By: Michael Dolinger, MD MBA

‍

Introduction to Intestinal Ultrasound (IUS)

Intestinal ultrasound (IUS) is a non-invasive imaging modality increasingly recognized for its value in assessing and managing inflammatory bowel disease with a growing interest in other gastrointestinal (GI) conditions too. Unlike more traditional imaging methods like CT and MRI, IUS offers the advantages of being radiation-free, repeatable, and conducive to real-time bedside assessments. This guide aims to provide a comprehensive introduction to IUS, emphasizing its utility, interpretation, and integration into clinical practice.

‍

Clinical Applications of IUS

1. Inflammatory Bowel Disease (IBD) Monitoring
   • Assessment of Disease Activity: IUS is effective in evaluating disease activity in both Crohn's disease and ulcerative colitis. Key sonographic features, such as bowel wall thickness (BWT), increased vascularity, and the presence of complications like abscesses or fistulas, can be accurately detected.
   • Treatment Response: IUS is useful as a diagnostic screening tool, for monitoring response to therapy, assessing disease activity and potential complications during symptomatic flares, and enabling timely adjustments in treatment regimens. Reduction in BWT and decreased vascularity often correlate with successful treatment response and ultimate clinical remission.
   • Postoperative Surveillance: In postoperative Crohn's disease, IUS can be employed to monitor for early recurrence, aiding in the prevention of clinical relapse. When combined with fecal calprotectin, IUS can be deployed in a non-invasive monitoring strategy to reduce, or expedite, colonoscopies as needed for decision-making.
2. Acute Abdominal Pain
   • Differential Diagnosis: IUS can rapidly distinguish between inflammatory and non-inflammatory causes of abdominal pain, aiding in the diagnosis of conditions such as appendicitis, diverticulitis, or intestinal obstruction.
   • Real-Time Evaluation: The ability to perform bedside assessments allows for immediate clinical decision-making in acute settings, regardless of previous IBD diagnostic status.
3. Pediatric Gastroenterology
   • Radiation-Free Imaging: Particularly valuable in pediatric populations, IUS provides a radiation-free alternative to CT scans, reducing long-term risks associated with repeated imaging. Unlike MRI, IUS does not requiring contrast and can be tolerated in young children, who may require anesthesia and sedation in order to successfully undergo MRI.
   • Disease Monitoring: In pediatric IBD, IUS is an excellent tool for routine monitoring, facilitating frequent assessments without exposure to ionizing radiation. In children, normalization of IUS in the setting of clinical remission and other non-invasive serum and stool biomarkers may obviate the need for repeat colonoscopy in the treat-to-target approach.

Technical Aspects of IUS

1. Equipment and Setup
   • Ultrasound Machines: Mid-High frequency linear transducers, and low-frequency convex transducers are typically used for IUS, offering detailed visualization of the bowel wall and surrounding structures. Lower-frequency probes may be employed for deeper or obese patients, but the linear probes are used for more detailed assessments.
   • Patient Preparation: There is no fasting or bowel preparation needed.. A full bladder may also be required for pelvic assessments and may increase the yield of the assessment of the rectum or of the J-Pouch in patients who have undergone surgery and a colectomy.
2. Imaging Protocol
   • Standard Views: Longitudinal and transverse views of the small and large intestines should be systematically obtained. Typically, this is performed akin to colonoscopy, from rectum/distal sigmoid colon proximally to the cecum and terminal ileum.
   • Doppler Imaging: Color Doppler is integral for assessing vascularity, which correlates with inflammation. Increased Doppler signal in the bowel wall often indicates active disease.
3. Interpretation of Findings
   • Bowel Wall Thickness (BWT): Normal bowel wall thickness is typically <3 mm in the colon and small intestine and <4 mm in the rectum. Increased BWT is a hallmark of inflammation and is measured from the lumen-mucosa interface to the muscularis propria-serosa interface of the anterior intestinal wall.
   • Stratification: Loss of the normal layered appearance (stratification) suggests active inflammation or chronic changes like fibrosis.
   • Vascularity: Increased vascularity on Doppler imaging indicates hyperemia associated with active inflammation.
   • Other Findings: Detection of complications such as strictures, abscesses, and fistulas, which can be identified through sonographic features like hypoechoic tracts, fluid collections, or narrowed segments of the bowel.

Integration into Clinical Practice

1. Combining IUS with Clinical Assessment
   • Routine Monitoring: Incorporating IUS into regular follow-up visits for IBD patients allows for real-time assessment of disease activity, complementing clinical indices like the Crohn’s Disease Activity Index (CDAI) or the Mayo score in adults or the Pediatric Crohn’s Disease Activity Index (PCDAI) or Pediatric Ulcerative Colitis Activity Index (PUCAI) in children.
   • Guiding Therapy: IUS findings should be integrated with laboratory markers (e.g., C-reactive protein, fecal calprotectin) to guide therapeutic decisions, including escalation or de-escalation of treatment.
2. Training and Proficiency
   • Skill Development: Proficiency in IUS requires dedicated training, typically achieved through hands-on courses, mentorship, and regular practice. Understanding normal anatomical variations and pathological findings is crucial for accurate interpretation.
   • Interdisciplinary Collaboration: Collaborating with radiologists and gastroenterologists experienced in IUS can enhance diagnostic accuracy and facilitate the learning curve for practitioners new to the technique.
3. Advantages and Limitations
   • Strengths: IUS is cost-effective, repeatable, and does not involve radiation. It is particularly advantageous for patients requiring frequent monitoring, such newly diagnosed IBD patients, symptomatic patients, and patients initiating treatment for numerous reasons. There are also certain patient populations whom it is imperative to reduce invasive monitoring and still achieve outcomes of remission, including young children, elderly, pregnant, and post-surgical IBD patients.
   • Limitations: Operator dependency and variability in image quality due to patient factors (e.g., obesity, bowel gas) are notable challenges. IUS may be less effective in deep or complete abdominal assessments compared to CT or MRI.

Future Directions

1. Technological Advancements
   • Elastography and Contrast-Enhanced Ultrasound: Emerging techniques like elastography, which assesses tissue stiffness, and contrast-enhanced ultrasound (CEUS), which improves visualization of vascular patterns, hold promise for enhancing the diagnostic capabilities of IUS.
2. Research and Validation
   • Clinical Trials: Ongoing research is essential to validate IUS findings against gold-standard techniques like endoscopy and cross-sectional imaging. Standardization of protocols and definitions will further establish IUS as a key tool in GI practice and clinical trials.

Conclusion

Intestinal ultrasound is a powerful, non-invasive tool that offers significant benefits in the diagnosis and management of GI disorders, particularly IBD. With appropriate training and integration into clinical workflows, IUS can enhance patient care by providing timely, accurate, and cost-effective assessments. As technology evolves and research progresses, IUS is poised to become an indispensable part of gastrointestinal practice.

‍

‍