The major MRI indication in perianal fistula is preoperative classification.
MRI allows a direct visualization of the fistolous tracts and abscesses,
combined with high soft tissue resolution.
MRI help to classify accurately fistolous tracts but also to identify the entire extention of the disease that otherwise would have been missed.
The most appropriate MR protocol used for evaluation of perianal fistulas consists of: oblique axial T1 and T2-weighted FSE,
oblique axial and coronal fat-suppressed T1 and T2-weighted FSE,
oriented perpendicular or parallel to the long axis of the anal canal,
with large and small FOV.
T1-weighted images provide an excellent anatomic overview of the sphincter complex,
levator plate,
and ischiorectal fossa.
Fistulous tracks,
inflammation,
and abscesses appear as areas of low to intermediate signal intensity and may not be distinguished from normal structures.
On T2-weighted images fistulous tracks have high signal intensity and can be well distinguished from sphincters and muscles that have low signal intensity.
Abscesses also have high signal intensity on T2 due to the presence of pus in the central cavity.
T1-weighted contrast-enhanced fat-suppressed MRI sequences can help differentiate inflammation from abscess.
Although the use of intravenous constrast-enhancement produces images that are visually appealing,
it may not be essential for all the cases and its additional value is not yet determined.
Initial classification of perianal fistulas was based on surgical anatomy described by Parks et al.
They described the course and relationship of the perianal fistulas primary tract’s to the external and internal sphincters with reference to the coronal plane.
There were four categories: intersphincteric,
transsphincteric,
suprasphincteric and extrasphincteric [4] (figure 2).
MR imaging findings are not included in the Parks classification,
so an MR imaging–based classification was proposed by radiologists: The St James’s University Hospital Classification.
It consists of five grades and relates the Parks surgical classification to anatomy seen at MR imaging in both axial and coronal planes.
The classification considers the primary fistulous track but also the secondary ramifications and associated abscesses.
It is easy to use because it utilizes axial anatomic landmarks familiar to radiologists.
Grade 1: simple linear intersphincteric fistula.
The fistulous track extends from the anal canal through the intersphincteric space to reach the skin of the perineum.
There is no ramification of the track within the sphincter complex.
The fistulous track is always observed in the intersphincteric space (figure 3).
Grade 2: intersphincteric with abscess or secondary track.
The primary track and a secondary track or abscess occur in the intersphincteric space,
they never cross the external sphincter.
Secondary fistulous tracks may be of the horseshoe type,
crossing the midline or they may ramify in the ipsilateral intersphincteric plane (figure 4).
Grade 3: transsphincteric.
These fistulas extends through both layers of the sphincter complex and reaches the skin through the ischiorectal and ischioanal fossae.
They are not complicated by secondary tracks or abscesses (figure 5).
Grade 4: transsphincteric with abscess or secondary track in ischiorectal or ischioanal fossa.
These fistulas are similar to Grade 3 but complicated by an abscess or a secondary extention in the ischiorectal or ischioanal fossae (figure 6).
Grade 5: supralevator and translevator.
Perianal fistulous disease extends above the insertion point of the levator ani muscle.