Quantitative intravascular ultrasound. Demonstration using a novel site targeted acoustic contrast agent

The authors´ goal is to apply the techniques of quantitative ultrasonic tissue characterization to intravascular ultrasound. They have developed a system to acquire radio frequency ultrasonic signals from a modified commercially available intravascular ultrasound scanner. This system incorporates a novel method of acoustic signal calibration. Experiments have been completed demonstrating the ability to make repeatable measurements of frequency-averaged backscattered power. In order to test and evaluate this system, intravascular ultrasound was used to characterize the enhancement of backscattered power from clot specifically targeted with an acoustic contrast agent recently developed in the authors´ laboratory. The agent has the unique physical attribute of low inherent echogenicity in suspension, while attaining high acoustic reflectivity when bound to a biologic surface or to itself. Clots from canine plasma were formed in vitro on nitrocellulose membranes. Clots were incubated with either a clot-specific (n=2) or a clot-nonspecific (n=2) contrast emulsion. Specimens were imaged with a 30 MHz intravascular ultrasonic catheter. Radio frequency signals backscattered from the clots were acquired, gated, and their power spectra obtained by FFT. Frequency-averaged power backscattered from the clots exposed to clot-specific contrast exceeded that from clots exposed to clot-nonspecific contrast by 5.6±0.7 dB (±SD,p<0.05). These data demonstrate the capability of this system to determine quantitative indexes of backscatter and suggest the feasibility of applying quantitative ultrasonic tissue characterization in clinical intravascular ultrasound