Importance of high-frequency and low-frequency three-dimensional endoanal ultrasound scan in diagnosing perianal fistula and abscess

Background: Three-dimensional endoanal ultrasound scanning (3D-EAUS) is the first line of investigation in the management of perianal fistula and abscess. High frequency (HF) and low frequency (LF) are 2 frequency modes in 3D-EAUS which have different resolutions and penetration depth. Our study aims to compare the 3D images of HF with LF modes. Methods: This is a retrospective study of the 3D-EAUS images of patients with perianal fistula and abscess (perianal sepsis) who underwent 3D-EAUS from January 2019 to December 2020 in our hospital. The scans with both high and LF setting images were included in our study Results: The 53 3D-EAUS images were randomly selected, out of them images with both HF and LF modes were included. The dimensions of both HF and LF 3D cuboid images were measured. The volume of tissue measured in HF and LF modes were 155 ml and 380 ml respectively. The LF images scored 22/24 points and the HF images scored 16/24 points. Conclusions: LF and HF modes complement each other. LF mode can better delineate tracts and abscesses in extra-sphincteric space than HF mode. HF mode can better delineate tracts and abscesses in inter-sphincteric space as well as internal openings. We recommend routine use of both frequency modes while performing 3D-EAUS. Operating surgeons should gain expertise in performing the scan and reading the 3D image.