On Signal to Noise Ratio Tradeoffs in fMRI

When planning an fMRI experiment, it is desirable to optimize the SNR to maximize the statistical return on scan time investment. At first glance, the tradeoffs may seem unclear. As more slices are acquired, the TR needed to fit in the slices increases linearly. With longer TR the flip angle and degree of magnetization recovery between pulses both increase, leading to higher signal. However, at constant scan time, fewer time frames can be acquired, leading to lower statistics in the post-processing analysis. It is not obvious which of these effects dominates: increasing signal with longer TR, or decreased overall SNR with fewer frames.