Experimental Quantification of Noise in Linear Ultrasonic Imaging

Self Cite

Delay-and-sum algorithms are imaging techniques in non-destructive testing which form images by summing backpropagated signals. Under this approach, a small number of high intensity signals, such as those from boundary reflections, may create artefacts which degrade the image and hinder defect detection. This paper introduces a probabilistic model of the summation which explains the origin of this effect and proposes to replace the summation in the imaging algorithm by the more statistically robust geometric median. As demonstrated on a experimental inspection using multi-view Total Focusing Method and Plane Wave Imaging, this novel technique effectively suppresses some artefacts, at the expense of an increase of the structural noise amplitude and additional diffraction artefacts at the ends of some structural features. As such the geometric median provides an alternative imaging approach that may improve performance in some circumstances.

“…or another delay-and-sum algorithm. As a consequence, I grain 1 (r) is Rayleigh distributed, which was experimentally validated on B-scans [18] and TFM images [20].…”

Section: A Theorymentioning

confidence: 71%

On the Use of the Geometric Median in Delay-and-Sum Ultrasonic Array Imaging

Budyn

2020

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“…The sensitivity model was derived in this paper from a two-dimensional model but could be similarly obtained from a three-dimensional model; whether the agreement would remain acceptable has not been studied. In parallel work, the authors characterised the structural noise present in multi-view TFM images [32]; this will be combined with the sensitivity model developed in the current paper to provide quantitative estimates of the signal to noise ratio for any defect at any location in any view. This is a key input into any future data-fusion algorithm for improved defect detection and characterisation from multi-view TFM images.…”

Section: Discussionmentioning

confidence: 99%

A Model for Multiview Ultrasonic Array Inspection of Small Two-Dimensional Defects

Budyn

Bevan

Zhang

et al. 2019

Self Cite

The multi-view total focusing method (TFM) is an imaging algorithm for ultrasonic full matrix array data that exploits internal reflections and mode conversions in the inspected object to create multiple images, the views. Modelling the defect response in multi-view TFM is an essential first step in developing new detection and characterisation methods which exploit the information present in these views. This paper describes a raybased forward model for small two-dimensional defects and compares its results against finite-element simulations and experimental data for the inspection of a side-drilled hole, a notch and a crack. A simpler version of this model, based on a singlefrequency approximation, is derived and compared. A good agreement with the multi-frequency model and a speed-up of several orders of magnitude are achieved.

“…A detection threshold is needed to determine the existence of a defect. It is assumed that the grain noise image amplitude follows a Rayleigh distribution [29]. If there are N independent pixels in the considered image area, then the probability of false alarm, p f a , is given by…”

Section: B Copper Samplementioning

confidence: 99%

Quantification of the Effect of Multiple Scattering on Array Imaging Performance

Velichko

2020

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