Calibration of three-dimensional ultrasound images for image-guided radiation therapy

A new technique of patient positioning for radiotherapy/radiosurgery of ext racranial tumours using three-dimensional (3D) ultrasound images has been d eveloped. The ultrasound probe position is tracked within the treatment roo m via infrared light emitting diodes (IRLEDs) attached to the probe, In ord er to retrieve the corresponding room position of the ultrasound image, we developed an initial ultrasound probe calibration technique for both 2D and 3D ultrasound systems. This technique is based on knowledge of points in b oth room and image coordinates. We first tested the performance of three algorithms in retrieving geometric al transformations using synthetic data with different noise levels. Closed form solution algorithms (singular value decomposition and Horn's quatemio n algorithms) were shown to outperform the Hooke and Jeeves iterative algor ithm in both speed and accuracy. Furthermore, these simulations show that f or a random noise level of 2.5, 5, 7.5 and 10 mm, the number of points requ ired for a transformation accuracy better than 1 mm is 25, 100, 200 and 500 points respectively. Finally, we verified the tracking accuracy of this sy stem using a specially designed ultrasound phantom. Since ultrasound images have a high noise level, we designed an ultrasound phantom that provides a large number of points for the calibration. This ti ssue equivalent phantom is made of nylon wires, and its room position is op tically tracked using IRLEDs. By obtaining multiple images through the nylo n wires, the calibration technique uses an average of 300 points for 3D ult rasound volumes and 200 for 2D ultrasound images, and its stability is very good for both rotation (standard deviation: 0.4 degrees) and translation ( standard deviation: 0.3 mm) transformations. After this initial calibration procedure, the position of any voxel in the ultrasound image volume can be determined in world space, thereby allowing real-time image guidance of th erapeutic procedures. Finally, the overall tracking accuracy of our 3D ultr asound image-guided positioning system was measured to be on average 0.2 mm , 0.9 mm and 0.6 mm for the AP, lateral and axial directions respectively.