MRI for assessment of anal fistula

The anal canal begins at the anal verge which corresponds to the lowermost portion of the external sphincter. The upper part of puborectalis muscle forms the radiologic upper boundary of the anal canal. Thus, the anus is the infralevator portion of the gastrointestinal tract, surrounded by the ischioanal fossa to each side. The ischioanal fossa is sometimes incorrectly termed the ischiorectal fossa.

The external sphincter muscle is a voluntary striated muscle, which continues 1.5–2 cm upward until it ends and the fibers of the puborectalis muscle continue as part of the pelvic floor [14]. For radiologic purposes, the levator ani muscle is the muscle that forms the pelvic floor.

The muscular propria layer, which covers the rectum, like most of the gastrointestinal tract, has two layers: the inner circular and the outer longitudinal layer. The circular is continuous with the internal sphincter muscle. This involuntary muscle provides 85% of the resting tone of the anus, the remainder is provided by the external sphincter. However, this muscle when contracted can prohibit defecation. Likewise, damage to this muscle can lead to fecal incontinence [15].

Almost mid-portion in the anal canal there is the dentate line. Here the anal glands extend to the intersphincteric plane. Three-fourths of the fistulas formed from this area extend through the intersphincteric plane to the skin. The dentate line is also where squamous epithelium meets columnar epithelium.

The anal clock is a transversal view of the canal, which corresponds to the radiologic view as well. At 12 o’clock, therefore, is the ventral portion of the anal canal, and at 3 o’clock is the left lateral part, and so on (Fig. 1).

The most common classification is that of Parks (Fig. 2), which is based on extensive study of 400 consecutive cases, many of which were more complex and severe than those seen at the practice of a general surgeon [16]. Parks described the primary tracts as following four patterns. The most common group is the intersphincteric type, where the primary track reaches the perianal skin through the intersphincteric plane (Fig. 3).

The next common type, or trans-sphincteric type, occurs when the track courses through the external sphincter muscle, usually involving the ischioanal fossa (Fig. 4). The external opening can be further away from the anus, meaning that the most distal part of the ischioanal fossa is involved. This should not necessarily mean that that the fistula has traversed the external sphincter. The level at which the external sphincter complex is traversed is most often at the mid-anal canal level and the internal opening is usually at around 6 o’clock, at the level of dentate line. This is, however, not invariable.

In the suprasphincteric fistula, in contrast to intersphincteric fistula, the fistula courses initially upward above the sphincter muscles (Fig. 5), and then coursing down to the perianal skin (Fig. 6).

The primary tracks described above can be complicated by secondary tracks. Supralevator, ischioanal and horseshoe extensions are common. Horseshoe extensions are circular extensions to both sides of the internal opening (Fig. 7). Any type of fistula may show a circumferential spread, but a typical horseshoe-shaped fistula has two tracts and one internal opening, often in the midline posteriorly at the level of the inferior border of the puborectalis muscle. Horseshoe fistulas can extend in the intersphincteric, ischioanal, or supralevator directions. A fistula medial to the levator plate or puborectalis muscle is supralevator, and a fistula lateral to these muscles is infralevator.

MRI has in several studies been shown to be highly predictive of patient outcome [17‐20]. At St James’s University Hospital the following MRI grading classification is used: 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. This MR grading has been shown to correlate with outcome: grades 1 and 2 are associated with favorable outcome (i.e., no recurrences and therefore no need for reoperations), while grades 3–5 are associated with less favorable outcome (leading to recurrences needing reoperations).

Any widening of the primary or the secondary tract could be considered an abscess. There is no clear-cut definition of when a fistula is large enough to be called an abscess; the arbitrary limit of 1 cm can be used.

Complex Crohn’s fistulas can consist of multiple tracts and abscesses [21]. A well-defined primary tract is often not recognizable. The tracts tend to be relatively large, often with extensions. The anal canal, the rectum, sigmoid colon, small bowel loops, and other pelvic organs maybe involved. The fistulas and abscesses can be located below, above or within the levator ani muscle. Unilateral thickening of the levator muscle may be reactive and is not always due to an intramuscular abscess. Proctitis and thickened perirectal fascia are commonly seen [22].

Fat-suppression should be employed if using turbo-spin-echo (TSE) T2-sequences, which could cause confusion as the high signal of the fistula could easily be missed due to high signal of surrounding fat. Therefore, most authors favor STIR (short-inversion-time inversion recovery) imaging since it combines fat-suppression and structural delineation [25]. With experience and time, however, TSE images are quite adequate, with the advantage of higher structural delineation. Therefore, we prefer, in contrast, TSE images for the delineation of fistulas. We have seen inexperienced radiologists interpret other high-signal findings due to fat for fistulas (Fig. 11).

Some normal pelvic structures have relatively high signal on STIR, such as the periprostatic or vaginal vessels and the internal sphincter. Asymmetry helps to distinguish between fistulas and normal higher signal intensity sometimes but not always. We use STIR in one plane only to demonstrate fluid channel more efficiently, but use TSE in three planes for adequate assessment of fistulas. These planes are the sagittal, semi-coronal (parallel to anal canal or perpendicular to pelvic floor) and perpendicular to anal canal. At least for one plane, we recommend imaging with a higher echo-time, which delineates fistulas very clearly. Another sequence that we have used recently is T2-weighted volume imaging with the capability of performing reformatted images. Though our studies are not finished yet, we believe that this single sequence can replace all T2-weighted imaging.

T1-weighted imaging may not always be necessary. It has, however, been sometimes beneficial to have these images [26]. Unenhanced T1-weighted imaging may help diagnose postoperative hemorrhage or fat-containing grafts. The tracks enhance vividly after gadolinium administration. Some normal structures (Fig. 12) and structures close to a fistula (Fig. 13) can cause confusion since they can have avid contrast enhancement. This could be at least partially explained by the timing of imaging (Fig. 14). We have not found any reports questioning the usefulness of postcontrast imaging, but only publications about more time-consuming measurements showing intensity curves correlating with degree of inflammation or, more commonly, patient’s symptoms. Those who have demonstrated the positive effect of contrast enhancement have usually used dynamic imaging looking at time intensity curves. There is a possibility that the timing of the contrast enhancement plays a role in the assessment of degree of inflammation (Fig. 14). Moreover, we are not aware of any studies evaluating the additive value of contrast enhancement to other sequences. Some radiologists, therefore, do not use contrast enhancement; whenever only surgery is planned, T2-weighted imaging is adequate. The parameters for T1-weighted imaging are very similar to other pelvic imaging protocols, with TR 9 ms, TE 4–5 ms, FOV 20–26 cm, matrix 256, flip angle 10, slice thickness 1 mm and number of slices 100. We use 3D-volume T1-weighted imaging with fat suppression (THRIVE in Philips, VIBE in Siemens and FAME in GE). Using volume imaging enables us to image in only one plane. Not all the authors use the same parameters, and there is no clear advantage of any of these. However, it seems prudent to use the same sequence before and after contrast enhancement for most reliable comparisons.

For T2-weighted imaging, most radiologists use a TSE or FSE (fast-spin-echo) T2 sequence. The parameters for this sequence are TR 3,000–4,000 ms, TE 120–150 ms, FOV 20–26 cm, slice thickness 3–5 mm, matrix 512, flip angle 90, number of slices 24 (for 4-mm slice thickness), and fold-over direction. Please note that the field of view does not need to be too large. Very seldom do fistulas extend beyond the true pelvis.

The location of external openings is of little importance, since both the patient and the surgeon can find it often easily themselves. Goodsall’s rule states that the external opening of a fistula situated behind the transverse anal line will open into the anal canal in the midline posteriorly. An anterior opening is usually associated with a radial tract, however. The exception to the rule is anterior fistulas lying more than 3 cm from the anus, which may have a curved track (similar to posterior fistulas) [27]. This rule is, however, not without many exceptions [28]. The internal opening will decide the extent of sphincter division during fistulotomy. Since the dentate line cannot be identified on MRI, its position must be inferred as the middle of the anal canal. If no internal opening can be detected, then a sinus should be diagnosed instead of a fistula.

Most authors use pelvic surface coils, the same that is used for almost all pelvic imaging. Endorectal and, more appropriately, endoanal probes are less commonly used. Though endoanal probes provide better depiction of internal opening than surface coils, their limited field of view is a problem (Fig. 15). Also, placement of these probes is not always easy or possible. Combining both coils probably provides the best diagnostic accuracy, yet is cumbersome to employ [29]. In our practice, we do not use anal probes for depiction of fistula anymore.

Final points: