Calibration of three-dimensional ultrasound images for image-guidedradiation therapy

Abstract

A new technique of patient positioning for radiotherapy/radiosurgery ofextracranial tumours using three-dimensional (3D) ultrasound images has beendeveloped. The ultrasound probe position is tracked within the treatment roomvia infrared light emitting diodes (IRLEDs) attached to the probe. In order toretrieve the corresponding room position of the ultrasound image, we developedan initial ultrasound probe calibration technique for both 2D and 3Dultrasound systems. This technique is based on knowledge of points in bothroom and image coordinates.We first tested the performance of three algorithms in retrieving geometricaltransformations using synthetic data with different noise levels. Closed formsolution algorithms (singular value decomposition and Horn's quaternionalgorithms) were shown to outperform the Hooke and Jeeves iterative algorithmin both speed and accuracy. Furthermore, these simulations show that for arandom noise level of 2.5, 5, 7.5 and 10 mm, the number of points required fora transformation accuracy better than 1 mm is 25, 100, 200 and 500 pointsrespectively. Finally, we verified the tracking accuracy of this system usinga specially designed ultrasound phantom.Since ultrasound images have a high noise level, we designed an ultrasoundphantom that provides a large number of points for the calibration. Thistissue equivalent phantom is made of nylon wires, and its room position isoptically tracked using IRLEDs. By obtaining multiple images through the nylonwires, the calibration technique uses an average of 300 points for 3Dultrasound volumes and 200 for 2D ultrasound images, and its stability is verygood for both rotation (standard deviation: 0.4°) and translation(standard deviation: 0.3 mm) transformations. After this initial calibrationprocedure, the position of any voxel in the ultrasound image volume can bedetermined in world space, thereby allowing real-time image guidance oftherapeutic procedures. Finally, the overall tracking accuracy of our3D ultrasound image-guided positioning system was measured to be on average0.2 mm, 0.9 mm and 0.6 mm for the AP, lateral and axial directions respectively.