Purpose: The increasing availability of 1.5T MR-Linacs brings renewed relevance of Fricke gel dosimetry for machine QA and patient-specific-QA. The primary method of imaging Fricke gels in the past has been T1-mapping; however, the current version of the Unity MR-Linac (Elekta AB) makes T1-mapping a challenge. In this work, we present solutions for overcoming the Unity’s issues of rescaling and acquiring T1-maps. These solutions include using T1-weighted imaging and using off-line processing to obtain T1-maps.
Methods: We created and optimized imaging protocols including T1-TSE and T1-weighted (T1-W) with varying flip angles (VFA) and TR time. Protocols were designed to avoid automatic rescaling between scans and allow the creation of T1-maps. The VFA images were converted to T1-maps and quantitative analysis of scans was performed using open-source pMRI software and in-house MATLAB code. Sample sizes were 1-cm², 3.5-mL cuvettes or 50-mL centrifuge tubes with 2.9-cm diameter. We modified standard FXG formulation by removing xylenol orange to increase sensitivity of the gels. Samples were placed in water, and half of each sample was irradiated to doses between 0-6Gy using a 7MV FFF beam.
Results: We evaluated our readout techniques by obtaining contrast-to-noise ratios (CNR) as a function of dose. We compared T1-W, T1-TSE and T1-map images. T1-map and T1-W images show the CNR increasing with dose, as expected. The T1-TSE images were obtained without rescaling-avoidance; as a result, the CNR increase with dose increase was masked. Preliminary data shows T1-TSE images are most sensitive in the <2Gy range, while T1-maps show the best linearity and contrast in the 3-6Gy range.
Conclusion: We present multiple methods for imaging Fricke gels in clinically relevant dose ranges, and tools for overcoming imaging limitations on a commercial 1.5T MR-Linac. Further work on T1 map generation and T1-TSE image optimization is currently in progress.