Changes in MRI signal intensity during hypercapnic challenge under conscious and anesthetized conditions

Abstract

Most functional magnetic resonance imaging (fMRI) studies in animals are conducted under anesthesia to minimize motion artifacts. However, methods and techniques have been developed recently for imaging fully conscious rats. Functional MRI studies on conscious animals report enhanced BOLD signal changes as compared to the anesthetized condition. In this study, rats were exposed to different concentrations of carbon dioxide (CO(2)) while conscious and anesthetized to test whether cerebrovascular reactivity may be contributing to these enhanced BOLD signal changes. Hypercapnia produced significantly greater increases in MRI signal intensity in fully conscious animals (6.7-13.3% changes) as when anesthetized with 1% isoflurane (3.2-4.9% changes). In addition, the response to hypercapnia was more immediate in the conscious condition (< 30s) with signal risetimes twice as fast as in the anesthetized state (60s). Both cortical and subcortical brain regions showed a robust, dose- dependent increase in MRI signal intensity with hypercapnic challenge while the animals were conscious but little or no change when anesthetized. Baseline variations in MRI signal were higher while animals were conscious but this was off set by greater signal intensity changes leading to a greater contrast-to-noise ratio, 13.1 in conscious animals, as compared to 8.0 in the anesthetized condition. In summary, cerebral vasculature appears to be more sensitive to hypercapnic challenge in the conscious condition resulting in enhanced T2* MRI signal intensity and the potential for better BOLD signal changes during functional imaging.

Fig. 1

Cortical (red) and subcortical (blue) regions of interest outlined on an anatomic scan.

Fig. 2

Time course of CO₂ inhalation under anesthetized and conscious conditions. Raw cortical signal intensities averaged for all animals are shown for 100 repetitions over 200 s. The bar indicates the time of exposure to CO₂.

Fig. 3

Percent change in MRI signal intensity in cortical regions under conscious and anesthetized conditions during hypercapnic challenge. There was a significant (p < 0.001) dose-dependent change in the conscious and anesthetized condition.

Fig. 4

Percent change in subcortical regions under anesthetized and conscious conditions during hypercapnic challenge. There was a significant (p < 0.001) dose-dependent change in the conscious but not in the anesthetized condition.