Effect of echo spacing and readout bandwidth on basic performances of EPI‐fMRI acquisition sequences implemented on two 1.5 T MR scanner systems

Purpose Although in EPI‐fMRI analyses typical acquisition parameters (TR, TE, matrix, slice thickness, etc.) are generally employed, various readout bandwidth (BW) values are used as a function of gradients characteristics of the MR scanner. Echo spacing (ES) is another fundamental parameter of EPI‐fMRI acquisition sequences but the employed ES value is not usually reported in fMRI studies. In the present work, the authors investigated the effect of ES and BW on basic performances of EPI‐fMRI sequences in terms of temporal stability and overall image quality of time series acquisition. Methods EPI‐fMRI acquisitions of the same water phantom were performed using two clinical MR scanner systems (scanners A and B) with different gradient characteristics and functional designs of radiofrequency coils. For both scanners, the employed ES values ranged from 0.75 to 1.33 ms. The used BW values ranged from 125.0 to 250.0 kHz/64pixels and from 78.1 to 185.2 kHz/64pixels for scanners A and B, respectively. The temporal stability of EPI‐fMRI sequence was assessed measuring the signal‐to‐fluctuation noise ratio (SFNR) and signal drift (DR), while the overall image quality was assessed evaluating the signal‐to‐noise ratio(SNR ts)and nonuniformity(NU ts)of the time series acquisition. Results For both scanners, no significant effect of ES and BW on signal drift was revealed. The SFNR,NU tsandSNR tsvalues of scanner A did not significantly vary with ES. On the other hand, the SFNR,NU ts, andSNR tsvalues of scanner B significantly varied with ES. SFNR (5.8%) andSNR ts(5.9%) increased with increasing ES. SFNR (25% scanner A, 32% scanner B) andSNR ts(26.2% scanner A, 30.1% scanner B) values of both scanners significantly decreased with increasing BW.NU tsvalues of scanners A and B were less than 3% for all BW and ES values. Nonetheless, scanner A was characterized by a significant upward trend (3% percentage of variation) of time series nonuniformity with increasing BW whileNU tsof scanner B significantly increased (19% percentage of variation) with increasing ES. Conclusions Temporal stability (SFNR and DR) and overall image quality (NU tsandSNR ts) of EPI‐fMRI time series can significantly vary with echo spacing and readout bandwidth. The specific pattern of variation may depend on the performance of each single MR scanner system in terms of gradients characteristics, EPI sequence calibrations (eddy currents, shimming, etc.), and functional design of radiofrequency coil. Our results indicate that the employment of low BW improves not only the signal‐to‐noise ratio of EPI‐fMRI time series but also the temporal stability of functional acquisitions. The use of minimum ES values is not entirely advantageous when the MR scanner system is characterized by gradients with low performances and suboptimal EPI sequence calibration. Since differences in basic performances of MR scanner system are potential source of variability for fMRI activation, phantom measurements of SFNR, DR,NU ts, andSNR tscan be executed before subjects acquisitions to monitor the stability of MR scanner performances in clinical group comparison and longitudinal studies.