AJR 2003; 181:1611-1613
© American Roentgen Ray Society
Digital Subtraction MR Fistulography: New Diagnostic Tool for the Detection of Fistula In Ano
Oliver Schaefer1,
Christian Lohrmann and
Mathias Langer
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).
Fistula in ano is a frequent disease that is often compromising for the
patient. Standard assessment of anal fistulas requires examination with the
patients under anesthesia [1].
Inadequate assessment of fistulas may result in simple fistulas being
converted into complex fistulas, and failure to recognize complex
ramifications can result in recurrent sepsis and an unnecessarily protracted
clinical course [2]. Pelvic MRI
is an accurate diagnostic tool for the detection of fistula in ano
[3]. The purpose of this
article is to present a new MRI protocol including a 3D contrast-enhanced,
high-resolution fast low-angle shot (FLASH) sequence in the axial plane and
digital image subtraction.
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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Fig. 1A. —38-year-old man with clinically diagnosed anal fistula.
Subtraction MR fistulogram detects translevator fistula (arrowhead)
with extension between puborectalis muscle and anal canal and tract in right
ischiorectal fossa (arrow).
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Fig. 1B. —38-year-old man with clinically diagnosed anal fistula.
Subtraction MR fistulogram reveals blind-ending intersphincteric fistula
(arrowhead) at 8-o'clock lithotomy position and secondary extension
of translevator fistula (arrow) with external opening adjacent to
natal cleft.
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Fig. 1D. —38-year-old man with clinically diagnosed anal fistula. Thin
coronal maximum-intensity-projection reconstruction of 3D FLASH sequence after
IV injection of gadobenate dimeglumine and image subtraction shows course of
translevator fistula in right ischiorectal fossa to perianal cutis (white
arrows) close to external sphincter muscle (asterisk). Fistula
traverses puborectalis muscle (arrowhead indicates right levator
muscle). Note blind-ending fistula extension (black arrow) into
intersphincteric space.
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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.
References
- Buchanan G, Halligan S, Williams A, et al. Effect of MRI on
clinical outcome of recurrent fistula in ano. Lancet2002; 360:1661
–1662[Medline]
- Chapple KS, Spencer JA, Windsor AC, Wilson D, Ward J, Ambrose NS.
Prognostic value of magnetic resonance imaging in the management of fistula in
ano. Dis Colon Rectum2000; 43:511
–516[Medline]
- Schwartz DA, Wiersema MJ, Dudiak KM, et al. A comparison of
endoscopic ultrasound, magnetic resonance imaging, and exam under anesthesia
for evaluation of Crohn's perianal fistulas.
Gastroenterology2001; 121:1064
–1072[Medline]
- Beets-Tan RG, Beets GL, van der Hoop AG, et al. Preoperative MR
imaging of anal fistulas: does it really help the surgeon?
Radiology2001; 218:75
–84[Abstract/Free Full Text]
- Hussain SM, Outwater EK, Joekes EC, et al. Clinical and MR imaging
features of cryptoglandular and Crohn's fistulas and abscesses.
Abdom Imaging2000; 25:67
–74[Medline]
- Spencer JA, Ward J, Beckingham IJ, Adams C, Ambrose NS. Dynamic
contrast-enhanced MR imaging of perianal fistulas. AJR1996; 167:735
–741[Abstract/Free Full Text]
- deSouza NM, Gilderdale DJ, Coutts GA, Puni R, Steiner RE. MRI of
fistula-in-ano: a comparison of endoanal coil with external phased array coil
techniques. J Comput Assist Tomogr1998; 22:357
–363[Medline]
- Scholefield JH, Berry DP, Armitage NC, Wastie ML. Magnetic
resonance imaging in the management of fistula in ano. Int J
Colorectal Dis 1997;12:276
–279[Medline]
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