Water diffusion and T₂ quantification in transient-state MRI: the effect of RF pulse sequence

In quantitative measurement of the $T_2$ value of tissues, the diffusion of water molecules has been recognized as a confounder. This is most notably so for transient-state quantitative mapping techniques, which allow simultaneous estimation of $T_1$ and $T_2$ . In prior work, apparently conflicting conclusions are presented on the level of diffusion-induced bias on the T₂ estimate. So far there is a lack of studies on the effect of the RF pulse angle sequence on the level of diffusion-induced bias. In this work, we show that the specific transient-state RF pulse sequence has a large effect on this level of bias. In particular, the bias level is strongly influenced by the mean value of the RF pulse angles. Also, for realistic values of the spoiling gradient area, we infer that the diffusion-induced bias is negligible for non-liquid human tissues; yet, for phantoms, the effect can be substantial (15% of the true $T_2$ value) for some RF pulse sequences. This should be taken into account in validation procedures.