Abstract
The role of hypoxia in renal disease and injury has long been suggested but much work still remains, especially as it relates to human translation. Invasive pO2 probes are feasible in animal models but not for human use. In addition, they only provide localized measurements. Histological methods can identify hypoxic tissue and provide a spatial distribution, but are invasive and allow only one-time point. Blood oxygenation level dependent (BOLD) MRI is a noninvasive method that can monitor relative oxygen availability across the kidney. It is based on the inherent differences in magnetic properties of oxygenated vs. deoxygenated hemoglobin. Presence of deoxyhemoglobin enhances the spin-spin relaxation rate measured using a gradient echo sequence, known as R2* (= 1/T2*). While the key interest of BOLD MRI is in the application to humans, use in preclinical models is necessary primarily to validate the measurement against invasive methods, to better understand physiology and pathophysiology, and to evaluate novel interventions. Application of MRI acquisitions in preclinical settings involves several challenges both in terms of logistics and data acquisition. This section will introduce the concept of BOLD MRI and provide some illustrative applications. The following sections will discuss the technical issues associated with data acquisition and analysis.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.
Highlights
Renal hemodynamics and oxygenation offer a number of striking differences vs. nonrenal tissues
Work using atrial natriuretic peptide (ANP), which was expected to cause some increase in blood flow, caused a significant decrease in both cortical and medullary O2 due to the net increase in O2 consumption related to sodium transport [6]
Two MR imaging techniques are used for renal Blood oxygenation level dependent (BOLD) Magnetic resonance imaging (MRI), namely, single shot echo planar imaging (EPI) [13] and multiple gradient echo sequences [4]
Summary
All organs exist in a state of dynamically balanced oxygen supply as determined by blood flow and arterial oxygen content and demand as determined by metabolism. Increased renal blood flow is, in general, accompanied by increased glomerular filtration rate, and necessitates increased energy-dependent tubular sodium reabsorption Another particularity is the highly heterogeneous blood perfusion and oxygenation within the kidney. For a comprehensive description of the causes and the degree of the heterogeneity of intrarenal perfusion and oxygenation see the chapter by Cantow K et al “Quantitative Assessment of Renal Perfusion and Oxygenation by Invasive Probes: Basic Concepts.” Given these particularities in renal oxygenation, there is an interest in understanding the physiological mechanisms involved in maintaining the oxygenation status within the medulla in health and how they may be affected in disease. Work using atrial natriuretic peptide (ANP), which was expected to cause some increase in blood flow, caused a significant decrease in both cortical and medullary O2 due to the net increase in O2 consumption related to sodium transport [6] This illustrates the distinction between monitoring oxygenation vs perfusion or blood flow and is especially relevant in the kidney.
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