M.Sc Thesis

M.Sc StudentLivne Anat
SubjectSubharmonic Imaging Using Ultrasound Contrast Agents
DepartmentDepartment of Biomedical Engineering
Full Thesis textFull thesis text - English Version


Ultrasound contrast agents are commercially available for over two decades, and are used in a variety of clinical applications, mainly imaging of vascularization. There has been, since then, a continuous effort to enhance the visualization of the contrast agents and increase the "contrast to tissue" ratio. The gas-filled microbubbles, upon exposure to acoustic field, generate subharmonics at half the incident frequency, and higher harmonics at multiples of the incident frequency. Subharmonics, generated by the microbubbles at half the incident frequency, are hardly generated by tissues, thus offer better contrast as opposed to fundamental harmonic of second harmonic imaging. These subharmonics are best generated when the transmitted pulse is at twice the resonance frequency of the bubbles.

The differentiation of contrast agent backscatter from tissue backscatter is problematic due to the spectrum overlap between the two. Many filtering techniques have been tried, though results are not satisfactory.

This research presents a different approach to the contrast enhancement - dual frequency excitation. This method was shown to induce subharmonics generation by the microbubbles, and as a result improve contrast to tissue ratio. The transmitted pulse consists of a superposition of two sine waves, approximating a sawtooth wave.

As part of this work, experiments were conducted to validate the efficiency of the method by comparison to traditional single frequency excitation. Then different pulse parameters were investigated and optimized to produce quality imaging.

Spectral analysis indicates improvement of the ratio between subharmonic and first harmonic signals of 2-3 dB, with comparison to traditional single frequency excitation. Additionally, increasing PRF and pulse duration further improves subharmonic emission.

This method appears suitable for integration in commercial ultrasound systems, and can offer subharmonic imaging with better contrast, with no need to compromise axial resolution or frame rate.