“…Conventional NDT technologies mainly include thermal imaging, ultrasound, computed tomography, eddy currents, and so on [7], [8], [9], [10], [11], [12]. Although these technologies have been successfully applied to various applications, requirements of them such as isolation from surrounding thermal radiation, coupling medium, expensive equipment and long measurement time limit their use in practical applications [13]. In this light, microwave imaging methods have advantages that it is relatively insensitive to the surrounding environment and consists of a simple and inexpensive measurement system [14].…”