Comparison of R1 Mapping Protocols: What are we measuring?

Purpose: Recent work highlights the breadth of reported spin-lattice relaxation rates ($R_1$) for individual tissues. One potential source of variation is the protocol used to determine $R_1$. The methodological dependence of R1 and relaxivity $r_1$ are investigated. Methods: $R_1$ is quantified in gel phantoms with varying concentration of MnCl2, and a small cohort of three healthy volunteers using different acquisition methods. Siemens inversion recovery (IR) and saturation recovery (SR) protocols are applied to phantoms and volunteers. Variable flip angle (VFA) protocols are additionally applied to phantoms. $R_1$ is quantified using single voxel fits, and distributions examined for regions in the thalamus, and cerebellum as well as grey and white matter. Phantoms exclude boundary fits and relaxivity is quantified across the full concentration range. Normality of $R_1$ distributions is assessed by Kolmogorov-Smirnov score, and inter-sequence agreement by two-sample t-test. Results: Phantom relaxivity is found to be 7.16 Hz/mM, 9.22 Hz/mM and 10.65 Hz/mM to 11.91 Hz/mM for IR, SR and VFA methods, respectively. In vivo $R_1$ exhibit low intra-participant variation for IR. SR $R_1$ are lower than IR values with inter- and intra-participant variation on the same order. Brain regions and phantoms mapped with different protocols varied significantly with t-test p-values between 0 and 5E-10. Conclusion: Results suggest a significant protocol dependence of $R_1$, and corresponding relaxivity, suggesting inter-method comparisons should be attempted tentatively, if at all.