P6C-11 In Vitro Study of Frequency-Dependent Blood Echogenicity Under Pulsatile Flow

Previous in vivo and in vitro studies have demonstrated that blood echogenicity varies under pulsatile flow, but such changes could not always be measured at physiological stroke rates. The apparent contradiction between these studies could be due to the use of different ultrasound frequencies. Backscattered signals from porcine blood were measured in a pulsatile Couette flow apparatus. Cyclic changes in shear rate for stroke rates of 20 to 70 beats per minute (BPM) were applied to the Couette system. To confirm that the cyclic echogenicity variations were observable, spectral analysis was performed to verify if changes in echo-amplitude corresponded to the stroke rate applied to the flow. Echogenicity was measured by two single-element transducers at 10 MHz and 35 MHz. At 35 MHz, cyclic variations in backscatter were observed from 20 to 70 BPM. However, at 10 MHz, they were detected only at 20 BPM. We conclude that cyclic variations in RBC aggregation exist at physiological stroke rates, unlike what has been demonstrated in previous in vitro studies at frequencies les 10 MHz. The increase sensitivity for small changes in aggregate size with the 35 MHz transducer might be the explanation provided for the better characterization of RBC aggregation at high stroke rates. Our results corroborate in vivo observations of blood echogenicity cyclic variations in patients using a 30 MHz intravascular ultrasound catheter.