Absorption and attenuation in soft tissues. I. Calibration and error analyses.

Error estimation for pulse decay absorption and radiation force insertion loss attenuation measurements are developed. In absorption measurements, significant difficulty lies with accurate determination of peak intensity, especially where sharply focused beams are utilized. An intensity calibration is developed based on radiation force measurement of total power and main lobe beam patterns, using embedded thermocouples and short bursts of ultrasound. The main lobe measurements are highly reproducible, but sidelobes are easily corrupted by noise. Thus, a theoretical extension of the main-lobe beam pattern to include sidelobes is utilized to estimate peak focal intensity. The theory is based on circular baffled piston sources with apodizing lenses, an ideal condition that was closely approximated in a specially constructed experimental apparatus. The approach enables estimates of peak intensity in situ with typical uncertainty of <5% and resulting absorption coefficient uncertainty of 10%. Similar analyses of attenuating measurement uncertainties show that errors of </=3% are possible where repeated measurements of radiation force insertion loss are made on relatively homogeneous materials.