Calibrated bold fMRI with an optimized ASL-BOLD dual-acquisition sequence

Area of Science:

• Neuroimaging
• Functional Magnetic Resonance Imaging (fMRI)
• Physiology

Background:

• Calibrated fMRI estimates task-induced changes in cerebral metabolic rate of oxygen (CMRO2) using simultaneous cerebral blood flow (CBF) and blood-oxygen-level-dependent (BOLD) signal measurements.
• Determining the calibration factor M requires measuring BOLD signal and CBF during gas challenges (CO2 or O2).
• Existing combined BOLD-ASL acquisition strategies have limitations in perfusion temporal signal-to-noise ratio (tSNR).

Purpose of the Study:

• To introduce and validate a novel ASL dual-acquisition sequence for improved fMRI calibration.
• To assess the sequence's performance in enhancing perfusion tSNR and its utility for M-mapping and CMRO2 quantification.

Main Methods:

• Development of a background-suppressed 3D-GRASE readout combined with a 2D multi-slice EPI sequence for ASL dual-acquisition.
• Separate optimization of acquisition parameters for CBF and BOLD data.
• Validation against a dual-echo EPI readout ASL sequence using a visual fMRI paradigm and hypercapnia gas challenge.

Main Results:

• The dual-acquisition sequence demonstrated a 3-fold increase in ASL time-series tSNR compared to the dual-echo sequence.
• BOLD tSNR was comparable, though slightly lower T-scores (30%) were observed in BOLD activation maps due to a longer TR.
• Average gray matter M-value of 8.71±1.03 and fractional CMRO2 changes of 12.5±5% were quantified during motor activation.

Conclusions:

• The proposed ASL dual-acquisition sequence significantly enhances perfusion tSNR, addressing limitations of previous combined BOLD-ASL methods.
• This improved tSNR is essential for accurate BOLD signal calibration and reliable CMRO2 quantification in fMRI.
• The sequence shows potential for precise M-mapping and CMRO2 change assessment in response to physiological challenges and tasks.