Suppression of multiple Scattering and imaging of nonlinear Scattering in Ultrasound imaging
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- Institutt for fysikk 
The work presented is based on an imaging method using dual-frequency band pulses for imaging of nonlinear scatters from stiff particles like micro calcifications. The pulse complex consist of a high-frequency imaging pulse and a low-frequency manipulation pulse which overlap in time. The frequency ratio between the two pulses is in the range of 10:1. This technique has already demonstrated to perform well while imaging resonant nonlinear scatters from such as micro bubbles. However, imaging of lower intensity nonlinear scatters from stiff particles has not yet been achieved. It can be shown that the magnitude and the polarity of the nonlinear scatters follows the magnitude and the polarity of the low-frequency pulse. Imaging of stiff particles requires higher manipulation pressure, but the manipulation pressure does not change the scattering only. The co-propagating high frequency pulse observes a non homogeneous low frequency pressure and becomes distorted. This distortion of the high frequency pulse masks the nonlinear scattering, and to achieve sufficient suppression of the linear scattering the distortion must be corrected for.In this thesis two agar based phantoms have been made. In both phantoms reverberation layer(s) of plastic was mounted. Steel wires with diameters 0.5mm, 0.3mm were positioned below the plastic layer(s) together with calcium particles of bead size 190 micrometer. Images of both phantoms have been compared using conventional B-mode imaging and the presented method. In the first phantom, Phantom A, the SNR for two steel wires and a calcium particle were increased by 8 - 9dB while class 3 reverberations were suppressed down to the backscatter noise level. Similarly for the second phantom, Phantom B, class 1,2 reverberations were suppressed and the SNR for two steel wires were increased by 11 - 12dB.