title: Detailing the relation between renal T2* and renal tissue pO2 using an integrated approach of parametric magnetic resonance imaging and invasive physiological measurements.
abstract: OBJECTIVE
  This study was designed to detail the relation between renal T2* and renal tissue pO2 using an integrated approach that combines parametric magnetic resonance imaging (MRI) and quantitative physiological measurements (MR-PHYSIOL).
  
  
  METHODS
  Experiments were performed in 21 male Wistar rats. In vivo modulation of renal hemodynamics and oxygenation was achieved by brief periods of aortic occlusion, hypoxia, and hyperoxia. Renal perfusion pressure (RPP), renal blood flow (RBF), local cortical and medullary tissue pO2, and blood flux were simultaneously recorded together with T2*, T2 mapping, and magnetic resonance-based kidney size measurements (MR-PHYSIOL). Magnetic resonance imaging was carried out on a 9.4-T small-animal magnetic resonance system. Relative changes in the invasive quantitative parameters were correlated with relative changes in the parameters derived from MRI using Spearman analysis and Pearson analysis.
  
  
  RESULTS
  Changes in T2* qualitatively reflected tissue pO2 changes induced by the interventions. T2* versus pO2 Spearman rank correlations were significant for all interventions, yet quantitative translation of T2*/pO2 correlations obtained for one intervention to another intervention proved not appropriate. The closest T2*/pO2 correlation was found for hypoxia and recovery. The interlayer comparison revealed closest T2*/pO2 correlations for the outer medulla and showed that extrapolation of results obtained for one renal layer to other renal layers must be made with due caution. For T2* to RBF relation, significant Spearman correlations were deduced for all renal layers and for all interventions. T2*/RBF correlations for the cortex and outer medulla were even superior to those between T2* and tissue pO2. The closest T2*/RBF correlation occurred during hypoxia and recovery. Close correlations were observed between T2* and kidney size during hypoxia and recovery and for occlusion and recovery. In both cases, kidney size correlated well with renal vascular conductance, as did renal vascular conductance with T2*. Our findings indicate that changes in T2* qualitatively mirror changes in renal tissue pO2 but are also associated with confounding factors including vascular volume fraction and tubular volume fraction.
  
  
  CONCLUSIONS
  Our results demonstrate that MR-PHYSIOL is instrumental to detail the link between renal tissue pO2 and T2* in vivo. Unravelling the link between regional renal T2* and tissue pO2, including the role of the T2* confounding parameters vascular and tubular volume fraction and oxy-hemoglobin dissociation curve, requires further research. These explorations are essential before the quantitative capabilities of parametric MRI can be translated from experimental research to improved clinical understanding of hemodynamics/oxygenation in kidney disorders.
authors.affiliation: From the *Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine; and †Institute of Physiology, Charité-Universitätsmedizin Berlin, Campus Mitte, and Center for Cardiovascular Research, Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Pohlmann
authors.forename: Karen
authors.initials: K
authors.lastname: Arakelyan
authors.forename: Jan
authors.initials: J
authors.lastname: Hentschel
authors.forename: Kathleen
authors.initials: K
authors.lastname: Cantow
authors.forename: Bert
authors.initials: B
authors.lastname: Flemming
authors.forename: Mechthild
authors.initials: M
authors.lastname: Ladwig
authors.forename: Sonia
authors.initials: S
authors.lastname: Waiczies
authors.forename: Erdmann
authors.initials: E
authors.lastname: Seeliger
authors.forename: Thoralf
authors.initials: T
authors.lastname: Niendorf
mesh_terms: D000465:Algorithms; D000818:Animals; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D007668:Kidney / Q000033:anatomy & histology* / Q000378:metabolism*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D010092:Oximetry / Q000379:methods*; D010100:Oxygen / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D018511:Systems Integration
chemical_list: D010100:Oxygen