Probing renal blood volume with magnetic resonance imaging

Abstract

Damage to the kidney substantially reduces life expectancy. Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. In vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Blood oxygenation level–dependent (BOLD) magnetic resonance imaging (MRI) is sensitive to changes in the effective transversal relaxation time (T2*) in vivo, and is non‐invasive and indicative of renal tissue oxygenation. However, the renal T2* to tissue pO2 relationship is not governed exclusively by renal blood oxygenation, but is affected by physiological confounders with alterations in renal blood volume fraction (BVf) being of particular relevance. To decipher this interference probing renal BVf is essential for the pursuit of renal MR oximetry. Superparamagnetic iron oxide nanoparticle (USPIO) preparations can be used as MRI visible blood pool markers for detailing alterations in BVf. This review promotes the opportunities of MRI‐based assessment of renal BVf. Following an outline on the specifics of renal oxygenation and perfusion, changes in renal BVf upon interventions and their potential impact on renal T2* are discussed. We also describe the basic principles of renal BVf assessment using ferumoxytol‐enhanced MRI in the equilibrium concentration regimen. We demonstrate that ferumoxytol does not alter control of renal haemodynamics and oxygenation. Preclinical applications of ferumoxytol enhanced renal MRI as well as considerations for its clinical implementation for examining renal BVf changes are provided alongside practical considerations. Finally, we explore the future directions of MRI‐based assessment of renal BVf.

Multimodal Functional Imaging for Cancer/Tumor Microenvironments Based on MRI, EPRI, and PET

“…For BOLD MRI, several review papers were already published and described technical details with numerous applications in functional MRI [ 80 , 81 , 82 ]. Paramagnetic deoxyhemoglobin has a ferrous iron on the heme.…”

Clinical and experimental approaches for imaging of acute kidney injury

“…Several currently available imaging techniques can evaluate renal blood flow in AKI [57]. Dynamic contrast-enhanced MRI, which uses iron oxide nanoparticles or gadoliniumbased contrast agents with low molecular weight, can determine the spatial distribution of renal perfusion and vascular reactivity [55,[58][59][60]. Of note, single-kidney GFR/ split function can be estimated using this method [61].…”

“…In addition to 1 H MRI, fluorine-19 MRI can be employed to quantify changes in blood partial pressure of oxygen (pO 2 ) and BV in kidneys [ 60 , 69 ]. The application of fluorinated emulsions prepared from perfluorocarbons to examine BV fraction and pO 2 in the renal microvasculature has been explored using mice with AKI [ 70 ].…”

Physiological system analysis of the kidney by high‐temporal‐resolution monitoring of an oxygenation step response

“…To be rendered a semi-quantitative physiological approach, MRI needs to be benchmarked with integrative physiological measurements, including invasive quantitative approaches and optical spectroscopy techniques, to detail the T * 2 dependence on O 2 saturation of Hb (StO 2 ) and the link between StO 2 , blood partial pressure of O 2 (pO 2 ), and tissue pO 2 . 13,17,26,[38][39][40] The confounding factors vascular and tubular volume fraction, hematocrit, and oxyhemoglobin dissociation may be one reason for the difficulty in establishing simple normal ranges for renal T * 2 together with guidelines for interpretation of deviations thereof. Other reasons include the variability in the measurement protocols across studies and sites, which has recently been addressed in consensus-based recommendations for renal BOLD MRI.…”

“…Advancing renal mapping into a reliable and clinically useful diagnostic tool remains challenging. To be rendered a semi‐quantitative physiological approach, MRI needs to be benchmarked with integrative physiological measurements, including invasive quantitative approaches and optical spectroscopy techniques, to detail the dependence on O 2 saturation of Hb (StO 2 ) and the link between StO 2 , blood partial pressure of O 2 (pO 2 ), and tissue pO 2 13,17,26,38‐40 . The confounding factors vascular and tubular volume fraction, hematocrit, and oxyhemoglobin dissociation may be one reason for the difficulty in establishing simple normal ranges for renal together with guidelines for interpretation of deviations thereof.…”