In vivo determination of absolute cerebral blood volume using hemoglobin as a natural contrast agent: An MRI study using altered arterial carbon dioxide tension

The ability of the magnetic resonance imaging transverse relaxation time, R -2 = 1/T-2, to quantify cerebral blood volume (CBV) without the need for an exogenous contrast agent was studied in cats (n = 7) under pentobarbital a nesthesia. This approach is possible because R-2 is directly affected by ch anges in CBF, CBV, CMRO2, and hematocrit (Hct), a phenomena better known as the blood-oxygenation-level-dependent (BOLD) effect. Changes in CBF and CB V were accomplished by altering the carbon dioxide pressure, PaCO2, over a range from 20 to 140 mm Hg. For each PaCO2 value, R-2 in gray and white mat ter were determined using MRI, and the whole-brain oxygen extraction ratio was obtained from arteriovenous differences (sagittal sinus catheter). Assu ming a constant CMRO2, the microvascular CBV was obtained from an exact fit to the BOLD theory for the spin-echo effect. The resulting CBV values at n ormal PaCO2 and normalized to a common total hemoglobin concentration of 6. 88 mmol/L were 42 +/- 18 mu L/g (n = 7) and 29 +/- 19 mu L/g (n = 5) for gr ay and white matter, respectively, in good agreement with the range of lite rature values published using independent methodologies. The present study confirms the validity of the spin-echo BOLD theory and, in addition, shows that blood volume can be quantified from the magnetic resonance imaging spi n relaxation rate R-2 using a regulated carbon dioxide experiment.