Quantification of the Effect of Multiple Scattering on Array Imaging Performance

A quantitative assessment of the detection limit is an important task in a range of fields, where imaging in a random scattering medium is performed. All images suffer, to varying extents, from coherent noise, including speckle caused by material microstructure. The quality of images can be greatly improved by using phased arrays because of the possibility to focus backscattered signals in transmission and reception. As a consequence, under the single scattering assumption, the signal-to-noise ratio (SNR) increases with frequency due to better focusing. However, in reality, material structural noise severely affects the detection performance, especially at high frequencies and large penetration depths. The actual detection limit depends on the type of imaged target and the material properties, but the underlying physical reason is the same and is related to the increase in the contribution of multiple scattering to the measured data. Thus, in this article, a method for estimating the proportion of the multiple scattering contribution in the total image intensity is proposed. Experimental results are presented for ultrasonic array immersion imaging of a collection of randomly distributed steel rods, as well as direct contact imaging of highly scattering polycrystalline materials. It is shown that the SNR as a function of frequency and imaging depth is directly correlated with the measured single scattering rate. Moreover, the detection limit corresponds to the onset of the dominant multiple scattering regime, when the multiple scattering rate approaches 100%.