Sampling Rate Effects on Resting State fMRI Metrics

Low image sampling rates used in resting state functional magnetic resonance imaging (rs-fMRI) may cause aliasing of the cardiorespiratory pulsations over the very low frequency (VLF) BOLD signal fluctuations which reflects to functional connectivity (FC). In this study, we examine the effect of sampling rate on currently used rs-fMRI FC metrics. Ultra-fast fMRI magnetic resonance encephalography (MREG) data, sampled with TR 0.1 s, was downsampled to different subsampled repetition times (sTR, range 0.3–3 s) for comparisons. Echo planar k-space sampling (TR 2.15 s) and interleaved slice collection schemes were also compared against the 3D single shot trajectory at 2.2 s sTR. The quantified connectivity metrics included stationary spatial, time, and frequency domains, as well as dynamic analyses. Time domain methods included analyses of seed-based functional connectivity, regional homogeneity (ReHo), coefficient of variation, and spatial domain group level probabilistic independent component analysis (ICA). In frequency domain analyses, we examined fractional and amplitude of low frequency fluctuations. Aliasing effects were spatially and spectrally analyzed by comparing VLF (0.01–0.1 Hz), respiratory (0.12–0.35 Hz) and cardiac power (0.9–1.3 Hz) FFT maps at different sTRs. Quasi-periodic pattern (QPP) of VLF events were analyzed for effects on dynamic FC methods. The results in conventional time and spatial domain analyses remained virtually unchanged by the different sampling rates. In frequency domain, the aliasing occurred mainly in higher sTR (1–2 s) where cardiac power aliases over respiratory power. The VLF power maps suffered minimally from increasing sTRs. Interleaved data reconstruction induced lower ReHo compared to 3D sampling (p < 0.001). Gradient recalled echo-planar imaging (EPI BOLD) data produced both better and worse metrics. In QPP analyses, the repeatability of the VLF pulse detection becomes linearly reduced with increasing sTR. In conclusion, the conventional resting state metrics (e.g., FC, ICA) were not markedly affected by different TRs (0.1–3 s). However, cardiorespiratory signals showed strongest aliasing in central brain regions in sTR 1–2 s. Pulsatile QPP and other dynamic analyses benefit linearly from short TR scanning

Sydämen autonominen säätely kylmässä

Opinnäytetyö oli osa Kohonnut verenpaine ja kylmä -tutkimushanketta, jossa tutkittiin verenpaineen säätelyä kylmässä hypertensiivisilla (kohonnut verenpaine) ja normotensiivisilla (normaali verenpaine) miehillä. Sykevaihtelu (HRV) ja barorefleksiherkkyys (BRS) ovat tärkeitä tekijöitä sydämen autonomisessa säätelyssä. Tiedetään, että kylmäaltistus nostaa verenpainetta ja laskee sykettä, mutta sen yhteyttä sykevaihteluun ja barorefleksiin ei tunneta tarkasti. Opinnäytetyön tavoitteena oli analysoida sykevaihtelun ja barorefleksiherkkyyden muutoksia kylmässä ja siitä palautumisen aikana normotensiivisilla tutkittavilla. Kylmäaltistusmittaukset tehtiin 51 hypertensiiviselle ja 32 normotensiiviselle tutkittavalle. Sykevaihtelun ja barorefleksiherkkyyden osalta mitattiin jatkuvana signaalina EKG, verenpaine sormesta ja hengitys rintakehän laajenemista mittaavalla pietsosähköisellä anturilla. Näistä aikasarjoista analysoitiin aika- ja taajuuskenttämenetelmin syke- ja verenpainevaihtelu sekä barorefleksiherkkyys perustasossa, kylmäaltistuksessa sekä siitä palautumisen aikana. Lopulliset analyysit tehtiin 16 tutkittavan aineistolle. Matalataajuinen (LF) sykevaihtelu kasvoi kylmäaltistuksen ja aikana ja pysyi koholla koko palautumisen ajan. Korkeataajuinen (HF) sykevaihtelu kasvoi ja pysyi merkitsevästi koholla palautumisen keskivaiheille asti. Barorefleksiherkkyys ei muuttunut merkitsevästi kylmässä, mikä todetaan seuraavista tuloksista: matalataajuinen barorefleksiherkkyys (p=0,083), korkeataajuinen barorefleksiherkkyys (p=0,163) ja keskiarvoinen barorefleksiherkkyys (p=0,086). Matalataajuinen barorefleksiherkkyys kuitenkin kasvoi kylmäaltistuksen aikana ja jäi hieman koholle palautumisen ajaksi. Systolinen verenpaine (yläpaine) kasvoi kylmäaltistuksen aikana lähes 26 mmHg ja diastolinen verenpaine (alapaine) lähes 11 mmHg ja jäi merkitsevästi koholle palautumisen 15 minuuttiin asti. Sykevaihtelu lisääntyi eli parasympaattisen hermoston aktiivisuus kasvoi kylmässä. Syke laski, mikä johtui osittain verenpaineen noususta ja kertoo barorefleksin aktiivisuudesta kylmäaltistuksen aikana. Terveillä henkilöillä barorefleksiherkkyys ei kylmän vuoksi heikentynyt, mikä kertoo sydämen säätelyjärjestelmän toimivan oikein kylmässä.The thesis is a part of Hypertension in Cold -project that was created for studying the effects of cold exposure on the regulation of blood pressure of hypertensive (high blood pressure) and normotensive (normal blood pressure) men. Heart rate variabilility (HRV) and baroreflex are important factors in autonomic regulation of the heart. Baroreflex sensitivity (BRS) is a dynamic gain, reflecting parallel fluctuations of arterial pressure and heart rate. Reflections of cold exposure to HRV and BRS are not known well enough. The aim of the Thesis was to analyze autonomic regulation of the heart especially focusing on HRV and BRS. The laboratory measurements included 56 hypertensive and 32 normotensive volunteers. For HRV and BRS it was needed to measure continuing ECG, blood pressure from finger and respiration using a respiratory belt. HRV and blood pressure variability were analyzed during the baseline, cold exposure and recovery. The final analysis of data was carried out for 16 cases. The low-frequency (LF) component of the HRV increased significantly (from 4,8 ± 1,2 to 5,9 ± 1,1, p=0,007) during cold exposure and remained higher to the end of the recovery (5,3 ± 1,2, p=0,009 and 5,6 ± 1,4, p=0,020). The high-frequency (HF) component of the HRV increased (from 3,3 ± 1,1 to 4,8 ± 1,3, p=0,001) remained (4,8 ± 1,3, p=0,023) part of the recovery. There were no significant effect of cold exposure or recovery on BRS (low-frequency BRS, p=0,083; high-frequency BRS, p=0,163; mean BRS p=0,086). Systolic blood pressure (SBP) increased 26 mmHg (p=0,000) and diastolic blood pressure by 11 mmHg (p=0,000), which describes that cold exposure rises blood pressure. HRV increased which repreresents higher activity of parasympathetic nervous system during cold exposure. A higher parasympathetic activity was also detected because of increasing blood pressure and decreasing heart rate, which describes the acitivity of the baroreflex. No baroreflex failure was observed in results which describes that the regulation of heart behaves properly with healthy volunteers during exposure to cold

Inverse correlation of fluctuations of cerebral blood and water concentrations in humans

Abstract Near-infrared spectroscopy (fNIRS) measures concentrations of oxygenated (HbO) and deoxygenated (HbR) hemoglobin in the brain. Recently, we demonstrated its potential also for measuring concentrations of cerebral water (cH₂O). We performed fNIRS measurements during rest to study fluctuations in concentrations of cH₂O, HbO and HbR in 33 well-rested healthy control subjects (HC) and 18 acutely sleep-deprived HC. Resting-state fNIRS signal was filtered in full-band, cardiac, respiratory, low-, and very-low-frequency bands. The sum of HbO and HbR constitutes the regional cerebral blood volume (CBV). CBV and cH₂O concentrations were analyzed via temporal correlation and phase synchrony. Fluctuation in concentrations of cH₂O and CBV was strongly anti-correlated across all frequency bands in both frontal and parietal cortices. Fluctuation in concentrations of cH₂O and CBV showed neither a completely synchronous nor a random phase relationship in both frontal and parietal cortices. Acutely sleep-deprived subjects did not show significant differences in temporal correlation or phase synchrony between fluctuations in cH₂O and CBV concentrations compared with well-rested HC. The reciprocal interrelation between fluctuations in CBV and cH₂O concentrations is consistent with the Munro–Kellie doctrine of constant intracranial volume. This coupling may constitute a functional mechanism underlying glymphatic circulation, which persists despite acutely disturbed sleep patterns

Infra-slow fluctuations in cortical potentials and respiration drive fast cortical EEG rhythms in sleeping and waking states

Objective: Infra-slow fluctuations (ISF, 0.008–0.1 Hz) characterize hemodynamic and electric potential signals of human brain. ISFs correlate with the amplitude dynamics of fast (>1 Hz) neuronal oscillations, and may arise from permeability fluctuations of the blood–brain barrier (BBB). It is unclear if physiological rhythms like respiration drive or track fast cortical oscillations, and the role of sleep in this coupling is unknown. Methods: We used high-density full-band electroencephalography (EEG) in healthy human volunteers (N = 21) to measure concurrently the ISFs, respiratory pulsations, and fast neuronal oscillations during periods of wakefulness and sleep, and to assess the strength and direction of their phase-amplitude coupling. Results: The phases of ISFs and respiration were both coupled with the amplitude of fast neuronal oscillations, with stronger ISF coupling being evident during sleep. Phases of ISF and respiration drove the amplitude dynamics of fast oscillations in sleeping and waking states, with different contributions. Conclusions: ISFs in slow cortical potentials and respiration together significantly determine the dynamics of fast cortical oscillations. Significance: We propose that these slow physiological phases play a significant role in coordinating cortical excitability, which is a fundamental aspect of brain function.Peer reviewe

Synchronous functional magnetic resonance eye imaging, video ophthalmoscopy, and eye surface imaging reveal the human brain and eye pulsation mechanisms

Abstract The eye possesses a paravascular solute transport pathway that is driven by physiological pulsations, resembling the brain glymphatic pathway. We developed synchronous multimodal imaging tools aimed at measuring the driving pulsations of the human eye, using an eye-tracking functional eye camera (FEC) compatible with magnetic resonance imaging (MRI) for measuring eye surface pulsations. Special optics enabled integration of the FEC with MRI-compatible video ophthalmoscopy (MRcVO) for simultaneous retinal imaging along with functional eye MRI imaging (fMREye) of the BOLD (blood oxygen level dependent) contrast. Upon optimizing the fMREye parameters, we measured the power of the physiological (vasomotor, respiratory, and cardiac) eye and brain pulsations by fast Fourier transform (FFT) power analysis. The human eye pulsated in all three physiological pulse bands, most prominently in the respiratory band. The FFT power means of physiological pulsation for two adjacent slices was significantly higher than in one-slice scans (RESP1 vs. RESP2; df = 5, p = 0.045). FEC and MRcVO confirmed the respiratory pulsations at the eye surface and retina. We conclude that in addition to the known cardiovascular pulsation, the human eye also has respiratory and vasomotor pulsation mechanisms, which are now amenable to study using non-invasive multimodal imaging of eye fluidics

The variability of functional MRI brain signal increases in Alzheimer's disease at cardiorespiratory frequencies

Biomarkers sensitive to prodromal or early pathophysiological changes in Alzheimer's disease (AD) symptoms could improve disease detection and enable timely interventions. Changes in brain hemodynamics may be associated with the main clinical AD symptoms. To test this possibility, we measured the variability of blood oxygen level-dependent (BOLD) signal in individuals from three independent datasets (totaling 80 AD patients and 90 controls). We detected a replicable increase in brain BOLD signal variability in the AD populations, which constituted a robust biomarker for clearly differentiating AD cases from controls. Fast BOLD scans showed that the elevated BOLD signal variability in AD arises mainly from cardiovascular brain pulsations. Manifesting in abnormal cerebral perfusion and cerebrospinal fluid convection, present observation presents a mechanism explaining earlier observations of impaired glymphatic clearance associated with AD in humans.Peer reviewe

Cardiovascular pulsatility increases in visual cortex before blood oxygen level dependent response during stimulus

Abstract The physiological pulsations that drive tissue fluid homeostasis are not well characterized during brain activation. Therefore, we used fast magnetic resonance encephalography (MREG) fMRI to measure full band (0–5 Hz) blood oxygen level-dependent (BOLDFB) signals during a dynamic visual task in 23 subjects. This revealed brain activity in the very low frequency (BOLDVLF) as well as in cardiac and respiratory bands. The cardiovascular hemodynamic envelope (CHe) signal correlated significantly with the visual BOLDVLF response, considered as an independent signal source in the V1-V2 visual cortices. The CHe preceded the canonical BOLDVLF response by an average of 1.3 (± 2.2) s. Physiologically, the observed CHe signal could mark increased regional cardiovascular pulsatility following vasodilation