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Analysis and correction of biases in cross‐relaxation MRI due to biexponential longitudinal relaxation

Cross‐relaxation imaging (CRI) is a family of quantitative magnetization transfer techniques that utilize images obtained with off‐resonance saturation and longitudinal relaxation rate (R1) maps reconstructed by the variable flip angle (VFA) method. It was demonstrated recently that a significant bias in an apparent VFA R1 estimation occurs in macromolecule‐rich tissues due to magnetization transfer (MT)‐induced biexponential behavior of longitudinal relaxation of water protons. The purpose of this article is to characterize theoretically and experimentally the resulting bias in the CRI maps and propose methods to correct it.