Intima-Media Thickness (IMT): Measurement Technique, Norms, and Difference from Plaque
What Ultrasound Shows: The Double Line Phenomenon
Histologically, the arterial wall consists of three layers: intima, media, and adventitia. Ultrasound visualizes not the layers themselves, but their boundaries — the so-called double line phenomenon. The measured intima-media complex (IMT) is composed of the intima and media.
Measurement Technique
Correct measurement is performed according to the following rules:
• On the far wall of the CCA, approximately 1 cm proximal to the bifurcation.
• In a straight segment without plaques.
• Strictly at the end of diastole (according to ECG).
• Only using the edge-detection method: manual caliper is unacceptable as it provides a single point, has poor reproducibility, and systematically overestimates the result.
According to the ASE consensus (2020), when characterizing plaque, measurement begins in the same plane as the carotid IMT measurement, which is necessary for consistency in identifying plaque exceeding the IMT threshold >1.5 mm. For standardization, measurement can be performed from any segment of the long and short axis of the carotid artery (bulb, ICA, CCA), with the projection and segment specified in the protocol.
Quality Index (QI)
Measurement should only be trusted with a good quality index (QI) — a small spread of values across cardiac cycles. If QI is insufficient, the study must be repeated.
IMT and Plaque — Different Findings
Plaque is a separate finding and is not included in IMT. According to the Mannheim Consensus (2011), plaque is defined as a focal structure protruding into the arterial lumen by at least 0.5 mm or 50% of the surrounding IMT value, or demonstrating a thickness >1.5 mm when measured from the media-adventitia boundary to the intima-lumen boundary.
| Plaque Criterion (Mannheim, 2011) | Value |
|---|---|
| Focal protrusion into the lumen | ≥ 0.5 mm |
| Relative to surrounding IMT | ≥ 50% |
| Absolute thickness | > 1.5 mm |
Threshold Norms
Norms are assessed by age and sex percentiles: a value above the 75th percentile is interpreted as increased risk. A rough clinical guide for the upper limit in middle-aged individuals is about 0.9 mm, but this should be considered approximate.
Example of conclusion formulation: on the far wall of the CCA on both sides, IMTmean is about 0.93–0.95 mm with IMTmax up to 1.1 mm, plaques are not visualized — this is interpreted as IMT thickening above the age norm, a sign of early vascular remodeling; risk factor correction and reassessment in 1–2 years are recommended.
Vascular Age
From age percentiles of IMT, a patient-friendly indicator — vascular (arterial) age — is derived. Vascular age and expected IMT are calculated through regression.
Features in Children
In children, the wall is thin, and absolute IMT values are significantly lower than in adults; they naturally increase with age and body size. Therefore, adult percentiles and IMT thresholds (as well as the author's Z-score for adult IMT) are not applicable to children.
Frequently asked questions
Where and when exactly should IMT be measured?
On the far wall of the CCA, approximately 1 cm proximal to the bifurcation, in a straight segment without plaques, strictly at the end of diastole according to ECG.
Why can't a manual caliper be used?
Manual measurement provides a single point, has poor reproducibility, and systematically overestimates the result. Only automatic edge-detection is used.
At what threshold is a finding considered plaque, not IMT?
According to the Mannheim Consensus (2011): focal protrusion ≥0.5 mm, or ≥50% of surrounding IMT, or thickness >1.5 mm. Plaque is not included in IMT.
What is the upper limit of normal for IMT?
Norms are based on age and sex percentiles (>75th percentile = increased risk). A rough clinical guide for middle age is about 0.9 mm, but this is approximate.
Can adult IMT thresholds be applied to children?
No. In children, the wall is thinner, values are lower and increase with age; adult percentiles, thresholds, and IMT Z-score are not applicable to children.