How is Carotid Intima-Media Thickness (CIMT) testing performed on adults with risk factors for cardiovascular disease, such as hypertension, diabetes, or a family history of heart disease?

How CIMT is Performed

CIMT is performed using high-frequency ultrasound (7-12 MHz linear-array transducer) with the patient supine and head turned 45° away from the side being examined, measuring the far wall of the common carotid artery 8-16 mm proximal to the flow divider, with measurements taken at end-diastole using standardized protocols that require well-trained operators to achieve reproducible results. 1

Equipment Requirements

The following equipment is essential for proper CIMT testing 1:

• High-frequency linear-array transducer (7-12 MHz) for optimal resolution 1
• 2D, color, and spectral Doppler imaging capabilities 1
• Electrocardiograph (ECG) for timing measurements at end-diastole 1
• Digital or super VHS recording for image storage and offline analysis 1

Patient Positioning and Preparation

Proper patient positioning is critical for obtaining high-quality images 1:

• Patient rests supine in a comfortable position 1
• Head turned 45° toward the side opposite the examination 1
• Lightest possible transducer pressure should be used to avoid obscuring natural vessel pulsations 1

Anatomical Segments and Measurement Sites

CIMT can be measured at multiple carotid segments, though the common carotid artery provides the most reproducible measurements 1:

Standard Measurement Protocol (ACAPS/ARIC Method)

The most widely validated approach measures three segments bilaterally 1:

• Common carotid artery (CCA): 8-16 mm proximal to the flow divider (bifurcation) 1
• Carotid bifurcation (bulb): Beginning at the flow divider and extending 8 mm proximally 1
• Internal carotid artery (ICA): Proximal 8 mm above the flow divider 1

Preferred Measurement Site

The far wall of the common carotid artery is the preferred standard site due to its superior reproducibility and ability to refine cardiovascular risk prediction 1. The common carotid can be adequately imaged in 89-92% of patients, compared to only 37-63% for the internal carotid artery 1.

Measurement Technique

Timing and Image Acquisition

• Measurements must be taken at end-diastole, synchronized with the R wave on ECG 1
• Both near wall (closest to skin) and far wall (farthest from skin) can be measured, though far wall measurements are more reproducible 1
• Multiple angles should be assessed to ensure comprehensive evaluation 1

What is Being Measured

CIMT represents the combined thickness of the intimal and medial arterial wall layers 1. The measurement extends from:

• The border between the echolucent vessel lumen and echogenic intima (inner boundary) 1
• To the border between the echolucent media and echogenic adventitia (outer boundary) 1

Calculation of Final Value

The standard approach calculates 1:

• Mean of maximal CIMT measurements from 3 segments (common carotid, bifurcation, internal carotid) 1
• Measured bilaterally from 2 scanning angles 1
• Resulting in a mean of 12 maximal measurements as the primary outcome 1

Image Analysis Methods

Offline vs. Online Measurement

Offline analysis of digitally stored images is strongly preferred over online caliper measurements by the sonographer 1:

• Automated edge-detection software reduces interobserver variability compared to manual measurements 1
• Sequential multiframe image processing (5-frame averaging) reduces measurement variability by 27% relative to single-frame measurement 1
• Separation of data acquisition and analysis improves study reproducibility because variability between sonographers is generally larger than between readers 1

Important Caveat

If ultrasound images have indistinct borders, automated systems may require manual override, eliminating the principal advantage of automated edge detection 1.

Critical Quality Requirements

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

• Well-trained operators are essential for achieving excellent intertest and interobserver reproducibility 1
• Provider competency and lab accreditation standards are needed to ensure quality imaging 1
• Uniform training and performance monitoring throughout the study period maintains high-quality, reproducible measurements 1

Reproducibility Considerations

The test has demonstrated highly accurate results with excellent reproducibility primarily in research settings using standardized protocols, though reproducibility is less consistent in practitioner-based settings 1. This emphasizes the critical importance of following standardized protocols and maintaining operator competency 1.

Additional Assessments

Plaque Detection

Beyond IMT measurement, sonographers should identify 1:

• Atherosclerotic plaque, operationally defined as focal increase in thickness ≥50% of surrounding IMT 1
• Plaque presence increases predicted coronary artery disease risk at any level of CIMT 1

Carotid Stiffness Parameters

B-mode-guided M-mode images can be obtained to assess carotid stiffness 1:

• Maximal and minimal lumen diameters are measured from the common carotid artery 1
• Combined with noninvasive blood pressure measurements to calculate stiffness parameters 1

Completeness of Measurements

Body composition significantly affects measurement completeness 2:

• Increased body mass index and waist circumference are significantly related to less complete data (p<0.01) 2
• Near wall measurements of the internal carotid artery are more difficult to obtain (86-96% completeness) compared to other segments (>97% completeness) 2
• With proper protocols, 89-96% of patients can have complete data on all twelve carotid segments 2