System Configuration
We setup the MMS with the configuration of a 6-DOF robotic PPS, a 3D optical tracking system, and a Special Measurement Tool (SMT), as shown in Fig.1. We also used a Leica laser tracking system to verify the MMS. All the tests of this study were performed in the treatment room of the SC200 proton therapy system [17][18], which was developed by Hefei CAS Ion Medical and Technical Devices Co., Ltd. (HFCIM, Hefei, China)
The PPS used by us was designed by LEONI CIA Cables System, France. This PPS is specifically designed for Proton Therapy System [19][20], with 6-DOF of serial robot's type, high manipulability and accuracy. The detailed specifications of this PPS are shown in Table 1.
Table 1
Main Characteristics of LEONI PPS
Specification
|
Value
|
Treatment Volume
|
100 x 50 x 40 cm
|
Max Payloads
|
Up to 285 kg on axis 6
|
Accuracy
|
0.5 mm, 0.2 deg
|
Degree of Freedom
|
6
|
Cartesian Velocity
|
0.1 m/s, 6 deg/s
|
Loading Position Height
|
52 cm without coupling device and without patient support
|
Daily QA
|
Less than 10 min
|
The 3D Optical Tracking System is Polaris Vega ST from NDI, Canada. This tracking system can track the standard tools (offered by NDI) and customized tools made by several sphere markers with a retro-reflective coating. The SMT is a customized tool and passive type in this study, can be detected by the tracking system with the result of SMT’s position (Tx, Ty, Tz) and orientation (Rx, Ry, Rz). NDI offers the application programming interface (API) to get the results with a sampling frequency of 30/60 Hz. The Volumetric Accuracy (RMS, Root Mean Squared) of this tracking system was 0.12 mm inside of the pyramid-shaped volume.
The SMT used in this study was designed by us. Two methods of measurement were integrated into the tool, with the Infrared Reflective (IR) markers and laser tracking markers/target holders, as shown in Fig.2(a). Four IR markers were located at the left of the SMT with the coordinate defined as SMTC, which can be configured by NDI 6D Architect (a software provided by NDI) and then can be identified and tracked by NDI Vega. Three laser tracking markers/target holders were located at the right of the SMT. They can be tracked by the Leica laser tracking system (we used the Leica Absolute Tracker AT960, from Hexagon Manufacturing, Great Britain) with the reflectors. With these three markers, the software can calculate the predefined coordinate system. For the SMT, we defined the coordinate system of the IR markers and laser tracking markers as the same one, so the SMTC can also be identified by the laser tracking markers.
Software (written in python) was developed for the study, with the functions of the following:
a) Controlling the PPS with the movement of point to point and linear motion. Based on the API from LEONI, we developed the code of connecting to PPS and command interaction. We also defined the 3D test points inside the treatment volume and the motion path with this code.
b) Capturing the movement and providing the real-time graphics of the PPS and SMT. Based on the API from NDI and LEONI, we developed the code to achieve this function by obtaining the position coordinate of the PPS (room coordinates) and SMTC tracked by NDI Vega in real-time. These values were used for further processing. We also applied the visualization coordinate system on the Graphical User Interface (GUI) to show the points and trajectories in real-time.
c) Calculating the coordinate transformation matrices in real-time. As the coordinate systems of NDI Vega, laser tracking system and PPS/room were not the same in our system, coordinate transformation should be calculated to fulfill the unification. We recorded the positions of three unique points tracked by NDI Vega, laser tracking system and PPS/room, and then our software could calculate the transformation matrices among them automatically. Finally, all the points tracked by NDI Vega were translated to the corresponding positions in the laser tracking coordinate system and room coordinate system in real-time. These positions and trajectories can be graphed on the GUI in real-time.
d) Calculating the real-time errors between NDI Vega and PPS. By capturing the coordinate of the SMT in NDI Vega and PPS/room, the software calculated the discrepancies automatically and showed the results on the GUI.
e) Tracking different tools. This function provided the possibility of multi-object tracking in real-time during the treatment. This is efficient for users who want to track several targets simultaneously, such as the detection of the patients’ movement, the value of couch sagging and the trajectory of the PPS.
Fig.3 shows the GUI of the software developed for this study. Fig.4 shows the brief framework of the software. All the data are real-time and can be exported for further processing.
Coordinate Transformation
In this study, the coordinate transformation should be considered between NDI Vega, laser tracker and PPS, which were the transformation matrices Tm1, Tm2, to convert SMTC from the NDI Vega coordinate system to PPS coordinate system (room coordinate system), and from the laser tracker coordinate system to PPS coordinate system, respectively. Tm1 and Tm2 were calculated using Singular Value Decomposition (SVD). The transformation description is shown in Eq. (1) (2).

where P represents the description of the target position in room coordinates and the S represents the description of the target position in NDI Vega/laser tracker coordinates. R, t represents the rotation and translation from NDI Vega/laser tracker coordinates to Room coordinates, respectively. We used 3 unique points to calculate the R and t, and used 48 unique points to calculate the errors of the transformation matrix Tm.
Verification of MMS
In our study, the most important procedure of the development was to verify the MMS, as all functions and outputs were based on the accuracy of the MMS. The methods to verify the MMS are listed below:
- Verify the stability of the SMT in NDI Vega tracking system. The SMT was used to verify the stability with the method: 60 points were defined to evenly distribute inside the pyramid-shaped volume; for each point, the sampling time of coordinate was more than 10s at a frequency of 30 Hz, and then calculate the mean deviations and standard deviation (mean ± SD); perform the sampling of all points and calculate the global mean ± SD.
- Verify the accuracy of Transformation Matrix Tm. To verify the Tm (calculated with SVD), we used the 3D CAD software with the method: define two coordinate systems (A and B) and measure 51 position points in these two different coordinate systems; use 3 points to calculate the Tm and 48 points to verify the accuracy of Tm (for each of these points, use the Tm to calculate the coordinate from A to B, and compare with the measured coordinate in B directly); calculate the mean ± SD
- Verify the MMS accuracy with laser tracking system. The accuracy of the MMS was impacted by a lot of factors, including the processing and assembly errors of the SMT, the inherent error of NDI Vega tracking system, the environment of the treatment room (such as vibration and temperature), the accuracy of the transformation matrix and so on. In this study, we verified the accuracy of the MMS with these factors by comparing the position data of the SMT measured by NDI Vega tracking system and laser tracking system. 83 points distributed inside the treatment volume were adopted, the step sizes on axis x, y, z were 25 mm, 25 mm, 20 mm, respectively. The treatment volume was defined from the isocenter coordinate system as follows: ± 250 mm, ± 500 mm, -100 mm to +300 mm along axis x, y, z, respectively. For pitch, yaw and roll, the step sizes were 1º,1º and 5º, respectively.
- Verify the synchronism/response time of the MMS. To verify the feasibility of gating treatment and accident prevention caused by the unexpected movement, a test plan with the reciprocating motion for the PPS was designed and executed. The details were planned a one-minute reciprocation motion along axis z with a distance of ± 10 mm and set the threshold value to 8 mm to trigger the simulation of gating. For the result, we used the values of 0, -1, 1 to denote inside of, out of the lower, and out of the upper safety space, respectively.
Trajectory Tracking
The trajectory of the SMT can represent the target movement by placing it at any desired position, e.g., the isocenter for system QA, near the chest and abdomen for respiratory capture, someplace on the couch top for couch sagging. We placed the SMT at the isocenter position to track its trajectories with different payloads (0 kg, 30 kg, 60 kg, 90 kg, 120 kg, 150 kg) on the couch top. To capture the obvious offset of the SMT, the motion compensation was not used in this study.