title: fMRI at 1.5, 3 and 7 T: characterising BOLD signal changes.
abstract: Blood oxygenation level dependent (BOLD) signal changes occurring during execution of a simple motor task were measured at field strengths of 1.5, 3 and 7 T using multi-slice, single-shot, gradient echo EPI at a resolution of 1x1x3 mm(3), to quantify the benefits offered by ultra-high magnetic field for functional MRI. Using four different echo times at each field strength allowed quantification of the relaxation rate, R(2)* and the change in relaxation rate on activation, DeltaR(2)*. This work adds to previous studies of the field strength dependence of BOLD signal characteristics, through its: (i) focus on motor rather than visual cortex; (ii) use of single-shot, multi-slice, gradient echo EPI for data acquisition; (iii) co-registration of images acquired at different field strengths to allow assessment of the BOLD signal changes in the same region at each field strength. DeltaR(2)* was found to increase linearly with field strength (0.51+/-0.06 s(-1) at 1.5 T; 0.98+/-0.08 s(-1) at 3 T; 2.55+/-0.22 s(-1) at 7 T), while the ratio of DeltaR(2)*/R(2), which dictates the accessible BOLD contrast was also found to increase (0.042+/-0.002 at 1.5 T; 0.054+/-0.002 at 3 T; 0.084+/-0.003 at 7 T). The number of pixels classified as active, the t-value calculated over a common region of interest and the percentage signal change in the same region were all found to peak at TE approximately T(2)* and increase significantly with field strength. An earlier onset of the haemodynamic response at higher field provides some evidence for a reduced venous contribution to the BOLD signal at 7 T.
authors.affiliation: Sir Peter Mansfield Magnetic Resonance Centre, School of Physics and Astronomy, University of Nottingham, Nottingham, UK.
authors.forename: Wietske
authors.initials: W
authors.lastname: van der Zwaag
authors.forename: Susan
authors.initials: S
authors.lastname: Francis
authors.forename: Kay
authors.initials: K
authors.lastname: Head
authors.forename: Andrew
authors.initials: A
authors.lastname: Peters
authors.forename: Penny
authors.initials: P
authors.lastname: Gowland
authors.forename: Peter
authors.initials: P
authors.lastname: Morris
authors.forename: Richard
authors.initials: R
authors.lastname: Bowtell
mesh_terms: D000328:Adult; D001931:Brain Mapping / Q000379:methods*; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*; D012680:Sensitivity and Specificity