Computed transmission ultrasound tomography

Beginning with the wave equation, the authors have derived the classic reconstruction equations, which assume the ultrasonic energy travels in a straight line. The straight line reconstruction methods result in images that are not absolutely quantitative, although they may be useful in delineating speed and attenuation within two-dimensional cross sections, especially in organs such as the breast. Aberrations associated with straight-line reconstruction images are results of the effects of refraction and of diffraction. In addition, these methods assume that the acoustic wave travels within a plane and not in three dimensions; thus the assumed dimensionality of the problem also gives aberrations in the final image. The effects of diffraction are very complex and, given the current methods of measuring arrival time and amplitude, cause aberrations in the image, which result in errors both in geometry and in magnitude of the reconstructed values. Correction of diffraction effects with techniques termed 'diffraction tomography' are being investigated and have resulted in some preliminary data.