Synthetic MRI for Radiotherapy Planning for Brain and Prostate Cancers: Phantom Validation and Patient Evaluation

PURPOSE: We aimed to evaluate the accuracy of T(1) and T(2) mappings derived from a multispectral pulse sequence (magnetic resonance image compilation, MAGiC(®)) on 1.5-T MRI and with conventional sequences [gradient echo with variable flip angle (GRE-VFA) and multi-echo spin echo (ME-SE)] compared to the reference values for the purpose of radiotherapy treatment planning. METHODS: The accuracy of T(1) and T(2) measurements was evaluated with 2 coils [head and neck unit (HNU) and BODY coils] on phantoms using descriptive statistics and Bland–Altman analysis. The reproducibility and repeatability of T(1) and T(2) measurements were performed on 15 sessions with the HNU coil. The T(1) and T(2) synthetic sequences obtained by both methods were evaluated according to quality assurance (QA) requirements for radiotherapy. T(1) and T(2) in vivo measurements of the brain or prostate tissues of two groups of five subjects were also compared. RESULTS: The phantom results showed good agreement (mean bias, 8.4%) between the two measurement methods for T(1) values between 490 and 2,385 ms and T(2) values between 25 and 400 ms. MAGiC(®) gave discordant results for T(1) values below 220 ms (bias with the reference values, from 38% to 1,620%). T(2) measurements were accurately estimated below 400 ms (mean bias, 8.5%) by both methods. The QA assessments are in agreement with the recommendations of imaging for contouring purposes for radiotherapy planning. On patient data of the brain and prostate, the measurements of T(1) and T(2) by the two quantitative MRI (qMRI) methods were comparable (max difference, <7%). CONCLUSION: This study shows that the accuracy, reproducibility, and repeatability of the multispectral pulse sequence (MAGiC(®)) were compatible with its use for radiotherapy treatment planning in a range of values corresponding to soft tissues. Even validated for brain imaging, MAGiC(®) could potentially be used for prostate qMRI.