MRI measurements of cerebral deoxyhaemoglobin concentration [dHb]--correlation with near infrared spectroscopy (NIRS).

Changes in physiological parameters such as cerebral blood flow, cerebral blood volume, oxygen extraction, and the size and distribution of cerebral blood vessels, result in changes in the local concentration of deoxyhaemoglobin ([dHb]). The purpose of this study was to quantitatively investigate the dependence of the R2* relaxation rate upon the [dHb] per voxel. Five neonatal piglets were studied in a 7 T/20 cm bore magnet. MRI was conducted using a 2.5 cm diameter surface coil placed over the parietal lobes. Four progressively T2*-weighted images were acquired, allowing the absolute quantitation of R2*. Simultaneous near infrared spectroscopy (NIRS) measurements were made from an area encompassing the MR imaging slice, and allowed the absolute quantitation of [dHb]. The arterial oxygen saturation (SaO2) of the piglet was lowered stepwise by decreasing the fractional inspired oxygen concentration (FiO2), which precipitated a change in [dHb]. NIRS and MRI measurements were made at each FiO2 step. The results demonstrate an extremely strong, linear relationship between R2* as determined by MRI and [dHb], as measured by NIRS. Whereas NIRS can only give us a global measure of [dHb], the results suggest the future use of MRI in producing high resolution relaxation rate maps related to the [dHb] distribution of the brain.