H.Tayar*i, A.Daghfou*s, F.Jabnoun**, K.Bouzaid**i, L.Rezgui Marhou*l Radiology services Trauma center*, Tunisia Taher Maamouris Hospita**l, Nabeul GI27

Anal fistula is a benign condition but may cause considerable distress to the patient and difficulty for the surgeon. Fistulae are intimately related to the anal sphincter complex, so that incision and drainage may damage these muscles to avariable degree with the risk of anal incontinence. The correct balance between eradication of infection and maintenance of continence depends upon accurate pre-operative assessment of fistula geography, namely the site and level of any internal opening, the anatomy of the primary track and the presence of any secondary ramifications. These questions are best answered by MRI, which is more accurate than all other pre-operative investigations.

Illustrate the contribution of magnetic resonnance imaging in the diagnosis and assessement of anal fistulas for providing valuable assisstance in conducting surgical.

Retrospective study. The study population comprised teen adult patients complaining of anal fistula and whose all received a clinical examination by a surgeon and a pelvic MRI. The protocol includes T1 and T2 weighted sequences in three planes, a sequence of diffusion and T1 Fat Sat gadolinuim injection in three planes.

Average age: 38 years. Sex ratio: 6 men/4women. All patients were followed for crohns disease. Pelvic MRI has objectified 6 complex fistula and 4 cases of simple fistula. Collections were observed in 5 cases.

a b Simple linear intersphincteric fistula. Axial T2-weighted (a) and STIR images (b) show fistulous tracks in the intersphincteric plane ( ). Coronal T1-weighted postcontrast image at the same level (c) demonstrates hyperenhancement in the same region, representing inflammation ( ). c

Complex intersphincteric fistula with horseshoe track. 43-year-old man with complex fistulating Crohns disease. The intersphincteric fistulous track ( in axial T2 Weighteraand STIRb images) crosses the midline in the anterior interhemispheric space ( in coronal T2-Weighter imagesc) forming a horse-shoe track. a bc

de f Enhancement on contrast administration is noted in the three plans axial (d), coronal (e) et sagittal (f) T1-weighted postcontrast images ( ): ACTIVE FISTULA

a c b Simple transphincteric fistula 29-year-old woman with long-standing Crohns disease. (a) STIR image showing a transsphincteric fistula. ( ) (b) Axial and ( c) coronal Sagittal T1-weighted postcontrast images in the same patient demonstrates hyperenhancement along fistulous tract. ( )

ab c Trans-sphincteric complex fistula with abscess There are axial T2-Weighted images: The trans-sphincteric track is seen entering the anal canal at 6 o clock ( ). In addition, an abscess in the left ischioanal fossa is seen ( ).

de f Axial T1-weighted postcontrast image (d) in the same patient demonstrates hyperenhancement along a contiguous fistulous tract to the skin ( ). Axial and coronal T1-weighted postcontrast images (e-f) shows partial enhancement of rim ( ), indicating presence of fluid in center with rim of inflammatory tissue: abcesses.

Complex fistula and voluminous abcesses (a) Axial T2-weighted image shows large abscess extending into right gluteus and levator ani muscles.( ) (b) Axial fat-saturated T2-weighted image shows abscess (a) more clearly because bright signal of fat, in which abscess is located, is suppressed. ( ) (c ) T1-weighted image after administration of IV contrast medium clearly shows rim enhancement of lesions on right ( ), indicating presence of large amount of pus. a bc

(d) Coronal sequence shows the course of the fistula ( ) from the canal anal to the left levator ani muscle. d

Anal fistula is a common disease that has long challenged surgeons skills. Perianal fistula, if not treated properly will result in one of two terrible complications, recurrence or incontinence. The key to successful management of fistula-in-ano lies in correctly identifying the full extent of disease and its relationship to the sphincter complex. Its the role of Magnetic Resonnance Imaging. This exam is more sensitive than even surgical exploration of the tract.

MRI imaging of perianal fistulae relies on the inherent high soft tissue contrast resolution and the multiplanar display of anatomy by this modality. Its especially useful in patients with fistulae associated with Crohns disease and those with reccurent fistulae, as these entities are associated with branching fistulous tracts. Missed extensions are the commonest cause of recurrence.

T2W images (TSE and fat-suppressed) provide good contrast between the hyperintense fluid in the tract and the hypointense fibrous wall of the fistula, while providing good delineation of the layers of the anal sphincter. Gadolinuim-enhanced T1W images are useful to differentiate a fluid-filled tract from an area of inflammation. The tract wall enhances, whereas the central portion is hypointense. Abscesses are also very well depicted on post- gadolinuim images.

The exact location of the primary tract (ischioanal or intersphincteric) is most easily visualized on axial images. The presence of disruption of the external anal sphincter differenciates a transsphincteric fistula from an intersphincteric one. The internal opening of the fistula is also best seen in this plane. Coronal images depict the levator plane, thereby allowing differentiation of supralevator from infralevator infection. A combination of an axial and a longitudinal series (coronal, sagittal or radial) will provide all the necessary details.

MRI also allows to classify anal fistulas in five grades according to: JAMESS UNIVERSITY HOSPITAL MR IMAGING CLASSIFICATION OF PERIANAL FISTULAS Grade Description 0 Normal appearance 1 Simple linear intersphincteric fistula 2 Intersphincteric fistula with intersphincteric abscess or secondary fistulous track 3 Trans-sphincteric fistula 4 Trans-sphincteric fistula with abscess or secondary track within the ischioanal or ischiorectal fossa 5 Supralevator and translevator disease

Magnetic resonance imaging has become a powerful tool in the evaluation of anal anatomy. In patients with complex disease, MRI is an important adjunct in delineating disease location and extent, its relationship to sphincter muscles, and in planning management. MRI also plays an important role in evaluating the response to medical and surgical therapies.