Biology subjects

Blockley, N. P.

Publications and source records attributed to Blockley, N. P..

4 recordsLinked to original sources

bioRxiv2020

Improving qBOLD based measures of oxygen extraction fraction using hyperoxia-BOLD derived measures of blood volume

PurposeStreamlined-qBOLD (sqBOLD) is a refinement of the quantitative BOLD (qBOLD) technique capable of producing non-invasive and quantitative maps of oxygen extraction fraction (OEF) in a clinically feasible scan time. However, sqBOLD measurements of OEF have been reported as being systematically lower than expected in healthy brain. Since the qBOLD framework infers OEF from the ratio of the reversible transverse relaxation rate (R2') and deoxygenated blood volume (DBV), this underestimation has been attributed the overestimation of DBV. Therefore, this study proposes the use of an independent measure of DBV using hyperoxia-BOLD and investigates whether this results in improved estimates of OEF. MethodsMonte Carlo simulations were used to simulate the qBOLD and hyperoxia-BOLD signals and to compare the systematic and noise related errors of sqBOLD and the new hyperoxia-qBOLD (hqBOLD) technique. Experimentally, sqBOLD and hqBOLD measurements were performed and compared with TRUST (T2 relaxation under spin tagging) based oximetry in the sagittal sinus. ResultsSimulations showed a large improvement in the uncertainty of DBV measurements leading to a much improved dynamic range for OEF measurements with hqBOLD. In a group of ten healthy volunteers, hqBOLD produced measurements of OEF in cortical grey matter (OEFhqBOLD = 38.1 {+/-} 10.1 %) that were not significantly different to TRUST oximetry measures (OEFTRUST = 40.4 {+/-} 7.7 %), whilst sqBOLD derived measures (OEFsqBOLD = 16.1 {+/-} 3.1 %) were found to be significantly different. ConclusionThe simulations and experiments in this study demonstrate that an independent measure of DBV provides improved estimates of OEF.

physiology↗

bioRxiv2018

Coupling between cerebral blood flow and cerebral blood volume: Contributions of different vascular compartments

A better understanding of the coupling between changes in cerebral blood flow (CBF) and cerebral blood volume (CBV) is vital for furthering our understanding of the BOLD response. The aim of this study was to measure CBF-CBV coupling in different vascular compartments during neural activation. Three haemodynamic parameters were measured during a visual stimulus. Look-Locker Flow-sensitive Alternating Inversion Recovery (LL-FAIR) was used to measure changes in CBF and arterial CBV (CBVa) using sequence parameters optimised for each contrast. Changes in total CBV (CBVtot) were measured using a Gadolinium based contrast agent technique. Haemodynamic changes were extracted from a region of interest based on voxels that were activated in the CBF experiments. The CBF-CBVtot coupling constant tot was measured as 0.16{+/-}0.14 and the CBF-CBVa coupling constant a was measured as 0.65{+/-}0.24. Using a two compartment model of the vasculature (arterial and venous), the change in venous CBV (CBVv) was predicted for an assumed value of baseline arterial and venous blood volume. These results will enhance the accuracy and reliability of applications that rely on models of the BOLD response, such as calibrated BOLD.

neuroscience↗

bioRxiv2017

Serial quantification of brain oxygenation in acute stroke using streamlined-qBOLD

It has been proposed that metabolic markers of baseline brain oxygenation have a role to play in the early identification of the ischemic penumbra. Streamlined-qBOLD is a magnetic resonance imaging technique that does not require exogenous contrast. It is a refinement of the quantitative BOLD methodology that provides a simplified approach to mapping and quantifying baseline brain oxygenation related parameters (reversible transverse relaxation rate (R2'), deoxygenated blood volume (DBV) and deoxyhaemoglobin concentration ([dHb])) in a clinically relevant manner. Streamlined-qBOLD was applied to an exploratory cohort of acute stroke patients in a serial imaging study. Detailed voxel-level analysis was used to quantify the metabolic profile of ischaemic tissue on presentation and investigate these metrics in relation to tissue outcome. Individual patient examples illustrate the appropriate interpretation of R2', DBV and [dHb] in acute stroke and demonstrate the ability of this method to deliver regional information related to oxygen metabolism in the ischaemic tissue. Regional analysis confirms that R2', DBV and [dHb] vary between regions of ischaemia with different tissue outcomes.

neuroscience↗

bioRxiv2017

Rapid cerebrovascular reactivity mapping: Enabling vascular reactivity information to be routinely acquired

Cerebrovascular reactivity mapping (CVR), using magnetic resonance imaging (MRI) and carbon dioxide as a stimulus, provides useful information on how cerebral blood vessels react under stress. This information has proven to be useful in the study of vascular disorders, dementia and healthy ageing. However, clinical adoption of this form of CVR mapping has been hindered by relatively long scan durations of 7 to 12 minutes. By replacing the conventional block presentation of carbon dioxide enriched air with a sinusoidally modulated stimulus, the aim of this study was to investigate whether more clinically acceptable scan durations are possible. Firstly, the conventional block protocol was compared with a sinusoidal protocol of the same duration of 7 minutes. Estimates of the magnitude of the CVR signal (CVR magnitude) and the relative timing of the CVR response (CVR phase) were found to be in good agreement between the stimulus protocols. Secondly, data from the sinusoidal protocol was reanalysed using decreasing amounts of data in the range 1 to 6 minutes. The CVR magnitude was found to tolerate this reduction in scan duration better than CVR phase. However, these analyses indicate that scan durations in the range of 3 to 5 minutes produce robust data.

neuroscience↗