fMRI (7T)

MRI · reads

Functional MRI at 7 tesla is magnetic resonance imaging of the human brain that infers neuronal activity from blood oxygenation. In gradient-echo BOLD imaging the signal falls as local deoxyhemoglobin rises; after neuronal activity increases, blood flow rises more than oxygen use, so deoxyhemoglobin drops near active sites and in downstream draining veins. What it reads is BOLD image intensity, an indirect measure of neuronal activity, and the venous contribution limits spatial specificity. At 7 T in humans, spin-echo BOLD gave narrower estimated point-spread functions than gradient-echo BOLD.

Spatial resolution

820 µm (point spread)

Temporal resolution

227 ms (sample interval)

Field of view

1.23 L (acquired volume)

Reads

BOLD MR image intensity; hemodynamic response timing and amplitude; resting-state functional connectivity (derived)

Writes

n/a

Destructive?

non-destructive

Prerequisites

other: Human 7 T safety screening for conductive objects and implants (medical devices, dental appliances, jewelry, metallic clothing fibers, ECG electrodes/pads, cosmetic pigments, tattoo ink) due to focal RF heating risk; general 7 T safety recommendation; other: Hearing protection at human 7 T: highest-rated ear plugs properly inserted into the external ear canal, plus additional padding where possible, since tight 7 T head coils make headphones difficult; general recommendation; other: External head coil positioning: 32-channel phased-array head coil (90-adult rest protocol); custom 32-channel receive array with birdcage transmit coil (28-adult visual protocol); other: Personalized padding to achieve a tight head fit (90-adult 7 T rest protocol); other: Awake instructions: keep eyes open at the start of each 10 min resting scan and press a button to indicate readiness (90-adult rest protocol); awake viewing of visual stimuli of varying durations or oscillatory stimuli with behavioral performance (28-adult visual protocol)

Theoretical spatial resolution

470 µm (point spread)

Theoretical field of view

1.23 L (reachable envelope)

Description

Human functional MRI infers local changes in neuronal activity from task-invoked metabolic and hemodynamic responses, so its signal is an indirect measure of neuronal activity. In gradient-echo blood oxygenation level dependent (BOLD) imaging, the signal is inversely proportional to local deoxyhemoglobin content. After neuronal activity rises, local arterial cerebral blood flow increases more than oxygen consumption, which lowers deoxyhemoglobin in capillaries. This change propagates from capillaries at active sites downstream to draining veins, so large veins distant from those sites contribute functional mapping signals. That venous contribution impairs spatial specificity and adds to any intrinsic imprecision in coupling between blood flow and neuronal activity. Intermediary non-neuronal events limit specificity in space and how that specificity changes over time. A spatio-temporal point-spread function in human gray matter describes these limits. At 7 T in humans, estimated point-spread functions were narrower for spin-echo BOLD than for gradient-echo BOLD. These estimates reflect realistically achievable specificity at that field strength but are only upper bounds on the true widths. In human resting-state 7 T fMRI, functional connectivity between regional signals may reflect both monosynaptic and polysynaptic connections. Higher-resolution acquisition, minimal smoothing and nuisance regression using signals from individual ventricles can mitigate low signal-to-noise, partial volume effects and physiological aliasing.

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