12 June 2026 via Shahriar Lahouti. Peer reviewed by Mojtaba Chardoli.

CONTENTS

• Preface
• Essentials for GI US
  • Anatomy
  • Scanning Technique
  • Correlated US findings of bowel wall histology
  • Key Sonographic Parameters for Diagnosing GI disease
• Inflammatory Bowel Disease Ultrasound
  • Clinical case scenario 
  • Rationals
  • Treat-To-Target Strategy
  • Monitoring IBD Activity
    • How and When to Monitor
  • IUS
    • Definition of response and remission
    • IUS vs Clinical and biochemical markers
    • IUS vs clinical signs & symptoms
    • IUS vs MRE
    • IUS vs Endo/colonoscopy
    • IUS Parameters
    • IUS scoring system in IBD
    • IUS in special population
  • Multimodal Monitoring for IBD activity
  • Treatment Target in IBD
  • Treatment escalation and de-escalation based on IUS
  • IUS-Guided tight control algorithm  
    
    • Clinical case example
• RECAP
• Going further
• Appendix
  • Classification of available therapy for IBD
  • 📊Ileal Crohn’s vs. sigmoid/colon Crohn’s disease
• Media

Preface

Ultrasonography is frequently used to diagnose various gastrointestinal diseases, in both acute and chronic settings. Its non-invasiveness, lack of ionizing radiation, low cost, and real-time capabilities make it an ideal first-line imaging tool.

Clinical implications of bowel ultrasound include its use as part of a routine abdominal scan for a broad range of non-specific abdominal symptoms, as well as its dedicated role in assessing acute conditions such as appendicitis, diverticulitis, intestinal obstruction, and perforation, as well as chronic gut-related disorders (discussed separately). In patients with Crohn’s disease, ultrasound contributes to the detection of disease, evaluation of its extent, identification of complications such as abscesses, fistulas, obstructions, and assessment of disease activity and follow-up.

In this article, the utility of ultrasound in inflammatory bowel disease is discussed.

Essentials for GI Ultrasound

Anatomy

◾️Small bowel

• Between stomach and large intestine
• ~ 4-7 meters in length
• Centrally located in abdomen
• Contents of jejunal loops are usually liquid and appear hypoechoic/anechoic.
• Intraperitoneal, except for 2nd-4th portions of duodenum, which are retroperitoneal.
  • 📍Retroperitoneal structures have a more fixed position and are easy to locate (Right below figure). 
• Intraluminal extensions/folds valvulae conniventes, increase surface area for absorption
• Normal measurement of small bowel caliber is < 3 cm.

◾️Large bowel

• Between the small bowel and the anus
  • ~ 1.5 meters in length
  • Peripherally located in the abdomen
  • Contain feces and gas
  • Cecum and colon identified by  haustral pattern.
    • Haustra are seen as prominent curvilinear echogenic arcs with posterior reverberation
    • Cecum identified by a curvilinear arc of hyperechogenicity (representing feces and gas) in the right lower quadrant, blind-ending caudally
  • Cecum and appendix, transverse colon, and rectosigmoid are intraperitoneal
  • Ascending colon, descending colon, and middle-to-distal rectum are retroperitoneal.
  • Normal caliber of large bowel:
    • Cecum < 9 cm
    • Transverse colon < 6 cm

◾️Peristalsis

• Refers to the coordinated rhythmic contractions of the bowel wall generated by the muscularis externa and regulated by neural and myogenic control systems, including the interstitial cells of Cajal. On real-time ultrasonography, peristalsis is readily assessed without the need for oral contrast agents.
  • Small bowel exhibits active, rapid peristalsis, while colonic motility is slower with predominantly mixing movements.
  • Normal bowel loops demonstrate appropriate peristaltic activity for their segment. Diminished or absent peristalsis suggests underlying pathology such as inflammation, ischemia, or obstruction, while hyperperistalsis may be observed in early mechanical obstruction or infectious enteritis.
• Real-time ultrasound offers a unique advantage over CT and MRI by enabling direct dynamic evaluation of bowel motility.

Scanning Technique

◾️Examination of the GI tract is performed after scanning the abdominal solid viscera by using a curvilinear probe.

◾️Probe selection

• Curvilinear probe (1–8 MHz):
  • Provides a panoramic overview and orientation to abdominal anatomy
  • Allows deeper penetration for obese patients and deeper bowel segments (rectum, retroperitoneal colon)
  • Used initially to identify major landmarks and gross abnormalities
• Linear probe (5–12 MHz):
  • Offers high-resolution visualization of bowel wall layers
  • Enables precise measurement of wall thickness and assessment of stratification
  • Essential for evaluating superficial bowel loops and mesenteric fat
  • Should be used after a curvilinear overview for detailed interrogation
• Limitations:
  • Curvilinear probe lacks sufficient resolution to distinguish the five wall layers
  • Linear probe has limited depth penetration (typically <4–6 cm), inadequate for deep or obese patients

◾️B-mode optimization

• 🛠️Technical settings:
  • Imaging depth: 8 cm or less (instead of the default 15 cm for liver)
  • Focus point positioned just below the target bowel loop
  • Lower gain setting preferred to visualize thin intestinal walls
  • Narrower dynamic range facilitates identification of wall layers
  • Harmonic imaging should be activated when available (improves delineation of wall layers)
  • Create a preset button named “Intestine” or “Bowel” for rapid adjustment
• Advantages:
  • Optimized settings improve visualization of thin bowel wall layers
  • Harmonic imaging reduces artifacts and enhances wall contrast
• Limitations:
  • Default abdominal settings often obscure fine bowel wall detail
  • Suboptimal settings may lead to false-negative or false-positive findings

◾️GI Tract scanning

• First, the abdominal US probe is used to get an overview before switching to a mid-range to high-frequency probe for a detailed examination.
  Then use a linear probe for a more detailed examination of the GI tract.
  Start with “Large bowel” scanning.
  • Cecum: Located in the right iliac fossa. Recognizable by its typical content of air and fecal material, as well as its relationship with the narrower ileal loop. This ileal loop usually contains more fluid and particulate content and exhibits noticeable peristalsis. Longitudinal view of the large bowel identifies haustration more easily.
  • Rest of the colon: Then, assess the colon systematically by performing a sweep along the entire path of the colon down to the sigmoid colon and rectum in the pelvis. The rectum can be visualized with a full bladder, which serves as an acoustic window.
  • Small bowel: Identify the terminal ileum and trace it as far as possible proximally. For the exploration of jejunal and ileal loops, parallel transverse scans are recommended, moving across the abdomen from one side to the other in a craniocaudal direction, encompassing the entire abdominal area.

◾️Slow graded compression

• Normal bowel is readily compressible, shows peristaltic activity, and dispels intraluminal gas.
• Slow graded compression is useful for the following reasons:
  • The compression helps displace endoluminal gas.
    • This can push away overlying bowel segments with gas or intra-abdominal fat and, in this way, enable the examiner to reach deeper with high-frequency probes, such as in the pelvis.
  • Assess the compressibility of the examined loop.
    • This is a crucial observation in contrast to abnormal bowel, which is usually thick-walled, rigid, demonstrates reduced peristalsis, and remains in a relatively fixed position.
  • Identify tender points that might disclose underlying pathology.
    • This technique also allows for the demonstration of localized peritonism, which increases the index of suspicion for underlying focal pathology.

◾️Doppler assessment

• Provides additional information regarding the vascularity of the bowel wall and adjacent fat; however, this technique has a limited sensitivity due to artifact.
• Increased vascularity is typically seen in infectious or inflammatory thickening, and the absence of flow can potentially indicate bowel ischemia. These findings require correlation with the underlying clinical context and baseline ultrasound images.
• 🛠️Technical settings:
  • Color or Power Doppler should be used to evaluate bowel wall vascularity.
  • Velocity scale set low: 2–5 cm/s (to detect slow-velocity flow in small mural vessels).
  • Wall filter adjusted to the lowest setting.
  • Color gain turned up until flash artifacts appear, then reduced slightly until they disappear.
  • Pulse repetition frequency (PRF): low setting to maximize sensitivity.
• 🧒Patient cooperation:
  • Patient should hold breath during image acquisition to minimize respiratory motion.
  • Breath-holding reduces flash artifacts and improves Doppler signal detection.
• Advantages:
  • Provides a semiquantitative assessment of bowel wall inflammation.
  • Hyperemia correlates with active inflammation (infectious or inflammatory).
  • Absence of flow in thickened bowel suggests ischemia (specific, though not highly sensitive).
  • Helps differentiate between inflammatory and fibrotic strictures (increased vs. absent flow).
  • Can be repeated serially to monitor treatment response without radiation.
• Limitations:
  • Limited sensitivity for slow or low-volume flow, particularly in deep bowel loops (>4 cm depth)
  • Flash artifacts from peristalsis and patient movement degrade image quality.
  • Operator-dependent for both acquisition and interpretation.
  • High body mass index reduces depth penetration and signal detection.
  • Normal bowel wall rarely demonstrates detectable Doppler signals, limiting baseline comparison.
  • Cannot detect capillary flow (only larger intramural vessels).

Correlated US findings of bowel wall histology

◾️The bowel wall throughout the GI tract has uniform general histology, comprised of 4 layers

1.  Mucosa
   • Functions for absorption and secretion
   • Composed of epithelium and loose connective tissue
   • Lamina propria
   • Muscularis mucosa (deep layer of mucosa)
2.  Submucosa (This is the thickest layer)
   • The submucosa contains vessels, nerves, and fat and is hyperechoic due to abundant loose connective tissue.
3. Muscularis externa
   • The muscularis is hypoechoic (due to muscular tissue), and the outer black layer is easy to identify. The muscular layer responsible for peristalsis. Contains Auerbach plexus
4.  Serosa
   • The epithelial lining is continuous with the peritoneum. The serosa or visceral peritoneum is the thin but tough outer hyperechoic layer, which usually blends with the hyperechoic fatty tissue of mesentery and omentum, surrounding the bowel.
   • If there is intraperitoneal fluid, the hyperechoic serosa (arrow) can be separately identified, as in these ileal loops.

◾️Ultrasound appearance of the bowel wall

• Stratified appearance of bowel wall on histology is depicted by 5 distinct layers on ultrasound as alternating echogenic/sonolucent (hypoechoic) appearance (gut signature).
  • Five sonographic layers are identified as alternating hyperechoic and hypoechoic layers:
    1. Interface of lumen and mucosa: Echogenic=White
    2. Muscularis mucosa: Hypoechoic=Black
    3. Submucosa: Echogenic=White
    4. Muscularis propria/externa: Hypoechoic=Black
    5. Serosa: Echogenic=White

• 🔎Transducer frequency and layer visualization
  • Curvilinear probe (3–5 MHz): Typically shows only 2–3 layers (inner hyperechoic submucosa + outer hypoechoic muscularis propria)
  • Linear probe (7–12 MHz): All 5 layers readily distinguished
  • Linear probe (12–18 MHz): Optimal for detailed layer analysis but limited depth penetration
• ✅ Normal wall thickness values by segments:
  • Small intestine: ≤ 3 mm
  • Colon: ≤ 4 mm
  • Stomach: ≤ 5–6 mm
  • Rectum: ≤ 7 mm

💡Normal bowel wall thickness is less than 3 mm, with an average maximum single-wall thickness of 3–5 mm depending on the degree of distension.

💡The number of visible layers depends on transducer frequency and proximity to the gut wall. With a curvilinear probe (5 MHz), the transverse section of the bowel demonstrates a stratified target-like appearance with an inner hyperechoic layer (submucosa) and a hypoechoic outer rim (muscularis propria).

✅Using a linear transducer (12–18 MHz), all five distinct layers can be readily appreciated; on axial imaging, a pattern of concentric rings with alternating hyperechoic and hypoechoic bands is seen.

• 💡Clinical pearl – loss of stratification
  • Preservation of the 5-layer signature suggests a benign or inflammatory etiology
  • Disruption or loss of stratification raises suspicion of severe transmural inflammation (e.g., active Crohn’s disease) or malignancy.
• Anisotropy artifact warning
  • Bowel wall layers appear most distinct when the ultrasound beam is perpendicular to the wall.
  • Oblique angles may artificially blur or obliterate layer distinction.

💡Features of small bowel loops include their central location, presence of valvulae conniventes, and active peristalsis.

💡In contrast, the colon demonstrates a heterogeneous haustral pattern associated with a prominent linear arc of gas and posterior reverberation artifact.

Key Sonographic Parameters for Diagnosing GI disease

• Physiological gut signature refers to a lumen that is normal for this segment, a wall structure with characteristic layering, well-defined wall thickness, and normal peristalsis.
• Pathological gut signature is characterized by changes in the normal appearance of the lumen, wall thickness, and/or peristalsis/pliability.

Wall thickness

• Measurement technique:
  • Measured perpendicularly from the lumen-mucosa interface to the serosa (White To White).
  • Use high-frequency linear probe (5–12 MHz) for accurate measurement.
  • Perform measurement under mild compression with transducer perpendicular to bowel wall to avoid tangential overestimation.
• Normal values 👉 Above
• Clinical significance of bowel wall thickening (BWT):
  • Thickening is the most common and reliable sign of bowel disease, but it is nonspecific.
  • Symmetric thickening: suggests an inflammatory or infectious etiology.
  • Asymmetric thickening: raises suspicion for neoplasm (e.g., lymphoma, adenocarcinoma).
  • The degree of thickening does not reliably differentiate between benign and malignant causes.
• Differential diagnosis of BWT:
  • Inflammatory: Crohn’s disease, ulcerative colitis, infectious enteritis, diverticulitis
  • Ischemic: bowel ischemia, ischemic colitis
  • Neoplastic: adenocarcinoma, lymphoma, GIST
  • Other: intramural hemorrhage, edema, radiation enteritis

Wall layer structure (layering)

• Normal appearance (STRATFIED APPEARANCE)
  • Five alternating hyperechoic and hypoechoic layers (gut signature), explained above.
  • Best visualized with a high-frequency linear probe (7–12 MHz).
• 🛠️Assessment technique:
  • Scan perpendicular to bowel wall to avoid anisotropy artifact.
  • Use minimal compression to preserve layer distinction.
  • Optimize focus point at the level of the bowel wall.
• Clinical implication
  • Preserved stratification:
    • Suggests a benign or inflammatory etiology.
    • Typically seen in: ulcerative colitis (mucosal involvement), mild Crohn’s disease, infectious enteritis, diverticulitis.
  • Loss of stratification:
    • Indicates severe transmural inflammation or malignancy.
    • Typically seen in: Active Crohn’s disease (transmural), lymphoma, adenocarcinoma, severe ischemia.
    • Associated with a higher risk of complications (fistulas, abscesses).
• 💡Clinical pearl:
  • Preservation of the 5-layer signature is a reassuring sign that argues against malignancy.
  • Complete loss of stratification in a thickened bowel loop warrants further investigation (cross-sectional imaging or endoscopy).

Wall stiffness (pliability)

• 🛠️Assessment technique:
  • Evaluated using graded compression with the ultrasound probe
  • Apply gradual pressure while following respiratory movements
  • Compare the compressibility of the abnormal segment to the adjacent normal bowel
• ✅Normal findings:
  • Normal bowel is readily compressible
  • Bowel walls approximate, or the lumen collapses completely under gentle pressure
  • Intraluminal gas is displaced, improving visualization of deeper structures
• ⚠️Abnormal findings (rigid, non-compressible):
  • Inflammatory: Active Crohn’s disease, severe diverticulitis, phlegmon
  • Fibrotic: Chronic fibrotic stricture (preserved layering, no hyperemia)
  • Neoplastic: Adenocarcinoma, lymphoma, GIST
  • Ischemic: Non-viable bowel, transmural infarction
• 💡Clinical significance:
  • Loss of compressibility is a key distinguishing feature between normal and abnormal bowel.
  • Helps differentiate between functional (dynamic) ileus and mechanical obstruction. 
  • Rigid, fixed loops with loss of peristalsis suggest transmural disease
  • Fibrotic strictures typically show preserved layering but increased stiffness compared to inflammatory strictures
• Limitations:
  • Limited efficacy in obese patients (increased abdominal wall thickness)
  • Patient discomfort may limit compression over tender areas
  • Overly aggressive compression may distort wall architecture

💡Clinical Pearl

Peristalsis

• Assessment technique:
  • Evaluated in real-time using B-mode ultrasound
  • Observe a segment of bowel for 10–20 seconds to assess contraction frequency and amplitude
  • Document findings with cine clips for serial comparison
• Normal findings:
  • Small bowel: active, coordinated, progressive contractions
  • Large bowel: slower, less frequent, predominantly mixing movements
  • Normal peristalsis helps displace intraluminal gas and contents

• Clinical significance:
  • Real-time ultrasound offers a unique advantage over CT and MRI for motility assessment
  • Diminished peristalsis is an indicator of unhealthy bowel
  • Absent peristalsis in a dilated, fluid-filled loop suggests paralytic ileus (📽️ videlow below).
    • 💡PEARL: Sonographic criteria for small bowel obstruction:
      • 3 or more dilated loops of bowels
      • Bowel dilation >2.5cm
      • Alternating (bidirectional) peristalsis (luminal contents cannot keep moving forward due to obstruction)
  • Hyperperistalsis proximal to a transition point suggests mechanical obstruction (more on this: 🔗here).
• Limitations:
  • Subjective assessment (operator-dependent)
  • Fasting reduces baseline small bowel motility
  • Deep or gas-obscured loops may be difficult to assess

Luminial diameter and contents

• 🛠️Assessment technique:
  • Luminal contents: Assess contents using B-mode ultrasound with graded compression
    • Note the presence, quantity, and echogenicity of intraluminal material
  • Luminal diameter: Assess in transverse and longitudinal planes:
    • Luminal diameter is measured from inner wall to inner wall (from the mucosal interface of one wall to the mucosal interface of the opposite wall) in a transverse or axial plane of the bowel loop.

• ✅Normal findings:
  • Luminal contents:
    • Small bowel: fluid and particulate content (hypoechoic/anechoic with some echogenic particles)
    • Colon: feces and gas (hyperechoic with posterior shadowing and reverberation artifacts)
  • Luminal diameter
    • Normal small bowel caliber: < 3 cm
    • Normal colonic caliber: cecum < 9 cm, transverse colon < 6 cm

• Clinical significance:
  • Dilated small bowel loops with fluid content and hyperperistalsis suggest early mechanical obstruction
  • Dilated loops with absent peristalsis suggest paralytic ileus
  • Fecal loading in the rectum with posterior shadowing suggests constipation
  • Bezoars may demonstrate a “twinkling artifact” on color Doppler, helping differentiate them from fecal material

Vascularity (blood flow signal)

• 🛠️Assessment technique:
  • Use Color or Power Doppler with low velocity scale (2–5 cm/s)
  • Wall filter set to lowest setting
  • Color gain adjusted until flash artifacts appear, then reduced slightly
  • Power Doppler may be more sensitive than Color Doppler for detecting slow flow in small mural vessels
  • Patient holds breath during acquisition to minimize motion artifact
  • Assess both the bowel wall and the adjacent mesenteric fat
• Normal findings:
  • Normal bowel wall rarely demonstrates detectable Doppler signals
  • Absence of flow in healthy, non-thickened bowel is expected
• Limberg Score
  • Vascularity score for MUC: 0 = absent; 1 = mild (spotty); 2 = moderate (long stretches within wall); 3 = severe (stretches extending into mesentery)

• 💡Clinical significance:
  • Hyperemia correlates with active inflammation and neovascularization
  • Useful for monitoring treatment response (decreased vascularity suggests response to anti-inflammatory therapy)
  • Differentiates inflammatory from fibrotic strictures (increased vs. absent flow)
  • Absence of flow in thickened bowel suggests ischemia, especially in the correct clinical context
• Limitations:
  • Limited sensitivity for slow or low-volume flow, particularly in deep bowel loops (>4 cm)
  • Flash artifacts from peristalsis and respiratory motion degrade image quality
  • High body mass index reduces depth penetration and signal detection
  • Cannot detect capillary flow (only larger intramural vessels)

Location/distribution

• Assessment technique:
  • Systematically scan all four quadrants using both curvilinear and linear probes
  • Trace abnormal bowel loops to document the length and pattern of involvement
  • Note the anatomic segment(s) affected (e.g., terminal ileum, ascending colon, sigmoid)
  • Document whether involvement is single or multifocal
• Normal findings:
  • Small bowel: centrally located, jejunum in the left upper/mid abdomen, ileum in the right lower abdomen
  • Colon: peripherally located (picture-frame distribution)
  • Retroperitoneal segments (ascending colon, descending colon, rectum) have fixed positions

• Clinical significance:
  • Anatomic location helps narrow the differential diagnosis
  • Terminal ileum involvement: Crohn’s disease, infectious ileitis, tuberculosis
  • Periappendiceal location: appendicitis, Crohn’s disease involving the cecum
  • Rectal sparing: Crohn’s disease (suggests against ulcerative colitis)
  • Segmental with skip lesions: highly suggestive of Crohn’s disease
  • Continuous from rectum proximally: suggestive of ulcerative colitis

Internal and external delineation

• 🛠️Assessment technique:
  • Evaluate clarity of the bowel wall layers (internal delineation)
  • Assess the serosal margin and its interface with surrounding tissues (external delineation)
  • Use high-frequency linear probe with optimal focus and gain settings
  • Compare affected segment to adjacent normal bowel
• ✅Normal findings:
  • Internal: Five distinct wall layers clearly visualized (when using high-frequency probe)
  • External: Smooth, well-defined hyperechoic serosal margin
  • Clear separation between bowel wall and adjacent mesenteric fat

• Clinical significance:
  • Good internal delineation favors a benign/inflammatory etiology
  • Poor internal delineation with loss of stratification raises suspicion for malignancy or severe transmural inflammation
  • Poor external delineation suggests disease extension beyond the bowel wall
  • Matted, inseparable loops indicate significant intra-abdominal pathology (adhesions, carcinomatosis, active IBD)
• Limitations:
  • Deep bowel loops or an obese body habitus reduce visualization quality
  • Overlying bowel gas may obscure segments
  • Operator-dependent for optimal image acquisition

Findings outside the wall

• Assessment technique:
  • Evaluate mesenteric fat adjacent to thickened bowel loops
  • Assess for lymphadenopathy, fluid collections, and free peritoneal fluid
  • Use graded compression to displace overlying bowel gas
  • Apply color Doppler to assess the vascularity of extramural lesions
• Normal findings:
  • Mesenteric fat: hypoechoic parallel layers, 7–12 mm thick
  • Lymph nodes: oval, elongated, hypoechoic; short axis < 4 mm in adults
  • No free fluid or collections

• 💡Clinical significance:
  • Inflamed hyperechoic fat is a useful clue, drawing attention to the underlying thickened bowel
  • Creeping fat is a highly specific feature of Crohn’s disease
  • Presence of an abscess or phlegmon indicates a complicated disease requiring intervention
  • CEUS helps differentiate between phlegmon (enhancing) and abscess (avascular)
  • Free fluid in obstruction suggests increased risk of strangulation and surgery

Inflammatory Bowel Disease Ultrasound

Clinical Case Scenario

Rationals

◾️Why ultrasound in IBD?

• IBD requires frequent objective monitoring — clinical symptoms alone are unreliable (up to 25% of asymptomatic patients have active inflammation)
• Endoscopy is invasive, costly, and impractical for serial follow-up
• CT exposes young patients to cumulative radiation
• IUS is non-invasive, radiation-free, low-cost, repeatable, and patient-preferred

⎮*ECCO-ESGAR (2019) and ACG (2025) recognize IUS as a first-line tool for assessing disease activity and complications in IBD*

◾️What IUS can do in IBD

• For Crohn’s disease:
  • Detect and localize small bowel and colonic inflammation
  • Assess disease extent (sensitivity 86–94%, specificity 94–97%)
  • Identify complications: strictures, fistulas, abscesses (comparable to MRE/CT)
  • Monitor response to biologic therapy as early as 4–12 weeks
  • Detect postoperative recurrence (BWT >3–5.5 mm predicts Rutgeerts ≥ i2b)
• For ulcerative colitis:
  • Assess colonic wall thickening (BWT >3 mm) and hyperemia
  • Evaluate disease extension (especially when colonoscopy is incomplete)
  • Monitor treatment response — changes seen as early as 2 weeks
  • Milan Ultrasound Criteria (MUC >6.2) predict endoscopic activity with 100% specificity
  • Transperineal US evaluates rectal inflammation (BWT ≥4 mm)

⚠️Limitations of IUS in IBD 

• Limited visualization of the rectum, proximal jejunum, and deep pelvis
• Operator-dependent (requires dedicated training)
• Less accurate for disease length measurement in extensive small bowel CD
• Not suitable for dysplasia surveillance
• Obesity may limit image quality

Treat-To-Target Strategy in IBD 

🎯Treatment Target Strategy in IBD: Deep Remission

• Deep Remission: This is the combination of clinical remission + endoscopic healing while being corticosteroid-free. Studies show this significantly reduces disease progression.
  • According to the latest STRIDE-II consensus and major society guidelines, the “gold standard” test for IBD remission is Endoscopic Healing (also known as Mucosal Healing).
    • This is the primary long-term target because it directly correlates with improved clinical outcomes, reduced hospitalization rates, and a lower risk of surgery.
• Emerging Targets
  • Transmural Healing (CD): Because Crohn’s affects the full thickness of the bowel wall, there is growing evidence that achieving Transmural Healing (normalization of the bowel wall on Ultrasound or MRI/MRE) leads to even better long-term outcomes than mucosal healing alone. However, it is currently considered an adjunctive target rather than a mandatory gold standard.
    • Endoscopy sees only the mucosa (inner lining).
    • MRE sees the entire bowel wall (mucosa + submucosa + muscularis + serosa) and surrounding mesentery.
      • MRE is the gold standard imaging test to confirm Transmural Healing.
      • Is MRE recommended for routine monitoring? Not for all patients. It is reserved for specific scenarios (see below).
  • Histologic Healing (UC): While STRIDE-II does not mandate it as a formal target, histological remission (normalization of tissue under the microscope) is emerging as a deeper goal for Ulcerative Colitis to predict even lower relapse rates

📍NOTE: Clinical Significance of Transmural Healing as a Therapeutic Target

📍MRE indications

Monitoring IBD Activity

How and when to monitor disease activity

Achieving IBD remission is no longer defined by a single test. Instead, it is a multi-tiered “Treat-to-Target” strategy that combines several layers of evidence to confirm disease control. Here are the gold standard tests for the key targets in IBD management:

🎯Long-Term Gold Standard: Colonoscopy

• This is the definitive test for remission. It involves direct visual inspection of the intestinal lining. Guidelines define remission by specific scores:
  • Crohn’s Disease (CD): SES-CD < 3 or the absence of ulcerations
  • Ulcerative Colitis (UC): Mayo Endoscopic Score (MES) of 0 (normal mucosa) or 1 (mild friability/erythema)

🎯Intermediate/Short-Term Targets (Non-Invasive)

• Since colonoscopy is invasive and costly, these tests are used for frequent monitoring to confirm that treatment is working.
  • Fecal Calprotectin: This is the most reliable non-invasive biomarker. It measures inflammation directly in the gut.
    • Target: Usually < 100–250 µg/g
  • Serum CRP: Normalization of C-Reactive Protein levels (typically < 5 mg/L)
  • Intestinal Ultrasound (IUS): A radiation-free, point-of-care tool gaining prominence. Remission is defined as Bowel Wall Thickness (BWT) ≤ 3 mm with no Doppler signal (normalization of wall structure)

🎯Clinical & Patient-Reported (PRO) Targets Are Immediate Goals

• While not sufficient alone to define remission (due to symptom-inflammation discordance), resolution of symptoms is a mandatory immediate goal.
  • CD: Harvey-Bradshaw Index (HBI) < 5 or PRO2 normalization (Abdominal pain ≤ 1, Stool frequency ≤ 3)
  • UC:  Partial Mayo Score < 3 with no rectal bleeding and normal/near-normal stool frequency

Treatment Targets in IBD: From Symptoms to Transmural Healing

IUS

◾️Definitions of response and remission based on IUS evaluation (IUS consensus 2022)

• US response (at 12–14 weeks):
  • Reduction in BWT by ≥25% OR ≥2.0 mm OR
  • Reduction ≥1.0 mm + decrease in Limberg score by ≥1 grade
• IUS transmural remission (at 26–52 weeks):
  • BWT ≤3 mm AND
  • Normal/absent color Doppler signal

⎮Transmural healing (TH) is associated with lower rates of surgery, hospitalization, and corticosteroid use

📖Evidence

◾️IUS vs. Biochemical Markers (CRP and Fecal Calprotectin)

• Why Biomarkers Alone Are Insufficient
  • CRP: Normal in 20–30% of patients with endoscopically active CD; nonspecific (elevated in infections, arthritis, etc.)
  • Fecal calprotectin (FC): More sensitive than CRP, but cannot localize disease, cannot detect complications (strictures, fistulas, abscesses), and levels vary by disease extent and location
  • When biomarkers are used for monitoring IBD activity, still 50% of patients don’t meet the primary outcome (i.e., deep remission: CDEIS <4 and no deep ulcers at 48 weeks) *.

◾️IUS vs. Clinical signs & symptoms

◾️IUS vs. MRE

◾️IUS vs. endo/colonoscopy

📍Note: No Total IUS Score Equivalent to Total SES-CD

• There is no IUS score equivalent of the total SES-CD score for the entire visualized bowel, and limits correlation between the two measures
• Segmental BWT on IUS has been shown to be accurate in detecting endoscopic inflammation based on the segmental SES-CD score
• Several segmental IUS activity scores have been recently developed, but lack sufficient external validation with the segmental SES-CD
• 🔗 IBD SCORES

◾️IUS Parameters 

• The full list of indices is listed in the following table.
• However, in practice, despite different scoring systems, 2 indices are used for IBD activity monitoring: BWT and bowel wall hyperemia (this is graded by the modified Limberg score).

◾️Ultrasound Scoring Systems in IBD

• To standardize disease activity assessment and treatment response, several semi-quantitative and quantitative scoring systems have been developed.
• Modified Limberg Score (Doppler Grading)
  • The most widely used semi-quantitative grading system for bowel wall vascularity 👇:

◾️IUS in Special Population

◾️Practical Approach. Multimodal Monitoring for IBD activity

Treatment Escalation and De-escalation Based on IUS

• Therapy Escalation and De-escalation Based on IUS
  • Clinical symptoms alone are unreliable for guiding therapy (up to 25% discordance with inflammation)
  • Biomarkers (CRP, FC) cannot localize disease or detect complications
  • Endoscopy is invasive and cannot be repeated frequently
  • IUS provides real-time, non-invasive, transmural assessment that enables timely treatment adjustments

⎮”Symptoms of CD do not correlate well with the presence of active inflammation and therefore should not be the sole guide for therapy.”
📖ACG Clinical Guideline: Management of Crohn’s Disease in Adults (2025), Key Concept #9

When to Escalate Therapy Based on IUS

When to De-escalate Therapy Based on IUS

De-escalation decisions should be individualized based on: disease history, prior complications (strictures, fistulas, surgery), patient preference, and risk of relapse.

IUS-Guided Tight Control Algorithm

◾️Summary: IUS in Therapeutic Decision-Making

• IUS enables real-time, non-invasive, transmural assessment that:
• Guides escalation when the response is inadequate
• Guides de-escalation when deep remission is achieved
• Detects complications (strictures, fistulas, abscesses) early
• Reduces unnecessary endoscopy and radiation exposure
• Improves patient compliance and shared decision-making.

Clinical Case Example

RECAP

• IUS is a non-invasive, radiation-free, real-time imaging tool that provides transmural assessment of IBD activity
• IUS is comparable to MRE and CTE for detecting disease activity and complications (strictures, fistulas, abscesses)
• IUS correlates well with endoscopy (r = 0.70–0.85) and is patient-preferred over repeated colonoscopies
• IUS adds value beyond clinical symptoms (which are unreliable in up to 25% of patients) and biomarkers (which cannot localize disease or detect complications)
• Transmural healing (BWT ≤3 mm + absent Doppler) is associated with lower rates of surgery, hospitalization, and corticosteroid use
• IUS enables early assessment of treatment response (as early as week 4–8) and guides therapy escalation or de-escalation in a tight control strategy
• IUS is particularly valuable in special populations: pregnancy (safe), pediatrics (no sedation), and postoperative CD (early recurrence detection)
• Limitations include operator dependency, limited visualization of rectum/proximal jejunum, and inability to screen for dysplasia
• IUS is not a replacement for endoscopy but a complementary tool that, when combined with symptoms and biomarkers, optimizes IBD management.
• Intestinal ultrasound is a transformative, non-invasive tool that provides real-time, transmural assessment of IBD activity. When integrated into a tight-control monitoring strategy alongside clinical assessment and biomarkers, IUS enables earlier treatment optimization, reduces unnecessary endoscopy and radiation exposure, improves patient compliance, and is associated with better long-term outcomes, including lower rates of surgery and hospitalization.

Going Further

• US of the GI tract – Normal Anatomy (Radiology Assistant)
• Crohn’s disease – role of Ultrasound (Radiology assistant)

Appendix

📊Classification of available therapy for IBD

📊Ileal Crohn’s vs. sigmoid/colon Crohn’s disease

📽️Media

• Paralytic ileus
  • Patient with acute appendicitis and peritonitis, showing a dilated jejunum (> 3 cm) with clear fluid content and without peristaltic activity of contractions over a length of> 10 cm

• Bowel Ultrasound Normal Vs Crohn’s Disease | Limberg Score, SUS-CD System & Other USG Findings

• Intestinal Ultrasound for IBD Monitoring