## Investigation of Choice for Anal Fistula Mapping **Key Point:** MRI pelvis with endorectal coil is the gold standard for preoperative assessment of anal fistulas because it provides superior soft tissue contrast and three-dimensional anatomical detail of the fistula tract, sphincter involvement, and associated complications. ### Why MRI is Superior MRI offers: - **High sensitivity and specificity** (>90%) for identifying primary and secondary tracts - **Multiplanar imaging** (axial, coronal, sagittal) to visualize complex fistula anatomy - **Sphincter assessment** — critical for sphincter-sparing surgical planning - **Detection of horseshoe tracts, abscesses, and recurrent disease** - **No radiation exposure** — important in young patients requiring repeat imaging ### Comparison with Other Modalities | Investigation | Sensitivity | Specificity | Limitations | |---|---|---|---| | MRI (endorectal coil) | 90–95% | 85–90% | Expensive, time-consuming, contraindicated with metallic implants | | Fistulography | 60–70% | 70–80% | Operator-dependent, misses secondary tracts, radiation exposure | | TRUS | 70–80% | 65–75% | Limited field of view, poor for complex/horseshoe tracts | | High-resolution anoscopy | 40–50% | 50–60% | Cannot visualize external tract anatomy | **Clinical Pearl:** MRI is particularly valuable in recurrent fistulas, where previous surgery may have altered anatomy, and in cases of suspected horseshoe or transsphincteric extension. **High-Yield:** The Parks classification of anal fistulas (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric) is best determined by MRI, which directly influences surgical approach and sphincter preservation strategy. [cite:Bailey & Love 27e Ch 78]
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