Introduction
MRI scanning technique
Technique for anal fistula imaging
Parameters | T2 TSE | T2 TSE | T2 TSE | T2w TIRM | T2 TIRM | T1 TSE FS | T1 TSE FS CM |
---|---|---|---|---|---|---|---|
Imaging planes | Oblique axial | Sagittal | Oblique coronal | Oblique axial | Oblique coronal | Oblique axial | Oblique axial |
TR/TE (msec) | 3020/10 | 5010/100 | 3800/100 | 4190/60 | 5340/60 | 545/10 | 545/10 |
FOV (cm) | 260 | 250 | 250 | 290 | 380 | 260 | 260 |
Section thickness (mm) | 4 | 4 | 4 | 4 | 4 | 4 | 4 |
Intersection gap (mm) | 0.8 × 0.8 × 4.0 | 0.8 × 0.8 × 4.0 | 0.8 × 0.8x4.0 | 0.9 × 0.9 × 4.0 | 1.2 × 1.2 × 4.0 | 1.0 × 1.0 × 4.0 | 1.0 × 1.0 × 4.0 |
Matrix | 320 × 256 | 320 × 256 | 320 × 256 | 320 × 256 | 320 × 256 | 320 × 256 | 320 × 256 |
Averages | 1 | 1 | 1 | 2 | 1 | 1 | 1 |
MRI anatomy of the anal canal
MRI classification of anal fistulas
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Intersphincteric AF accounts for 45% of tracts. It penetrates the IAS and runs in the intersphincteric space to its external perianal opening (although it can have a blind subcutaneous ending).
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Transsphincteric AF accounts for 30% of tracts. It penetrates the IAS and then EAS at various levels and runs through the ischiorectal fossa to its external skin opening (it can be blind and end subcutaneously or in the ischiorectal fossa).
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Suprasphincteric AF accounts for 20% of tracts. It penetrates the IAS. At first, it runs upwards in the intersphincteric space to the supralevator space, crosses the PR, and bends downwards in the ischiorectal fossa to terminate in its external perianal opening (it can be blind and end subcutaneously or in the ischiorectal fossa).
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Extrasphincteric AF accounts for 5% of tracts. It opens internally to the rectum (although it can be blind, i.e., does not penetrate the rectal wall) as a complication of pelvic inflammation, trauma or surgery. It has a peripheral course outside of the sphincters in the ischiorectal fossa down to its external skin opening (it can also end blindly subcutaneously or in the ischiorectal fossa).
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Grade 1: Simple linear intersphincteric fistula (as above in the Parks classification).
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Grade 2: Intersphincteric fistula with intersphincteric abscess and secondary fistulous tract.
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Grade 3: Transsphincteric fistula (as above in the Parks classification).
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Grade 4: Transsphincteric fistula with an abscess and secondary tract within the ischioanal or ischiorectal fossa. Abscesses can develop at any part of the fistula or its extension, but below the levator ani level.
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Grade 5: Supralevator and translevator disease (incorporates suprasphincteric and extrasphincteric fistulas from the Parks classification), i.e., all fistulas above the levator ani.
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Subcutaneous fistulas that in our practice are identified very rarely. Their etiology can be associated with purulent perianal conditions, which are not necessarily associated with anal crypt infection. In MRI, they are typically found medially to the IAS in the epithelial layer (i.e., do not penetrate the IAS) [11].
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Rectovaginal fistulas that usually develop due to an obstetric trauma. Axial and sagittal planes of contrast-enhanced MRI can detect even narrow (1–2 mm) fistulas, which are the most difficult to diagnose.
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Rectoperineal fistulas, with an etiology usually associated with perineal trauma or childbirth.