Pelvic Cross-Sectional Anatomy & Gynecologic/Urologic Pathology

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Pelvic Boundaries, Compartments & Peritoneal Spaces

Cross-sectional pelvic imaging requires a comprehensive understanding of three-dimensional pelvic osteology, muscular boundaries, fascial planes, and peritoneal reflections. The pelvis houses the terminal gastrointestinal tract, the lower urinary system, the internal reproductive organs, and an extensive network of neurovascular structures. The NMTCB(CT) technologist must be skilled at identifying pelvic anatomy, mapping fluid collections and drop metastases, and distinguishing urologic and gynecologic pathologies from acute traumatic disruptions.

Skeletal and Muscular Boundaries

• Pelvic Inlet (Pelvic Brim): Formed by the sacral promontory, the arcuate lines of the ilium, the pectineal lines, and the pubic crest. It delineates the greater (false) pelvis superiorly (which is continuous with the abdominal cavity) from the lesser (true) pelvis inferiorly.
• Pelvic Diaphragm (Floor): A funnel-shaped muscular sling closing the pelvic outlet, pierced by the urethra, rectum, and (in females) the vagina. It is formed by the levator ani muscle complex and the coccygeus (ischiococcygeus) muscle:
  1. Puborectalis: Originates from the posterior pubic bone and forms a dynamic U-shaped muscular sling around the anorectal junction. Tonic contraction creates the anorectal angle ($80^\circ\text{ to }90^\circ$), which is the primary mechanism maintaining fecal continence.
  2. Pubococcygeus: Middle portion of the levator ani, extending from the pubis to the anococcygeal ligament.
  3. Iliococcygeus: Lateral, posterior sheet arising from the tendinous arch of the obturator fascia and inserting onto the coccyx.
• Lateral Pelvic Wall Muscles: The obturator internus covers the inner surface of the obturator membrane, and the piriformis arises from the anterior sacrum and exits through the greater sciatic foramen. Both serve as key landmarks for identifying pelvic sidewall lymph nodes and neurovascular bundles.

Peritoneal Reflections and Dependent Spaces

In the supine patient, gravity directs intraperitoneal free fluid (ascites, hemoperitoneum, bile, pus) and malignant exfoliated tumor cells into the lowest anatomical recesses:

• Rectouterine Pouch (Pouch of Douglas): In females, the peritoneum reflects from the posterior surface of the uterus onto the upper anterior rectum. This represents the lowest, most dependent space of the abdominopelvic peritoneal cavity in females in both the supine and upright positions. Pathological fluid, pelvic abscesses, and "drop metastases" (carcinomatosis from gastric, ovarian, or colonic cancer) can accumulate here; distribution depends on position, source and peritoneal anatomy.
• Vesicouterine Pouch: The anterior female peritoneal reflection between the superior dome of the urinary bladder and the anterior uterine isthmus. It is relatively shallow.
• Rectovesical Pouch: In males, the peritoneum drapes over the superior surface of the bladder and reflects directly onto the anterior wall of the middle third of the rectum, creating the rectovesical pouch, which is the most dependent peritoneal space in males.
• Space of Retzius (Retropubic Space): An extraperitoneal space filled with loose areolar fat and the Santorini prevesical venous plexus, situated immediately posterior to the pubic symphysis and anterior to the urinary bladder. It is completely outside the peritoneal cavity. Extraperitoneal bladder ruptures and pubic ramus fractures bleed and extravasate contrast directly into this confined space.

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Female Pelvic Anatomy & Gynecologic Pathology

Normal Cross-Sectional Anatomy

• Uterus: Located centrally between the bladder and rectum. Composed of the central endometrium (variable thickness: $1\text{--}4\text{ mm}$ postmenopausal, up to $14\text{--}16\text{ mm}$ secretory phase), the thick smooth-muscle myometrium (enhances intensely post-contrast), and the cervix projecting into the upper vagina.
• Ovaries and Adnexa: Normal ovaries lie in the ovarian fossae along the lateral pelvic sidewalls, suspended by the broad, utero-ovarian, and infundibulopelvic (suspensory) ligaments. Normal post-pubertal ovarian volume ranges from $6\text{ to }10\text{ cc}$, containing multiple small fluid-attenuation functional follicles ($<2.5\text{ cm}$). In postmenopausal women, ovaries atrophy ($<3\text{ cc}$) and become difficult to distinguish from adjacent bowel loops.

Adnexal torsion is a clinical emergency for which ultrasound is commonly the initial imaging test. CT obtained for another pelvic question may show an enlarged ovary, edema or altered enhancement, but apparently preserved enhancement does not exclude torsion. Communicate concern promptly for the appropriate clinical assessment.

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Male Pelvic Anatomy & Urologic Pathology

Prostate Gland and Seminal Vesicles

• Prostate Gland: An inverted conical fibromuscular and glandular organ situated immediately inferior to the bladder neck, superior to the urogenital diaphragm, and anterior to the rectum. Normal young adult prostate volume is $<25\text{ to }30\text{ cc}$.
• Seminal Vesicles: Paired, multi-lobulated, fluid-filled tubular structures located superior to the prostate base and posterior to the bladder trigone, forming a characteristic "bow-tie" appearance on axial CT slices. The fat-filled retrovesical space separates the prostate/seminal vesicles from the anterior rectal wall.

Pathological Urologic Conditions on CT

• Benign Prostatic Hyperplasia (BPH): Nodular hyperplasia originating within the transition (periurethral) zone. On CT, the prostate enlarges symmetrically or asymmetrically ($>30\text{ cc}$, commonly $50\text{ to }100\text{ cc}$ or greater). The median lobe frequently hypertrophies superiorly, elevating and indenting the floor of the urinary bladder ("fishhooking" of the distal ureters). Chronic bladder outlet obstruction triggers compensatory detrusor hypertrophy (bladder wall thickening $>5\text{ mm}$), detrusor trabeculation, and bladder diverticula.
• Prostate Adenocarcinoma: The vast majority (frequency varies by population) arise in the peripheral zone. While multiparametric MRI (mpMRI) is the gold standard for intraprostatic tumor detection and local staging (T-staging), CT is utilized primarily for:
  • Nodal staging: Detecting pathological enlargement of pelvic lymph nodes (obturator, internal iliac, external iliac, presacral); short-axis size informs suspicion but does not prove malignancy.
  • Bone metastases: Prostate cancer characteristically produces osteoblastic (sclerotic / dense, $>+500\text{ HU}$) osseous metastases within the pelvic bones and spine.
• Urinary Bladder Neoplasms: The normal distended bladder wall is smooth, uniform, and measures $\le 3\text{ mm}$. Urothelial (transitional cell) carcinoma accounts for $>90\%$ of bladder malignancies, presenting on contrast-enhanced CT as a focal, irregular, polypoid intraluminal soft-tissue mass or asymmetric wall thickening that demonstrates early nodular enhancement.

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Pelvic Trauma: Ring Fractures, Vascular Injury & Bladder Rupture

High-energy pelvic trauma (motor vehicle collisions, motorcycle crashes, pedestrian strikes, falls from height) carries an acute mortality rate between $10\%$ and $50\%$, primarily driven by rapid retroperitoneal exsanguination.

Pelvic Ring Biomechanics & Classifications

The pelvic ring is a rigid mechanical ring composed of three bones (sacrum, two innominate bones) bound by dense posterior sacroiliac ligaments. Mechanically, the pelvic ring behaves like a rigid pretzel: a visible injury should prompt assessment of the rest of the ring for another fracture or ligamentous disruption; an isolated fracture is possible.

• Young-Burgess Trauma Classification:
  1. Anteroposterior Compression (APC): Direct impact from anterior to posterior. Results in pubic symphysis diastasis ($>2.5\text{ cm}$), tearing of the sacrospinous and sacrotuberous ligaments, and disruption of anterior sacroiliac joints ("open-book fracture"). Causes substantial pelvic volume expansion and tearing of the presacral venous plexus and internal iliac arterial branches.
  2. Lateral Compression (LC): Most common pelvic fracture mechanism (frequency varies by population), caused by lateral T-bone impacts. Produces horizontal pubic rami fractures and ipsilateral or contralateral sacral compression crush fractures. Pelvic volume decreases; severe hemorrhage is less common than in APC.
  3. Vertical Shear (VS): High-energy vertical axial load (fall onto an extended lower extremity). Complete disruption of the anterior and posterior ring, tearing all sacroiliac, sacrotuberous, and sacrospinous ligaments with cranial displacement of the entire hemipelvis. Severe neurovascular disruption and life-threatening hemorrhage.

Pelvic trauma can cause arterial, venous and osseous bleeding. Contrast extravasation and hematoma distribution help the team assess injury, but absence of a visible blush does not exclude dangerous hemorrhage. Resuscitation, stabilization, embolization and other interventions depend on the complete trauma assessment. Pelvic lymph-node size similarly informs suspicion without proving malignant involvement.

Traumatic bladder rupture and retrograde CT cystography

A traumatic rupture question requires adequate retrograde bladder distension; passive excretion of IV contrast into the bladder does not exclude a leak. Before catheter insertion, the clinical team must address suspected urethral injury, particularly blood at the urethral meatus. Do not blindly place a Foley catheter through a potentially injured urethra.

Use the ordered dilute water-soluble agent and gravity filling, generally at least 300 mL in an adult or the patient's tolerance under AUA guidance. The protocol specifies scan coverage and any postdrainage images. Do not force filling or make a scanner acquisition while assuming an inadequately filled bladder is sufficient.

Extraperitoneal leakage remains in perivesical, retropubic and related tissues. Intraperitoneal contrast can outline bowel loops and peritoneal surfaces after a dome injury. Intraperitoneal traumatic rupture generally requires operative repair. Uncomplicated extraperitoneal rupture may be treated with catheter drainage, often two to three weeks, whereas complex injuries can require repair. Avoid invented universal prevalence percentages and automatic conservative management for all extraperitoneal injuries.

Reference: AUA urotrauma guideline.