Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation.

TLDR

Neuronal activity induces local changes in cerebral blood flow, volume, and oxygenation, and high‑speed echo‑planar MRI techniques were developed to detect these hemodynamic changes. The authors obtained noninvasive tomographic maps of brain activity by applying visual and motor stimuli while acquiring continuous gradient‑echo or spin‑echo inversion‑recovery images sensitive to blood oxygenation and flow changes. Cine subtraction images revealed activity‑induced MR signal changes at second‑level resolution, with 8‑Hz photic stimulation producing ~1.8 % signal increases in V1 (rise times 4.4 s GE, 8.9 s IR) and similar responses in M1 during hand squeezing, demonstrating functional MRI’s ability to map brain physiology.

Abstract

Neuronal activity causes local changes in cerebral blood flow, blood volume, and blood oxygenation. Magnetic resonance imaging (MRI) techniques sensitive to changes in cerebral blood flow and blood oxygenation were developed by high-speed echo planar imaging. These techniques were used to obtain completely noninvasive tomographic maps of human brain activity, by using visual and motor stimulus paradigms. Changes in blood oxygenation were detected by using a gradient echo (GE) imaging sequence sensitive to the paramagnetic state of deoxygenated hemoglobin. Blood flow changes were evaluated by a spin-echo inversion recovery (IR), tissue relaxation parameter T1-sensitive pulse sequence. A series of images were acquired continuously with the same imaging pulse sequence (either GE or IR) during task activation. Cine display of subtraction images (activated minus baseline) directly demonstrates activity-induced changes in brain MR signal observed at a temporal resolution of seconds. During 8-Hz patterned-flash photic stimulation, a significant increase in signal intensity (paired t test; P less than 0.001) of 1.8% +/- 0.8% (GE) and 1.8% +/- 0.9% (IR) was observed in the primary visual cortex (V1) of seven normal volunteers. The mean rise-time constant of the signal change was 4.4 +/- 2.2 s for the GE images and 8.9 +/- 2.8 s for the IR images. The stimulation frequency dependence of visual activation agrees with previous positron emission tomography observations, with the largest MR signal response occurring at 8 Hz. Similar signal changes were observed within the human primary motor cortex (M1) during a hand squeezing task and in animal models of increased blood flow by hypercapnia. By using intrinsic blood-tissue contrast, functional MRI opens a spatial-temporal window onto individual brain physiology.