The following review will aid readers in providing an overview of scale-free dynamics and monofractal analysis, as well as its applications and potential in functional magnetic resonance imaging (fMRI) neuroscience and clinical research. Like natural phenomena such as the growth of a tree or crashing ocean waves, the brain expresses scale-invariant, or fractal, patterns in neural signals that can be measured. While neural phenomena may represent both monofractal and multifractal processes and can be quantified with many different interrelated parameters, this review will focus on monofractal analysis using the Hurst exponent (H). Monofractal analysis of fMRI data is an advanced analysis technique that measures the complexity of brain signaling by quantifying its degree of scale-invariance. As such, the H value of the blood oxygenation level-dependent (BOLD) signal specifies how the degree of correlation in the signal may mediate brain functions. This review presents a brief overview of the theory of fMRI monofractal analysis followed by notable findings in the field. Through highlighting the advantages and challenges of the technique, the article provides insight into how to best conduct fMRI fractal analysis and properly interpret the findings with physiological relevance. Furthermore, we identify the future directions necessary for its progression towards impactful functional neuroscience discoveries and widespread clinical use. Ultimately, this presenting review aims to build a foundation of knowledge among readers to facilitate greater understanding, discussion, and use of this unique yet powerful imaging analysis technique.
Pre-eclampsia is a serious complication of pregnancy, and maternal nutritional factors may play protective roles or exacerbate risk. The tendency to focus on single nutrients as a risk factor obscures the complexity of possible interactions, which may be important given the complex nature of pre-eclampsia. An evidence review was conducted to compile definite, probable, possible and indirect nutritional determinants of pre-eclampsia to map a nutritional conceptual framework for pre-eclampsia prevention. Determinants of pre-eclampsia were first compiled through an initial consultation with experts. Secondly, an expanded literature review was conducted to confirm associations, elicit additional indicators and evaluate evidence. The strength of association was evaluated as definite RR<0.40 or ≥3.00, probable RR 0.40-0.69 or 1.50-2.99, possible RR 0.70-0.89 or 1.10-1.49 or not discernible RR 0.90-1.09. The quality of evidence was evaluated using GRADE. Twenty-five nutritional factors were reported in two umbrella reviews and 22 meta-analyses. Of these, 14 were significantly associated with pre-eclampsia incidence. Maternal vitamin D deficiency and higher serum iron emerged as definite nutritional risk factors for pre-eclampsia incidence across populations, while low serum zinc was a risk factor in Asia and Africa. Calcium and/or vitamin D supplementation were probable protective nutritional factors. Healthy maternal dietary patterns were possibly associated with lower risk of developing pre-eclampsia. Potential indirect pathways of maternal nutritional factors and pre-eclampsia may exist through obesity, maternal anaemia and gestational diabetes mellitus. Research gaps remain on the influence of household capacities and sociocultural, economic and political contexts, as well as interactions with medical conditions.
Cervical and upper-thoracic spinal cord injury (SCI) commonly results in autonomic cardiovascular impairments. These impairments can lead to alterations in blood flow, cerebral perfusion pressure and ultimately tissue perfusion, which can lead to an elevated risk of stroke and global cognitive deficits. The aim of this study was to assess cerebrovascular reactivity (CVR) in both the grey matter (GM) and brainstem using functional magnetic resonance imaging (fMRI) in participants with SCI compared to non-injured controls. CVR represents the capacity of brain parenchyma to change cerebral blood flow in response to a vasoactive stimulus (e.g. carbon dioxide, CO2) or altered metabolic demand [e.g. neurovascular coupling (NVC)]. Thirteen participants (7 chronic SCI (all male, median age of 42 years), 6 controls (all male, median age of 33 years) were studied cross-sectionally. CVR was measured by assessing the MRI-blood oxygen level–dependent signal with hypercapnic challenge (controlled CO2 inhalation). The CVR outcome measure was assessed in three ways. Initially, CVR was calculated as is standard, via the linear, least-squares fit across the whole gas challenge protocol (CVRwhole). In addition, CVR was further decomposed into its dynamic (tau) and static components (steady state CVR; ssCVR). A 24-hour ambulatory blood pressure monitor was worn to capture free-living blood pressure outcomes. Our results showed a longer tau in the GM of SCI participants compared to controls (median of the difference = 3.0 seconds; p<0.05). Time since injury (TSI) displayed negative correlations with ssCVR in the GM and brainstem of SCI participants: RS=-0.77, p=0.041 and RS=-0.76, p=0.049, respectively, where RS is the Spearman’s rank Correlation Coefficient. Neurological level of injury (NLI), modified into an ascending, continuous numeric variable, was positively correlated with GM CVRwhole (RS=0.85, p=0.016), GM ssCVR (RS=0.95, p=0.001) and brainstem ssCVR (RS=0.90, p=0.006). Lower CVRwhole and ssCVR in the SCI-cohort was significantly (P<0.05) correlated with lower daytime blood pressure (RS≥ 0.81) and a higher frequency of hypotensive episodes (RS≥ -0.83). Thus, living with a SCI for a longer period of time, having a higher NLI and lower blood pressure are linked with poorer CVR outcomes. Our preliminary findings reveal an important difference between the cohorts in the dynamic CVR component, tau. Collectively, these results may partially explain the increased cerebrovascular health burden in individuals with SCI.HighlightsCVR is the change in blood flow in response to a vasodilatory stimulus (e.g., hypercapnia).Impaired CVR is linked with increased stroke risk and cognitive deficits.We investigated the dynamic and steady-state components of CVR using fMRI in individuals with a SCI.The dynamic component was significantly different compared to non-injured controls.CVR is significantly correlated with time since injury, level of injury and ambulatory daytime blood pressure.
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