Kidneys

This randomized crossover study aims to validate measurements of renal perfusion in healthy males with PET/CT, using 15-oxygen labeled water as tracer, against the validated multiparametric MRI technique. The study will determine: * Total renal blood flow and regional renal perfusion at baseline (rest) and during isometric muscle exercise of the upper arm, using PET/CT (with 15-oxygen labeled water as a tracer) and MRI. * The coefficient of variation of renal blood flow and regional renal perfusion measurements at baseline and during isometric muscle exercise of the upper arm, using PET/CT (with 15-oxygen labeled water as a tracer) and MRI. * Regional renal oxygenation and oxygen extraction at baseline and during isometric muscle exercise of the upper arm using MRI. * The coefficient of variation on regional renal oxygenation and oxygen extraction measurements at baseline and during isometric muscle exercise of the upper arm using MRI.