Non-contrast with contrast-enhanced three-dimensional endoanal ultrasound in preoperative assessment of anal fistula: A comparative study

Transcription

1 Available online at doi: /biij.9.2.e7 biij Biomedical Imaging and Intervention Journal ORIGINAL ARTICLE Non-contrast with contrast-enhanced three-dimensional endoanal ultrasound in preoperative assessment of anal fistula: A comparative study Low SF *, 1, Maimunah A 1, Syazarina-Sharis O 1, Sagap I 2, Hamzaini AH 1 1 Department of Radiology, Universiti Kebangsaan Malaysia Medical Centre, Kuala Lumpur, Malaysia 2 Department of Surgery, Universiti Kebangsaan Malaysia Medical Centre, Kuala Lumpur, Malaysia Received 3 June 2012; received in revised form 10 January 2012; accepted 23 January 2013 ABSTRACT Purpose: The purpose of this study was to compare the accuracy of non-contrast-enhanced three-dimensional endoanal ultrasound (NC-3D-EAS) with contrast-enhanced three-dimensional endoanal ultrasound (CE-3D-EAS) in preoperative assessment of anal fistula. Materials and Methods: A total of 28 patients (30 primary tracts) with anal fistula underwent 3D-EAS assessments. Three-dimensional volume displays were acquired in 3 steps: before (NC-3D-EAS), immediately after (CE-3D-EAS) and 10 minutes after 3% hydrogen peroxide administration (delayed CE-3D-EAS). Fistula classification and the presence of internal opening were determined via NC-3D-EAS and CE-3D-EAS. The abscess cavity and secondary tract were determined using all 3 steps. The 3D-EAS findings were compared with surgical findings, which served as the reference standard. Results: CE-3D-EAS was found to be more accurate than NC-3D-EAS. In the classification of the primary tract, there was good agreement between the surgical finding and NC-3D-EAS (Kappa = 0.674), and very good agreement between the surgical finding and CE-3D-EAS (Kappa = 0.815). The sensitivity and specificity of NC-3D-EAS and CE- 3D-EAS for detection of the internal opening were 75%, 81%, 95% and 91%, respectively. The sensitivity and specificity for NC-3D- EAS, CE-3D-EAS and delayed-ce-3d-eas in the detection of abscess cavity were identical, at 95% and 91%, respectively. There were 6 secondary tracts detected on NC-3D-EAS but only 4 were seen in CE-3D- EAS and delayed CE-3D-EAS. The other two hypoechoic lines were perianal scarrings. Delayed CE-3D-EAS allowed for better delineation of the secondary tract. Conclusion: CE-3D-EAS increases the accuracy of pre-operative anal fistula assessment, particularly in recurrent or complex fistula. Delayed CE-3D-EAS provides extra information about the extension of the secondary tract by allowing more time for the tract to fill up with hydrogen peroxide Biomedical Imaging and Intervention Journal. All rights reserved. Keywords: Anal fistula, three dimensional endoanal ultrasound (3D-EAS), primary tract, internal opening, secondary tract, abscess cavity. * Corresponding author. Address: Department of Radiology, UKMMC, Jalan Yaacob Latiff, Bandar Tun Razak, 56000, Cheras, Kuala Lumpur, Malaysia. Fax: ; soofinlow@gmail.com (Soo-Fin Low). Part of the content has been presented at the 19th United European Gastroenterology Week in Stockholm, Sweden, on October INTRODUCTION Anal fistula is an inflammatory perianal tract. Anorectal infection is the major cause of this condition. Anal fistula complicated with perianal abscess has a tendency

2 Low et al. Biomed Imaging Interv J 2013; 9(2):e7 2 to form a new outflow tract, resulting in the formation of a complex fistula. Almost all anal fistulae require surgical treatment. Rarely, the anal fistula heals spontaneously [1]. Surgery is the gold standard for the assessment of anal fistula. However, complex fistula is associated with a secondary tract or abscess cavity. Failure to identify these extensions during surgery is a well-known reason of recurrence [2]. The goals for the treatment of anal fistula include: identification of the course of anal fistula, drainage of the collections, eradication of the fistula, prevention of recurrence, and preservation of the sphincteric integrity to maintain continence. Preoperative imaging can disclose the hidden secondary tract that could go undetected during examination under anesthesia (EUA). The information about the course of the primary tract, the position of the internal opening, the amount of sphincter muscle involvement, secondary tract and abscess formation are readily accessible in preoperative imaging. In current practice, endoanal ultrasound (EAS) is a widely accepted technique for imaging of the anal sphincter complex. The accuracy of the EAS is improved with three-dimensional (3D) reconstruction [3]. The data set of the 3D endoanal ultrasound (3D-EAS) can be reviewed in axial, coronal, sagittal or even any oblique planes. 3D-EAS is more cost-effective and less timeconsuming compared to Magnetic Resonance Imaging (MRI). However, there is limited data regarding the value of the 3D-EAS in assessment of anal fistula. The aim of this study was to compare the accuracy of non-contrast-enhanced 3D endoanal ultrasound (NC- 3D-EAS) with contrast-enhanced 3D endoanal ultrasound (CE-3D-EAS) in pre-operative assessment of anal fistula. The assessment includes classification of the primary tract, and identification of the internal opening, abscess cavity and secondary tract. The CE-3D-EAS was divided into 2 steps, namely immediately, and 10 minutes, after administration of hydrogen peroxide. Hydrogen peroxide was used as a contrast agent in this study. METHODOLOGY This was a prospective cross-sectional study conducted from June 2008 to January The study received approval from the ethics committee. The study included patients who were followed-up at the surgery clinic in Universiti Kebangsaan Malaysia Medical Centre (UKMMC) and required surgical treatment for anal fistula. Clinically, they presented with perianal discharge, perianal induration and visualised external opening. The study protocol was explained to all patients and consent was obtained. Patients underwent pre-operative 3D-EAS and surgery. Patients who did not complete either 3D- EAS or surgery were excluded. There were a total of 32 patients in the study. Only 28 patients were recruited for statistical analysis. Four patients with healed fistula diagnosed on 3D-EAS were excluded from the study. Of the 28 patients, 22 were males and 6 were females. Their age ranged from 19 to 62 years with a mean of 42 years. Fifteen patients were new cases and 13 patients were recurrent cases. All the recurrent cases had previous perianal surgery and 2 of them had 2 primary tracts. Thus, there were a total 30 primary tracts for analysis. There was no ano-vaginal fistula or fistula secondary to inflammatory bowel disease. Three-Dimensional Endoanal Ultrasound (3D-EAS) 3D-EAS was performed using a rotating endoanal probe (type 2050, B-K Medical, Herlev, Denmark), which permits a 360-degree image mode. The frequency of the probe ranged from 6 16 MHz, depending on the depth of the field of view. The software facilitated 2D and 3D reconstruction images. Figure 1 Parks classification of anal fistula. Blue line = intersphincteric fistula, purple line = transphincteric fistula, red line = extrasphincteric fistula, green line = suprasphincteric fistula, IAS = internal anal sphincter, EAS = external anal sphincter, Puborec = puborectalis muscle, LA = levator ani, * = intersphincteric space. Bowel preparation was not required prior to 3D- EAS. 3D-EAS was performed with the patient in the left lateral position. Digital rectal examination was performed prior to 3D-EAS. The endoanal probe was enclosed in a latex condom containing degassed gel to achieve maximum image quality. 3D-EAS was performed in 3 steps by an experienced consultant radiologist. Firstly, the NC-3D-EAS was performed to identify the primary tract, internal opening, secondary tract and abscess cavity. Secondly, about 1 to 3 ml of contrast (3% diluted hydrogen peroxide) was injected into the fistula via the external opening using a venofix until there was reflux of contrast from the external opening. The fistula and the related findings were again analysed immediately post-contrast (CE-3D-EAS). Finally, the anal canal was imaged again 10 minutes after contrast injection (delayed CE-3D-EAS). Threedimensional reconstruction images were acquired for all

3 Low et al. Biomed Imaging Interv J 2013; 9(2):e7 3 3 steps. The accuracy of the NC-3D-EAS, CE-3D-EAS and delayed CE-3D-EAS were determined after comparison with the surgical findings. In delayed CE- 3D-EAS, the focus was on identification of the secondary tract and abscess cavity. The Parks classification system of anal fistula as intersphincteric, transphincteric, suprasphincteric and extrasphincteric [4] was used (Figure 1). Superficial fistulae were included in the study. The tract with an identifiable internal opening was recognised as an anal fistula. The tract without an identifiable internal opening was designated as a perianal sinus. There are 3 accepted criteria to diagnose the internal opening in NC-EAS [5]. Firstly, the site of the internal opening is the hypoechoic root-like budding formed by the fistula, which is in contact with the internal anal sphincter (IAS). Secondly, there should be the presence of a root-like budding with an IAS defect. Thirdly, the subepithelial defect connects the fistula through an IAS defect (Figure 2). Surgery Surgical findings were used as the gold standard for assessment of anal fistula. The imaging findings were considered true-positive only when they were revealed surgically. Surgery was performed under spinal anaesthesia. The patients were positioned in lithotomy, prone Jack-knife or Lloyd Davies positions, depending on the location of the anal fistula and the type of surgical approach. EUA was performed and findings were documented. Hydrogen peroxide and mammary probe were introduced into the fistula via the external opening. The presence of internal opening, extension or cavity was identified and followed by fistulectomy. Vessel loop was inserted into the fistula as a draining seton. Fistula tract was sent for histology assessment. Data Collection The findings from NC-3D-EAS, CE-3D-EAS and surgery were recorded separately in the standardised forms. The presence of a primary tract [intersphincteric (IS), trans-sphincteric (TS), supra-sphincteric (SS) and extra-sphincteric (ES), superficial fistula (SF) and perianal sinus (PS)], perianal scarring, internal opening, abscess cavity, and secondary tract(s) were documented. In delayed CE-3D-EAS, the presence of secondary tract, scarring and abscess cavity were documented. Data Analysis and Statistics The agreement between the NC-3D-EAS, CE-3D- EAS and surgical findings in classification of the primary tracts were analysed with Kappa values using SPSS version 17. Sensitivity and specificity of the NC- 3D-EAS and CE-3D-EAS in identifying the internal opening and abscess cavity were compared to the surgical findings. The sensitivity and specificity of delayed CE-3D-EAS in identifying the abscess cavity was also calculated. Statistical analysis of the secondary tract was not performed, as the secondary tracts were not described in the surgical notes, but were labelled as complex fistula. RESULTS A total of 30 primary tracts were analysed. 3D-EAS were performed ranging from the same day as the surgery to 150 days prior to surgery with a mean interval of 46 days. The most common site for external opening was the 6 o clock position (57%, 17 out of 30). Occurrence of external opening at the 11 1 o clock positions, 2 3 o clock positions and 9 10 o clock positions were 27% (n = 8), 13% (n = 4) and 3% (n = 1) respectively. Figure 2 (A) NC-3D-EAS image shows an ill-defined hypoechoic tract (arrow) that breaches the hypoechoic internal anal sphincter (IAS) and extends into the subepithelial layer, consistent with internal opening. (B) CE-3D-EAS image shows the hydrogen peroxide breaching the IAS (black arrow) and extending into the subepithelial layer (white arrow). The normal intersphincteric space (*) is echogenic in nature and external anal sphincter (EAS) is of mixed hypoechoic to hyperechoic.

4 NC-3D-EAS Low et al. Biomed Imaging Interv J 2013; 9(2):e7 4 Classification of the primary tract A total of 19 primary fistulae, 9 perianal sinuses and 2 perianal scarrings were identified at surgery (Table 1). Out of 19 primary fistulae, there were 11 intersphincteric fistulae, 2 transphincteric fistulae, 2 extrasphincteric fistulae and 4 superficial fistulae. There was no suprasphincteric fistula. On NC-3D-EAS, 9 out of 11 intersphincteric fistulae were correctly classified and 2 were diagnosed as perianal sinus. All 2 transphincteric fistulae and 2 extrasphincteric fistulae were correctly diagnosed by NC-3D-EAS. Only 1 out of 4 superficial fistulae was correctly classified by NC-3D-EAS. The other 2 superficial fistulae were diagnosed as intersphincteric fistula and 1 superficial fistula as perinal sinus. Eight out of 9 perianal sinuses were correctly identified by NC- 3D-EAS and 1 was diagnosed as intersphincteric fistula. NC-3D-EAS diagnosed 1 perianal scarring correctly, but another was diagnosed as intersphinteric fistula. CE-3D-EAS achieved 100% correct diagnoses for classification of intersphincteric fistula, transphincteric fistula, extrasphincteric fistula and perianal scarring. However, only 25% (1 of 4) of superficial fistulae were correctly classified. Another 3 superficial fistulae were diagnosed as intersphincteric fistulae in 2 patients and perianal sinus in one patient. The strength of agreement between the NC-3D-EAS and surgical findings was good, with Kappa value of (Table 2) while agreement between CE-3D-EAS and surgical findings was very good, with a Kappa value of (Table 3). Identification of the internal opening Out of 30 primary tracts, 19 internal openings (63%) were identified at surgery. In the identification of the internal opening, NC-3D-EAS and CE-3D-EAS had 2 and 1 false positive results, and 4 and 1 false negative results, respectively (Table 4 and Table 5). The sensitivity and specificity of NC-3D-EAS were 75% and 81% (15 of 19 internal openings), respectively. However, with CE-3D-EAS, the sensitivity and specificity increased to 95% and 91% (18 out of 19), respectively. Identification of the abscess cavity There were 6 abscess cavities detected at surgery. Five of 6 cavities were detected with all 3 steps of 3D- EAS. One small cavity was not detected on 3D-EAS, even with all 3 steps. One abscess cavity, which was seen with all 3 steps of 3D-EAS, was not detected at surgery. A total of 23 cases with concordant findings between 3D-EAS and surgical findings comprised the true negative cases. The sensitivity and specificity of all 3 steps of 3D-EAS in abscess cavity identification were 83% and 96%, respectively. Table 1 Classification of anal fistula with NC-3D-EAS, CE-3D-EAS and surgery of 30 primary tracts. NC-3D-EAS CE-3D-EAS Surgery Frequency Percentage Frequency Percentage Frequency Percentage IS TS ES SF PS S Total (IS: intersphincteric fistula, TS: transphincteric fistula, ES: extrasphincteric fistula, SF: superficial fistula, PS: perianal sinus, S: perianal scarring) Table 2 Agreement of anal fistula classification between NC-3D-EAS and surgery. SURGERY IS TS ES SF PS S Total IS TS ES SF PS S Total Kappa value = 0.674

5 CE-3D-EAS Low et al. Biomed Imaging Interv J 2013; 9(2):e7 5 Table 3 Agreement of anal fistula classification between CE-3D-EAS and surgery. SURGERY IS TS ES SF PS S Total IS TS ES SF PS S Total Kappa value = Table 4 Identification of internal opening by NC-3D-EAS and surgery. NC-3D-EAS Surgery Identified Not identified Total Identified Not identified Total Table 5 Identification of internal opening by CE-3D-EAS and surgery. CE-3D-EAS Surgery Identified Not identified Total Identified Not identified Total Identification of the secondary tract Using NC-3D-EAS, 6 secondary hypoechoic tracts were detected and labelled as secondary tracts. Upon performing CE-3D-EAS, only 4 secondary tracts were found. The other two were scar tissues or fibrous tracts. Similarly, delayed CE-3D-EAS also detected 4 secondary tracts with more distal migration of hydrogen peroxide. However, secondary tracts were not precisely documented in the surgical notes. Instead, these cases were described as complex fistulae. DISCUSSION Based on a review of the literature, the accuracy of EAS in assessing anal fistula ranges from % [6 8]. Some studies show that there is increased accuracy of diagnosis of up to 95% with CE-EAS [1, 2]. Many other studies also show that CE-EAS increases the diagnostic value for anal fistula [5, 9 11]. The usage of NC-EAS and CE-EAS in 126 patients with anal fistula has been compared [12]. The sensitivities of NC-EAS and CE-EAS for detecting the internal opening were 65% and 89%, respectively [12]. In this study, the internal opening of subcutaneous and extrasphincteric fistulae were not seen on NC-EAS. Detection of both the superior and extrasphincteric internal opening was much more promising with CE- EAS. This paper concluded that CE-EAS is superior to NC-EAS [12]. A more recent study, conducted in 2009, reported that CE-3D-EAS was able to detect a few more primary tracts and secondary extensions, compared to NC-3D- EAS [13]. However, there was no statistically significant difference between the NC-3D-EAS and CE-3D-EAS in classifying the primary tract, internal opening and secondary tracts, due to the high accuracy of NC-3D- EAS. However, an earlier study from 2005 shows that CE-3D-EAS can improve diagnostic accuracy [14]. This study confirmed that CE-3D-EAS is superior in classifying primary tracts and in identifying internal openings, particularly in recurrent and complex anal fistulae which were usually associated with secondary extension and scarring. The findings of both NC-3D- EAS and CE-3D-EAS in detecting abscess cavity were identical. Unlike CE-3D-EAS, NC-3D-EAS was not satisfactory in differentiating scarring from fistula or sinus tract. One patient was thought to have recurrent anal fistula from the results of NC-3D-EAS, but was

6 Low et al. Biomed Imaging Interv J 2013; 9(2):e7 6 shown on CE-3D-EAS to have perianal scarring, which was then confirmed at surgery. Another 2 patients with complex fistulae were initially diagnosed to have abscess with secondary tract on NC-3D-EAS. However, CE-3D- EAS revealed that they had abscesses with adjacent scarring. These linear hypoechogenicities that mimic secondary tracts were not opacified with hydrogen peroxide (Figure 3). Using the 3 accepted criteria to identify internal opening [5], the sensitivity of the NC-3D-EAS in identification of internal opening was 75%. In CE-3D- EAS, the sensitivity went up to 95%. Three internal openings, which were not detected on NC-3D-EAS, were detected after introducing hydrogen peroxide (Figure 4). This study also found that scarring in recurrent anal fistula mimicked internal opening on NC-3D-EAS. The scarring, which appeared as hypoechogenicity, could blend with the IAS or extend into the subepithelial space, resulting in over-reporting of internal opening on NC- 3D-EAS. Otherwise, NC-3D-EAS was accurate in detecting internal opening in new cases using Cho DY criteria [5]. There was only one internal opening of a superficial fistula not seen on CE-3D-EAS. Identifying the low-lying position of the internal opening is challenging as it could be obscured by the trapped air between the endo-probe and anal canal. In addition, a small internal opening might not be detected due to probe compression. In CE-3D-EAS, if the external Figure 3 (A) NC-3D-EAS image shows a large cavity (white arrow) with a hypoechoic tract (black arrow) abutting the internal anal sphincter at the 7 o clock position. Another hypoechoic tract extends to the o clock position, mimicking a secondary extension (yellow arrow). (B) The CE-3D-EAS image shows accumulation of hydrogen peroxide in the cavity without extension into the hypoechoic tract. The finding is consistent with tissue scarring instead of fistula. Figure 4 (A) The internal opening is not seen on axial NC-3D-EAS. (B) Axial view CE-3D-EAS image at the same level shows that hydrogen peroxide breaches the internal anal sphincter and extends into the submucosa in keeping with the site of internal opening (arrow).

7 Low et al. Biomed Imaging Interv J 2013; 9(2):e7 7 opening is close to the anal verge, the contrast will escape into the anal canal and obscure the findings. In this study, neither NC-3D-EAS nor CE-3D-EAS could accurately classify the superficial fistula. Retrospective review of the 3D-EAS images showed the fistula were in contact with the inferior edge of the IAS. Thus, we diagnosed these 3 superficial fistulae as intersphincteric fistulae. However, surgical findings confirmed there was no involvement of the IAS. The tendency to overestimate the height of the fistula, such that the superficial and intersphincteric fistulae were reported as transphincteric and suprasphincteric fistulae, has been previously noted [15]. Apart from that, superficial, suprasphincteric and extrasphinteric fistula were the most difficult to diagnose on EAS [16]. There was no suprasphincteric fistula in this study. The small field of view is one of the pitfalls of 3D-EAS as remote structures away from the anal canal cannot be demonstrated [17]. Inadequate acquisition of the upper anal canal would miss the extrasphincteric and suprasphincteric tract. In the case of superficial fistula, inadequate acquisition of the low anal canal would miss the tract. The endo-anal probe should be parallel to the anal canal. Improper angulation of the probe in relation to the anal canal could distort the anatomy of the anal sphincter complex and lead to interpretation error. Sensitivity and specificity in detecting abscess cavity with all 3 steps of 3D-EAS were high, at 83% and 96%, respectively. A dry cavity was detected at surgery but not seen on 3D-EAS; this was most likely due to the small size of the cavity. There was an abscess cavity detected with all 3 steps of 3D-EAS, but not in surgery. As there was an 8-week interval between 3D-EAS and surgery, the cavity had most likely resolved with Figure 5 (A) CE-3D-EAS image of an intersphincteric fistula shows hydrogen peroxide at 6 o clock position (vertical arrows). (B) Delayed CE- 3D-EAS image shows migration of contrast to 4 o clock position (horizontal arrow). Figure 6 Coronal oblique view on NC-3D-EAS (A) and CE-3D-EAS images (B) show a transphincteric fistula with secondary tract extending into the high intersphincteric space. This secondary extension was not revealed surgically.

8 Low et al. Biomed Imaging Interv J 2013; 9(2):e7 8 antibiotics prior to surgery. In addition, the presence of internal and external openings communicating with the abscess also facilitated the spontaneous drainage of the pus. The smallest cavity detected in this study measured 8 mm in diameter. Most of the published studies did not specify the timing between the image acquisition and administration of contrast, except one study which documented that image acquisition started 20 seconds after contrast administration [13]. A study reported that the contrast injection had to be performed in two steps [18]. A small amount of contrast is injected first and then followed by greater injection pressure to detect the presence of secondary tracts that might be missed initially [18]. In this study, CE-3D-EAS was performed immediately and 10 minutes after contrast administration. The delayed CE-3D-EAS was designed to detect potential hidden secondary tract and abscess cavity. Both CE-3D-EAS and delayed CE-3D-EAS could identify similar number of secondary tract. However, delayed CE-3D-EAS was extremely helpful to demonstrate the extension and length of the secondary tract. During immediate scanning in CE-3D-EAS, the secondary tract was only partially filled up by hydrogen peroxide. In the 10-minute delayed scanning, the contrast migrated distally, demonstrating more peripheral and superior extension of the secondary tract (Figure 5). The findings from delayed CE-3D-EAS alerted the surgeon to look for the secondary tract to minimise the risk of recurrence. Three patients with secondary tracts showed good outcome after surgery. They were followed-up at the surgery clinic for 9 to 12 months after surgery and remained asymptomatic. One patient was found to have transphincteric fistula and secondary extension in all 3 steps but not during EUA (Figure 6). During surgery, this patient was diagnosed to have only transphincteric fistula. However, pre-operative MRI confirmed the 3D-EAS findings. This patient remained symptomatic post-operatively. It had been reported that MRI may actually be superior to surgery in the detection of anal fistula [19]. The authors found that 2 patients who had normal findings during surgical review subsequently developed infection at the site, which had been previously reported on MRI [19]. Based on that study, another published article used surgical findings guided by MRI to ensure the reference standard was as accurate as possible [20]. In a follow-up study, body coil MRI was shown to be more accurate than initial surgical exploration in predicting the outcome [21]. Surgical findings can be difficult to define as the true reference standard because the surgeons who performed the assessment can have different levels of experience. Instead of using surgical findings as a reference standard, post-surgical clinical outcome as a was used as the reference standard to minimise the potential biases [22]. The main limitation of this study was the small sample size and limited types of primary fistulae. Most of the tracts are intersphincteric fistulae and perianal sinuses, which were relatively easy to diagnose via EAS, thus potentially outweighing the other types of fistulae. CONCLUSION 1. CE-3D-EAS significantly increases the accuracy of pre-operative anal fistula assessment, particularly in recurrent or complex anal fistula. 2. CE-3D-EAS is superior to NC-3D-EAS in differentiating recurrent fistula from scarring. 3. In recurrent fistula, scarred tissues mimic internal opening in NC-3D-EAS. CE-3D-EAS improves the diagnostic accuracy in this condition. 4. Delayed CE-3D-EAS should be performed in all cases with complex fistula for better delineation of secondary tract. ACKNOWLEDGEMENT The authors thank Ms. Siti Farizwana Mohd Ridzwan for her contribution in formatting and editing the article. REFERENCES 1. Shouler PJ, Grimley RP, Keighley MR and Alexander-Williams J. Fistula-in-ano is usually simple to manage surgically. Int J Colorectal Dis 1986; 1(2): Seow-Choen F and Nicholls RJ. Anal fistula. Br J Surg 1992; 79(3): Santoro GA and Di Falco G. Bening anorectal diseases : diagnosis with endoanal and endorectal ultrasound and new treatment options. Berlin: Springer; 2006: xv, Parks AG, Gordon PH and Hardcastle JD. A classification of fistula-in-ano. Br J Surg 1976; 63(1): Cho DY. Endosonographic criteria for an internal opening of fistula-in-ano. Dis Colon Rectum 1999; 42(4): Poen AC, Felt-Bersma RJ, Eijsbouts QA, Cuesta MA and Meuwissen SG. Hydrogen peroxide-enhanced transanal ultrasound in the assessment of fistula-in-ano. Dis Colon Rectum 1998;41(9): Yee LF, Birnbaum EH, Read TE, Kodner IJ and Fleshman JW. Use of endoanal ultrasound in patients with rectovaginal fistulas. Dis Colon Rectum 1999; 42(8): Law PJ, Talbot RW, Bartram CI and Northover JM. Anal endosonography in the evaluation of perianal sepsis and fistula in ano. Br J Surg 1989; 76(7): Kruskal JB, Kane RA and Morrin MM. Peroxide-enhanced anal endosonography: technique, image interpretation, and clinical applications. Radiographics 2001;21 Spec No:S173 S Sloots CE, Felt-Bersma RJ, Poen AC and Cuesta MA. Assessment and classification of never operated and recurrent cryptoglandular fistulas-in-ano using hydrogen peroxide enhanced transanal ultrasound. Colorectal Dis 2001; 3(6): Lengyel AJ, Hurst NG and Williams JG. Pre-operative assessment of anal fistulas using endoanal ultrasound. Colorectal Dis 2002; 4(6): Sudol-Szopinska I, Szczepkowski M, Panorska AK, Szopiński T and Jakubowski W. Comparison of contrast-enhanced with noncontrast endosonography in the diagnostics of anal fistulas. Eur Radiol 2004; 14(12): Kim Y and Park YJ. Three-dimensional endoanal ultrasonographic assessment of an anal fistula with and without H(2)O(2) enhancement. World J Gastroenterol 2009; 15(38): Ratto C, Grillo E, Parello A, Costamagna G and Doglietto GB. Endoanal ultrasound-guided surgery for anal fistula. Endoscopy 2005; 37(8): Gustafsson UM, Kahvecioglu B, Aström G, Ahlström H and Graf W. Endoanal ultrasound or magnetic resonance imaging for preoperative assessment of anal fistula: a comparative study. Colorectal Dis 2001; 3(3): Choen S, Burnett S, Bartram CI and Nicholls RJ. Comparison between anal endosonography and digital examination in the evaluation of anal fistulae. Br J Surg 1991; 78(4):

9 Low et al. Biomed Imaging Interv J 2013; 9(2):e Halligan S. Imaging fistula-in-ano. Clin Radiol 1998; 53(2): Navarro-Luna A, Garcia-Domingo MI, Rius-Macias J and Marco- Molina C. Ultrasound study of anal fistulas with hydrogen peroxide enhancement. Dis Colon Rectum 2004; 47(1): Lunniss PJ, Armstrong P, Barker PG, Reznek RH and Phillips RK. Magnetic resonance imaging of anal fistulae. Lancet 1992; 340(8816): Buchanan GN, Halligan S, Bartram CI, William AB, Tarroni D and Cohen CR. Clinical examination, endosonography, and MR imaging in preoperative assessment of fistula in ano: comparison with outcome-based reference standard. Radiology 2004; 233(3): Spencer JA, Chapple K, Wilson D, Ward J, Windsor AC and Ambrose NS. Outcome after surgery for perianal fistula: predictive value of MR imaging. Am J Roentgenol 1998;171(2): Buchanan G, Halligan S, Williams A, Cohen CR, Tarroni D, Phillips RK and Bartram CI. Effect of MRI on clinical outcome of recurrent fistula-in-ano. Lancet 2002; 360(9346):