Development of Frameless SRS using Real-Time 6D Facial Surface Monitoring and Continuous Adaptive Head Motion Correction

Abstract

Purpose/Objective(s)To achieve sub-mm precision for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, invasiveness of the ring can be highly uncomfortable and not well tolerated. We report on a method for performing highly accurate head positioning without the use of a head ring or other mechanical restraining devices placed around the head or over the face. This method relies on real-time 6D patient head motion tracking to guide a stepper motor controlled 3D head motion compensation stage. Two techniques are evaluated to provide head position monitoring; use of several infrared markers attached to a bite block or 6D facial surface monitoring based on VisionRT.Materials/MethodsThe proposed system consisted of a central control computer, optical patient motion tracking systems and a 3D motion compensation stage. The optical tracking systems include a Polaris position sensor unit used to monitor four optical reflective markers attached to a bite block and a VisionRT to track highly contoured surfaces of the patients face. A LINAC head ring couch mount was mechanically modified to replace all the manual control hand knobs with stepper motors. Taking into account micro-stepping and further gear reduction by the mechanical axis controls on the frame, the resolution of position adjustment of the frame was measured to be approximately 0.0013 mm. The motion feedback of the markers is processed by the control computer. If the lesion deviates beyond a preset 3D motion tolerance, a relay switch is activated and the MV beam is turned off.ResultsThe percentage of time that a patient was under a particular distance from target for the 15 min period is calculated both with and without the use of correction. Preliminary results favor a bite block due to its < 0.25 mm tracking accuracy compared to the > 1 mm accuracy of VisionRT. With a bite block, using a 0.5 mm target position tolerance, test subject 1 was over tolerance ∼ 95% of the time without motion correction, whereas with motion correction, this was reduced to ∼ 3%. However, problems with the bite block include: an invasive initial setup procedure where the subject must bite into the viscous dental paste and wait several minutes for it to cure into an elastic solid, possible position reproducibility issues as the bite block is taken out and inserted into the patient's mouth, and not being suitable for patients lacking upper teeth.ConclusionsThe main advantage of the method is the complete removal of all head restraining devices such as head rings or face masks while still maintaining high sub-millimeter geometric accuracies.Key words: SRS, frameless Purpose/Objective(s)To achieve sub-mm precision for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, invasiveness of the ring can be highly uncomfortable and not well tolerated. We report on a method for performing highly accurate head positioning without the use of a head ring or other mechanical restraining devices placed around the head or over the face. This method relies on real-time 6D patient head motion tracking to guide a stepper motor controlled 3D head motion compensation stage. Two techniques are evaluated to provide head position monitoring; use of several infrared markers attached to a bite block or 6D facial surface monitoring based on VisionRT. To achieve sub-mm precision for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, invasiveness of the ring can be highly uncomfortable and not well tolerated. We report on a method for performing highly accurate head positioning without the use of a head ring or other mechanical restraining devices placed around the head or over the face. This method relies on real-time 6D patient head motion tracking to guide a stepper motor controlled 3D head motion compensation stage. Two techniques are evaluated to provide head position monitoring; use of several infrared markers attached to a bite block or 6D facial surface monitoring based on VisionRT. Materials/MethodsThe proposed system consisted of a central control computer, optical patient motion tracking systems and a 3D motion compensation stage. The optical tracking systems include a Polaris position sensor unit used to monitor four optical reflective markers attached to a bite block and a VisionRT to track highly contoured surfaces of the patients face. A LINAC head ring couch mount was mechanically modified to replace all the manual control hand knobs with stepper motors. Taking into account micro-stepping and further gear reduction by the mechanical axis controls on the frame, the resolution of position adjustment of the frame was measured to be approximately 0.0013 mm. The motion feedback of the markers is processed by the control computer. If the lesion deviates beyond a preset 3D motion tolerance, a relay switch is activated and the MV beam is turned off. The proposed system consisted of a central control computer, optical patient motion tracking systems and a 3D motion compensation stage. The optical tracking systems include a Polaris position sensor unit used to monitor four optical reflective markers attached to a bite block and a VisionRT to track highly contoured surfaces of the patients face. A LINAC head ring couch mount was mechanically modified to replace all the manual control hand knobs with stepper motors. Taking into account micro-stepping and further gear reduction by the mechanical axis controls on the frame, the resolution of position adjustment of the frame was measured to be approximately 0.0013 mm. The motion feedback of the markers is processed by the control computer. If the lesion deviates beyond a preset 3D motion tolerance, a relay switch is activated and the MV beam is turned off. ResultsThe percentage of time that a patient was under a particular distance from target for the 15 min period is calculated both with and without the use of correction. Preliminary results favor a bite block due to its < 0.25 mm tracking accuracy compared to the > 1 mm accuracy of VisionRT. With a bite block, using a 0.5 mm target position tolerance, test subject 1 was over tolerance ∼ 95% of the time without motion correction, whereas with motion correction, this was reduced to ∼ 3%. However, problems with the bite block include: an invasive initial setup procedure where the subject must bite into the viscous dental paste and wait several minutes for it to cure into an elastic solid, possible position reproducibility issues as the bite block is taken out and inserted into the patient's mouth, and not being suitable for patients lacking upper teeth. The percentage of time that a patient was under a particular distance from target for the 15 min period is calculated both with and without the use of correction. Preliminary results favor a bite block due to its < 0.25 mm tracking accuracy compared to the > 1 mm accuracy of VisionRT. With a bite block, using a 0.5 mm target position tolerance, test subject 1 was over tolerance ∼ 95% of the time without motion correction, whereas with motion correction, this was reduced to ∼ 3%. However, problems with the bite block include: an invasive initial setup procedure where the subject must bite into the viscous dental paste and wait several minutes for it to cure into an elastic solid, possible position reproducibility issues as the bite block is taken out and inserted into the patient's mouth, and not being suitable for patients lacking upper teeth. ConclusionsThe main advantage of the method is the complete removal of all head restraining devices such as head rings or face masks while still maintaining high sub-millimeter geometric accuracies.Key words: SRS, frameless The main advantage of the method is the complete removal of all head restraining devices such as head rings or face masks while still maintaining high sub-millimeter geometric accuracies.