Magnetic Resonance Cartography of Renal Tubule Volume Fraction During Diuretic Intervention

The renal tubular volume fraction (TVF) fluctuates under physiological conditions, and is altered in several renal diseases. Tools that enable noninvasive assessment of TVF are currently lacking. Magnetic Resonance (MR) TVF cartography is a novel approach for unraveling renal (patho-)physiology. Here, we employ MR-TVF cartography to monitor changes in response to the diuretic furosemide, and examine its role for the interpretation of renal oxygenation assessed by mapping the MRI relaxation time T₂*. We hypothesize that furosemide increases TVF.

In anesthetized rats (n = 7) the MRI relaxation times T₂, T₂*, T₂' and kidney size were obtained before/following an i.v. bolus of furosemide using a 9.4 Tesla MRI scanner. Spectral analysis of the T₂ signal decay was performed to estimate the number of T₂ components in renal tissue. TVF cartographies were calculated using voxel-wise bi-exponential fit of the T₂ decay. Near Infrared Spectroscopy (NIRS, n = 9) was used to assess the total hemoglobin concentration (HbT) as a surrogate of renal blood volume.

Furosemide induced changes in renal MRI and NIRS parameters relative to baseline: TVF_CORTEX = 31.1%, TVF_{OUTER_MEDULLA} = 30.7%, T_{2_CORTEX} = 13.0% and T_{2_OUTER_MEDULLA} = 20.6%. HbT_CORTEX was reduced by 2.7%. HbT_MEDULLA declined by 8.6%. Kidney size showed a modest increase of 2.9%. T₂*_{OUTER_MEDULLA} and T₂´_{OUTER_MEDULLA} rose by 20.5% and 20.2%. T₂*_CORTEX and T₂´_CORTEX remained unchanged. T₂* and TVF were strongly correlated in the outer medulla and moderately in the cortex.

MR-TVF cartography is highly relevant for elucidating mechanisms of renal (patho-)physiology, including the role of renal oxygenation assessed by MRI mapping of renal T₂*.