A Primer on Functional Magnetic Resonance Imaging (original) (raw)
Related papers
Comparison of spatial and temporal pattern for fMRI obtained with BOLD and arterial spin labeling
Journal of Neural Transmission, 2006
Summary. Functional magnetic resonance imaging (fMRI) is presently either performed using blood oxygenation level-dependent (BOLD) contrast or using cerebral blood flow (CBF), measured with arterial spin labeling (ASL) technique. The present fMRI study aimed to provide practical hints to favour one method over the other. It involved three different acquisition methods during visual checkerboard stimulation on nine healthy subjects: 1) CBF
Journal of Cerebral Blood Flow & Metabolism, 2010
Blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) is the most widely used method for mapping neural activity in the brain. The interpretation of altered BOLD signals is problematic when cerebral blood flow (CBF) or cerebral blood volume change because of aging and/or neurodegenerative diseases. In this study, a recently developed quantitative arterial spin labeling (ASL) approach, bolus-tracking ASL (btASL), was applied to an fMRI experiment in the rat brain. The mean transit time (MTT), capillary transit time (CTT), relative cerebral blood volume of labeled water (rCBV lw ), relative cerebral blood flow (rCBF), and perfusion coefficient in the forelimb region of the somatosensory cortex were quantified during neuronal activation and in the resting state. The average MTT and CTT were 1.939 ± 0.175 and 1.606 ± 0.106 secs, respectively, in the resting state. Both times decreased significantly to 1.616±0.207 and 1.305±0.201 secs, respectively, during activation. The rCBV lw , rCBF, and perfusion coefficient increased on average by a factor of 1.123±0.006, 1.353±0.078, and 1.479±0.148, respectively, during activation. In contrast to BOLD techniques, btASL yields physiologically relevant indices of the functional hyperemia that accompanies neuronal activation.
T1ρ contrast in functional magnetic resonance imaging
Magnetic Resonance in Medicine, 2005
The application of T 1 in the rotating frame (T 1ρ ) to functional MRI in humans was studied at 3 T. Increases in neural activity increased parenchymal T 1ρ . Modeling suggested that cerebral blood volume mediated this increase. A pulse sequence named spin-locked echo planar imaging (SLEPI) that produces both T 1ρ and T 2 * contrast was developed and used in a visual functional MRI (fMRI) experiment. Spin-locked contrast significantly augments the T 2 * blood oxygen level-dependent (BOLD) contrast in this sequence. The total functional contrast generated by the SLEPI sequence (1.31%) was 54% larger than the contrast (0.85%) obtained from a conventional gradient-echo EPI sequence using echo times of 30 ms. Analysis of image SNR revealed that the spin-locked preparation period of the sequence produced negligible signal loss from static dephasing effects. The SLEPI sequence appears to be an attractive alternative to conventional BOLD fMRI, particularly when long echo times are undesirable, such as when studying prefrontal cortex or ventral regions, where static susceptibility gradients often degrade T 2 *-weighted images.
Real-time functional MRI using pseudo-continuous arterial spin labeling
Magnetic Resonance in Medicine, 2011
The first implementation of real-time acquisition and analysis of arterial spin labeling-based functional magnetic resonance imaging time series is presented in this article. The implementation uses a pseudo-continuous labeling scheme followed by a spiral k-space acquisition trajectory. Real-time reconstruction of the images, preprocessing, and regression analysis of the functional magnetic resonance imaging data were implemented on a laptop computer interfaced with the MRI scanner. The method allows the user to track the current raw data, subtraction images, and the cumulative t-statistic map overlaid on a cumulative subtraction image. The user is also able to track the time course of individual time courses and interactively selects a region of interest as a nuisance covariate. The pulse sequence allows the user to adjust acquisition and labeling parameters while observing their effect on the image within two successive pulse repetition times. This method is demonstrated by two functional imaging experiments: a simultaneous finger-tapping and visual stimulation paradigm, and a bimanual finger-tapping task. Magn Reson Med 65:1570-1577, Conventional functional magnetic resonance imaging (fMRI) collects blood oxygen level-dependent (BOLD)contrast MR images of a subject's brain while performing a cognitive task, whereas subsequent image reconstruction and analysis are performed offline (i.e., on a separate computer after the experiment is completed). Real-time fMRI is an exciting extension to conventional fMRI techniques that enables the user to analyze fMRI data as it is being collected. Thus, in real-time fMRI, the results are immediately available as the subject is being scanned, and the results can be used to reveal and guide the subject's cognitive processes. It can also facilitate the experimenter's parameter selections or a clinician's interventions (1).
Experimental data up to 7.0 T show that the blood oxygenation level-dependent (BOLD) signal of functional magnetic resonance imaging (fMRI) increases with higher magnetic field strength. Although several studies at 11.7 T report higher BOLD signal compared with studies at 7.0 T, no direct comparison at these two field strengths has been performed under the exact same conditions. It therefore remains unclear whether the expected increase of BOLD effect with field strength will still continue to hold for fields7.0 T. To examine this issue, we compared the BOLD activation signal at 7.0 and 11.7 T with the two common sequences, spin-echo (SE) and gradient-echo (GE) echo planar imaging (EPI). We chose the physiologically well controlled rat model of electrical forepaw stimulation under medetomidine sedation. While a linear to superlinear increase in activation with field strengths up to 7.0 T was reported in the literature, we observed no significant activation difference between 7.0 and 11.7 T with either SE or GE. Discussing the results in light of the four-component model of the BOLD signal, we showed that at high field only two extravascular contributions remain relevant, while both intravascular components vanish. Constancy of the BOLD effect is discussed due to motional narrowing, i.e., susceptibility gradients become so strong that phase variance of diffusing spins decreases and therefore the BOLD signal also decreases. This finding will be of high significance for the planning of future human and animal fMRI studies at high fields and their quantitative analysis.
Dynamic magnetic resonance imaging of cerebral blood flow using arterial spin labeling
Methods Mol Biol, 2009
Modern functional neuroimaging techniques, including functional MRI, positron emission tomography and optical imaging of intrinsic signals, rely on a tight coupling between neural activity and cerebral blood flow (CBF) to visualize brain activity using CBF as a surrogate marker. Because the spatial and temporal resolution of neuroimaging modalities is ultimately determined by the spatial and temporal specificity of the underlying hemodynamic signals, characterization of the spatial and temporal profiles of the hemodynamic response to focal brain stimulation is of paramount importance for the correct interpretation and quantification of functional data. The ability to properly measure and quantify CBF with MRI is a major determinant of progress into our understanding of brain function. This chapter reviews the dynamic arterial spin labeling (DASL) method to measure CBF and the CBF functional response with high temporal resolution.
Magnetic Resonance Imaging, 2012
Accurate localization of brain activity using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) has been challenged because of the large BOLD signal within distal veins. Arterial spin labeling (ASL) techniques offer greater sensitivity to the microvasculature but possess low temporal resolution and limited brain coverage. In this study, we show that the physiological origins of BOLD and ASL depend on whether percent change or statistical significance is being considered. For BOLD and ASL fMRI data collected during a simple unilateral hand movement task, we found that in the area of the contralateral motor cortex the centre of gravity (CoG) of the intersubject coefficient of variation (CV) of BOLD fMRI was near the brain surface for percent change in signal, whereas the CoG of the intersubject CV for Z-score was in close proximity of sites of brain activity for both BOLD and ASL. These findings suggest that intersubject variability of BOLD percent change is vascular in origin, whereas the origin of inter-subject variability of Z-score is neuronal for both BOLD and ASL. For longer duration tasks (12 s or greater), however, there was a significant correlation between BOLD and ASL percent change, which was not evident for short duration tasks (6 s). These findings suggest that analyses directly comparing percent change in BOLD signal between pre-defined regions of interest using short duration stimuli, as for example in event-related designs, may be heavily weighted by large-vessel responses rather than neuronal responses.
Quantitative analysis of arterial spin labeling FMRI data using a general linear model
Magnetic Resonance Imaging, 2010
Arterial spin labeling techniques can yield quantitative measures of perfusion by fitting a kinetic model to difference images (tagged-control). Because of the noisy nature of the difference images investigators typically average a large number of tagged versus control difference measurements over long periods of time. This averaging requires that the perfusion signal be at a steady state and not at the transitions between active and baseline states in order to quantitatively estimate activation induced perfusion. This can be an impediment for functional magnetic resonance imaging task experiments. In this work, we introduce a general linear model (GLM) that specifies Blood Oxygenation Level Dependent (BOLD) effects and arterial spin labeling modulation effects and translate them into meaningful, quantitative measures of perfusion by using standard tracer kinetic models. We show that there is a strong association between the perfusion values using our GLM method and the traditional subtraction method, but that our GLM method is more robust to noise. for time 1, … , . The first regressor, the baseline vector 1 and its coefficient parameter, the scalar , indicate the baseline signal, and can be interpreted as a measure of spin density. The second regressor describes the baseline difference between control and tagged images a Functional MRI laboratory, University of Michigan, MI 48109, USA
Comparison of diffusion-weighted high-resolution CBF and spin-echo BOLD fMRI at 9.4 T
Magnetic Resonance in Medicine, 2002
The quantification of blood oxygenation-level dependent (BOLD) functional MRI (fMRI) signals is closely related to cerebral blood flow (CBF) change; therefore, understanding the exact relationship between BOLD and CBF changes on a pixelby-pixel basis is fundamental. In this study, quantitative CBF changes induced by neural activity were used to quantify BOLD signal changes during somatosensory stimulation in ␣-chloralose-anesthetized rats. To examine the influence of fast-moving vascular spins in quantifying CBF, bipolar gradients were employed. Our data show no significant difference in relative CBF changes obtained with and without bipolar gradients. To compare BOLD and CBF signal changes induced by neural stimulation, a spin-echo (SE) sequence with long SE time of 40 ms at 9.4 T was used in conjunction with an arterial spin labeling technique. SE BOLD changes were quantitatively correlated to CBF changes on a pixel-by-pixel and animal-by-animal basis. Thus, SE BOLD-based fMRI at high magnetic fields allows a quantitative comparison of functional brain activities across brain regions and subjects. Magn Reson Med 47: 736-741, 2002.
Functional magnetic resonance imaging of the human brain: data acquisition and analysis
1998
Abstract It is now feasible to create spatial maps of activity in the human brain completely non-invasively using magnetic resonance imaging. Magnetic resonance imaging (MRI) images in which the spin magnetization is refocussed by gradient switching are sensitive to local changes in magnetic susceptibility, which can occur when the oxygenation state of blood changes. Cortical neural activity causes increases in blood flow, which usually result in changes in blood oxygenation.