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Preoperative MR Imaging of Anal Fistulas: Does It Really Help the Surgeon?¹

¹ From the Departments of Radiology (R.G.H.B.T., R.F.A.V., J.M.A.v.E.), Surgery (G.L.B., A.G.v.d.H., C.G.M.I.B.), and Clinical Epidemiology (A.G.H.K.), University Hospital of Maastricht, P. Debyelaan 25, 6229 HX Maastricht, the Netherlands. From the 1999 RSNA scientific assembly. Received December 2, 1999; revision requested December 30; final revision received April 24, 2000; accepted May 8. Address correspondence to R.G.H.B.T. (e-mail: rbe@rdia.azm.nl).

View larger version (50K): [in a new window]   Figure 1. St Mark’s Hospital fistula surgery form, based on the Parks classification (18). (Reprinted, with permission, from reference 9.)

View larger version (64K): [in a new window]   Figure 2. Chart shows ROC curves and area under the curve (AUC), or Az values, for the three observers in the detection of fistula tracks, abscesses, horseshoe fistulas, and internal openings. The 95% CIs of the estimated Az values for fistula tracks were as follows: Az1: 0.97, 1.00; Az2: 0.92, 1.00; and Az3: 0.94, 1.00. For abscesses, Az1: 0.92, 1.00; Az2: 0.88, 1.00; and Az3: 0.88, 1.00. For horseshoe fistulas, Az1: 1.00, 1.00; Az2: 0.88, 1.00; and Az3: 0.82, 1.00. For internal openings, Az1: 0.89, 0.99; Az2: 0.85, 0.97; and Az3: 0.86, 0.97.

View larger version (171K): [in a new window]   Figure 3a. Transsphincteric fistula. (a, b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense fistula track (white arrowhead) posterior to the anal canal (black arrowhead). The track crosses the sphincter muscles (white arrow in a) posteriorly. (c, d) Transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show the track (black arrow in c) first coursing laterodorsally from the left external sphincter muscles (arrowhead in c), then crossing the external sphincter at a midanal level (black arrow in d). A transsphincteric fistula track was diagnosed at MR imaging and confirmed at surgery. The internal opening was correctly predicted as being at the five o’clock position dorsally, at midanal level (white arrow in d). Note that the internal opening is not directly visualized but can be inferred only from the course and proximity of the fistula track in the sphincter muscle compartments. The arrowhead in d indicates the internal sphincter muscle.

View larger version (162K): [in a new window]   Figure 3b. Transsphincteric fistula. (a, b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense fistula track (white arrowhead) posterior to the anal canal (black arrowhead). The track crosses the sphincter muscles (white arrow in a) posteriorly. (c, d) Transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show the track (black arrow in c) first coursing laterodorsally from the left external sphincter muscles (arrowhead in c), then crossing the external sphincter at a midanal level (black arrow in d). A transsphincteric fistula track was diagnosed at MR imaging and confirmed at surgery. The internal opening was correctly predicted as being at the five o’clock position dorsally, at midanal level (white arrow in d). Note that the internal opening is not directly visualized but can be inferred only from the course and proximity of the fistula track in the sphincter muscle compartments. The arrowhead in d indicates the internal sphincter muscle.

View larger version (130K): [in a new window]   Figure 3c. Transsphincteric fistula. (a, b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense fistula track (white arrowhead) posterior to the anal canal (black arrowhead). The track crosses the sphincter muscles (white arrow in a) posteriorly. (c, d) Transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show the track (black arrow in c) first coursing laterodorsally from the left external sphincter muscles (arrowhead in c), then crossing the external sphincter at a midanal level (black arrow in d). A transsphincteric fistula track was diagnosed at MR imaging and confirmed at surgery. The internal opening was correctly predicted as being at the five o’clock position dorsally, at midanal level (white arrow in d). Note that the internal opening is not directly visualized but can be inferred only from the course and proximity of the fistula track in the sphincter muscle compartments. The arrowhead in d indicates the internal sphincter muscle.

View larger version (134K): [in a new window]   Figure 3d. Transsphincteric fistula. (a, b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense fistula track (white arrowhead) posterior to the anal canal (black arrowhead). The track crosses the sphincter muscles (white arrow in a) posteriorly. (c, d) Transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show the track (black arrow in c) first coursing laterodorsally from the left external sphincter muscles (arrowhead in c), then crossing the external sphincter at a midanal level (black arrow in d). A transsphincteric fistula track was diagnosed at MR imaging and confirmed at surgery. The internal opening was correctly predicted as being at the five o’clock position dorsally, at midanal level (white arrow in d). Note that the internal opening is not directly visualized but can be inferred only from the course and proximity of the fistula track in the sphincter muscle compartments. The arrowhead in d indicates the internal sphincter muscle.

View larger version (166K): [in a new window]   Figure 4a. Abscess and fistula track missed at surgery. (a) Coronal and (b) transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense lesion with a hypointense rim (arrowhead in a) above the pelvic floor, which suggests a small abscess in the supralevator space. Note the layering of debris within the fluid collection (arrow in b). Also note the hyperintense structure between the sphincter muscles (arrow in a) that extends toward the abscess collection. MR imaging was used to correctly predict the presence of a left intersphincteric blind-ending fistula track leading toward a supralevator abscess collection on the left side. Both the track and abscess were missed at initial surgical exploration because the patient was thought to have only a sinus on the contralateral side (not shown).

View larger version (183K): [in a new window]   Figure 4b. Abscess and fistula track missed at surgery. (a) Coronal and (b) transverse T2-weighted two-dimensional turbo SE (3,427/150) MR images show a hyperintense lesion with a hypointense rim (arrowhead in a) above the pelvic floor, which suggests a small abscess in the supralevator space. Note the layering of debris within the fluid collection (arrow in b). Also note the hyperintense structure between the sphincter muscles (arrow in a) that extends toward the abscess collection. MR imaging was used to correctly predict the presence of a left intersphincteric blind-ending fistula track leading toward a supralevator abscess collection on the left side. Both the track and abscess were missed at initial surgical exploration because the patient was thought to have only a sinus on the contralateral side (not shown).

View larger version (176K): [in a new window]   Figure 5a. Horseshoe fistula and rectal opening missed at surgery. (a-d) Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR images from posterior (a) to anterior (d) show two hyperintense fluid collections that cross the midline, one located infralevatorly (arrow in a and b) and the other at the level of the pelvic floor (arrowhead in a and b). A secondary track (arrow in c) ascends from the highest collection toward the rectal lumen. The rectal opening (arrowhead in d) is clearly visualized. MR imaging findings suggested the presence of two horseshoe fistulas at different levels, with connection to the rectal lumen. The surgeons identified the lower horseshoe fistulas but missed the higher extensions because the problem was thought to have been solved by draining the lower extension. Because of the additional MR imaging information, surgical exploration was extended, and the higher extensions were found and properly treated.

View larger version (173K): [in a new window]   Figure 5b. Horseshoe fistula and rectal opening missed at surgery. (a-d) Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR images from posterior (a) to anterior (d) show two hyperintense fluid collections that cross the midline, one located infralevatorly (arrow in a and b) and the other at the level of the pelvic floor (arrowhead in a and b). A secondary track (arrow in c) ascends from the highest collection toward the rectal lumen. The rectal opening (arrowhead in d) is clearly visualized. MR imaging findings suggested the presence of two horseshoe fistulas at different levels, with connection to the rectal lumen. The surgeons identified the lower horseshoe fistulas but missed the higher extensions because the problem was thought to have been solved by draining the lower extension. Because of the additional MR imaging information, surgical exploration was extended, and the higher extensions were found and properly treated.

View larger version (178K): [in a new window]   Figure 5c. Horseshoe fistula and rectal opening missed at surgery. (a-d) Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR images from posterior (a) to anterior (d) show two hyperintense fluid collections that cross the midline, one located infralevatorly (arrow in a and b) and the other at the level of the pelvic floor (arrowhead in a and b). A secondary track (arrow in c) ascends from the highest collection toward the rectal lumen. The rectal opening (arrowhead in d) is clearly visualized. MR imaging findings suggested the presence of two horseshoe fistulas at different levels, with connection to the rectal lumen. The surgeons identified the lower horseshoe fistulas but missed the higher extensions because the problem was thought to have been solved by draining the lower extension. Because of the additional MR imaging information, surgical exploration was extended, and the higher extensions were found and properly treated.

View larger version (178K): [in a new window]   Figure 5d. Horseshoe fistula and rectal opening missed at surgery. (a-d) Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR images from posterior (a) to anterior (d) show two hyperintense fluid collections that cross the midline, one located infralevatorly (arrow in a and b) and the other at the level of the pelvic floor (arrowhead in a and b). A secondary track (arrow in c) ascends from the highest collection toward the rectal lumen. The rectal opening (arrowhead in d) is clearly visualized. MR imaging findings suggested the presence of two horseshoe fistulas at different levels, with connection to the rectal lumen. The surgeons identified the lower horseshoe fistulas but missed the higher extensions because the problem was thought to have been solved by draining the lower extension. Because of the additional MR imaging information, surgical exploration was extended, and the higher extensions were found and properly treated.

View larger version (172K): [in a new window]   Figure 6a. (a) True-positive diagnosis of a fistula track. Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a hyperintense tubular structure in the left ischiorectal fossa that is surrounded by a hypointense rim (black arrow) and was correctly diagnosed as an extrasphincteric fistula track. The track ends in a small abscess (white arrow) in the left side of the pelvic floor. (b) False-positive diagnosis of a fistula track. Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a hypointense track (large arrow) that courses to the left sphincter muscle complex. This hypointense structure was mistaken for an inactive fistula track. At surgery, only fibrosis was found. Note the detailed anatomy of the anal sphincter complex: the external sphincter muscle (arrowhead a), the puborectalis muscle (arrowhead b), the pelvic floor muscles (arrowhead c), and the longitudinal muscle in the intersphincteric space (small arrow).

View larger version (165K): [in a new window]   Figure 6b. (a) True-positive diagnosis of a fistula track. Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a hyperintense tubular structure in the left ischiorectal fossa that is surrounded by a hypointense rim (black arrow) and was correctly diagnosed as an extrasphincteric fistula track. The track ends in a small abscess (white arrow) in the left side of the pelvic floor. (b) False-positive diagnosis of a fistula track. Coronal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a hypointense track (large arrow) that courses to the left sphincter muscle complex. This hypointense structure was mistaken for an inactive fistula track. At surgery, only fibrosis was found. Note the detailed anatomy of the anal sphincter complex: the external sphincter muscle (arrowhead a), the puborectalis muscle (arrowhead b), the pelvic floor muscles (arrowhead c), and the longitudinal muscle in the intersphincteric space (small arrow).

View larger version (171K): [in a new window]   Figure 7a. Intrapelvic disease in two patients with Crohn perianal fistulas. (a) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a presacral fluid collection (arrow) with connection to the bladder (not shown) and sigmoid colon (arrowhead) that was correctly diagnosed as a presacral abscess that drained to the bladder and sigmoid colon. (b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR image depicts a fistula track (arrowhead) that connects the rectal lumen (bullet arrow) with the vaginal lumen (arrow), which is suggestive of a rectovaginal fistula and was confirmed at surgery.

View larger version (153K): [in a new window]   Figure 7b. Intrapelvic disease in two patients with Crohn perianal fistulas. (a) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR image shows a presacral fluid collection (arrow) with connection to the bladder (not shown) and sigmoid colon (arrowhead) that was correctly diagnosed as a presacral abscess that drained to the bladder and sigmoid colon. (b) Sagittal T2-weighted two-dimensional turbo SE (3,427/150) MR image depicts a fistula track (arrowhead) that connects the rectal lumen (bullet arrow) with the vaginal lumen (arrow), which is suggestive of a rectovaginal fistula and was confirmed at surgery.