P5B-6 In Vitro Pressure Estimation Obtained from Subharmonic Contrast Microbubble Signals

The long term goal of this project is to monitor and quantifyintra-cardiac pressures via contrast-enhanced subharmonic imaging. Our group has proposed the concept of subharmonic aided pressure estimation (SHAPE) utilizing microbubble-based contrast agent signals for the noninvasive estimation of hydrostatic blood pressures in heart cavities and major blood vessels. An experimental system for in vitro SHAPE was constructed based on two single-element transducers assembled confocally at a 60deg angle to each other. Changes in the first, second, and subharmonic amplitudes of the contrast agents Sonazoid, Optison, Definity, PRC-1 and Levovist were measured in vitro over a pressure range from 0 to 186 mmHg using a sealed 2.25 L water tank that can sustain pressure changes in excess of 200 mmHg. Over the pressure range studied the first and second harmonic amplitudes reduced only approximately 2.4 dB and 1.8 dB in vitro. Over the same pressure range, the subharmonic amplitudes of the agents decreased between 9.6 and 13 dB. Excellent linear regressions (r > 0.98; p > 0.001) were achieved between the subharmonic amplitude and hydrostatic pressure for all five agents tested. In conclusion, the subharmonic microbubble signal component is a good indicator of the hydrostatic pressure variation in vitro. SHAPE offers the possibility of allowing pressure gradients in the heart to be obtained noninvasively. Future studies will include in vivo pressure measurements.