Process to determine the level of the primary internal opening of anal fistula on magnetic resonance imaging

Pankaj Garg; Baljit Kaur; G. Mahak

doi:10.3393/ac.2024.00948.0135

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Technical Note Anorectal benign disease Process to determine the level of the primary internal opening of anal fistula on magnetic resonance imaging: Pankaj Garg¹, Baljit Kaur², G. Mahak¹; Annals of Coloproctology 2025;41(3):253-258.
DOI: https://doi.org/10.3393/ac.2024.00948.0135
Published online: June 24, 2025

¹Garg Fistula Research Institute, Panchkula, India

²Department of Radiology, SSRD MRI Centre, Chandigarh, India

Correspondence to: Pankaj Garg, MS, FASCRS Garg Fistula Research Institute, 1042, Sector-15, Panchkula 134113, India Email: drgargpankaj@yahoo.com

• Received: December 17, 2024 • Revised: April 23, 2025 • Accepted: April 27, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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INTRODUCTION
TECHNIQUE
DISCUSSION
ARTICLE INFORMATION
REFERENCES

INTRODUCTION

Anal fistula is a challenging condition to manage [1]. The fistula traverses 2 circular muscle layers: the internal anal sphincter (IAS) and the external anal sphincter (EAS). Surgical management of fistulas always entails some risk to these sphincters, which can result in deterioration of continence [2]. Therefore, to prevent sphincter injury and ensure proper management, accurately locating the primary internal opening is essential [3]. Failure to accurately identify this opening has been associated with a particularly high risk of fistula recurrence [3, 4].

Perianal abscesses and fistulas are presumed to originate from the anal glands [5, 6]. These glands are located at the base of the columns of Morgagni and, via their ducts, open into the crypts at the dentate (pectinate) line of the anal canal [7]. From these openings, the gland ducts enter the submucosa, where approximately two-thirds extend into the IAS and about half terminate in the intersphincteric space [8]. When a duct becomes obstructed, perianal abscess formation and/or anal fistula development can occur. Since the anal glands open at the dentate line, the primary internal opening of a cryptoglandular anal fistula is also situated at this line [9].

The anatomical anal canal is defined as the space between the dentate line and the anal verge. However, in clinical practice, the term “surgical anal canal” is more commonly used. This refers to the segment of bowel between the anorectal sling and the anal verge [9–11]. Although these landmarks are palpable and visible on clinical examination, they are not readily distinguishable on magnetic resonance imaging (MRI). Consequently, data on the boundaries and extent of the surgical anal canal on MRI are scarce. The relationship between the primary internal opening and the dentate line is of critical importance for the operating surgeon. However, the dentate line is a visual landmark in the anal mucosa, identified intraoperatively, and cannot be visualized on endoanal ultrasound or MRI. Consequently, radiology reports typically omit any reference to the dentate line and its relationship to the primary internal opening.

To address this gap, a multidisciplinary team—consisting of a surgeon, a radiologist, and a data scientist—with extensive expertise in anal fistula management [12–14] and MRI evaluation (BK and PG have each reviewed more than 8,000 anal fistula MRIs over the past 20 years) [15] developed a standardized method for determining the level of the internal opening and the dentate line on MRI.

It is important to identify and differentiate the IAS and EAS on MRI. The IAS is composed of smooth muscle and represents a thickened extension of the rectum’s inner circular layer, whereas the EAS is a skeletal muscle structure that represents a downward extension of the levator ani. Anatomically, the IAS terminates just proximal to the lower part of the EAS (Fig. 1), and the subcutaneous portion of the EAS curves around the distal aspect of the IAS, ending inferior to its lower edge (Figs. 1, 2). On axial T2-weighted images, the IAS appears moderately hyperintense and homogeneous. The EAS is relatively hypointense (darker) with a striated appearance, thus appearing less homogeneous than the IAS. This differentiation becomes more challenging on short tau inversion recovery (STIR) sequences, where fat and soft tissue signals are suppressed. Following intravenous contrast administration, the IAS demonstrates greater enhancement than the EAS, reflecting its richer blood supply.

TECHNIQUE

Calculation of the level of the primary internal opening on MRI

Step 1: Understanding and utilizing biplanar mode on MRI

MRI images are acquired in 3 orthogonal planes: axial (transverse), coronal, and sagittal. However, most MRI analysis applications also provide biplanar (2 sections) or multiplanar (3 sections) viewing modes (Fig. 3). These modes enable simultaneous assessment of 2 planes and facilitate correlation between them. For example, when axial and coronal sections are displayed together, clicking on the axial image projects a horizontal line onto the coronal view, indicating the height of that axial slice. Conversely, selecting a point in the coronal view draws a corresponding horizontal line on the axial image, marking the anteroposterior position of the coronal slice (Fig. 3A).

This capability can also be used to identify the coronal slice at the exact midpoint of the anal canal. In biplanar mode, with both coronal and axial views displayed, clicking on the coronal section will project its position as a horizontal line on the axial view. Then, the coronal section can be slid anteriorly or posteriorly until that horizontal line bisects the anal canal into anterior and posterior halves (Fig. 1). At this position, the coronal slice corresponds precisely to the center of the anal canal (Fig. 3A).

Step 2: Normal anatomy of the anorectum on MRI

On MRI, 2 key landmarks must be identified: the anorectal sling and the anal verge. The distance between these points defines the length of the surgical anal canal [9].

The anorectal sling appears on MRI as the inferior border of the levator ani muscle (Figs. 1C, 2). The anal verge corresponds to the medial termination of the subcutaneous portion of the EAS (Figs. 1C, 2). This subcutaneous EAS descends lateral to the IAS, curves around its inferior border (Fig. 2), and ends a few millimeters medial to the IAS. The point of its termination defines the anal verge (Figs. 1, 2).

Step 3: Measuring the length of the surgical anal canal

After identifying the anorectal sling and the anal verge, the distance between these landmarks on MRI can be measured using the scale tool in RadiAnt DICOM Viewer (Medixant) (Figs. 1, 4).

Step 4: Calculating the level of the dentate line and primary internal opening

In biplanar mode, an axial section should be displayed on one side (Fig. 4C) and the corresponding coronal section on the other (Fig. 4D). First, the axial slice is selected at the level of the primary internal opening (Fig. 4C). This action highlights the level of the opening on the coronal section (yellow line in Fig. 4D). Next, on the coronal image, the boundaries of the surgical anal canal are delineated by drawing 2 parallel horizontal lines at the anorectal sling and the anal verge (red horizontal lines in Fig. 4D). Finally, a vertical line is drawn along the length of the surgical anal canal (red vertical line in Fig. 4D).

The intersection of the horizontal yellow line (marking the level of the internal opening) and the vertical red axis of the surgical anal canal precisely locates the primary internal opening in relation to the surgical anal canal. The distance from the intersection to the anal verge (the lower red horizontal line) represents the height of the opening above the verge. Dividing this value by the total length of the surgical anal canal (the red vertical line) yields the relative position of the primary opening within the canal (Fig. 4D).

Step 5: Reporting the level of the primary internal opening

Once the opening’s position has been determined, it can be reported either in relation to the length of the surgical anal canal or relative to the dentate line. To express its relation to the dentate line, the following method is employed.

The dentate line is an anatomical landmark on the mucosal surface. Although visible intraoperatively, it cannot be seen on MRI. Anatomically, it lies approximately 25% to 33% up the surgical anal canal—that is, at the junction of the lower one-third and upper two-thirds [9], a position previously confirmed by intraoperative assessment of several hundred patients [15]. Therefore, on MRI the dentate-line region is defined as the segment spanning 25% to 33% of the canal length measured from the anal verge. The level of the primary internal opening is then evaluated relative to this dentate-line zone.

If the measurement falls between 25% and 33% of the surgical anal canal length, the primary internal opening is considered to be at the level of the dentate line. If it is less than 25%, the primary opening lies below the dentate line. If it is greater than 33%, the primary opening lies above the dentate line.

The sphincter is shorter in the anterior direction, especially in women. Therefore, in addition to the above calculation, the sagittal view is also helpful for determining the height of the internal opening and the point at which the fistula tract penetrates the external sphincter. Similarly, in patients with previous surgery, resultant fibrosis should be considered when calculating the opening’s height. In the present study, patients who had undergone any prior proctological procedure were excluded.

MRI imaging protocol

All MRI examinations were acquired on the same 1.5-Tesla scanner (Achieva, Philips Medical Systems) using a phased-array 4-channel SENSE body coil (Philips Healthcare). First, the long axis of the anal canal was identified using a midline sagittal localizing sequence. Next, transverse and coronal STIR sequences were determined for this axis. Additional axial T1-weighted and coronal T2-weighted images along the canal axis were obtained for all patients. Sagittal T1-weighted images were acquired in approximately 15% of cases, when the STIR sequences suggested presacral extension. No intravenous contrast, endorectal or endoanal receiver coil, or 3-dimensional reconstruction was used. Established MRI criteria for fistula-in-ano diagnosis were applied, and the radiologist recorded the presumed internal opening (location and height), number of fistula tracts, associated findings, and fistula classification per existing systems.

DISCUSSION

This is the first report to describe a method for assessing the level (or height) of the primary internal opening and evaluating its relationship to both the surgical anal canal and the dentate line. This approach is particularly useful for surgeons who may not be fully experienced in MRI interpretation but who need to determine the level of the primary opening. Further prospective studies are required to corroborate these findings.

ARTICLE INFORMATION

Conflict of interest

No potential conflict of interest relevant to this article was reported.

Funding

None.

Author contributions

Conceptualization: all authors; Methodology: PG, BK; Visualization: PG, GM; Writing–original draft: all authors; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Fig. 1.

Schematic diagram. (A, B) Normal anatomy. (C) The level of the anorectal sling and anal verge, with the surgical anal canal extending between these landmarks.

Fig. 2.

Coronoal view. (A) Schematic diagram. (B) On a magnetic resonance imaging (MRI) scan. LA, levator ani muscle; AS, anorectal sling; C, the subcutaneous portion of the external anal sphincter, which curves around the inferior border of the internal anal sphincter (its medial edge marks the anal verge on MRI); AV, anal verge.

Fig. 3.

Magnetic resonance images showing (A) biplanar mode (axial and coronal planes in 1 frame) and (B) multiplanar mode (axial, coronal, and sagittal planes in 1 frame). STIR, short tau inversion recovery.

Fig. 4.

A 35-year-old man with multiple posterior fistula tracts and a primary internal opening in the mid-anal canal at the posterior midline. Schematic diagrams of (A) axial and (B) coronal views. (C) Axial magnetic resonance imaging at the level of the primary internal opening (arrow). (D) Coronal magnetic resonance imaging at the level of the opening (yellow line). Horizontal red lines mark the anorectal sling (upper) and anal verge (lower), and the vertical red line indicates the length of the surgical anal canal. The intersection of the vertical and the horizontal red lines denotes the level of the internal opening relative to the canal.

REFERENCES

1. Aryanti C, Faruk M. Optimum management for complex anal fistula: a network meta-analysis of randomized controlled trials. Surg Open Sci 2024;18:117–22. Article PubMed PMC
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7. Yassin NA, Dardanov D, Phillips RK. Sepsis, CT, and the deep postanal space: a riddle, wrapped in a mystery, inside an enigma. Dis Colon Rectum 2015;58:1111–3. Article PubMed
8. Lilius HG. Fistula-in-ano, an investigation of human foetal anal ducts and intramuscular glands and a clinical study of 150 patients. Acta Chir Scand Suppl 1968;383:7–88. PubMed
9. Lee JM, Kim NK. Essential anatomy of the anorectum for colorectal surgeons focused on the gross anatomy and histologic findings. Ann Coloproctol 2018;34:59–71. Article PubMed PMC PDF
10. Zhang H, Zhou ZY, Hu B, Liu DC, Peng H, Xie SK, et al. Clinical significance of 2 deep posterior perianal spaces to complex cryptoglandular fistulas. Dis Colon Rectum 2016;59:766–74. Article PubMed
11. Courtney H. The posterior subsphincteric space; its relation to posterior horseshoe fistula. Surg Gynecol Obstet 1949;89:222–6. PubMed
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Process to determine the level of the primary internal opening of anal fistula on magnetic resonance imaging

Ann Coloproctol. 2025;41(3):253-258. Published online June 24, 2025

DOI: https://doi.org/10.3393/ac.2024.00948.0135

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Figure

Process to determine the level of the primary internal opening of anal fistula on magnetic resonance imaging

Fig. 1. Schematic diagram. (A, B) Normal anatomy. (C) The level of the anorectal sling and anal verge, with the surgical anal canal extending between these landmarks.

Fig. 2. Coronoal view. (A) Schematic diagram. (B) On a magnetic resonance imaging (MRI) scan. LA, levator ani muscle; AS, anorectal sling; C, the subcutaneous portion of the external anal sphincter, which curves around the inferior border of the internal anal sphincter (its medial edge marks the anal verge on MRI); AV, anal verge.

Fig. 3. Magnetic resonance images showing (A) biplanar mode (axial and coronal planes in 1 frame) and (B) multiplanar mode (axial, coronal, and sagittal planes in 1 frame). STIR, short tau inversion recovery.

Fig. 4. A 35-year-old man with multiple posterior fistula tracts and a primary internal opening in the mid-anal canal at the posterior midline. Schematic diagrams of (A) axial and (B) coronal views. (C) Axial magnetic resonance imaging at the level of the primary internal opening (arrow). (D) Coronal magnetic resonance imaging at the level of the opening (yellow line). Horizontal red lines mark the anorectal sling (upper) and anal verge (lower), and the vertical red line indicates the length of the surgical anal canal. The intersection of the vertical and the horizontal red lines denotes the level of the internal opening relative to the canal.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Process to determine the level of the primary internal opening of anal fistula on magnetic resonance imaging