Histological validation of fast macromolecular proton fraction mapping as a quantitative myelin imaging method in the cuprizone demyelination model

Abstract

Cuprizone-induced demyelination in mice is a frequently used model in preclinical multiple sclerosis research. A recent quantitative clinically-targeted MRI method, fast macromolecular proton fraction (MPF) mapping demonstrated a promise as a myelin biomarker in human and animal studies with a particular advantage of sensitivity to both white matter (WM) and gray matter (GM) demyelination. This study aimed to histologically validate the capability of MPF mapping to quantify myelin loss in brain tissues using the cuprizone demyelination model. Whole-brain MPF maps were obtained in vivo on an 11.7T animal MRI scanner from 7 cuprizone-treated and 7 control С57BL/6 mice using the fast single-point synthetic-reference method. Brain sections were histologically stained with Luxol Fast Blue (LFB) for myelin quantification. Significant (p < 0.05) demyelination in cuprizone-treated animals was found according to both LFB staining and MPF in all anatomical structures (corpus callosum, anterior commissure, internal capsule, thalamus, caudoputamen, and cortex). MPF strongly correlated with quantitative histology in all animals (r = 0.95, p < 0.001) as well as in treatment and control groups taken separately (r = 0.96, p = 0.002 and r = 0.93, p = 0.007, respectively). Close agreement between histological myelin staining and MPF suggests that fast MPF mapping enables robust and accurate quantitative assessment of demyelination in both WM and GM.

Quantitative Assessment of Normal Fetal Brain Myelination Using Fast Macromolecular Proton Fraction Mapping

“…1,23,24 A relatively high MPF and its correlation with GA in the thalamus are also in concordance with early myelination of this structure commencing at the 25th week. 25 While the myelin content provides the main determinant of the MPF in WM according to our results and other studies, [6][7][8][9][10] subtle effects of other tissue properties on this parameter are also discernible. An increased MPF in the germinal matrix observed in young fetuses suggests that unmyelinated brain tissue creates a low-signal background highlighting fine distinctions in the macromolecular content not related to myelin and probably associated with cellularity.…”

“…The MPF is a biophysical parameter describing the amount of macromolecular protons in tissues involved in cross-relaxation with free water showed promising clinical results in adult brain studies 11,12 and has been histologically validated as an accurate quantitative tool for measuring demyelination. 10 The single-point technique 4 also allowed designing an ultrafast MPF mapping protocol for nonbrain applications. 13 In utero brain imaging is a promising area of MPF mapping applications where this method could identify the earliest brain tissue changes associated with myelin development and potentially lead to new diagnostic approaches aimed at detecting delayed myelination of the fetal brain.…”

Iron-Insensitive Quantitative Assessment of Subcortical Gray Matter Demyelination in Multiple Sclerosis Using the Macromolecular Proton Fraction

“…At the same time, these correlations are not very strong and explain only 30-40% of data dispersion within either univariate or multivariate models. In view of good inter-observed agreement and low variability in MPF measurements, 17,24 this observation cannot be completely attributed to measurement noise. It also should be pointed out that the correlations between MPF in subcortical GM structures and disability scales appeared systematically weaker than those for cortical GM and more similar in strength to the correlations for NAWM reported earlier for the same patient population.…”

“…35 However, iron remains the dominant factor determining T2* in brain tissues even with a low background iron content. 35,36 Third, while myelin content has been established as the main histological determinant of MPF in brain tissues 21,24 , minor effects of other pathological factors, such as inflammation, gliosis, and loss of neuronal arborization on this parameter cannot be completely excluded. Fourth, we did not use a contrast agent due to the research nature of the imaging protocol and safety considerations.…”

“…MPF is a key parameter determining MT effect in tissues and defined within the two-pool model of MT as a relative amount of macromolecular protons involved into cross-relaxation with water protons. 16 A number of animal studies have demonstrated close associations between MPF and myelin content in both WM and GM, 20-24 thus supporting clinical applications of this parameter as a myelin biomarker. It has been demonstrated that MPF measurements are practically insensitive to large variations in T1 caused by a paramagnetic contrast agent in an animal tumor model.…”