Accuracy of maximum velocity estimates made using Doppler ultrasound systems

Abstract

This study was performed in order to provide quantitative data on the estimation of maximum velocity made using modern Doppler ultrasound systems. This is important since the degree of stenosis within arteries is commonly assessed from the maximum velocity. A string phantom was used as the source of Doppler signals. This enables direct comparison between the Doppler estimated maximum velocity and the true filament velocity. Six modern commercial Doppler systems were used. Measurements were made under standard conditions for each probe. In addition a number of factors were varied in turn (beam-filament angle, filament depth, filament velocity and Doppler aperture position). Under standard conditions the maximum velocity was overestimated in all cases (0-29% error). For all measurements maximum velocity errors ranged from -4% to 47%. There was a large intraprobe variation in maximum velocity estimation (mean variation of 25%), and a large interprobe variation (mean variation of 25%), and a large interprobe variation (mean variation of 18%). These results indicate that, at present, errors in maximum velocity estimation may be directly translated into significant errors in the estimate of the degree of arterial stenosis made from velocity measurements. As a consequence, some patients may be incorrectly categorized. Consideration should be given to applying angle dependent correction factors to maximum velocity measurements, and to the use of conversion from Doppler frequency shift to velocity using the angle derived from the edge of the Doppler aperture.

An audit of a hospital-based Doppler ultrasound quality control protocol using a commercial string Doppler phantom

“…For each ultrasound transducer presented in Figure 4, a consistent overestimation of the set velocity was evident, reflecting similar findings from a previous study 6 , suggesting that little improvement has been made in the Doppler ultrasound maximum velocity measurement process implemented by ultrasound scanner manufacturers.…”

“…Calibration of the filament velocity, necessary for ensuring annual consistency of measurement, was performed using a tachometer; however, where such a device is not accessible the true filament velocity may be determined by dividing the string's length by the time measured for one filament revolution, as has been detailed previously 6 . A further improvement, to be implemented at the commissioning stage, would be to employ a simple retort stand instead of the CIRS clamp.…”

“…It is imperative that these ultrasound systems are capable of accurately measuring blood flow velocities to ensure correct diagnosis and appropriate patient treatment; however, previous studies have reported an overestimation in measurements of the maximum velocity by up to 40% in some systems 4 . This has been attributed to geometric spectral broadening effects, which are related to the active Doppler aperture size of the transducer and the angle correction used 5,6 .…”

Rapid Quantitation of Aortic Valve Flow Using Spiral Fourier Velocity Encoded MRI

“…Doppler flow measurements may be inaccurate when the ultrasound beam cannot be properly aligned with the vessel axis, requiring measured velocities to be "angle-corrected" by the operator. Peak-velocity overestimation on the order of 18-40% have been reported in the literature (Hoskins, 1996;Winkler & Wu, 1995), usually due to spectral broadening at large insonation angles, and to Doppler gain settings.…”

Assessment of Carotid Flow Using Magnetic Resonance Imaging and Computational Fluid Dynamics

“…The current clinical gold standard for cardiovascular flow measurement is Doppler ultrasound. However, evaluation by ultrasound is inadequate when there is fat, air, bone, or surgical scar in the acoustic path, and flow measurement is inaccurate when the ultrasound beam cannot be properly aligned with the axis of flow (Hoskins, 1996;Winkler & Wu, 1995). Two alternative approaches to 3D flow assessment are currently available to the researcher and clinician: (i) direct, model-independent velocity mapping using flow-encoded magnetic resonance imaging (MRI), and (ii) model-based computational fluid dynamics (CFD) calculations.…”