Functional magnetic resonance imaging
Emerging acquisition strategies

Balanced Steady-State Free Precession (bSSFP) fMRI: bSSFP offers high SNR efficiency due to optimized magnetization usage, leading to high tSNR. It also features reduced distortion because of its short readouts, minimizing susceptibility-induced distortion and signal dropout compared to GRE-EPI. It's compatible with advanced short-readout strategies like multi-shot, spiral, and radial trajectories. However, bSSFP is prone to banding artifacts from off-resonance effects and has a complex contrast mechanism influenced by T₂/T₁, diffusion, and off-resonance. It also tends to have a higher SAR compared to GRE sequences. Pass-Band bSSFP: This variant has a flat magnitude response over a broad frequency range (approximately 75% of 1/TR). Its contrast is T₂-based, driven by diffusion in susceptibility gradients and intravascular T₂ changes, with low sensitivity to frequency shifts. While offering lower contrast than long-TE GRE, it provides superior robustness to off-resonance and shim variations, making efficient whole-brain coverage achievable with minimal phase cycling. Larger Flip angles could be used to operate in Passband region.Transition-Band bSSFP: This type operates within a narrow frequency range (around 10–15% of 1/TR) with a steep magnitude and phase response. Its BOLD sensitivity is frequency-driven, where small frequency shifts cause large signal changes, potentially yielding high functional contrast (10–40%). However, this makes it highly dependent on the baseline resonance frequency of the voxel and critical of shimming quality, challenging whole-brain coverage without multiple acquisitions.Applications: bSSFP techniques can be beneficial for layer-specific imaging or studies requiring high SNR and excellent temporal resolution, especially in low-field or low-SNR brain regions.
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