Imaging in perianal fistulising Crohn’s disease: A practical guide for the gastroenterologist

Abstract

Perianal fistulising Crohn’s disease is a challenging complication that can affect up to 20% of patients with Crohn’s disease and is associated with significant morbidity. Despite advances in medical therapies, particularly anti-tumor necrosis factor agents, the majority of patients still require surgical intervention. Accurate diagnosis and monitoring are essential to optimise outcomes and guide multidisciplinary management. Although clinical scoring systems such as the perianal disease activity index are widely used, their subjective application limits their reproducibility and reliability, underscoring the need for more objective methods of evaluating perianal fistulising Crohn’s disease activity. Imaging has thus become central to the objective assessment of perianal fistulising Crohn’s disease, with magnetic resonance imaging (MRI) recognised as the gold standard in view of its ability to provide clear, detailed images of the perianal region in a radiation-free manner. Guidelines also endorse the use of imaging modalities such as endoanal ultrasound and transperineal ultrasound as viable alternatives to MRI for the assessment of perianal fistulising Crohn’s disease in centres with appropriate expertise. This article aims to evaluate and compare the diagnostic accuracy and clinical utility of MRI, endoanal ultrasound, and transperineal ultrasound in the assessment of perianal fistulising Crohn’s disease, highlighting their respective strengths, limitations, and roles in clinical practice.

Key Words: Crohn’s disease; Perianal; Fistulising; Imaging; Magnetic resonance imaging; Ultrasound

Core Tip: Perianal fistulising Crohn’s disease affects up to 20% of patients and frequently requires combined medical and surgical management. Accurate and objective disease assessment is essential for the diagnosis, monitoring, and guidance of therapeutic interventions. Magnetic resonance imaging of the pelvis is the gold standard imaging modality for the evaluation of perianal fistulising Crohn’s disease. However, modalities such as endoanal ultrasound and transperineal ultrasound represent practical and accessible alternatives, particularly for repeated short to medium-term disease reassessment. This article aims to evaluate and compare the diagnostic accuracy and clinical utility of magnetic resonance imaging, endoanal ultrasound, and transperineal ultrasound in the assessment of perianal fistulising Crohn’s disease.

INTRODUCTION

Perianal fistulising disease is a common and debilitating complication of Crohn’s disease, affecting nearly 1 in 5 patients within the first decade of diagnosis[1]. While advances in medical therapy have improved clinical outcomes, around two-thirds of patients with perianal fistulising Crohn’s disease still require surgery[2]. Accurate diagnosis and monitoring strategies are essential to optimise outcomes and guide multidisciplinary care[3,4]. Although clinical scoring systems such as the perianal disease activity index are widely used, their subjective application limits their reproducibility and reliability, underscoring the need for more objective methods of evaluating disease activity[5,6]. Imaging has thus become central to the objective assessment of perianal fistulising Crohn’s disease, with magnetic resonance imaging (MRI) recognised as the gold standard due to its radiation-free, high-resolution, multiplanar capabilities[7,8].

The therapeutic landscape of perianal fistulising Crohn’s disease continues to evolve, increasingly favouring a personalised, multimodal approach that integrates medical, surgical, and novel adjunctive therapies. Anti-tumor necrosis factor (anti-TNF) agents, specifically adalimumab and infliximab, remain cornerstone of medical treatment, with improved perianal fistula outcomes associated with higher drug levels[9-12]. Newer biologic agents such as ustekinumab[13] and vedolizumab[14] have also demonstrated effectiveness in perianal fistulising Crohn’s disease, with emerging data regarding the utility of small molecule janus kinase inhibitors[15]. Nevertheless, surgery remains a key component of management, often in combination with medical therapy, with procedures involving seton placement, fistulotomy, and mucosal advancement flaps, commonly utilised[16,17]. Adjunctive therapies such as mesenchymal stem cell injections and hyperbaric oxygen therapy have shown promise, particularly in treatment-resistant cases, although barriers such as accessibility, high cost, and standardised treatment protocols, limit widespread adoption[18,19].

Imaging plays an essential role in the diagnosis, classification, and longitudinal assessment of perianal fistulising Crohn’s disease, offering objective insights that inform clinical management[20]. Among available modalities, pelvic MRI is now established as the first-line tool for both initial evaluation and ongoing monitoring of perianal fistulising Crohn’s disease activity[21]. However, timely access to pelvic MRI, particularly for the assessment of acute complications, such as perianal abscesses, may be limited by constraints on healthcare resources[22]. This highlights the need for alternative, non-invasive imaging modalities that are accurate, accessible, and suitable for repeated use, particularly in short- to medium- timeframes, for assessments of treatment response and disease progression. Endoanal ultrasound (EAUS) and transperineal ultrasound (TPUS) have emerged as valuable complementary imaging modalities to pelvic MRI in this context[23]. Their clinical utility is recognised in joint consensus statements from the European Crohn’s and Colitis Organisation (ECCO) and the European Society of Gastrointestinal and Abdominal Radiology (ESGAR), which endorse both modalities as effective adjuncts to MRI in centres with appropriate expertise[7,24].

This article aims to evaluate and compare the diagnostic accuracy and clinical utility of MRI, EAUS, and TPUS in the assessment of perianal fistulising Crohn’s disease, highlighting their respective strengths, limitations, and roles in clinical decision-making. The article will also examine the role of historical and emerging imaging modalities, offering insight into how these tools may complement and enhance current imaging strategies.

DEFINING RADIOLOGICAL RESPONSE AND REMISSION IN PERIANAL CROHN’S DISEASE

Perianal fistulising Crohn’s disease is a chronic and debilitating manifestation of Crohn’s disease, driving both patients and clinicians to pursue treatment targets linked to improved quality of life and favourable long-term outcomes[25]. This underscores the need to clearly define treatment outcomes in a manner that allows reliable and reproducible assessment of response and remission. In clinical practice, clinical response is often defined as a reduction of 50% or more in externally draining fistulas from baseline, while remission is generally defined as the complete absence or closure of all externally draining fistulas[12,13,26]. These definitions are reflected in perianal disease activity scores such as the perianal disease activity index and fistula drainage assessment[27]. However, these scores rely heavily on subjective clinical evaluation and have not been rigorously validated, raising concerns about their consistency, reproducibility, and applicability across various clinical settings and between different healthcare providers[28]. This highlights the need for more objective, standardised criteria to accurately monitor disease progression and inform clinical decision-making.

Radiological outcomes, specifically radiological response and remission, remain poorly defined in the context of perianal fistulising Crohn’s disease across all imaging modalities, including MRI[7,29]. This uncertainty is exemplified by the lack of consensus on which radiological features are most important, and how to optimally combine and apply these parameters into radiological disease activity scores that can reliably quantify disease activity and evaluate interval change[30]. As a result, clinicians and researchers encounter difficulties when relying on imaging as a standardised tool for outcome assessment in perianal fistulising Crohn’s disease. In an effort to address these gaps, a recently published international Delphi consensus from the Treatment Optimisation and CLASSification (TOpClass) consortium introduced the first set of standardised MRI-based definitions for radiological improvement and remission in perianal Crohn’s disease[16]. This framework offers a reproducible, objective approach for interpreting MRI findings, representing a key step toward consistency in clinical trials and practice. However, similar standardised definitions for EAUS and TPUS are still lacking, highlighting a critical need for further research and consensus development in this area.

IMAGING MODALITIES FOR DIAGNOSING AND MONITORING PERIANAL FISTULISING CROHN’S DISEASE

Multi-modal imaging plays a crucial role in the diagnosis, classification, and monitoring of perianal Crohn’s disease[23]. These techniques have the ability to provide valuable clinical insight into the extent, severity, and activity of perianal fistulising Crohn’s disease, enabling clinicians to assess fistula characteristics, detect abscesses, and evaluate treatment response without having to subject patients to the risks associated with examination under anaesthesia (EUA). Hence, the use of imaging remains essential in guiding clinical decision-making, reducing patient discomfort, and improving long-term treatment outcomes in patients with perianal fistulising Crohn’s disease[31].

Several imaging techniques have been proposed to diagnose and monitor perianal fistulising Crohn’s disease. These range from non-invasive methods, such as MRI and TPUS, to semi-invasive techniques like EAUS (Table 1)[24,32]. Each of these imaging modalities plays an important role in clinical practice, with relative strengths and limitations related to their practical application, diagnostic accuracy, and ability to evaluate interval change (Table 2). The following section will evaluate these characteristics across the most commonly applied imaging modalities in perianal fistulising Crohn’s disease.

Table 1 Comparison of key imaging modalities in perianal fistulising Crohn’s disease¹.

	Pelvic MRI	Transperineal ultrasound	Endoanal ultrasound
Fistula detection	Gold standard for detecting complex and deep fistula tracts	Optimal for superficial and anterior fistula tracts	Optimal for simple, low fistula tracts
Abscess detection	Capable of identifying deep and pelvic abscesses	Capable of detecting local abscesses, may miss deeper pelvic collections	Capable of detecting local abscesses, may miss deeper pelvic collections
Assessment of extent of fistula tract	Provides extensive pelvic mapping, delineating the full course of fistula tracts and associated complications	Limited, particularly for supralevator tracts and deeper extension	Limited, confined to anal canal and surrounding tissues
Sphincter involvement	Visualises damage and inflammation to sphincters	Limited less detailed visualisation of sphincter complex	High resolution images of internal and external sphincter anatomy; utility for assessment of sphincter integrity
Pre-surgical planning	Ideal for mapping complex fistula anatomy and planning surgical interventions	Limited, assessment of superficial disease	Optimally reserved for low, simple fistulising disease to aid surgical decision-making
Post-treatment monitoring	Optimal for long-term monitoring and assessment of deep healing; not feasible for frequent repetition due to cost and availability	Easy and repeatable, even at short intervals; useful for monitoring superficial disease	Capable, but invasive, may not be suitable for frequent use
Patient comfort	Comfortable, non-invasive; longer examination time	Comfortable, non-invasive; requires perineal exposure	Less comfortable; involves probe insertion; not suitable in setting of rectal stenosis
Access	May be limited to high resource healthcare settings	May be limited by expertise	May be limited by expertise
Cost	High	Low	Moderate; may increase if endoscopy required
Operator dependency	Low to moderate; standardised protocols reduce variability	High	High
Advantages	Comprehensive evaluation of complex anatomy, superior soft tissue contrast	Non-invasive, accessible and cost-effective; repeatable at short- and medium- intervals	High-resolution imaging of anal sphincter complex, effective for simple fistulas
Disadvantages	High cost, resource intensive; difficult to repeat at short intervals	Limited in assessment of deep or complex fistulas; operator dependent	Invasive; limited field of view; operator dependent

¹The information presented in this table reflects the authors’ interpretation and views based on the available evidence. It may not represent a consensus or official position of affiliated institutions.

MRI: Magnetic resonance imaging.

Table 2 Scenario specific imaging choices in perianal Crohn’s disease¹.

Clinical scenario	Preferred imaging modality	Rationale
Initial evaluation at diagnosis	MRI	Designated gold standard imaging modality capable of facilitating comprehensive assessment of complex perianal anatomy
Anal stenosis or severe anal pain	MRI	Avoids discomfort associated with EAUS
Fistula with suspected abscess	MRI or TPUS	MRI provides superior soft tissue contrast for deep abscess detection, however, TPUS may also be suitable for assessment of superficial pathology
Assessment of sphincter complex	EAUS	Provides high resolution images of internal and external sphincter
Point-of-care re-evaluation	TPUS	Suitable for serial point-of-care evaluation in view of being minimally invasive
Post-treatment follow-up	MRI or TPUS	MRI for deep healing, TPUS for superficial closure
Contraindication to MRI	EAUS or TPUS	Ultrasound is safe with few absolute contraindications

¹The information presented in this table reflects the authors’ interpretation and views based on the available evidence. It may not represent a consensus or official position of affiliated institutions.

MRI: Magnetic resonance imaging; TPUS: Transperineal ultrasounds; EAUS: Endoanal ultrasound.

MRI IN PERIANAL CROHN’S DISEASE

MRI is considered the gold standard imaging modality for the assessment of perianal fistulas in Crohn’s disease[33]. The ECCO guidelines recommend that MRI be applied immediately following the diagnosis of perianal fistulising Crohn’s disease, except in circumstances where urgent drainage of sepsis is required and waiting for an MRI would delay timely intervention[34]. Even in cases of perianal sepsis, timely pelvic MRI can provide a valuable surgical roadmap for intervention, enabling optimal drainage to be achieved on the first occasion[35].

Pelvic MRI provides high-resolution multiplanar images of the perianal region without the need for ionising radiation[36]. This makes MRI an ideal imaging modality in chronic lifelong diseases such as perianal fistulising Crohn’s disease which have a young age of onset, including during childbearing years. Imaging protocols of pelvic MRI scans typically encompass T2-weighted sequences with and without fat saturation, and contrast-enhanced T1-weighted sequences, which collectively allow for detailed visualisation of active inflammation, fluid collections, and fibrosis[32] (Figures 1 and 2). Moreover, pelvic MRI is well tolerated by patients for monitoring perianal fistulising disease, with an international survey documenting that patients prefer MRI due to its regularity and non-invasive nature when compared to colonoscopy[37].

Figure 1 Magnetic resonance imaging of the pelvis demonstrating an intersphincteric fistula with an associated collection. A: Axial T2 fat-saturated image: Intersphincteric collection (“a” sign) with T2 hyperintense content; B: Axial T1 fat-saturated post-gadolinium image: This collection demonstrates rim enhancement surrounding a central low-signal area, consistent with pus; C: Axial T2 fat-saturated image: Intersphincteric tract (arrow) demonstrates a central hyperintense signal with a hypointense rim; D: Axial T1 fat-saturated post-gadolinium image: The tract contents show enhancement without rim enhancement, consistent with granulation tissue within the tract and a fibrous tract wall.

Figure 2 Magnetic resonance imaging of the pelvis demonstrating a posterior branching fistula. A: Axial T2-weighted fat-saturated; B: Coronal T2-weighted fat-saturate; C: Axial T1-weighted fat-saturated post-contrast T1-weighted. Posterior branching fistula with internal opening posteriorly at 6 o’clock. A fibrotic right-sided branch is noted at 7 o’clock demonstrating low T2 signal of the tract (A and B, arrows) and no contrast enhancement (C, arrow). An inflamed left-sided branch is noted at 5 o’clock demonstrating high T2 signal within the tract (A and B, arrowheads) and pronounced rim enhancement with central low signal related to fluid/pus (C, arrowhead).

Diagnostic accuracy

A meta-analysis by Siddiqui et al[38] comparing the diagnostic performance of MRI and EAUS against EUA, for both idiopathic and Crohn’s disease related perianal fistulas, reported that MRI had a sensitivity and specificity of 87% [95% confidence interval (CI): 0.63-0.96] and 69% (95%CI: 0.51-0.82), respectively, for the detection of perianal fistulas. When evaluating specific features such as the location of the internal opening, MRI showed a sensitivity of 96%, specificity of 90%, a positive predictive value of 90%, and a negative predictive value of 96%[39]. These metrics underscore the strength of MRI in the diagnosis and classification of perianal fistulas in Crohn’s disease.

Disease monitoring and response to therapy

Following diagnosis, MRI plays a crucial role in monitoring disease activity. However, studies have shown that radiological outcomes on MRI typically lag clinical outcomes[40]. In a meta-analysis by Lee et al[29], which included 9 studies and 259 patients with perianal fistulising Crohn’s disease, 47% of patients achieved clinical remission of their fistula at 52 weeks, however only 25% achieved radiological healing, defined as the absence of high-signal tracts on T2-weighted scans, with an odds ratio (OR) of 0.43. This was also demonstrated in study by Ng et al[41] in which 26 patients with perianal fistulising Crohn’s disease treated with infliximab or adalimumab exhibited clinical remission before achieving radiological healing, defined as the absence of high-signal tracks on fat saturated T2 sequences. Notably, five patients achieved radiological healing at 6 months, all of whom maintained clinical remission at 12 and 18 months of follow up[41]. These findings reflect that radiological outcomes on MRI represent more difficult endpoints to achieve than clinical outcomes alone, in patients with perianal Crohn’s disease.

The lack of standardised MRI-specific treatment targets for perianal fistulising Crohn’s disease was recently addressed by the TOpClass consortium, which established expert consensus definitions for radiological healing and improvement. According to their criteria, radiological healing is defined as the absence of T2-weighted hyperintensity in the fistula tract, coupled with a completely fibrotic tract and, where contrast is used, the absence of contrast enhancement on post-contrast T1-weighted images[16]. This definition emphasises a full transition from inflammation to fibrosis as the key indicator of fistula healing. In contrast, radiological improvement was less stringently defined, requiring one or more of an increasingly fibrotic tract or a clear reduction in T2 hyperintensity, fistula diameter, fistula length, abscess size, or contrast enhancement of the fistula tract[16]. This broader definition allows for the detection of partial therapeutic responses, capturing changes in both inflammatory activity and anatomical features of the fistula, thereby offering a more nuanced assessment of treatment efficacy in clinical practice and trials. These standardised definitions are crucial for unifying clinical endpoints, enabling consistent evaluation across studies and improving communication among multidisciplinary teams. The TOpClass consensus recommends that MRI be repeated between 6 and 12 months following definitive surgical intervention or initiation of advanced medical therapies, such as biologics or small molecules, with 6 months being more appropriate for assessing early treatment response and 12 months for confirming radiological healing[16]. Earlier imaging may be warranted in cases of new or persistent symptoms, or when there is a change in therapeutic strategy.

Another emerging therapeutic goal in the management of perianal Crohn’s disease is complete or deep tissue remission, defined as clinical remission in conjunction with the absence of contrast enhancement and T2 hyperintensity on MRI[42]. In a study of 49 patients with perianal fistulising Crohn’s disease who received maintenance anti-TNF therapy over 40 months, one-third of the cohort achieved deep tissue remission, with longer duration of anti-TNF exposure notably associated with deep tissue remission[43]. Chambaz et al[44] followed 48 patients with perianal Crohn’s disease exposed to anti-TNF treatment over a median duration of 62 months; finding that flare-free and perianal event-free survival were significantly prolonged in patients who achieved deep tissue remission. Moreover, patients with persistent T2 hyperintensity and contrast enhancement on MRI required four times as many perianal surgeries compared to those who achieved deep tissue remission.

MRI may also serve as a valuable tool in predicting clinical outcomes. A retrospective study by Shenoy-Bhangle et al[45], including 36 adult and paediatric patients with perianal Crohn’s disease who underwent serial pelvic MRIs, identified that a fistula length of less than 2.5 cm predicted treatment response (OR = 5; 95%CI: 1.2-20.9), while an aggregate fistula length greater than 2.5 cm was associated with disease progression (OR = 9.2; 95%CI: 0.5-161.9). Similarly, Karmiris et al[46] reported that in a cohort of 59 patients starting infliximab therapy, one third of the patients required surgical intervention and it was noted that those with branched single fistula (P < 0.0001) and collections (P = 0.006) at baseline MRI were more likely to undergo surgery.

MRI disease activity scores

Three MRI-based scoring systems have been proposed to assess radiological disease activity in adult patients with perianal Crohn’s disease. These include the Van Assche Index (VAI), the modified VAI (mVAI), and the MRI-based index for Crohn’s disease (MAGNIFI-CD) Index[47-49] (Table 3). The aim of these disease activity scores is to identify and integrate key radiological parameters associated with favourable long-term treatment outcomes, and their development reflects an effort to quantify disease activity and, in some cases, qualitatively assess disease severity in a more standardised and reproducible manner.

Table 3 Summary of diagnostic properties of magnetic resonance imaging-based indices for perianal Crohn’s disease activity.

	Van Assche Index	Modified Van Assche Index	MAGNIFI-CD
Score range	0-22	0-19.5	0-25
Contrast needed	No	Yes	Yes
AUROC[96]	0.925	0.908	0.869
Intra-rater ICC[49,50]	0.86 (0.81-0.90)	0.90 (0.86-0.93)	0.85 (0.77-0.90)
Inter-rater ICC[49,50]	0.66 (0.52-0.76)	0.67 (0.55-0.75)	0.74 (0.63-0.80)

AUROC: Area under the receiver operating characteristics; ICC: Intraclass correlation coefficient; MAGNIFI-CD: Magnetic resonance imaging-based index for Crohn’s disease.

The VAI, developed in 2003, is a radiological scoring system designed to assess treatment response in patients with perianal fistulising Crohn’s disease[47]. The VAI consists of six parameters that assess both the anatomical characteristics and inflammatory activity of fistula tracts. Studies have shown that while individual parameters within the VAI may not strongly correlate with short-term treatment outcomes, when combined, included parameters are associated with significant improvement over time, especially T2-weighted hyperintensity[47]. This has led to the VAI being widely adopted in clinical research[16]. However, limitations include its reduced sensitivity to changes in fistula size and inability to capture new disease activity, such as newly formed fistulas or abscesses[50]. To address these limitations, the mVAI was developed in 2017 using a Delphi-like consensus method[48]. It retained most components of the original VAI but added two new features, T1 post-contrast hyperintensity and the dominant tract feature, to enhance its clinical correlation and inter-rater consistency. External validation revealed that while the mVAI could differentiate between responders and non-responders, the results were mixed, with only a subset of clinical responders achieving a decrease in mVAI score[50]. Building further on these efforts, the MAGNIFI-CD index, introduced in 2019, incorporated variables with the highest predictive accuracy from a large trial cohort[49]. This score integrates both structural and inflammatory markers and demonstrated strong external validation. This has led to the MAGNIFI-CD being endorsed by the TOpClass consensus as the best available index for the assessment of perianal fistulising Crohn’s disease based on existing clinical evidence[16].

Strengths and limitations

MRI does have several limitations which are important to acknowledge. High cost and limited access may represent significant barriers to pelvic MRI, particularly in low resource healthcare settings, and in the context of undertaking serial imaging at short to medium-term intervals to evaluate change or appraise treatment response. Moreover, patient populations, such as those with pacemakers, metallic implants, renal impairment and those who suffer from claustrophobia, may have absolute or relative MRI contraindications. MRI also has limited sensitivity for the diagnosis of non-fistulising perianal disease, making it less effective in patients with non-fistulising manifestations of perianal Crohn’s disease[51].

EAUS

EAUS is a valuable imaging modality in the diagnosis and management of perianal fistulising Crohn’s disease, offering detailed assessment of the anal sphincter complex and surrounding tissue[52]. Typically performed using a 360° rotating axial endoprobe, EAUS generates high-resolution cross-sectional images of the anorectal region and is often performed in conjunction with lower gastrointestinal endoscopy[23,53] (Figure 3). The examination is usually performed with the patient in the left lateral decubitus or prone position, with the endoprobe gently inserted into the distal rectum and gradually withdrawn through the anal canal, allowing for real-time imaging of the internal and external anal sphincters, levator ani muscle, and perineal tissues[54]. The application of contrast agents, such as hydrogen peroxide, into fistula tracts have been used to enhance the echogenicity of fistulous pathways and improve tract visualisation[55]. However, recent expert consensus guidelines have questioned the clinical utility of this approach, concluding that the routine use of such contrast agents does not provide additional diagnostic benefit and is thus not recommended in standard EAUS protocols[32].

Figure 3 Evaluating perianal fistulising Crohn’s disease using endoanal ultrasound. A: Large intersphincteric abscess with gas content (reverberation artefact from gas) posterior; B: Anterior intersphincteric abscess; C and D: Posterior midline transphincteric fistula with corresponding magnetic resonance imaging; E and F: Low transphincteric fistula with corresponding magnetic resonance imaging. Arrows delineate perianal pathology seen on endoanal ultrasound.

Diagnostic accuracy

A meta-analysis comparing EAUS to the gold standard of EUA in terms of diagnostic accuracy demonstrated that EAUS has a sensitivity and specificity of 0.87 (95%CI: 0.70-0.95) and 0.43 (95%CI: 0.21-0.69), respectively, for the detection of idiopathic and Crohn’s disease related perianal fistulas[38]. The comparatively high sensitivity in ruling out perianal fistula reflects that EAUS may represent a useful adjunct to clinical assessment in the context of pre-operative planning to determine whether an EUA is needed. This was exemplified by Spradlin et al[56] in a small randomised controlled study of 10 Crohn’s disease patients with known perianal fistulas, all of whom underwent EAUS at baseline followed by an EUA and were then randomised to having either an EAUS at weeks 22 and 38 or clinical assessment. At 54 weeks, 80% of patients in the EAUS group achieved complete cessation of drainage compared to 20% in the control group, suggesting that EAUS-guided management may lead to more effective and targeted interventions in this setting.

One of EAUS’s key diagnostic advantages is its high accuracy in identifying internal fistula openings, with a reported diagnostic accuracy of 93%[57]. This led the ESGAR consensus to recommend EAUS over MRI in this specific setting due to its greater spatial resolution in this region[32]. Technological advancements, particularly the incorporation of three-dimensional (3D) imaging, have also enhanced the application of EAUS in the assessment of perianal fistulas. This is supported by findings of a meta-analysis of 12 studies evaluating the diagnostic performance of 3D-EAUS, reporting that 3D-EUAS had an overall accuracy in detecting perianal fistulas of 91% across all fistula types[58]. Notably, 3D-EAUS demonstrated high sensitivity (97%) but relatively low specificity (61%) for detecting the internal opening[58]. In a comparative study evaluating 3D-EAUS and MRI in the context of pre-operative fistula assessment, both modalities demonstrated comparable accuracy for simple fistulas, while MRI was superior in the evaluation of complex fistulising disease[59]. EAUS also has utility in differentiating fistula-in-ano from Crohn’s-related fistulas[60]. Zawadzki et al[61] described the “Crohn’s ultrasound fistula sign”, characterised by a hypoechogenic tract encircled by a distinct hyperechogenic area with a thin hypoechogenic rim. This sign was further validated by Zbar et al[62] who calculated the sensitivity, specificity, positive predictive value, negative predictive value and accuracy of the “Crohn’s ultrasound fistula sign” to be 43.6%, 97.5%, 80.9%, 87.5% and 86.8%, respectively.

Disease monitoring and response to therapy

While MRI remains the most commonly used and evidence-based imaging modality for the assessment of fistula activity and healing in perianal fistulising Crohn’s disease, multiple clinical guidelines, including the joint ECCO-ESGAR consensus, recognise EAUS as a reliable and effective alternative, particularly when performed by experienced operators[7]. However, there is currently a lack of clinical consensus standards to guide clinicians in differentiating between active and inactive or fibrotic fistulas using EAUS[63,64]. To address this, de la Portilla et al[65] developed a novel ultrasonographic classification system for perianal Crohn’s fistulae, validated against operative findings over 150 scans. This study demonstrated an 81.3% concordance between 3D-EAUS and surgical findings, with the classification system showing high inter-observer reliability (k = 0.86; 95%CI: 0.67-1.05). Nonetheless, this system still requires external validation[65]. Recent studies have also explored strategies to improve the accuracy of EAUS in the evaluation of perianal fistulising Crohn’s disease activity. Losco et al[66] demonstrated that computerised analysis of gray-scale tones on EAUS showed significant differences between active and inactive disease, with an area under the curve of 0.871 (95%CI: 0.765-0.938), indicating that this may represent a future strategy to evaluate disease activity.

EAUS has also been shown to influence treatment decisions. In a study by Wiese et al[67], patients receiving EAUS-guided treatment with adalimumab had a significantly higher rate of drainage cessation at 24 weeks compared to the control group (78% vs 27%, P = 0.04). Although this difference was not sustained at 48 weeks (P = 0.44), EAUS was still valuable in guiding treatment choices. Similarly, a study by Lahat et al[68] showed that EAUS influenced management in 86% of patients with perianal Crohn’s disease.

Strengths and limitations

Among techniques used to evaluate the perianal region, EAUS provides the highest spatial and temporal resolution, enabling real-time visualisation of submillimeter structures[69]. It is also useful in patients who cannot undergo or tolerate an MRI, including those with renal dysfunction given that it does not require intravenous contrast. Despite these strengths, EAUS has several limitations. It is semi-invasive, requiring probe insertion through the anal canal, which may be uncomfortable or unfeasible in patients with anal strictures or perianal abscess. The ultrasound beam also has limited penetration beyond the external sphincter, which restricts its ability to detect deeper extensions of fistulas, such as ischioanal or supralevator fistulas[70]. Moreover, distinguishing between fibrosis and active inflammation remains a diagnostic challenge, especially in recurrent disease where both may coexist within the same tract[7]. Finally, expertise in EAUS is not universal, and operator experience is likely to impact its clinical utility and application.

TPUS

TPUS is a non-invasive and increasingly adopted imaging technique in the assessment of perianal fistulising Crohn’s disease, particularly in the evaluation of rectovaginal and anovaginal fistulae and superficial perianal lesions[71]. It offers a well-tolerated alternative to EAUS, particularly for patients experiencing significant perianal discomfort or those unable to tolerate endocavitary procedures[70]. The examination is performed using linear transducers positioned externally over the perineum, directly above the anal verge. These probes typically provide a scanning depth of 5 to 8 cm, allowing for adequate visualisation of superficial and slightly deeper perianal structures[69]. In some protocols, contrast agents such as hydrogen peroxide or SonoVue (sulfur hexafluoride microbubbles) are introduced into the fistulous tract to enhance echogenicity and tract delineation[69].

Diagnostic accuracy

A meta-analysis comparing TPUS to MRI and EUA for the detection of fistulas in perianal Crohn’s disease found TPUS to have an overall sensitivity of 99% (95%CI: 96%-100%) compared to 96% and 99% for MRI and EUA, respectively. Using TPUS to detect the internal fistula opening has a sensitivity of 87% (95%CI: 77%-94%), while using TPUS to detect perianal abscesses had a sensitivity of 88% (95%CI: 62%-100%)[72]. Several studies have advocated for the use of TPUS as a preliminary tool in patients with suspected perianal fistulas[73-75]. This is supported by the European Federation of Societies in Ultrasound in Medicine and Biology[69] which endorsed TPUS as an effective imaging modality in patients with perianal fistulising Crohn’s disease. TPUS is valuable for identifying the presence and location of internal openings, assessing the relationship between fistulas and the sphincter complex, detecting perianal abscesses, and monitoring fistula progression over time[72,76]. These capabilities make TPUS a useful tool for both initial diagnosis and ongoing monitoring of perianal fistulising disease.

Disease monitoring and response to therapy

There is no universally agreed-upon criterion for assessing disease activity using TPUS[72]. However, it is generally accepted that an active fistula is associated with low echogenicity on TPUS. As the fistula heals, the echogenicity increases, correlating with closure of the tract and better long-term outcomes[74]. Regular TPUS assessments during treatment can provide valuable insight into fistula healing. In a study by Rasul et al[76], 35 patients with perianal Crohn’s disease were assessed by TPUS before and up to 48 weeks after treatment with infliximab. TPUS identified more fistulas than were appreciated clinically and detected unsuspected fluid collections. At 8 weeks, clinical assessments showed complete fistula closure in approximately half of the patients, while TPUS only showed complete healing in 2 out of 35 patients. This suggests that clinical improvement does not always correlate with complete ultrasonographic closure of fistulas, highlighting the potential utility of using TPUS to confirm clinical assessment, including assessments of treatment response[76].

Strengths and limitations

TPUS is a low-cost, high-resolution imaging tool that offers real-time, multiplanar views of the perineal and peri-rectal regions[71]. It is especially useful for patients who cannot tolerate probe insertion associated with EAUS, such as those with anal stenosis, as it is non-invasive and well-tolerated[77]. TPUS can detect a wide range of fistula types, including superficial, inter-sphincteric, transphincteric, extra-sphincteric, and supra-sphincteric variants. However, its penetration is limited, restricting its ability to assess deeper pelvic structures or complex fistulas[70]. It also has limited capacity to evaluate deeper muscles including the external anal sphincter and levator ani[78]. In such cases, complementary imaging with EAUS or MRI may be beneficial. Despite these limitations, TPUS remains a practical and valuable tool for initial assessment, treatment planning, and follow-up, particularly when MRI is not available, or frequent, non-invasive monitoring capable of evaluating interval change is valued.

COMPARING AND COMBINING IMAGING MODALITIES

Studies have compared the diagnostic capabilities of different imaging modalities in the context of diagnosing and monitoring perianal fistulising Crohn’s disease (Table 4). Buchanan et al[79] performed a large prospective clinical trial that compared pre-operative digital rectal examination (DRE), EAUS, and MRI. MRI was reported to be superior to both EAUS and DRE in terms of abscess detection and accuracy of fistula classification, with 90% of patients correctly classified by MRI, compared to 81% and 61% by EAUS and DRE, respectively[79]. In another comparative study by Bor et al[80], 23 patients with perianal fistulising Crohn’s disease underwent MRI, EAUS, and TPUS prior to EUA. TPUS showed superior diagnostic accuracy, with 100% sensitivity for both fistulas and abscesses. In comparison, MRI and EAUS had 84.6% sensitivity for fistulas, and 58.8% and 92.8% sensitivity for abscesses, respectively[80]. TPUS detected all 10 surgically confirmed abscesses, while MRI identified only 3[80]. Similarly, both MRI and EAUS misclassified one-third of fistulas and failed to detect 4 cases. Overall, these studies underscore the respective strengths and limitations of imaging modalities used to diagnose and classify perianal fistulas in Crohn’s disease. While MRI remains the gold standard for comprehensive evaluation of perianal fistulising Crohn’s disease, TPUS and EAUS are highly valuable in specific clinical settings, particularly for detecting abscesses and delineating fistula types. In a prospective blinded study by Schwartz et al[81], diagnostic accuracies for EUA, MRI, and EAUS were reported as 91%, 87%, and 91%, respectively, with diagnostic accuracy reaching 100% when any two modalities were combined. These findings highlight the potential advantages of a multi-modal imaging approach, though further research is needed to explore the utility of this approach in the longitudinal monitoring and assessment interval changes in perianal fistulising Crohn’s disease.

Table 4 Diagnostic performance of imaging modalities in perianal Crohn’s disease.

	Pelvic MRI[39,95]	TPUS[73,74]	EAUS[39]
Fistula detection	Sensitivity: 87%-100%; specificity: 69%-86%	Sensitivity: 85%-99.0%; specificity: 80%	Sensitivity: 87%; specificity: 43%
Internal opening detection	Sensitivity: 19%-97%; specificity: 71%-100%	Sensitivity: 87%-95%; specificity: 83%	Sensitivity: 88%-91%; specificity: 43%-100%
Perianal abscess detection	Sensitivity: 0%-60%; specificity: 97%-100%	Sensitivity: 86%; specificity: 100%	Sensitivity: 0%-40%; specificity: 97%-100%

MRI: Magnetic resonance imaging; TPUS: Transperineal ultrasound; EAUS: Endoanal ultrasound.

OTHER MODALITIES

Various non-traditional imaging modalities, such as X-ray fistulography, computed tomography (CT) fistulography, CT pelvis, and fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT, have been explored in the assessment of perianal Crohn’s disease. These techniques serve as alternative diagnostic options when standard imaging methods like MRI, EAUS, or TPUS are not available, unfeasible, contraindicated, or when expertise is lacking. Although these imaging methods can provide valuable insights, their diagnostic accuracy is generally lower compared to traditional imaging modalities such as MRI and ultrasonography. Moreover, all of these techniques involve ionising radiation, which limits their use in younger patients, particularly those of child-bearing age, and as part of long-term disease monitoring. Nevertheless, despite these limitations, these non-traditional imaging techniques remain useful in specific scenarios, making them important options for clinicians to be aware of.

X-ray fistulography

X-ray fistulography is one of the earliest techniques used to evaluate perianal fistulas. It involves cannulation of the external fistula opening and injection of contrast to visualise the tract through radiographs[63]. However, it has limited diagnostic value in perianal Crohn’s disease due to its poor anatomical resolution, low sensitivity (as low as 16%), and a notable false-positive rate[82]. As a result, it is now considered largely obsolete in clinical practice. A modified version of X-ray fistulography, which includes the use of a Foley catheter and a metal ring as a marker, has demonstrated improved diagnostic capability, with internal openings identified in 74.2% of cases and abscesses in 87.8%[83]. However, despite these improvements, this technique still lacks the precision of more advanced imaging modalities and is thus infrequently used.

CT fistulography

CT fistulography combines standard CT imaging with the injection of water-soluble contrast either per rectum or through the fistula opening. It allows for cross-sectional imaging of the perianal region and can be useful in emergencies or when traditional imaging modalities are not available, and imaging is time critical. However, it has a lower diagnostic accuracy than MRI, with studies reporting a 73% accuracy in classifying fistulas compared to 92% for MRI[84]. While CT fistulography may still play a role in acute settings or resource-limited environments, it is not recommended as the primary imaging modality for routine assessment of perianal fistulising Crohn’s disease[32,33].

CT pelvis

CT pelvis has a limited role in the assessment of perianal fistulising Crohn’s disease, but may be useful when MRI is unavailable, contraindicated, or when rapid evaluation of perianal symptoms is required. While MRI remains the gold standard due to its superior soft tissue contrast and ability to delineate fistula anatomy and inflammation, CT can be useful for detecting acute complications such as abscesses, or pelvic collections in patients with perianal fistulising Crohn’s disease[85]. However, CT is less sensitive than MRI in the evaluation of the extent and complexity of fistula tracts, particularly in the setting of active inflammation or subtle secondary extensions.

FDG-PET/CT

FDG-PET/CT is a functional imaging technique that assesses metabolic activity to detect inflammation. By using 18F-FDG uptake, this modality offers the potential to identify active inflammatory changes in perianal fistulas which may not be clearly visible on CT imaging alone[86]. Preliminary studies evaluating the utility of PET-CT in perianal fistulas have shown interesting results. However, it is important to note that these findings were derived from incidental fistula detection, reflecting that imaging was not undertaken specifically to identify and evaluate perianal fistulas in patients with Crohn’s disease[87]. Despite its promise, the use of FDG-PET/CT in diagnosing and monitoring perianal fistulising Crohn’s disease is limited by several factors including radiation exposure, access, cost, a lack of standardised imaging protocols, and insufficient longitudinal data. Hence, further research is therefore required to validate its clinical utility and explore its integration with conventional imaging techniques, such as MRI.

EMERGING IMAGING MODALITIES

Emerging imaging technologies such as 3D modelling, assessment of perianal fistula volumes, and artificial intelligence (AI) are reshaping the diagnostic and monitoring landscape of perianal Crohn’s disease. These modalities aim to overcome the limitations of conventional imaging by offering more detailed visualisation, objective disease quantification, and enhanced interpretation of perianal imaging.

3D modelling

3D modelling enhances standard two-dimensional MRI by transforming anatomical data into colour-coded, simplified 3D representations of the anorectal region. These reconstructions improve the visualisation of complex perianal fistula anatomy and enable clinicians to better appreciate the spatial relationships of fistula tracts and associated structures[88]. These models serve as valuable communication tools between clinicians and patients[89]. Researchers have also demonstrated the effectiveness of integrating AI algorithms with 3D models to accurately identify and document intricate and often complex fistula tracts, offering potential for improved diagnosis and treatment planning[90].

Assessment of perianal fistula volumes

Volumetric MRI, which quantifies fistula tract volume through 3D reconstruction, offers a promising approach for the assessment disease activity, including treatment response, in perianal fistulising Crohn’s disease[91]. Preliminary studies suggest reductions in fistula volume may correlate with symptomatic improvement more accurately than traditional clinical indices alone[92,93]. This was exemplified in a recent retrospective study by Ordás Jiménez et al[92] which investigated changes in 3D volumetry of perianal fistulas in patients with Crohn’s disease using pelvic MRI. The study demonstrated that reductions in the volumetric size of the active fistula component, defined by T2-weighted hyperintensity, were predictive of clinical remission following biological therapy. Notably, a ≥ 16% reduction in the active volume was associated with high sensitivity (84.6%) and specificity (81%) for remission, highlighting the potential of volumetric assessment of perianal fistula volumes as an objective tool for monitoring treatment response. When combined with standardised imaging protocols and radiological disease activity scores across serial assessments, volumetric analysis may serve as a valuable complement to existing scoring systems. In addition, volumetric assessment enables 3D reconstruction of complex fistulas, offering a novel way to enhance patient understanding of this challenging condition by illustrating how their disease has changed over time following initiation of medical therapy.

AI in image interpretation

The application of AI in enhancing MRI interpretation for perianal Crohn’s disease holds considerable promise. Deep learning models, particularly convolutional neural networks, have shown strong capability in accurately identifying relevant anatomical features and disease-specific abnormalities. This was exemplified by reports that AI algorithms were able to achieve high rates of detecting perianal abscesses (84%), fistula tracts (80%), and internal openings (92%) on MRI[94]. These predictive tools have the potential to support radiological assessments of response and healing. They may also play an important role in individualised treat-to-target strategies aimed at improving long-term outcomes in patients with perianal fistulising Crohn’s disease.

PROPOSED IMAGING ALGORITHM

Selecting the most appropriate imaging modality to image perianal fistulas in Crohn’s disease should be guided by the specific diagnostic question, disease complexity, patient-related factors, and the need and interval for disease reassessment. Although MRI remains the gold standard, EAUS and TPUS are valuable adjuncts and may even be preferable in specific clinical scenarios. Each modality has distinct strengths and limitations across three key stages of care: Diagnosis, monitoring, and assessment of interval change (Figure 4).

Figure 4 Imaging in perianal fistulising Crohn’s disease. MRI: Magnetic resonance imaging; TOpClass: Treatment Optimisation and CLASSification; VAI: Van Assche Index; mVAI: Modified Van Assche Index; MAGNIFI-CD: Magnetic resonance imaging-based index for Crohn’s disease. ¹Sooner if persistent symptoms, or planned treatment change; ²Supported by TOpClass.

At diagnosis or first presentation of perianal symptoms, pelvic MRI is the preferred first-line modality, particularly for complex or deep-seated fistulising disease. MRI is capable of providing comprehensive pelvic mapping, delineating the extent of fistulous tracts, and helping to inform surgical planning[95]. While timely access to MRI is ideal, delays should not hinder appropriate clinical management, particularly in the setting of acute perianal pathologies including abscesses. In patients with contraindications to MRI, EAUS offers high-resolution imaging of the anal sphincter complex and is especially useful for detecting internal openings and characterising simple, low-lying fistula tracts. However, the use of EAUS is limited by its invasive nature, operator dependency, and restricted field of view. TPUS is optimal for superficial or anterior fistulas and for identifying associated superficial abscesses. It is non-invasive, inexpensive, and relatively quick to perform, making it a suitable tool for point-of-care use. Both EAUS and TPUS complement MRI and may be more appropriate in certain clinical scenarios, specifically resource-limited and urgent care settings.

For monitoring following medical or surgical intervention, MRI remains the optimal modality for long-term evaluation, with repeat imaging typically recommended at 6 to 12 months to assess response and healing, respectively. However, regular MRI may not be feasible due to cost or access limitations. In these instances, EAUS and TPUS present practical alternatives in centres with appropriate expertise and volume. EAUS is particularly valuable for the assessment of sphincter anatomy, while TPUS is well suited to frequent or serial evaluation of superficial fistulous disease and abscesses.

When appraising treatment response and fistula healing, MRI remains the preferred imaging modality, particularly in cases of complex or refractory disease. Consensus frameworks such as TOpClass support MRI-based definitions of response, with radiological healing typified by the transition from active inflammation to fibrosis. Several MRI-based scoring systems have been developed to quantify inflammatory activity and assess therapeutic response[96], with the TOpClass consortium favouring the MAGNIFI-CD score. In contrast, EAUS and TPUS lack validated criteria or established scoring systems capable of objectifying treatment response and are therefore primarily qualitative in nature. Nonetheless, EAUS and TPUS remain valuable adjuncts, particularly for detecting early tract closure, monitoring for disease-related complications, and evaluating sphincter involvement, especially in circumstances where MRI is unavailable, delayed, or when repeated imaging and a multimodal strategy may be clinically advantageous.

Future research efforts should focus on establishing objective and reproducible ultrasound scoring systems for perianal Crohn’s disease. Potential directions include defining quantifiable sonographic parameters (e.g., fistula tract thickness, abscess size, number of branches, internal opening dimensions, and vascularity on Doppler) that can be correlated with clinical indices and MRIbased scores, adopting advanced techniques such as colour Doppler and shearwave elastography to measure vascularity and tissue stiffness and distinguish active from fibrotic disease, and standardising acquisition protocols, including probe type, scanning planes, patient positioning and the use of contrast agents, to minimise operator variability. Candidate scoring systems should then undergo prospective multicentre validation to assess inter and intraobserver reproducibility, define normal parameter ranges and evaluate their relationship with treatment outcomes. Finally, a multidisciplinary consensus process involving gastroenterologists, colorectal surgeons, radiologists and sonographers will be essential to agree on core imaging variables and their weighting within a composite ultrasoundbased activity index.

CONCLUSION

Perianal fistulising Crohn’s disease remains a complex and debilitating manifestation of inflammatory bowel disease, that requires accurate, reproducible, and accessible imaging tools to inform multidisciplinary medical and surgical decision-making. While MRI remains the gold standard for the diagnosis, classification, and monitoring of perianal fistulas, limitations including cost, accessibility, and contraindications, represent important considerations. This highlights the need for alternative imaging modalities, such as EAUS and TPUS, that are accurate and capable of being repeated at short-to-medium intervals to complement assessments of treatment response using MRI. EAUS offers high spatial resolution, particularly in the detection of internal fistula openings and sphincteric involvement, while TPUS provides a non-invasive, well-tolerated, and highly sensitive imaging modality for the detection of superficial fistulas and abscesses. These characteristics highlight that patient selection and understanding the nature of their disease remains central to determining which imaging modality is optimal to evaluate perianal fistula in Crohn’s disease.

The recent development of consensus MRI-specific definitions for radiological healing and improvement represents an important first-step in the standardisation of radiological treatment targets in perianal Crohn’s disease. However, equivalent standardised criteria for ultrasound modalities such as TPUS and EAUS are lacking, illuminating a potential gap in the current literature. Addressing this gap remains particularly relevant given that a multimodal imaging strategy, that leverages the relative strengths of MRI, EAUS, and TPUS, may provide the optimal framework for short, medium, and long-term radiological assessment of perianal fistulising Crohn’s disease. Future research should focus on the external validation of imaging-based activity indices, the development of ultrasound-specific treatment targets, and the integration of advanced imaging technologies, including 3D modelling, volumetric fistula assessment, and AI, into clinical practice.