Effects of intracellular pH, blood, and tissue oxygen tension on T1ρ relaxation in rat brain

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Abstract
The effects of intracellular pH (pHi), paramagnetic macroscopic, and microscopic susceptibility on T1 in the rotating frame (T1ρ) were studied in rat brain. Intracellular acidosis was induced by hypercapnia and pHi, T1ρ, T2, diffusion, and cerebral blood volume (CBV) were quantified. Taking into account the CBV contribution, a prolongation of parenchymal T1ρ by 4.5% was ascribed to a change in tissue water relaxation caused by a one unit drop in pHi. Blood T1ρ was found to prolong linearly with blood oxygenation saturation (Y). The macroscopic susceptibility contribution to parenchymal T1ρ was assessed both through BOLD and an iron oxide contrast agent, AMI‐227. The T1ρ data from these experiments could be described by intravascular effects with insignificant effects of susceptibility gradients on tissue water. Tissue oxygen tension (PtO2) was manipulated and monitored with microelectrodes to assess its plausible contribution to microscopic susceptibility and relaxation. Parenchymal T1ρ was virtually unaffected by variations in the PtO2, but T1 was shortened in hyperoxia and T2 showed a negative BOLD effect in hypoxia. It is demonstrated that pHi directly modulates tissue T1ρ, possibly through its effect on proton exchange; however, neither BOLD nor PtO2 directly influence tissue T1ρ. The observations are discussed in the light of physicochemical mechanisms contributing to the ischemic T1ρ changes. Magn Reson Med 48:470–477, 2002.
Funding Information
  • Academy of Finland
  • Sigrid Juselius Foundation
  • Finnish Cultural Foundation
  • Research Foundation of Leiras

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