What is the recommended protocol for a CT (Computed Tomography) scan of the perineum?

CT Protocol for Perineal Imaging

For CT imaging of the perineum, intravenous contrast is essential with complete coverage from the lower abdomen through the entire perineum and anal sphincter complex. 1

Key Components of the Protocol

Contrast Administration

• Intravenous contrast: Required to delineate rim-enhancement of fluid collections, diagnose abscess formation, and visualize fistulous tracts 2, 1
• Rectal contrast (optional):
  • Water-soluble rectal contrast should be considered when there is suspicion of:
    • Rectal perforation
    • Recent surgery
    • Need for fistula delineation 2, 1
  • Water-soluble contrast is preferred over barium to avoid complications if spillage occurs into the peritoneal cavity 2

Scan Parameters

• Coverage: Lower abdomen through the entire perineum and anal sphincters 2, 1
• Slice thickness: 2-3 mm 2
• Timing: Contrast-enhanced imaging should be initiated 50-70 seconds after beginning the intravenous contrast injection (between enteric and portal phases) 2
• Acquisition technique: Should be adapted to patient size with consideration for low-dose techniques 2
  • Tube potential selection
  • Automatic exposure control
  • Iterative reconstruction

Patient Positioning

• Standard supine position
• Ensure complete visualization of the perineal region

Clinical Applications

Specific Indications

• Perianal abscesses and fistulae 2, 3
• Rectovaginal and rectovesicular fistulae 2
• Perineal trauma, including obstetric injuries 1
• Inflammatory conditions (e.g., Crohn's disease with perianal involvement) 2
• Perineal masses and malignancies 4, 5, 6

Diagnostic Capabilities

• CT provides important information in diagnosing the underlying etiology of fistulae and detecting their course and locations 2
• Effective at identifying abscesses with reported sensitivity of approximately 77% 1, 7
• Can detect complications such as perforation or leak in patients with trauma or recent surgery 2

Limitations and Considerations

Technical Limitations

• Limited spatial resolution in the pelvis compared to MRI 1
• Lower accuracy (24%) for classifying perianal fistulae compared to endoanal ultrasound (82%) or MRI 1
• Difficulty differentiating between fistula tract and inflammation 1

Alternative Imaging Modalities

• MRI: Superior for complex fistula evaluation and has the highest detection rates for anorectal abscesses, but limited availability in emergency settings 2, 1
• Endoanal ultrasound: Good for fistula detection but requires special skills and may be poorly tolerated by patients with acute pain 2, 1

Common Pitfalls to Avoid

• Relying solely on CT for detailed fistula anatomy when planning complex surgical repairs (MRI is superior for this purpose) 1
• Assuming small abscesses will always be detected on CT (sensitivity is approximately 77%, not 100%) 1
• Failing to include the entire perineum in the scan field, which can result in underestimation of disease extent 5
• Not using IV contrast, which significantly reduces diagnostic accuracy for inflammatory conditions and abscesses 2, 1

By following this protocol, clinicians can optimize CT imaging of the perineum for accurate diagnosis and appropriate management of various perineal conditions.