Canadian Neurovascular Health Society Board member Dr. Bernhard Juurlink comments on the recent UBC paper "Anisotropic cerebral vascular architecture causes orientation dependency in cerebral blood flow and volume measured with dynamic susceptibility contrast magnetic resonance imaging."

The first part of the commentary deals with the paper. The second part has nothing to do with the paper but a commentary on mean transit time.

Paper Commentary:

The calculations of blood flow in the brain using MRI techniques usually are done under the assumption that the blood vessels in the brain run in all directions in a random fashion, that is the blood vessels are isotropic. But it is known that a large fraction of blood vessels in the white matter run in the same direction as the nerve fibres, thus here the blood vessels are anisotropic. What these authors show is that one must be careful in interpreting MRI data with respect to blood flow and blood volume and must take into account whether the tissue is anisotropic. Of note is that one thing not affected by anisotropy is mean transit time of blood flow through the brain, i.e., the time a bolus of blood enters the brain from either the internal cerebral arteries or vertebral veins and when it leaves via the venous system. It is well known that mean transit time is increased in people with MS.

General Commentary on Mean Transit Time:

There are two possible reasons for an increased mean transit time. One is that there is a lower pressure forcing blood through the vascular bed and the second possibility is that there is increased resistance to the blood flow. Imagine the flow of water through a garden hose when the tap is only partially open and the pressure pushing the through the hose is low or when the tap is wide open but you put a kink in the hose. In both cases the amount flowing through the hose is slower (i.e. has a greater mean transit time) than maximum possible flow and the amount of water leaving (and entering) the hose is lower but for different reasons. In people with MS the blood pressure in the arteries is normal – your physician will ensure this. So this leaves only one possibility why the there is increased mean blood transit time – there is increased resistance to blood flow. Indeed, Dr Zamboni and colleagues have shown that there is increased resistance in venous return from the head using cervical plethysmography.

Link to paper Abstract. Full paper is subscription only.
http://jcb.sagepub.com/content/early/2016/06/02/0271678X16653134.abstract

*Thanks to Florence for this photo of Dr. Juurlink presenting at ISNVD