Perianal fistulization is the result of a chronic inflammation of the perianal tissues. A wide spectrum of clinical manifestations, ranging from simple to complex fistulas, can be seen, the latter especially in patients with Crohn disease. Failure to detect secondary tracks and hidden abscesses may lead to therapeutic failure, such as insufficient response to medical treatment and relapse after surgery. Currently, magnetic resonance (MR) imaging is the preferred technique for evaluating perianal fistulas and associated complications. Initially used most often in the preoperative setting, MR imaging now also plays an important role in evaluating the response to medical therapy.
Key points
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MR imaging has been shown to accurately show the anatomy of the perianal region.
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Currently, MR imaging is also a reliable technique to assess the outcome of medical therapy using the anti-TNF agent infliximab in patients with Crohn disease with fistula-in-ano occurring during the first year of follow-up.
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Another important advantage of MR imaging is the multiplanar assessment. However, imaging planes must be correctly aligned to the anal canal.
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Despite the closure of draining external orifices after infliximab therapy, fistula tracks persist with varying degrees of residual inflammation, which may cause recurrent fistulas and pelvic abscesses.
Introduction
Perianal fistulas are a major cause of morbidity. Fistulas are defined as an abnormal communication between 2 epithelium-lined surfaces. In the case of a perianal fistula, the connection is between the mucosal layer of the anal canal and the perianal skin.
Perianal fistulas predominantly affect young adults, especially men in their fourth decade.
Treatment of perianal fistulizing disease is medical or surgical. Patients with Crohn disease are first treated with antibiotics, immunosuppressive agents, or anti–tumor necrosis factor (anti-TNF) antibodies. Fistulas not related to Crohn disease are usually treated with surgery.
Recurrence after therapy is the most common problem. To avoid recurrence after medical or surgical therapy, detailed information must be obtained about the location of any fistula track and the affected pelvic structures. High-resolution magnetic resonance (MR) imaging allows precise assessment of the relationship of the fistula track to the pelvic floor structures, and identification of secondary fistulas or abscesses.
Introduction
Perianal fistulas are a major cause of morbidity. Fistulas are defined as an abnormal communication between 2 epithelium-lined surfaces. In the case of a perianal fistula, the connection is between the mucosal layer of the anal canal and the perianal skin.
Perianal fistulas predominantly affect young adults, especially men in their fourth decade.
Treatment of perianal fistulizing disease is medical or surgical. Patients with Crohn disease are first treated with antibiotics, immunosuppressive agents, or anti–tumor necrosis factor (anti-TNF) antibodies. Fistulas not related to Crohn disease are usually treated with surgery.
Recurrence after therapy is the most common problem. To avoid recurrence after medical or surgical therapy, detailed information must be obtained about the location of any fistula track and the affected pelvic structures. High-resolution magnetic resonance (MR) imaging allows precise assessment of the relationship of the fistula track to the pelvic floor structures, and identification of secondary fistulas or abscesses.
Normal anal canal anatomy
Underneath the mucosa, the anal canal consists of an internal layer of circular smooth muscle (the internal sphincter) and an outer striated muscle layer (the external sphincter). The 2 sphincters are separated by the intersphincteric space, which contains predominantly fat ( Fig. 1 ). This space forms a natural plane of lower resistance in which fistulas can easily spread. The external sphincter is surrounded by the fat-containing ischiorectal and ischioanal space.
The internal sphincter is continuous with the circular smooth muscle of the rectum. It is responsible for 85% of the anal resting tone. In most individuals, disruption of the sphincter will not cause loss of continence.
The external sphincter is continuous with the puborectal and levator ani muscles ( Fig. 2 ). It contributes only 15% of the anal resting tone, but its strong voluntary contractions resist defecation. A disruption of the external sphincter can lead to incontinence.
MR imaging has been shown to accurately show the anatomy of the perianal region. On axial T2-weighted images, the internal and external anal sphincter appear as circular structures with low signal intensity.
After intravenous administration of gadolinium, the internal and external sphincter can be easily distinguished on T1-weighted images by their different contrast enhancement. The internal sphincter muscle enhances to a higher degree than the external sphincter muscle ( Fig. 3 ).
Cause of perianal fistulas
In patients without Crohn disease, perianal fistulas usually arise from infected or obstructed intersphincteric anal glands (cryptogenic fistulas). The anal glands lie at the level of the dentate line in the mid-anal canal and can penetrate the internal sphincter toward the intersphincteric plane (intersphincteric fistula). From this space, the infection may track down the intersphincteric plane to the skin. Alternatively, infection may pass both layers of the anal sphincter to enter the ischiorectal space (transsphincteric fistula).
The cause of perianal fistulas in Crohn disease may be a fistula arising from inflamed or infected anal glands, and/or penetration of fissures or ulcers in the rectum or anal canal.
Diagnosis: accuracy and application of MR imaging
The use of MR imaging in the evaluation of perianal fistulas has been reported in many studies, showing it to be the preferred technique for preoperative evaluation of perianal fistulas and improved patient outcome. MR imaging has a high sensitivity and specificity in the detection of primary and secondary tracks, abscesses, and internal openings.
Endoscopic ultrasound can be equivalent to MR imaging in complementing examination under anesthesia. In clinical practice, MR imaging is used more frequently than endoscopic ultrasound. Endoscopic ultrasound is operator-dependent, and in patients with severe proctitis or anal strictures, its tolerability is suboptimal. Furthermore, the limited field of view is a considerable inconvenience, precluding the use of endoscopic ultrasound to assess suprasphincteric and extrasphincteric tracks or secondary extensions.
Several studies have reported that preoperative pelvic MR imaging findings change surgical management in 10% to 15% of patients, or reveal important additional information in 21% of patients, particularly those with Crohn disease.
In a larger study of patients with a recurrent anal fistula, the postoperative recurrence rate was as low as 16% when surgeons always acted based on the MR imaging findings. The rate of recurrence was 30% when surgeons occasionally acted based on MR imaging results, and 57% when MR imaging results were ignored.
Currently, MR imaging is also a reliable technique to assess the outcome of medical therapy using the anti-TNF agent infliximab in patients with Crohn disease with fistula-in-ano occurring during the first year of follow-up ( Fig. 4 ). This result was confirmed by 3 similar studies, and also by another study using endoscopic ultrasound.
However, in the long-term follow-up, the improvements observed at MR imaging correlate with the clinical and endoscopic response to infliximab in only half of the patients.
MR imaging protocol/technique
Various MR imaging techniques have been described.
Imaging Coils
Two types of coils can be used: the endoanal and the external phased array coils. Use of endoanal coils was initially proposed to improve MR imaging evaluation of perianal fistulas, but these coils are poorly tolerated in symptomatic patients.
Advantages of the external phased array coil include the larger field of view, which prevents fistula extensions from being overlooked, especially in patients with Crohn disease, and the wide availability of these coils. Furthermore, MR imaging with phased array surface coils requires no patient preparation and is well tolerated.
Imaging Sequences
On T2-weighted MR sequences, active fistulas and abscesses are hyperintense.
T1-weighted contrast-enhanced fat-suppressed MR imaging sequences are used to further improve the contrast of pelvic MR imaging and to distinguish inflamed tissue from normal perineal tissues. Furthermore, images with fat suppression better illustrate the activity of the fistulas.
A gadolinium-enhanced T1-weighted sequence is helpful in differentiating between fluid and granulation tissue, which is important in abscesses. Pus has high signal intensity on T2-weighted images, and thus cannot be reliably distinguished from edema and inflammation. The use of dynamic contrast-enhanced MR imaging for determining the degree of activity in perianal Crohn disease might be helpful in selecting a subpopulation of patients with perianal Crohn disease who should be monitored more closely for development of more extensive disease.
Another recent development is the introduction of 3.0-Tesla (T) imaging. 3.0-T imaging further improves spatial resolution and secondary diagnostic accuracy. The finer detail helps in detecting and characterizing even smaller fistula tracks. However, comparative studies with 1.5-T or 3.0-T have not been reported.
Use of diffusion-weighted sequences for evaluating perianal fistulas has been reported. Because inflammatory tissues usually have high signal intensity at diffusion-weighted imaging, this technique is used as an adjunct to T2-weighted imaging for diagnosing anal fistulas.
Imaging Planes
Another important advantage of MR imaging is the multiplanar assessment. However, imaging planes must be correctly aligned to the anal canal. Therefore, a sequence in the sagittal plane is first performed. The transverse and coronal sequences must be aligned with the anal canal at the sagittal sequence, oriented perpendicularly (transverse) and parallel (sagittal) to the long axis of the anal canal.
The specific protocol and sequence parameters applied at the authors’ institution are provided in detail in the first table: sagittal fat-suppressed T2-weighted turbo spin echo (TSE), axial fat-suppressed T2-weighted TSE, axial oblique T2-weighted TSE (short axis), coronal oblique T2-weighted TSE (long axis), and axial oblique and coronal oblique fat-suppressed T1-weighted TSE with gadolinium ( Table 1 ).
| Scan Parameters | Scan Name | |||||
|---|---|---|---|---|---|---|
| T2_TSE_sagittal_FS | T2_TSE_axial_FS | T2_TSE_axial oblique | T2_TSE_coronal oblique | T1_TSE_axial oblique_FS | T1_TSE_coronal oblique_FS | |
| Sequence type | Turbo spin echo (TSE) | Turbo spin echo (TSE) | Turbo spin echo (TSE) | Turbo spin echo (TSE) | Turbo spin echo (TSE) | Turbo spin echo (TSE) |
| Orientation | Sagittal | Axial | Axial oblique | Coronal oblique | Axial oblique | Coronal oblique |
| Number of slices | 20 | 30 | 20 | 20 | 26 | 20 |
| Slice thickness (mm) | 4 | 6 | 6 | 6 | 4 | 4 |
| Slice gap (mm) | 0.8 | 1.2 | 1.26 | 1.26 | 0 | 0 |
| Field of View (mm) | 259 × 360 | 230 × 320 | 230 × 320 | 270 × 320 | 309 × 380 | 380 × 380 |
| TR (ms) | 8870 | 6960 | 8870 | 8780 | 730 | 580 |
| TE (ms) | 134 | 134 | 134 | 134 | 11 | 11 |
| Number of averages | 3 | 4 | 3 | 2 | 3 | 3 |
| Fat suppression | Yes | Yes | No | No | Yes | Yes |
| Matrix | 368 × 512 | 368 × 512 | 368 × 512 | 432 × 512 | 416 × 512 | 512 × 512 |
| Pixel resolution (mm) | 0.7 × 0.7 × 4.0 | 0.6 × 0.6 × 6.0 | 0.6 × 0.6 × 6.0 | 0.6 × 0.6 × 6.0 | 0.7 × 0.7 × 4.0 | 0.7 × 0.7 × 4.0 |
| Acquisition time (min:s) | 1:39 | 3:16 | 1:30 | 1:38 | 5:48 | 5:39 |
| Extra information | Orientation axial to anal canal | Orientation coronal to anal canal | Acquired twice, once before and once after contrast administration, axial to anal canal | After contrast administration, coronal to anal canal | ||
| Less commonly mentioned, but can be added if desired | ||||||
| Parallel imaging | GRAPPA, factor 2 | GRAPPA, factor 2 | GRAPPA, factor 2 | GRAPPA, factor 2 | GRAPPA, factor 2 | GRAPPA, factor 2 |
| Bandwidth (Hz/Px) | 305 | 305 | 305 | 305 | 195 | 195 |
| Turbo factor | 61 | 61 | 61 | 61 | 3 | 3 |
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