Frame Slippage Verification in Stereotactic Radiosurgery

Purpose: To develop a method for detecting frame slippage in stereotactic radiosurgery by interactively matching in three dimensions Digitally Reconstructed Radiographs (DRRs) to portal images.Methods and Materials: DRRs are superimposed over orthogonal edge-detected portal image pairs obtained prior to treatment. By interactively manipulating the CT data in three dimensions (rotations and translations) new DRRs are generated and overlaid with the orthogonal portal images. This method of matching is able to account for ambiguities due to rotations and translations outside of the imaging plane. The matching procedure is performed with anatomical structures, and is used in tandem with a fiducial marker array attached to the stereotactic frame. The method is evaluated using portal images simulated from patient CT data and then tested using a radiographic head phantom.Results: For simulation tests a mean radial alignment error of 0.82 mm was obtained with the 3D matching method compared to a mean error of 3.52 mm when using conventional matching techniques. For the head phantom tests the mean alignment displacement error for each of the stereotactic coordinates was found to be Δx = 0.95 mm, Δy = 1.06 mm, Δz = 0.99 mm, with a mean error radial of 1.94 mm (SD = 0.61 mm).Conclusion: Results indicate that the accuracy of the system is appropriate for stereotactic radiosurgery, and is therefore an effective tool for verification of frame slippage.