Fig. 4: The r1-r2* relaxivity has unique biological correlates compared to R1 and R2* across the brain and in aging.
From: Non-invasive assessment of normal and impaired iron homeostasis in the brain
R1, R2* and the r1-r2* relaxivity were measured in vivo across younger (aged 23–63 years, N = 26) and older (aged 65–77 years, N = 13) subjects (different marker shapes) in 11 brain regions (different colors, WM white matter, CTX cortex). Each row presents the correlations of these qMRI measurements with a different in vivo or ex vivo histological feature (fitted model and 95% confidence bounds are presented for significant correlations): (a) qMRI vs. the macromolecular tissue volume (MTV), an in vivo myelin-sensitive marker, measured for younger (aged 23–63 years, N = 26) and older (aged 65–77 years, N = 13) subjects. Unlike R1 and R2*, the r1-r2* relaxivity is not linearly related to this myelin-sensitive marker (See Supplementary Figs. 21, 22 for additional in vivo myelin-sensitive markers). b qMRI vs. the iron concentration (postmortem, from the literature5, 7) measured for younger (aged 27–64 years, N > = 7) and older (aged 65−88 years, N > = 8) subjects (for precise sample sizes see Table 2). Notably, the r1-r2* relaxivity is not significantly correlated with the iron content when excluding the outlier values in the globus pallidum while the R2* correlation survives this exclusion (Supplementary Section 12). c qMRI vs. the iron mobilization capacity (transferrin (TF) /iron ratio), an iron homeostasis marker (postmortem, from the literature5, 7, same subjects as in b). Only the r1-r2* relaxivity is significantly correlated with the iron mobilization capacity, implying for its sensitivity to the iron homeostasis across brain regions and in aging. For all panels, data points show mean values and error bars show the mean absolute deviation across subjects, p-values are for one-sided F-test corrected for multiple comparisons (FDR), full statistics are in supplementary table 2.
