Ah Belcher

Spatial and temporal performance of 3D optical surface imaging for real‐time head position tracking

PURPOSE The spatial and temporal tracking performance of a commercially available 3D optical surface imaging system is evaluated for its potential use in frameless stereotactic radiosurgery head tracking applications. METHODS Both 3D surface and infrared (IR) marker tracking were performed simultaneously on a head phantom mounted on an xyz motion stage and on four human subjects. To allow spatial and temporal comparison on human subjects, three points were simultaneously monitored, including the upper facial region (3D surface), a dental plate (IR markers), and upper forehead (IR markers). RESULTS For both static and dynamic phantom studies, the 3D surface tracker was found to have a root mean squared error (RMSE) of approximately 0.30 mm for region-of-interest (ROI) surface sizes greater than 1000 vertex points. Although, the processing period (1/fps) of the 3D surface system was found to linearly increase as a function of the number of ROI vertex points, the tracking accuracy was found to be independent of ROI size provided that the ROI was sufficiently large and contained features for registration. For human subjects, the RMSE between 3D surface tracking and IR marker tracking modalities was 0.22 mm left-right (x-axis), 0.44 mm superior-inferior (y-axis), 0.27 mm anterior-posterior (z-axis), 0.29° pitch (around x-axis), 0.18° roll (around y-axis), and 0.15° yaw (around z-axis). CONCLUSIONS 3D surface imaging has the potential to provide submillimeter level head motion tracking. This is provided that a highly accurate camera-to-LINAC frame of reference calibration can be performed and that the reference ROI is of sufficient size and contains suitable surface features for registration.

Development of a 6DOF robotic motion phantom for radiation therapy.

PURPOSE The use of medical technology capable of tracking patient motion or positioning patients along 6 degree-of-freedom (6DOF) has steadily increased in the field of radiation therapy. However, due to the complex nature of tracking and performing 6DOF motion, it is critical that such technology is properly verified to be operating within specifications in order to ensure patient safety. In this study, a robotic motion phantom is presented that can be programmed to perform highly accurate motion along any X (left-right), Y (superior-inferior), Z (anterior-posterior), pitch (around X), roll (around Y), and yaw (around Z) axes. In addition, highly synchronized motion along all axes can be performed in order to simulate the dynamic motion of a tumor in 6D. The accuracy and reproducibility of this 6D motion were characterized. METHODS An in-house designed and built 6D robotic motion phantom was constructed following the Stewart-Gough parallel kinematics platform archetype. The device was controlled using an inverse kinematics formulation, and precise movements in all 6 degrees-of-freedom (X, Y, Z, pitch, roll, and yaw) were performed, both simultaneously and separately for each degree-of-freedom. Additionally, previously recorded 6D cranial and prostate motions were effectively executed. The robotic phantom movements were verified using a 15 fps 6D infrared marker tracking system and the measured trajectories were compared quantitatively to the intended input trajectories. The workspace, maximum 6D velocity, backlash, and weight load capabilities of the system were also established. RESULTS Evaluation of the 6D platform demonstrated translational root mean square error (RMSE) values of 0.14, 0.22, and 0.08 mm over 20 mm in X and Y and 10 mm in Z, respectively, and rotational RMSE values of 0.16°, 0.06°, and 0.08° over 10° of pitch, roll, and yaw, respectively. The robotic stage also effectively performed controlled 6D motions, as well as reproduced cranial trajectories over 15 min, with a maximal RMSE of 0.04 mm translationally and 0.04° rotationally, and a prostate trajectory over 2 min, with a maximal RMSE of 0.06 mm translationally and 0.04° rotationally. CONCLUSIONS This 6D robotic phantom has proven to be accurate under clinical standards and capable of reproducing tumor motion in 6D. Such functionality makes the robotic phantom usable for either quality assurance or research purposes.

Technical Note: High temporal resolution characterization of gating response time

PURPOSE Low temporal latency between a gating ON/OFF signal and the LINAC beam ON/OFF during respiratory gating is critical for patient safety. Here the authors describe a novel method to precisely measure gating lag times at high temporal resolutions. METHODS A respiratory gating simulator with an oscillating platform was modified to include a linear potentiometer for position measurement. A photon diode was placed at linear accelerator isocenter for beam output measurement. The output signals of the potentiometer and diode were recorded simultaneously at 2500 Hz with an analog to digital converter for four different commercial respiratory gating systems. The ON and OFF of the beam signal were located and compared to the expected gating window for both phase and position based gating and the temporal lag times extracted. RESULTS For phase based gating, a real-time position management (RPM) infrared marker tracking system with a single camera and a RPM system with a stereoscopic camera were measured to have mean gate ON/OFF lag times of 98/90 and 86/44 ms, respectively. For position based gating, an AlignRT 3D surface system and a Calypso magnetic fiducial tracking system were measured to have mean gate ON/OFF lag times of 356/529 and 209/60 ms, respectively. CONCLUSIONS Temporal resolution of the method was high enough to allow characterization of individual gate cycles and was primary limited by the sampling speed of the data recording device. Significant variation of mean gate ON/OFF lag time was found between different gating systems. For certain gating devices, individual gating cycle lag times can vary significantly.

Use of proximal operator graph solver for radiation therapy inverse treatment planning

Purpose Most radiation therapy optimization problems can be formulated as an unconstrained problem and solved efficiently by quasi‐Newton methods such as the Limited‐memory Broyden‐Fletcher‐Goldfarb‐Shanno (L‐BFGS) algorithm. However, several next generation planning techniques such as total variation regularization‐ based optimization and MV+kV optimization, involve constrained or mixed‐norm optimization, and cannot be solved by quasi‐Newton methods. Using standard optimization algorithms on such problems often leads to prohibitively long optimization times and large memory requirements. This work investigates the use of a recently developed proximal operator graph solver (POGS) in solving such radiation therapy optimization problems. Methods Radiation therapy inverse treatment planning was formulated as a graph form problem, and the proximal operators of POGS for quadratic optimization were derived. POGS was exploited for the first time to impose hard dose constraints along with soft constraints in the objective function. The solver was applied to several clinical treatment sites (TG119, liver, prostate, and head&neck), and the results were compared to the solutions obtained by other commercial and non‐commercial optimizers. Results For inverse planning optimization with nonnegativity box constraints on beamlet intensity, the speed of POGS can compete with that of LBFGSB in some situations. For constrained and mixed‐norm optimization, POGS is about one or two orders of magnitude faster than the other solvers while requiring less computer memory. Conclusions POGS was used for solving inverse treatment planning problems involving constrained or mixed‐norm formulation on several example sites. This approach was found to improve upon standard solvers in terms of computation speed and memory usage, and is capable of solving traditionally difficult problems, such as total variation regularization‐based optimization and combined MV+kV optimization.

Spatial and rotational quality assurance of 6DOF patient tracking systems

PURPOSE External tracking systems used for patient positioning and motion monitoring during radiotherapy are now capable of detecting both translations and rotations. In this work, the authors develop a novel technique to evaluate the 6 degree of freedom 6(DOF) (translations and rotations) performance of external motion tracking systems. The authors apply this methodology to an infrared marker tracking system and two 3D optical surface mapping systems in a common tumor 6DOF workspace. METHODS An in-house designed and built 6DOF parallel kinematics robotic motion phantom was used to perform motions with sub-millimeter and subdegree accuracy in a 6DOF workspace. An infrared marker tracking system was first used to validate a calibration algorithm which associates the motion phantom coordinate frame to the camera frame. The 6DOF positions of the mobile robotic system in this space were then tracked and recorded independently by an optical surface tracking system after a cranial phantom was rigidly fixed to the moveable platform of the robotic stage. The calibration methodology was first employed, followed by a comprehensive 6DOF trajectory evaluation, which spanned a full range of positions and orientations in a 20 × 20 × 16 mm and 5° × 5° × 5° workspace. The intended input motions were compared to the calibrated 6DOF measured points. RESULTS The technique found the accuracy of the infrared (IR) marker tracking system to have maximal root-mean square error (RMSE) values of 0.18, 0.25, 0.07 mm, 0.05°, 0.05°, and 0.09° in left-right (LR), superior-inferior (SI), anterior-posterior (AP), pitch, roll, and yaw, respectively, comparing the intended 6DOF position and the measured position by the IR camera. Similarly, the 6DOF RSME discrepancy for the HD optical surface tracker yielded maximal values of 0.46, 0.60, 0.54 mm, 0.06°, 0.11°, and 0.08° in LR, SI, AP, pitch, roll, and yaw, respectively, over the same 6DOF evaluative workspace. An earlier generation 3D optical surface tracking unit was observed to have worse tracking capabilities than both the IR camera unit and the newer 3D surface tracking system with maximal RMSE of 0.69, 0.74, 0.47 mm, 0.28°, 0.19°, and 0.18°, in LR, SI, AP, pitch, roll, and yaw, respectively, in the same 6DOF evaluation space. CONCLUSIONS The proposed technique was found to be effective at evaluating the performance of 6DOF patient tracking systems. All observed optical tracking systems were found to exhibit tracking capabilities at the sub-millimeter and subdegree level within a 6DOF workspace.

Robotic stage for head motion correction in stereotactic radiosurgery

A novel 4 Degree-of-freedom (4D) robotic system was developed to correct both translational and rotational head motion deviations during image-guided stereotactic radiosurgery procedures. The correction was non-invasive, and was performed in real-time with high positioning accuracy. An optical system was used for real-time 6D head position tracking, and a decoupling control law was designed to control the 4D stage for the motion correction in xyz direction and pitch angle. Both experiments on an phantom and volunteers was used to demonstrate the control performance. With motion correction, on average, over 99% of the time the translational error was within 0.5 mm, and the pitch angle error was within 0.2 degrees.

Robotic real-time translational and rotational head motion correction during frameless stereotactic radiosurgery

PURPOSE To develop a control system to correct both translational and rotational head motion deviations in real-time during frameless stereotactic radiosurgery (SRS). METHODS A novel feedback control with a feed-forward algorithm was utilized to correct for the coupling of translation and rotation present in serial kinematic robotic systems. Input parameters for the algorithm include the real-time 6DOF target position, the frame pitch pivot point to target distance constant, and the translational and angular Linac beam off (gating) tolerance constants for patient safety. Testing of the algorithm was done using a 4D (XY Z + pitch) robotic stage, an infrared head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a four-axis motor controller, through which four stepper motors were driven to perform motion compensation. RESULTS The control of the translation of a brain target was decoupled with the control of the rotation. For a phantom study, the corrected position was within a translational displacement of 0.35 mm and a pitch displacement of 0.15° 100% of the time. For a volunteer study, the corrected position was within displacements of 0.4 mm and 0.2° over 98.5% of the time, while it was 10.7% without correction. CONCLUSIONS The authors report a control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets the submillimeter and subdegree accuracy required by SRS.

SU‐E‐J‐157: Simulation and Design of a Real‐Time 6D Head Motion Compensation Platform Based on a Stewart Platform Approach

PURPOSE Real-time sub-millimeter head motion compensation during frameless SRS delivery has the potential to achieve the accuracy of frame-based SRS while being significantly less invasive. Previously, we demonstrated real-time 6D head motion monitoring using an optical camera, however, at the time we were limited to only 3D (x-y-z) of head motion correction due to mechanical restrictions of the head platform. In this work we investigate the feasibility of using a compact 6D robotic Stewart platform (hexapod) placed under the patients head to perform both translational and rotational motion compensation in real-time. Benefits of a hexapod approach over a conventional serial kinematics stage include less flex, compactness, high force to weight ratio, and fast response times. METHODS A hexapod is a parallel robotics device consisting of two platforms connected by six linear actuators oriented at particular angles. To provide accurate motion in 6D, the desired position of the top platform (head) was ascertained using inverse kinematics. MATLAB was used to simulate the six actuator positions for performing motion along x-y-z-phi -theta-psi. Prior recorded 6D human volunteer head motion data was used as an input for simulation of motion compensation. Six Firgelli L12-P linearservo actuators, together with a PCI-7344 motion controller and Labview software, were used for initial construction of a hexapod prototype. RESULTS The necessary actuator lengths over time were computed for this data, simulating the required 6D movement of the hexapod for motion correction. Simulations on previously collected volunteer data indicate a hexapod system is capable of responding to subject head motion with corrections of precise movements, and solutions to the linear system can be computed at near real-time speeds. CONCLUSIONS Based on simulated results, it was successfully demonstrated that a hexapod device can compensate for small patient head motions along all six degrees of freedom.

Constrained quadratic optimization for radiation treatment planning by use of graph form ADMM

The alternating direction method of multipliers (ADMM) is applied to constrained quadratic optimization of intensity-modulated radiation therapy (IMRT) planning. The proximal operator for the ADMM algorithm is developed, and the algorithm is applied to different clinical treatment sites (liver, prostate, and head & neck). The results are compared with the solutions obtained by several other commercial and non-commercial optimizers. The ADMM solver is about one to two orders of magnitude faster than the other solvers, moreover, it requires less computer memory.

Use of a laser-guided collimation system to perform direct kilovoltage x-ray spectra measurements on a linear accelerator onboard imager

PURPOSE The increased use of image-guided radiation therapy (IGRT) has led to increased use of kV on board imaging (OBI) devices. At present, directly measured OBI beam quality data have only been reported in terms of half-value layers (HVL). However, the HVL metric alone does not give the full OBI energy spectra as needed for accurate beam modeling. Although direct kV spectrometer devices exist they typically suffer from detector pile-up when used with OBI sources. We therefore present, for the first time, a novel laser-guided collimation system that allows direct measurement of the full energy spectrum for clinical OBI systems. METHODS Several clinically relevant spectra (80, 100, and 125 kVp), with and without the half bow-tie filter, were measured using a thermoelectric cooled cadmium telluride (CdTe) detector paired with a multichannel analyzer. To prevent detector saturation, the photon flux at the detector was reduced by use of an in-house designed laser-guided collimation system. After applying energy bin corrections, direct spectroscopic measurements were compared to Monte Carlo (MC) simulated spectra in order to verify accuracy of collected data. Both percent depth dose (PDD) curves and digitally reconstructed radiographs (DRR) were compared using the measured vs MC spectra. RESULTS Measured and MC spectra agree with RMSD between 1.96% and 3.29%. PDD curves generated from the measured and MC spectra were found to match except for in the small buildup region, with an overall match for the six beams ranging between 0.3% and 2.7% RMSD. DRRs matched well with a maximum difference in contrast of 1.1% and RMSD of 0.46% contrast for various materials in DRRs. CONCLUSIONS The use of a laser-guided collimation system provided a method for quickly obtaining highly accurate kV spectrum data from OBI sources. For kV dose or DRR calculation, it was found that both spectra produced similar results.