IBUS-SAS

How to use it

Score each segment you scanned, at the most affected part of that segment. IBUS-SAS is a segmental score: there is no total across the bowel. The large number in the result is the most affected segment, and each segment is listed below it.

IBUS-SAS = 4 × BWT (mm) + 15 × i-fat + 7 × colour Doppler + 4 × stratification, capped at 100 (Novak 2021).

• Inflammatory mesenteric fat (i-fat): 0 absent · 1 uncertain · 2 present
• Colour Doppler signal (modified Limberg): 0 none · 1 short signals · 2 long signals inside the bowel wall · 3 long signals inside and outside the bowel wall
• Bowel wall stratification: 0 normal · 1 uncertain · 2 focal loss (≤3 cm) · 3 extensive loss (>3 cm)

Worked example (Novak 2021, Figure 4): BWT (7.8 + 7.1 + 8.5 + 8.4) / 4 = 7.95, rounded to 8.0 mm; i-fat present (2); no Doppler signal (0); focal loss of stratification (2). Score = 32 + 30 + 0 + 8 = 70.

Measuring bowel wall thickness

BWT carries the most weight, and it is the most reliable parameter (inter-rater ICC 0.96).

• Take four measurements: two in cross-section (more than 90° apart) and two longitudinal (at least 1 cm apart), in the most affected part of the segment. Use the “4 measurements” switch to let the tool average them.
• Place the calipers perpendicular to the wall (Novak 2021, Figure 1 shows valid and invalid placements).
• The tool rounds BWT to 0.1 mm, as in the worked example.

Experts set >3 mm as abnormal for every segment when building the score (Novak 2021). A sigmoid wall up to 4 mm can be normal, especially with diverticular disease (Ilvemark 2022), and the 2025 trial consensus left the sigmoid normal range (3–4 mm) unresolved (Allocca 2025).

Reading the result

There is no validated activity cut-off. Published thresholds come from single studies with different reference standards:

Use the score to follow the same segment over time, and read it with the clinical picture, calprotectin and endoscopy.

Treatment targets (Ilvemark 2022 consensus):

• IUS response: BWT falls by more than 25%, or by more than 2.0 mm, or by more than 1.0 mm with the Doppler grade falling by at least one. Enter a baseline BWT (and baseline Doppler grade) in each segment’s “Compare with an earlier scan” section.
• Transmural remission: BWT ≤3 mm with no Doppler signal.

The 2025 international consensus for trials uses a slightly different response (a fall of 25% or more in BWT, or a smaller BWT fall combined with a fall of at least one grade in Doppler or another IUS parameter) and remission (BWT ≤3 mm, or normalisation of all parameters). It recommends assessing response at weeks 4–8 for the colon and week 12 for the terminal ileum (Allocca 2025).

Teaching points for IUS

• A normal segment doesn’t score 0. A 2 mm wall with nothing else scores 8. The score’s 0 is theoretical.
• i-fat is heavily weighted (15 points per grade) and agreement on it is only moderate (kappa 0.51). If unsure, score “uncertain” rather than guess.
• Stratification is the least reliable item (kappa 0.39), so read it alongside Doppler and BWT.
• Doppler depends on machine settings. The IBUS group standardised the settings, cine-loop length and colour box size for scoring. Keep yours consistent between scans so serial scores are comparable.
• Strictures and fistulae aren’t in the score. Report prestenotic dilatation, fistulae, phlegmon and abscess separately.
• UC: IBUS-SAS was built for Crohn’s disease. Early studies in UC (mostly the sigmoid) show good correlation with the Mayo endoscopic subscore and the UCEIS (Innocenti 2025; Fischer 2025), but the trial consensus prefers the Milan Ultrasound Criteria for UC.
• Pair the scan with the endoscopic score: SES-CD for Crohn’s disease, UCEIS or Mayo for UC.