High-frequency harmonic imaging with coded excitation: Implications for the assessment of coronary atherosclerosis

Abstract

The adventitial vasa vasorum grows abnormally in life-threatening atherosclerotic plaques. Harmonic intravascular ultrasound (H-IVUS) could help assess the vasa vasorum by nonlinear imaging of microbubble contrast agents. However, the harmonics generated in tissue at high acoustic pressures compromise the specificity of H-IVUS - a trait that has hampered its clinical use. Therefore, H-IVUS should be conducted at low pressure amplitudes; but the resulting decrease in signal-to-noise ratio (SNR) could limit the sensitive detection of the vasa vasorum. In this study, we investigated the feasibility of improving the SNR of H-IVUS imaging with chirp-coded excitation. Numerical simulations and experiments were conducted to assess the harmonic response of the commercial contrast agent Targestar-p™, to sine-burst and chirp-coded excitation (center frequencies 10 and 13 MHz, peak-pressures 100 to 300 kPa). We employed 1) a single-element transducer pair, and 2) a dual-peak frequency transducer for our studies. Our experimental results demonstrated that exciting the agent with chirp-coded pulses can improve the harmonic SNR by 7 to 14 dB. Further, the axial resolution obtained with chirp-coded excitation was within 10% of that expected for sine-burst excitation. Therefore, we envisage that chirp-coded excitation may be a viable strategy to visualize the vasa vasorum with H-IVUS imaging.