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Clin Ultrasound. 2025;10(2):75-81

Publication date (electronic) : 2025 November 30

doi : https://doi.org/10.18525/cu.2025.10.2.75

Kwang Woo Kim

Department of Internal Medicine, Seoul Metropolitan Government Seoul National University Boramae Medical Center, Seoul National University College of Medicine, Seoul, Korea

김광우

서울대학교 의과대학 서울특별시보라매병원 소화기내과

Address for Correspondence: Kwang Woo Kim, M.D. Department of Internal Medicine, Seoul Metropolitan Government Seoul National University Boramae Medical Center, Seoul National University College of Medicine, 20 Boramae-ro 5-gil, Dongjak-gu, Seoul 07061, Korea Tel: +82-2-870-3435, Fax: +82-2-831-2826 E-mail: labottw5@snu.ac.kr

Received 2025 November 5; Accepted 2025 November 13.

Trans Abstract

INTRODUCTION

Intestinal ultrasound (IUS) has become an integral imaging modality in the management of inflammatory bowel disease (IBD), offering real-time and radiation-free evaluation at the point of care [1-4]. Its bedside availability and high reproduc-ibility allow for rapid assessment of disease activity and complications, making IUS particularly suited to clinical scenarios requiring immediate therapeutic decisions [2,5]. In the era of treatto-target strategies, timely and objective monitoring of inflammation is a key determinant of long-term outcomes in Crohn’s disease (CD) and ulcerative colitis (UC) [6]. IUS enables direct visualization of bowel wall abnormalities including transmural inflammation, mesenteric vascularity, and even some extra-intestinal manifestations, thereby complementing endoscopy, other cross-sectional imaging, and biochemical markers [3,4].

Prospective studies and meta-analyses demonstrate that IUS has high accuracy for identifying disease activity and complications in CD and UC while effectively tracking therapeutic response and predicting relapse [7,8]. Standardized protocols—including bowel wall thickness (BWT), bowel wall vascularity, bowel wall stratification (BWS), and mesenteric fat assessment— have been endorsed by international societies and incorporated into consensus-based scoring systems such as International Bowel Ultrasound Segmental Activity Score (IBUS-SAS) [9,10]. Ongoing international initiatives continue to refine reporting, training, and competency assessment to support widespread adoption in routine IBD practice [11,12].

This review summarizes practical scanning approaches, interpretive criteria, diagnostic performance, and the expanding role of IUS in tight monitoring for IBD.

PRACTICAL SCANNING TECHNIQUE AND KEY PARAMETERS

Optimal performance of IUS in IBD requires standardized technique, structured acquisition, and consistent interpretation, particularly as bowel wall measurements and Doppler signals are operator-dependent and influence treatment decisions [3,4]. Conventional practice has recommended short fasting of approximately 2 to 4 hours prior to ultrasonography for improved visualization; however, this is not mandatory for IUS [13]. A systematic evaluation does not require a predetermined starting point and may begin either at the terminal ileum in the right lower quadrant or at the sigmoid colon in the left lower quadrant, depending on the examiner's preference or training. The key principle is to ensure complete assessment without missing any segments [11,13].

Both low-frequency curvilinear probes (2–5 MHz) for deep loops and high-frequency linear probes (7–12 MHz) for mural detail are recommended [3,4]. Gentle graded compression enhances bowel wall visualization, displaces gas, and helps differentiate mural thickening from stool or gas artifacts [13]. Real-time sweeping and orthogonal planes are necessary to avoid underestimation of focal inflammation and to assess motility, which may support differentiation between inflammatory and fibrostenotic disease [14].

Core parameters include BWT, BWS, color Doppler signal (CDS), and mesenteric fat proliferation (Table 1) [15]. BWT remains the most reproducible marker and correlates with endoscopic disease severity [3,7]. Cut-offs typically applied are >3 mm in the colon and >2 mm in the terminal ileum, although physiologic sigmoid wall thickness may approach 3–4 mm [3,8]. Loss of stratification reflects active transmural disease or fibrosis, whereas preserved layering can be seen in mild inflammation or functional symptoms [3,8]. CDS assessment using the Limberg score provides objective vascularity grading that corresponds to disease activity and predicts endoscopic inflamma-tion [16,17]. Mesenteric fat proliferation is a hallmark feature of transmural activity, particularly in CD, and align with radiologic and histologic markers of deep inflammation [9,18]. Importantly, recent work demonstrates that UC may also exhibit deeper-layer changes—including submucosal thickening and fat proliferation—even in endoscopic remission, challenging the traditional “mucosal-only” paradigm and underscoring the need to assess extra-mucosal activity in UC [19].

Table 1.

Key IUS parameters for IBD [15]

DIAGNOSTIC ACCURACY AND PERFORMANCE

IUS demonstrates high diagnostic accuracy for assessing inflammatory activity and complications in IBD, with performance comparable to cross-sectional imaging modalities such as magnetic resonance enterography (MRE) and computed tomography enterography [3,4]. Meta-analyses report pooled sensitivities of approximately 85–94% and specificities of approximately 88–97% for detecting active CD inflammation using BWT, CDS, and BWS as core criteria [20]. IUS parameters, particularly BWT and CDS, demonstrate strong correlation with endoscopic disease activity scores in both CD and UC, with correlation coefficients consistently exceeding 0.7–0.8 in prospective studies [7,18]. Representative sonographic and corresponding endoscopic images are shown in Fig. 1. In UC, IUS similarly correlates strongly with endoscopic severity, with recent data demonstrating high discriminatory ability across disease extents and severities [18,21].

Figure 1.

Practical correlation between intestinal ultrasound and endoscopic images in inflammatory bowel disease. (A) Endoscopic image showing ulcerative colitis with Mayo endoscopic subscore of 1. (B, C) Corresponding intestinal ultrasound images of the same segment demonstrating: (B) bowel wall thickening with measurement, and (C) color doppler signal corresponding to a Limberg score of 2.

The diagnostic value of IUS extends beyond luminal inflammation. It reliably identifies penetrating disease, including fistulas and abscesses, with sensitivity of approximately 81–89% and specificity of approximately 93–97% in expert hands [7,22]. Importantly, IUS performs equivalently to MRE in detecting abscesses and is often superior for bedside diagnosis and early decision-making in acute presentations [23]. UC has traditionally been considered a mucosal disease, yet accumulating evidence now highlights extracolonic and transmural components. Deep-layer abnormalities—including submucosal thickening, fat proliferation, and persistent Doppler signal—have been observed even in patients achieving endoscopic remission [19]. This evolving concept reinforces the value of IUS in complementing mucosal targets and identifying deeper residual inflammation that may influence relapse risk. The recent multicenter validation of IBUS-SAS in UC reported excellent prediction of endoscopic activity (area under the curve 0.91), with sensitivity of 100% and specificity 80% at a threshold of approximately 15.9 [10].

Emerging IUS scoring systems in IBD consistently incorporate BWT and CDS as core markers of inflammation. In CD, seven indices have been described, with IBUS-SAS, Bowel Ultrasound Severity Score (BUSS), and Simplified Ultrasound Score for Crohn’s Disease (SUS-CD) demonstrating the strongest correlations with endoscopic activity indices. IBUS-SAS also includes bowel wall stratification and mesenteric inflammatory fat, improving accuracy but requiring more time to apply. In UC, the IBUS-SAS, Milan Ultrasound Criteria (MUC), and Ulcerative Colitis-Intestinal Ultrasound Score (UC-IUS) are available, with IBUS-SAS and MUC validated against Mayo Endoscopic Subscore (MES) and Ulcerative Colitis Endoscopic Index of Severity (UCEIS), demonstrating high predictive performance. While these scores enhance standardization and responsiveness in clinical assessment, practical adoption may be limited by calculation complexity, highlighting the future role of automation and integrated reporting tools [4].

Collectively, these findings establish IUS as a highly accurate, dynamic, and patient-centered modality that complements endoscopy and advanced imaging, particularly in settings prioritizing rapid evaluation, tight control, and advanced therapy-era optimization of care.

MONITORING, TREATMENT RESPONSE, AND TREAT-TO-TARGET

IUS is increasingly recognized as a core component of treatto-target strategies in IBD, enabling early evaluation of therapeutic response and prediction of long-term outcomes. Early reduction in BWT, improvement in CDS, and restoration of BWS correlate with clinical, biochemical, and endoscopic improvements [3,4,6,8]. Beyond baseline assessment, IUS is increasingly integrated into treat-to-target strategies and early therapeutic response monitoring. Changes in BWT and Doppler activity correlate with biochemical and endoscopic improvements as early as week 4-8 in colonic disease and week 12 in terminal ileitis [2,8].

A 2025 systematic review found that BWT reduction and normalization of Doppler vascularity were the most frequent sonographic markers used to define treatment response and remission across clinical studies [8]. Normalization of BWT to ≤3 mm and CDS grade 0–1 are widely accepted remission thresholds, while ≥25% BWT reduction or composite improvement (BWT + CDS) represent pragmatic markers of response in both CD and UC [2]. The 2025 international IUS trial consensus formally endorsed these criteria and recommended standardized assessment intervals at weeks 4–8 and week 12, followed by regular monitoring during maintenance therapy [2].

Ultrasound treatment response predicts favorable outcomes beyond symptom relief, including reduced steroid use, higher likelihood of biologic persistence, and decreased hospitalization and surgery rates [24,25]. IUS remission has been associated with lower long-term relapse risk and enhanced durability of remission compared with clinical response alone [26,27]. Moreover, the ability of IUS to detect transmural residual inflammation— even in endoscopically quiescent disease—supports its integration into deeper remission targets and aligns with modern goals of preventing fibrosis, fistula formation, and progressive bowel damage [14,19].

Given its bedside availability, absence of radiation, and reproducibility when performed by trained operators, IUS is increasingly implemented in routine clinical algorithms. Optimal practice incorporates baseline evaluation, early response assessment, targeted therapy escalation when response is insufficient, and regular surveillance during biologic maintenance. Such “tight monitoring” protocols harness IUS to accelerate therapeutic decisions, minimize cumulative inflammation burden, and align with treat-to-target principles in IBD.

SPECIAL CLINICAL SCENARIOS

CONCLUSION

IUS has evolved into an essential imaging modality in IBD care, supporting rapid, accurate, and radiation-free assessment across diverse clinical settings. Its ability to directly visualize bowel wall inflammation, mesenteric changes, vascularity, and complications allows clinicians to make timely therapeutic decisions and detect early treatment response. Evidence now supports routine early reassessment, enabling tight monitoring and aligning IUS with treat-to-target principles in the advanced therapy era.

Validated scoring systems with consensus-based definitions of response and remission are bringing consistency to clinical trials and real-world practice. IUS additionally offers unique value in specific clinical situations, reinforcing its versatility. As understanding of transmural disease deepens—including in UC—ultrasound contributes to comprehensive disease control beyond mucosal endpoints.

Future priorities include continued standardization, quality assurance, and expansion of training pathways, as well as integration of elastography, radiomics, and artificial intelligence to enhance pathologic characterization and improve diagnostic accuracy. With increasing global adoption and maturing evidence, IUS is poised to remain a central tool for achieving deep, durable remission and improving long-term outcomes in patients with IBD.

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