An in vitro study of a myocardial perfusion contrast agent

Ultrasound contrast agent applications have expanded in recent years to the assessment of perfusion and microvascularity. A new system that measures the acoustic properties of single microbubbles can provide information that can shed new light on contrast enhanced imaging. A polymer shelled agent, BiSphere® (Point Biomedical, San Carlos, CA, USA) was used. A very diluted hydrodynamically focused flow of microbubbles was exposed to ultrasound beams of 1 to 4 MHz transmit frequency and 0.2 to 1 MPa peak negative pressure. RF data were captured using an ultrasound scanner (Sonos5500, Philips, Andover, MA, USA). The microbubbles were subjected to consecutive triggered pulses and their survival rate was also assessed. At 0.5 MPa fundamental Scattering Cross Section (SCS) was maximum below 1.5 MHz, while the second harmonic SCS was constant up to 2.3 MHz and greater than the fundamental between 2 and 2.4 MHz. A strong ultraharmonic was present in the majority of the scattered echoes above 0.8 MPa. At 0.5 MPa the microbubble echoes disappeared after the first insonation, suggestive of rapid dissolution. In conclusion BiSphere™ displayed compatibility with myocardial perfusion requirements, but most importantly the investigated scattered pulses provided spectral and temporal characteristics that could be used to optimise ultrasound examinations.