Transperineal ultrasound: Role in inflammatory bowel disease management

Abstract

Transperineal ultrasound (TPUS) is a non-invasive, real-time imaging technique increasingly utilized for the evaluation of anorectal and pelvic floor disorders in patients with inflammatory bowel disease (IBD). In ulcerative colitis, it enables accurate assessment of rectal wall thickness and vascularity, which correlate closely with both endoscopic and histological inflammation. This makes it a practical alternative to sigmoidoscopy, especially in settings such as pregnancy, childhood, or long-term disease monitoring. In Crohn’s disease, TPUS offers high diagnostic accuracy for detecting, classifying, and monitoring perianal fistulas and abscesses. It is also effective for evaluating rectovaginal fistulas and pouch-related complications following restorative proctocolectomy. Advancements such as color Doppler imaging, contrast-enhanced ultrasound, three-dimensional reconstruction, and computer-assisted grayscale analysis further enhance its diagnostic and functional capabilities. Compared to magnetic resonance imaging and endoscopic ultrasound, TPUS serves as a practical, patient-friendly, and cost-effective frontline or complementary imaging tool for comprehensive disease evaluation and treatment planning-particularly useful in children, pregnant women, and resource-limited settings. Its bedside feasibility, repeatability, and expanding role in treatment monitoring underscore its value in modern IBD care pathways.

Key Words: Transperineal ultrasound; Inflammatory bowel disease; Ulcerative colitis; Crohn’s disease; Proctitis; Fistula

Core Tip: Transperineal ultrasound (TPUS) is a non-invasive, bedside imaging tool for assessing anorectal inflammation and fistulizing disease in patients with inflammatory bowel conditions. It enables real-time evaluation of rectal inflammation in ulcerative colitis and offers reliable detection and monitoring of perianal fistulas and abscesses in Crohn’s disease. Its utility extends to children, pregnant women, and patients with pouch-related complications after surgery. Enhanced techniques-such as Doppler imaging, contrast administration, three-dimensional ultrasound, and computerized analysis-expand its diagnostic power. TPUS serves as a practical, repeatable, and cost-effective imaging option, especially as a first-line screening tool or when magnetic resonance imaging is unavailable, contraindicated, or impractical.

INTRODUCTION

Anorectal and perianal complications are among the most challenging manifestations of inflammatory bowel disease (IBD), particularly Crohn’s disease (CD) and ulcerative colitis (UC). These conditions often require repeated assessments to guide therapeutic decisions, detect complications, and monitor treatment response. While endoscopy and magnetic resonance imaging (MRI) remain the established standards for evaluation, they are limited by cost, accessibility, patient discomfort, and repeatability-particularly in scenarios such as pediatric care, pregnancy, or urgent outpatient settings. Transperineal ultrasound (TPUS), in contrast, offers a non-invasive, real-time, and bedside imaging solution that is repeatable, well-tolerated, and resource-efficient. In this context, TPUS has gained increasing attention as a non-invasive, real-time, and bedside tool capable of visualizing both structural and functional aspects of anorectal disease[1].

Despite its growing use, the role of TPUS in IBD remains under-recognized and inconsistently applied across clinical settings. Clinicians often lack clear guidance on its technical implementation, interpretation of findings, or how it compares with other imaging modalities such as MRI or endoscopic ultrasound (EUS)[2,3]. Moreover, the expanding applications of advanced features-such as use of microconvex probe, color Doppler, contrast enhancement, three-dimensional (3D) imaging, and computer-assisted quantification-have not been comprehensively reviewed in a single framework to support informed clinical decision-making[1,4,5].

In this review, we synthesize the current evidence and evolving applications of TPUS in IBD. We emphasize its potential as a frontline imaging modality in specific populations (children, pregnant women) and outpatient scenarios where rapid, repeatable, and radiation-free assessment is valuable. We cover its use in CD for the detection, classification, and monitoring of perianal fistulas and abscesses; in UC for assessing rectal inflammation; and in post-surgical pouch evaluation. We also highlight novel adjuncts such as Doppler imaging, contrast-enhanced ultrasound (CEUS), 3D reconstruction, and computer-aided interpretation. By contextualizing these findings within clinical practice, we aim to clarify the utility, limitations, and future directions of this promising imaging modality.

TPUS TECHNIQUE

Patient preparation and positioning

TPUS should begin with verbal consent, and the procedure is ideally performed in the presence of a female assistant when examining female patients. Bowel preparation is generally not required unless TPUS is scheduled on the same day as colonoscopy, in which case 1-2 L of polyethylene glycol solution may be used. To ensure patient comfort and modesty, disposable colonoscopy pants are recommended. The patient is positioned in the left lateral decubitus position with the right leg gently flexed at the hip and knee to allow full exposure of the perineum (Figure 1A)[1]. This standardized, reproducible positioning enhances both operator comfort and imaging consistency.

Figure 1 Transperineal ultrasound. A: Schematic demonstrating transperineal ultrasound probe placement in the left lateral decubitus position; B: High-frequency linear probe (e.g., LA2-14A) for superficial structures and rectal wall imaging; C: Microconvex probe (CA4-10M), offering superior maneuverability within the perineal cleft.

Equipment and imaging setup

The examination can be performed using high-end ultrasound (US) systems, equipped with high-frequency linear (2-14 or 2-18 MHz) (Figures 1B and 2A), or microconvex (4-10 MHz) (Figures 1C and 2B-D). The advantage of micro convex probe is easy manoeuvrability in the anal cleft[1]. Harmonic imaging should be enabled for optimal resolution. The probe should be covered with a condom or saran wrap filled with coupling gel to prevent contamination. For color Doppler imaging, the gain should be set between 3-5 MHz with flow velocities calibrated between 5-8 m/second (Figures 2D and 3A). A low pulse repetition frequency (approximately 800 Hz) is ideal for detecting slow-flow signals associated with inflammatory activity in the perianal region[6]. Use of Doppler and CEUS should be guided by the clinical question, e.g., activity monitoring or detection of secondary tracts.

Figure 2 Perineal ultrasound for measuring rectal thickness using various probes. A: Sagittal view on transperineal ultrasound with linear probe showing the anal canal and rectum; B: Axial image showing hypoechoic internal anal sphincter and hyperechoic external anal sphincter; C: Axial image showing rectal wall thickness; D: Doppler-enhanced axial view showing color flow within perianal structures (Modified Limberg scoring in progress). AC: Anal canal; R: Rectum.

Figure 3 Perineal ultrasound in anovaginal fistula. A: Sagittal view showing anterior anorectal anatomy including bladder and vagina; B: Doppler imaging depicting perianal vascularity suggestive of active inflammation; C: High-frequency linear probe image showing superficial anovaginal fistula, with air tracking inside fistula; D: Volumetric three-dimensional transperineal ultrasound reconstruction of fistula. AR: Anterior anorectal; B: Bladder; V: Vagina; F: Fistula.

Technique and anatomical landmarks

The probe is initially positioned horizontally anterior to the anus, with the vertical marker on probe oriented towards the operator. In this orientation, the sagittal section of the anorectum is visualised (Figure 3A and B). The superficial, vertical part of the anorectum is the anal canal (AC) and the horizontal, deeper part is the rectum (Figure 2A). The inner, hypoechoic part of the AC is the internal anal sphincter and the outer, hyperechoic part is the external anal sphincter (Figure 2B). In this section, the anterior structures like vagina and bladder with urethra are seen on the right of the screen anterior to anorectum (Figure 3B). To obtain coronal views, the probe can be rotated 90 degrees counterclockwise (Figure 2C). In this position, orienting the probe tip towards the coccyx can help obtain the axial section (Figure 2D). To simplify spatial orientation, the 12 o’clock position on a traditional clock is rotated to 3 o’clock in TPUS left lateral decubitus position (Figure 4), enhancing intuitive correlation with clinical examination. Tilting the probe anteriorly or posteriorly allows visualization of tracts extending toward the scrotum (12 o’clock) or coccyx (6 o’clock), respectively. Likewise, upward or downward tilting allows visualization of lateral extensions (3 o’clock or 9 o’clock on the clock face). Sometimes a gliding movement in perineum may be warranted to follow the external opening of a long fistula tract. High-frequency linear probes (e.g., 2-18 MHz) are especially useful for superficial tracts such as anovaginal or anovulvar fistulas (Figure 3C)[1]. The 3D probe can be used to obtain 3D image of the fistula keeping the cursor at the fistula level (Figure 3D).

Figure 4 Clock-face orientation comparison for fistula reporting: Lithotomy position (left) vs transperineal ultrasound in left lateral decubitus (right). In transperineal ultrasound, 12 o’clock is anterior (screen right), and 6 o’clock is posterior (screen left).

Assessment and reporting standards

Fistulas should be described using Parks classification (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric), and clock-face orientation should be used to report internal and external openings. In left lateral position, the 12 o’clock reference corresponds to the anterior midline (3 o’clock with respect to operator)[1]. Routine parameters assessed include bowel wall thickness (BWT) (normal < 3 mm in colon and < 4 mm in rectum), wall stratification (preserved, obscure, or lost), presence of abscesses, and internal openings. Color Doppler signal is interpreted using the Modified Limberg score to grade vascularity[6]. When available, contrast agents like diluted hydrogen peroxide may be used to enhance visualization of fistulous tracts and identify secondary branches or cavities[7]. Reporting templates should include tract type, location, length, internal and external openings, associated abscess, and Doppler/CEUS findings.

Image acquisition and interpretation

Images should be obtained in both sagittal and axial planes. Dynamic clips or cine loops are encouraged to document tract extension and real-time features such as fluid motion or Doppler activity. All images should be stored in a archiving platform to facilitate surgical planning or follow-up comparisons. TPUS findings should always be interpreted in conjunction with clinical findings and may serve as a baseline for longitudinal imaging in chronic disease monitoring.

Operator considerations

TPUS may have reduced accuracy in imaging supralevator or deeply located lesions due to limited probe penetration beyond 5-6 cm. Air artifacts from perineal skin folds may occasionally interfere with image quality. Operator experience plays a crucial role in interpretation; thus, training in standardized probe manipulation, anatomical correlation, and Doppler optimization is essential for consistent, high-quality TPUS assessments[8]. A structured learning curve and incorporation into point-of-care training modules are essential for reliable TPUS adoption in clinical practice.

TPUS IN CD

Fistula

Fistula detection: In CD, TPUS is valuable for screening, characterizing, serial follow up and guiding therapy in perianal fistulas and abscesses. TPUS is a highly accurate, non-invasive modality for detecting perianal fistulas. According to a meta-analysis by Maconi et al[9], TPUS achieved a pooled sensitivity of 98% and a positive predictive value (PPV) of 95% for fistula detection, with even higher sensitivity (99%) in CD-specific cohorts. Its point-of-care accessibility and minimal patient preparation requirements position it as an ideal first-line tool for bedside evaluation.

Fistula classification: TPUS is also effective in classifying fistulas according to Parks’ criteria. Correlation for fistula classification in reference to operative findings is 85.7% for both cryptoglandular and CD fistula[10]. Systematic review reported a pooled classification sensitivity of 92% (95%CI: 85%-97%) and PPV of 92%[9]. Classification accuracy was marginally lower when compared against MRI vs exam under anesthesia, highlighting the importance of multimodal correlation. TPUS is especially well-suited to identifying low and intermediate tracts, with reduced accuracy for complex, high, or branching configurations[9]. Integration with Doppler and contrast-enhanced imaging can improve delineation in borderline cases.

Most commonly, TPUS identifies intersphincteric (Figure 5A) and transsphincteric tracts (Figure 5B and C), which appear as hypoechoic linear structures between or across sphincters. These tracts may be visualized dynamically by tilting the linear probe in horizontal position. On tilting if they are going towards the right or left buttock they are around 9 o’clock and 3 o’clock respectively. Gliding the probe anteriorly and posteriorly may be required to trace the external opening towards the scrotum and coccyx respectively in the 12 o’clock and the 6 o’clock position. Trans-sphincteric fistula traverses both the sphincters after arising from the AC and draining into the external opening. While superficial fistulas are occasionally observed, extrasphincteric and suprasphincteric tracts are more challenging and may necessitate adjunctive MRI or transrectal US (Figure 5D) for full delineation[2,9].

Figure 5 Inter- and trans- sphincteric fistula. A: Transperineal ultrasound sagittal image showing intersphincteric fistula between internal anal sphncter and extenal anal sphicter (EAS); B: Transsphincteric fistula originating from anal canal and extending through EAS (12 o’clock); C: Axial view showing orientation of the long tract going anteriorly; D: Complex transsphincteric tract with anterior extension seen on transrectal ultrasound. F: Fistula; EAS: Extenal anal sphincter; IAS: Internal anal sphincter.

Internal opening: Internal openings are a critical target for fistula mapping. TPUS demonstrated a pooled sensitivity of 91% (95%CI: 84%-97%) and a PPV of 87% (95%CI: 76%-95%) in detecting internal openings. In CD-specific studies, the sensitivity for internal opening detection remained robust at 87%, although these findings are more variable due to operator dependency and interference from air or fibrosis[9]. In cryptoglandular cases, abscesses with associated acoustic shadowing precluded identification of internal opening site[10]. Carefully guided probe positioning and experience are essential for confident identification. Structured training and probe manipulation techniques enhance accuracy.

Abscess

For perianal abscesses, TPUS provides real-time identification of hypoechoic or anechoic fluid collections with posterior acoustic enhancement, often accompanied by peripheral hyperemia on color Doppler. The pooled sensitivity across studies is 86% (95%CI: 67%-99%) with a PPV of 90%, although slightly lower in MRI-validated cohorts[9]. TPUS is particularly effective in detecting low-lying or intersphincteric abscesses and is valuable in bedside monitoring after surgical drainage or seton placement. However, its diagnostic yield declines for deeper collections (e.g., ischiorectal or supralevator), where adjunctive MRI may be required[2].

TPUS in pediatric perianal CD

TPUS is increasingly recognized as a valuable, well-tolerated imaging tool in children with perianal CD (PACD). In a preliminary study by Hwang et al[11], TPUS successfully identified fistulas and abscesses in pediatric patients with minimal discomfort, showing active disease in 47% of exams and abscesses in 19%, with good correlation to clinical symptoms. Lee et al[12] reported a sensitivity of 76% for fistulas and 56% for abscesses on TPUS compared to MRI, with better concordance than colonoscopy, especially for abscess detection. In a study comparing TPUS with MRI in treatment monitoring, Jung et al[13] found TPUS had excellent specificity (93.3%) and PPV (95.0%) for detecting treatment response, though with modest sensitivity (63.3%). Notably, superficial and thinner fistulas were more likely to be missed[13]. Together, these findings support the use of TPUS as a practical first-line and monitoring tool in pediatric PACD, particularly for intersphincteric and low transsphincteric disease, though MRI remains necessary for complex or supralevator tracts (Table 1).

Table 1 Studies evaluating transperineal ultrasound in pediatric perianal Crohn’s disease.

Ref.	Patients (n)	Main findings	Concordance or accuracy vs MRI
Hwang et al[11], 2014	43	TPUS feasible in all; 47% with active fistula, 19% with abscess; well tolerated	No MRI comparison; pain severity correlated with activity
Lee et al[12], 2018	38	TPUS sensitivity 76% (fistula), 56% (abscess); better concordance with MRI than colonoscopy	κ = 0.296 for fistula, 0.624 for abscess
Jung et al[13], 2022	29	TPUS detected 80% of fistulas; excellent PPV (95%) and specificity (93%) for response	κ = 0.486; sensitivity 63%, accuracy 73%
Jung et al[13], 2022 (MRI/TPUS) analysis	56 fistulas analyzed	TPUS more likely to miss superficial/thin tracts; intersphincteric better detected	Detection correlated with thickness (P = 0.009); 80.4% of MRI fistulas seen on TPUS

TPUS: Transperineal ultrasound; MRI: Magnetic resonance imaging; PACD: Perianal Crohn’s disease; PPV: Positive predictive value, κ: Cohen’s kappa coefficient.

TPUS for rectovaginal fistulas

Rectovaginal fistulas (RVFs) are a debilitating and often under-recognized complication of CD, typically requiring accurate anatomical delineation to guide therapy (Figure 6). In a prospective comparative study, Maconi et al[14] demonstrated that TPUS successfully detected all 9 RVFs and 2 anovulvar fistulas among 46 CD patients, with 84.9% sensitivity and 86.5% PPV compared to endoanal US (EUS) as the reference standard. TPUS provided clear visualization of the fistulous tract between the rectum and vagina (Figure 6A-C), without the need for invasive or painful procedures, making it especially advantageous in patients with anal stenosis or pain. While MRI and EUS remain standard for pre-surgical mapping, TPUS offers a practical, non-invasive alternative for screening and follow-up, particularly when access to cross-sectional imaging is limited.

Figure 6 Rectovaginal fistula. A: Intersphincteric fistula at 6 o’clock; B: Annotated image showing fistulous connection between rectum and vagina; C: Superficial anovaginal fistula; D and E: Transperineal ultrasound axial images demonstrating posterior intersphicteric fistulous extension; F: Endoscopic image of perineum. EAS: Extenal anal sphincter; IAS: Internal anal sphincter.

Computer-assisted assessment of fistula activity

Quantifying perianal fistula activity remains a clinical challenge, particularly for monitoring treatment response. Two studies by Caprioli et al[15] and Losco et al[4] demonstrated that computer-assisted analysis of anal US images, based on grayscale tone measurement, can objectively distinguish active from inactive fistulas in CD. Active tracts appear more hypoechoic, with a defined grayscale cutoff (mean < 117-118) discriminating inflammation with good diagnostic accuracy [area under the curve (AUC) approximately 0.87-0.90][4]. This approach improved agreement with clinical and MRI findings from fair (κ = 0.26-0.29) to good levels (κ = 0.61-0.77)[15]. While not yet routine in clinical practice, these studies highlight the potential role of computer-enhanced US as a reproducible, operator-independent tool for evaluating perianal disease activity, especially when combined with clinical indices such as pouchitis disease activity index (PDAI) or fistula drainage assessment.

Comparison of MRI and TPUS in PACD

Multiple studies have compared TPUS with MRI in assessing PACD, highlighting its strengths as a frontline, non-invasive imaging tool. In a prospective adult cohort, Maconi et al[2] reported excellent agreement between TPUS and MRI for fistula classification (κ = 0.78), with TPUS achieving 94% sensitivity and correctly classifying over 90% of fistulas. However, its sensitivity was reduced for suprasphincteric and extrasphincteric tracts and deep abscesses. Bor et al[16] similarly reported 100% sensitivity for both fistulas and abscesses using TPUS, outperforming MRI and transrectal US in a surgical validation study, recommending TPUS as a first-line tool in complex cases. Terracciano et al[17] further demonstrated strong agreement between TPUS and MRI using both Parks (κ = 0.67) and American Gastroenterological Association (κ = 0.83) classifications. Wedemeyer et al[18] validated the use of TPUS with conventional probes in 25 CD patients, achieving excellent agreement with MRI (κ > 0.83), and detecting 90% of fistulas and 86% of abscesses. Importantly, TPUS was feasible and well tolerated even in patients with anal stenosis, providing real-time, high-resolution imaging without the need for sedation or preparation.

In pediatric populations, Jung et al[13] observed moderate agreement between TPUS and MRI (κ = 0.49) for treatment response, with TPUS showing excellent specificity (93%) and PPV (95%). Lee et al[12] demonstrated that TPUS had better concordance with MRI than colonoscopy in detecting both fistulas and abscesses, though sensitivity remained modest.

Overall, these studies position TPUS as an effective modality for characterizing and monitoring low-lying perianal fistulas. While MRI remains superior for deep, supralevator, complex secondary, high tracts or horseshoe extensions, TPUS offers a rapid, accessible, and patient-friendly option for triaging and guiding treatment in PACD (Table 2).

Table 2 Summary of studies comparing transperineal ultrasound with magnetic resonance imaging or surgical findings in perianal Crohn’s disease and inflammatory bowel disease.

Ref.	No. of patients	Patient characteristics	Concordance rate (TPUS vs MRI or surgery)	Main findings
Maconi et al[2], 2013	59	Adults with CD, prospective	κ = 0.78 (fistulae classification)	TPUS sensitivity 94%, PPV 93%; less accurate for extrasphincteric/suprasphincteric tracts and deep abscesses
Bor et al[16], 2016	23	Adults with complicated CD	Not specified	TPUS 100% sensitivity for fistulae and abscesses; outperformed MRI and TRUS
Jung et al[13], 2022	29	Pediatric CD on biologics	κ = 0.49	TPUS had high specificity (93%) and PPV (95%) for assessing treatment response vs MRI
Lee et al[12], 2018	38	Pediatric CD with PACD	κ = 0.30-0.62	TPUS better concordance with MRI than colonoscopy; sensitivity 76% for fistulae, 56% for abscesses
Wedemeyer et al[18], 2004	25	Adults with active PACD	κ > 0.83	TPUS detected 90% of fistulas and 86% of abscesses; comparable to MRI; especially useful in anal stenosis
Terracciano et al[17], 2016	28	IBD patients	Sensitivity 100% for RVF and superficial abscess; κ = 0.34 for deep abscess	TPUS highly concordant with MRI for Parks (κ = 0.67) and AGA (κ = 0.83) classifications; superior for RVF and superficial abscesses

TPUS: Transperineal ultrasound; MRI: Magnetic resonance imaging; CD: Crohn’s disease; PACD: Perianal Crohn’s disease; TRUS: Transrectal ultrasound; RVF: Rectovaginal fistula; AGA: American Gastroenterological Association, κ: Cohen’s kappa statistic; PPV: Positive predictive value.

TPUS in rectal inflammation

TPUS can also be used to assess rectal inflammation in CD, although its primary utility is in UC. Rectal wall thickening, increased vascularity on Doppler, and loss of stratification may be visualized[1]. TPUS can be used serially in patients on therapy to assess response, especially when sigmoidoscopy is not feasible. Its limited depth penetration necessitates caution in patchy or high rectal disease.

Monitoring of seton placement and healing

In CD patients with complex fistulas, TPUS offers a non-invasive method to monitor the position and effect of setons. The seton appears as a linear hyperechoic structure traversing the hypoechoic fistulous tract (Figure 7). TPUS can assess for residual tracts, early closure, secondary tracts, or ongoing inflammation around the seton, which may necessitate further intervention. It also allows assessment of epithelialization of fistulous tracts over time, guiding the decision for seton removal or escalation of biologic therapy[1]. It enables safe, serial follow-up without repeat MRI.

Figure 7 Seton on transperineal ultrasound. Hyperechoic linear seton visualized within hypoechoic fistulous tract, indicating treated perianal disease under follow-up.

TPUS IN UC

TPUS is now recognized as a robust, non-invasive tool for evaluating rectal inflammation in UC, with validated applications across disease activity monitoring, treatment response, and mucosal healing. Early prospective work demonstrated that a rectal BWT ≥ 4 mm predicted endoscopic and histological healing with > 90% accuracy, while reduction in BWT as early as one week into therapy predicted week 8 remission, outperforming C-reactive protein and fecal calprotectin[6,19].

Building on this, the TRINITY study by Pal et al[3] compared TPUS, transabdominal US (TAUS), and EUS in a large cohort of 142 patients (Figure 8). While EUS showed the highest accuracy for predicting endoscopic and histologic remission, TPUS achieved moderate diagnostic performance (AUC 0.73 and 0.66, respectively). Notably, combining TAUS and TPUS improved performance modestly, highlighting the complementary value of a dual-modality approach in resource-constrained or endoscopy-limited settings[3]. These findings support TPUS as a frontline option in outpatient surveillance, especially in patients for whom EUS or repeated endoscopy is not feasible.

Figure 8 Transperineal ultrasound in ulcerative colitis. A and B: Transabdominal ultrasound showing rectal wall thickness with Doppler signal; C: Transperineal ultrasound (TPUS) axial image assessing rectal inflammation (bowel wall thickness > 4 mm); D: TPUS Doppler showing modified Limberg score 2; E and F: Endoscopic ultrasound showing rectal mucosa and layered wall; Doppler overlay (F) confirms vascular activity.

In pediatric UC, Tokushima et al[20] used superb microvascular imaging with TPUS and found that the combination of rectal wall thickening > 4.5 mm and microvascular signal at wall circumference achieved 100% sensitivity and specificity for detecting active disease. Meanwhile, in pregnancy, Greeve et al[21] confirmed TPUS as a feasible and well-tolerated modality, capable of influencing management even in the third trimester. These findings underscore TPUS’s unique value in vulnerable populations where endoscopy and MRI may be impractical.

Collectively, these studies establish TPUS as a practical, scalable, and repeatable tool for longitudinal UC care, particularly in distal colitis and vulnerable populations like children and pregnant women[20,21]. To fully standardize its use, further research should aim to validate segment-specific cutoffs, assess reproducibility across platforms, and define TPUS-guided therapeutic thresholds (Table 3).

Table 3 Studies evaluating transperineal ultrasound in ulcerative colitis.

Ref.	No. of patients	Patient characteristics	Concordance rate	Main findings
Sagami et al[6], 2020	84	UC patients undergoing induction therapy; correlation with endoscopy and histology	BWT ≥ 4 mm correlated with MES > 1; sensitivity > 90%	TPUS can predict mucosal and histological healing with high accuracy
Sagami et al[19], 2022	100	UC patients on induction therapy; early TPUS response at week 1 vs remission at week 8	ΔBWT at week 1 predictive of week 8 remission (AUC = 0.75)	ΔBWT ≥ 1 mm at 1 week predicted treatment success; more accurate than CRP/FC
Greeve et al[21], 2024	8	Pregnant women with ulcerative proctitis	Feasibility and acceptability in pregnancy; no adverse events	TPUS feasible and safe during all pregnancy trimesters, including 3rd trimester
Tokushima et al[20], 2024	131	Pediatric patients with UC (MES 0-3) and non-IBD proctitis	RWT > 4.5 mm and MSWC positive had 100% sensitivity and specificity for active UC vs proctitis	TPUS with mSMI can differentiate UC from non-IBD proctitis with accuracy similar to colonoscopy
Pal et al[3], 2025	142	Adults with UC (proctitis, proctosigmoiditis, extensive colitis); compared TPUS, TAS, and EUS	TPUS AUC 0.73 (endoscopic remission), 0.66 (histologic remission)	TPUS moderately accurate for distal disease; TAS+TPUS improved diagnostic value but still inferior to EUS

TPUS: Transperineal ultrasound; UC: Ulcerative colitis; MES: Mayo endoscopic subscore; BWT: Bowel wall thickness; ΔBWT: Change in bowel wall thickness; CRP: C-reactive protein; FC: Fecal calprotectin; TAS: Transabdominal ultrasound; EUS: Endoscopic ultrasound; RWT: Rectal wall thickness; MSWC: Microvascular signal at wall circumference; mSMI: Microvascular superb microvascular imaging; AUC: Area under the curve.

TPUS IN ILEAL POUCH DISORDERS

TPUS has emerged as a non-invasive, accessible tool to assess pouch-related inflammation in patients with an ileal pouch-anal anastomosis (IPAA) following colectomy for UC (Figure 9). In a prospective cross-sectional study of 44 pouch patients, Ardalan et al[22] demonstrated that TPUS had good diagnostic accuracy for pouchitis, with a pouch wall thickness ≥ 4.0 mm achieving 87% specificity and correlating strongly with endoscopic and histologic inflammation (AUC 0.79). A cutoff of < 3.0 mm reliably excluded pouchitis (88% sensitivity) and defined a state of "sonographic remission"[22].

Figure 9 Transperineal ultrasound in ileal pouch. A: Transperineal ultrasound (TPUS) image showing normal pouch wall; B: Endoscopic view of normal pouch mucosa; C and D: Transabdominal ultrasound showing prestenotic dilatation with stricture; E: Color Doppler showing increased vascularity in the pouch; F: TPUS sagittal image of thickened pouch with adjacent vagina; G: Endoscopic balloon dilation under image guidance; H: TPUS view of anorectum and vagina for assessing anovaginal fistula in in ileal pouch case. P: Pouch wall.

Importantly, pouch wall thickness assessed via TPUS correlated with total PDAI (r = 0.51), endoscopic subscore (r = 0.45), and histological subscore (r = 0.52), suggesting TPUS can be used both for initial diagnosis and longitudinal response assessment[22]. While TPUS Doppler parameters showed limited discrimination-likely due to probe depth and pelvic vascular noise-the structural findings were reproducible, especially when using convex or microconvex probes.

The transabdominal approach was more effective for evaluating the pre-pouch ileum. In cases of moderate-severe pre-pouch ileitis, BWT ≥ 3.0 mm and Limberg score ≥ 1 provided AUCs of 0.78 and 0.79, respectively[22]. These sonographic changes were significantly more frequent in patients with chronic antibiotic-refractory or immune-mediated pouchitis. Therefore, combining transabdominal and transperineal gastrointestinal US allows localization of inflammation, particularly in complex or recurrent pouch dysfunction.

Faecal calprotectin (FC) served as a complementary test: < 100 μg/g ruled out inflammatory pouch disorders (sensitivity 89%), while ≥ 350 μg/g reliably ruled them in (specificity 100%). When combined with TPUS, this approach could have reduced the need for pouchoscopy by up to 70%, reserving it for equivocal or discordant cases[22].

In summary, TPUS-particularly when combined with TAUS and FC-offers a rapid, repeatable, and well-tolerated method to differentiate pouchitis, pre-pouch ileitis, and irritable pouch syndrome. Its use can streamline follow-up and reduce dependence on invasive procedures, provided operators are adequately trained and reporting frameworks are standardized.

ADVANCED APPLICATIONS OF TPUS IN IBD

Color Doppler

The integration of color Doppler imaging into TPUS significantly enhances the evaluation of PACD by visualizing hypervascularity (Figure 3A) in and around fistulas and abscesses-serving as a marker of active inflammation. In a surgical validation study by Mallouhi et al[23], the addition of color Doppler to grayscale TPUS increased diagnostic confidence and overall accuracy for detecting perianal inflammatory disease (AUC improved from 0.86 to 0.89; P = 0.002). Hypervascularity at the lesion’s periphery was identified as an independent predictor of perianal inflammation, with an odds ratio of 8.5, supporting its value in distinguishing active from quiescent disease[23].

Color Doppler can differentiate active granulating tissue from fibrotic tracts and is particularly useful in guiding management during biologic therapy by identifying persistent subclinical inflammation or fluid collections not apparent on examination[1]. Routine use of color Doppler in longitudinal TPUS protocols can aid objective assessment of therapeutic response and early detection of recurrence.

CEUS

To enhance detection and delineation, CEUS using diluted hydrogen peroxide has shown promising utility. When introduced into the external opening, the agent generates microbubbles that convert hypoechoic tracts to echogenic, thereby improving visualization of fistulous ramifications and small abscesses within inflammatory masses. This technique has been shown to be particularly helpful in mapping complex, ramified, or supralevator fistulas[1]. In a study by Sloots et al[7], Hydrogen peroxide ultarsound (HPUS) revealed that only 22% of Crohn’s fistulas were simple, while the remaining 78% were complex, including suprasphincteric, extrasphincteric, or anovaginal types. HPUS was superior to physical exam, computed tomography, and conventional US in demonstrating the full extent of fistula anatomy, emphasizing its value before surgery or biologic therapy. The safety profile, rapid acquisition, and bedside applicability of CEUS make it an attractive enhancement in routine TPUS-guided fistula characterization.

Sonoelastography

Sonoelastography is an emerging adjunctive technique that evaluates tissue stiffness. In the context of PACD, it may help differentiate between active, inflamed fistulas (which appear softer and more compressible) and fibrotic or sclerotic tracts (which are stiffer and more resistant to pressure)[1]. While early evidence is promising, larger validation studies are needed before widespread clinical adoption. Incorporation of sonoelastography into TPUS protocols holds promise for individualized monitoring of fibrosis progression and therapeutic response.

Three-dimensional transperineal and EUS in PACD

The 3D US has added significant value to the assessment of PACD by offering enhanced spatial resolution, multiplanar reconstruction, and improved delineation of complex fistula anatomy. Unlike conventional 2D imaging, 3D US allows for volumetric assessment of sphincter involvement, fistula branching, and associated abscesses, often revealing lesions that are clinically silent (Figure 3D).

In a prospective study of 95 patients with PACD, de la Portilla et al[5] demonstrated that 3D EUS influenced therapeutic decision-making in nearly half of all cases examined. Abscesses were detected in 17.7% of patients-many of which were unsuspected clinically-and the imaging findings correlated with surgical findings in 81.3% of cases. Importantly, 3D US identified sphincter defects and allowed accurate classification of fistulas using a novel sonographic scheme tailored for PACD. The study reported high inter-observer agreement (κ = 0.86), suggesting that 3D imaging is both reproducible and clinically impactful in guiding surgical timing, seton removal, or biologic therapy[5].

Beyond morphological assessment, 3D US has diagnostic specificity. Zawadzki et al[24] introduced the Crohn’s US fistula sign (CUFS)-a characteristic 3D US feature defined by a hypoechoic tract surrounded by a hyperechoic halo with a thin hypoechoic outer rim. In their study of 157 patients with anal fistulae, CUFS had a specificity of 98% and a PPV of 87% for CD, enabling differentiation from cryptoglandular fistulas. Remarkably, in 12 cases, CUFS prompted the diagnosis of CD prior to the appearance of intestinal symptoms[24].

Incorporating 3D US, especially with contrast enhancement (e.g., hydrogen peroxide), offers a non-invasive, bedside-compatible, and radiation-free modality to assess fistula complexity, guide interventions, and potentially identify CD in undiagnosed patients. While MRI remains the standard for deep-seated or supralevator disease, 3D TPUS/EAUS serves as a practical, first-line imaging strategy with growing diagnostic and prognostic utility in PACD.

Image guided interventions

TPUS can be used to guide therapeutic interventions, such as abscess drainage, seton placement or endoscopic balloon dilation, by providing real-time anatomical orientation (Figures 9G and 10). In patients on anti-tumor necrosis factor therapy, TPUS enables serial assessment to detect silent abscesses, confirm persistent tracts despite external closure, and guide the duration of medical therapy. It also facilitates precision-targeted interventions and minimizes procedural risks by delineating real-time anatomy at bedside.

Figure 10  Transperineal ultrasound guided fistulotomy. A: Transsphincteric fistula extending from anal canal, rectum; B: Fistula extending posteriorly to coccyx; C: Endoscopic fistulotomy done guided by pre-procedure transperineal ultrasound. F: Fistula; AC: Anal canal; R: Rectum; S: Sacrum.

CONCLUSION

TPUS has transitioned from an ancillary technique to a core imaging tool in the management of IBD. Its non-invasive nature, bedside feasibility, and expanding diagnostic applications-including rectal inflammation in UC, perianal fistulas in CD, and complications post-IPAA-make TPUS a versatile modality. Adjunctive tools such as color Doppler, CEUS, elastography, and 3D imaging enhance both anatomical and functional insights. TPUS compares favorably with MRI and EUS in many scenarios and excels as a point-of-care strategy, particularly in pediatrics, pregnancy, and resource-limited settings. With growing evidence and structured training, TPUS is poised for wider integration into IBD care algorithms-as a first-line, repeatable, and cost-effective modality. Standardized training programs, validated reporting templates, and multicenter reproducibility data will be crucial next steps to ensure consistent, high-quality implementation. Ultimately, TPUS offers the potential to transform disease monitoring from hospital-based imaging to real-time, bedside precision.