title: In vivo quantification of cerebral r2*-response to graded hyperoxia at 3 tesla.
abstract: OBJECTIVE
  This study aims to quantify the response of the transverse relaxation rate of the magnetic resonance (MR) signal of the cerebral tissue in healthy volunteers to the administration of air with step-wise increasing percentage of oxygen.
  
  
  METHODS
  The transverse relaxation rate (R2*) of the MR signal was quantified in seven volunteers under respiratory intake of normobaric gas mixtures containing 21, 50, 75, and 100% oxygen, respectively. End-tidal breath composition, arterial blood saturation (SaO2), and heart pulse rate were monitored during the challenge. R2* maps were computed from multi-echo, gradient-echo magnetic resonance imaging (MRI) data, acquired at 3.0T. The average values in the segmented white matter (WM) and gray matter (GM) were tested by the analysis of variance (ANOVA), with Bonferroni post-hoc correction. The GM R2*-reactivity to hyperoxia was modeled using the Hill's equation.
  
  
  RESULTS
  Graded hyperoxia resulted in a progressive and significant (P < 0.05) decrease of the R2* in GM. Under normoxia the GM-R2* was 17.2 ± 1.1 s(-1). At 75% O2 supply, the R2* had reached a saturation level, with 16.4 ± 0.7 s(-1) (P = 0.02), without a significant further decrease for 100% O2. The R2*-response of GM correlated positively with CO2 partial pressure (R = 0.69 ± 0.19) and negatively with SaO2 (R = -0.74 ± 0.17). The WM showed a similar progressive, but non-significant, decrease in the relaxation rates, with an increase in oxygen intake (P = 0.055). The Hill's model predicted a maximum R2* response of the GM, of 3.5%, with half the maximum at 68% oxygen concentration.
  
  
  CONCLUSIONS
  The GM-R2* responds to hyperoxia in a concentration-dependent manner, suggesting that monitoring and modeling of the R2*-response may provide new oxygenation biomarkers for tumor therapy or assessment of cerebrovascular reactivity in patients.
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland ; Klinikum Dritter Orden, Center for Radiology and Nuclear Medicine, Munich, Germany.
authors: Grigorios
authors: G
authors: Gotzamanis
authors: Department of Anesthesiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Roman
authors: R
authors: Kocian
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland ; Institute for Biomedical Engineering, Eidgenössische Technische Hochschule (ETH), Zurich, Switzerland.
authors: Pinar S
authors: PS
authors: Özbay
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Manuel
authors: M
authors: Redle
authors: Department of Neuroradiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Spyridon
authors: S
authors: Kollias
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Christian
authors: C
authors: Eberhardt
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Andreas
authors: A
authors: Boss
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Daniel
authors: D
authors: Nanz
authors: Department of Diagnostic and Interventional Radiology, University Hospital of Zurich, Zurich, Switzerland.
authors: Cristina
authors: C
authors: Rossi
keywords: Blood oxygen level dependent (BOLD); R2*; graded hyperoxia; respiratory challenge