Notes
Original Sonographic NASCET index (SNI)
- The highest measured angle-adjusted velocity in each segment should be used for calculating the SNI
- SNI is calculated using the following formula:
- SNI = [1 – (PSVd / PSVp)0.5] x 100
- Where
- PSVd = maximum angle-adjusted PSV in the normal distal internal carotid artery (ICA)
- PSVp = maximum angle-adjusted PSV at the maximum narrowing of the ICA
- SNI can be used to aid the identification of significant (>70%) ICA stenosis but is neither a standalone tool, nor the substitute for grading systems such as e.g. the SRU criteria
- It has been shown that an SNI threshold value of 45.5 has a high specificity (94%) but modest sensitivity (80%) in identifying >70% ICA narrowing, while a threshold value of 33.3 has a greater sensitivity (93%) with an associated drop in sensitivity (82%)
- Note that since flow velocity parameters become inaccurate in critical stenosis or near-occlusions SNI should not be relied upon in these cases
Last updated: 2022-02-19