Noninvasive evaluation of kidney hypoxia and fibrosis using magnetic resonance imaging

Role of Chemical Exchange Saturation Transfer and Magnetization Transfer MRI in Detecting Metabolic and Structural Changes of Renal Fibrosis in an Animal Model at 3T

Objective

To investigate the value of combined chemical exchange saturation transfer (CEST) and conventional magnetization transfer imaging (MT) in detecting metabolic and structural changes of renal fibrosis in rats with unilateral ureteral obstruction (UUO) at 3T MRI.

Materials and Methods

Thirty-five Sprague-Dawley rats underwent UUO surgery (n = 25) or sham surgery (n = 10). The obstructed and contralateral kidneys were evaluated on days 1, 3, 5, and 7 after surgery. After CEST and MT examinations, ¹⁸F-labeled fluoro-2-deoxyglucose positron emission tomography was performed to quantify glucose metabolism. Fibrosis was measured by histology and western blots. Correlations were compared between asymmetrical magnetization transfer ratio at 1.2 ppm (MTR_asym(1.2ppm)) derived from CEST and maximum standard uptake value (SUV_max) and between magnetization transfer ratio (MTR) derived from MT and alpha-smooth muscle actin (α-SMA).

Results

On days 3 and 7, MTR_asym(1.2ppm) and MTR of UUO renal cortex and medulla were significantly different from those of contralateral kidneys (p < 0.05). On day 7, MTR_asym(1.2ppm) and MTR of UUO renal cortex and medulla were significantly different from those of sham-operated kidneys (p < 0.05). The MTR_asym(1.2ppm) of UUO renal medulla was fairly negatively correlated with SUV_max (r = −0.350, p = 0.021), whereas MTR of UUO renal medulla was strongly negatively correlated with α-SMA (r = −0.744, p < 0.001).

Conclusion

CEST and MT could provide metabolic and structural information for comprehensive assessment of renal fibrosis in UUO rats in 3T MRI and may aid in clinical monitoring of renal fibrosis in patients with chronic kidney disease.

CuATSM PET to diagnose age-related diseases: a systematic literature review

Purpose

Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone) positron emission tomography (CuATSM PET) is a non-invasive imaging technique that can be used to detect hypoxia and inform prognosis in cancer. Hypoxia and oxidative stress are also hallmarks of various age-related diseases. Whether CuATSM PET has a role in the evaluation of hypoxia and oxidative stress in age-related diseases has yet to be established. The aim of this systematic review is to evaluate the utility of CuATSM PET in the diagnosis and management of age-related diseases.

Methods

EMBASE, Medline, Scopus, Web of Science and Psychinfo were systematically searched for articles published between January 1st 1997 and February 13th 2020. We included articles published in English reporting the use of CuATSM PET in the diagnosis and management of age-related diseases in humans or animals.

Results

Nine articles were included describing CuATSM PET measures in neurological and cardiovascular disease. There was higher CuATSM uptake in diseased compared to control subjects in Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), myocardial ischemia (MI), cardiac dysautonomia and atherosclerosis. Higher CuATSM uptake was seen in diseased compared to control anatomical areas in PD, cerebrovascular disease (CVD), MI and atherosclerosis. CuATSM uptake was associated with disease severity in PD, ALS, CVD and atherosclerosis. An association between CuATSM uptake and disease duration was shown in atherosclerosis.

Conclusion

CuATSM uptake is higher in neurological and cardiovascular diseases and associated with disease severity and duration. Further investigations using CuATSM PET in other age-related diseases are needed.

Detection of water-molecular-motion configuration in patients with lupus nephritis: a primary study using diffusion-weighted imaging

BACKGROUND

Lupus nephritis (LN) is one of most common types of secondary glomerulonephritis, which is characterized by longitudinal pathological changes. Microstructural lesions of LN will impact the motion of water molecules, which can be detected by diffusion-weighted imaging (DWI). There are few reported measurements of water diffusion in patients with LN, and the nature of water diffusion across the entire depth of the renal parenchyma remains largely unknown.

METHODS

Twenty adult patients with LN and 11 healthy volunteers underwent DWI inspection. Renal biopsy samples were characterized based on the revised ISN/RPS 2003 classification. The apparent-diffusion coefficient (ADC) was calculated via fitting into a mono-exponential model. To compare the ADC level across the entire renal parenchyma between the two groups, repeated-measures analysis of variance (RM-ANOVA) was performed. ADC data derived from DWI pictures were transformed into tridimensional maps by MATLAB software.

RESULTS

Compared with data from healthy volunteers, lower average ADC values with major undulatory magnitudes were found in patients with LN, especially in the cortical zone. Tridimensional maps of patients with LN displayed geographic terrain-like canyons and/or valleys that were different from the corresponding terrain-like flatlands and/or plateaus in healthy volunteers. A heterogeneity of ADC values was found in bilateral kidneys. Left kidneys predominated higher ADC values in patients with LN. The ADC values across the entire renal parenchyma exhibited statistically significant differences among the three identified pathological subclasses (P < 0.001).

CONCLUSIONS

Analysis of the motion of water molecules across the entire renal parenchyma may be helpful for better understanding the pathological conditions of LN, for which microstructural and functional heterogeneity may be detected and visualized via DWI.

p53 upregulated by HIF-1α promotes hypoxia-induced G2/M arrest and renal fibrosis in vitro and in vivo

Hypoxia plays an important role in the genesis and progression of renal fibrosis. The underlying mechanisms, however, have not been sufficiently elucidated. We examined the role of p53 in hypoxia-induced renal fibrosis in cell culture (human and rat renal tubular epithelial cells) and a mouse unilateral ureteral obstruction (UUO) model. Cell cycle of tubular cells was determined by flow cytometry, and the expression of profibrogenic factors was determined by RT-PCR, immunohistochemistry, and western blotting. Chromatin immunoprecipitation and luciferase reporter experiments were performed to explore the effect of HIF-1α on p53 expression. We showed that, in hypoxic tubular cells, p53 upregulation suppressed the expression of CDK1 and cyclins B1 and D1, leading to cell cycle (G2/M) arrest (or delay) and higher expression of TGF-β, CTGF, collagens, and fibronectin. p53 suppression by siRNA or by a specific p53 inhibitor (PIF-α) triggered opposite effects preventing the G2/M arrest and profibrotic changes. In vivo experiments in the UUO model revealed similar antifibrotic results following intraperitoneal administration of PIF-α (2.2 mg/kg). Using gain-of-function, loss-of-function, and luciferase assays, we further identified an HRE3 region on the p53 promoter as the HIF-1α-binding site. The HIF-1α–HRE3 binding resulted in a sharp transcriptional activation of p53. Collectively, we show the presence of a hypoxia-activated, p53-responsive profibrogenic pathway in the kidney. During hypoxia, p53 upregulation induced by HIF-1α suppresses cell cycle progression, leading to the accumulation of G2/M cells, and activates profibrotic TGF-β and CTGF-mediated signaling pathways, causing extracellular matrix production and renal fibrosis.

The utility of magnetic resonance imaging for noninvasive evaluation of diabetic nephropathy

BACKGROUND

Noninvasive quantitative measurement of fibrosis in chronic kidney disease (CKD) would be desirable diagnostically and therapeutically but standard radiologic imaging is too variable for clinical usage. By applying a vibratory force, tissue shear wave stiffness can be measured by magnetic resonance elastography (MRE) that may correlate with progression of kidney fibrosis. Since decreased kidney perfusion decreases tissue turgor and stiffness, we combined newly available three-dimensional MRE shear stiffness measurements with MR arterial spin labeling (ASL) kidney blood flow rates to evaluate fibrosis in diabetic nephropathy.

METHODS

Thirty individuals with diabetes and Stage 0-5 CKD and 13 control individuals without CKD underwent noncontrast MRE with concurrent ASL blood flow measurements.

RESULTS

MRE cortical shear stiffness at 90 Hz was decreased significantly below controls in all CKD stages of diabetic nephropathy. Likewise, ASL blood flow decreased progressively from 480 ± 136 mL/min/100 g of cortical tissue in controls to 302 ± 95, 229 ± 7 and 152 ± 32 mL/min/100 g in Stages 3, 4 and 5 CKD, respectively. A magnetic resonance imaging (MRI) surrogate for the measured glomerular filtration fraction [surrogate filtration fraction = estimated glomerular filtration rate (eGFR)/ASL] decreased progressively from 0.21 ± 0.07 in controls to 0.16 ± 0.04 in Stage 3 and 0.10 ± 0.02 in Stage 4-5 CKD.

CONCLUSIONS

In this pilot study, MRI with ASL blood flow rates can noninvasively measure decreasing kidney cortical tissue perfusion and, with eGFR, a decreasing surrogate filtration fraction in worsening diabetic nephropathy that appears to correlate with increasing fibrosis. Differing from the liver, MRE shear stiffness surprisingly decreases with worsening CKD, likely related to decreased tissue turgor from lower blood flow rates.

Quantitative assessment of renal allograft pathologic changes: comparisons of mono-exponential and bi-exponential models using diffusion-weighted imaging

Background

Diffusion-weighted imaging (DWI) can noninvasively assess renal allograft pathologic changes that provide useful information for clinical management and prognostication. However, it is still unknown whether the bi-exponential model analysis of DWI signals is superior to that of the mono-exponential model.

Methods

Pathologic and DWI data from a total of 47 allografts were prospectively collected and analyzed. Kidney transplant interstitial fibrosis was quantified digitally. The severity of acute and chronic pathologic changes was semi-quantified by calculating the acute composite scores (ACS) and chronic composite score (CCS). Mono-exponential total apparent diffusion coefficient (ADCT), and the bi-exponential parameters of true diffusion (D) and perfusion fraction (fp) were acquired. The diagnostic performances of both mono-exponential and bi-exponential parameters were assessed and compared by calculating the area under the curve (AUC) from receiver-operating characteristic (ROC) curve analysis.

Results

ADCT, D, and fp were all significantly correlated with interstitial fibrosis, ACS, and CCS. Cortical fp discriminated mild from moderate and severe ACS with the largest AUC of 0.89 [95% confidence interval (CI), 0.77-0.96]. Noticeably, only cortical fp could differentiate severe ACS from mild-to-moderate ACS (P<0.001) with an AUC of 0.80 (95% CI, 0.65-0.90) and a sensitivity of 100% (95% CI, 66.4-100%). Strikingly, the joint use of D and fp in either the cortex or the medulla could achieve a sensitivity of 100% for identifying either mild or severe interstitial fibrosis. Meanwhile, the serial use of cortical D and cortical fp showed the largest specificity for identifying both mild [88.9% (95% CI, 70.8-97.6%)] and severe [84.4% (95% CI, 67.2-94.7%)] interstitial fibrosis. For identifying mild CCS, the AUC of medullary ADCT (0.90, 95% CI, 0.78-0.97) was similar to that of cortical D (0.81, 95% CI, 0.67-0.91) and fp (0.86, 95% CI, 0.73-0.94), but statistically larger than that of medullary D (P=0.005) and fp (P=0.01). Furthermore, the parallel use of cortical D and cortical fp could increase the sensitivity to 95.0% (95% CI, 75.1-99.9%), whereas serial use of medullary D and medullary fp could increase the specificity to 100% (95% CI, 87.2-100%). The AUCs for differentiating severe from mild and moderate CCS were statistically insignificant among all parameters in the cortex and medulla (P≥0.15).

Conclusions

Cortical fp was superior to the ADCT for identifying both mild and severe acute pathologic changes. Nevertheless, ADCT was equal to or better than single D or fp for evaluating chronic pathologic changes. Thus, both monoexponential and bi-exponential analysis of DWI images are complementary for evaluating kidney allograft pathologic changes, and the combined use of D and fp can increase the sensitivity and specificity for discriminating allograft pathologic changes severity.

Apparent Diffusion Coefficient in the Resolution of Renal Ischemia after Angioplasty on Diffusion-weighted Imaging: Renal Artery Stenosis Caused by Progressive Thrombosis in Residual Chronic Aortic Dissection

We report a case in which diffusion-weighted magnetic resonance imaging (DWI) demonstrated renal artery stenosis-related renal ischemia and the therapeutic efficacy of revascularization. The patient was a 73-year-old man, who underwent descending thoracic aortic replacement due to DeBakey IIIb chronic aortic dissection, and who showed progressive renal dysfunction due to right renal artery stenosis caused by false lumen thrombosis. DWI demonstrated a decreased apparent diffusion coefficient (ADC) in the right kidney, indicating renal ischemia. Angioplasty with stenting restored renal perfusion and improved the renal function, resulting in the normalization of the decreased ADC in the treated kidney. Thus, DWI can be used to monitor renal ischemia in cases involving advanced renal artery stenosis.

Kidney Functional Magnetic Resonance Imaging and Change in eGFR in Individuals with CKD

BACKGROUND AND OBJECTIVES

Kidney functional magnetic resonance imaging (MRI) requires further investigation to enhance the noninvasive identification of patients at high risk of CKD progression.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this exploratory study, we obtained baseline diffusion-weighted and blood oxygen level-dependent MRI in 122 participants of the CKD Optimal Management with Binders and Nicotinamide trial, which was a multicenter, randomized, double-blinded, 12-month, four-group parallel trial of nicotinamide and lanthanum carbonate versus placebo conducted in individuals with eGFR 20-45 ml/min per 1.73 m². Lower values of apparent diffusion coefficient (ADC) on diffusion-weighted MRI may indicate increased fibrosis, and higher values of relaxation rate (R2*) on blood oxygen level-dependent MRI may represent decreased oxygenation. Because there was no effect of active treatment on eGFR over 12 months, we tested whether baseline kidney functional MRI biomarkers were associated with eGFR decline in all 122 participants. In a subset of 87 participants with 12-month follow-up MRI data, we evaluated whether kidney functional MRI biomarkers change over time.

RESULTS

Mean baseline eGFR was 32±9 ml/min per 1.73 m², and mean annual eGFR slope was -2.3 (95% confidence interval [95% CI], -3.4 to -1.1) ml/min per 1.73 m² per year. After adjustment for baseline covariates, baseline ADC was associated with change in eGFR over time (difference in annual eGFR slope per 1 SD increase in ADC: 1.3 [95% CI, 0.1 to 2.5] ml/min per 1.73 m² per year, ADC×time interaction P=0.04). This association was no longer significant after further adjustment for albuminuria (difference in annual eGFR slope per 1 SD increase in ADC: 1.0 (95% CI, -0.1 to 2.2) ml/min per 1.73 m² per year, ADC×time interaction P=0.08). There was no significant association between baseline R2* and change in eGFR over time. In 87 participants with follow-up functional MRI, ADC and R2* values remained stable over 12 months (intraclass correlation: 0.71 and 0.68, respectively).

CONCLUSIONS

Baseline cortical ADC was associated with change in eGFR over time, but this association was not independent of albuminuria. Kidney functional MRI biomarkers remained stable over 1 year.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

CKD Optimal Management with Binders and Nicotinamide (COMBINE), NCT02258074.

The role of renal hypoxia in the pathogenesis of diabetic kidney disease: a promising target for newer renoprotective agents including SGLT2 inhibitors?

Diabetic kidney disease is the most common cause of end-stage kidney disease and poses a major global health problem. Finding new, safe, and effective strategies to halt this disease has proven to be challenging. In part that is because the underlying mechanisms are complex and not fully understood. However, in recent years, evidence has accumulated suggesting that chronic hypoxia may be the primary pathophysiological pathway driving diabetic kidney disease and chronic kidney disease of other etiologies and was called the chronic hypoxia hypothesis. Hypoxia is the result of a mismatch between oxygen delivery and oxygen demand. The primary determinant of oxygen delivery is renal perfusion (blood flow per tissue mass), whereas the main driver of oxygen demand is active sodium reabsorption. Diabetes mellitus is thought to compromise the oxygen balance by impairing oxygen delivery owing to hyperglycemia-associated microvascular damage and exacerbate oxygen demand owing to increased sodium reabsorption as a result of sodium-glucose cotransporter upregulation and glomerular hyperfiltration. The resultant hypoxic injury creates a vicious cycle of capillary damage, inflammation, deposition of the extracellular matrix, and, ultimately, fibrosis and nephron loss. This review will frame the role of chronic hypoxia in the pathogenesis of diabetic kidney disease and its prospect as a promising therapeutic target. We will outline the cellular mechanisms of hypoxia and evidence for renal hypoxia in animal and human studies. In addition, we will highlight the promise of newer imaging modalities including blood oxygenation level-dependent magnetic resonance imaging and discuss salutary interventions such as sodium-glucose cotransporter 2 inhibition that (may) protect the kidney through amelioration of renal hypoxia.

The New Biology of Diabetic Kidney Disease-Mechanisms and Therapeutic Implications

Diabetic kidney disease remains the most common cause of end-stage kidney disease in the world. Despite reductions in incidence rates of myocardial infarction and stroke in people with diabetes over the past 3 decades, the risk of diabetic kidney disease has remained unchanged, and may even be increasing in younger individuals afflicted with this disease. Accordingly, changes in public health policy have to be implemented to address the root causes of diabetic kidney disease, including the rise of obesity and diabetes, in addition to the use of safe and effective pharmacological agents to prevent cardiorenal complications in people with diabetes. The aim of this article is to review the mechanisms of pathogenesis and therapies that are either in clinical practice or that are emerging in clinical development programs for potential use to treat diabetic kidney disease.

Munc18-1-interacting protein 3 mitigates renal fibrosis through protection of tubular epithelial cells from apoptosis

BACKGROUND

Tubulointerstitial fibrosis is a hallmark of chronic kidney disease (CKD), and is initiated by tubular epithelial cell (TEC) injury. Hypoxia promotes tubular cell death, fibrosis and CKD progression. Munc18-1-interacting protein 3 (Mint3) is a molecule that activates hypoxia-inducible factors (HIFs) by binding and suppressing factor inhibiting HIF-1 (FIH). However, the role of Mint3 in tubulointerstitial fibrosis remains unknown.

METHODS

We induced fibrosis of the kidney after unilateral ischemia-reperfusion injury (uIRI) in Mint3-knockout and littermate wild-type mice. The duration of ischemia was 23 min and the kidneys were harvested at 24 h and 7 days after ischemia-reperfusion. The function of Mint3 was further investigated by using mouse cortical tubular (MCT) cells, which were treated with Mint3 and/or FIH small interfering RNA and exposed to normoxia or hypoxia.

RESULTS

Knockout of Mint3 did not affect the acute injury induced by uIRI, but exacerbated the tubulointerstitial fibrosis, accompanied by an increase in TEC apoptosis. Consistently, hypoxia-induced apoptosis of MCT cells was aggravated by Mint3 knockdown. Unexpectedly, the additional knockdown of FIH did not suppress the increase in apoptosis by Mint3 knockdown, demonstrating the irrelevance of the FIH/HIF pathway. Therefore, we next focused on nuclear factor (NF)-κB, which has an anti-apoptotic role. Indeed, not only the expression of the inhibitory NF-κB p50 but also the DNA-binding activity of p50/p50 homodimer was increased by knockdown of Mint3 in the TECs, along with the decreased expressions of the NF-κB-targeted anti-apoptotic genes. An increase in NF-κB p50 was also confirmed in Mint3-knockout kidneys.

CONCLUSIONS

Mint3 in epithelial cells protects the cells from apoptosis by up-regulating anti-apoptotic effects of NF-κB, leading to fibrosis suppression. This new pathophysiology of tubulointerstitial fibrosis could be a target of future therapy for CKD.

Spin-lock relaxation rate dispersion reveals spatiotemporal changes associated with tubulointerstitial fibrosis in murine kidney

PURPOSE

To develop and evaluate a reliable non-invasive means for assessing the severity and progression of fibrosis in kidneys. We used spin-lock MR imaging with different locking fields to detect and characterize progressive renal fibrosis in an hHB-EGF^Tg/Tg mouse model.

METHODS

Male hHB-EGF^Tg/Tg mice, a well-established model of progressive fibrosis, and age-matched normal wild type (WT) mice, were imaged at 7T at ages 5-7, 11-13, and 30-40 weeks. Spin-lock relaxation rates R_{1 ρ} were measured at different locking fields (frequencies) and the resultant dispersion curves were fit to a model of exchanging water pools. The obtained MRI parameters were evaluated as potential indicators of tubulointerstitial fibrosis in kidney. Histological examinations of renal fibrosis were also carried out post-mortem after MRI.

RESULTS

Histology detected extensive fibrosis in the hHB-EGF^Tg/Tg mice, in which collagen deposition and reductions in capillary density were observed in the fibrotic regions of kidneys. R₂ and R_{1 ρ} values at different spin-lock powers clearly dropped in the fibrotic region as fibrosis progressed. There was less variation in the asymptotic locking field relaxation rate R 1 ρ ∞ between the groups. The exchange parameter S_ρ and the inflection frequency ω_infl changed by larger factors.

CONCLUSION

Both S_ρ and ω_infl depend primarily on the average exchange rate between water and other chemically shifted resonances such as hydroxyls and amides. Spin-lock relaxation rate dispersion, rather than single measurements of relaxation rates, provides more comprehensive and specific information on spatiotemporal changes associated with tubulointerstitial fibrosis in murine kidney.

Ultrasound super-resolution imaging provides a noninvasive assessment of renal microvasculature changes during mouse acute kidney injury

Acute kidney injury (AKI) is a risk factor for the development of chronic kidney disease (CKD). One mechanism for this phenomenon is renal microvascular rarefaction and subsequent chronic impairment in perfusion. However, diagnostic tools to monitor the renal microvasculature in a noninvasive and quantitative manner are still lacking. Ultrasound super-resolution imaging is an emerging technology that can identify microvessels with unprecedented resolution. Here, we applied this imaging technique to identify microvessels in the unilateral ischemia-reperfusion injury mouse model of AKI-to-CKD progression in vivo. Kidneys from 21 and 42 day post- ischemia-reperfusion injury, the contralateral uninjured kidneys, and kidneys from sham-operated mice were examined by ultrasound super-resolution and histology. Renal microvessels were successfully identified by this imaging modality with a resolution down to 32 μm. Renal fibrosis was observed in all kidneys with ischemia-reperfusion injury and was associated with a significant reduction in kidney size, cortical thickness, relative blood volume, and microvascular density as assessed by this imaging. Tortuosity of the cortical microvasculature was also significantly increased at 42 days compared to sham. These vessel density measurements correlated significantly with CD31 immunohistochemistry (R²=0.77). Thus, ultrasound super-resolution imaging provides unprecedented resolution and is capable of noninvasive quantification of renal vasculature changes associated with AKI-to-CKD progression in mice. Hence, this technique could be a promising diagnostic tool for monitoring progressive kidney disease.

Bariatric surgery and kidney disease outcomes in severely obese youth

Bariatric surgery, an emerging treatment for severely obese youth with and without T2D, provides marked improvement in insulin resistance, beta-cell function, and central adiposity. Further, preliminary data suggest that bariatric surgery also results in significant improvement in markers of obesity-related nephropathy and DKD, beyond that which can be achieved with current medical interventions. Yet, the mechanisms whereby bariatric surgery attenuates kidney disease remain unclear. This review summarizes the data on the effects of bariatric surgery on obesity-related nephropathy and DKD in youth with and without T2D, in addition to potential mechanisms underlying the nephroprotective effects of weight loss surgery and how these may differ in Roux-en-Y gastric bypass vs. vertical sleeve gastrectomy. Finally, we discuss potential future non-surgical therapies to mitigate kidney disease.

Consensus-based technical recommendations for clinical translation of renal BOLD MRI

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field.

Effects of additional vasodilatory or nonvasodilatory treatment on renal function, vascular resistance and oxygenation in chronic kidney disease: a randomized clinical trial

AIM

Progression of chronic kidney disease (CKD) may be accelerated by tissue hypoxia due to impaired blood supply. This could be induced by small artery narrowing resulting in abnormally high intrarenal vascular resistance (RVR). We investigated whether a reduction in RVR achieved by adding vasodilating medical therapy (AVT) is superior to adding nonvasodilating medical therapy (AnonVT) regarding tissue oxygenation and preservation of kidney function.

METHODS

Eighty-three grade 3 and 4 CKD patients [estimated glomerular filtration rate (GFR) 34.6 ml/min per 1.73 m] were randomized to either AVT with amlodipine and/or renin angiotensin blockade or AnonVT with the nonvasodilating beta-blocker metoprolol. Investigations were performed at baseline and after 18 months of therapy. Systemic vasodilation was documented in the forearm vasculature using resting venous occlusion plethysmography. GFR was measured as Chrome-EDTA plasma clearance. Using MRI, renal artery blood flow was measured for calculation of RVR and for estimating renal oxygenation (R2*).

RESULTS

AVT and AnonVT achieved as planned similar blood pressure levels throughout the study. At follow-up, resistance had decreased by 7% (P < 0.05) and RVR by 12% (P < 0.05) in the AVT group, whereas in the AnonVT group, resistance increased by 39% (P < 0.01), whereas RVR remained unchanged. At follow-up, no significant differences in cortical or medullary R2* values between AVT and AnonVT were observed, and the GFR decline was similar in the two groups (3.0 vs. 3.3 ml/min per 1.73 m).

CONCLUSION

Long-term intensified vasodilation treatment reduced peripheral and RVR, but this was not associated with improvement of R2* or protection against loss of kidney function in CKD patients.

Noninvasive evaluation of renal oxygenation in children with chronic kidney disease using blood-oxygen-level-dependent magnetic resonance imaging

BACKGROUND

Renal hypoxia is considered a final pathway in the progression of chronic kidney disease (CKD). Blood-oxygen-level-dependent magnetic resonance imaging (BOLD-MRI) has shown merit for evaluating renal oxygenation in adults.

OBJECTIVE

To investigate renal cortical and medullary R2* values by CKD stage and by renal function index in children with chronic kidney disease.

MATERIALS AND METHODS

Twenty-one children with CKD Stage 1-3, 16 children with CKD Stage 4-5, and 6 healthy volunteers underwent a renal MRI using multigradient recalled-echo sequence with 16 echoes. We measured the R2* values of the renal cortex and medulla on BOLD-MRI.

RESULTS

The cortical R2* value was ranked as CKD Stage 4-5 > CKD Stage 1-3 > healthy controls, and the medullary R2* value was ranked as CKD Stage 4-5 > CKD Stage 1-3. There was no significant difference in the medullary R2* value between CKD Stage 1-3 patients and the healthy controls. There was a positive correlation between the R2* values in the renal cortex (r=0.73) and medulla (r=0.89), and the serum creatinine level (P<0.001), and the renal cortical and medullary R2* values were negatively correlated with the estimated glomerular filtration rate (r=-0.71 and r=-0.89, respectively; P<0.001).

CONCLUSION

BOLD-MRI might contribute to noninvasive assessment of renal oxygenation in children with CKD in vivo but it did not reflect renal function in our sample.

The role of oxidative stress and hypoxia in renal disease

Oxygen is required to sustain aerobic organisms. Reactive oxygen species (ROS) are constantly released during mitochondrial oxygen consumption for energy production. Any imbalance between ROS production and its scavenger system induces oxidative stress. Oxidative stress, a critical contributor to tissue damage, is well-known to be associated with various diseases. The kidney is susceptible to hypoxia, and renal hypoxia is a common final pathway to end stage kidney disease, regardless of the underlying cause. Renal hypoxia aggravates oxidative stress, and elevated oxidative stress, in turn, exacerbates renal hypoxia. Oxidative stress is also enhanced in chronic kidney disease, especially diabetic kidney disease, through various mechanisms. Thus, the vicious cycle between oxidative stress and renal hypoxia critically contributes to the progression of renal injury. This review examines recent evidence connecting chronic hypoxia and oxidative stress in renal disease and subsequently describes several promising therapeutic approaches against oxidative stress.

Functional MRI as a Tool for Evaluating Interstitial Fibrosis and Prognosis in Kidney Disease

Background

Kidney fibrosis is a key driver of progression of kidney diseases. Renal biopsies remain the gold-standard approach to specifically diagnose and stage renal fibrosis at present. However, there is a lack of multi-dimensional pictures showing renal function, histology, and imaging of the fibrotic kidney.

Summary

Magnetic resonance imaging (MRI) strategies have been rapidly evolving during the past couple of decades, especially driven by advances in technology. Recently, several pioneer and remarkable studies demonstrated that advanced functional MRI (fMRI) tools could be useful for the evaluation of kidney fibrosis and progression, which provides more opportunities to benefit from the significant value of fMRI tools for clinical implementation.

Key Messages

fMRI will be a novel approach to evaluate interstitial fibrosis and prognosis in kidney disease.

Consensus-based technical recommendations for clinical translation of renal BOLD MRI

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field.

Magnetization Transfer Imaging Is Unaffected by Decreases in Renal Perfusion in Swine

OBJECTIVES

Multiparametric renal magnetic resonance imaging (MRI), including diffusion-weighted imaging, magnetic resonance elastography, and magnetization transfer imaging (MTI), is valuable in the noninvasive assessment of renal fibrosis. However, hemodynamic changes in diseased kidneys may impede their ability to measure renal fibrosis. Because MTI assesses directly tissue content of macromolecules, we test the hypothesis that MTI would be insensitive to renal hemodynamic changes in swine kidneys with acute graded ischemia.

MATERIALS AND METHODS

Seven domestic pigs underwent placement of an inflatable silicone cuff around the right renal artery to induce graded renal ischemia. Multiparametric MRI was performed at baseline, 50%, 75%, and 100% renal artery stenosis as well as reperfusion. Measurements included regional perfusion, R2*, apparent diffusion coefficient (ADC), stiffness, and magnetization transfer ratio (MTR) using arterial spin-labeled MRI, blood oxygenation-dependent MRI, diffusion-weighted imaging, magnetic resonance elastography, and MTI, respectively. Histology was performed to rule out renal fibrosis.

RESULTS

During graded ischemia, decreases in renal perfusion were accompanied with elevated R2*, decreased ADC, and stiffness, whereas no statistically significant changes were observed in the MTR. No fibrosis was detected by histology. After release of the obstruction, renal perfusion showed only partial recovery, associated with return of kidney R2*, ADC, and stiffness to baseline levels, whereas cortical MTR decreased slightly.

CONCLUSIONS

Renal MTI is insensitive to decreases in renal perfusion and may offer reliable assessment of renal structural changes.

Noninvasive quantitative magnetization transfer MRI reveals tubulointerstitial fibrosis in murine kidney

Excessive tissue scarring, or fibrosis, is a critical contributor to end stage renal disease, but current clinical tests are not sufficient for assessing renal fibrosis. Quantitative magnetization transfer (qMT) MRI provides indirect information about the macromolecular composition of tissues. We evaluated measurements of the pool size ratio (PSR, the ratio of immobilized macromolecular to free water protons) obtained by qMT as a biomarker of tubulointerstitial fibrosis in a well-established murine model with progressive renal disease. MR images were acquired from 16-week-old fibrotic hHB-EGF^Tg/Tg mice and normal wild-type (WT) mice (N = 12) at 7 T. QMT parameters were derived using a two-pool five-parameter fitting model. A normal range of PSR values in the cortex and outer stripe of outer medulla (CR + OSOM) was determined by averaging across voxels within WT kidneys (mean ± 2SD). Regions in diseased mice whose PSR values exceeded the normal range above a threshold value (tPSR) were identified and measured. The spatial distribution of fibrosis was confirmed using picrosirius red stains. Compared with normal WT mice, scattered clusters of high PSR regions were observed in the OSOM of hHB-EGF^Tg/Tg mouse kidneys. Moderate increases in mean PSR (mPSR) of CR + OSOM regions were observed across fibrotic kidneys. The abnormally high PSR regions (% area) detected by the tPSR were significantly increased in hHB-EGF^Tg/Tg mice, and were highly correlated with regions of fibrosis detected by histological fibrosis indices measured from picrosirius red staining. Renal tubulointerstitial fibrosis in OSOM can thus be assessed by qMT MRI using an appropriate analysis of PSR. This technique may be used as an imaging biomarker for chronic kidney diseases.

Non-invasive assessment of early stage diabetic nephropathy by DTI and BOLD MRI

OBJECTIVE

Patients with diabetes mellitus, diabetic nephropathy (DN) and healthy donor were analyzed to test whether the early DN patients can be detected using both blood oxygenation level dependent (BOLD) and diffusion tensor imaging.

METHODS

This study was approved by the Ethics Committee of our hospital. MR images were acquired on a 3.0-Tesla MR system (Discovery MR750, General Electric, Milwaukee, WI). 30 diabetic patients were divided into NAU (normal to mildly increased albuminuria, N = 15) and MAU (moderately increased albuminuria, N = 15) group based on the absence or presence of microalbuminuria. 15 controls with sex- and age-matched were enrolled in the study. Prior to MRI scan, all participants were instructed to collect their fresh morning urine samples for quantitative measurement of urinary microalbumin and urinary creatinine. Then, the estimations of serum creatinine, serum uric acid, HbAlc and fasting plasma glucose as well as fundus examinations were performed in all subjects. Then, the values of albumin-creatinine ratio (ACR) and estimated glomerular filtration rate were also calculated. All subjects underwent renal diffusion tensor imaging (DTI) and BOLD acquisition after fasting for 4 h. Regions of interest were placed in renal medulla and cortex for evaluating apparent diffusion coefficient (ADC), fractional anisotropy (FA) and R2* values by two experienced radiologists. The consistency between the two observations was estimated using intragroup correlation coefficients. To test differences in ADC, FA and R2* values across the three groups, the data were analyzed using separate one-way ANOVAs. Post-hoc pair wise comparisons were then performed using t-test. To investigate the clinical relevance of imaging parameters in both regions across the three groups, the correlations of values of the ACR/estimated glomerular filtration rate and of the ADC/FA/R2* were calculated.

RESULTS

There was a high level of consistency of those ADC, FA and R2* values across the three groups on both renal cortex and medulla measured by the two doctors. The FA value of medulla in MAU group was lower than that in control (p < 0.01). The R2* value of medulla in the NAU group was higher than that in the control (p < 0.01), and the R2* value of medulla in the MAU group was lower than that in the control (p = 0.009) . Moreover, the current study revealed a decreasing trend in FA values of the renal medulla from the control group to NAU and MAU groups. Finally, a weak negatively correlation between medullary R2* and ACR was found in current study.

CONCLUSION

Medullary R2* value might be a new more sensitive predictor of early DN. Meanwhile, BOLD imaging detected the medullary hypoxia at the simply diabetic stage, while DTI didn't identify the medullary directional diffusion changes at this stage. Based on our assumption mentioned above, it's presumable that BOLD imaging may be more sensitive for assessment of the early renal function changes than DTI. These imaging techniques are more accurate and practical than conventional tests.

ADVANCES IN KNOWLEDGE

Non-invasive MRI was used to detect renal function changes at early DN stage.

Dynamic nuclear polarization magnetic resonance imaging and the oxygen-sensitive paramagnetic agent OX63 provide a noninvasive quantitative evaluation of kidney hypoxia in diabetic mice

Renal hypoxia may play an important role in the progression of diabetic nephropathy. However, tools that noninvasively and quantitatively measure oxygen tension in the kidney are lacking. Here, we evaluated the feasibility of a noninvasive and quantitative imaging technique using dynamic nuclear polarization magnetic resonance imaging (DNP-MRI) in combination with the oxygen-sensitive paramagnetic agent OX63 for measuring oxygen tension in the kidney. Our results demonstrate that the DNP-MRI technique can yield quantitative maps of oxygen tension in the mouse renal cortex. Using this procedure, we also showed that oxygen tension was less elevated in the renal cortex of both streptozotocin-induced type 1 diabetic mice and db/db mice, a model of type 2 diabetes, than in the renal cortex of age-matched control mice of each respective model. Oxygen tension in streptozotocin-exposed mice was significantly improved by insulin treatment. Thus, the noninvasive and quantitative DNP-MRI technique appears to be useful for studying the pathophysiological role of hypoxia.

Human renal response to furosemide: Simultaneous oxygenation and perfusion measurements in cortex and medulla

Aim

Disturbances of renal medullary perfusion and metabolism have been implicated in the pathogenesis of kidney disease and hypertension. Furosemide, a loop diuretic, is widely used to prevent renal medullary hypoxia in acute kidney disease by uncoupling sodium metabolism, but its effects on medullary perfusion in humans are unknown. We performed quantitative imaging of both renal perfusion and oxygenation using Magnetic Resonance Imaging (MRI) before and during furosemide. Based on the literature, we hypothesized that furosemide would increase medullary oxygenation, decrease medullary perfusion, but cause minor changes (<10%) in renal artery flow (RAF).

Methods

Interleaved measurements of RAF, oxygenation (T₂*) and perfusion by arterial spin labelling in the renal cortex and medulla of 9 healthy subjects were acquired before and after an injection of 20 mg furosemide. They were preceded by measurements made during isometric exercise (5 minutes handgrip bouts), which are known to induce changes in renal hemodynamics, that served as a control for the sensitivity of the hemodynamic MRI measurements. Experiments were repeated on a second day to establish that the measurements and the induced changes were reproducible.

Results

After furosemide, T₂* values in the medulla increased by 53% (P < 0.01) while RAF and perfusion remained constant. After hand‐grip exercise, T₂* values in renal medulla increased by 22% ± 9% despite a drop in medullary perfusion of 7.2% ± 4.7% and a decrease in renal arterial flow of 17.5% ± 1.7% (P < 0.05). Mean coefficients of variation between repeated measurements for all parameters were 7%.

Conclusion

Furosemide induced the anticipated increase in renal medullary oxygenation, attributable exclusively to a decrease in renal oxygen consumption, since no change of RAF, cortical or medullary perfusion could be demonstrated. All measures and the induced changes were reproducible.

Found in Translation: Reasons for Optimism in the Pursuit to Prevent Chronic Kidney Disease After Acute Kidney Injury

Purpose of review:

The current review will discuss on the progress of studying the transition phase between acute kidney injury (AKI) and chronic kidney disease (CKD) through improved animal models, common AKI and CKD pathways, and how human studies may inform different translational approaches.

Sources of information:

PubMed and Google Scholar.

Methods:

A narrative review was performed using the main terms “acute kidney injury,” “chronic kidney disease,” “end-stage renal disease,” “animal models,” “review,” “decision-making,” and “translational research.”

Key findings:

The last decade has shown much progress in the study of AKI, including evidence of a pathophysiological link between AKI and CKD. We are now in a phase of redesigning animal models and discovering mechanisms that can replicate the pathological conditions of the AKI-to-CKD continuum. Translating these findings into the clinic is a barrier that must be overcome. To this end, current efforts include prediction of AKI onset and maladaptive repair, detecting patients susceptible to the progression of chronic maladaptive repair, and understanding shared signaling mechanisms between AKI and CKD.

Limitations:

This is a narrative review of the literature that is partially influenced by the knowledge, perspectives, and experiences of the authors and their research background.

Implications:

Overall, this new knowledge from the AKI-to-CKD continuum will help bridge the discontinuity that exists between animal models and patients, resulting in more effective translational biomarkers and therapeutics to test in known AKI pathologies thereby preventing the chronicity of kidney injury progression.

Combination of Functional Magnetic Resonance Imaging and Histopathologic Analysis to Evaluate Interstitial Fibrosis in Kidney Allografts

BACKGROUND AND OBJECTIVES

Recent developments indicated that functional magnetic resonance imaging (MRI) could potentially provide noninvasive assessment of kidney interstitial fibrosis in patients with kidney diseases, but direct evidence from histopathology is scarce. We aimed to explore the diagnostic utilities of functional MRI for the evaluation of kidney allograft interstitial fibrosis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We prospectively examined 103 kidney transplant recipients who underwent for-cause biopsies and 20 biopsy-proven normal subjects with functional MRI. Histomorphometric analyses of interstitial fibrosis and peritubular capillary densities were performed on digitally scanned Masson's trichrome- and CD34-stained slides, respectively. The performances of functional MRI to discriminate interstitial fibrosis were assessed by calculating the area under the curve using receiver-operating characteristic curve.

RESULTS

Main pathologic findings in this single-center cohort were representative of common diagnostic entities in the kidney allografts, with rejection (32%) and glomerulonephritides (31%) accounting for the majority of diagnoses. Apparent diffusion coefficient from diffusion-weighted imaging correlated with interstitial fibrosis (ρ=-0.77; P<0.001). Additionally, decreased arterial spin labelings were accompanied by peritubular capillary density reductions (r=0.77; P<0.001). Blood oxygen level-dependent (BOLD) imaging demonstrated cortical hypoxia with increasing interstitial fibrosis (ρ=0.61; P<0.001). The area under the curve for the discrimination of ≤25% versus >25% interstitial fibrosis and ≤50% versus >50% interstitial fibrosis were 0.87 (95% confidence interval [95% CI], 0.79 to 0.93) and 0.88 (95% CI, 0.80 to 0.93) by apparent diffusion coefficient, 0.92 (95% CI, 0.85 to 0.97) and 0.94 (95% CI, 0.87 to 0.98) by arterial spin labeling, 0.81 (95% CI, 0.72 to 0.88) and 0.86 (95% CI, 0.78 to 0.92) by perfusion fraction, 0.79 (95% CI, 0.69 to 0.87) and 0.85 (95% CI, 0.76 to 0.92) by BOLD imaging, respectively.

CONCLUSIONS

Functional MRI measurements were strongly correlated with kidney allograft interstitial fibrosis. The performances of functional MRI for discriminating ≤50% versus >50% interstitial fibrosis were good to excellent.

Evaluating Renal Fibrosis with R2* Histogram Analysis of the Whole Cortex in a Unilateral Ureteral Obstruction Model

RATIONALE AND OBJECTIVES

The aim of this study was to use histogram analysis to assess the correlation between blood oxygen-level dependent magnetic resonance imaging (BOLD-MRI) and renal fibrosis induced by unilateral ureteral obstruction (UUO) in an animal model for a long experimental period.

MATERIALS AND METHODS

The rabbits were randomly divided into a control group (n = 6) and a UUO group (n = 30). The rabbits in the UUO group underwent left ureteral obstruction surgery. BOLD-MRI examinations were performed at 2, 4, 6, and 8 weeks after ligation. After the examinations, nephrectomy was performed for histologic evaluation. Histogram analysis of the left renal cortex (C) R2* values was performed to measure the mean, median, 10th percentile, 90th percentile, skewness, and kurtosis for all kidneys. Masson trichrome staining was used to assess the percentage of fibrotic area.

RESULTS

The histogram R2* values of the mean, median, 10th percentile, and 90th percentile at week 2 were all lower than those at baseline. Over the course of UUO progression, there were statistical differences between the histogram R2* values at any other two time points, except between weeks 4 and 6, and weeks 6 and 8. A close correlation was found between the percentage of fibrotic area and R2* values (mean: F = 21.49, p = 0.0001, R² = 0.49, median: F = 30.07, p < 0.0001, R² = 0.58, 10th percentile: F = 31.02, p < 0.0001, R² = 0.59, 90th percentile: F = 24.13, p < 0.0001, R² = 0.52).

CONCLUSION

BOLD-MRI could reflect the formation and progression of renal fibrosis in a rabbit UUO model; however, the value of BOLD-MRI in the long-term evaluation of fibrosis is limited.

Noninvasive assessment of renal fibrosis by magnetic resonance imaging and ultrasound techniques

Renal fibrosis is a useful biomarker for diagnosis and guidance of therapeutic interventions of chronic kidney disease (CKD), a worldwide disease that affects more than 10% of the population and is one of the major causes of death. Currently, tissue biopsy is the gold standard for assessment of renal fibrosis. However, it is invasive, and prone to sampling error and observer variability, and may also result in complications. Recent advances in diagnostic imaging techniques, including magnetic resonance imaging (MRI) and ultrasonography, have shown promise for noninvasive assessment of renal fibrosis. These imaging techniques measure renal fibrosis by evaluating its impacts on the functional, mechanical, and molecular properties of the kidney, such as water mobility by diffusion MRI, tissue hypoxia by blood oxygenation level dependent MRI, renal stiffness by MR and ultrasound elastography, and macromolecule content by magnetization transfer imaging. Other MR techniques, such as T1/T2 mapping and susceptibility-weighted imaging have also been explored for measuring renal fibrosis. Promising findings have been reported in both preclinical and clinical studies using these techniques. Nevertheless, limited specificity, sensitivity, and practicality in these techniques may hinder their immediate application in clinical routine. In this review, we will introduce methodologies of these techniques, outline their applications in fibrosis imaging, and discuss their limitations and pitfalls.

The Accuracy of Renal Function Measurements in Obstructive Hydronephrosis Using Dynamic Contrast-Enhanced MR Renography

OBJECTIVE. The objective of our study was to assess the accuracy of glomerular filtration rate (GFR) evaluation in patients with obstructive hydronephrosis using dynamic contrast-enhanced MR renography (DCE-MRR). MATERIALS AND METHODS. A group of 28 adult volunteers were enrolled in this study: 13 without hydronephrosis, eight with low-grade hydronephrosis, and seven with high-grade hydronephrosis. The GFR obtained from DCE-MRR (GFR_MRR) and the GFR obtained from renal scintigraphy (GFR_RS) were compared with the reference GFR (GFR_Ref) acquired using the two plasma sample method. The correlation and agreement between GFR_MRR and GFR_Ref, GFR_RS and GFR_Ref, and single-kidney GFR_MRR (skGFR_MRR) and single-kidney GFR_RS (skGFR_RS) were assessed. The interrater reliability of DCE-MRR and the interrater reliability of renal scintigraphy (RS) were measured. RESULTS. Both GFR_MRR and GFR_RS correlated well with GFR_Ref. In patients with hydronephrosis, DCE-MRR and RS overestimated GFR by 12.8 ± 13.9 mL/min (mean ± SD) and 11.5 ± 12.3 mL/min, respectively. The skGFR_RS was higher than skGFR_MRR by 5.7 ± 3.8 mL/min in high-grade hydronephrotic kidneys (p = 0.004). Good interrater reliability was observed for skGFR_MRR (intraclass correlation coefficient [ICC] = 0.82-0.92) and skGFR_RS (ICC = 0.79-0.90) for both nonhydronephrotic kidneys and hydronephrotic kidneys. The overall mean SDs of repeated measurements from three investigators were 4.0 and 3.8 mL/min for skGFR_MRR and skGFR_RS, respectively. CONCLUSION. Both DCE-MRR and RS tend to overestimate GFR in patients with hydronephrosis. RS-derived skGFR is slightly higher than that of DCE-MRR in kidneys with high-grade hydronephrosis. DCE-MRR is comparable to RS and may serve as an alternative noninvasive method for GFR measurement.

Evaluation of Renal Pathophysiological Processes Induced by an Iodinated Contrast Agent in a Diabetic Rabbit Model Using Intravoxel Incoherent Motion and Blood Oxygenation Level-Dependent Magnetic Resonance Imaging

OBJECTIVE

To examine the potential of intravoxel incoherent motion (IVIM) and blood oxygen level-dependent (BOLD) magnetic resonance imaging for detecting renal changes after iodinated contrast-induced acute kidney injury (CI-AKI) development in a diabetic rabbit model.

MATERIALS AND METHODS

Sixty-two rabbits were randomized into 2 groups: diabetic rabbits with the contrast agent (DCA) and healthy rabbits with the contrast agent (NCA). In each group, 6 rabbits underwent IVIM and BOLD imaging at 1 hour, 1 day, 2 days, 3 days, and 4 days after an iohexol injection while 5 rabbits were selected to undergo blood and histological examinations at these specific time points. Iohexol was administrated at a dose of 2.5 g I/kg of body weight. Further, the apparent transverse relaxation rate (R2*), average pure molecular diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f) were calculated.

RESULTS

The D and f values of the renal cortex (CO) and outer medulla (OM) were significantly decreased compared to baseline values in the 2 groups 1 day after the iohexol injection (p < 0.05). A marked reduction in the D* values for both the CO and OM was also observed after 1 hour in each group (p < 0.05). In the OM, a persistent elevation of the R2* was detected for 4 days in the DCA group (p < 0.05). Histopathological changes were prominent, and the pathological features of CI-AKI aggravated in the DCA group until day 4. The D, f, and R2* values significantly correlated with the histological damage scores, hypoxia-inducible transcription factor-1α expression scores, and serum creatinine levels.

CONCLUSION

A combination of IVIM and BOLD imaging may serve as a noninvasive method for detecting and monitoring CI-AKI in the early stages in the diabetic kidney.

Continuous prospectively navigated multi-echo GRE for improved BOLD imaging of the kidneys

The objective of this study is to develop improved methods for renal blood oxygenation level dependent (BOLD) imaging. T2* mapping of the kidneys, or renal BOLD imaging, may depict renal oxygen levels and may be valuable as a noninvasive means of following the progression of renal disease. Current renal BOLD data is limited by imaging in a single breath hold, which results in low resolution and low signal-to-noise ratio (SNR). We compare a new free-breathing renal BOLD method with conventional breath-hold BOLD (BH-BOLD). A multi-echo GRE sequence with continuous prospective respiratory navigation and real-time feedback was developed that allows high resolution and high SNR renal BOLD imaging with constant sequence repetition time (TR) during free-breathing BOLD (FB-BOLD). The sequence was evaluated in 10 normal volunteers and compared with conventional BH-BOLD. Scan time for the FB-BOLD sequence was approximately three minutes, compared with 15 seconds for the BH-BOLD sequence. SNR of source images and residual error of T2* fitting were compared between the two methods. The FB-BOLD sequence produced motion-free T2* maps of the kidneys with SNR 1.9 times higher than BH-BOLD images. Residual error of T2* fitting was consistently lower in the right kidney with FB-BOLD (30% less than BH-BOLD) but higher in the left kidney (80% more than BH-BOLD), likely related to placement of the navigator on the right hemidiaphragm. A free-breathing prospectively navigated renal BOLD sequence allows flexible tradeoff between scan time, resolution, and SNR.

Application of BOLD-MRI in the classification of renal function in chronic kidney disease

PURPOSE

The purpose of the study was to explore the application of blood oxygenation level-dependent functional magnetic resonance imaging (BOLD-MRI) in classification of chronic kidney disease (CKD).

METHODS

Twenty-nine cases with CKD and 27 healthy volunteers underwent renal BOLD-MRI. Cases of CKD were divided into two groups according to the estimated glomerular filtration rate (eGFR). The R2* values were measured in renal cortex and medulla, respectively. The difference of R2* between renal cortex and medulla was compared, and the correlations of R2* value in renal cortex and medulla with eGFR were analyzed.

RESULTS

Twenty-nine cases of CKD were divided into two groups, with 13 cases of mild renal impairment and 16 cases of moderate to severe renal impairment. In the control and mild renal impairment group, the R2* of renal cortex was significantly lower than that of medulla (P < 0.001). In the control group, mild renal impairment and moderate to severe renal impairment group, the R2* value of cortex increased, while the R2* value of medulla gradually decreased. The eGFR of patients was positively correlated with R2* of medulla (r = 0.81, P < 0.001), while displayed no correlation with R2* of cortex (r = - 0.32, P > 0.05). When the threshold of R2* of medulla was set at 28.4 Hz, the sensitivity and specificity to distinguish normal and mild renal impairment group were 92.31% and 85.19%, respectively.

CONCLUSION

The change of blood oxygen in renal cortex and medulla could be detected with BOLD-MRI, so as to evaluate the renal function and anoxic injury of CKD.

Cortical Perfusion and Tubular Function as Evaluated by Magnetic Resonance Imaging Correlates with Annual Loss in Renal Function in Moderate Chronic Kidney Disease

BACKGROUND

Chronic hypoxia is a well-recognized factor in the pathogenesis of chronic kidney disease (CKD). Loss of microcirculation is thought to lead to enhanced renal hypoxia, which in turn results in the development of fibrosis, a hallmark of progressive CKD. To evaluate the role of functional magnetic resonance imaging (MRI), we performed perfusion, oxygenation, and diffusion MRI measurements in individuals with diabetes and stage 3 CKD.

METHODS

Fifty-four subjects (41 individuals with diabetes and stage 3 CKD and 13 healthy controls) participated in this study. Data with blood oxygenation level dependent (BOLD), arterial spin labeling perfusion and diffusion MRI were acquired using a 3T scanner.

RESULTS

Renal cortical perfusion was reduced in CKD compared to the controls (109.54 ± 25.38 vs. 203.17 ± 27.47 mL/min/100 g; p < 0.001). Cortical apparent diffusion coefficient showed no significant reduction in CKD compared to controls (1,596.10 ± 196.64 vs. 1,668.72 ± 77.29 × 10-6 mm2/s; p = 0.45) but was significantly associated with perfusion. Cortical R2* values were modestly increased in CKD (20.76 ± 4.08 vs. 18.74 ± 2.37 s-1; p = 0.12). Within the CKD group, R2*_Medulla and R2*_Kidney were moderately and negatively associated with estimated glomerular filtration rate. There was a significant association between cortical perfusion and medullary response to furosemide with annual loss of renal function, used as an estimate of CKD progression.

CONCLUSIONS

Subjects with a moderate degree of CKD had significantly lower renal perfusion. Diffusion and BOLD MRI showed more modest differences between the groups. Individuals with progressive CKD had lower perfusion and response to furosemide.

Hepatocyte Growth Factor-Secreting Mesothelial Cell Sheets Suppress Progressive Fibrosis in a Rat Model of CKD

BACKGROUND

Although hepatocyte growth factor (HGF) has antifibrotic effects and is involved in angiogenesis and vasodilation, systemic administration of HGF to prevent kidney fibrosis is not a feasible strategy for suppressing interstitial fibrosis in patients with CKD.

METHODS

We investigated a novel therapy involving HGF transgenic cell sheets grown in culture from human mesothelial cells and administered to rats with unilateral ureteral obstruction (UUO). We compared progression of fibrosis in rats with UUO that received one of five interventions: HGF-transgenic mesothelial cell sheets transplanted to the kidney surface, HGF-transgenic mesothelial cell sheets transplanted to thigh, mesotherial cell sheets transplanted to kidney, no sheets, or HGF injections.

RESULTS

HGF transgenic cell sheets transplanted to the kidney strongly suppressed the induction of myofibroblasts and collagen in the kidney for 28 days; other interventions did not. Additionally, the HGF-secreting cell sheets ameliorated loss of peritubular capillaries and maintained renal blood flow.

CONCLUSIONS

These findings suggest that cell sheet therapy is a novel and promising strategy for inhibiting progressive fibrosis in CKD.

Comparison of blood oxygen level-dependent imaging and diffusion-weighted imaging in early diagnosis of acute kidney injury in animal models

BACKGROUND

Early diagnosis of acute kidney injury (AKI) has clinical importance. Current methods are neither adequately sensitive nor specific. Blood oxygen level-dependent (BOLD) imaging and diffusion-weighted imaging (DWI) may help to assess AKI in the early phase.

PURPOSE

To investigate the feasibility of BOLD imaging and DWI in the assessment of AKI and compare the sensitivities of both techniques in early detection of renal damage.

STUDY TYPE

Prospective animal study.

ANIMAL MODEL

Thirty New Zealand white rabbits.

FIELD STRENGTH/SEQUENCE

3 T clinical MRI/BOLD and DWI.

ASSESSMENT

Thirty rabbits were divided into three groups (severe AKI group, mild AKI group, and control group). Transarterial renal embolization with different doses of microspheres was performed to create severe and mild AKI disease models. All the MRI scans of kidneys were conducted within 2 hours after the embolization procedure. Histological examinations with hematoxylin and eosin staining were performed to validate renal damage.

STATISTICAL TESTS

Analysis of variance (ANOVA) for comparisons between groups, and paired t-test for tests within the same group. P < 0.05 was considered statistically significant.

RESULTS

Both R2* and apparent diffusion coefficient (ADC) showed significant differences between the severe AKI group (56.34 ± 3.45 s-1 for R2*, 1.14 ± 0.23 mm² /s for ADC) and the control group (28.24 ± 2.26 s-1 for R2*, 1.94 ± 0.33 mm² /s for ADC, both P < 0.01). However, the ADC values did not show significant differences (P = 0.41) between mild AKI group (1.88 ± 0.31 mm² /s for ADC) and the control group (1.94 ± 0.33 mm² /s for ADC), while R2* was still useful in differentiating the two groups (52.32 ± 4.1 s-1 vs. 28.24 ± 2.26 s-1 for R2*, P < 0.01). The histopathologic results were found to be correlated with MRI findings.

DATA CONCLUSION

BOLD contrast and DW images are both effective in detecting AKI noninvasively, but BOLD imaging is more sensitive in early detection of mild ischemia than DWI.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:719-724.

Validation of the corticomedullary difference in magnetic resonance imaging-derived apparent diffusion coefficient for kidney fibrosis detection: a cross-sectional study

Background

Kidney cortical interstitial fibrosis (IF) is highly predictive of renal prognosis and is currently assessed by the evaluation of a biopsy. Diffusion magnetic resonance imaging (MRI) is a promising tool to evaluate kidney fibrosis via the apparent diffusion coefficient (ADC), but suffers from inter-individual variability. We recently applied a novel MRI protocol to allow calculation of the corticomedullary ADC difference (ΔADC). We here present the validation of ΔADC for fibrosis assessment in a cohort of 164 patients undergoing biopsy and compare it with estimated glomerular filtration rate (eGFR) and other plasmatic parameters for the detection of fibrosis.

Methods

This monocentric cross-sectional study included 164 patients undergoing renal biopsy at the Nephrology Department of the University Hospital of Geneva between October 2014 and May 2018. Patients underwent diffusion-weighted imaging, and T1 and T2 mappings, within 1 week after biopsy. MRI results were compared with gold standard histology for fibrosis assessment.

Results

Absolute cortical ADC or cortical T1 values correlated poorly to IF assessed by the biopsy, whereas ΔADC was highly correlated to IF (r=−0.52, P &lt; 0.001) and eGFR (r = 0.37, P &lt; 0.01), in both native and allograft patients. ΔT1 displayed a lower, but significant, correlation to IF and eGFR, whereas T2 did not correlate to IF nor to eGFR. ΔADC, ΔT1 and eGFR were independently associated with kidney fibrosis, and their combination allowed detection of extensive fibrosis with good specificity.

Conclusion

ΔADC is better correlated to IF than absolute cortical or medullary ADC values. ΔADC, ΔT1 and eGFR are independently associated to IF and allow the identification of patients with extensive IF.

Ultrasound Elastography Evaluations in Patient Populations With Various Kidney Diseases

Renal elastography is an ultrasonographic method that has been recently found to provide information on renal functions and fibrosis. In this study, we aimed to investigate the relationship between elastography scores and renal functions and proteinuria levels in patient populations with various kidney diseases. Seventy-five diabetic nephropathy patients, 66 kidney transplant patients, and 45 glomerulonephritis patients were included in the study. The amount of proteinuria was measured according to the protein-to-creatinine ratio in spot urine samples. The estimated glomerular filtration rate (eGFR) was calculated according to the Modification of Diet in Renal Disease formula. Ultrasound elastography scores were measured in each patient group by a radiologist. The mean age of diabetic nephropathy patients was 61 ± 10 years. The mean elastography score was 0.96 ± 0.30. Elastography score was positively correlated with serum blood urea nitrogen and creatinine levels and was negatively correlated with eGFR value. The mean age of kidney transplant patients was 42 ± 12 years. The mean elastography score was 1.10 ± 0.38. There was a significant relationship between elastography score and proteinuria level. The mean age of glomerulonephritis patients was 37 ± 13 years. The mean elastography score was 0.91 ± 0.41. Elastography score was positively correlated with serum blood urea nitrogen and creatinine levels. However, there was no relationship between elastography score and eGFR value and proteinuria level. Although renal elastography provides information on renal functions and proteinuria in patients with diabetic nephropathy, renal transplant, and glomerulonephritis, there is a need for studies with a larger number of patients on this subject.

Evaluation of renal dysfunction using texture analysis based on DWI, BOLD, and susceptibility-weighted imaging

OBJECTIVE

To explore the value of texture analysis based on diffusion-weighted imaging (DWI), blood oxygen level-dependent MRI (BOLD), and susceptibility-weighted imaging (SWI) in evaluating renal dysfunction.

METHODS

Seventy-two patients (mean age 53.72 ± 13.46 years) underwent MRI consisting of DWI, BOLD, and SWI. According to their estimated glomerular filtration rate (eGFR), the patients were classified into either severe renal function impairment (sRI, eGFR < 30 mL/min/1.73 m²), non-severe renal function impairment (non-sRI, eGFR ≥ 30 mL/min/1.73 m², and < 80 mL/min/1.73 m²), or control (CG, eGFR ≥ 80 mL/min/1.73 m²) groups. Thirteen texture features were extracted and then were analyzed to select the most valuable for discerning the three groups with each imaging method. A ROC curve was performed to compare the capacities of the features to differentiate non-sRI from sRI or CG.

RESULTS

Six features proved to be the most valuable for assessing renal dysfunction: 0.25Quantile_DWI, 0.5Quantile_DWI, Homogeneity_DWI, Entropy_BOLD, Skewness_SWI, and Correlation_SWI. Three features derived from DWI (0.25Quantile_DWI, 0.5Quantile_DWI, and Homogeneity_DWI) were smaller in sRI than in non-sRI; Entropy_BOLD and Correlation_SWI were smaller in non-sRI than in CG (p < 0.05). 0.25Quantile_DWI, 0.5Quantile_DWI, and Homogeneity_DWI showed similar capacities for differentiating sRI from non-sRI. Similarly, Entropy_BOLD and Correlation_SWI showed equal capacities for differentiating non-sRI from CG.

CONCLUSION

Texture analysis based on DWI, BOLD, and SWI can assist in assessing renal dysfunction, and texture features based on BOLD and SWI may be suitable for assessing renal dysfunction during early stages.

KEY POINTS

• Texture analysis based on MRI techniques allowed for assessing renal dysfunction. • Texture features based on BOLD and SWI, but not DWI, may be suitable for assessing renal function impairment during early stages. • SWI exhibited a similar capacity to BOLD for assessing renal dysfunction.

Assessment of renal fibrosis in murine diabetic nephropathy using quantitative magnetization transfer MRI

PURPOSE

Renal fibrosis is a hallmark of progressive renal disease; however, current clinical tests are insufficient for assessing renal fibrosis. Here we evaluated the utility of quantitative magnetization transfer MRI in detecting renal fibrosis in a murine model of progressive diabetic nephropathy (DN).

METHODS

The db/db eNOS-/- mice, a well-recognized model of progressive DN, and normal wild-type mice were imaged at 7T. The quantitative magnetization transfer data were collected in coronal plane using a 2D magnetization transfer prepared spoiled gradient echo sequence with a Gaussian-shaped presaturation pulse. Parameters were derived using a two-pool fitting model. A normal range of cortical pool size ratio (PSR) was defined as Mean±2SD of wild-type kidneys (N = 20). The cortical regions whose PSR values exceeded this threshold (threshold PSR) were assessed. The correlations between the PSR-based and histological (collagen IV or picrosirius red stain) fibrosis measurements were evaluated.

RESULTS

Compared with wild-type mice, moderate increases in mean PSR values and scattered clusters of high PSR region were observed in cortex of DN mouse kidneys. Abnormally high PSR regions (% area) that were detected by the threshold PSR were significantly increased in renal cortexes of DN mice. These regions progressively increased on aging and highly correlated with histological fibrosis measures, while the mean PSR values correlated much less.

CONCLUSION

Renal fibrosis in DN can be assessed by the quantitative magnetization transfer MRI and threshold analysis. This technique may be used as a novel imaging biomarker for DN and other renal diseases.

Recent advances in renal imaging

Kidney diseases can be caused by a wide range of genetic, hemodynamic, toxic, infectious, and autoimmune factors. The diagnosis of kidney disease usually involves the biochemical analysis of serum and blood, but these tests are often insufficiently sensitive or specific to make a definitive diagnosis. Although radiologic imaging currently has a limited role in the evaluation of most kidney diseases, several new imaging methods hold great promise for improving our ability to non-invasively detect structural, functional, and molecular changes within the kidney. New methods, such as dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and blood oxygen level-dependent (BOLD) MRI, allow functional imaging of the kidney. The use of novel contrast agents, such as microbubbles and nanoparticles, allows the detection of specific molecules in the kidney. These methods could greatly advance our ability to diagnose disease and also to safely monitor patients over time. This could improve the care of individual patients, and it could also facilitate the evaluation of new treatment strategies.

Reduced oxygenation but not fibrosis defined by functional magnetic resonance imaging predicts the long-term progression of chronic kidney disease

Background

Although chronic hypoxia and fibrosis may be a key to the progression of chronic kidney disease (CKD), a noninvasive means of measuring these variables is not yet available. Here, using blood oxygen level–dependent (BOLD) and diffusion-weighted (DW) magnetic resonance imaging (MRI), we assessed changes in renal tissue oxygenation and fibrosis, respectively, and evaluated their correlation with prognosis for renal function.

Methods

The study was conducted under a single-center, longitudinal, retrospective observational design. We examined the prognostic significance of T2* values of BOLD-MRI and apparent diffusion coefficient (ADC) values on DW-MRI and other clinical parameters. The rate of decline in estimated glomerular filtration rate (eGFR) was calculated by linear regression analysis using changes in eGFR during the observation period.

Results

A total of 91 patients were enrolled, with a mean age of 55.8 ± 15.6 years. Among patients, 51 (56.0%) were males and 38 (41.8%) had diabetes mellitus. The mean eGFR was 49.2 ± 28.9 mL/min/1.73 m2 and the mean observation period was 5.13 years. ADC values of DW-MRI but not T2* values of BOLD-MRI were well correlated with eGFR at the initial time point. The mean annual rate of decline in eGFR during the 5-year observation period was −1.92 ± 3.00 mL/min/1.73 m2. On multiple linear regression analysis, the rate of decline in eGFR was significantly correlated with eGFR at the start point, period average amount of proteinuria and T2* values, but not with ADC values (t = 2.980, P = 0.004).

Conclusions

Reduced oxygenation as determined by low T2* values on BOLD-MRI is a clinically useful marker of CKD progression.

The determinants, biomarkers, and consequences of microvascular injury in kidney transplant recipients

Independent of the initial cause of kidney disease, microvascular injury to the peritubular capillary network appears to play a central role in the development of interstitial fibrosis in both native and transplanted kidney disease. This association is explained by mechanisms such as the upregulation of profibrotic genes and epigenetic changes induced by hypoxia, capillary leakage, endothelial and pericyte transition to interstitial fibroblasts, as well as modifications in the secretome of endothelial cells. Alloimmune injury due to antibody-mediated rejection and ischemia-reperfusion injury are the two main etiologies of microvascular damage in kidney transplant recipients. The presence of circulating donor-specific anti-human leukocyte antigen (HLA) antibodies, histological findings, such as diffuse C4d staining in peritubular capillaries, and the extent and severity of peritubular capillaritis, are commonly used clinically to provide both diagnostic and prognostic information. Complement-dependent assays, circulating non-HLA antibodies, or evaluation of the microvasculature with novel imaging techniques are the subject of ongoing studies.