Best Imaging Approach for Colonic Prostatic Fistula
MRI pelvis without and with IV contrast is the optimal imaging modality for evaluating a suspected colonic prostatic fistula due to its superior soft tissue resolution and ability to accurately characterize fistulous tracts. 1
Primary Imaging Options
MRI Pelvis (First-Line)
- Protocol: Without and with IV gadolinium contrast
- Benefits:
- Highest diagnostic accuracy for fistula detection and classification
- Superior soft tissue resolution for visualizing fistulous tracts
- No radiation exposure
- Excellent for detecting associated abscesses with 100% sensitivity and 90% specificity 1
- Provides high diagnostic confidence with gadolinium-enhanced sequences 1
- Can accurately visualize the internal opening and extent of fistulae
CT Pelvis with IV Contrast (Alternative)
- Protocol: With IV contrast (water-soluble rectal contrast optional)
- Benefits:
- More widely available and faster than MRI
- Good for detecting associated abscesses (sensitivity 86%, specificity 88%) 1
- Can identify air within fistulous tracts
- Useful in acute settings or when MRI is contraindicated
- Limitations:
- Radiation exposure
- Lower soft tissue resolution than MRI
- Less accurate for fistula classification (only 24% correctly classified vs 82% with endoanal ultrasound) 1
Secondary/Adjunctive Imaging Options
CT Cystography
- Consider when standard CT findings are ambiguous
- Provides additional information regarding size and location of fistula for presurgical planning 1
- Not typically needed for initial evaluation as contrast-enhanced CT alone can usually diagnose fistulae based on enhancing tracts extending between structures 1
Fluoroscopic Cystography
- May be useful in specific circumstances for preoperative planning
- Lower sensitivity than cross-sectional imaging
- Consider when CT findings are ambiguous 1
Imaging Findings Suggestive of Colonic Prostatic Fistula
On MRI:
- Enhancing tract(s) between colon and prostate
- Active inflammation in fistulous tracts showing avid enhancement with contrast
- Rim enhancement of associated abscesses
- Diffusion restriction in active inflammatory tracts
On CT:
- Air in the bladder or prostatic urethra
- Focal thickening of involved structures
- Contrast material passing between colon and prostate/bladder
- Extraluminal mass that may contain air
Clinical Pearls and Pitfalls
- Important: Do not delay drainage if a perianal or prostatic abscess is suspected clinically; examination under anesthesia with drainage should not be postponed if imaging is not immediately available 1
- Endoscopic evaluation of the rectum is essential to determine the most appropriate management strategy 1
- Combining imaging modalities (MRI + examination under anesthesia) improves diagnostic accuracy 1
- Prostatic abscesses can fistulize to the rectum and may be difficult to diagnose due to nonspecific symptoms 2
- Consider the underlying etiology (diverticulitis, prostatic abscess, trauma, iatrogenic injury) as this may affect imaging findings 3, 4
Algorithm for Imaging Approach
- First choice: MRI pelvis without and with IV contrast
- If MRI contraindicated or unavailable: CT pelvis with IV contrast
- If findings are equivocal: Consider adjunctive studies (CT cystography, fluoroscopic cystography)
- If abscess suspected and patient unstable: Proceed directly to examination under anesthesia without waiting for imaging
MRI's superior soft tissue contrast resolution makes it the ideal modality for evaluating suspected colonic prostatic fistulae, with CT serving as a reasonable alternative when MRI is contraindicated or unavailable.