Non-invasive ultrasound measurements of vascular impedance: Rekindling an old in-house concept on in vivo measurements

Despite its introduction in the early 1950´s, vascular impedance has yet to be applied in clinical use to predict early stages of cardiovascular disease (CVD). It is defined as the ratio between blood pressure and flow and describes the opposition to blood flow within the cardiovascular system.

During this thesis project, local pulse wave velocity (PWV) and vascular impedance was estimated from in vivo ultrafast plane wave imaging recordings of the right common carotid artery in eight healthy human volunteers. Both estimations were based on the tracking of the arteries radial displacement. Motion tracking was performed by using two tracking methods: phase shift (PS)- and iterative phase shift (IPS)-tracking. The IPS-tracking is a brand new radial motion tracking method on vessels created during this project by combining existing phase shift- and block matching motion tracking methods in use at the Department of Biomedical Engineering, Lund University.

Results showed local PWV-estimations to vary, occasionally to a substantial degree, between, as much as within, volunteer recordings. For the incident pulse wave, probable PWV values totally varied between 1.58 – 7.60 m/s. For the bifurcation wave reflection, probable PWV values totally ranged between 1.06 – 16.09 m/s in the opposite direction. The vascular impedance, defined as vascular characteristic impedance used to estimate vascular input impedance, was in partial successfully determined in two volunteers. The results, however, suggest both local PWV- and vascular impedance estimations to be sensitive to slope fitting of the progressive change in cardiac cycle locations and phase between consecutive scanlines. Furthermore, vascular impedance estimations seem to require longer in vivo recordings containing multiple heart cycles to gain more certain results.