Research On Coded Excitation Methods For Medical Ultrasonic Elastography

Ultrasound elastography is a non-invasive imaging technique for the detection of lesions based on elasticity.The tissues are excited by specific methods and the resultant deformation or vibration is detected by some ultrasonic pulse-echo technique.An image of elasticity can be achieved by analyzing vibration signals on a pixel-by-pixel basis.One of the main difficulties in ultrasound elastography is that the amplitude of the vibration induced in the tissue(often involving shear wave)is weak,and the shear wave is significantly attenuated as it propagates in the human body.The high frequency component of the shear wave is especially strongly attenuated resulting in a very limited propagation distance of a few millimeters.Therefore,ultrasound elastography suffers from poor signal-to-noise ratio(SNR),shallow detection depth,and failure to obtain elasticity data at high frequency.Coded excitation is a widely used technique in ultrasound imaging.It can enhance SNR without sacrifying resolution,thereby improving image quality.In this thesis,I investigated the application of coding detection techniques in ultrasound elastography.The detection of weak shear wave signals was achieved by using encoding detection technology,which improved the detection performance.The main contributions of this thesis include the following:1.Improved performance of transient elastography(TE)using coding technique.A platform for transient elastography with coding detection was constructed.A pork belly phantom experiment and an in-vitroliver experiment were designed.A conventional short-pulse and a variable-length coded pulse were employed in transient elastography.The experiment results showed that coded pulses outperformed the short-pulse detection by providing superior detection SNR,robust measurement and deeper detection depth.2.Improved performance of 2-D elastography using plane wave coded excitation technique.The plane wave coding detection technique is a combination of coded excitation and Stolt’s f-k migration beam-forming algorithm.A pork belly phantom experiment and in-vitroliver experiment were used to evaluate the performance of the plane wave coding detection.The experiment results showed that plane wave coded excitation technique can provide higher detection SNR and detection depth in 2-D elastography.3.Two aspects of detection and excitation to improve the detection of high-frequency shear wave.High-frequency shear wave were detected by several methods and the results showed that the detection ability of the high-frequency shear wave can be improved by coded excitation.Replacing the existing piston oscillator with a ring,the experiment found that the ring vibrator can increase the detection limit of high-frequency shear wave to about 500 ~ 900 Hz.In summary,coded excitation technique can improve the quality of ultrasonic elastography by improving the shear wave SNR,which is demonstrated by both theory and experiments.The combination of coded excitation technique and Stolt’s f-k migration beam-forming was successfully applied to elastography.A novel method of transient elastography with a ring vibrator was studied and induction of high frequency shear wave within elastic phantoms was demonstarted.