This study was a prospective study, approved by the ethics committee at our institute during the period from august 2019 to august 2021. The findings were correlated with operative findings (considered as the standard reference).

Patients

It included 120 patients who were referred from the anorectal surgery department and subjected to endoanal ultrasound and MRI fistulography prior to surgery.

Inclusion criteria

Patient presented with clinically diagnosed perianal fistula.

Exclusion criteria

Patients that didn’t proceed to operative intervention thus lacked operative confirmation of the findings, patient with general contraindications to MRI examination as (claustrophobic patients, those with cardiac pacemaker or metallic foreign body in their eye or aneurysm clips in their brain). About 20 patients were excluded.

Procedure

Informed consent was obtained from the all patients before they were subjected to the following.

History taking

Detailed history taking was obtained; every patient had to answer several questions as the following:

  • Symptoms of perianal discharge, pain, discomfort.

  • Possible risk factors as recent operation or previous perianal fistula.

Endoanal ultrasound

Technique

EAUS was performed by bK medial systems scanner 1202 (BK, Herlev, Denmark) with a model 2052 transducer equipped with automated multifrequency crystals (11.9 MHz), with 360 mechanical rotation, Fractional bandwith 96.2 % and stainless steel reflector. All patients were evaluated in the lateral decubitus position. The transducer is inserted within the anal canal after being coated with a condom and properly lubricated. No bowel preparation or sedation were required.

The transducer was advanced till the U-shaped sling of the puborectalis, then automatically withdrawn to the superficial perianal plane.

The transducer was positioned so that the anterior aspect of the anal canal is superior on the screen at the 12-o’clock position; the left aspect at 3 o’clock, the posterior aspect at 6 o’clock, and the right aspect at 9 o’clock. Three scan planes were acquired:

  1. 1.

    The deep plane represents the anal canal’s upper third, where the hyperechoic puborectalis muscle has a distinctive U-shaped sling appearance.

  2. 2.

    The iso to hyperechoic external anal sphincter (EAS) and inner hypoechoic internal anal sphincter (IAS), as well as the transverse perineal muscle marks the intermediate plane.

  3. 3.

    The hyperechoic layer of the subcutaneous section of the external anal sphincter marks the superficial plane, which represented the lower extremity of the anal canal.

Post processing and image analysis

Ultrasound was performed by two radiologists (the first with 10 years of experience and the second with 6 years of experience) in the same setting and the final diagnosis was reached by their agreement (in consensus). Both radiologists were blinded to the results of the MRI.

Further 3D processing of the images was done using B.K 3D viewer software version 7.0.0.519.

The primary tract appears as a hypoechoic tract/band passing adjacent or through to the external sphincter. Its internal opening is identified as the site of the interruption of the hypoechogenic texture of the internal sphincter (Fig. 1).

Fig. 1
figure 1

a, b MRI axial T2 and STIR WI images showing two fluid filled intersphincteric fistulous tracts eliciting high signal and abutting the internal sphincter at 1 o’clock (yellow arrows) and 6–7 o’clock (red arrows). c Coronal T2 MRI WI images showing left intersphincteric fluid filled fistulous tract (blue arrow). d Ultrasound images show a hypoechoic fistulous tract seen within the left anterior intersphincteric space abutting the internal anal sphincter (internal opening) opposite 1 o’clock (yellow arrow). e Another hypoechoic fistulous tract seen within the right posterior intersphincteric space abutting the internal sphincter opposite 6–7 o’clock (Red arrow). (f) Ultrasound coronal images show left short intersphincteric fistulous tract breaching the internal sphincter at the level of the superficial portion of the external sphincter (blue arrow)

The anatomy of the fistula was identified considering:

  • The primary tract.

  • Location of the fistula in relation to the sphincters.

  • The clock position of the internal opening.

  • Secondary extensions and complications (abscess or collections).

MRI examination

MRI was performed using 1.5 Tesla magnet scanners by two devices (Intera and Achieva, Philips medical system). All patients were examined in the supine position using a phased array surface coil. Total study time ranged from 20 to 30 minutes. No sedation was used.

MRI protocol

Different MRI techniques have been utilised in the imaging of perianal fistula. All provide multiplanar high resolution images of the perianal region in surgically relevant planes. Thus, the most important aspect of image acquisition is related to obtaining axial and coronal oblique images, which are orthogonal and parallel to the anal canal as the anal canal is oriented at an angle of about 45o anteriorly in the sagittal plane. To achieve such orthogonal orientation of the anal canal, sagittal fast spin-echo (FSE) T2W sequences should be performed as a localiser scan, providing an overview of the pelvis and displaying the correct axis of imaging. Imaging were done in coronal, axial and sagittal projections using T1 and T2 and T2 STIR sequence parameters, with a field of vision (FOV) of 450, a 4 mm slice thickness, a 196×256 matrix, and 2 mm interslice gap. To delineate the anal canal and separate the mucosal walls, a small enema tip was used for distension of the lower rectum, air was administered through this enema tip (however it was not applicable in all patients).

If intravenous contrast is used, suppression of background fat with SPIR (spectral presaturation with inversion recovery).Images post contrast are obtained in axial and coronal planes with FOV of 350, 1 mm slice thickness and 0.48 mm interslice gap. Such parameters are those used in elkasr alainy that we apply for all patient (Table 1).

Table 1 Parameters of sequences used in MRI scans of anal fistulas

MRI images interpretation

The MR images were evaluated by two radiologists (the first with 10 years of experience and the second with 6 years of experience) in the same setting and the final diagnosis was reached by their agreement (in consensus). Both radiologists were blinded to the results of the ultrasound. The anatomy of the fistula was identified considering:

  • The primary tracts: were classified as fluid-filled if they had low signal intensity on T1 weighted images and high signal intensity on T2 and STIR images, and as fibrotic if they elicit low signal intensity on all three weighted images. Inflamed granulation tissue in the walls of abscess and fistulas enhances whereas chronic fistulas usually do not.

  • Location of the fistula in relation to the sphincters.

  • The clock position of the internal opening.

  • Secondary extensions and complications (abscess or collections) seen eliciting low T1, high T2 and STIR signal with post contrast enhancement in case of abscess.

Surgical producers

The operative technique is chosen according to the fistula tract and its relation to the anal sphincter. The surgical techniques are as follows:

Fistulectomy and fistulotomy

Fistulectomy is recommended for low anal fistulas, as the success rate is high with this procedure and with minimal risk of incontinence. A tract is cannulated and excised in to leave the wound to heal by secondary intention or closed primarily.

Seton drainage

Placement of a seton drain is another frequently employed technique in anal fistula surgery. The material used is either a strong braided non-resorbable suture or a plastic (vessel loop, etc.) suture thread.

Anodermal advancement flap

Another option for covering the inner fistula cavity is the use of anodermal flaps. Advancement flaps consist of mucosa, submucosa, and part of the internal sphincter. The flap is lifted, edge of the flap containing internal opening. The underlying fistulous tract is excised up to the level of the internal sphincter. Here, the tract is transfixed. The flap is then advanced and sutured to close the internal defect. The outer part of the track can be curetted.

Statistical analysis

The statistical tool SPSS (Statistical Package for the Social Sciences) version 26 was used to code and enter the data (IBM Corp., Armonk, NY, USA).In quantitative data, the mean, standard deviation, minimum, and maximum were used to summarise the data, while categorical data was summarised using frequency (count) and relative frequency (%). Cohen test was used to assess the agreement between EAUS and MR and expressed as k values. Cohen suggested the Kappa result be interpreted as follows: values ≤ 0 as indicating no agreement and 0.01–0.20 as none to slight, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1.00 as almost perfect agreement. A statistically significant P value was less than 0.05.

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