Quantification of the Effect of Multiple Scattering on Array Imaging Performance

Abstract

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 increases with frequency due to better focusing. However, in reality, material structural noise severely affects the detection performance, and 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 paper 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 signal-to-noise ratio (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%.A. Velichko is with the

Reflection Matrix Approach for Quantitative Imaging of Scattering Media

“…Thus, the multiplescattering rates combined with the measurement of FðrÞ provide a sensitive local mapping of the heterogeneities in the medium which includes both small-and large-scale variations of the refractive index. In this context, we mention the recent work of Velichko [67], which measures a quantity similar to ρðrÞ as a function of depth and frequency. While noise is not treated separately from multiple scattering, their results emphasize the clear relation between local measurements of multiple scattering and the reliability of ultrasound images.…”

Optimal Extraction of Ultrasonic Scattering Features in Coarse Grained Materials

“…In effect, the power of the proposed method is based on the possibility of the rapid extraction of the local array data, which could be post-processed for different purposes. For example, additional signal processing might include imaging enhancement via eigendecomposition of the time reversal operator [39], [40], or to estimate the rate of single and multiple scattering in the data [38].…”

“…The level of multiple scattering can be controlled by applying different frequency filters to the array data before imaging in order to determine the relative robustness of the different scattering matrix extraction methods. The single scattering rate can be estimated from the specific properties of the generalized image [38]. The main contribution from single scattering is located around the main diagonal of the generalized image (when x T = x R ).…”

Comparison of Time Domain and Frequency-Wavenumber Domain Ultrasonic Array Imaging Algorithms for Non-Destructive Evaluation

“…As seen in Figure 6 a the API varies only slightly with (for ) which is a distinctly different trend from the aluminium specimen (i.e., Figure 4 and Figure 5 ). This difference is due to the combined effect of the larger defect geometry used in this case to ensure detection (i.e., a 2 mm SDH) and the multiple scattering from the grain structure which distorts the reflected signals and adds backscattered noise [ 32 ]. The effect of defect geometry is explored further in Figure 6 e which compares the simulated API as a function of for SDHs with various diameters.…”