Sammendrag
Nonlinear propagation of sound has been exploited in the last fifteen years in medical ultrasound imaging through tissue harmonic imaging (THI). THI creates an image by filtering the received ultrasound echo around the second harmonic frequency band. This technique produces images of enhanced quality due to reduced body wall reverberation, lower perturbations from off-axis echoes, and multiple scattering of reduced amplitude. In order to optimize the image quality it is essential to be able to predict the amplitude level and spatial distribution of the propagating ultrasound pulse. A method based on the quasi-linear approximation has been developed to quickly provide an estimate of the ultrasound pulse. This method does not need to propagate the pulse stepwise from the source plane to the desired depth, it directly computes a transverse profile at any depth from the definitions of the transducer and the pulse. The computation handles three spatial dimensions which allows for any transducer geometry. A comparison of pulse forms, transverse profiles, as well as axial profiles between this method, the KZKTexas code, and Abersim shows a satisfactory match. The computation time for the quasi-linear method is also smaller than the time required by the other methods. The fast estimates produced by this method could be used when reconstructing harmonic image from partial images at different focus depths. They predict the pulse amplitude at any depth and allow for an adequate intensity scaling of each partial image.
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