Quick Answer: How to Calculate ICA CCA Ratio
To calculate the ICA/CCA ratio, divide the peak systolic velocity in the internal carotid artery (ICA PSV) by the peak systolic velocity in the common carotid artery (CCA PSV):
Example: If ICA PSV is 210 cm/s and CCA PSV is 70 cm/s, the ratio is 210 ÷ 70 = 3.0.
What Is the ICA/CCA Ratio?
The ICA/CCA ratio is a Doppler ultrasound metric used in carotid duplex studies to support estimation of internal carotid artery stenosis severity. It compares flow velocity in the ICA with flow velocity in the ipsilateral CCA. Because it is a ratio, it helps normalize some patient-to-patient variation in absolute velocities.
This ratio is not interpreted in isolation. Sonographers and interpreting clinicians combine it with ICA PSV thresholds, ICA end-diastolic velocity (EDV), plaque morphology, and grayscale/color Doppler findings.
Core Formula
Most labs use PSV measured at standard angle correction, often near 60 degrees, with technique consistency across measurements.
Step-by-Step Calculation
- Measure ICA PSV on one side (right or left) using proper Doppler angle correction.
- Measure CCA PSV on that same side under consistent settings.
- Confirm both values use the same unit (usually cm/s).
- Divide ICA PSV by CCA PSV.
- Round the result as needed (for example, to two decimals).
If CCA PSV is zero or extremely low due to technical or physiologic issues, the ratio may be unstable or non-interpretable. In that case, repeat acquisition and rely on full duplex criteria.
Worked Examples
Example 1
ICA PSV = 130 cm/s, CCA PSV = 80 cm/s
Example 2
ICA PSV = 250 cm/s, CCA PSV = 65 cm/s
Example 3
ICA PSV = 340 cm/s, CCA PSV = 70 cm/s
ICA/CCA Ratio Interpretation Ranges
Reference ranges vary by institution and protocol, but commonly used carotid duplex frameworks include the following approximate guide:
| ICA/CCA Ratio | Typical Interpretation Context | How It Is Used |
|---|---|---|
| < 2.0 | Often consistent with lower-grade stenosis (<50%) when paired with lower ICA PSV and supportive imaging findings. | Usually reassuring, but still interpreted with plaque and waveform data. |
| 2.0 to 4.0 | Frequently seen in moderate stenosis ranges (commonly 50–69%) when other criteria agree. | Requires integration with ICA PSV and EDV. |
| > 4.0 | Can support severe stenosis (commonly ≥70%) in conjunction with elevated ICA PSV/EDV and plaque findings. | Used as supporting evidence, not stand-alone diagnosis. |
Important: Exact cutoffs can differ between laboratories, accreditation standards, and reporting systems. Always follow your local validated criteria.
Common Pitfalls When Calculating ICA/CCA Ratio
- Mixing right and left side measurements in the same ratio.
- Using inconsistent Doppler angle correction between ICA and CCA velocities.
- Using a non-representative CCA sample (for example, turbulence or poor alignment).
- Relying on ratio alone without PSV, EDV, and plaque visualization.
- Ignoring hemodynamic confounders such as contralateral disease, low cardiac output, or arrhythmia.
Best Practice Tips
- Document exactly where each spectral sample was obtained.
- Use consistent machine presets and angle correction standards.
- Repeat suspicious measurements before final reporting.
- Correlate with clinical symptoms and prior imaging when available.
Frequently Asked Questions
Is ICA/CCA ratio the same as ICA PSV?
No. ICA PSV is a single velocity value in the ICA. ICA/CCA ratio is a derived value: ICA PSV divided by CCA PSV.
Can I calculate ICA/CCA ratio with EDV?
The standard ICA/CCA ratio uses PSV values. EDV is interpreted separately as part of stenosis grading.
What if the ratio looks high but plaque is minimal?
Re-check technique and sampling. Hemodynamic factors or technical variation can alter velocities. Final interpretation should integrate full duplex findings.
Do all labs use the same ratio thresholds?
No. Thresholds are often similar but may vary. Use institution-specific validated criteria and accreditation guidance.
Bottom Line
If you are asking, “How do you calculate ICA CCA ratio?” the calculation is straightforward: divide ICA PSV by CCA PSV from the same side. The challenge is interpretation, which should always include the complete carotid duplex context rather than one number alone.