Peak Velocity Overestimation and Linear‐Array Spectral Doppler

Ultrasound Instruments are used to evaluate blood flow velocities in the human body. Most clinical instruments perform velocity calculations based on the Doppler principle and measure the frequency shift of a reflected ultrasound beam. Doppler‐only instruments use single‐frequency, s1ngle‐crystal transducers. Linearand annular‐array multiple‐crystal transducers are used for duplex scanning (simultaneous B‐mode image and Doppler). Clinical interpretation relies primarily on determination of peak velocities or frequency shifts as identified by the Doppler spectrum. Understanding of the validity of these measurements is important for Instruments in clnical use. The present study exammed the accuracy with which several ultrasound instruments could estimate velocities based on the Identification of the peak of the Doppler spectrum, across a range of different angles of insonation, on a Doppler string phantom. The stnng was running in a water tank at constant speeds of 50, 100, and 150 em/sec and also in a sine wave pattern at 100ߚ or 150ߚcm/sec amplitude. Angles of msonatlon were 30, 45, 60, and 70 degrees. The single‐frequency, single‐crystal transducers (PC Dop 842, 2‐M Hz pulsed‐wave, 4‐MHz continuous‐wave) provided acceptably accurate velocity estimates at all tested velocities independent of the angle of insonation. All duplex Doppler instruments with linear‐array transducers (Philips P700, 5.0‐MHz; Hewlett‐Packard Sonos 1000, 7.5‐MHz; ATL Ultramark 9 HDI, 7 5‐MHz) exhibited a consistent overestimation of the true flow velocity due to mcreasing intrinsic spectral broadening with increasing angle of insonation. At an angle of insonation of 60 degrees the peak of the Doppler spectrum overestimated the true flow velocity by 25% and at 70 degrees, by 33%. The most likely explanation of this phenomenon is the wide aperture of these probes, with potential angle diversity of the reflected ultrasound beams along the surface area (footprint) of the probe. The use of an Intensity‐weighted peak velocity (centroid), with calculation of the velocity which includes 95% of the spectral points, rather than the absolute peak of the spectrum, proved to be more accurate and was angle Independent in these Instruments.