A focused transducer will concentrate its signal energy at its focus, producing a clearimage of objects at that depth. However, spatial resolution and signal strength deteriorate awayfrom the transducer???s focus. Therefore, having only one focal length places a great restriction onultrasound imaging, especially with a single-element transducer. To extend the region of focus,three techniques were examined. The first was the synthetic aperture focusing technique (SAFT),a well-studied method that allows focus at every distance instead of just one distance. Using thismethod, lateral resolution is restored and signal-to-noise ratio (SNR) is increased due to thecompounding of several scan lines. SAFT was combined with a virtual source technique tofurther extend the region of focus. The second technique examined was coded excitation, ameans of improving SNR while maintaining the axial resolution. The third technique was tissueharmonic imaging, which produces a narrower beamwidth and reduced sidelobes, and thereforecan improve spatial resolution and contrast of images.Each technique is individually known to improve ultrasound image qualities, with its ownstrengths and drawbacks. This study combines all three. In the final stage of this study using thevirtual source technique, harmonic imaging was implemented using coded excitation beyond thetransducer focus. Because of the low signal strength received under the synthetic aperturetechnique, harmonic imaging has never been attempted using SAFT and a virtual source. Therehas been no previous research to determine whether the techniques will build on each other ordetract from each other.This study included simulations and experiments with the techniques applied to a singlescatterer, as well as experiments with a tissue-mimicking phantom. Implementing codedexcitation with SAFT led to a final SNR higher than was observed when applying SAFT alone.Although coding helped to increase SNR, the sidelobes became much more visible.Additionally, it was found that synthetic aperture was not a linear process and could causedistortion when the transmitted pulse is very long. This problem was eliminated when timecompression was applied before SAFT. When applying tissue harmonic imaging, the sidelobeswere less prominent than before. However, the speckle increased and the SNR decreased, likelydue to the decreased signal strength of the transducer at the harmonic frequencies.iiiThis study has shown that synthetic aperture with a virtual source, coded excitation, andtissue harmonic imaging can be combined to image beyond the focal length of a transducer.However, there was no research on the maximum depth at which the technique can be practical.Additionally, this study did not test the effectiveness of the techniques when applied to realtissue.
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