The physics of functional magnetic resonance imaging (fMRI)

Buxton, Richard B.

doi:10.1088/0034-4885/76/9/096601

Cited by 206 publications

(169 citation statements)

References 221 publications

(306 reference statements)

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“…The BOLD phenomenon is a highly complex process (Buxton, 2013). Fluctuations in neural activity produce dynamic changes in oxygen metabolism, blood flow, and blood volume which, in turn, determine the quantity and distribution of deoxyhemoglobin in an imaging voxel at a given moment in time.…”

Section: Theorymentioning

confidence: 99%

Understanding the dynamic relationship between cerebral blood flow and the BOLD signal: Implications for quantitative functional MRI

Simon

Buxton

2015

NeuroImage

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Calibrated BOLD imaging, in which traditional measurements of the BOLD signal are combined with measurements of cerebral blood flow (CBF) within a BOLD biophysical model to estimate changes in oxygen metabolism (CMRO2), has been a valuable tool for untangling the physiological processes associated with neural stimulus-induced BOLD activation. However, to date this technique has largely been applied to the study of essentially steady-state physiological changes (baseline to activation) associated with block-design stimuli, and it is unclear whether this approach may be directly extended to the study of more dynamic, naturalistic experimental designs. In this study we tested an assumption underlying this technique whose validity is critical to the application of calibrated BOLD to the study of more dynamic stimuli, that information about fluctuations in venous cerebral blood volume (CBVv) can be captured indirectly by measuring fluctuations in CBF, making the independent measurement of CBVv unnecessary. To accomplish this, simultaneous arterial spin labeling and BOLD imaging was used to measure the CBF and BOLD responses to flickering checkerboards with contrasts that oscillated continuously with frequencies of ~0.02-0.16Hz. The measurements were then fit to a dynamic physiological model of the BOLD response in order to explore the range of consistent CMRO2 and CBVv responses. We found that the BOLD and CBF responses were most consistent with relatively tight dynamic coupling between CBF and CMRO2 and a CBVv response that was an order of magnitude slower than either CBF or CMRO2. This finding suggests that the assumption of tight flow-volume coupling may not be strictly valid, complicating the extension of calibrated BOLD to more naturalistic experimental designs.

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Section: Theorymentioning

confidence: 99%

Understanding the dynamic relationship between cerebral blood flow and the BOLD signal: Implications for quantitative functional MRI

Simon

Buxton

2015

NeuroImage

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“…The basic physical phenomenon underlying the BOLD effect is that deoxyhemoglobin is paramagnetic, and its presence reduces the MR signal slightly (Buxton, 2013). If the blood becomes more oxygenated, the MR signal goes up.…”

Section: The Challenge Of Interpreting the Bold Response In A Quantitmentioning

confidence: 99%

Variability of the coupling of blood flow and oxygen metabolism responses in the brain: a problem for interpreting BOLD studies but potentially a new window on the underlying neural activity

et al. 2014

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Recent studies from our group and others using quantitative fMRI methods have found that variations of the coupling ratio of blood flow (CBF) and oxygen metabolism (CMRO2) responses to a stimulus have a strong effect on the BOLD response. Across a number of studies an empirical pattern is emerging in the way CBF and CMRO2 changes are coupled to neural activation: if the stimulus is modulated to create a stronger response (e.g., increasing stimulus contrast), CBF is modulated more than CMRO2; on the other hand, if the brain state is altered such that the response to the same stimulus is increased (e.g., modulating attention, adaptation, or excitability), CMRO2 is modulated more than CBF. Because CBF and CMRO2 changes conflict in producing BOLD signal changes, this finding has an important implication for conventional BOLD-fMRI studies: the BOLD response exaggerates the effects of stimulus variation but is only weakly sensitive to modulations of the brain state that alter the response to a standard stimulus. A speculative hypothesis is that variability of the coupling ratio of the CBF and CMRO2 responses reflects different proportions of inhibitory and excitatory evoked activity, potentially providing a new window on neural activity in the human brain.

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“…Most fMRI relies on the blood-oxygen-level dependent (BOLD) signal: local microvascular hyperemia leads to less paramagnetic deoxyhemoglobin, thus reducing magnetic field distortion and increasing the MR signal observed during a gradient-recalled echo sequence (Buxton, 2013). The BOLD signal is, therefore, directly related to local transient changes in the oxygen supplied by CBF.…”

Section: Introductionmentioning

confidence: 99%

Arterial impulse model for the BOLD response to brief neural activation

Kim

Ress

2016

NeuroImage

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The blood oxygen level dependent (BOLD) signal evoked by brief neural stimulation, the hemodynamic response function (HRF), is a critical feature of neurovascular coupling. The HRF is directly related to local transient changes in oxygen supplied by cerebral blood flow (CBF) and oxygen demand, the cerebral metabolic rate of oxygen (CMRO2). Previous efforts to explain the HRF have relied upon the hypothesis that CBF produces a non-linear venous dilation within the cortical parenchyma. Instead, the observed dynamics correspond to prompt arterial dilation without venous volume change. This work develops an alternative biomechanical model for the BOLD response based on the hypothesis that prompt upstream dilation creates an arterial flow impulse amenable to linear description. This flow model is coupled to a continuum description of oxygen transport. Measurements using high-resolution fMRI demonstrate the efficacy of the model. The model predicts substantial spatial variations of the oxygen saturation along the length of capillaries and veins, and fits the varied gamut of measured HRFs by the combined effects of corresponding CBF and CMRO2 responses. Three interesting relationships among the hemodynamic parameters are predicted. First, there is an offset linear correlation with approximately unity slope between CBF and CMRO2 responses. Second, the HRF undershoot is strongly correlated to the corresponding CBF undershoot. Third, late-time-CMRO2 response can contribute to a slow recovery to baseline, lengthening the HRF undershoot. The model provides a powerful mathematical framework to understand the dynamics of neurovascular and neurometabolic responses that form the BOLD HRF.

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The physics of functional magnetic resonance imaging (fMRI)

Cited by 206 publications

References 221 publications

Understanding the dynamic relationship between cerebral blood flow and the BOLD signal: Implications for quantitative functional MRI

Understanding the dynamic relationship between cerebral blood flow and the BOLD signal: Implications for quantitative functional MRI

Variability of the coupling of blood flow and oxygen metabolism responses in the brain: a problem for interpreting BOLD studies but potentially a new window on the underlying neural activity

Arterial impulse model for the BOLD response to brief neural activation

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