Blood flow assessment by ultrasound-induced destruction of echocontrast agents using harmonic power Doppler imaging: Which parameters determine contrast replenishment curves?

Objective: To evaluate the feasibility of flow determinations by contrast r eplenishment using harmonic power Doppler imaging (H-PDI). Background: The application of indicator dilution principles on contrast echocardiography i s limited by numerous methodical problems. Recently, a new method was intro duced that relies on ultrasound-mediated microbubble destruction and evalua tion of the contrast replenishment. Methods: Definity, a perfluorocarbon-de rived contrast agent under development, was continuously infused into a ste ady flow phantom and H-PDI registrations were performed within a silicone t ube (d = 8 mm). Replenishment interval between destruction and imaging fram e was varied from 0.04-2 seconds. Nonlinear curve fitting was performed usi ng an exponential mathematical model. Results: Strong linear correlation be tween contrast dose and maximum signal intensity as well as between flow an d the slope variable beta of the replenishment curve was found for all sett ings (r > 0.96). Maximum signal intensity and contrast replenishment rate w ere found to be a function of emission power and were significantly influen ced by depth and focus position. Conclusion: The feasibility of flow assess ment using replenishment curves obtained by H-PDI was demonstrated. However in experimental conditions, flow analysis was severely influenced by ultra sound system settings and imaging conditions such as emission power sound f ield geometry, and investigation depth. For a clinical use of this promisin g approach, algorithms that take specific system settings and imaging condi tions into account have to be found. Imaging modalities that enable a most homogeneous scan field are best suited for the assessment of contrast reple nishment.