Spatial Resolution Improvement for Ultrasonic Imaging of an Object Consisting of Multiple Planes with Level Differences Using Contour Extraction

The time of flight of sound is often used to measure the shape of an object. Ultrasonic imaging has an advantage of being non-destructive sensing. There is a close geometrical relationship between an ultrasonic beam and the spatial resolution of an acoustic image. Therefore, the realization of a high spatial resolution is difficult without adequate beam forming. In this paper, the spatial resolution improvement for the ultrasonic imaging of an object consisting of multiple planes with level differences in water is described. This imaging technique uses contour extraction and does not require focusing and beam forming. The ultrasonic transducer, used for the experiments, consists of a piezoelectric ceramic disk with a diameter of 4λ at the frequency of 3.5 MHz. The ultrasonic beam has a diameter of about 30λ at a distance of 200λ from the transducer. The resolution was improved by some countermeasures to compensate for the beam expansion. A contour extraction algorithm has been used in this experiment for image processing, in order to improve the spatial resolution around the edges of an object. A distinct advantage in using this algorithm is that the computational complexity is less than that required by other methods such as beam forming, neural networks and tomography algorithms. A distance error of 0.15 mm and a spatial error of ±0.5 mm were achieved using this system.