Techniques for BOLD and blood volume weighted fMRI

Functional MRI (fMRI) generally employs gradient-echo EPI to measure BOLD signal changes that result from changes in tissue relaxation time T2* between activation and rest. As T2* strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T2*, imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artefacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T2* coverage are optimised by pixel-wise weighted echo summation based on measured local T2*, or contrast-to-noise measurements. The method is evaluated both using an approach that allows differential BOLD CNR to be calculated without stimulation, and a Stroop experiment. Results at 3 T show that BOLD sensitivity improves by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artefacts. Using parallel imaging markedly reduces image distortion. The method should hence find widespread application in functional brain imaging. Poser BA, Versluis MJ, Hoogduin JM, Norris DG. Magn Reson Med 55(6): 1227-1235; 2006