The intensity reflection coefficient: a complementary method for investigating blood backscattering properties with ultrasound.

Parametric imaging of backscatter indices is an active field of research in ultrasound (US) tissue characterization. The US intensity reflection coefficient (IRC) is introduced to characterize porcine blood in a Couette shear flow system. US properties of red blood cells (RBCs) were investigated at shear rates of 5 s(-1) and 500 s(-1) for hematocrits ranging from 0 to 54%. The IRC was determined and compared to the integrated backscatter power (IBP) calculated between 8 MHz and 12 MHz. The frequency dependence of both parameters was also explored using their spectral slopes determined in the same frequency bandwidth. The experimental results showed that the IRC is a mirror function of the IBP and their spectral slopes had similar behavior. At a shear rate of 500 s(-1), selected to disaggregate RBCs, the IRC exhibited a bi-modal variation with a minimum at 23% hematocrit. The minimum was less pronounced and moved down to 13-17% hematocrits in the case of a shear rate of 5 s(-1) that promoted aggregation. The IBP presented a sharp peak at 25% hematocrit for the high shear-rate value, while a plateau appeared after 13% hematocrit in the case of the low shear rate. Furthermore, the transition from lower to higher shear rate was accompanied by a diminution in the level of both indices. In the frequency domain, the spectral slope of the backscatter power presented a Rayleigh scattering behavior (value around 4) for all hematocrits at a shear rate of 500 s(-1). It dropped to a value of 2 beyond 13% hematocrit when the shear rate was decreased to 5 s(-1). At a shear rate of 500 s(-1), the spectral slope of the IRC was constant and close to 3.8 below 28% hematocrit and decreased at higher hematocrits. At 5 s(-1), it was constant (between 3.7 and 3.9) for hematocrits lower than 17% and then decreased. This study showed the potential of the intensity reflection coefficient to investigate the RBC aggregation phenomenon. An advantage of IRC is its easy computation.