Modified time-of-flight based calibration approach for ultrasonic computed tomography

In the previous paper [11], a method for geometrical and transducer-time-delay auto-calibration of an ultrasonic computed tomography (USCT) system has been described, aiming at calibration of individual ultrasonic (US) transducer positions. The present contribution describes a novel modification of the method utilizing the particular USCT system concept: the exactly known spatial relations among transducers grouped in each of the transducer array systems (TASes). The algorithms used for the calibration remain based on the principles similar to the global positioning system (GPS) navigation, however, the positions and orientations of complete TASes are calibrated, rather than individual positions of transducers. This way, the number of unknowns is substantially reduced while the number of available equations remains unchanged. Consequently, a solution substantially more robust with respect to measurement noise can be obtained based on this highly overdetermined equation system. The method is capable of calibrating the individual positions of all ultrasonic transducers via their positions in TASes as well as their individual time delays at once during sc. empty measurement, without a need for any particular arrangements, e.g. calibration phantoms.