Quantification of Blood Velocity and Vascular Wall Shear Rate From Ultrasound Radio Frequency Signals and Its Relationship to Vascular Mechanical Properties and Potential Clinical Applications.

Abstract

This study evaluates a novel measurement method of determining vascular wall strain and wall shear rate, which are interrelated physiologic parameters fundamentally important in vascular disease. Wall strains during vascular wall dilation were performed using ultrasound 2D speckle tracking; vascular wall edges and vascular wall shear rate were determined using decorrelation based velocity measurement method for in-vitro and in-vivo flow measurement. These experiments and measurements were performed to investigate both the novel measurement methods as well as the relationship between the vascular wall shear rate and vascular wall dilation. First, this study measures arterial wall strains using the ultrasound radio-frequency (RF) signals. Strains in the arterial wall during arterial dilation (from diastole to systole) were determined using a 2D speckle tracking algorithm. These ultrasound results were compared with measurements of arterial strain as determined by finite-element analysis (FEA) models with and without the effects from surrounding tissue, which was represented by homogenous material with fixed elastic modulus. Under pressure equalization, the strain levels predicted by FEA model without surrounding tissue were considerably greater than the strain levels measured by both ultrasound and the FEA model with surrounding tissue. Second, this research aims to measure wall edges and wall shear rate for in-vitro flow experiment using decorrelation ultrasound based velocity measurement. The flow velocity was obtained by multiplying the speckle movement in two consecutive frames by the acoustic frame rate. The wall edge was determined using B-mode images and 2nd order gradient of flow velocity profiles. The wall shear rate was measured at the wall edge and evaluated by comparison with velocity gradients from parabolic flow velocity profile based on Poiseuille theory. Third, this research measures the vascular wall shear rate in the brachial artery for healthy and renal disease subjects using the decorrelation based ultrasound velocity measurement. The vascular wall shear rate and vascular diameter pre-, during- and post-vascular occlusion with pressure cuffs were compared for the healthy and renal disease subjects at top and bottom wall edges. The mean vascular wall shear rate change between pre- and post-vascular occlusion was significantly different for the healthy versus renal disease subjects.