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, 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 microL/g (n = 7) and 29+/-19 microL/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.
Ulatowski
J A
JA
Department of Anesthesiology, Johns Hopkins University Medical School, Baltimore, Maryland 21205, USA.
Oja
J M
JM
Suarez
J I
JI
Kauppinen
R A
RA
Traystman
R J
RJ
van Zijl
P C
PC
D000818:Animals; D001810:Blood Volume; D002245:Carbon Dioxide; D002415:Cats; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006454:Hemoglobins; D008279:Magnetic Resonance Imaging; D008297:Male
D006454:Hemoglobins; D002245:Carbon Dioxide