Real‐time RF pulse adjustment for B₀ drift correction

Effect of Fat-saturation Pulse on EPI Time Series in the Presence of B0 Drift

“…3) such as reported in previous studies. [2][3][4][5][6][7] In Fig. 3, the change in B 0 was relatively stable and small during theˆrst several minutes of scanning.…”

“…Signiˆcant B 0 drifts ranging from approximately one to 2.5 Hz/ min have been observed with various pulse sequences and scanner systems from diŠerent manufacturers. [2][3][4][5][6][7] In particular, the EPI sequence most commonly used for fMRI produces large B 0 drifts as a result of heating from the higher load on its gradient coil [2][3][4] and its narrow sampling bandwidth (BW) in the phase encoding direction. The value for this direction is typically 20 Hz per pixel, 8 which is much lower than that for its frequency encoding direction (1,280 Hz per pixel for 64×64 matrix).…”

“…A large linear trend resulting from the contamination of fat signals in a voxel (pixel) time series in fat tissue has been reported. 5 In contrast, we often encounter nonlinear ‰uctuation in fMRI time series, 17,18 and the source is not limited to fat tissue. This observation suggests a more complex underlying mechanism than one that simply results in the progressive contamination of fat signals due to the ineŠectiveness of fat-saturation pulses.…”

System Stability of a 3T-MRI During Continuous EPI Scan

“…4c; see first 10 min and last 10 min of TSNR images . The B0 fluctuation of MRI during the system load has often been reported, [11][12][13][14][15] and this causes image shifts, where the degree of shift and direction depend on the imaging sequence. In the EPI sequence, the B0 change can affect the image position in both phase and frequency directions simultaneously.…”

“…The EPI sequence has been widely used for f-MRI. Although this sequence can detect a subtle magnetic field change resulting from activations of the local brain, it is easily affected by physiological alterations [8][9][10] such as respiration, pulsation, and subject motion in addition to system instability due to a main magnetic fi eld B0 change [11][12][13][14][15] throughout data acquisition.…”