extension | Suspends ovary; continuous with ovarian surface epithelium |
The uterine artery crosses the ureter at the base of the broad ligament ("water under the bridge") - a critical surgical landmark. The round ligament of the uterus and ligament of the ovary are both enclosed within the broad ligament related to the uterus and ovary respectively.
Pelvic Peritoneum
The peritoneum at the pelvic inlet is continuous with abdominal peritoneum. In the pelvis it drapes over the superior surfaces of pelvic viscera but does not reach the pelvic floor in most regions. Bladder is anterior, rectum is posterior; in women the uterus lies between them.
- Females: peritoneum covers the uterine fundus, reflects anteriorly onto the bladder (vesico-uterine pouch) and posteriorly onto the rectum (recto-uterine pouch / pouch of Douglas). The broad ligaments serve as the anterior boundary of the recto-uterine pouch.
- Males: peritoneum covers the bladder dome, drapes over the superior poles of the seminal vesicles, then reflects onto the anterior and lateral surfaces of the rectum, forming the rectovesical pouch.
Umbilical Folds (Anterior Abdominal Wall - Internal Surface)
| Fold | Structure Contained | Position |
|---|---|---|
| Median umbilical fold | Urachus (obliterated) | Single, midline |
| Medial umbilical folds | Obliterated umbilical arteries | Paired, paramedian |
| Lateral umbilical folds | Inferior epigastric vessels | Paired, lateral |
These folds define the inguinal fossae relevant to hernia classification: indirect hernia passes lateral to the lateral (inferior epigastric) fold; direct hernia passes medial to it (between medial and lateral folds).
Pelvic Walls and Openings
The walls of the true pelvis consist of the sacrum, coccyx, and inferior half of the pelvic bones, completed by the obturator internus and piriformis muscles.
Key Ligaments of the Pelvic Walls
| Ligament | Attachment | Function |
|---|---|---|
| Sacrospinous | Sacrum/coccyx → ischial spine | Converts greater sciatic notch → greater sciatic foramen |
| Sacrotuberous | Sacrum/coccyx/PSIS → ischial tuberosity | Converts lesser sciatic notch → lesser sciatic foramen; resists sacral rotation |
Communications Between Pelvis and Adjacent Regions
| Aperture | Communicates With | Key Structures Passing Through |
|---|---|---|
| Pelvic inlet | Abdomen ↔ pelvis | Sigmoid colon, ureters, major vessels, nerves, lymphatics; ductus deferens (males); ovarian vessels/nerves (females) |
| Greater sciatic foramen (above piriformis) | Pelvis → gluteal region | Superior gluteal nerve/vessels, piriformis |
| Greater sciatic foramen (below piriformis) | Pelvis → gluteal region | Inferior gluteal nerve/vessels, sciatic nerve, pudendal nerve/internal pudendal vessels, posterior femoral cutaneous nerve |
| Lesser sciatic foramen | Gluteal region ↔ perineum | Pudendal nerve, internal pudendal vessels (re-enter perineum), obturator internus tendon |
| Obturator canal | Pelvis → adductor region of thigh | Obturator nerve, obturator artery and vein |
| Gap between pubic symphysis and perineal membrane | Pelvic cavity ↔ perineum | Dorsal vein of penis/clitoris |
| Gap between inguinal ligament and pelvic bone | Abdomen → thigh | Femoral artery/vein, femoral nerve, lateral cutaneous nerve of thigh, lymphatics, iliopsoas, pectineus |
Pelvic Floor and Pelvic Diaphragm
The pelvic diaphragm consists of the levator ani and coccygeus muscles, completed posteriorly by coccygeus overlying the sacrospinous ligaments.
Muscles of the Pelvic Diaphragm
| Muscle | Origin | Insertion | Innervation | Function |
|---|---|---|---|---|
| Levator ani (pubococcygeus, puborectalis, iliococcygeus) | Posterior pubis → tendinous arch of obturator internus fascia → ischial spine | Anterior part: superior surface of perineal membrane; posterior part: perineal body, anococcygeal ligament, coccyx | Direct branches from anterior ramus S4; inferior rectal branch of pudendal nerve (S2-S4) | Pelvic floor support; maintains anorectal angle (puborectalis); urethral/vaginal sphincter function; reinforces external anal sphincter |
| Coccygeus | Ischial spine and pelvic surface of sacrospinous ligament | Lateral coccyx and inferior sacrum | Branches from anterior rami S3-S4 | Completes pelvic floor posteriorly; pulls coccyx forward after defecation |
The levator ani has a U-shaped anterior defect - the urogenital hiatus - through which the urethra (both sexes) and vagina (females) pass. The anal canal passes through a posterior circular orifice. The margins of the hiatus merge with the walls of the associated viscera and with muscles in the deep perineal pouch below.
The tendinous arch (arcus tendineus levator ani) is a thickening of obturator internus fascia along which levator ani originates.
The pelvic floor is supported anteriorly by the perineal membrane and muscles of the deep perineal pouch.
Perineal Body
A poorly defined fibromuscular node at the centre of the perineum, midway between the ischial tuberosities. Converging structures include: - Levator ani muscles of the pelvic diaphragm - Bulbospongiosus, external anal sphincter, and other perineal muscles (urogenital and anal triangles)
Imaging of the Pelvic Floor
On MRI (T2-weighted axial and coronal): - Levator ani: intermediate signal muscle; best on coronal and axial oblique images - Puborectalis: sling posterior to anorectal junction; maintains the anorectal angle (~90°) - Perineal body: low T2 signal fibromuscular node at intersection of perineal structures - Dynamic MRI (defaecography/pelvic floor MRI): assesses pelvic floor descent, organ prolapse, rectocele, cystocele, levator ani defects
Perineum
Boundaries
The perineum is a diamond-shaped region inferior to the pelvic floor, whose margin is defined by the pelvic outlet:
| Boundary | Structure |
|---|---|
| Anterior | Inferior border of pubic symphysis |
| Posterior | Tip of coccyx |
| Anterolateral | Ischiopubic rami |
| Posterolateral | Sacrotuberous ligaments |
| Lateral points | Ischial tuberosities |
Triangular Subdivisions
A transverse line connecting the ischial tuberosities divides the perineum into:
- Urogenital triangle (anterior): contains roots of external genitalia; urethral orifice and vaginal orifice (females); distal urethra within erectile tissue of penis, opening at glans (males)
- Anal triangle (posterior): contains anal aperture, external anal sphincter, ischio-anal (ischiorectal) fossae
Perineal Membrane and Deep Perineal Pouch
The perineal membrane is a thick triangular fascial sheet spanning the space between the arms of the pubic arch, with a free posterior border. It: - Supports the pelvic floor anteriorly - Lies inferior to the deep perineal pouch - Is traversed by the urethra (and vagina in females)
The pelvic cavity communicates with the perineum through a small gap between the pubic symphysis and the perineal membrane, transmitting the dorsal vein of the penis/clitoris.
Ischio-anal Fossae
Inverted wedge-shaped fat-filled spaces on either side of the anal canal, formed as levator ani courses medially from the lateral pelvic wall:
| Wall | Structure |
|---|---|
| Lateral | Ischium, obturator internus muscle, sacrotuberous ligament |
| Medial | Levator ani |
| Apex | Where levator ani attaches to obturator internus fascia (superiorly) |
| Floor | Skin of perineum |
- The fossae communicate posterior to the anal canal, permitting horseshoe abscess formation
- Contain the pudendal nerve and internal pudendal vessels in Alcock's canal on the lateral wall
- The anterior recess extends superior to the perineal membrane into the deep perineal pouch
- Function: allow movement of the pelvic diaphragm and expansion of the anal canal during defecation
- The ischiorectal fossa is the anatomical landmark of the perineum on cross-sectional imaging
Pelvic Fascial Architecture
| Fascia / Septum | Location | Clinical Relevance |
|---|---|---|
| Rectovaginal septum | Between rectum and vagina (females) | Deficiency → rectocele; invasion in posterior cervical cancer |
| Rectovesical septum | Between prostate/bladder base and rectum (males) | Continuous with prostatic fascia; disrupted in posterior prostate cancer |
| Prostatic fascia | Around anterior and lateral prostate | Contains prostatic venous plexus; important in prostate cancer staging |
| Transverse cervical (cardinal) ligament | Cervix → pelvic sidewall | Primary uterine support; most important ligament; at risk in hysterectomy |
| Uterosacral ligament | Cervix → sacrum | Secondary support; site of endometriosis/cervical cancer invasion |
Major Vessels
| Vessel | Origin | Territory / Notes |
|---|---|---|
| Abdominal aorta | Diaphragm → L4 bifurcation | → Common iliac arteries |
| External iliac artery | Common iliac | → Femoral artery below inguinal ligament; lower limb supply |
| Internal iliac artery | Common iliac (at pelvic inlet) | Pelvis, perineum, gluteal region |
| Superior gluteal artery | Internal iliac (largest branch) | Gluteal muscles; exits greater sciatic foramen above piriformis |
| Inferior gluteal artery | Internal iliac | Gluteal region; exits greater sciatic foramen below piriformis |
| Internal pudendal artery | Internal iliac | Perineum; exits greater sciatic foramen → lesser sciatic foramen → runs in Alcock's canal |
| Uterine artery | Internal iliac | Uterus; crosses ureter at base of broad ligament ("water under the bridge") |
| Obturator artery | Internal iliac | Medial thigh; traverses obturator canal |
| Inferior epigastric artery | External iliac | Rectus sheath; forms lateral umbilical fold; anastomoses with superior epigastric |
| Inferior vena cava | Confluence of common iliac veins (L5) | Left renal vein crosses aorta anteriorly |
Major Nerves
| Plexus | Spinal Levels | Key Branches | Exit Point |
|---|---|---|---|
| Lumbar plexus | L1-L4 | Femoral (L2-L4), obturator (L2-L4), lateral cutaneous nerve of thigh (L2-L3), genitofemoral (L1-L2), iliohypogastric (L1), ilioinguinal (L1) | Below inguinal ligament (femoral); obturator canal (obturator) |
| Lumbosacral trunk | L4-L5 | Descends over pelvic brim to join sacral plexus on piriformis | - |
| Sacral plexus | L4-S3 (with lumbosacral trunk) | Sciatic (L4-S3), superior gluteal (L4-S1), inferior gluteal (L5-S2), pudendal (S2-S4), posterior femoral cutaneous, nerves to coccygeus/levator ani/external anal sphincter | Greater sciatic foramen (most branches) |
| Pudendal nerve | S2-S4 | Inferior rectal, perineal branch, dorsal nerve of penis/clitoris | Greater sciatic foramen → lesser sciatic foramen → Alcock's canal |
| Pelvic splanchnic nerves | S2-S4 | Parasympathetics to hindgut and pelvic viscera | Directly into pelvis (do not exit via sciatic foramen) |
| Coccygeal plexus | S4-Co | Anococcygeal nerves (skin over coccyx) | - |
Pelvic Compartments on Cross-Sectional Imaging
| Compartment | Extent | Contents |
|---|---|---|
| Peritoneal cavity (pelvic) | To level of vagina (pouch of Douglas, females) or seminal vesicles (rectovesical pouch, males) | Loops of bowel; uterus/ovaries partly covered; free fluid collects here |
| Extraperitoneal pelvic space | Pelvic inlet to pelvic diaphragm; continuous with retroperitoneum | Bladder, seminal vesicles, prostate, distal ureters, fat, vessels; includes retropubic space of Retzius |
| Perineum | Below pelvic diaphragm | Ischio-anal fossae, external genitalia, external sphincters |
The retropubic space (of Retzius) is an extraperitoneal prevesical space between the pubic symphysis anteriorly and the bladder posteriorly; accessible surgically without entering the peritoneal cavity.
Imaging Appearances
CT
- Peritoneal fluid: collects in Morrison's pouch, paracolic gutters, and pouch of Douglas; HU ~0-15 (simple fluid), >30 HU (blood), higher with pus or tumour cells
- Peritoneal enhancement: thin uniform = normal/physiological; nodular/irregular = carcinomatosis or infection
- Levator ani: paired symmetric soft tissue density muscles forming the pelvic floor
- Ischio-anal fossae: bilateral symmetric fat-density spaces flanking anal canal; asymmetric soft tissue density → abscess or tumour
- Pelvic ligaments: cardinal and uterosacral ligaments visible as soft tissue bands with good contrast; thickening/irregularity suggests malignant infiltration
- Umbilical folds: raised peritoneal folds on inner anterior abdominal wall visible on axial CT
MRI
- T2-weighted sequences optimal for peritoneal, fascial, and pelvic floor anatomy
- Levator ani: intermediate T2 signal; best on coronal and axial oblique images
- Peritoneal deposits: intermediate T1/T2 signal; enhance with gadolinium
- Pouch of Douglas: small physiological fluid is T2 bright, T1 dark; endometriosis deposits may be T1 bright (haemorrhagic)
- Pelvic fasciae: low T2 signal fibrous structures; disruption indicates invasive disease
- Dynamic pelvic floor MRI: assesses pelvic organ prolapse, levator defects, rectocele, cystocele
- Obstetric pelvimetry: T2 sagittal MRI measures conjugate diameter, transverse inlet, bispinous outlet, sagittal outlet without radiation risk
Ultrasound
- Transabdominal US: detects free fluid; properitoneal fat stripe visible as echogenic flank band
- Transvaginal US: most sensitive for small volumes of fluid in the pouch of Douglas; detects endometriosis, ovarian pathology, tubo-ovarian abscess
- FAST exam: evaluates Morrison's pouch, splenorenal recess, and pelvic cul-de-sac for haemoperitoneum; extended FAST includes pleural and pericardial spaces
- Transperineal/endoanal US: ischio-anal fossa assessment for fistula-in-ano characterisation