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Dinâmica da alteração perfusional induzida por estado de apnéia utilizando fMRI; Dynamic of brain perfusion changes induced by breath-holding fMRI.

Andrade, Katia Cristine
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 30/05/2006 Português
Relevância na Pesquisa
27.35%
O mecanismo de contraste mais utilizado em imagens funcionais por ressonância magnética (functional Magnetic Resonance Imaging, fMRI), também conhecido por sinal BOLD (Blood Oxygenation Level Dependent) mede indiretamente a atividade neural, sendo sensível a mudanças no fluxo cerebral sangüíneo (Cerebral Blood Flow, CBF), na taxa cerebral metabólica do oxigênio (Cerebral Metabolic Rate of Oxygen, CMRO2) e no volume cerebral sanguíneo (Cerebral Blood Volume, CBV) e, em princípio, ele pode ser utilizado para mapear perfusão cerebral. Desse modo, o objetivo principal deste trabalho foi investigar, quantitativamente, alterações perfusionais no cérebro humano mapeadas pelas mudanças do sinal BOLD em resposta à indução transitória do estado de apnéia. Para isso, imagens por ressonância magnética foram obtidas através de um scanner de 1.5 T Siemens (Magneton Vision) com seqüências do tipo EPI-BOLD. Nesta pesquisa, foi analisada a influência da duração da apnéia no sinal BOLD. Observou-se, também, a diferença ocasionada no sinal em duas situações: apnéia iniciando-se após a inspiração ou após a expiração. Além disso, foi estudada a propagação deste sinal BOLD pelas diferentes regiões cerebrais. Por último...

Ressonância magnética funcional: mapeamento do córtex motor através da técnica BOLD

Nunes, Patrícia Ribeiro
Fonte: Escola Superior de Tecnologia da Saúde de Lisboa Publicador: Escola Superior de Tecnologia da Saúde de Lisboa
Tipo: Dissertação de Mestrado
Publicado em //2012 Português
Relevância na Pesquisa
37.08%
A ressonância magnética funcional (RMf) é hoje uma ferramenta fundamental na investigação funcional do cérebro humano, quer em indivíduos saudáveis quer em pacientes com patologias diversas. É uma técnica complexa que necessita de uma aplicação cuidada e rigorosa, e uma compreensão dos mecanismos biofísicos a ela subjacentes, de modo a serem obtidos resultados fiáveis e com melhor aceitação clínica. O efeito BOLD (Blood Oxygenation Level Dependent) é o método mais utilizado para medir e estudar a actividade cerebral e baseia-se nas alterações das propriedades magnéticas da molécula hemoglobina. Com este Projecto propomo-nos optimizar um protocolo de RMf realizada com o efeito BOLD, em voluntários saudáveis, de modo a que este possa ser aplicado em futuros estudos de pacientes com patologias. ABSTRACT - Nowadays functional magnetic resonance imaging (fMRI) is a fundamental tool for the research of human brain function of healthy subjects or patients with several pathologies. It is a complex technique that requires a careful and rigorous application, and an understanding of its biophysical mechanisms, so that reliable results can be obtained with better clinical acceptance. The BOLD effect (Blood Oxygenation Level Dependent) is the most widely used method to measure and study the brain activity and its based on changes in magnetic properties of the hemoglobin molecule. The aim of this project was to optimize a BOLD fMRI protocol on healthy subjects...

Ressonância magnética funcional: mapeamento do córtex motor através do efeito BOLD

Nunes, Patrícia R.; Tecelão, Sandra R.; Nunes, Rita G.
Fonte: Escola Superior de Tecnologia da Saúde de Lisboa Publicador: Escola Superior de Tecnologia da Saúde de Lisboa
Tipo: Artigo de Revista Científica
Publicado em /06/2014 Português
Relevância na Pesquisa
37.3%
Introdução – A ressonância magnética funcional (RMf) é hoje uma ferramenta fundamental na investigação funcional do cérebro humano, quer em indivíduos saudáveis quer em doentes com patologias diversas. É uma técnica complexa que necessita de uma aplicação cuidada e rigorosa e uma compreensão dos mecanismos biofísicos, de modo a serem obtidos resultados fiáveis e com melhor aceitação clínica. O efeito BOLD (Blood Oxygenation Level Dependent), que se baseia nas propriedades magnéticas da hemoglobina, é o método mais utilizado para medir a atividade cerebral por RMf. Objetivos – Otimizar um protocolo de RMf por efeito BOLD em voluntários saudáveis para mapeamento do córtex motor, de modo a que possa ser aplicado no futuro em doentes com patologias diversas. Metodologia – Foram estudados 34 voluntários saudáveis divididos em 2 grupos de estudo: BOLD 1 e BOLD 2. Com vista à otimização, foram testados no subgrupo BOLD 1 diferentes paradigmas e no subgrupo BOLD 2 foi estudada a influência do tempo de eco (TE). Para as várias condições foram comparados os volumes da região ativada e os níveis de ativação obtidos. Resultados/Discussão – O córtex motor foi identificado em todos os voluntários estudados. Não foram detetadas diferenças estatisticamente significativas quando comparados os resultados obtidos com os diferentes parâmetros de aquisição. Conclusão – O protocolo foi otimizado tendo em conta o nível de conforto reportado pelos voluntários. Uma vez que se pretende aplicar este mesmo protocolo no estudo de doentes...

Epileptogenic focus localization and complexity analysis of its BOLD signal

Tavares, Vânia Sofia Santos
Fonte: Universidade de Lisboa Publicador: Universidade de Lisboa
Tipo: Dissertação de Mestrado
Publicado em //2014 Português
Relevância na Pesquisa
27.36%
Tese de mestrado integrado em Engenharia Biomédica e Biofísica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2014; Epilepsia é considerada a mais importante doença neurológica crónica a nível mundial. Esta afeta mais de 50 milhões de pessoas de todas as idades, e dessa população apenas 70% dos casos são controláveis com fármacos anti-epiléticos. Dos restantes 30%, 10% beneficiam da ressecação cirúrgica da região responsável pela atividade epilética e os restantes 20% não conseguem controlar adequadamente as suas crises. De entre as razões que justificam o baixo impacto da cirurgia encontra-se o facto de se desconhecer, na maioria dos casos, o foco desta atividade elétrica anormal. Por isso, a deteção deste foco é importante tanto para o diagnóstico como para o controlo das crises. O foco epiletogénico é um conceito teórico, consistindo na descreve a região cerebral que é necessário remover para deixar o doente livre de crises. Este é caracterizado por dois tipos de atividade epiléptica: a ictal e a interictal. A primeira diz respeito à atividade elétrica gerada durante as crises epiléticas e a segunda à atividade gerada entre as crises. A primeira é caracterizada uma intensa descarga elétrica que pode ter uma duração até alguns minutos. Já a segunda forma de atividade epiletogénica é...

Dynamic functional connectivity of BOLD fMRI signal during both rest and task execution states

Brito, Joana Paula Fontinha de
Fonte: Universidade de Lisboa Publicador: Universidade de Lisboa
Tipo: Dissertação de Mestrado
Publicado em //2014 Português
Relevância na Pesquisa
36.78%
Tese de mestrado integrado em Engenharia Biomédica e Biofísica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2014; A conectividade cerebral é um tema muito atual na área das neurociências. A dinâmica a conectividade tem sido um tema muito explorado ultimamente com o objetivo de se descobrir mais sobre os processos cerebrais relacionados com sinais neuronais em bandas restritas de frequência e ainda compreender as diferenças entre repouso e execução de tarefas. O objetivo principal do presente trabalho consistiu no desenvolvimento de um procedimento para o estudo da conectividade dinâmica baseado na análise da coerência através da Transformada "Wavelet" que proporciona especificidade no tempo e na frequência. A abordagem implementada baseou-se em duas hipóteses diferentes de comunicação neuronal. A primeira considera que dois sinais neuronais oscilatórios comunicam durante períodos de coerência de magnitude elevada e a segunda considera que a comunicação neuronal ocorre em períodos de acoplamento de fase. O uso das duas hipóteses permitiu obter, respectivamente, dois perfis de comunicação neuronal. Uma vez que, as distribuições nulas dos perfis de coerência de magnitude e de acoplamento de fase são desconhecidas...

Processamento de imagens BOLD de ressonância magnética do músculo

Pereira, Ana Rita Lindo dos Santos Justo
Fonte: Faculdade de Ciências e Tecnologia Publicador: Faculdade de Ciências e Tecnologia
Tipo: Dissertação de Mestrado
Publicado em //2012 Português
Relevância na Pesquisa
37.13%
Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção de Grau de Mestre em Engenharia Biomédica; A presente dissertação foi desenvolvida com a colaboração da Faculdade de Motricidade Humana (FMH/UTL) e a Clínica de Ressonância Magnética de Caselas.; A presente dissertação de mestrado pretende avaliar as alterações fisiológicas de vários músculos da perna quando submetidos a exercício físico intenso, através da técnica de ressonância magnética funcional BOLD (Blood Oxygen Level Dependent). As diferentes propriedades magnéticas da oxi-hemoglobina (diamagnética) e da desoxihemoglobina(paramagnética), associadas aos factores hemodinâmicos, geram contraste nas imagens de ressonância magnética, durante a activação muscular, possibilitando a visualização dos músculos recrutados com a realização do exercício. Sete jovens atletas saudáveis participaram neste estudo. As imagens BOLD foram adquiridas antes e imediatamente após o exercício-físico. O paradigma de exercício usado foi o salto unipedal, executado em posição ortostática, permitindo atingir a fadiga muscular em poucos minutos. A análise das imagens dos músculos Tibial Anterior...

Processamento de imagens BOLD de Ressonância Magnética do músculo através da técnica ICA

Gomes, Maria Jakelyn Dias de Faria
Fonte: Faculdade de Ciências e Tecnologia Publicador: Faculdade de Ciências e Tecnologia
Tipo: Dissertação de Mestrado
Publicado em //2013 Português
Relevância na Pesquisa
37.01%
Dissertação para obtenção do Grau de Mestre em Engenharia Biomédica; A presente dissertação de mestrado, tem como objectivo o processamento de imagens Blood Oxygen Level Dependent (BOLD), dos músculos da perna, quando submetidos a exercício intenso até ser atingida a fadiga muscular, recorrendo à técnica de processamento Independent Component Analysis (ICA). A ICA é um método estatístico utilizado para descobrir factores escondidos, procura decompor os dados em mapas espaciais independentes e em gráficos da variação no tempo, nos quais idealmente cada um representaria diferentes artefactos ou padrões de activação. Esta técnica foi aplicada com carácter exploratório no presente estudo, pretendendo-se encontrar dados relevantes, que de outro modo não seriam possíveis. As imagens BOLD utilizadas neste estudo foram adquiridas durante a realização de uma dissertação anterior a esta, realizada por Ana Rita Pereira com o tema: "Processamento de Imagens BOLD de Ressonância Magnética do Músculo". O paradigma utilizado foi o salto unipedal, executado em posição ortostática, permitindo atingir a fadiga muscular em poucos minutos. O pré-processamento das imagens foi feito com o software OsiriX e FMRIB Software Library (FSL) e o processamento foi feito com o software FSL. Os resultados permitiram verificar que a análise efectuada não permitiu separar cada componente em diferentes mapas espaciais. Verificou-se que as variáveis (músculos e fluxo sanguíneo nos vasos sanguíneos) não são independentes. Desta forma...

Quantitative aspects of brain perfusion dynamic induced by BOLD fMRI

Andrade,Katia C.; Pontes-Neto,Octavio M.; Leite,Joao P.; Santos,Antonio Carlos; Baffa,Oswaldo; Araujo,Draulio B. de
Fonte: Academia Brasileira de Neurologia - ABNEURO Publicador: Academia Brasileira de Neurologia - ABNEURO
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2006 Português
Relevância na Pesquisa
36.92%
The increase of relative cerebral blood flow (rCBF) may contribute for a change in blood oxygenation level dependent signal (BOLD). The main purpose of this study is to investigate some aspects of perfusional alterations in the human brain in response to a uniform stimulation: hypercapnia induced by breath holding. It was observed that the BOLD signal increased globally during hypercapnia and that it is correlated with the time of breath holding. This signal increase shows a clear distinction between gray and white matter, being greater in the grey matter.

Negative BOLD fMRI Response in the Visual Cortex Carries Precise Stimulus-Specific Information

Bressler, David; Spotswood, Nicole; Whitney, David
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 02/05/2007 Português
Relevância na Pesquisa
27.29%
Sustained positive BOLD (blood oxygen level-dependent) activity is employed extensively in functional magnetic resonance imaging (fMRI) studies as evidence for task or stimulus-specific neural responses. However, the presence of sustained negative BOLD activity (i.e., sustained responses that are lower than the fixation baseline) has remained more difficult to interpret. Some studies suggest that it results from local “blood stealing” wherein blood is diverted to neurally active regions without a concomitant change of neural activity in the negative BOLD regions. However, other evidence suggests that negative BOLD is a result of local neural suppression. In both cases, regions of negative BOLD response are usually interpreted as carrying relatively little, if any, stimulus-specific information (hence the predominant reliance on positive BOLD activity in fMRI). Here we show that the negative BOLD response resulting from visual stimulation can carry high information content that is stimulus-specific. Using a general linear model (GLM), we contrasted standard flickering stimuli to a fixation baseline and found regions of the visual cortex that displayed a sustained negative BOLD response, consistent with several previous studies. Within these negative BOLD regions...

Improved spatial localization of post-stimulus BOLD undershoot relative to positive BOLD

Zhao, Fuqiang; Jin, Tao; Wang, Ping; Kim, Seong-Gi
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.23%
The negative blood oxygenation level dependent (BOLD) signal following the cessation of stimulation (post-stimulus BOLD undershoot) is observed in functional magnetic resonance imaging (fMRI) studies. However, its spatial characteristics are unknown. To investigate this, gradient-echo BOLD fMRI in response to visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T. Since the middle cortical layer (layer 4) is known to have the highest metabolic and cerebral blood volume (CBV) responses, images were obtained to view the cortical cross-section. Robust post-stimulus BOLD undershoot was observed in all studies, and lasted longer than 30 sec after the cessation of 40-60 sec stimulation. The magnitude of post-stimulus BOLD undershoot was linearly dependent on echo time with little intercept when extrapolating to TE = 0, indicating the T2* change is the major cause of the BOLD undershoot. The post-stimulus BOLD undershoot was observed within the cortex and near the surface of the cortex, while the prolonged CBV elevation was observed only at the middle of the cortex. Within the cortex, the largest post-stimulus undershoot was detected at the middle of the cortex, similar to the CBV increase during the stimulation period. Our findings demonstrate that...

Investigating the Source of Nonlinear BOLD Response to Ultra-short Visual Stimulus

Zhang, Nanyin; Zhu, Xiao-Hong; Chen, Wei
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.27%
Several studies have demonstrated significant nonlinearity in the blood-oxygenation-level-dependent (BOLD) signal. The source of BOLD nonlinearity observed in these studies can originate from vascular and/or neuronal responses (Birn and Bandettini, 2005). These nonlinear effects in the BOLD signal can be a potential problem in rapid event-related (ER) fMRI design. In attempt to examine and solve this problem, and also with the aim at understanding the nature of BOLD signal, it is essential to study the source of BOLD nonlinearity because the obscurity in this issue not only impedes accurate modeling of BOLD nonlinearity, but also limits generalization of the conclusions regarding BOLD nonlinearity. In the present study, we employed a paired-stimulus paradigm composed of two ultra-short visual stimuli separated by a variable inter-stimulus interval (ISI). ISIs were chosen (1, 2, 4, 6 and 8 seconds) to ensure neuronal activity is invariant to all stimuli. This is validated by a previous study demonstrating that the refractory period of visual response to ultra-short flashing stimulation is not longer than ~600–800ms (Zhang et al., 2007). Under this circumstance, we have found that the BOLD response to the second of paired stimuli is significantly reduced when ISI is relatively short...

Inter-subject Variability in Hypercapnic Normalization of the BOLD fMRI Response

Liau, Joy; Liu, Thomas T.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.31%
In the application of hypercapnic normalization to functional magnetic resonance imaging (fMRI) studies, the blood oxygenation level dependent (BOLD) response to a functional stimulus is typically divided by the BOLD response to a hypercapnic challenge. While some prior studies have shown that hypercapnic normalization can reduce inter-subject BOLD variability, other studies have found an increase in inter-subject variability. In this study we used measures of baseline cerebral blood flow (CBF) and the functional BOLD and CBF responses to both visual stimuli and hypercapnia to assess the effect of hypercapnic normalization on inter-subject variability. We found that the functional and hypercapnic BOLD and CBF responses all exhibited a significant inverse dependence on baseline CBF. In contrast, the maximum BOLD response was independent of baseline CBF and was not a major source of inter-subject BOLD variability. Division of the functional BOLD response by the hypercapnic BOLD response increased inter-subject variability in the normalized responses as compared to the original responses, reflecting the presence of a systematic bias term that was inversely dependent on the hypercapnic BOLD response. This systematic bias resulted from a positive intercept term in the linear relationship between the functional and hypercapnic BOLD responses. This positive intercept term reflected a steeper inverse dependence of the hypercapnic CBF response on baseline CBF...

Focal BOLD-fMRI changes in bicuculline-induced tonic-clonic seizures in the rat

DeSalvo, Matthew N.; Schridde, Ulrich; Mishra, Asht M.; Motelow, Joshua E.; Purcaro, Michael J.; Danielson, Nathan; Bai, Xiaoxiao; Hyder, Fahmeed; Blumenfeld, Hal
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.25%
Generalized tonic-clonic seizures cause widespread physiological changes throughout the cerebral cortex and subcortical structures in the brain. Using combined blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) at 9.4 T and electroencephalography (EEG) these changes can be characterized with high spatiotemporal resolution. We studied BOLD changes in anesthetized Wistar rats during bicuculline-induced tonic-clonic seizures. Bicuculline, a GABAA receptor antagonist, was injected systemically and seizure activity was observed on EEG as high amplitude, high-frequency polyspike discharges followed by clonic paroxysmal activity of lower frequency, with mean electrographic seizure duration of 349 s. Our aim was to characterize the spatial localization, direction, and timing of BOLD signal changes during the pre-ictal, ictal and post-ictal periods. Group analysis was performed across seizures using paired t-maps of BOLD signal superimposed on high resolution anatomical images. Regional analysis was then performed using volumes of interest to quantify BOLD timecourses. In the pre-ictal period we found focal BOLD increases in specific areas of somatosensory cortex (S1, S2) and thalamus several seconds before seizure onset. During seizures we observed BOLD increases in cortex...

Retinotopic mapping with Spin Echo BOLD at 7 Tesla

Olman, Cheryl A.; Van de Moortele, Pierre-Francois; Schumacher, Jennifer F.; Guy, Joe; Uğurbil, Kâmil; Yacoub, Essa
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.23%
For blood oxygenation level-dependent (BOLD) functional MRI experiments, contrast-to-noise ratio (CNR) increases with increasing field strength for both gradient echo (GE) and spin echo (SE) BOLD techniques. However, susceptibility artifacts and non-uniform coil sensitivity profiles complicate large field-of-view fMRI experiments (e.g., experiments covering multiple visual areas instead of focusing on a single cortical region). Here, we use SE BOLD to acquire retinotopic mapping data in early visual areas, testing the feasibility of SE BOLD experiments spanning multiple cortical areas at 7 Tesla. We also use a recently developed method for normalizing signal intensity in T1-weighted anatomical images to enable automated segmentation of the cortical gray matter for scans acquired at 7T with either surface or volume coils. We find that the CNR of the 7T GE data (average single-voxel, single-scan stimulus coherence: 0.41) is almost twice that of the 3T GE BOLD data (average coherence: 0.25), with the CNR of the SE BOLD data (average coherence: 0.23) comparable to that of the 3T GE data. Repeated measurements in individual subjects find that maps acquired with 1.8 mm resolution at 3T and 7T with GE BOLD and at 7T with SE BOLD show no systematic differences in either the area or the boundary locations for V1...

Contrast Response Functions for Single Gabor Patches: ROI-Based Analysis Over-Represents Low-Contrast Patches for GE BOLD

Schumacher, Jennifer F.; Thompson, Serena K.; Olman, Cheryl A.
Fonte: Frontiers Research Foundation Publicador: Frontiers Research Foundation
Tipo: Artigo de Revista Científica
Publicado em 24/03/2011 Português
Relevância na Pesquisa
27.27%
Important for the interpretation of BOLD fMRI data is a linear relationship between the BOLD response and the underlying neural activity: increased BOLD responses should reflect proportionate increases in the underlying neural activity. While previous studies have demonstrated a linear relationship between the peak amplitude of the BOLD response and neural activity in primary visual cortex (V1), these studies have used stimuli that excite large areas of cortex, and the linearity of the BOLD response has not been demonstrated when only a small patch of cortex is stimulated. The BOLD response to isolated Gabor patches of increasing contrast was measured with gradient echo (GE) BOLD and spin echo (SE) BOLD at 7 T. Our primary finding is notable spatial heterogeneity of the BOLD contrast response, particularly for the GE BOLD data, resulting in a more reliably linear relationship between BOLD data and estimated neural responses in the center of the cortical representations of the individual Gabor patches than near the edges. A control experiment with larger sinusoidal grating patches confirms that the observed sensitivity to voxel selection in the regions of interest-based analysis is unique to the small stimuli.

Anatomical and functional assemblies of brain BOLD oscillations

Baria, Alexis T.; Baliki, Marwan N.; Parrish, Todd; Apkarian, A. Vania
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 25/05/2011 Português
Relevância na Pesquisa
27.23%
Brain oscillatory activity has long been thought to have spatial properties, the details of which are unresolved. Here we examine spatial organizational rules for the human brain oscillatory activity as measured by blood oxygen level-dependent (BOLD). Resting state BOLD signal was transformed into frequency space (Welch’s method), averaged across subjects, and its spatial distribution studied as a function of four frequency bands, spanning the full bandwidth of BOLD. The brain showed anatomically constrained distribution of power for each frequency band. This result was replicated on a repository dataset of 195 subjects. Next, we examined larger-scale organization by parceling the neocortex into regions approximating Brodmann Areas (BAs). This indicated that BAs of simple function/connectivity (unimodal), vs. complex properties (transmodal), are dominated by low frequency BOLD oscillations, and within the visual ventral stream we observe a graded shift of power to higher frequency bands for BAs further removed from the primary visual cortex (increased complexity), linking frequency properties of BOLD to hodology. Additionally, BOLD oscillation properties for the default mode network demonstrated that it is composed of distinct frequency dependent regions. When the same analysis was performed on a visual-motor task...

Linear coupling of undershoot with BOLD response in ER-fMRI and nonlinear BOLD response in rapid-presentation ER-fMRI

Zong, Xiaopeng; Huang, Jie
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.3%
In event-related (ER) BOLD-fMRI brain activation studies, understanding the relationship between the elicited BOLD signal and its underlying neuronal activity is essential for any quantitative interpretation of the neural events from the BOLD measurements. This requires a better understanding of the dynamic BOLD response. Besides the neuronal activity-induced positive BOLD response, the dynamic response is also characterized by a profound post-stimulus undershoot. The relationship between the positive response and the post-stimulus undershoot, however, remains poorly understood. Earlier studies using block-design paradigms with long stimulation durations (>10 s) do not suggest a quantitative relationship. Using an ER paradigm, this study revealed a linear coupling between the positive BOLD response and the post-stimulus undershoot across the human visual cortex. The voxelwise linear coupling across the visual cortex strongly supports a homogeneous hemodynamic response in ER paradigms, though the BOLD response magnitude varies substantially over a wide range across the visual cortex. Although underlying neuronal activity is responsible for a BOLD response, the blood volume fraction affects the magnitude of the BOLD response; the larger the blood volume fraction...

Differentiating BOLD and Non-BOLD Signals in fMRI Time Series Using Multi-Echo EPI

Kundu, Prantik; Inati, Souheil J.; Evans, Jennifer W.; Luh, Wen-Ming; Bandettini, Peter A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.32%
A central challenge in the fMRI based study of functional connectivity is distinguishing neuronally related signal fluctuations from the effects of motion, physiology, and other nuisance sources. Conventional techniques for removing nuisance effects include modeling of noise time courses based on external measurements followed by temporal filtering. These techniques have limited effectiveness. Previous studies have shown using multi-echo fMRI that neuronally related fluctuations are Blood Oxygen Level Dependent (BOLD) signals that can be characterized in terms of changes in R2* and initial signal intensity (S0) based on the analysis of echo-time (TE) dependence. We hypothesized that if TE-dependence could be used to differentiate BOLD and non-BOLD signals, non-BOLD signal could be removed to denoise data without conventional noise modeling. To test this hypothesis, whole brain multi-echo data were acquired at 3 TEs and decomposed with Independent Components Analysis (ICA) after spatially concatenating data across space and TE. Components were analyzed for the degree to which their signal changes fit models for R2* and S0 change, and summary scores were developed to characterize each component as BOLD-like or not BOLD-like. These scores clearly differentiated BOLD-like “functional network” components from non BOLD-like components related to motion...

A BOLD Perspective on Age-Related Neurometabolic-Flow Coupling and Neural Efficiency Changes in Human Visual Cortex

Hutchison, Joanna Lynn; Shokri-Kojori, Ehsan; Lu, Hanzhang; Rypma, Bart
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 03/05/2013 Português
Relevância na Pesquisa
27.25%
Age-related performance declines in visual tasks have been attributed to reductions in processing efficiency. The neural basis of these declines has been explored by comparing the blood-oxygen-level-dependent (BOLD) index of neural activity in older and younger adults during visual task performance. However, neural activity is one of many factors that change with age and lead to BOLD signal differences. We investigated the origin of age-related BOLD changes by comparing blood flow and oxygen metabolic constituents of BOLD signal. Subjects periodically viewed flickering annuli and pressed a button when detecting luminance changes in a central fixation cross. Using magnetic resonance dual-echo arterial spin labeling and CO2 ingestion, we observed age-equivalent (i.e., similar in older and younger groups) fractional cerebral blood flow (ΔCBF) in the presence of age-related increases in fractional cerebral metabolic rate of oxygen (ΔCMRO2). Reductions in ΔCBF responsiveness to increased ΔCMRO2 in elderly led to paradoxical age-related BOLD decreases. Age-related ΔCBF/ΔCMRO2 ratio decreases were associated with reaction times, suggesting that age-related slowing resulted from less efficient neural activity. We hypothesized that reduced vascular responsiveness to neural metabolic demand would lead to a reduction in ΔCBF/ΔCMRO2. A simulation of BOLD relative to ΔCMRO2 for lower and higher neurometabolic-flow coupling ratios (approximating those for old and young...

Studying the Spatial Distribution of Physiological Effects on BOLD Signals Using Ultrafast fMRI

Tong, Yunjie; Frederick, Blaise deB.
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Português
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The blood-oxygen-level dependent (BOLD) signal in functional MRI (fMRI) reflects both neuronal activations and global physiological fluctuations. These physiological fluctuations can be attributed to physiological low frequency oscillations (pLFOs), respiration, and cardiac pulsation. With typical TR values, i.e., 2 s or longer, the high frequency physiological signals (i.e., from respiration and cardiac pulsation) are aliased into the low frequency band, making it hard to study the individual effect of these physiological processes on BOLD. Recently developed multiband EPI sequences, which offer full brain coverage with extremely short TR values (400 ms or less) allow these physiological signals to be spectrally separated. In this study, we applied multiband resting state scans on nine healthy participants with TR = 0.4 s. The spatial distribution of each physiological process on BOLD fMRI was explored using their spectral features and independent component analysis (ICA). We found that the spatial distributions of different physiological processes are distinct. First, cardiac pulsation affects mostly the base of the brain, where high density of arteries exists. Second, respiration affects prefrontal and occipital areas, suggesting the motion associated with breathing might contribute to the noise. Finally...