BOLD specificity and dynamics evaluated in humans at 7 T: comparing gradient-echo and spin-echo hemodynamic responses

Area of Science:

• Neuroimaging
• Functional Magnetic Resonance Imaging (fMRI)
• Human Brain Anatomy

Background:

• High-field gradient-echo (GE) BOLD fMRI offers high resolution for brain function studies.
• Increased spatial resolution in GE BOLD fMRI can introduce signal confounds from larger, non-capillary vessels.
• Spin-echo (SE) fMRI is generally considered more sensitive to microvascular signals.

Purpose of the Study:

• To assess the microvascular contribution to the GE BOLD response across cortical depth in the human brain.
• To compare the hemodynamic impulse response (HRF) characteristics between GE and SE BOLD fMRI.
• To determine the vascular origin of the early GE BOLD signal in deep cortical layers.

Main Methods:

• Utilized spin-echo (SE) and gradient-echo (GE) BOLD fMRI in human cortex.
• Measured hemodynamic impulse responses (HRFs) using very short stimuli (0.25 s) and a fast event-related paradigm.
• Analyzed BOLD responses across different cortical depths, focusing on layers III-IV.

Main Results:

• The onset (≈1.25 s) and rising slope of GE and SE HRFs were remarkably similar in deep gray matter (layers III-IV).
• This similarity suggests that the early GE BOLD signal in these layers originates predominantly from microvasculature.
• The findings indicate reduced contamination from large vessels in the early GE BOLD response in metabolically active deep cortical regions.

Conclusions:

• The early onset of the GE BOLD HRF in deep cortical gray matter is strongly associated with microvascular signals.
• GE BOLD fMRI, even at high resolution, can provide reliable information about microvascular function in specific brain regions.
• This study validates the use of GE BOLD fMRI for investigating neural activity linked to microvascular changes in the human cortex.