Pooling quantitative MRI data: A multi-protocol study of healthy subcortical ageing

Quantitative MRI (qMRI) measures relaxation rates, exchange rates and proton densities that reflect the biophysical properties of tissue and are ideally free from protocol- and scanner-dependence. In practice, qMRI has not yet achieved this level of independence from sequence and hardware choice, and quantitative measurements often differ across sites and acquisition schemes. At the same time pooling data across different sources can be beneficial to statistical power of longitudinal, cross-sectional or case-control studies. Here we investigate how protocol and hardware differences can affect pooling data from difference sources in large ultra high field (UHF) qMRI studies in the context of healthy aging. We combine the openly available ageing UHF qMRI MP2RAGEME-based dataset with two different MPM-based sets of qMRI data. We evaluate how pooling affects age dependence of qMRI parameters and investigate protocol-related biases, with a particular focus on subcortical structures. We focus the analysis, first, on replication and expansion of the reference qMRI dataset on normative aging, second, the examination of the protocol influence on the measured qMRI values, and third, on detecting the protocol effect on the age dependence inferred from the data. We find that the age-related changes for R1, R2* and volume detected by different protocols were of the same order of magnitude supporting the idea that age-related inter-individual variability can be correctly grasped across sites and protocols using qMRI approaches. We further observe larger relative difference across protocols for R1 and volume, while R2* remains more consistent for most regions. We therefore provide assessment of the pooling effects in the ultra-high resolution UHF qMRI data performed over a relatively large participant cohort, comprising data from different sites collected with different quantitative protocols.