“…Thresholding selection techniques can be classified into two categories: bi-level and multi-level. A variety of thresholding approaches exist for image segmentation, ranging from conventional methods to intelligent techniques (Ma and Liu, 2016;Iskandarani, 2017;Suganya et al, 2013;Jung et al, 2017;Geng et al, 2015;Santhi and Wahida Banu, 2015). The interaction between the ultrasound wave and defects within the tested structure is a function of the defect size, defect orientation and propagation mode among other factors A material boundary constitutes a change in the acoustic impedance.…”