Abstract
BOLD-MRI (blood oxygenation-level dependent magnetic resonance imaging) allows non-invasive measurement of renal tissue oxygenation in humans, without the need for contrast products. BOLD-MRI uses the fact that magnetic properties of hemoglobin depend of its oxygenated state:: the higher local deoxyhemoglobin, the higher the so called apparent relaxation rate R2* (sec−1), and the lower local tissue oxygen content. Several factors other than deoxyhemoglobin (such as hydration status, dietary sodium intake, and susceptibility effects) influence the BOLD signal, and need to be taken into account when interpreting results. The last 5 years have witnessed important improvements in the standardization of these factors, and the appearance of new, highly reproducible analysis techniques of BOLD-images, that are reviewed in this article. Using these new BOLD-MRI analysis techniques, it has recently been shown that persons suffering from chronic kidney diseases (CKD) have lower cortical oxygenation than normotensive controls, thus confirming the chronic hypoxia hypothesis. The acute alterations in R2* after the administration of furosemide are smaller in CKD, and represent an estimate of the oxygen-dependent tubular transport of sodium. BOLD-MRI-alone or in combination with other functional MRI methods- can be used to monitor the renal effects of drugs, and is increasingly used in the preclinical setting. The near future will tell whether or not BOLD-MRI represents a new tool to predict renal function decline an adverse renal outcome.
Highlights
Chronic kidney disease (CKD), defined as an estimated glomerular filtration rate below 60 ml/min/1.73 m2 and/or the presence of albuminuria, has become a major public health problem with a global prevalence in the general population of ∼10% (Ponte et al, 2013)
We review the technical hurdles that had to be overcome, others that still need to be resolved, the main results of clinical studies and the perspectives of BOLD-Magnetic Resonance Imaging (MRI)
BOLD-MRI should identify patients at increased risk of CKD progression, expecting that those with the lowest renal oxygenation, or the lowest R2∗ change after furosemide, have the highest risk of progression. These data are lacking, hampering the introduction of BOLD-MRI in clinical practice. Another issue is the fact that BOLD-MRI alone cannot establish whether a high R2∗ value is the result of low oxygen delivery, high oxygen consumption, or both
Summary
Chronic kidney disease (CKD), defined as an estimated glomerular filtration rate below 60 ml/min/1.73 m2 and/or the presence of (micro) albuminuria, has become a major public health problem with a global prevalence in the general population of ∼10% (Ponte et al, 2013). In a third method called the fractional tissue hypoxiatechnique, the whole renal parenchyma is selected, and the percentage of R2∗ values above a certain threshold (usually 30 s−1 or >2.5 standard deviations of the average R2∗ value) is reported (Saad et al, 2013) This technique provides a variable— the percentage of hypoxic tissue- that can be interpreted by clinicians, but it does not differentiate between cortex and medulla (Figure 1, right image). Recent BOLD-MRI studies have demonstrated that CKD patients have higher R2∗ values (corresponding to lower renal tissue oxygenation) as compared to controls, confirming the findings of animal studies. The mean R2∗ is higher in CKD patients, differences are small, and according to some authors mainly driven by a minority of CKD patients with high R2∗ values (Prasad et al, 2015) This finding illustrates that renal tissue oxygenation is rather tightly controlled in the majority of individuals.
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