The impact of sound speed errors on medical ultrasound imaging

The results of a quantitative study of the impact of sound speed errors on the spatial resolution and amplitude sensitivity of a commercial medical ul trasound scanner are presented in the context of their clinical significanc e. The beamforming parameters of the scanner were manipulated to produce so und speed errors ranging over +/-8% while imaging a wire target and an atte nuating, speckle-generating phantom. For the wire target, these errors prod uced increases in lateral beam width of up to 320% and reductions in peak e cho amplitude of up to 10.5 dB. In the speckle-generating phantom, these er rors produced increases in speckle intensity correlation cell area of up to 92% and reductions in mean speckle brightness of up to 5.6 dB. These resul ts are applied in statistical analyses of two detection tasks of clinical r elevance. The first is of low contrast lesion detectability, predicting the changes in the correct decision probability as a function of lesion size, contrast, and sound speed error. The second is of point target detectabilit y, predicting the changes in the correct decision probability as function o f point target reflectivity and sound speed error. Representative results o f these analyses are presented and their implications for clinical imaging are discussed. In general, sound speed errors have a more significant impac t on point target detectability over lesion detectability by these analyses , producing up to a 22% reduction in correct decisions for a typical error. (C) 2000 Acoustical Society of America. [S0001-4966(00)01206-6].