1. ¹Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
2. ²Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York, USA
3. ³Department of Neurosurgery, Tokyo Medical and Dental University, Tokyo, Japan
4. ⁴Department of Neurobiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
5. ⁵Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA
6. ⁶Ogawa Laboratories for Brain Function Research, Hamano Life Science Research Foundation, Tokyo, Japan

Correspondence: Dr S-G Kim, Department of Neurobiology, University of Pittsburgh Medical School, 3025 East Carson Street, Pittsburgh, PA 15203, USA. E-mail: kimsg@ pitt.edu

Received 7 July 2005; Revised 20 October 2005; Accepted 25 October 2005; Published online 14 December 2005.

Abstract

The magnitude of the blood oxygenation level-dependent (BOLD) signal depends on cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen (CMRO₂). Thus, it is difficult to separate CMRO₂ changes from CBF and CBV changes. To detect the BOLD signal changes induced only by CMRO₂ responses without significant evoked CBF and CBV changes, BOLD and CBV functional magnetic resonance imaging (fMRI) responses to visual stimulation were measured under normal and hypotension conditions in isoflurane-anesthetized cats at 4.7 T. When the mean arterial blood pressure (MABP) decreased from 8910 to 501 mm Hg (meanstandard deviation, n=5) by infusion of vasodilator sodium nitroprusside, baseline CBV in the visual cortex increased by 28.4%8.3%. The neural activity-evoked CBV increase in the visual cortex was 10.8%3.9% at normal MABP, but was negligible at hypotension. Positive BOLD changes of +1.8%0.5% (gradient echo time=25 ms) at normal MABP condition became prolonged negative changes of -1.2%0.3% at hypotension. The negative BOLD response at hypotension starts approximately 1 sec earlier than positive BOLD response, but similar to CBV change at normal MABP condition. Our finding shows that the negative BOLD signals in an absence of CBV changes are indicative of an increase in CMRO₂. The vasodilator-induced hypotension model simplifies the physiological source of the BOLD fMRI signals, providing an insight into spatial and temporal CMRO₂ changes.