A fixed-distance plane wave method for estimating the ultrasound coefficient of nonlinearity

A practical method is proposed to assess the ultrasound coefficient of nonlinearity of a medium by measuring the fundamental and 2nd harmonic in the near field of a plane piston source for varying source pressure. The method uses the Fubini solution to extract the slope of the linear dependency of the ratio harmonic/fundamental on the fundamental pressure measured at the same location. It eliminates the need for a motion stage, required by methods observing harmonic growth with source distance. It also excludes the need to measure the pressure at the source, since, in the current experiment, conducted in distilled water, it neglects depletion of the fundamental due to attenuation and energy transfer to higher harmonics. The variability of the estimated beta was evaluated with 9 measurements for which the setup was mounted anew. This was performed for 4 different distances from the source. The estimated beta slightly decreased with increasing distance from the source, possibly due to focusing effects. The average beta estimated over all measurements was 3.48+−0.43, showing good agreement with previously reported values. The reproducibility and accuracy of the proposed method is relevant for its adoption aimed at beta measurements in tissue samples for clinical diagnostic research.