The ICA/CCA ratio is a non-invasive diagnostic metric used in vascular ultrasound to assess the degree of narrowing, or stenosis, within the carotid arteries. This calculation provides essential information about the dynamics of blood flow in the neck vessels that supply the brain. By comparing flow speeds between two connected arteries, the ratio helps physicians determine the severity of plaque buildup, which is a significant factor in stroke risk. This method is a key component of carotid duplex ultrasound, offering a standardized way to evaluate arterial health.
Anatomical Purpose of the ICA/CCA Ratio
The ICA/CCA ratio is based on the anatomical relationship between the Common Carotid Artery (CCA) and the Internal Carotid Artery (ICA). The CCA serves as the main trunk, which then splits into the ICA, which supplies the majority of blood flow to the brain, and the External Carotid Artery (ECA), which supplies the face and neck. Stenosis typically occurs at the ICA origin, making it the segment of interest for potential disease.
The ratio normalizes the ICA measurement by comparing it to the CCA, which is assumed to be free of significant disease. This normalization accounts for systemic factors, such as the patient’s cardiac output or blood pressure, which cause absolute blood flow speed to vary widely among individuals. Without this comparison, a high absolute speed in the ICA might be misinterpreted as severe stenosis. The ratio isolates the change in flow speed caused by localized narrowing in the ICA, making the diagnostic criteria more reliable across different patients.
Identifying the Necessary Velocity Measurements
The specific measurement required to calculate the ICA/CCA ratio is the Peak Systolic Velocity (PSV). PSV represents the fastest speed the blood reaches in the artery during the contraction phase of the heart, known as systole. Doppler ultrasound technology is employed to obtain these precise velocity measurements by transmitting sound waves and analyzing the frequency shift as they reflect off moving red blood cells.
Two velocity measurements are needed for the ratio. One is the PSV from the narrowest point of the Internal Carotid Artery, which is the site of the suspected stenosis, where blood flow accelerates significantly. The second measurement is the PSV taken from a standardized, healthy segment of the distal Common Carotid Artery, typically two to four centimeters below the bifurcation. This distal CCA measurement avoids turbulent flow near the branch point, ensuring the baseline velocity is accurate and representative of the main arterial flow.
Step-by-Step Ratio Calculation
The calculation of the ICA/CCA ratio is a straightforward division, utilizing the two Peak Systolic Velocity (PSV) measurements obtained from the ultrasound study. The formula is simply the ICA PSV divided by the CCA PSV. This procedure compares the highest speed of blood flow found within the suspected narrowed segment to the baseline speed in the common artery feeding it.
For instance, if the highest PSV measured in the ICA is 250 centimeters per second (cm/s), and the baseline PSV in the CCA is 60 cm/s, the ratio is calculated as 250 divided by 60. This results in an ICA/CCA ratio of approximately 4.17. Since the units (cm/s) cancel out during division, the resulting number is dimensionless. This single, unitless number is then used to grade the severity of the carotid stenosis.
Interpreting the Ratio and Stenosis Severity
The calculated ICA/CCA ratio is a direct indicator used to grade the severity of carotid artery stenosis, with higher ratios corresponding to greater degrees of narrowing. Standardized thresholds, such as those established by the Society of Radiologists in Ultrasound (SRU) consensus, are used to translate the ratio into a clinical diagnosis. A ratio of less than 2.0, for example, suggests a normal artery or a stenosis of less than 50%.
A ratio falling between 2.0 and 4.0 indicates a moderate stenosis, usually categorized as 50% to 69% narrowing of the artery’s diameter. When the ratio exceeds 4.0, it suggests a severe stenosis, often defined as 70% or greater narrowing. This level of severe stenosis carries an elevated risk of stroke and often warrants more aggressive treatment, such as surgical intervention or stenting.
The ratio is one piece of the overall diagnostic picture, and various factors can lead to an over- or underestimation of the stenosis. For instance, an unusually low CCA velocity due to heart failure could artificially inflate the ratio, while heavy calcification might limit the ability to get the most accurate velocity reading. When used alongside other ultrasound parameters, such as the ICA End-Diastolic Velocity (EDV) and direct visualization of the plaque, the ICA/CCA ratio provides a robust and reliable basis for clinical decision-making.