M Cavusoglu

Comparison of pulsed arterial spin labeling encoding schemes and absolute perfusion quantification

Arterial spin labeling (ASL) using magnetic resonance imaging (MRI) is a powerful noninvasive technique to investigate the physiological status of brain tissue by measuring cerebral blood flow (CBF). ASL assesses the inflow of magnetically labeled arterial blood into an imaging voxel. In the last 2 decades, various ASL sequences have been proposed which differ in their ease of implementation and their sensitivity to artifacts. In addition, several quantification methods have been developed to determine the absolute value of CBF from ASL magnetization difference images. In this study, we evaluated three pulsed ASL sequences and three absolute quantification schemes. It was found that FAIR-QUIPSSII implementation of ASL yields 10-20% higher signal-to-noise ratio (SNR) and 18% higher CBF as compared with PICORE-Q2TIPS (with FOCI pulses) and PICORE-QUIPSSII (with BASSI pulses). In addition, quantification schemes employed can give rise to up to a 35% difference in CBF values. We conclude that, although all quantitative ASL sequences and CBF calibration methods should in principle result in the similar CBF values and image quality, substantial differences in CBF values and SNR were found. Thus, comparing studies using different ASL sequences and analysis algorithms is likely to result in erroneous intra- and intergroup differences. Therefore, (i) the same quantification schemes should consistently be used, and (ii) quantification using local tissue proton density should yield the most accurate CBF values because, although still requiring definitive demonstration in future studies, the proton density of blood is assumed to be very similar to the value of gray matter.

Functional localization in the human brain: gradient-echo, spin-echo, and arterial spin-labeling fMRI compared with neuronavigated TMS

A spatial mismatch of up to 14 mm between optimal transcranial magnetic stimulation (TMS) site and functional magnetic resonance imaging (fMRI) signal has consistently been reported for the primary motor cortex. The underlying cause might be the effect of magnetic susceptibility around large draining veins in Gradient‐Echo blood oxygenation level‐dependent (GRE‐BOLD) fMRI. We tested whether alternative fMRI sequences such as Spin‐Echo (SE‐BOLD) or Arterial Spin‐Labeling (ASL) assessing cerebral blood flow (ASL‐CBF) may localize neural activity closer to optimal TMS positions and primary motor cortex than GRE‐BOLD. GRE‐BOLD, SE‐BOLD, and ASL‐CBF signal changes during right thumb abductions were obtained from 15 healthy subjects at 3 Tesla. In 12 subjects, tissue at fMRI maxima was stimulated with neuronavigated TMS to compare motor‐evoked potentials (MEPs). Euclidean distances between the fMRI center‐of‐gravity (CoG) and the TMS motor mapping CoG were calculated. Highest SE‐BOLD and ASL‐CBF signal changes were located in the anterior wall of the central sulcus [Brodmann Area 4 (BA4)], whereas highest GRE‐BOLD signal changes were significantly closer to the gyral surface. TMS at GRE‐BOLD maxima resulted in higher MEPs which might be attributed to significantly higher electric field strengths. TMS‐CoGs were significantly anterior to fMRI‐CoGs but distances were not statistically different across sequences. Our findings imply that spatial differences between fMRI and TMS are unlikely to be caused by spatial unspecificity of GRE‐BOLD fMRI but might be attributed to other factors, e.g., interactions between TMS‐induced electric field and neural tissue. Differences between techniques should be kept in mind when using fMRI coordinates as TMS (intervention) targets. Hum Brain Mapp, 2011.

Quantifying the Link between Anatomical Connectivity, Gray Matter Volume and Regional Cerebral Blood Flow: An Integrative MRI Study

Background In the graph theoretical analysis of anatomical brain connectivity, the white matter connections between regions of the brain are identified and serve as basis for the assessment of regional connectivity profiles, for example, to locate the hubs of the brain. But regions of the brain can be characterised further with respect to their gray matter volume or resting state perfusion. Local anatomical connectivity, gray matter volume and perfusion are traits of each brain region that are likely to be interdependent, however, particular patterns of systematic covariation have not yet been identified. Methodology/Principal Findings We quantified the covariation of these traits by conducting an integrative MRI study on 23 subjects, utilising a combination of Diffusion Tensor Imaging, Arterial Spin Labeling and anatomical imaging. Based on our hypothesis that local connectivity, gray matter volume and perfusion are linked, we correlated these measures and particularly isolated the covariation of connectivity and perfusion by statistically controlling for gray matter volume. We found significant levels of covariation on the group- and regionwise level, particularly in regions of the Default Brain Mode Network. Conclusions/Significance Connectivity and perfusion are systematically linked throughout a number of brain regions, thus we discuss these results as a starting point for further research on the role of homology in the formation of functional connectivity networks and on how structure/function relationships can manifest in the form of such trait interdependency.

Fat intake modulates cerebral blood flow in homeostatic and gustatory brain areas in humans

BACKGROUND The hypothalamus is the central homeostatic control region of the brain and, therefore, highly influenced by nutrients such as glucose and fat. Immediate and prolonged homeostatic effects of glucose ingestion have been well characterized. However, studies that used stimulation with fat have mainly investigated immediate perceptional processes. Besides homeostatic processes, the gustatory cortex, including parts of the insular cortex, is crucial for the processing of food items. OBJECTIVE The aim of this study was to investigate the effect of high- compared with low-fat meals on the hypothalamus and the insular cortex. DESIGN Eleven healthy men participated in a single-blinded, functional MRI study of high- and low-fat meals on 2 measurement days. Cerebral blood flow (CBF) was measured before and 30 and 120 min after intake of high- and low-fat yogurts. Hunger was rated and blood samples were taken before each CBF measurement. RESULTS High-fat yogurt induced a pronounced decrease in CBF in the hypothalamus, and the corresponding CBF change correlated positively with the insulin change. Furthermore, insular activity increased after 120 min in the low-fat condition only. The CBF change in both regions correlated positively in the high-fat condition. CONCLUSIONS The decrease in hypothalamic activity and the interaction with the insular cortex elicited by fat may contribute to an efficient energy homeostasis. Therefore, fat might be a modulator of homeostatic and gustatory brain regions and their interaction. This trial was registered at clinicaltrials.gov as NCT01516021.

Differential effects of intranasal insulin and caffeine on cerebral blood flow

Insulin is an important modulator of brain functions such as memory and appetite regulation. Besides the effect on neuronal activity, it is also possible that insulin has a direct vasodilatory effect on cerebral blood flow (CBF). We investigated the impact of increased insulin levels in the central nervous system on basal and task‐induced CBF as well as blood oxygenation level‐dependent (BOLD) response in the visual cortex using pulsed arterial spin‐labeling MRI. An intranasal insulin application was used to avoid peripheral hyperinsulinaemia, which would lead to a cascade of hormonal changes. In a control experiment, caffeine was applied due to its well‐known impact on the vasculature of the brain leading to a reliable reduction of CBF. Eight lean subjects were included in the study. On 2 separate days, intranasal human insulin or caffeine tablets were given to the subjects after fasting over night. On each day, basal CBF and task‐induced CBF were measured before and 30 min after application of insulin or caffeine in each subject. During the task condition, a flickering checkerboard was presented. Insulin had no effect on basal CBF and task‐induced CBF in comparison with drug‐free baseline measurement in the visual cortex and control regions. After caffeine application, however, there was a significant decrease of CBF during stimulation in the visual cortex. The BOLD response was not altered by insulin or caffeine between pre‐ and postdose measurements. In conclusion, we found no evidence for a direct vasodilatory effect of intranasal insulin on the cerebral vascular system in this study. Hum Brain Mapp, 2012.

Regional effects of magnetization dispersion on quantitative perfusion imaging for pulsed and continuous arterial spin labeling

Most experiments assume a global transit delay time with blood flowing from the tagging region to the imaging slice in plug flow without any dispersion of the magnetization. However, because of cardiac pulsation, nonuniform cross‐sectional flow profile, and complex vessel networks, the transit delay time is not a single value but follows a distribution. In this study, we explored the regional effects of magnetization dispersion on quantitative perfusion imaging for varying transit times within a very large interval from the direct comparison of pulsed, pseudo‐continuous, and dual‐coil continuous arterial spin labeling encoding schemes. Longer distances between tagging and imaging region typically used for continuous tagging schemes enhance the regional bias on the quantitative cerebral blood flow measurement causing an underestimation up to 37% when plug flow is assumed as in the standard model. Magn Reson Med, 2013.

Spectral Envelope Analysis for Simple Snorers and Obstructive Sleep Apnea Patients

In recent years, several studies have shown the relationship between snoring and obstructive sleep apnea syndrome (OSAS). Instead of time domain analysis of snoring signal, the spectral features and shapes of snores have been found different in simple snorers and OSAS patients. In this study, we propose a method to differentiate simple snorers and OSAS patients based on spectral envelope estimation of snoring signals. Formant frequencies and bandwidths are computed for both groups and the variation and consistency of the formant distributions are determined. A total of 1400 snoring episodes from 7 simple snorer and 7 OSAS patients were analyzed. Significant differences are found in the variation of formant frequencies of both groups. The results are interpreted from a pathophysiological view.

Analysing Snoring Sounds For Obstructive Sleep Apnea (OSA) Patients

Several studies have done in order to determine the relationship between snoring and obstructive sleep apnea syndrome (OSAS). One of the common problem that is faced during the medical treatment of the apnea is the undetermination of the efficiency of the applied treatment in terms of objective criteria. It is needed to automatically detect each snoring episode in order to estimate the spectral features and determine the snoring sound intensity. In this study, an automatic detection system of acoustic snoring signals has been designed, to work with long duration respiratory sound recordings. The system was designed to select snoring episodes from simple snorers and OSAS patients and to reject the undesired waveforms. The sound recordings were taken from patients that are suspected of OSAS pathology while they were connected to the polysomnography in Gulhane Military Medical Academy (GMMA) Sleep Studies Laboratory. In order to validate the system, 500 snores were analysed taken from 30 patients with different apnea/hypopnea index (AHI) . Results were compared with manual annotations done by a medical doctor and the average sensitivity of the system is determined as 86%