A biomechanical model is presented for the dynamic changes in deoxyhemoglobin content during brain activation. The model incorporates the conflicting effects of dynamic changes in both blood oxygenation and blood volume. Calculations based on the model show pronounced transients in the deoxyhemoglobin content and the blood oxygenation level dependent (BOLD) signal measured with functional MRI, including initial dips and overshoots and a prolonged poststimulus undershoot of the BOLD signal. Furthermore, these transient effects can occur in the presence of tight coupling of cerebral blood flow and oxygen metabolism throughout the activation period. An initial test of the model against experimental measurements of flow and BOLD changes during a finger-tapping task showed good agreement.

http://ece-research.unm.edu/vcalhoun/courses/fMRI_Spring12/Buxton_1998_dynamics_in_blood_flow.pdf

Dynamics of blood flow and oxygenation changes during brain activation: the balloon model.

We describe here experimental considerations in the implementation of quantitative perfusion imaging techniques for functional MRI using pulsed arterial spin labeling. Three tagging techniques: EPISTAR, PICORE, and FAIR are found to give very similar perfusion results despite large differences in static tissue contrast. Two major sources of systematic error in the perfusion measurement are identified: the transit delay from the tagging region to the imaging slice; and the inclusion of intravascular tagged signal. A modified technique called QUIPSS II is described that decreases sensitivity to these effects by explicitly controlling the time width of the tag bolus and imaging after the bolus is entirely deposited into the slice. With appropriate saturation pulses the pulse sequence can be arranged so as to allow for simultaneous collection of perfusion and BOLD data that can be cleanly separated. Such perfusion and BOLD signals reveal differences in spatial location and dynamics that may be useful both for functional brain mapping and for study of the BOLD contrast mechanism. The implementation of multislice perfusion imaging introduces additional complications, primarily in the elimination of signal from static tissue. In pulsed ASL, this appears to be related to the slice profile of the inversion tag pulse in the presence of relaxation, rather than magnetization transfer effects as in continuous arterial spin labeling, and can be alleviated with careful adjustment of inversion pulse parameters. © 1997 John Wiley & Sons, Ltd.

/pdf/implementation-of-quantitative-perfusion-imaging-techniques-41mxkyx6s3.pdf

Implementation of quantitative perfusion imaging techniques for functional brain mapping using pulsed arterial spin labeling.

http://dbic.dartmouth.edu/haxby/papers/Clark_NeuroImage_1996.pdf

Functional magnetic resonance imaging of human visual cortex during face matching: a comparison with positron emission tomography

The linearity of the cerebral perfusion response relative to stimulus duration is an important consideration in the characterization of the relationship between regional cerebral blood flow (CBF), cerebral metabolism, and the blood oxygenation level dependent (BOLD) signal It is also a critical component in the design and analysis of functional neuroimaging studies To study the linearity of the CBF response to different duration stimuli, the perfusion response in primary motor and visual cortices was measured during stimulation using an arterial spin labeling technique with magnetic resonance imaging (MRI) that allows simultaneous measurement of CBF and BOLD changes In each study, the perfusion response was measured for stimuli lasting 2, 6, and 18 sec The CBF response was found in general to be nonlinearly related to stimulus duration, although the strength of nonlinearity varied between the motor and visual cortices In contrast, the BOLD response was found to be strongly nonlinear in both regions studied, in agreement with previous findings The observed nonlinearities are consistent with a model with a nonlinear step from stimulus to neural activity, a linear step from neural activity to CBF change, and a nonlinear step from CBF change to BOLD signal change

/pdf/nonlinear-temporal-dynamics-of-the-cerebral-blood-flow-2gtwzyggav.pdf

Nonlinear temporal dynamics of the cerebral blood flow response.

https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(98)05923-6.pdf

Spatial and temporal mapping of neural activity associated with auditory hallucinations.

In this review, the clinical utility of echoplanar techniques in MRI of the brain is discussed. Comparison of high-resolution EPI with SE/turbo-SE shows high image quality of EPI in the supratentorial brain. In the infratentorial region, however, susceptibility artifacts limit image quality. For the assessment of neuronal brain activation utilizing the intrinsic contrast of blood (BOLD), EPI has definite advantages over other techniques of functional MRI. Due to its superior temporal resolution and multislice capabilities, EPI allows for analysis of complex neuronal activation patterns. Diffusion imaging benefits from the lack of bulk motion artifacts and serves primarily to detect early stroke. Three methods of perfusion imaging (rel. blood volume, rel. blood flow) are discussed: the susceptibility artifact method (T2*), the relaxitivity method (T1), and the signal-labelling technique (STAR). Perfusion imaging may have a clinical impact in the assessment of brain tumors and cerebral ischemia.

[Echo-planar imaging of the brain].

Functional magnetic resonance imaging (MRI) allows to directly visualize regional activity of the visual cortex during stimulation. The value of the method to evaluate physiologic and pathologic conditions is elaborated on in comparison with positron emission tomography (PET). Aspects as sequence selection and stimulus paradigms are discussed. Subtle activity in the primary cortex (V1), processing areas as MT/V5 and subcortical areas are visualized by MRI. Cooperation of the subjects is a essential factor. Despite promising initial results in clinical trials the focus of research in clinical trials the focus of research in the visual cortex yet remains in the preclinical field.

Functional magnetic resonance tomography of the visual cortex

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[Echo-planar imaging of the brain].

Functional magnetic resonance tomography of the visual cortex