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

M. E. Brevard, T. Q. Duong, J. A. King, C. F. Ferris

Research output: Contribution to journalArticlepeer-review

84 Scopus citations


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 (CO2) 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.

Original languageEnglish (US)
Pages (from-to)995-1001
Number of pages7
JournalMagnetic Resonance Imaging
Issue number9
StatePublished - Nov 2003
Externally publishedYes


  • Bold signal intensity
  • Functional magnetic resonance imaging
  • Regional cerebral blood flow
  • Vascular reactivity

ASJC Scopus subject areas

  • Biophysics
  • Biomedical Engineering
  • Radiology Nuclear Medicine and imaging


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