Fast 3-D Ultrasonic Imaging Using Time-Domain Synthetic Aperture Focusing Techniques Based on Circular Scan Conversions

Drawing-arc-based synthetic aperture focusing technique (DAB-SAFT) is capable of greatly improving the imaging speed by reinterpreting the focusing process in the forward direction, but it is only suitable for a two-dimensional (2-D) ultrasonic testing. By extending DAB-SAFT to fast 3-D ultrasonic imaging, this paper proposes both spherical scan-conversion-based SAFT and a spherical-surface-conversion-based SAFT (SSCB-SAFT) for a 3-D nondestructive testing. In SSCB-SAFT, the entire imaging procedure is converted to multiple scan-conversion operations of spherical surfaces. To obtain the coordinates of pixels on a given spherical surface, the spherical scan-conversion algorithm is presented based on circular scan conversions to speed up the imaging procedure by avoiding the time-consuming computations of root-mean-square distances. Besides, the performance of DAB-SAFT is further improved based on the relative positions between different scanning positions, which largely reduce the number of circular arc scan-conversion operations. This acceleration strategy is also applicable to SSCB-SAFT. The simulation experiments show that the improved SSCB-SAFT speeds up the imaging procedure four times while maintaining the same results.