Correction of phase rotation in pulse spectrum method for scanning acoustic microscopy and its application to measurements of cells

Scanning acoustic microscopy (SAM) can measure the mechanical properties, such as sound speed, thickness, and density, of biological tissues, by using the pulse spectrum method. However, the estimation method needs to be modified because of increases in the center frequency of acoustic transducers. In this paper, we proposed a new estimation method combining a time-of-flight method by Wiener filtering with the pulse spectrum method. First, an optimal control parameter β for Wiener filter was chosen based on a simulation by k-wave MATLAB toolbox. Setting the thickness of a layer to be 1.95 μm, a bias error between the estimated and true thickness was 0.0016% and the control parameter β was chosen to be 0.01 based on the simulated result and previous research. Next, the thickness of a film sample was measured by the time-of-flight method with Wiener filtering and was compared with an optically-measured thickness to confirm the estimation accuracy. Thickness was estimated to be 18.3 ± 0.025 μm at a center frequency of 120 MHz and agreed with the optically-measured thickness. Finally, the parameter n, the number of phase rotation in Gaussian plane, is calculated from the thickness and sound speed, and the pulse spectrum method with the correction of the parameter n is applied to the cellular measurements. Also, the mechanical properties estimated by the proposed method was compared with these by the conventional method.