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

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.

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.

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%).

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.