• MR: TECHNICAL AND  IMAGING CHARACTERISTICS

Magnetic field strength does not seem to be critical for results.  Although a magnetic ressonance unit with greater number of Teslas could be better to discriminate the anatomy from pathological findings,  it has not proved a real benefit in diagnosis, so a field of 1.5 Teslas is adequate for diagnosis.

The small size of the lesser pelvis requires the use of a surface coil, which increases the signal to noise-ratio and resolution space.
In our centre, we use two surface coils (phased array) in the pubic region above and below the patient and both oriented with a longitudinal axis parallel to magnetic field. This preparation is well tolerated, and allows greater duration of the sequences to obtain a better image quality. It uses a field of view (FOV) of 180 to 200 mm and a slice thickness of 1-3 mm, which includes the levator ani and presacral space. The use of anal endoluminal coil provides high spatial resolution with smaller field of view. Nevertheless, it has limited use due to rectal preparation requirement, and intolerance in patients with symptomatic stenosis. Many articles have compared the images obtained with endorectal and surface coil, but they have not found great differences, despite the smaller FOV in the first one.

MRI examination is performed in the three spacial planes, and coronal and axial planes are tilted with the anal canal axis. For studying fistulas, axial plane is the best to assess the internal orifice in intersphincteric fistula type. Coronal images show accurately the distance between the anal margin and internal openings. The levator ani is also best evaluated in coronal images. Thus, this plane  helps in the surgical planning for transsphincteric, extrasphincteric and suprasphincteric fistulas.

MRI protocol is based primarily on T2 turbo spin-echo weighted images, allowing optimum contrast between different sphincter components, fistulas and scars. Internal sphincter has a homogeneous morphology and it is slightly hyperintense regarding the external sphincter. Intersphincteric space shows a signal similar to the fat in ischiorectal fossa, but is so fine that cannot be displayed unless it is occupied by a fistula. External anal sphincter, puborectalis muscle and levator ani are relatively hypointense.
Similarly, we can distinguish the hypointensity of fibrotic wall in the fistulas with respect to the high signal  of the internal fluid. In contrast, T2-weighted image with fat suppression in the axial plane allows better visibility of fluid and inflammatory changes in the periphery of the fistula.
Finally, a 3D T1 sequence with fat suppression after administration of contrast material is useful for differentiating between inflammatory changes and an abscess. It should be noted that structures of anorectal ring, not substantially enhance after contrast administration except for internal anal sphincter and blood vessels.

In conclusion, fat suppression techniques and intravenous contrast administration increase discrimination of small tracts and possible complications such as multiple tracts, abscesses and / or adjacent organ involvement. Fistulas are more easily identified in fat suppression images, whereas T2 sequence allows us to evaluate the relation between the fistula and the anatomical structures that surround it. Furthermore, administration of endovenous contrast allows better discrimination of postoperative seton.

Fig. 6: MR Imaging protocol in our hospital.

We enclose examples of patients with Crohn's disease and perianal disease at our center in the last five years, as an example to characterize perianal lesions with MR imaging. (Fig. 8), ( Fig. 9), ( Fig. 10), ( Fig. 11), ( Fig. 12), ( Fig. 13), ( Fig. 14), ( Fig. 15), and ( Fig. 16)

• REPORT CONTENTS

A MRI report should help characterize perianal disease deciding the optimal treatment, and assessing the magnitude and characteristics of recurrences after treatment.
Description of findings in this disease is complicated and can be difficult to classify and summarize. So we will try to outline the key points that must be contained in a MRI report in order to be complete and understandable for surgeons and clinicians.
1. Primary tracts should be reported as single or multiple fistulas, as complete or incomplete (Sinus tract) when external orifice is absent.
2. Determining location of tracts, localizing and describing the internal orifice, the fistula course and the external orifice. In this term,  we must use the nomenclature of surgeons, consisting in a spherical orientation according to the anal clock and in classic lithotomy position.

Fig. 7: Anal clock.

A description in quadrants is an alternative.

3. Determining the thickness and length of the fistula, and  the distance to the anal margin.
4. Reporting the degree of complexity, describing extensions or recurrent fistulas. In case of multiple tracts, it is important to verify if there is any communication between the tracts describing the location.  Closed or fibrotic tracts should also be described.
5. Alluding or classifying according to Parks’ classification (in relation to anal sphincters). In this term, it is important to remember that abscesses and fistulas located above the levator ani muscles are much more difficult to detect clinically and they have a bad access prior to the surgery.
6. Presence of abscesses and their spread to adjacent organs  should be mentioned.

Finally, we must know some pelvic structures in order to avoid common mistakes and to improve the report. At this point we mention perianal veins, the pilonidal sinus and hemorrhoids. Veins are thin-walled , tortuous and symmetrical structures; the pilonidal sinus occupies the presacral space and does not extend into intersphincteric space; Finally, hemorrhoids can simulate small submucosal fluid collections, but are easily diagnosed on clinical examination.

• MEDICAL TREATMENT

Simple perianal fistulas

It is important to know if they are symptomatic, in absence of symptoms we should not do anything. Antibiotic is the first choice in medical therapy, the second option is azathioprine/6-mercaptopurine, and as third treatment is used infliximab (with lower evidence). When a perianal fistula in Crohn's disease is symptomatic, a combined medical and surgical strategy is recommended. Pain in patients with a unique fistula should raise the suspicion of a complication, mainly  an abscess emergence, and therefore it should be excluded with MRI or endoanal ultrasound. If  an abscess is diagnosed, it should be drained urgently and represents a surgical emergency.

Complex perianal disease

The lack of relevant clinical trials does not allow a clear consensus. I

As a first treatment, most groups use the same type of options like in the simple perianal fistulas. But in complex disease the identification and drainage of abscesses are vital. Antibiotics should represent the first-line therapy, but always accompanied by surgical drainage and AZA (azathioprine) as maintenance treatment.

The threshold for using biological treatments varies. Some groups consider these treatments as first line use in the treatment of perianal complex disease, but a combination of anti-TNF (tumor necrosis factor) and surgical treatment is the most preferred option. Although the results obtained with the anti-TNF therapy are better than any other treatment, it is evidenced that this therapy only heals fistulas in a small minority. In this point, MRI can assess post-treatment changes and has come to demonstrate: when treatment is stopped some tracts could persist and supurate again.
In conclusion, the two main anti-TNF drugs are infliximab or adalimumab and should be reserved as a second line of medical treatment.

Tacrolimus and cyclosporine can also be used in selected cases.

• SURGICAL TREATMENT

Sometimes surgery is necessary for simple fistulas but is always necessary for complex perianal disease. Drainage of abscesses and placing a seton are included. Fistulectomy and fistulotomy should be carried out selectively due to risk of postsurgical incontinence if external sphincter is injured. Finally, severe refractory disease can require proctectomy and discharge stoma. There are no controlled trials suggesting that local injection of infliximab can close fistulas and may be beneficial for nonresponders or intolerant to intravenous drugs.
During the last five years several small cohort studies have shown that the combination of seton placiement and infliximab exceeds the use of any of these strategies alone, probably due to a better drainage of the abscesses and fistulas.

In conclusion, the absence of draining a septic perianal process may injure any sphincteric perianal structures and therefore optimum management of Crohn’s disease should include colorectal surgeons, gastroenterologists and radiologists.