Biophysical principles of vascular diagnosis

Visual inspection of the spectral composition of the Doppler signal as a function of time (sonogram) has been very helpful in detecting the presence of stenoses with substantial lumen narrowing causing abnormal flow patterns. Attempts to grade a stenosis based on the spectral width at peak systole were less successful because of the obscuring effects of the ultrasound beam width with respect to lumen diameter, dimensions of the sample volume, angle of observation, and spectral broadening due to vessel branching and bends. The introduction of color flow imaging has put emphasis on the width of the velocity distribution and the consistency of flow patterns within the region of interest. This technique requires a high resolution in space, velocity, and time necessitating the development of new velocity estimation algorithms. The observed flow patterns can be related to the echogenicity and local wall thickness of peripheral vessels. In addition, the displacement behavior of arterial walls over time provides information about the elasticity of the wall. Knowing the instantaneous velocity of arterial walls, it becomes possible to suppress selectively and adaptively the arterial wall contribution, allowing for the assessment of low blood flow velocities close to the wall and, hence, of wall shear rate. The latter development enables the study of the interaction of blood velocities and the metabolism and structure of the walls, providing possible clues for atherogenesis.