Functional MRI impulse response for BOLD and CBV contrast in rat somatosensory cortex.

The contrast mechanism in functional MRI (fMRI) results from several vascular processes with different time scales, thus establishing a finite temporal resolution to fMRI experiments. In this work we measured the blood oxygen level-dependent (BOLD) and iron-oxide-derived cerebral blood volume (CBV) impulse response (IR) in a rat model of somatosensory brain activation at 11.7T. A binary m-sequence probe method was used to obtain high-sensitivity single-pixel estimates of the IR, from which two parameters-the full width at half maximum (FWHM) and the time to peak (TTP)-were determined as indices of the temporal resolution of the hemodynamic response (HDR). The results (N = 11) show that the CBV IR (N = 5, subset) is significantly narrower (FWHM = 1.37 +/- 0.11 s), and peaks earlier (TTP = 1.65 +/- 0.15 s) than the BOLD IR (N = 11, FWHM = 1.92 +/- 0.22 s and TTP = 2.18 +/- 0.14 s, respectively). These findings indicate that neurovascular control mechanisms have a temporal resolution better than 1.5 s FWHM, and point to a substantial contribution to BOLD of the dispersive transit of oxygenated hemoglobin across the rat vasculature, bringing important implications for the ultimately attainable temporal resolution of fMRI.