Subharmonic contrast intravascular ultrasound

The feasibility of subharmonic contrast intravascular ultrasound (IVUS) imaging was investigated using a prototype nonlinear IVUS system and the commercial contrast agent Definity (TM). The system employed a mechanically scanned commercial catheter with a custom transducer element fabricated to have dual frequency peak sensitivity in the vicinity of 15 and 30 MHz. Experiments were conducted at a fundamental frequency of 30 MHz (F30; 25% bandwidth) with on-axis pressures ranging from 0.12 to 0.79 MPa, as measured with needle a hydrophone. In vitro characterization experiments demonstrated the detection of 15 MHz subharmonic signals (SH15) when pressure levels reached 360 kPa. The formation of SH15 images was shown, with tissue signals suppressed to near the noise floor and contrast to tissue ratios improved by up to 30 dB relative to F30, depending on the depth. In vivo experiments were performed using the atherosclerotic rabbit aorta model. Following the bolus injection of contrast agent upstream of the imaging catheter, agent was detected within the aorta, vena cava, and within the adventitia. Histology confirmed the presence of adventitial microvessels. These results provide a first in vivo demonstration of subharmonic contrast IVUS, and suggest its potential as a new technique for imaging the vasa vasorum.