Image quality issues

The requirement of a fMRI pulse sequence is BOLD sensitivity, which means predominantly T2*-weighted sequences such as GE-EPI, although spin echo sequences such as spin echo EPI (SE-EPI) can also be used (Bandettini et al. 1994; Norris et al. 2002; Schmidt et al. 2005). SE-EPI suffers from reduced volume coverage relative to GE-EPI due to the longer echo times required, and generally shows smaller signal changes (Bandettini et al. 1994; Schmidt et al. 2005). However, spin echo sequences do not suffer from signal dropout and therefore perform much better in areas of large through-plane susceptibility gradients (Norris et al. 2002). There is mixed opinion as to whether, or in which parts of the brain, spin echo sequences may be advantageous (Parkes et al. 2005; Schmidt et al. 2005), but they are likely to be most usefully employed at field strengths of >3 T (Duong et al. 2002; Nair and Duong 2004). Some considerable research has recently been devoted to using steady-state free-precession sequences for fMRI (Miller et al. 2003, 2004, 2006; Zhong et al. 2007). While these sequences have shown promise, they are generally employed for niche applications (Miller et al. 2006). It is also worth noting that hybrid approaches using spin and gradient echoes such as GRASE (Feinberg et al. 1995; Fernandez-Seara et al. 2005) are also capable of being used to obtain high-quality fMRI data.