From the Guidelines
MRI is the best imaging modality to assess for a rectal fistula, as it provides excellent soft tissue contrast and can accurately delineate the primary fistulous tract, secondary extensions, and associated abscesses 1. For optimal results, a dedicated pelvic MRI protocol with a small field of view and high-resolution T2-weighted sequences should be used, often with the addition of fat suppression techniques. Contrast enhancement with gadolinium can help distinguish between active inflammation and fibrosis.
Key Points
- MRI has superior sensitivity (up to 90-95%) and specificity compared to other modalities like endoanal ultrasound or CT scanning 1.
- It can precisely identify the internal opening, the course of the tract, and its relationship to the sphincter complex, which is crucial for surgical planning.
- MRI can detect complications such as abscesses or secondary tracts that might be missed by other imaging techniques.
- The use of a multichannel phased array body coil provides better visualization of the extent of fistulae and those located in the supralevator space and ischiorectal fossa 1.
- The addition of diffusion-weighted sequences increases the conspicuity of fistulae over T2-weighted FSE sequences (100% sensitive) and discriminates between inflammatory mass and abscess (100% sensitivity and 90% specificity) 1.
Imaging Protocol
- A dedicated pelvic MRI protocol with a small field of view and high-resolution T2-weighted sequences should be used.
- Contrast enhancement with gadolinium can help distinguish between active inflammation and fibrosis.
- The use of a phased array body coil provides better visualization of the extent of fistulae and those located in the supralevator space and ischiorectal fossa.
Comparison with Other Modalities
- CT scanning has a lower sensitivity (77%) for anorectal abscess and is less effective in differentiating subtle attenuation changes to diagnose small abscesses and fistulae 1.
- Endoanal ultrasound has a higher sensitivity (82%) for perianal fistulae, but is limited in its ability to visualize the extent of fistulae and associated complications 1.
From the Research
Imaging Modalities for Rectal Fistula Assessment
The best imaging modality to assess for a rectal fistula is a crucial aspect of diagnosis and treatment. Several studies have investigated the effectiveness of different imaging modalities in this context.
Magnetic Resonance Imaging (MRI)
- MRI is considered the gold standard imaging technique for the evaluation of perianal fistulas, as it provides excellent anatomic detail of the anal sphincters and shows higher concordance with surgical findings than other imaging evaluations 2.
- A study comparing volumetric contrast-enhanced three-dimensional T1-weighted (CE 3D T1) turbo spin-echo (TSE) with conventional CE-two-dimensional (2D)-T1 and 2D-T2-weighted imaging (WI) sequences found that CE 3D T1 sequences outperformed conventional 2D sequences in evaluating perianal fistulas 3.
- MRI fistulogram (MRI-FG) and MRI colostogram (MRI-DPCG) have been shown to be effective in assessing anorectal malformations, with results comparable to those of contrast fistulogram (FG) and distal pressure colostogram (DPCG) 4.
- MRI is also useful in assessing disease extension and predicting prognosis in patients with anal fistula, helping to make therapy decisions and monitor therapy 5.
Other Imaging Modalities
- While other imaging modalities, such as computed tomography, ultrasound, and endoscopy, may be used in the diagnosis and treatment of enterocutaneous fistulas, MRI is generally considered the best imaging modality for assessing rectal fistulas 6.
Key Findings
- MRI provides excellent anatomic detail and shows higher concordance with surgical findings than other imaging evaluations.
- CE 3D T1 sequences outperform conventional 2D sequences in evaluating perianal fistulas.
- MRI-FG and MRI-DPCG are effective in assessing anorectal malformations.
- MRI is useful in assessing disease extension and predicting prognosis in patients with anal fistula.