High-resolution CMRO2 mapping in rat cortex: A multiparametric approach tocalibration of BOLD image contrast at 7 Tesla

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) method, which is sensitive to vascular paramagnetic deoxyhem oglobin, is dependent on regional values of cerebral metabolic rate of oxyg en utilization (CMRO2), blood flow (CBF), and volume (CBV). Induced changes in deoxyhemoglobin function as an endogenous contrast agent, which in turn affects the transverse relaxation rates of tissue water that can be measur ed by gradient-echo and spin-echo sequences in BOLD fMRI. The purpose here was to define the quantitative relation between BOLD signal change and unde rlying physiologic parameters. To this end, magnetic resonance imaging and spectroscopy methods were used to measure CBF, CMRO2, CBV, and relaxation r ates (with gradient-echo and spin-echo sequences) at 7 Tesla in rat sensori motor cortex, where cerebral activity was altered pharmacologically within the autoregulatory range. The changes in tissue transverse relaxation rates were negatively and linearly correlated with changes in CBF, CMRO2, and CB V. The multiparametric measurements revealed that CBF and CMRO2 are the dom inant physiologic parameters that modulate the BOLD fh IRI signal, where th e ratios of (Delta CMRO2/CMRO2)/(Delta CBF/ CBF) and (Delta CBV/CBV)/(Delta CBF/CBF) were 0.86 +/- 0.02 and 0.03 +/- 0.02, respectively. The calibrate d BOLD signals (spatial resolution of 48 mu L) from gradient-echo and spin- echo se quences were used to predict changes in CMRO2 using measured change s in CBF, CBV, and transverse relaxation rates. The excellent agreement bet ween measured and predicted values for changes in CMRO2 provides experiment al support of the current theory of the BOLD phenomenon. In gradient-echo s equences, BOLD contrast is affected by reversible processes such as static inhomogeneities and slow diffusion, whereas in spin-echo sequences these ef fects are refocused and are mainly altered by extravascular spin diffusion. This study provides steps by which multiparametric MRI measurements can be used to obtain high-spatial resolution CMRO2 maps.