Introduction In the healthy brain, arteries are compliant to fluctuations in pulsatile blood flow to provide a continuous flow to the capillary bed of the local tissue. When vessels stiffen in the case of normal aging and disease, they become less compliant which causes subsequent fluctuations in blood pressure thereby leading to downstream deterioration of the vessel walls1. This vascular deterioration is known to increase with age and has been associated with some of the cognitive deficits such as a diminished executive functioning and processing speed2, functions thought to be preserved by physical exercise3. Whereas arterial compliance has previously been probed using trans-cranial doppler (TCD) imaging methods4, recent advances in arterial spin labelling (ASL) methods mean that compliance can be assessed using fMRI, allowing more precise quantification of the local arterial wall properties rather than those distal to the site of measurement5. It was hypothesised that individuals with higher physical fitness would demonstrate better arterial compliance. Methods Eight healthy male volunteers (20-23years) underwent a VO2MAX fitness test at the University of South Wales in which participants underwent an incremental cycling exercise test to volitional exhaustion on a semi-recumbent cycle ergometer6. In order to recruit a wide fitness range, participants were recruited from running and cycling clubs as well as through general University wide advertisement. An MRI scan was performed on a separate session at Cardiff University at 3 Tesla. Arterial compliance was measured by quantifying changes in blood volume (aBV) within the bilateral middle cerebral arteries (MCA) over the cardiac cycle (systole/diastole), as determined by the simultaneous measurement of brachial artery blood pressure recordings6 (see equation 1). A multi-inversion time (MTI) PICORE7 pulsed arterial spin labelling (PASL) acquisition was performed with acquisition of CBF at short (TI’s = 250, 350, 450, 550, 650ms) medium (TI’s = 750, 850) and long (TI’s = 1,000, 1,500 and 2000ms) inversion times in order to accurately model arterial perfusion. Images were acquired using a spiral gradient echo sequence (TE=2.7ms) with a variable repetition time (1,000ms to 3,400ms). Eight control–tag pairs were acquired for each inversion time, for 12 slices, slice gap 1mm, voxel size=3x3x7mm resulting in a total acquisition time of ~18 minutes. Physiological noise correction was carried out on the raw data using a modified RETROICOR pipeline8 whereby the respiratory trace was regressed from the signal. Data was fitted to allow for a 225ms delay between the cerebral and finger pulse and normalised for the aBV in diastole to produce values of percentage change in aBV per mmHg increase in blood pressure (%/mm Hg). Results A simple linear regression revealed that physical fitness is able to predict arterial compliance. Bootstrapping of 1000 samples was carried out in order to improve confidence in this small sample. A significant regression was found (F(1,7)=9.4, p<.05), with an R2 of .61 (figure 1). Arterial compliance, measured increased by .01 for each ml/min/kg unit increase in VO2MAX. Conclusions In this cross-sectional study it was observed that young adults with higher VO2MAX have significantly higher arterial compliance of their middle cerebral arteries. This is the first experiment to use MRI to estimate arterial compliance in relation to fitness and provides promising clues towards the mechanisms by which exercise exerts its beneficial effects on the brain. For example, exercise may reduce arterial stiffening by signalling vascular endothelial growth9 and build up of smooth muscle in the major arteries, potentially reducing the effects of high pressure on downstream capillaries, thereby providing a constant flow of oxygen and nutrients to metabolising tissue. Our results complement non-MRI findings for an increase in arterial compliance seen systemically in fitter individuals10 suggesting a global effect of exercise on vascular health. Future research will apply this pASL method to patients with neurodegenerative disease where improving cerebrovascular health through exercise may delay symptom onset.
|Statws||Cyhoeddwyd - 2016|
|Digwyddiad||22nd Annual Meeting of the Organization for Human Brain Mapping - Palexpo Exhibition and Congress Centre, Geneva, Swistir|
Hyd: 26 Jun 2016 → 30 Jun 2016
|Cynhadledd||22nd Annual Meeting of the Organization for Human Brain Mapping|
|Teitl cryno||OHBM 2016|
|Cyfnod||26/06/16 → 30/06/16|