Estimation of T2* in functional spectroscopy during visual stimulation

Changes in T2* of activated pixels were estimated using single voxel and 1D chemical shift imaging (CSI) proton spectroscopy during visual stimulation. A single voxel was angulated and positioned to enclose activated pixels as determined by a corresponding functional MRI study. Eddy currents in angulated voxel studies produced a bump in the water peak, which was removed by modeling a biexponential function with two frequencies to fit the time domain data. The results show approximately 1.8% T2* change when averaged over five subjects. Non-angulated voxels, which do not enclose the activated pixels as closely as the angulated voxels but are free from eddy current artifacts were also studied. It was possible to separate the contributions from brain tissue and cerebrospinal fluid (csf) in the non-angulated voxels through a two-compartment model. Changes in the T2* during stimulation were detected for the tissue compartment only. Fitting also determined the amplitude of the water signal which is directly related to any T1 or in-flow related changes. The results show negligible changes in the amplitude. The height of the water peak in frequency domain, which is directly related to T2*, was found to increase during stimulation in a proportionate manner. The peak height was also used to localize activation in the visual cortex in 1D CSI experiments. It is concluded that T2* changes reflecting a blood oxygenation level dependent or BOLD effect are demonstrated in these studies.