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Technical Innovation |
1 All authors: Department of Radiology, University Hospital of Freiburg, Hustetter Str. 55, Freiburg i. Br. 79106, Germany.
Received February 20, 2003;
accepted after revision June 6, 2003.
Address correspondence to O. Schaefer
(schaefer{at}mrs1.ukl.uni-freiburg.de).
The study was approved by the local institutional review board. Over a period of 18 months between 2001 and 2002, 36 patients with the clinical diagnosis of anal fistula or abscess who were scheduled for surgical exploration underwent subtraction MR fistulograpghy.
Pelvic MRI was performed on a 1.5-T unit (Symphony, Siemens, Erlangen, Germany) with a phased array body coil. All patients were placed in a supine head-first position with the coil centered on the hip joints. No bowel preparation or catheterization of the anal canal or fistula was required. Subtraction MR fistulography is based on abnormal enhancement of the inflamed fibrous walls of fistulas or abscesses after IV administration of gadobenate dimeglumine (Gd-BOPTA) on T1-weighted images. Therefore, a high-resolution, fat-saturated T1-weighted gradient-echo sequence was obtained in the axial plane before and after IV administration of the contrast agent (3D FLASH; TR/TE, 7/2.7; flip angle, 45°; field of view, 280 mm; image matrix, 282 x 512; slice thickness, 2 mm; slab thickness, 140 mm; acquisition time, 2 min 32 sec). Each patient received 0.15 mmol/kg of Gd-BOPTA (Multihance, Bracco-Altana, Konstanz, Germany) as the standard dose, which was administered via an injection system (Spectris, Medrad, Pittsburgh, PA) at a rate of 1 mL/sec.
To optimize visualization of fistulas and abscesses in the pelvirectal and perianal region, we performed fat subtraction of the unenhanced from the contrast-enhanced sequences. For exact planning of the T1-weighted slab, which covers the levator muscle, the anal canal, the sphincter muscles, and the ischiorectal fossa, and to evaluate for presence of supralevator disease, we began the examination with a STIR sequence in the coronal plane (5289/60; flip angle, 180°; field of view, 250 mm; image matrix, 264 x 512; slice thickness, 4 mm; acquisition time, 4 min 53 sec).
The examination protocol was defined as subtraction MR fistulography because image subtraction resulted in visualization of fistulas as high-signal tubular structures containing varying degrees of low-signal fluid. The surrounding fat appeared dark on the images. Because of the 3D data set, we could perform multiplanar reconstruction (MPR) and maximum intensity projection (MIP), thereby generating an image impression similar to fistulography. Fluid-filled fistulas appeared as hyperintense tracts adjacent to the anal canal on STIR images. All patients tolerated subtraction MR fistulography, and there were no motion artifacts in the whole series, with consistently excellent image quality.
We present imaging findings of a 38-year-old man with the clinical diagnosis of anal fistula.
Subtraction MR fistulography of this patient revealed a translevatoric fistula with a blind-ending intersphincteric tract and a course in the right ischiorectal fossa with a secondary extension and two external openings (Figs. 1A, 1B, 1C, 1D). The imaging findings were confirmed at surgery.
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The diagnostic assessment of patients with fistula in ano is challenging even for specialized colorectal surgeons. One reason is the inability to directly visualize fistulas and abscesses. A number of reports [3–8] concerning pelvic MRI reflect its value in diagnosing fistula in ano. In the recent literature, MRI protocols for "anal fistula" consist of spin-echo, turbo spin-echo, STIR, and gradient-echo sequences. The commonly used image matrix is 166–205 x 256. The slice thickness of the sequences varies between 3 and 8 mm. In one study, gadoteridol was used as the IV contrast agent; in other studies a standard dose of 0.1 mmol/kg of gadopentetate dimeglumine was injected. In just a few reports, a dynamic contrast-enhanced imaging protocol was performed without fat saturation [3–8]. High-resolution imaging was presented in this report.
For exact planning of the axial 3D FLASH sequence with a slice thickness of 2 mm, we preferred a coronal STIR sequence, which enabled identification of the levator plate and abnormal fluid collections in relationship to the sphincter muscles, anal canal, and rectum. Image subtraction allowed us to rule out the disturbing fat signal superior relative to the fat saturation in our series and therefore facilitated detection of abnormal contrast enhancement surrounding fistulous tracks and abscesses. We believe that the presented MRI protocol is time-saving and provides excellent image quality. Three-dimensional gradient-echo imaging offers the possibility of image reconstruction like MPR and MIP, providing additional information from one data set. In addition, with the introduction of Gd-BOPTA, the internal sphincter muscle enhances to a higher degree than the external sphincter muscle. Therefore, the delineation of intersphincteric fistulous tracts and horseshoe extensions in the intersphincteric space may be improved.
In conclusion, high-resolution indirect MR fistulography may be an important complement to surgical exploration. The MRI protocol we presented is reproducible, multifunctional, and time-saving. It has a high acceptance among surgeons and gastroenterologists and is well tolerated by the patients.
Subtraction MR fistulography is routinely performed in all patients who are sent to our department with the clinical suspicion of perianal fistula and abscess before therapy is initiated.
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