Design, fabrication, and in vitro evaluation of an in vivo ultrasonic Doppler wall shear rate measuring device

In vivo wall shear rates have been obtained based on estimates from either volume flow rate or single-point velocity measurements along with the wall no slip assumption and a simple linear regression. Recent results (R.S. Fatemi and S.E. Rittgers, 1994) have shown that, under pulsatile flow conditions, wall shear rates are more accurately predicted by using up to four velocity points and a second- or third-order polynomial curve fit. The authors evaluate the accuracy of a new, in vivo transducer capable of determining wall shear rates nonintrusively from velocities at three points along a line perpendicular to the vessel wall. Three 20-MHz ultrasound crystals were embedded in an elastomer at distances of 1.5 and 2.1 mm with beam angles of 30°, 15°, and 60° to the horizontal plane. Microscopic examination showed that intercrystal spacings were within 1.5% of the design and the crystal angles were placed within 2.0%. In vitro calibration was performed under steady and pulsatile flow conditions with average shear rates being within 4.3±17.3% and 0.2±0.6.0% respectively, of the theoretically predicted values. Furthermore, peak and oscillatory shear rates were within -5.6±2.2% and -2.4±5.7% accuracy, respectively. Results from this study show this device to be capable of providing accurate wall shear rates in vivo.