Assessing arterial stiffness and cardiac pulsations in the brain at sub-millimetre resolution using MRI

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The human brain requires a constant smooth flow of blood to maintain good neurological health. As the heart beats it causes blood to ‘pulsate’ through the large arteries, speeding up when the heart contracts, and slowing down when it relaxes. It is the elastic structure of the walls of large arteries that modify this pulsatile flow, and it is important as it protects the cerebral microcirculation from excessive pulsatile energy. However, as we age, changes occur in the structure of the artery wall that cause them to stiffen. As the brain is a high-flow organ, it is unsurprising that increased arterial stiffness is associated with cerebrovascular disease. For this reason, assessing the stiffness and pulsatility of arteries is highly clinically desirable, however there are currently very few methods to do this in the brain. In this project we propose to develop a novel ultra-high field magnetic resonance imaging (MRI) method for assessing both stiffness and pulsatility of cerebral arteries. Existing MRI methods ‘encode’ blood flow velocity into the MRI signal, but this requires additional magnetic fields to be generated by the scanner, which takes time. Our new approach is based on exploiting the strong sensitivity to blood flow velocity inherent in the MRI signal under certain conditions, and thus allows us to measure pulsatile flow in cerebral arteries with high temporal resolution.The proposed project includes an MRI sequence development theme and an applications theme. For the sequence development theme, we will develop a new method for assessing cerebral large arterial stiffness by measuring the speed of the cardiac pressure wave (i.e. pulse wave velocity). This measurement technique will provide a quantitative measure of stiffness in the internal carotid and middle cerebral artery segments. Then we will focus on measuring the pulsatility in the small lenticulostriate arteries that branch directly from the middle cerebral artery. These newly developed techniques will allow us to explore the relationship between the stiffness of large cerebral arteries and the associated pulsatility downstream in the small arteries. In the applications theme we will apply these new measurement techniques in a cohort of healthy individuals across a broad age range and in a cohort of patients with cerebral small vessel disease. This programme of experiments will allow us to characterise how cerebral large artery stiffness and downstream pulsatility evolve with age, and how this process is altered in patients with small vessel pathology.