Abstract
Brain tissue undergoes viscoelastic deformation and volumetric strain as it expands over the cardiac cycle due to blood volume changes within the underlying microvasculature. Volumetric strain measurements may therefore provide insights into small vessel function and tissue viscoelastic properties. Displacement encoding via stimulated echoes (DENSE) is an MRI technique that can quantify the submillimetre displacements associated with brain tissue motion. Despite previous studies reporting brain tissue displacements using DENSE and other MRI techniques, a complete picture of brain tissue volumetric strain over the cardiac cycle has not yet been obtained. To address this need we implemented 3D cine‐DENSE at 7 T and 3 T to investigate the feasibility of measuring cardiac‐induced volumetric strain as a marker for small vessel blood volume changes. Volumetric strain over the entire cardiac cycle was computed for the whole brain and for grey and white matter tissue separately in six healthy human subjects. Signal‐to‐noise ratio (SNR) measurements were used to determine the voxel‐wise volumetric strain noise. Mean peak whole brain volumetric strain at 7 T (mean ± SD) was (4.5 ± 1.0) × 10−4 (corresponding to a volume expansion of 0.48 ± 0.1 mL), which is in agreement with literature values for cerebrospinal fluid that is displaced into the spinal canal to maintain a stable intracranial pressure. The peak volumetric strain ratio of grey to white matter was 4.4 ± 2.8, reflecting blood volume and tissue stiffness differences between these tissue types. The mean peak volumetric strains of grey and white matter tissue were found to be significantly different (p < 0.001). The mean SNR at 7 T and 3 T of the DENSE measurements was 22.0 ± 7.3 and 7.0 ± 2.8 respectively, which currently limits a voxel‐wise strain analysis at both field strengths. We demonstrate that tissue specific quantification of volumetric strain is feasible with DENSE. This metric holds potential for studying blood volume pulsations in the ageing brain in healthy and diseased states.
Highlights
ADAMS ET AL.The brain is mechanically coupled to the heart, which results in pulsatile viscoelastic deformation and volumetric strain of the brain tissue over the cardiac cycle
It may permit the study of the brain in the transition from healthy to diseased states, such as that which occurs in the development of cerebral small vessel disease, where damage to the small vessels causes further damage to the tissue
Unlike the larger arteries of the brain or the macroscopic tissue damage that arises from cerebral small vessel disease (cSVD), the small vessels themselves are difficult to study in vivo with current neuroimaging techniques
Summary
The goal was threefold: first, to obtain (in healthy subjects) whole brain volumetric strain measurements covering the entire cardiac cycle, which we propose as an important metric reflecting small vessel blood volume pulsations; second, to demonstrate the potential of DENSE in practice by assessing the physiological pulsatile differences between grey and white matter tissue (the volumetric strain values obtained in this study were evaluated against literature values of cerebrospinal fluid (CSF) stroke volumes and against literature values of the relative blood volume difference between grey and white matter), and third, to investigate the SNR performance of DENSE when applied to the measurement of cardiac‐induced whole brain tissue motion. The gain in SNR at 7 T compared with 3 T was determined, and from this a voxel‐wise uncertainty in the computed volumetric strain map was calculated
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