Backscatter measurements on whole blood using a real-time scanner

Backscatter from porcine whole blood under steady laminar flow has been measured as a function of the radial position in a polyurethane tube (1.27 cm outer diameter and 0.96 cm inner diameter) of a mock flow loop at different flow rates and hematocrits using a real-time scanner (Aloka 280 SL). The scanner has been modified to be capable of acquiring the RF data with a 7.5 MHz linear array under the control of a computer. For each experiment 250 frames of data were collected. Each frame consisted of 20 RF A-line signals which were digitized at 250 MHz sampling rate. Each A-line was divided into 20 segments. The range of the mean shear rate investigated was between 52 and 160 s^-1, whereas the hematocrit was between 4 and 40%. The results show that the location where maximal backscatter occurs and the variation of backscatter as a function of the radial position in the flow conduit may vary depending upon the hematocrit and flow rate. This observation deviates from the common belief that the flow profile is symmetrical and the shear rate is minimal at the center stream under steady laminar where blood echogenicity is maximal. A hypoechoic region or 'black hole' surrounded by a bright ring, unlike the 'black hole' phenomenon previously observed that occur only at lower shear rates, was observed to occur at high shear rates and hematocrits. When pulsatile flow was introduced, backscatter dropped significantly and under certain conditions, a nearly parabolic flow profile could be readily observed.