Accuracy of a two-sensor sonic digitizer.

Abstract

Digitizing devices are typically used in radiotherapy computer treatment planning for entering patient anatomy, the locations of internal radioactive source, and the outlines of irregularly shaped external beams. The errors encountered in the use of a large-area two-sensor sonic digitizer for computer input have been studied. Conversion of data from triangular to Cartesian coordinates makes the precision of the digitizer nonuniform over the sensitive area. The response of each senor has been measured and found to be a nonlinear function of distance. The assumption of linearity in computing the triangular distances from the sensor readings produces errors in the computed distances of up to 0.8%. An alternative method of computing the distances using a fitted cubic function reduces the errors to less than 0.1%. For a test pattern, the maximum position error was reduced from 0.5 to 0.1 cm.