An automatic six-degree-of-freedom image registration algorithm for image-guided frameless stereotaxic radiosurgery

A frameless radiosurgical treatment system has been developed by coupling an orthogonal pair of real-time x-ray cameras to a robotically manipulated linear accelerator to guide the therapy beam to treatment sites within a patient's cranium. The two cameras observe the position and orientation of the patient's head in the treatment system coordinate frame. An image registration algorithm compares the two real-time radiographs to a corresponding pair of digitally synthesized radiographs derived from a CT study of the patient. The algorithm determines all six degrees of translational and rotational difference between the position of the head in the CT coordinate frame and its position in the treatment room coordinate frame. This allows translation of treatment planning coordinates into treatment room coordinates without rigidly fixing the patient's head position during either the CT scan or treatment. In this paper the image registration algorithm is described and measurements of the precision and speed with which the process can determine the patient's position are reported. The tests have demonstrated translational uncertainty of 0.5-1.0 mm per axis and rotational uncertainty of 0.6-1.3 degrees per axis, accomplished in approximately 2 s elapsed time.