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MR Imaging Classification of Perianal Fistulas and Its Implications for Patient Management¹

¹ From the Departments of Clinical Radiology (J.M., J.A.S.) and Surgery (N.S.A.), St James's University Hospital, Beckett St, Leeds LS9 7TF, England. Recipient of a Cum Laude award for a scientific exhibit at the 1998 RSNA scientific assembly. Received May 13, 1999; revision requested June 22 and received July 30; accepted August 11. Address reprint requests to J.A.S. (e-mail: wilsonspencer@compuserve.com).

View larger version (14K): [in a new window]   Figure 1.   Drawing illustrates the anal clock, which is the surgeon's view of the perianal region when the patient is in the lithotomy position. The anterior perineum (p) is at the 12 o'clock position, and the natal cleft (n) is at the 6 o'clock position; 3 o'clock refers to the left lateral aspect, and 9 o'clock, the right lateral aspect of the anal canal. This schema exactly corresponds to the orientation of axial MR images of the perianal region.

View larger version (47K): [in a new window]   Figure 2.   Line diagram shows the normal anatomy of the perianal region in the coronal plane. a = anal canal, IAF = ischioanal fossa, IRF = ischiorectal fossa, R = rectum.

View larger version (65K): [in a new window]   Figure 3.   Line diagram shows the normal anatomy of the perianal region in the axial plane.

View larger version (148K): [in a new window]   Figure 4a.   Normal perianal anatomy. Coronal T1-weighted (a) and axial T2-weighted (b) MR images show the normal anatomy of the perianal region. a = anal canal, es = external sphincter, iaf = ischioanal fossa, irf = ischiorectal fossa, R = rectum, straight arrow in a = the levator plate.

View larger version (144K): [in a new window]   Figure 4b.   Normal perianal anatomy. Coronal T1-weighted (a) and axial T2-weighted (b) MR images show the normal anatomy of the perianal region. a = anal canal, es = external sphincter, iaf = ischioanal fossa, irf = ischiorectal fossa, R = rectum, straight arrow in a = the levator plate.

View larger version (86K): [in a new window]   Figure 5a.   Grade 1 perianal fistula. (a) Line diagram of the coronal view shows a right intersphincteric fistula (yellow track) extending from the dentate line down to the skin through the intersphincteric plane. (b) Line diagram of the axial view shows the posterior midline intersphincteric fistulous track (yellow spot) confined by the external sphincter.

View larger version (99K): [in a new window]   Figure 5b.   Grade 1 perianal fistula. (a) Line diagram of the coronal view shows a right intersphincteric fistula (yellow track) extending from the dentate line down to the skin through the intersphincteric plane. (b) Line diagram of the axial view shows the posterior midline intersphincteric fistulous track (yellow spot) confined by the external sphincter.

View larger version (131K): [in a new window]   Figure 6.   Grade 1 perianal fistula. Coronal dynamic contrast-enhanced MR image shows a right intersphincteric fistula entering the anal canal in the midline posteriorly (arrow).

View larger version (170K): [in a new window]   Figure 7.   Grade 1 perianal fistula. Axial T2-weighted MR image shows a posterior midline intersphincteric fistula (arrowhead).

View larger version (99K): [in a new window]   Figure 8.   Grade 2 horseshoe perianal fistula. Line diagram of the axial view shows an intersphincteric horseshoe fistula (yellow track, arrow) confined by the external sphincter.

View larger version (124K): [in a new window]   Figure 9.   Grade 2 horseshoe perianal fistula. Axial T2-weighted image shows an intersphincteric horseshoe fistula (arrow).

View larger version (91K): [in a new window]   Figure 10.   Grade 2 perianal fistula with an abscess. Line diagram of the coronal view shows a left intersphincteric abscess (a).

View larger version (157K): [in a new window]   Figure 11.   Grade 2 perianal fistula with an abscess. Coronal dynamic contrast-enhanced MR image shows a left intersphincteric abscess cavity (arrowhead) above the primary intersphincteric track (curved arrow). The enteric entry point is suggested by a medial track (straight arrow).

View larger version (154K): [in a new window]   Figure 12a.   Grade 2 perianal fistula with an abscess. Coronal (a), axial (b), and sagittal (c) dynamic contrast-enhanced MR images show an intersphincteric abscess, which is peripherally enhanced (curved arrow) and contains a central focus of nonenhancing pus (straight arrow). As viewed in all three planes, the fistula is confined by the external sphincter and the ischiorectal fossa is unaffected.

View larger version (140K): [in a new window]   Figure 12b.   Grade 2 perianal fistula with an abscess. Coronal (a), axial (b), and sagittal (c) dynamic contrast-enhanced MR images show an intersphincteric abscess, which is peripherally enhanced (curved arrow) and contains a central focus of nonenhancing pus (straight arrow). As viewed in all three planes, the fistula is confined by the external sphincter and the ischiorectal fossa is unaffected.

View larger version (142K): [in a new window]   Figure 12c.   Grade 2 perianal fistula with an abscess. Coronal (a), axial (b), and sagittal (c) dynamic contrast-enhanced MR images show an intersphincteric abscess, which is peripherally enhanced (curved arrow) and contains a central focus of nonenhancing pus (straight arrow). As viewed in all three planes, the fistula is confined by the external sphincter and the ischiorectal fossa is unaffected.

View larger version (134K): [in a new window]   Figure 13.   Grade 2 perianal fistula with an abscess. Axial T2-weighted MR image of a patient with Crohn disease shows an intersphincteric abscess (arrow) containing gas bubbles (arrowheads).

View larger version (89K): [in a new window]   Figure 14.   Grade 3 perianal fistula. Line diagram of the coronal view shows a right trans-sphincteric fistula (yellow track) crossing the ischiorectal fossa and piercing both layers of the sphincter complex.

View larger version (172K): [in a new window]   Figure 15.   Grade 3 perianal fistula. Coronal dynamic contrast-enhanced MR image shows a right trans-sphincteric fistula (arrow) and inflammatory change in the right ischiorectal fossa. Note the entry site in the middle third of the anal canal.

View larger version (175K): [in a new window]   Figure 16.   Grade 3 perianal fistula. Axial dynamic contrast-enhanced MR image shows a left trans-sphincteric fistula within the ischiorectal fossa and piercing the external sphincter (arrow).

View larger version (101K): [in a new window]   Figure 17.   Grade 4 perianal fistula with an ischiorectal fossa abscess. Line diagram of the coronal view shows a left trans-sphincteric fistula with a left ischiorectal fossa abscess (a).

View larger version (153K): [in a new window]   Figure 18.   Grade 4 perianal fistula with an ischiorectal fossa abscess. Coronal dynamic contrast-enhanced MR image shows a left trans-sphincteric fistula (arrow) with a left ischiorectal fossa abscess (arrowheads) containing nonenhancing pus. Note the contraction of the ischiorectal fossa.

View larger version (85K): [in a new window]   Figure 19.   Grade 4 perianal fistula with an abscess. Line diagram of the axial view shows a left trans-sphincteric fistula and left ischioanal fossa abscess (a).

View larger version (143K): [in a new window]   Figure 20a.   Grade 4 perianal fistula with an abscess. Axial T2-weighted (a) and dynamic contrast-enhanced (b) MR images show a left trans-sphincteric fistula (straight arrow) with a left ischioanal fossa abscess (curved arrow) and nonenhancing pus in the cavity (arrowhead).

View larger version (153K): [in a new window]   Figure 20b.   Grade 4 perianal fistula with an abscess. Axial T2-weighted (a) and dynamic contrast-enhanced (b) MR images show a left trans-sphincteric fistula (straight arrow) with a left ischioanal fossa abscess (curved arrow) and nonenhancing pus in the cavity (arrowhead).

View larger version (87K): [in a new window]   Figure 21.   Grade 4 perianal fistula with an abscess. Line diagram of the axial view shows intersphincteric and ischioanal fossa components of the abscess (a).

View larger version (134K): [in a new window]   Figure 22.   Grade 4 perianal fistula with an abscess. Axial T2-weighted MR image shows a left trans-sphincteric fistula (straight arrow) with intersphincteric and left ischioanal fossa components of the abscess (curved arrows).

View larger version (97K): [in a new window]   Figure 23.   Grade 5 perianal fistula with an abscess. Line diagram of the coronal view shows a pelvic abscess (a) with a translevator fistula traversing the ischiorectal fossa.

View larger version (145K): [in a new window]   Figure 24.   Grade 5 perianal fistula. Coronal dynamic contrast-enhanced MR image shows a right translevator fistula (straight arrow) with extensive supralevator horseshoe ramification (curved arrows).