| Summary: | 碩士 === 國立清華大學 === 生醫工程與環境科學系 === 96 === Detection of ultrasound contrast agents (UCAs) based on their nonlinear characteristics are popular for imaging blood perfusion. Various nonlinear imaging techniques have been reported that to distinguish UCAs from tissue by exploiting their harmonic signal is efficiently in image. For microcirculation imaging, a higher frequency system (> 10 MHz) is required to obtain high spatial resolution. However, aforementioned nonlinear techniques usually suffer from insufficient signal-to-noise ratio (SNR) for intense attenuation of high-frequency nature and the limited resonance frequency range of commercialized UCAs.
In this study, we proposed a novel approach named as dual-frequency excitation to induce nonlinear scatterings from UCAs based on high-frequency ultrasound. This proposed method provides both high lateral and contrast resolution in imaging UCAs. According to our investigations, when the difference frequency component of dual-frequency excitation is close to the resonance frequency of UCAs, the oscillation of UCAs is maximized and the echo is therefore enhanced. Note that the induced difference frequency component is beyond the frequency response of the high-frequency transducer, which excite the oscillation of UCAs more efficiency at low frequency.
Besides, it has been demonstrated that by destroying the drug-encapsulated UCAs, the drug release within specific area, thus the drug delivery is therefore achieved. Therefore, we proposed an acoustic-based method to observe the destruction of UCAs under dual-frequency excitation and discuss the lateral resolution. Results showed the dual-frequency excitation has the better destruction ability and smaller lateral resolution than the tone-burst excitation.
This thesis demonstrated the applications of dual-frequency excitation in contrast imaging and discussed the effects of insonation parameters on nonlinear contrast imaging. Results showed that the dual-frequency excitation ultrasound not only provides the high resolution in imaging, but also effectively induce the destruction of UCAs under specific insonation parameters. The proposed method is available for common ultrasound system, which is easy to implement on existed setup. In addition to the high-frequency perfusion imaging, the dual-frequency excitation is also potentially useful in drug delivery studies in the future.
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