BOLD sensitivity and vessel size specificity along CPMG and GRASE echo trains

Area of Science:

• Magnetic Resonance Imaging
• Neuroimaging
• Biophysics

Background:

• Functional BOLD (Blood-Oxygen-Level-Dependent) imaging is crucial for neuroscience research.
• Understanding vessel size specificity in BOLD signal acquisition is key to improving spatial resolution and interpretation.
• Rapid pulse sequences like CPMG and GRASE are explored for enhanced BOLD imaging efficiency.

Purpose of the Study:

• To evaluate the vessel size specificity and sensitivity of CPMG and GRASE sequences for BOLD imaging.
• To investigate the impact of various acquisition parameters (echo train length, echo spacing, field strength, flip angle) on BOLD signal.
• To analyze signal behavior before and after refocusing time points in CPMG and GRASE.

Main Methods:

• Monte Carlo simulations of magnetization evolution in artificial cylinder networks.
• Experimental measurements on microspheres to validate simulation results.
• Analysis of signal changes across different echo train lengths and refocusing flip angle schemes.

Main Results:

• Reduced refocusing flip angles shift vessel size sensitivity towards larger radii with increasing echo time.
• BOLD signal changes along the echo train are influenced by the refocusing flip angle scheme and do not always correlate with echo amplitudes.
• Signals acquired off-refocusing time points exhibit contributions from larger vessels, similar to gradient echo sequences.

Conclusions:

• CPMG and GRASE can be employed with low refocusing flip angles without compromising BOLD sensitivity.
• BOLD signal evolution along the echo train aids in designing optimal k-space reordering strategies.
• Short echo spacing minimizes gradient echo contributions from large vessels while preserving spin echo contributions from small vessels.