## Investigation of Choice for Anorectal Fistula ### Why MRI Pelvis with Endorectal Coil is Gold Standard **Key Point:** MRI with endorectal coil is the gold standard imaging modality for preoperative assessment of anorectal fistulas because it provides superior soft tissue contrast and multiplanar imaging without radiation. **High-Yield:** MRI accurately identifies: - Primary tract location and course - Secondary/branching tracts - Internal and external openings - Associated abscess cavities - Relationship to anal sphincters - Horseshoe extensions ### Comparison of Imaging Modalities | Investigation | Sensitivity | Specificity | Advantages | Limitations | |---|---|---|---|---| | **MRI (endorectal coil)** | 90–95% | 85–90% | Best soft tissue detail, multiplanar, no radiation, detects complex anatomy | Expensive, time-consuming, contraindicated with metallic implants | | Fistulography | 60–70% | Variable | Simple, quick | Poor soft tissue detail, radiation, misses branching tracts | | Transrectal ultrasound | 75–85% | 80% | Real-time, no radiation, operator-dependent | Limited field of view, cannot assess external sphincter involvement clearly | | CT pelvis | 70–80% | 75% | Fast, good for abscess | Poor soft tissue contrast, radiation, inferior to MRI for fistula anatomy | **Clinical Pearl:** Preoperative MRI reduces recurrence rates by enabling complete tract identification and treatment, especially in complex fistulas with secondary tracts or horseshoe configuration. ### Indications for MRI in Fistula Assessment 1. Complex or recurrent fistulas 2. Suspicion of horseshoe or branching tracts 3. Preoperative planning for sphincter-preserving surgery 4. Failed primary treatment 5. Inflammatory bowel disease–associated fistulas
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