| B-scanning using Phased Arrays is a modality of choice for imaging deep structures in medical ultrasound.; One of the main assumptions in medical ultrasound imaging is that the speed of sound is constant and known throughout the medium of propagation and that the difference in phase between signals departing from a single source at the same time is due solely to the distance to the source, the frequency and the speed of sound. This is a key factor for electronically focused arrays operating in a phase sensitive detection mode, because they depend on a known constant speed of sound for beam steering and dynamic focusing.; In the clinical setting, patient variability, organ boundaries, blood vessels and intervening tissue will introduce phase shifts in the signals arriving to the elements of the imaging array that are responsible for unwanted refraction and diffraction effects in the wavefront propagation.; In adaptive optics a method has been proposed to compensate the phase distortion caused by atmospheric turbulence for astronomical imaging. The goal of the Image Sharpening method is to maximize global properties of the image intensity.; In the astronomical case the imaging is incoherent, however ultrasound imaging with a Linear Phased Array is done with partially coherent radiation. This thesis presents the conditions necessary to apply the image sharpening method to correct wavefront distortion in ultrasound.; It is also shown that the sharpening functions reach their respective ABSOLUTE maxima in the absence of phase aberration.; The method does not require a user-supplied model and maintains the non-invasive nature of diagnostic ultrasound. The system adapts to sharpen the image of bright areas in the image.; If the image has distinct features the method converges, however in the clinical setting the image will suffer from noise, speckle and also from motion artifacts because the LPA probe is hand held and there is intrinsic patient motion. An analysis of the image sharpening method is done under these conditions and it is concluded that the appropriate selection of the domain of integration and the use of spatially independent sharpening integrals results in compensation even under those circumstances. (Abstract shortened with permission of author.)... |
