Experimental quantification of noise in linear ultrasonic imaging

An efficient procedure for experimental-based quantification of statistical distributions of both the random and micro-structural speckle noise within an ultrasonic image is presented. This is of particular interest in the multi-view total focusing method, which enables many images (views) of the same region to be obtained by utilising alternative ray paths and mode conversions. For example, in an immersion configuration, 21 separate views of the same region of a sample can be formed by exploiting direct and skip paths. These views can be combined through some form of data fusion algorithm, to improve defect detection and characterization performance. However, the noise level is different in different views and this should be accounted for in any data fusion algorithm. It is shown that by using only one set of experimental data from a single measurement location, rather than numerous independent locations, it is possible to obtain accurate noise parameters at an imaging level. This is achieved by accounting for the spatial variation in the noise parameters within the image, due to beam spread, directivity and attenuation with a simple empirical correction. An important feature of the process is the suppression of image artefacts caused by signal responses from other ray paths with the use of image masking. This masking process incorporates knowledge of the expected auto-correlation length (ACL) of image speckle noise and high amplitude cluster suppression. The expected ACL is determined via a simple ray-based forward model of a single point scatterer. Compared to the estimates obtained using multiple independent locations, the speckle noise parameters estimated from a single measurement location were within 0.4dB.