"As if this wasn't potentially confusing enough, superimposed on these metabolic differences between subcortical U-fibres and deeper white matter, is that merely by their location, subcortical U-fibres are relatively protected from chronic small vessel ischaemic change 5. As the hemispheric white matter receives dual blood supply, superficially from penetrating arteries that pass through the cortex as well as deeper vessels that ascend from the perforators, it stands to reason that the white matter closest to either source of blood should be least affected. Indeed this is what is observed, with chronic small vessel ischaemic change relatively sparing both the periventricular and subcortical white matter 5. It is important to note that this does not apply to large vessel occlusion which will involve the cortex as well as the underlying subcortical white matter."
"In diseases in which the abnormality is one of myelin metabolism/turnover as these fibres have very slow metabolism, they tend to be affected last. Note: often the word 'spared' is used, which is correct in so far as at the time one is imaging these patients there is little involvement of these fibres, however, this should not be taken to mean that they cannot be involved; in fact, in many instances they are involved eventually."