In Vivo Determination of Absolute Cerebral Blood Volume Using Hemoglobin as a Natural Contrast Agent: An MRI Study Using Altered Arterial Carbon Dioxide Tension

Abstract

The ability of the magnetic resonance imaging transverse relaxation time, R₂ = 1/T₂, to quantify cerebral blood volume (CBV) without the need for an exogenous contrast agent was studied in cats (n = 7) under pentobarbital anesthesia. This approach is possible because R₂ is directly affected by changes in CBF, CBV, CMRO₂, and hematocrit (Hct), a phenomena better known as the blood-oxygenation-level-dependent (BOLD) effect. Changes in CBF and CBV were accomplished by altering the carbon dioxide pressure, Paco₂, over a range from 20 to 140 mm Hg. For each Paco₂ value, R₂ in gray and white matter were determined using MRI, and the whole-brain oxygen extraction ratio was obtained from arteriovenous differences (sagittal sinus catheter). Assuming 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 normal Paco₂ and normalized to a common total hemoglobin concentration of 6.88 mmol/L were 42 ±18 μL/g (n = 7) and 29 ±19 μL/g (n = 5) for gray and white matter, respectively, in good agreement with the range of literature 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 spin relaxation rate R₂ using a regulated carbon dioxide experiment.