Abstract
Full angle spatial compounding is of interest for imaging small organs (breast, testicles) and small animals. Besides phase aberration and out of plane refraction the variations in speed of sound, usually assumed to be constant, also cause registration error. This error may be negligible regarding the visual appraisal of B-mode images; its effect on spatial compounding of the individual scans may, however, be detrimental to the spatial and contrast resolution depending on the geometry and the variations of the acoustic speed. This effect was studied on the basis of analytical modeling as well as measured data. An exact compensation of the refraction artifacts is achieved via ray tracing for known speed of sound distribution. An alternative approach is to carry out axial correction disregarding the bending due to refraction. The purpose of this paper is to study the characteristics of the axial correction and the extent to which the correction is sufficiently accurate. The effect of the speed of sound variation in spatial compounding was studied with the help of the point spread function (PSF). A geometrical acoustic model was derived with eikonal approximation to calculate the compounding PSF with axial correction as a function of the speed of sound variations and the location of the point scatterer. The effect of the speed of sound variation and the scatterer location on the spatial compounding PSF with axial correction was studied and the results were compared to those without correction. Axial correction was also applied to data measured on a high precision setup on phantoms corresponding to the modeled objects, in order to validate the efficacy of the simulation techniques. The results were found to conform to those predicted by the model. The object was chosen to be of cylindrical shape having a constant velocity either above or below the velocity of the surrounding medium and containing only one scatterer. The PSF of the compounding was shown to be generally of elliptical shape whose extent along the radial direction remains constant for a given speed of sound distribution and whose azimuthal extent decreases with increasing radial distance from the center of the cylinder. For a speed of sound 1550 m/s inside the cylinder of radius 3 cm immersed in water at 22 degrees C, the axial and azimuthal extents of the compounding PSF of a point located 2 cm from the center without correction was 2.32 mm and 1.76 mm respectively considering an ideal B-mode PSF. The axial correction results in PSF radial and azimuthal widths of 1.18 mm and 0.88 mm respectively. It may be concluded that refraction tends to produce circular and elliptical structures with decreasing azimuthal extents for higher radial coordinates. The axial time delay compensation works at the best in the vicinity of the center of rotation and may even add to azimuthal spreading in the worst case for increasing radial coordinates.