In a previous study, an annular-array transducer was employed to characterize homogeneous scattering phantoms and excised rat livers using backscatter envelope statistics and frequency domain analysis. A sound field correction method was also applied to take into account the average attenuation of the entire scattering medium. Here, we further generalized the evaluation of backscatter coefficient (BSC) using the annular array in order to study skin tissues with a complicated structure. In layered phantoms composed of two types of media with different scattering characteristics, the BSC was evaluated by the usual attenuation correction method which revealed an expected large difference from the predicted BSC. In order to improve the BSC estimate, a correction method that applied the attenuation of each layer as a reference combined with a method that corrects based on the attenuation of the analysis position were applied. It was found that the method using the average attenuation of each layer is the most effective. This correction method is well adapted to the extended depth of field provided by an annular array.

We investigated the differences between the transmission (Tx)/reception (Rx) sound fields for target and reference signals using a reference phantom method (RPM) to assess the stability of backscattering coefficient (BSC) evaluation. A clinical ultrasound scanner and two types of phased linear array transducer with low and high frequencies were used to evaluate the BSCs for two types of homogenous phantom with different attenuation coefficients and BSCs. Different Tx/Rx sound fields were reproduced using different combinations of Tx focus depths and aperture sizes. Target signals with Tx conditions that were both the same as and different from those for the reference signals were used to produce signals with different Tx/Rx sound fields. The differences in the Tx/Rx sound fields affected the depth dependence of the evaluated BSC. It was concluded that this can be a factor creating variation in the BSC for homogenous targets.

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