Magnetic Resonance Cartography of Renal Tubule Volume Fraction During Diuretic Intervention.

AIM: 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.

METHODS

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.

RESULTS

Furosemide induced changes in renal MRI and NIRS parameters relative to baseline: TVFCORTEX = 31.1%, TVFOUTER_MEDULLA = 30.7%, T2_CORTEX = 13.0% and T2_OUTER_MEDULLA = 20.6%. HbTCORTEX was reduced by 2.7%. HbTMEDULLA 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.

CONCLUSION

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*.