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Functional magnetic resonance imaging based on changes in vascular space occupancy.

Hanzhang Lu1, Xavier Golay, James J Pekar

  • 1Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.

Magnetic Resonance in Medicine
|July 24, 2003
PubMed
Summary

A novel functional MRI (fMRI) method, vascular space occupancy (VASO)-fMRI, measures cerebral blood volume changes without contrast agents. This technique reveals microvascular adjustments during brain activity, offering improved gray matter localization compared to BOLD fMRI.

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Area of Science:

  • Neuroimaging
  • Physiology
  • Medical Physics

Background:

  • Brain activation involves dynamic microvascular adjustments in oxygen delivery.
  • Existing functional MRI (fMRI) methods like BOLD and CBF-based fMRI have limitations in characterizing these microvascular changes.
  • A need exists for non-invasive fMRI techniques sensitive to microvascular dynamics independent of blood oxygenation and flow.

Purpose of the Study:

  • To introduce and validate a new fMRI methodology, vascular space occupancy (VASO)-fMRI, for assessing microvascular adjustments.
  • To demonstrate the feasibility of VASO-fMRI in response to visual stimulation, hypercapnia, and hypocapnia.
  • To compare the hemodynamic responses and localization properties of VASO-fMRI with CBF-based and BOLD fMRI.

Main Methods:

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  • Developed an fMRI contrast mechanism that eliminates the blood signal independently of oxygenation and flow.
  • Measured changes in parenchymal tissue signal (extravascular water) to infer cerebral blood volume (CBV) changes.
  • Validated the approach using visual stimulation, breath-hold (hypercapnia), and voluntary hyperventilation (hypocapnia) paradigms.

    • Main Results:

    • VASO-fMRI successfully detected inverse signal correlations with visual stimulation and hypercapnia, consistent with vasodilation.
    • The signal response was reversed during hyperventilation, indicating vasoconstriction.
    • VASO-fMRI demonstrated both arteriolar and venular temporal characteristics, with negligible effects from water exchange rate and CSF contamination.
    • At standard fMRI resolution, VASO-fMRI showed comparable contrast-to-noise ratio (CNR) to CBF-fMRI but was lower than BOLD-fMRI.
    • Arguments suggest superior gray matter localization for VASO-fMRI compared to BOLD-fMRI.

    Conclusions:

    • VASO-fMRI is a feasible non-invasive technique for assessing cerebral blood volume changes related to microvascular activity.
    • The method provides insights into both vasodilatory and vasoconstrictive responses in the brain vasculature.
    • VASO-fMRI offers potential advantages in gray matter localization for functional neuroimaging studies.