Blood oxygen level-dependent MR in renal disease: moving toward clinical utility

A feasibility study of using noninvasive renal oxygenation imaging for the early assessment of ischemic acute kidney injury in an embolization model

PURPOSE

To investigate the feasibility of using MRI based oxygenation imaging for early assessment of ischemic acute kidney injury (AKI) in an embolization model.

METHODS

Ischemic AKI model was induced in 40 rabbits by injection of microspheres into the right renal arteries. Animals were grouped according to the dose of microspheres: Severe AKI group, 2.0 mg (N = 10); Moderate AKI group, 1.0 mg (N = 10); Mild AKI group, 0.5 mg (N = 10); Control group, saline without microspheres (N = 10). A serial MRI examination was performed at intervals of 1 h, 1 day, 1 week and 4 weeks to evaluate the deterioration of renal function. A multi-echo ASE sequence was implemented for renal oxygenation measurement 1 h after surgery. Pathological examinations were performed 4 weeks after the surgery.

RESULTS

In renal cortex, renal oxygen extraction fraction (OEF) raised significantly after embolization procedures in all experimental groups (severe AKI: 0.39 ± 0.05, P < 0.05; moderate AKI: 0.36 ± 0.03, P < 0.05; mild AKI: 0.34 ± 0.02, P < 0.05) compared to the control group (0.29 ± 0.02). In outer medulla, significant difference was observed between control group (0.29 ± 0.03) and severe AKI group (0.35 ± 0.03, P < 0.05), and between control group and moderate AKI group (0.34 ± 0.04, P < 0.05). Corresponding lesions were found in pathological examinations 4 weeks after the procedure.

CONCLUSION

This study demonstrates the feasibility of using oxygenation imaging to assess the embolization induced ischemic AKI at an early stage.

Methods of Blood Oxygen Level-Dependent Magnetic Resonance Imaging Analysis for Evaluating Renal Oxygenation

Blood oxygen level-dependent magnetic resonance imaging (BOLD MRI) has recently been utilized as a noninvasive tool for evaluating renal oxygenation. Several methods have been proposed for analyzing BOLD images. Regional ROI selection is the earliest and most widely used method for BOLD analysis. In the last 20 years, many investigators have used this method to evaluate cortical and medullary oxygenation in patients with ischemic nephropathy, hypertensive nephropathy, diabetic nephropathy, chronic kidney disease (CKD), acute kidney injury and renal allograft rejection. However, clinical trials of BOLD MRI using regional ROI selection revealed that it was difficult to distinguish the renal cortico-medullary zones with this method, and that it was susceptible to observer variability. To overcome these deficiencies, several new methods were proposed for analyzing BOLD images, including the compartmental approach, fractional hypoxia method, concentric objects (CO) method and twelve-layer concentric objects (TLCO) method. The compartmental approach provides an algorithm to judge whether the pixel belongs to the cortex or medulla. Fractional kidney hypoxia, measured by using BOLD MRI, was negatively correlated with renal blood flow, tissue perfusion and glomerular filtration rate (GFR) in patients with atherosclerotic renal artery stenosis. The CO method divides the renal parenchyma into six or twelve layers of thickness in each coronal slice of BOLD images and provides a R2* radial profile curve. The slope of the R2* curve associated positively with eGFR in CKD patients. Indeed, each method invariably has advantages and disadvantages, and there is generally no consensus method so far. Undoubtedly, analytic approaches for BOLD MRI with better reproducibility would assist clinicians in monitoring the degree of kidney hypoxia and thus facilitating timely reversal of tissue hypoxia.

Renal Blood Oxygenation Level-Dependent Magnetic Resonance Imaging: A Sensitive and Objective Analysis

OBJECTIVES

The aim of this study was to determine a robust (sensitive and objective) method for analyzing renal blood oxygenation level-dependent magnetic resonance imaging data.

MATERIALS AND METHODS

Forty-seven subjects (30 with chronic kidney disease [CKD] and 17 controls) were imaged at baseline and after furosemide with a multiecho gradient recalled echo sequence. Conventional analysis consisted of regional segmentation (small cortex, large cortex, and medulla), followed by computing the mean of each region. In addition, we segmented the entire parenchyma and computed the mean (μ1) plus higher moments (μ2, μ3, and μ4). Two raters performed each of the segmentation steps, and agreement was assessed with intraclass correlation coefficients (ICCs). We used a measure of effect size (Cohen's d value), in addition to the usual measure of statistical significance, P values, for determining significant results.

RESULTS

The mean of the renal parenchyma showed the highest agreement between raters (ICC, 0.99), and the higher parenchyma moments were on par with large cortical region of interest (ROI) ICC. The renal parenchymal mean also exhibited significant sensitivity to changes after furosemide administration in healthy subjects (P = 0.002, d = 0.84), in agreement with medullary ROIs (P = 0.002, d = 1.59). When comparing controls and subjects with CKD at baseline, cortical ROI showed a significant difference (P = 0.015, d = -0.69), whereas the parenchyma ROI did not (P = 0.152, d = 0.39). Post-furosemide data in all regions resulted in a significant difference (large cortex: P = 0.026, d = -0.51; medulla: P = 0.019, d = -0.61) with the renal parenchyma ROI resulting in the largest effect size (P = 0.003, d = -0.75). Higher moments of the renal parenchyma showed similar significant differences as well.

CONCLUSIONS

Overall, our data support the use of the entire parenchyma to evaluate changes in the medulla after administration of furosemide, a widely used pharmacological maneuver. Changes in higher moments indicate that there is more than just a shift in the mean renal R2* and may provide clinically relevant information without the need for subjective regional segmentation. For evaluating differences between controls and subjects with CKD at baseline; large cortical ROI provided the highest sensitivity and objectivity. A combination of renal parenchyma assessment and large cortical ROI may provide the most robust method of evaluating renal blood oxygenation level-dependent magnetic resonance imaging data.

Current MRI techniques for the assessment of renal disease

PURPOSE OF REVIEW

Over the past decade, a variety of MRI methods have been developed and applied to many kidney diseases. These MRI techniques show great promise, enabling the noninvasive assessment of renal structure, function and injury in individuals. This review will highlight the current applications of functional MRI techniques for the assessment of renal disease and discuss future directions.

RECENT FINDINGS

Many pathological (functional and structural) changes or factors in renal disease can be assessed by advanced MRI techniques. These include renal vascular structure and function (contrast-enhanced MRI, arterial spin labelling), tissue oxygenation (blood oxygen level dependent MRI), renal tissue injury and fibrosis (diffusion or magnetization transfer imaging, magnetic resonance elastography), renal metabolism (chemical exchange saturation transfer, spectroscopic imaging), nephron endowment (cationic-contrast imaging), sodium concentration (23Na-MRI) and molecular events (targeted-contrast imaging).

SUMMARY

Current advances in MRI techniques have enabled the noninvasive investigation of renal disease. Further development, evaluation and application of the MRI techniques should facilitate better understanding and assessment of renal disease, and the development of new imaging biomarkers, enabling the intensified treatment of high-risk populations and a more rapid interrogation of novel therapeutic agents and protocols.

Significant perturbation in renal functional magnetic resonance imaging parameters and contrast retention for iodixanol compared with iopromide: an experimental study using blood-oxygen-level-dependent/diffusion-weighted magnetic resonance imaging and computed tomography in rats

OBJECTIVES

The objective of this study was to investigate the renal changes after intravenous administration of a high dose of either iodixanol or iopromide using functional magnetic resonance imaging (MRI) and computed tomography (CT).

MATERIALS AND METHODS

The study was approved by the institutional committee on animal research. Seventy-two male Sprague-Dawley rats were divided into 5 cohorts, comprising normal saline (NS), iopromide, iopromide + NS, iodixanol, and iodixanol + NS. Intravenous contrast was administrated at 8 g iodine/kg of body weight. Renal CT, quantitative functional MRI of blood-oxygen-level-dependent (BOLD) imaging and diffusion-weighted imaging (DWI), and histologic examinations were performed for 18 days after contrast administration. Statistical analysis was performed by using 1-way analysis of variance, Mann-Whitney test, and regression analysis.

RESULTS

In the renal cortex, BOLD showed persistent elevation of R2* and DWI showed persistent suppression of apparent diffusion coefficient after iodixanol administration for 18 days. Compared with iopromide, adjusted ΔR2* (ΔR2*adj) was significantly higher in the iodixanol group from 1 hour to 18 days (P < 0.04) after contrast; adjusted ΔADC (ΔADCadj) was significantly more pronounced at day 6 (P = 0.01) after contrast. The iodixanol cohort also exhibited persistently higher attenuation in the renal cortex on CT and more severe microscopic renal cortical vacuolization up to 18 days. Intravenous hydration decreased the magnetic resonance changes in both groups but more markedly with iodixanol.

CONCLUSIONS

At high doses, iodixanol induced greater changes in renal functional MRI (BOLD and DWI) relative to iopromide. Combined with longer contrast retention within the kidney, this suggests that iodixanol may produce more severe and longer-lasting contrast-induced renal damage.

Renal artery stenosis-when to screen, what to stent?

Renal artery stensosis (RAS) continues to be a problem for clinicians, with no clear consensus on how to investigate and assess the clinical significance of stenotic lesions and manage the findings. RAS caused by fibromuscular dysplasia is probably commoner than previously appreciated, should be actively looked for in younger hypertensive patients and can be managed successfully with angioplasty. Atheromatous RAS is associated with increased incidence of cardiovascular events and increased cardiovascular mortality, and is likely to be seen with increasing frequency. Evidence from large clinical trials has led clinicians away from recommending interventional revascularisation towards aggressive medical management. There is now interest in looking more closely at patient selection for intervention, with focus on intervening only in patients with the highest-risk presentations such as flash pulmonary oedema, rapidly declining renal function and severe resistant hypertension. The potential benefits in terms of improving hard cardiovascular outcomes may outweigh the risks of intervention in this group, and further research is needed.

Diffusion weighted imaging and blood oxygen level-dependent MR imaging of kidneys in patients with lupus nephritis

BACKGROUND

Lupus nephritis (LN) is one of most common secondary glomerulonephritis. There is no ideal method to simultaneously assess renal structure and function in patients with LN. The aim of this study is to investigate the utility of diffusion weighted imaging (DWI) and blood oxygen level-dependent (BOLD) MR imaging in the assessment of renal involvement and pathological changes in patients with LN.

METHODS

Sixty-five patients with LN and 16 healthy volunteers underwent coronal echo-planar DWI and BOLD MR imaging of the kidneys. The apparent diffusion coefficient (ADC) and R2* values of the kidneys were calculated with b values of 0 and 500 s/mm(2). The relationship between the renal injury variables and the ADCs or R2* values were evaluated. And 16 of 65 patients with LN underwent a repeated evaluation after the induction treatment for 9 to 12 months.

RESULTS

The mean ADC values of kidneys in patients with LN were 2.40 ± 0.25 × 10(-3) mm(2)/s, the mean R2* values of the renal cortex and medulla were 11.03 ± 1.60/sec and 14.05 ± 3.38/sec respectively, which were all significantly lower than that in volunteers. In patients with LN, the mean ADC values were correlated with eGFR (r = 0.510, p < 0.01). There was a negative correlation between the mean ADC values and renal pathology chronicity indexes (r = -0.249, p < 0.05), the R2* values of the renal medulla and proteinuria (r = -0.244, p < 0.05), and the degree of tubulointerstitial lesions (r = -0.242, p < 0.05). The ADC and R2* values of kidneys were significantly higher than those of pre-treatment in complete remission patients.

CONCLUSIONS

DWI and BOLD MR imaging of kidneys may be used to noninvasively monitor the disease activity and evaluate therapeutic efficacy in lupus nephritis.