Susceptibility induced gray–white matter MRI contrast in the human brain

Abstract

MR phase images have shown significantly improved contrast between cortical gray and white matter regions compared to magnitude images obtained with gradient echo sequences. A variety of underlying biophysical mechanisms (including iron, blood, myelin content, macromolecular chemical exchange, and fiber orientation) have been suggested to account for this observation but assessing the individual contribution of these factors is limited in vivo.For a closer investigation of iron and myelin induced susceptibility changes, postmortem MRI of six human corpses (age range at death: 56–80 years) was acquired in situ. Following autopsy, the iron concentrations in the frontal and occipital cortex as well as in white matter regions were chemically determined. The magnetization transfer ratio (MTR) was used as an indirect measure for myelin content. Susceptibility effects were assessed separately by determining R2* relaxation rates and quantitative phase shifts. Contributions of myelin and iron to local variations of the susceptibility were assessed by univariate and multivariate linear regression analysis.Mean iron concentration was lower in the frontal cortex than in frontal white matter (26 ± 6 vs. 45 ± 6 mg/kg wet tissue) while an inverse relation was found in the occipital lobe (cortical gray matter: 41 ± 10 vs. white matter: 34 ± 10 mg/kg wet tissue). Multiple regression analysis revealed iron and MTR as independent predictors of the effective transverse relaxation rate R2* but solely MTR was identified as source of MR phase contrast. R2* was correlated with iron concentrations in cortical gray matter only (r = 0.42, p < 0.05).In conclusion, MR phase contrast between cortical gray and white matter can be mainly attributed to variations in myelin content, but not to iron concentration. Both, myelin and iron impact the effective transverse relaxation rate R2* significantly. Magnitude contrast is limited because it only reflects the extent but not the direction of the susceptibility shift.

Effect of Age on MRI Phase Behavior in the Subcortical Deep Gray Matter of Healthy Individuals

“…8 Interestingly, the globus pallidus showed an inverse relationship of age with mean phase, suggesting that in this structure, the overall phase increases over time. This finding is in contrast to previous histologic publications 1 and may be due to the effect of the relatively high myelin content of the structure on MR imaging phase 38 or the presence of diamagnetic substances (eg, calcium) in elderly subjects. 39 However, a recent phase-imaging study also found slightly increased phase in older subjects.…”

Striatal Iron Content Predicts Its Shrinkage and Changes in Verbal Working Memory after Two Years in Healthy Adults

“…Future longitudinal studies may benefit from including several iron imaging methods to allow potentially more sensitive and more specific measurements (Haacke et al, 2015). Validity of R2* as an index of iron content may be also threatened by regional differences in myelination (Fukunaga et al, 2010;Langkammer et al, 2012;Lodygensky et al, 2012), although myelin-related bias in estimates of age differences in striatal iron is likely negligible (Daugherty and Raz, 2013). In contrast to the striatum, Hc may contain noniron components that affect R2*, e.g., myelin or amyloid (Quintana et al, 2006).…”

Analysis of Susceptibility-Weighted Images of Cortico-Medullary Junction

“…Nevertheless, it has to be noted that diamagnetic substances do not always show low signal on phase images; when considering the relative susceptibilities of substances to water susceptibility, substances less diamagnetic than water (such as lipids) might cause a quasi-paramagnetic effect and show high signal on PHA images. 31 However, very few studies using SWI have involved the CMJ, 14,25 and to the best of our knowledge, there has been no report of the local histological characteristics of interlobar differences in the CMJ.…”

“…10 The presence of deoxyhemoglobin in the venules of the CMJ will affect both MAG and PHA images. [11][12][13] Effects of myelin and other histological components at the CMJ appear mainly on PHA images, 14 and a recent study reported the disappearance of the CMJ signal on PHA images of demyelinated mice brains. 15 Water-macromolecule exchange has been reported as another cause of contrast on PHA images.…”