Intestinal Ultrasound (IUS) in Inflammatory Bowel Disease (IBD): A New Skill for Gastroenterologists

Vol. 29, Issue 4 (December 2024)

Partha Pal, MD
Department of Medical Gastroenterology and IBD
Asian Institute of Gastroenterology
Hyderabad, India

 

Mohammad Abdul Mateen, MD
Department of Diagnostic Radiology and Ultrasound
Asian Institute of Gastroenterology
Hyderabad, India

 

Introduction

The use of intestinal ultrasound (IUS) for the evaluation of inflammatory bowel disease (IBD) was first reported in the medical literature in 1979. Despite this early introduction, it did not become a widely adopted tool for gastroenterologists. Several factors contributed to this, including concerns over the accuracy of ultrasound compared to other imaging modalities like computed tomography (CT) or magnetic resonance imaging (MRI), and the absence of training programs tailored to the needs of gastroenterologists. Over the last decade, however, there has been a renewed interest in IUS, particularly as a point-of-care tool that gastroenterologists can use directly in their clinical practice.

The ability of IUS to provide real-time information about disease activity, complications, and treatment response, combined with its noninvasive nature, has made it a valuable addition to the clinical armamentarium for managing IBD. Unlike other diagnostic techniques, IUS is well-tolerated by patients, provides immediate results, and avoids the risks associated with radiation exposure and sedation.

Currently, the primary clinical indications for IUS in IBD include suspected IBD, monitoring IBD activity, assessing therapeutic response, detecting complications (strictures/abscesses), and predicting postoperative recurrence in Crohn’s disease (CD). In this article, we’ll examine the technology behind IUS, its clinical applications, its advantages over traditional imaging methods, and the potential challenges that remain.

The Ultrasound Machine and Probes

Performing IUS requires a machine equipped with both a low-frequency curvilinear probe and a high-frequency linear probe (≥7 MHz). Each type of probe serves a different purpose: the low-frequency curvilinear probe penetrates deep into the body, making it suitable for identifying complications such as deep-seated abscesses, while the high-frequency linear probe provides better resolution for assessing the structure of the bowel wall.

Advances in ultrasound technology have resulted in the development of bowel-specific probes, which employ single-crystal technology to improve image clarity and resolution across different depths. Unlike conventional probes that rely on multiple piezoelectric crystals, these dedicated bowel ultrasound probes provide higher contrast and uniform resolution, enhancing the accuracy of IUS.¹

Modern ultrasound machines also come with adjustable settings for depth, focus, color Doppler gain, and contrast, allowing clinicians to tailor the scan to the patient’s anatomy. For example, increasing the depth setting allows visualization of deeper structures, but this comes at the cost of reduced frame rate and line density. Meanwhile, adjusting the color Doppler gain helps minimize background noise when evaluating bowel vascularization, and contrast adjustments help to better delineate the bowel wall layers.

IUS Examination Technique

A systematic approach is crucial when performing an IUS examination, especially in patients with suspected or known IBD. The patient should expose their abdomen up to the inguinal ligament to allow full access to the abdominal and pelvic regions.

The examination typically begins in the left iliac fossa, where the sigmoid colon is visualized by identifying the iliac vessels over the psoas muscle. From there, the operator traces the colon upward to examine the descending colon, splenic flexure, transverse colon, hepatic flexure, ascending colon, cecum, and terminal ileum. The small intestine is examined using a method known as “lawn mowing,” where the clinician systematically scans in vertical and horizontal planes to ensure that no part of the bowel is missed.²

During the examination, the small intestine can be distinguished from the large bowel by its characteristic peristalsis and the presence of valvulae conniventes in the jejunum, located in the left upper quadrant. In cases of Crohn’s disease, the terminal ileum is a common site of inflammation and is typically visualized in the right iliac fossa, above the iliac vessels.

Key Ultrasound Parameters

Several key parameters are assessed during an IUS examination to evaluate disease activity and detect complications. These include:

1. Bowel Wall Thickness (BWT): BWT is the most critical parameter in assessing IBD activity. A normal bowel wall is typically less than 3 mm thick. BWT is measured at multiple points along the bowel to ensure an accurate assessment. The average of two measurements 1 cm apart in the longitudinal plane and two measurements 90 degrees apart in the cross-sectional plane is considered the most reliable (Figure 1A).
2. Color Doppler Signal (CDS): The intensity of bowel vascularity is graded using color Doppler ultrasound (Figure 1B). The modified Limberg scale (MLS) categorizes vascularity from 0 (no vascularity) (Figure 2A), 1 (few Doppler signals in bowel wall) (Figure 2B), 2 (streaks of Doppler signals in wall) (Figure 2C) to 3 (extensive vascularity extending into the mesentery) (Figure 2D). Increased vascularity correlates with active inflammation.
3. Bowel Wall Stratification (BWS): The layered appearance of the bowel wall on IUS resembles the structure of an “Oreo cookie” (inner hypoechoic mucosa, middle hyperechoic submucosa and outer hypoechoic muscularis). Loss of this layer pattern indicate active disease (Figure 1C).
4. Mesenteric Fat and Lymph Nodes: Inflammation of the mesenteric fat, known as “creeping fat,” is a hallmark of Crohn’s disease (Figure 1F). IUS can detect the presence and extent of mesenteric fat inflammation, which correlates with disease severity. Enlarged lymph nodes, often seen in areas of active disease, can also be evaluated using IUS.
5. Complications: IUS is highly effective in detecting complications of IBD, particularly in Crohn’s disease. Strictures (Figure 1D), fistulas (Figure 3A), and abscesses (Figure 3B-C) can be identified with high sensitivity and specificity. Strictures appear as segments of narrowed bowel lumen, often accompanied by pre-stenotic dilation (Figure 1E) and hyper-peristalsis. Fistulas and abscesses are typically visualized as hypoechoic areas adjacent to the bowel wall (Figure 3A-C).

Indications and Applications of IUS in IBD

IUS has a range of applications in IBD, from diagnosing suspected cases to monitoring disease progression and therapeutic response. Below are some of the key clinical applications of IUS:

1. Diagnosis of Suspected IBD: In patients presenting with gastrointestinal symptoms, IUS can be used to triage those who are likely to have IBD versus those with functional disorders like irritable bowel syndrome (IBS). Sensitivity and specificity for detecting IBD are reported to be around 85% and 95%, respectively, with IUS being particularly effective in identifying inflammation in the ileum and colon (Figure 4). However, its sensitivity for detecting rectal and jejunal lesions is lower, necessitating additional imaging or endoscopy in some cases.³
2. Assessment of Disease Severity: In patients with known IBD, IUS can be used to assess the severity of disease activity (Figure 4). Several scoring systems have been developed to quantify disease activity using IUS parameters. For example, the Milan Ultrasound Criteria (MUC) for ulcerative colitis (UC) [MUC = 1.4 × BWT (in mm) + 2 × CDS, where CDS absent = 0 and present, = 1, MUC>6.3 active disease] and the International Bowel Ultrasound Segmental Activity Score (IBUS-SAS) for CD [ = 4 × BWT + 15 × inflammatory fat + 7 × CDS + 4 × BWS, at the worst-affected segment, >48.7 active disease] provide objective measurements of inflammation.^{4, 5} These scores correlate well with endoscopic findings and can be used to monitor disease progression over time.
3. Monitoring Therapeutic Response: In ulcerative colitis (UC), a reduction in bowel wall thickness (BWT) of over 25% can occur within two weeks of treatment, preceding clinical and biomarker improvements. This response varies with the agent, such as infliximab and tofacitinib (2 weeks) and vedolizumab (6 weeks), with no difference after 8 weeks (Figure 4). A 30% BWT decrease correlates with a Mayo score reduction of ≥1. In acute severe colitis, a >20% BWT reduction in the sigmoid after 48 hours of intravenous steroids predicts clinical response and the need for rescue therapy by day 7.⁶
   In Crohn’s disease (CD), early changes in IUS at 4–8 weeks predict endoscopic response at 12–34 weeks. Transmural healing (BWT <3 mm, normal vascularity) occurs in 42.4% by 1 year, linked to steroid-free remission and fewer surgeries (Figure 4).⁷ Infliximab levels >5 mcg/ml and adalimumab >10.5 mcg/ml at week 14 are associated with IUS response.⁸ Studies have shown that early sonographic changes precede clinical and biomarker responses, making IUS a useful tool for guiding treatment decisions.
4. Detection of Postoperative Recurrence: In patients with Crohn’s disease who have undergone surgical resection, IUS can be used to detect postoperative recurrence. Increased BWT in the neo-terminal ileum (>3 mm has high accuracy: 87.5% to detect endoscopic recurrence), along with elevated CDS, suggests early recurrence, allowing for timely adjustments in therapy.⁹
5. Evaluation of Complications: IUS is highly effective in detecting complications such as strictures, fistulas, and abscesses, particularly in CD. The sensitivity and specificity of IUS for detecting strictures and fistulas are reported to be around 90%, making it a reliable alternative to more invasive procedures like CT enterography or MRI.²

Special Applications of IUS

In addition to its use in adult patients, IUS has several special applications, including in pediatric populations, pregnancy, and postoperative Crohn’s disease:

1. Pediatric IBD: IUS is particularly useful in pediatric patients, where radiation exposure from CT scans is a major concern. Dynamic sonographic imaging can provide valuable information about disease activity and complications, though BWT cut offs may be lower as compared to adult patients. Further validation of pediatric-specific IUS scores is needed to optimize its use in this population.
2. Pregnancy: The non-invasive nature of IUS makes it ideal for use in pregnant patients with IBD. While the gravid uterus can hinder the evaluation of the sigmoid colon and terminal ileum in the later stages of pregnancy, IUS can still provide valuable information about disease activity and guide treatment decisions without exposing the fetus to radiation.
3. Postoperative Crohn’s Disease: In patients with Crohn’s disease who have undergone surgical resection, IUS can be used to detect postoperative recurrence. Increased BWT in the neo-terminal ileum (>5.5 mm indicate ≥ Rutgeerts i3 recurrence).⁹ IUS with fecal calprotectin can predict up to 75% of postoperative endoscopic recurrence at 1 year.¹⁰

Advantages of IUS Over Other Imaging Modalities

IUS has several advantages over other imaging modalities like CT, MRI, and colonoscopy, particularly in the context of IBD management:

1. Non-invasive and Patient-friendly: IUS does not require bowel preparation, sedation, or invasive procedures like colonoscopy, making it more comfortable and ideal for frequent follow-ups in IBD management.
2. Real-time Results: IUS provides immediate feedback, allowing for quicker clinical decision-making compared to MRI or CT, which require longer scheduling and reporting times.
3. Cost-effective: IUS is less expensive than MRI or CT and does not require contrast agents or sedation, making it ideal for resource-limited settings. This makes IUS an ideal tool for low- and middle-income countries where access to advanced imaging may be limited.¹¹
4. No Radiation Exposure: IUS uses sound waves, avoiding radiation exposure, making it safer for repeated use, particularly for children and pregnant patients.
5. Widespread Availability: Ultrasound machines are accessible in most clinical settings, including remote areas, making IUS highly available for timely diagnosis and monitoring. The portability of ultrasound devices allows IUS to be performed at the bedside, in outpatient clinics, or during hospital rounds.¹¹

Challenges in Adopting IUS

Despite its many advantages, there are several challenges that have hindered the widespread adoption of IUS in clinical practice, particularly among gastroenterologists:

1. Operator Dependency: Accuracy depends on the operator’s skill, leading to variability. Proper training is necessary to ensure consistent results.
2. Learning Curve: While ultrasound is commonly used by radiologists, it is less familiar to many gastroenterologists. However, the learning curve for competency in IUS ranged from 100-200 cases based on prior exposure.¹² A robust training infrastructure and formal certification programs are needed to overcome this challenge.
3. Limited Availability of Training Programs: Although interest in IUS has grown, there are still relatively few structured training programs that offer comprehensive education in IUS for gastroenterologists. Some international organizations, like the International Bowel Ultrasound Group (IBUS), have begun offering hands-on courses and workshops, but more widespread training initiatives are needed.
4. Limited Access to Advanced Ultrasound Equipment: Some settings may lack high-quality machines with specialized bowel probes, which are essential for optimal imaging in IBD. However, feasibility of IUS with existing mid-end machines have been shown in a recent study.¹

Future Direction in IUS

As IUS continues to evolve, several advancements and innovations are on the horizon that could further enhance its utility in managing IBD:

1. Elastography: Elastography can help to assess bowel “stiffness,” quantitatively based on the strain ratio (ratio of deformability to applied stress in bowel wall and mesenteric fat). A strain ratio >2 predicted fibrotic stricture and need for surgery.¹³ Stiffness >21 kPa on shear wave elastography was shown to predict fibrotic strictures with good accuracy (~88%).¹⁴
2. Contrast-enhanced Ultrasound (CEUS): CEUS involves the use of intravenous microbubble contrast agents to enhance the visualization of blood flow within the bowel wall. This technique could improve the assessment of bowel vascularity and help differentiate between active inflammation and other causes of bowel thickening (based on peak enhancement, wash-in rates and time-to-peak etc.). CEUS has shown promise in several studies, but its role in routine clinical practice is still being explored.
3. Artificial Intelligence (AI) and Machine Learning: AI could standardize IUS interpretations, reduce operator dependency, and objectively quantify disease activity.
4. Expansion of Training Programs: More international societies are developing and expanding formal IUS training programs, potentially increasing its adoption in clinical practice.
5. Telemedicine and Remote IUS: Remote consultation of IUS images could be incorporated into telemedicine, especially in resource-limited areas.

Conclusion

IUS has emerged as a valuable tool in IBD, offering a non-invasive, real-time, and patient-friendly alternative to traditional imaging modalities. Its ability to assess disease activity, monitor therapeutic response, and detect complications has made it an indispensable tool for gastroenterologists, particularly in the management of IBD. Despite some challenges related to operator dependency and limited access to training programs, the growing interest in IUS and advancements in ultrasound technology are likely to increase its adoption in clinical practice.

As the field continues to evolve, innovations such as elastography, contrast-enhanced ultrasound, and artificial intelligence hold the potential to further enhance the accuracy and utility of IUS. With the expansion of training programs and increased awareness of its benefits, IUS is poised to play an increasingly central role in the personalized management of IBD, improving outcomes and quality of life for patients around the world.

References

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