Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging.

Abstract

We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce highspatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.Magnetic-resonance imaging (MRI) of rodent brains at high (7-T) magnetic-field strength shows proton signal-intensity alterations related to blood oxygenation in regions close to local blood vessels (1-3). We have termed this phenomenon blood oxygenation-level-dependent (BOLD) contrast and have demonstrated that the underlying mechanism is a magnetic-susceptibility variation caused by deoxyhemoglobin, an endogenous paramagnetic contrast agent. It was further demonstrated that this magnetic-susceptibility effect could be used to measure in vivo changes in hemodynamics. For example, pharmacologically induced changes in cerebral blood flow and oxygen utilization produce measurable changes in BOLD contrast in the rat cerebral cortex. Similar results have recently been demonstrated in cat brain (4).There is increased evidence that a local elevation in human-brain venous-blood oxygenation accompanies an increase in neuronal activity (5-8). For example, positron emission tomography imaging experiments demonstrate stimulation-produced increases in regional cerebral blood flow without significantly changing local oxygen use, thus predicting an elevation in venous-blood oxygenation (6, 7). This result suggested that BOLD contrast imaging could be used to map human mental operations. To examine whether detectable intrinsic magnetic-susceptibility changes are produced in the human brain in response to neuronal activation, we studied the effect of visual stimulation on gradient echo images of human visual cortex acquired at high-magneticfield strength. In general, high-field strength increases the magnitude of susceptibility contrast effects, accentuating BOLD contrast. MATERIALS AND METHODSMRI experiments were done with a 4-T whole-body imaging system with actively shielded gradient coils [Sisco (Sunnyvale, CA)/Siemens (Erlangen, F.R.G.)]. Approval for these human experiments was obtained from the institutional review board of the University of Minnesota Medical School. Radiofrequency power deposition was kept two orders of magnitude below Food and Drug Administration specificabsorption rate guidelines. A snugly fitted head holder with a curved-surface radiofrequency coil (14 cm in diameter) was used to limit...

In vivo17O NMR approaches for brain study at high field

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Real-time electrochemical monitoring of brain tissue oxygen: A surrogate for functional magnetic resonance imaging in rodents

“…Such changes form the basis of imaging techniques such as blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) (Bandettini et al, 1992;Ogawa et al, 1992). BOLD fMRI is a key tool in neuroscience because it provides a powerful non-invasive way of mapping the regional activity of the brain during the performance of sensory, motor or cognitive tasks.…”

How accurate is magnetic resonance imaging of brain function?

“…The most commonly used fMRI approach was introduced in 1992 [2][3][4] and is based on imaging the regional alterations in deoxyhemoglobin that accompany changes in neuronal activity. Magnetic resonance techniques can also generate functional maps that are based on changes in tissue perfusion [3,5 -8] or cerebral blood volume (CBV) [9][10][11] that are coupled to neuronal activity; in the case of CBV, this involves use of contrast agents that remain in the vasculature for long periods.…”