What is Intima-Media Thickness (IMT) in carotid ultrasound?

Intima-Media Thickness (IMT) in Carotid Ultrasound

Intima-Media Thickness (IMT) is a noninvasive ultrasound measurement of the combined thickness of the intimal and medial layers of the carotid artery wall, which serves as an important marker of subclinical atherosclerosis and cardiovascular risk assessment. 1

Definition and Technical Aspects

IMT is measured using high-resolution B-mode ultrasound imaging of the carotid artery wall. The measurement specifically represents:

• The distance between the lumen-intima interface and the media-adventitia interface of the arterial wall
• Most commonly measured in the far wall of the common carotid artery, though it can also be measured in other carotid segments (bulb, internal carotid)
• Typically expressed in millimeters (mm)

The procedure involves:

• Patient positioned supine with neck slightly extended and head turned 45° away from the side being examined
• Use of high-frequency (7-12 MHz) linear-array transducer
• Imaging the carotid artery in its long axis with multiple scanning angles
• Magnification of the vessel wall to identify the intimal-medial complex 1

Clinical Significance

IMT measurement has significant clinical value:

1. Risk Assessment: Increased carotid IMT independently predicts future risk for ischemic coronary events and stroke

   • Risk increases approximately 15% per 0.10-mm increase in carotid IMT 1
   • For carotid IMT values ≥1 mm, there is a 2-fold greater risk of acute MI over 3 years 1

2. Cardiovascular Risk Stratification: The 2010 ACCF/AHA guidelines state that:

   • Measurement of carotid artery IMT is reasonable for cardiovascular risk assessment in asymptomatic adults at intermediate risk (Class IIa recommendation) 1
   • IMT correlates with traditional cardiovascular risk factors including age, systolic blood pressure, serum lipids, and smoking 2

3. Monitoring Disease Progression/Treatment: IMT can be used to:

   • Track progression of atherosclerosis
   • Evaluate regression or slowing of progression in response to antihypertensive and lipid-lowering medications 2

Normal Values and Interpretation

Normal IMT values vary by age:

• Young adults (20-30 years): approximately 0.5 mm
• Older adults (60-70 years): approximately 0.9 mm 2

For specific age groups, upper limits (97.5 percentile) have been established:

• 35-39 years: 0.60 mm
• 40-49 years: 0.64 mm
• 50-59 years: 0.71 mm
• 60+ years: 0.81 mm 3

Important Considerations in IMT Assessment

1. Measurement Location: IMT at the carotid bifurcation is typically higher than at the common carotid artery 3

2. Standardization: Published recommendations on equipment, technical approach, and operator training must be followed to achieve high-quality results 1

3. Distinction from Plaque: The presence of atherosclerotic plaque (defined as a focal increase in thickness ≥50% of the surrounding IMT) increases cardiovascular risk beyond IMT alone 1

4. Screening Applications: According to guidelines, carotid IMT screening is:

   • Not recommended for low Framingham risk score patients
   • Uncertain utility for intermediate Framingham risk score patients
   • Uncertain utility for high Framingham risk score patients 1

Limitations and Practical Considerations

• Variability in measurement techniques between laboratories can affect results
• Requires well-trained operators and standardized protocols
• Automated computerized measurement systems may provide better reproducibility than manual measurements 2
• IMT measurement is distinct from standard carotid duplex screening, which assesses for stenosis and plaque but does not typically include formal IMT measurement 1

IMT measurement represents an important tool in cardiovascular risk assessment that provides valuable information beyond traditional risk factors, particularly for patients at intermediate risk of cardiovascular disease.