MSc projects 2021 001
The influence of measured diagnostic ultrasound equipment performance on the uncertainties in Doppler velocity estimation used for carotid vascular disease grading.
Pulsed wave (PW) Doppler ultrasound is commonly used to evaluate vascular carotid disease & to provide a numerical grading of vascular stenosis. It is increasingly used as the sole imaging modality for this assessment. PW velocity overestimation due to ultrasound system performance is known & reported & often stated as having the potential to lead to an incorrect stenosis grading. In 2009 reporting recommendations were published to unify clinical measurements of peak systolic velocity (PSV), ratios of PSV (PSVR) & ratios of PSV to end diastolic velocity – the St, Mary’s ratio (SMR) . The purpose of this work was to evaluate the effect of PW velocity estimation on these recommended grading thresholds.
In line with current ultrasound QA recommendations a string test object was used to measure PW velocity estimation for an expanded range of velocities (10cms-1 – 200cms-1), the linear relationship of the measured velocities was then used to extrapolate the data to include the expected range of velocities derived from the recommendations. These values were then propagated through the recommended grading scheme & the differences between expected & measured stenosis grades evaluated.
The overestimation of maximum velocities was confirmed in all measurements (mean values of +16% & +15.3% for the 2 systems evaluated) & exceeded the manufacturer’s published specifications (although the mean velocity estimation compared well, +2.3% & +2.5%). Significant overestimating effects were found for the direct measurement of PSV at >70% stenosis (p = 0.05) & at >90% stenosis (p = 0.01). For the PSVR & SMR ratios there was an underestimation (-1% to -1.5% for PSVR & -5.9% to -9% for SMR).
Given the positive systematic nature of the PW maximum velocity estimation this information is likely to be clinical departments where direct velocity measurements (rather than velocity ratios) are commonly used for diagnosis or management.
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