SU‐C‐103‐06: Isocenter Calibration Concept and Feasibility for MR‐Guided Radiation Therapy (MRgRT)
Bibliographic record
Abstract
Purpose: To describe and assess feasibility of a novel MR‐to‐Linac isocenter calibration tool for magnetic resonance guided radiotherapy (MRgRT™). Methods: The MRgRT system co‐developed by IMRIS and Varian employs a movable MR system, which travels into a Linac vault. To support MR‐based couch corrections based on MR‐to‐MR registration, we developed a software tool to calibrate the MR‐to‐Linac coordinate system transformation and apply it to MR images acquired for treatment guidance. To assess feasibility, we quantified repeatability of the movable MRI system at MR isocenter by securing an ACR phantom to a fixed IMRIS interventional table, and moving the MR system between the two adjacent rooms simulate normal function. We performed 18 cycles over two days in which we acquired 3D MRI datasets with 1 mm isotropic resolution. We assessed position variability by registering the MR volume for each cycle to the first volume, and tabulating transform parameters. Separately, we investigated if a calibrated position encoder on the magnet mover could minimize the z‐direction shift variability. Results: Greatest isocenter variability was observed in the z direction, with mean shift of 0.21 +/− 0.12 mm over both days. The greatest rotational variability was observed about the z‐axis, with mean rotation of 0.8 +/− 0.8 mrad over both days. Maximum shift was 0.51 mm in the z direction, and maximum rotation was 1.7 mrad about the z‐axis. In the separate repeatability experiment we found that the calibrated position encoder reduced the mean z‐shift variability from 0.59 +/− 0.35 mm to 0.17 +/− 0.13 mm. Conclusion: We showed that the movable MRI system provides a consistent within‐day and between‐day isocenter location, and z‐shift variability can be minimized with a position encoder. This work demonstrates the feasibility of using an MR‐to‐Linac isocenter calibration tool for the MRgRT system. Jeff Winter, Michael Westmore and Meir Dahan are all employees of IMRIS, the company co‐developing the MRgRT system. Funding for the MRgRT project at Princess Margaret Hospital was provided by the Canadian Foundation for Innovation.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.003 | 0.005 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".