Blood-oxygen-level-dependent (BOLD) magnetic resonance imaging (MRI) can provide regional measurements of oxygen content using deoxyhemoglobin paramagnetic characteristics. The apparent relaxation rate or R2*(=1/T2*) can be determined from the slope of log (intensity) versus echo time and is directly proportional to the tissue content of deoxyhemoglobin. Thus, as the level of deoxyhemoglobin increases, T2* will decrease, leading to an increase in R2*. Chronic kidney disease (CKD) can affect oxygenation levels in renal parenchyma, which influences the clinical course of the disease. The goal of this study was to detect and assess renal oxygenation levels in CKD using BOLD MRI.
Fifteen healthy subjects and 11 patients with CKD underwent a renal scan using multigradient-recalled-echo sequence with eight echoes. R2* (1/s) of the renal cortex and medulla was measured on BOLD images. Of the 11 patients, nine had biopsy-proven chronic glomerulonephritis, and two had a similar diagnosis based on clinical symptoms and investigations.
Mean medullary R2* (MR2*) and cortex R2* (CR2*) levels were significantly higher in patients (22 kidneys, MR2*=24.79±4.84 s(-1), CR2*=18.97±2.72 s(-1)) than in controls (30 kidneys, MR2*=19.98±1.19 s(-1), CR2*=16.03±1.23 s(-1)) (P<.01), and MR2* was increased more than CR2*. Medullary to cortical R2* ratios (MCR2*) of patients were significantly increased when compared with those of controls (P<.01). In the patient group, estimated glomerular filtration rate levels were greater than or equal to 60 ml/min/1.73 m(2) in six patients (12 kidneys), whose MR2* and CR2* were also significantly higher than those of controls (P<.01). Serum creatinine levels were normal in seven patients (14 kidneys), whose MR2*, CR2* and MCR2* were also higher than those of controls (P<.01).
BOLD MRI can be used to evaluate changes in renal oxygenation in CKD, suggesting that it has the potential to be an excellent noninvasive tool for the evaluation of renal function.
[Show abstract][Hide abstract] ABSTRACT: In functional renal magnetic resonance imaging (MRI), advanced techniques are applied to obtain information on a functional and molecular level from the kidney tissue beyond pure morphology. Techniques such as diffusion-weighted and diffusion tensor imaging, arterial spin labelling, and blood oxygenation level-dependent imaging provide potential biomarkers of organ function. Moreover, dynamic contrast-enhanced techniques after the intra-venous injection of gadolinium-chelates may be used to assess glomerular filtration and urinary excretion. This review summarizes recent developments of contrast- and non-contrast-enhanced MRI techniques for assessment of renal function in a clinical setting. The physiological background and the sequence techniques are described in detail. Potential clinical applications of the different techniques are discussed regarding their potential usefulness in the assessment of parenchymal diseases, urinary tract anomalies, transplant kidney function, and renal masses.
[Show abstract][Hide abstract] ABSTRACT: Oxygenation defects may contribute to renal disease progression but the chronology of events is difficult to define in vivo without recourse to invasive methodologies. BOLD MRI provides an attractive alternative but the R2* signal is physiologically complex. Post-acquisition data analysis often relies on manual selection of region(s) of interest. This approach excludes from analysis significant quantities of biological information and is subject to selection bias. We present a semi-automated, anatomically unbiased approach to compartmentalize voxels into two quantitatively related clusters. In control F344 rats, low R2* clustering was located predominantly within the cortex and higher R2* clustering within the medulla (70.96±1.48 versus 79.00±1.50; 3 scans per rat; n=6; P<0.01) consistent anatomically with a cortico-medullary oxygen gradient. An intravenous bolus of acetylcholine caused a transient reduction of the R2* signal in both clustered segments (P<0.01). This was nitric oxide dependent and temporally distinct from the hemodynamic effects of acetylcholine. Rats were then chronically infused with angiotensin II (60ng/min) and rescanned three days later. Clustering demonstrated a disruption of the cortico-medullary gradient, producing less distinctly segmented mean R2* clusters (71.30±2.00; versus 72.48±1.27; n=6; NS). The acetylcholine-induced attenuation of the R2* signal was abolished by chronic angiotensin II infusion, consistent with reduced nitric oxide bioavailability. This global map of oxygenation, defined by clustering individual voxels on the basis of quantitative nearness might be more robust in defining deficits in renal oxygenation than the absolute magnitude of R2* in small, manually selected regions of interest defined exclusively by anatomical nearness.
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