Magnetic field quality conversion factors experimentally measured in clinical MR‐linac beams for seven MR‐compatible ionization chamber models
Bibliographic record
Abstract
Abstract Purpose The purpose of this work was to experimentally quantify MR‐compatible ionization chamber response for 1.5T Elekta Unity and 0.35T ViewRay MRIdian MR‐linac systems through the determination of the magnetic field quality conversion factor, k B,Q . Methods Seven MR‐compatible ionization chamber models from Standard Imaging and PTW were evaluated. Both the quality conversion factor k Q and the magnetic field quality conversion factor k B,Q were experimentally determined through a cross‐calibration method. Specifically, the ratio of absorbed dose measured with a reference A1SL chamber under reference conditions to corrected output measured with each test chamber at the same point of measurement allowed for the determination of k B,Q . The angular dependence of the magnetic field quality conversion factor for MR‐compatible chamber models was assessed for the 1.5T Elekta Unity system by measuring k B,Q with the chamber axis and magnetic field direction aligned at cardinal angles (0°, 90°, 180°, 270°). Results Beam quality conversion ( k Q ) factors for MR‐compatible ionization chambers measured in a standard linac beam showed an average percent difference of −0.09 ± 0.18% compared to computed k Q values for their conventional chamber versions. Similarly, magnetic field quality conversion ( k B,Q ) factors for corresponding MR and non‐MR ionization chamber models measured using the same cross‐calibration technique demonstrated average percent differences of −0.1 ± 0.3% and 0.0 ± 0.2% for the Elekta Unity and ViewRay MRIdian, respectively. Investigation of the angular dependence of this correction factor demonstrated identical chamber response for equivalent MR‐compatible and conventional chamber models. Conclusions This work provides critical experimental validation of MR‐compatible ionization chamber performance, with a direct comparison of measured k B,Q values to corresponding conventional chamber models demonstrating nearly equivalent chamber response. k B,Q values determined using our experimental method will serve as an important reference for upcoming MR‐linac reference dosimetry protocols and ultimately represent an important step towards accurate output calibration of MR‐linac systems.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".