title: Normal tissue quantitative T1 and T2* MRI relaxation time responses to hypercapnic and hyperoxic gases.
abstract: OBJECTIVE
  Longitudinal (T(1)) and effective transverse (T(2)*) magnetic resonance (MR) relaxation times provide noninvasive measures of tissue oxygenation. The objective for this study was to quantify independent effects of inhaled O(2) and CO(2) on normal tissue T(1) and T(2)* in rabbit liver, kidney, and paraspinal muscle.
  
  
  METHODS
  Three gas challenges (100% O(2), 10% CO(2) [balance air], and carbogen [90% O(2) + 10% CO(2)]) were delivered to the rabbits in random order to isolate the effects of inspired O(2) and CO(2). During each challenge, quantitative T(1) and T(2)* maps were collected on a 1.5 Tesla MR imaging. Mean changes in T(1) (ΔT(1)) and T(2)* (ΔT(2)*) were calculated from regions of interest in each organ.
  
  
  RESULTS
  Greatest ΔT(1) and ΔT(2)* changes were observed in liver for 10% CO(2) and in kidney for 100% O(2). ΔT(1) and ΔT(2)* generally followed predicted patterns when transitioning from air breathing: lower T(1)/higher T(2)* with inspired O(2), higher T(1)/lower T(2)* with inspired CO(2), and variable T(1)/T(2)* changes in the presence of both (ie, carbogen). New observations also emerged: 1) between-gas-challenge transitions revealed the greatest significance in ΔT(2)* for the liver and kidney resulting from the isolation of independent O(2) and CO(2) effects; 2) ΔT(2)* provided the best sensitivity and detected both tissue oxygenation and blood volume modulation; and 3) ΔT(1) sensitivity was restricted mainly to tissue oxygenation in the absence of counteracting vasodilatation.
  
  
  CONCLUSIONS
  Robust use of MR relaxation times as noninvasive biomarkers requires an understanding of their relative sensitivity to organ-specific physiological responses.
authors.affiliation: The Research Institute and Diagnostic Imaging, The Hospital for Sick Children, Toronto, Ontario, Canada.
authors.forename: Jeff D
authors.initials: JD
authors.lastname: Winter
authors.forename: Marvin
authors.initials: M
authors.lastname: Estrada
authors.forename: Hai-Ling Margaret
authors.initials: HL
authors.lastname: Cheng
mesh_terms: D000280:Administration, Inhalation; D000818:Animals; D001784:Blood Gas Analysis; D002245:Carbon Dioxide / Q000008:administration & dosage / Q000378:metabolism*; D006935:Hypercapnia / Q000378:metabolism; D018496:Hyperoxia / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008279:Magnetic Resonance Imaging / Q000379:methods*; D018482:Muscle, Skeletal / Q000378:metabolism; D010100:Oxygen / Q000008:administration & dosage / Q000378:metabolism*; D011817:Rabbits
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen