A real-time technique for selective molecular imaging and drug delivery in large blood vessels

Current clinical imaging approaches are limited to detecting atherosclerosis when it manifests as an anatomic change. An ultrasound based real-time molecular imaging technique that assists clinicians in determining the presence or extent of the disease may play a critical role in guiding therapeutic drug delivery. Resonant-stimulation Pulse inversion (RSPI) uses dual frequency excitation pulses for simultaneously guiding and selectively imaging (adherent) targeted microbubbles in real-time. In vitro and ex vivo experiments were conducted to assess the performance of the proposed technique. For constant flow (10 cm/s) and microbubble concentration, the performance of the method was evaluated in saline with varying viscosity and blood with varying hematocrit. Freshly excised swine carotids of 5 mm internal diameter were placed in saline bath. Microbubbles possessing a shell comprising Dil (fluorescent dye) embedded with a lipid base were diluted in saline and blood and flowed through the arterial lumen. An RSPI sequence was used for imaging and subsequently for destruction of the microbubbles. The histological specimens of the arterial sections were then imaged for Dil fluorescence and compared with the corresponding ultrasound images from the RSPI sequence. The performance of RSPI was better (2 dB) in viscous saline (4 cp) than in blood with varying levels of hematocrit, underlining the significance of the interaction between microbubbles and red blood cells. The ultrasound image of the longitudinal section of the excised artery at the end of the RSPI sequence corresponded well with the composite bright field-fluorescent image of the arterial cross-section confirming Dil delivery to the lower wall of the artery.