The ICA/CCA ratio is a standard, non-invasive measurement used to assess blood flow in the neck arteries and diagnose carotid artery stenosis. This ratio compares blood flow velocities in two major vessels to determine the degree of narrowing caused by plaque buildup. Carotid stenosis restricts blood flow to the brain and is a significant factor in stroke risk. Calculating this ratio is a fundamental part of the vascular ultrasound examination, providing information about the severity of arterial obstruction.
Understanding the Carotid Arterial System
The carotid arterial system begins with the Common Carotid Artery (CCA), the main vessel traveling up the neck. At the upper thyroid cartilage, the CCA divides into two branches: the External Carotid Artery (ECA) and the Internal Carotid Artery (ICA). This division point, the carotid bifurcation, is the most frequent site for atherosclerotic plaque development. The ECA supplies blood to the face, scalp, and neck structures, and is generally not the focus of stroke risk assessment.
The ICA is the artery that continues to ascend into the skull to supply the anterior and middle sections of the brain. Because the ICA is the primary conduit for oxygenated blood to the brain, any narrowing is a serious concern. The ICA typically has a slightly wider diameter than the ECA at its origin, forming a slight dilation called the carotid bulb.
Measuring Blood Flow with Doppler Ultrasound
The data needed to calculate the ICA/CCA ratio are collected using Doppler ultrasound, a non-invasive imaging technique that measures the speed and direction of blood flow. This method utilizes the principle that sound waves reflect off moving red blood cells, and the change in frequency (the Doppler shift) correlates directly with blood velocity. This technique allows quantification of blood movement in centimeters per second (cm/sec).
The specific metric extracted for the ratio calculation is the Peak Systolic Velocity (PSV), the fastest speed the blood reaches during the heart’s contraction phase (systole). When an artery develops a blockage, the blood must squeeze through a smaller opening, causing velocity to increase dramatically at the point of greatest narrowing. By measuring the PSV in both the ICA and the CCA, the ultrasound provides the two numbers necessary to assess the degree of stenosis.
Calculating the ICA/CCA Ratio
The ICA/CCA ratio is calculated by dividing the Peak Systolic Velocity measured in the Internal Carotid Artery (ICA PSV) by the Peak Systolic Velocity measured in the Common Carotid Artery (CCA PSV). The formula is expressed simply as: ICA PSV / CCA PSV. For example, if the ICA PSV is 250 cm/sec and the CCA PSV is 50 cm/sec, the resulting ratio is 5.0.
The clinical rationale for using a ratio, rather than relying solely on the ICA PSV, is to account for systemic variables that can affect blood flow speed. Factors such as low cardiac output or heart function variability can alter the absolute velocity measured in the CCA. Using the CCA PSV as the denominator normalizes the reading, creating a baseline velocity for that specific patient. This normalization makes the ratio a more reliable indicator of localized narrowing in the ICA.
Interpreting Stenosis Results
The numerical value of the ICA/CCA ratio is directly correlated with the estimated degree of carotid artery stenosis, allowing clinicians to categorize the severity of the disease. A ratio of less than 2.0 is associated with minimal or less than 50% stenosis, often alongside an ICA PSV below 125 cm/sec. This range suggests that the narrowing is not yet considered hemodynamically significant, meaning it is not severely impacting blood flow to the brain.
The range indicating moderate stenosis is defined by an ICA/CCA ratio between 2.0 and 4.0. A ratio in this range, typically paired with an ICA PSV between 125 and 230 cm/sec, corresponds to a 50-69% diameter reduction, which may prompt closer monitoring and medical management. When the ratio exceeds 4.0, it suggests a severe stenosis of 70% or greater, often with a PSV greater than 230 cm/sec. Ratios in this higher range indicate a significant risk of stroke and may lead to discussions about surgical intervention, such as a carotid endarterectomy or stenting.