Perianal Fistulae Medical Imaging Maps: A Pictorial Diagnostic Review

Perianal fistula, or anal fistula, is an abnormal granulomatous tract connecting the anal canal or rectum to the surrounding skin. It causes perianal pain and fistulous discharge, negatively impacting patients' quality of life. The mechanism underlying fistula formation remains incompletely understood. Medical imaging is essential for accurate preoperative assessment of the shape, number and branching of fistulous tracts, identification of internal openings and Parks' classification. This pictorial review provides an image-based overview of the key medical imaging concepts essential for the diagnosis of perianal fistulae and summarises the roles, advantages and limitations of the available imaging modalities for evaluating perianal fistulae. The current evidence on X-ray fistulography, computed tomography (CT) fistulography, transcutaneous ultrasonography (TCUS), endoanal ultrasonography (EAUS) and pelvic magnetic resonance imaging (MRI) was reviewed, emphasising on their diagnostic performance and clinical applications. Conventional X-ray fistulography has progressively been abandoned because it is invasive, is uncomfortable for patients, provides poor soft tissue contrast and inadequately visualises anal sphincters, complex fistulous tracts, secondary branches and abscesses. CT fistulography is useful for evaluating perianal abscesses and fistula when MRI and EAUS are unavailable. TCUS is widely available, non-invasive and radiation-free but is operator-dependent and has limited ability to evaluate high perianal fistulae. EAUS accurately evaluates fistulous tracts, can be performed at the bedside or intraoperatively and is reliable when MRI is contraindicated. However, EAUS is operator-dependent, is contraindicated in patients with severe anal stenosis or acute anal pain, and has a small field of view. Pelvic MRI is the gold standard, providing excellent visualisation of anal canal anatomy, superior soft tissue contrast, accurate mapping of fistulous tracts, localisation of internal openings, assessment of the involvement of anal sphincters and detection of secondary tracts, abscesses and associated complications. Consequently, MRI is a preferred imaging modality, which can guide treatment decisions and monitoring of treatment response.