Abstract
PurposeTo implement a statistical framework for assessing the precision of several quantitative MRI metrics sensitive to myelin in the human spinal cord: T1, Magnetization Transfer Ratio (MTR), saturation imposed by an off-resonance pulse (MTsat) and Macromolecular Tissue Volume (MTV).MethodsThirty-three healthy subjects within two age groups (young, elderly) were scanned at 3T. Among them, 16 underwent the protocol twice to assess repeatability. Statistical reliability indexes such as the Minimal Detectable Change (MDC) were compared across metrics quantified within different cervical levels and white matter (WM) sub-regions. The differences between pathways and age groups were quantified and interpreted in context of the test-retest repeatability of the measurements.ResultsThe MDC was respectively 105.7ms, 2.77%, 0.37% and 4.08% for T1, MTR, MTsat and MTV when quantified over all WM, while the standard-deviation across subjects was 70.5ms, 1.34%, 0.20% and 2.44%. Even though particular WM regions did exhibit significant differences, these differences were on the same order as test-retest errors. No significant difference was found between age groups for all metrics.ConclusionWhile T1-based metrics (T1 and MTV) exhibited better reliability than MT-based measurements (MTR and MTsat), the observed differences between subjects or WM regions were comparable to (and often smaller than) the MDC. This makes it difficult to determine if observed changes are due to variations in myelin content, or simply due to measurement error. Measurement error remains a challenge in spinal cord myelin imaging, but this study provides statistical guidelines to standardize the field and make it possible to conduct large-scale multi-center studies.
Highlights
IntroductionThe Magnetization Transfer Ratio (MTR) has shown high correlation with histopathology of myelin in multiple sclerosis patients [2,3]
The Minimal Detectable Change (MDC) was respectively 105.7ms, 2.77%, 0.37% and 4.08% for T1, Magnetization Transfer Ratio (MTR), MTsat and Macromolecular Tissue Volume (MTV) when quantified over all white matter (WM), while the standard-deviation across subjects was 70.5ms, 1.34%, 0.20% and 2.44%
Measurement error remains a challenge in spinal cord myelin imaging, but this study provides statistical guidelines to standardize the field and make it possible to conduct largescale multi-center studies
Summary
The Magnetization Transfer Ratio (MTR) has shown high correlation with histopathology of myelin in multiple sclerosis patients [2,3]. MTR consists of various contributions (T1 and fraction F of exchanging protons bound to macromolecules) [6,7], which in some cases work against each other, reducing its sensitivity to myelin [2,8]. In this perspective, the quantification of the saturation imposed by an off-resonance pulse (MTsat) has been proposed to minimize T1 effects and increase the specificity to myelin [6]
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