Surgery is the most fundamental treatment option for anal fistula. However, the recurrence rate of anal fistula is high and frequently requires numerous operations. When treating an anal fistula, the major goal is to lower the recurrence rate and protect anal function. As a result, precisely identifying the site of the fistula, the main perianal fistula type, and the anatomy of the internal opening prior to surgery is critical to improving the success rate of one-off surgery. Some of the most popular imaging assessment modalities include fistulography, endoscopic ultrasound, CT, MRI, and B-ultrasound.
Fistulography is limited due to its low diagnostic accuracy[10]. It cannot show the anal sphincters or determine their relationship to the fistula[11]. Endosonography can visualize the sphincter complex with high spatial resolution and can be used to classify fistulas[12]. However, the endosonograph’s limited field of view prevents the detection of suprasphincteric or secondary tracts[11]. Conventional two-dimensional CT images cannot fully depict the distribution of subtle fistulas and abscesses because of their poor capacity for soft tissue differentiation[13, 14]. In addition, CT subjects patients to ionizing radiation[15]. Magnetic resonance(MR) imaging is a highly accurate noninvasive modality that can detect and characterize the presence and location of the primary fistulous track, secondary extension and accompanying abscess, as well as delineate its extent[16, 17]. Because of its excellent soft tissue contrast and spatial resolution in the perianal region, it has emerged as the imaging modality of choice for the evaluation of perianal fistulizing disease [18]. Preoperative MR imaging has been shown to influence subsequent surgery and, as a result, significantly reduce the risk of recurrence[12]. However, it is still limited by cost and accessibility[19]. Furthermore, obtaining multiple sequences to depict the fistula in detail is time-consuming[6]. The activity of fistula or abscess is also thought to be an important factor in determining the surgical treatment strategy. DW-MRI is a viable tool for assessing anal fistula activity, but it is prone to artifacts that degrade image quality[20].
Ultrasound has been developed as an alternative imaging study to detect and monitor anal fistulas and abscesses due to its low cost, high resolution, and real-time performance, which can be used not only in preoperative planning and intraoperative navigation, but also to guide drainage of deep pelvic abscesses. Due to the limitations of viewing images in only one plane, three-dimensional ultrasound was recently introduced, allowing for high-resolution imaging of the anal canal and anal sphincter anatomy on multiple planes. The number and location of fistulas and internal openings, a focal defect in the mucosa of the anal canal, and its communication with a large superficial abscess can all be seen clearly. By providing detailed multiplanar reconstruction of the anal canal, three-dimensional anorectal ultrasound has improved perianal fistula diagnosis. It also raises the sensitivity of fistula tracks, internal openings and anal sphincter injury detection. When patients are evaluated using a three-dimensional intracavitary probe, however, their pain can be worsened. Pelvic three-dimensional ultrasound, which does not require anal expansion, can be used to lessen pain and operate around the subcutaneous abscess.
This study shows that using pelvic three-dimensional ultrasound to assess perianal fistula can provide multiplane preoperative mapping of the perianal fistula, identify all the components, such as the position and type of the primary and secondary tracts, and internal opening, quantify the length of the injured sphincter muscle, display the relationship between the sphincter and fistula, and classify perianal fistulas.
This method is well tolerated and minimally invasive. As a result, the accuracy rates of pelvic 3D US and MRI to evaluate perianal fistulas (97.01%, 94.03%), internal openings(97.92%, 94.79%) and those under Parks classification (97.53%, 93.83%), respectively. There was no statistically significant difference between the above results (all P > 0.05), indicating that both diagnostic methods are equally effective in diagnosing anal fistula.
However, ultrasound has limitations in detecting and imaging deep lesions due to transonic beam penetration limitations and potential air interference[21]. Supralevator fistulas are the most complex of perianal fistulas and have a low incidence. Additional research should be conducted to assess the availability of pelvic three-dimensional imaging in the diagnosis of high perianal fistula. Additionally, further research is required to determine the efficacy of pelvic 3D US in assessing the activity of fistula or abscess.