Abstract 2898: Subharmonic Contrast Microbubble Signals for Noninvasive Pressure Estimation: an In Vitro Study
This project aims to monitor and quantify intra-cardiac pressures via contrast-enhanced subharmonic imaging. We have proposed subharmonic aided pressure estimation (SHAPE; U.S. Patent 6,302,845) utilizing microbubble-based contrast agent signals for the noninvasive estimation of hydrostatic blood pressures in the heart cavities and major blood vessels. An experimental system for SHAPE was constructed based on two single-element transducers assembled confocally at a 60° angle to each other. A transducer with a bandwidth of 38% and a center frequency of 2.2 MHz (Staveley, East Hartford, CT) was used as the transmitter and a second transducer with a bandwidth of 86% and a center frequency of 3.6 MHz (Etalon Inc., Lebanon, IN) was used as the receiver. Amplified signals were acquired using a digital oscilloscope equipped with mathematical functions (Model 9350AM, Lecroy, Chestnut Ridge, NY). Changes in hydrostatic pressure and signal amplitudes were analyzed via linear regressions using Stata 9.0 (College Station, TX). Changes in the first, second, and subharmonic amplitudes of the contrast agents Sonazoid (GE Healthcare, Oslo, Norway), Optison (GE Healthcare, Princeton, NJ) and Levovist (Schering AG, Berlin, Germany) were measured in vitro at different hydrostatic pressures (0–186 mmHg). The first, second, and subharmonic amplitudes were measured within a sealed 2.25 L water tank kept at room temperature (around 25°C). The pressure inside the tank was monitored by a pressure gauge (OMEGA Engineering Inc., Stamford, CT). Over the pressure range studied in vitro the first and second harmonic amplitudes reduced only ~2 dB. Over the same pressure range, the subharmonic amplitude decreased by up to 10 dB (Sonazoid 9.15±0.29 dB, Optison 10.04±0.15 dB, and Levovist 9.5±0.64 dB). Excellent linear regressions between the subharmonic amplitude and hydrostatic pressure were achieved for Sonazoid (r2=0.98, p<0.001,), Optison (r2=0.99, p<0.001), and Levovist (r2=0.98, p<0.001). This shows that the subharmonic component is a good indicator of the hydrostatic pressure variation. SHAPE offers the possibility of allowing pressure gradients in the heart to be obtained noninvasively. Future studies will include in vivo pressure measurements.