Myelin water imaging and R₂^* mapping in neonates: Investigating R₂^* dependence on myelin and fibre orientation in whole brain white matter

Abstract

R 2* relaxation provides a semiquantitative method of detecting myelin, iron and white matter fibre orientation angles. Compared with standard histogram-based analyses, angle-resolved analysis of R 2 * has previously been shown to substantially improve the detection of subtle differences in the brain between healthy siblings of subjects with multiple sclerosis and unrelated healthy controls. Neonates, who are born with very little myelin and iron, and an underdeveloped connectome, provide researchers with an opportunity to investigate whether R 2 * is intimately linked with fibre-angle or myelin content as it is in adults, which may in future studies be explored as a potential white matter developmental biomarker. Five healthy adult volunteers (mean age [±SD] = 31.2 [±8.3] years; three males) were recruited from Vancouver, Canada. Eight term neonates (mean age = 38.6 ± 1.2 weeks; five males) were recruited from the Children's Hospital of Chongqing Medical University neonatal ward. All subjects were scanned on identical 3 T Philips Achieva scanners equipped with an eight-channel SENSE head coil and underwent a multiecho gradient echo scan, a 32-direction DTI scan and a myelin water imaging scan. For both neonates and adults, bin-averaged R 2 * variation across the brain's white matter was found to be best explained by fibre orientation. For adults, this represented a difference in R 2 * values of 3.5 Hz from parallel to perpendicular fibres with respect to the main magnetic field. In neonates, the fibre orientation dependency displayed a cosine wave shape, with a small R 2 * range of 0.4 Hz. This minor relationship in neonates provides further evidence for the key role myelin probably plays in creating this fibre orientation dependence later in life, but suggests limited clinical application in newborn populations. Future studies should investigate fibre-orientation dependency in infants in the first 5 years, when substantial myelin development occurs. K E Y W O R D S magnetic susceptibility, myelin water imaging, neonates, R 2 * , white matter development, white matter fibre orientation

Myelin water imaging depends on white matter fiber orientation in the human brain

“…A strong correlation between the MWF and independent measures of myelin has been demonstrated in various postmortem studies. [8][9][10] Over the past two decades, myelin water imaging (MWI) has been applied to mild traumatic brain injury, 11 aging, 12 spinal cord injury, 13 neonates, 14 and multiple sclerosis, [15][16][17][18] among others.…”

Myelin water imaging depends on white matter fiber orientation in the human brain

“…A strong correlation between the MWF and independent measures of myelin has been demonstrated in various post-mortem studies (8)(9)(10). Over the past two decades, myelin water imaging (MWI) has been applied to mild traumatic brain injury (11), aging (12), spinal cord injury (13), neonates (14), and multiple sclerosis (15)(16)(17)(18), among others. The human brain's white matter is highly anisotropic.…”

Myelination may be impaired in neonates following birth asphyxia

“…Another imaging technique that has shown good specificity for myelin is myelin water imaging (MWI), which obtains a myelin water fraction (MWF) by estimating the short-T2 component of the water T2 relaxation-time spectrum. Recently, this technique has been applied to neonatal imaging ( Webber et al, 2019 ), but one important downside is the comparatively low signal-to-noise ratio, which makes imaging a challenge with respect to the low myelin concentration found in the neonatal brain. MWF also can be estimated using an approach based on multi-component relaxation (MCR) ( Deoni et al, 2011 , Dubois et al, 2014 ).…”