Abstract
Microbubbles detection techniques such as phase inversion and nonlinear harmonic methods have become popular in blood perfusion imaging due to its capability to distinguish bubbles from background tissue. The aforementioned techniques require the microbubbles to effeciently oscillate by insonation with pulse transmission near to the resonance frequency of the contrast agents. However, for most commercial ultrasound contrast agents (UCAs), they are originally designed to resonate at lower frequencies ranging from 2-3 MHz, so it makes difficult in imaging by high frequency ultrasound, thus limiting the spatial resolution of imaging. To oversome this problem, a dual-frequency difference excitation tenique has been proposed in our previos studies. The proposed dual-frequency (DF) excitation waveform is an amplitude modulated wave comprising two sinusoids (f1 and f2), it can be transmitted at high frequency band while produce low frequency driving force to excite microbubbles by resultant envelope component at frequency of (f1-f2). Based on the advantages of this technique, the thesis further investigates their potential applications. First, we concentrate the energy of nonlinear scattering induced by DF excitation to enhance the contrast to tissue ratio (CTR) of sub-harmonic imaging. In the study, the f2 at twice of the resonance frequency of UCAs is adopted to efficiently generate sub-harmonic component, and f1 is included as an ehancing component to induce high-order nonlinearity of UCAs at sub-harmonic frequency. The second and third-order nonlinear components related to envelope component would coincide at sub-harmonic frequency if a proper set of f1 and f2 are selected. We further optimize the sub-harmonic generation by tuning the phase between second and third-order component. The results show that, with dual-frequency excitation, the sub-harmonic CTR improves as compared to conventional method. Moreover, the CTR changes periodically with the phase of dual-frequency excitation, leading to a difference up to 9.1 dB between the maximal and minimal CTR. Moreover, the echo produced from the envelope component seems to be specific for UCAs and thus the proposed method has the potentials to improve both SNR (signal-to-noise ratio) and CTR in sub-harmonic imaging. Second, we focus on reconstructing the DF technique degraded axial resolution, because pulse length has to be enlongated to provide sufficient driving force at envelope frequency. To achieve this goal, we propose a method called as dual-frequency chirp (DF chirp) excitation which comprised two linear chirp signals, the resultant envelope component is modified from a single frequency tone bursts into the chirp form for pulse compression. This method is based on selectively extracting and compressing the second order nonlinear response by a matched filter with same center frequency and bandwidth as envelope component. The results show that DF chirp is feasible to improve the axial resolution and increase SNR of conventional DF excitation technique. However, no matter the DF chirp or DF tone bursts excitations, the second-order nonlinear response appeared at region of tissue background as acoustical pressure up to 800 kPa. To solve this problem, we further discuss the feasibilities of two methods including fourth-order nonlinear compression and chirp reversal two techniques for tissue suppression.