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

Assessment of Acute Kidney Injury using MRI

There has been growing interest in using quantitative magnetic resonance imaging (MRI) to describe and understand the pathophysiology of acute kidney injury (AKI). The ability to assess kidney blood flow, perfusion, oxygenation, and changes in tissue microstructure at repeated timepoints is hugely appealing, as this offers new possibilities to describe nature and severity of AKI, track the time-course to recovery or progression to chronic kidney disease (CKD), and may ultimately provide a method to noninvasively assess response to new therapies. This could have significant clinical implications considering that AKI is common (affecting more than 13 million people globally every year), harmful (associated with short and long-term morbidity and mortality), and currently lacks specific treatments. However, this is also a challenging area to study. After the kidney has been affected by an initial insult that leads to AKI, complex coexisting processes ensue, which may recover or can progress to CKD. There are various preclinical models of AKI (from which most of our current understanding derives), and these differ from each other but more importantly from clinical AKI. These aspects are fundamental to interpreting the results of the different AKI studies in which renal MRI has been used, which encompass different settings of AKI and a variety of MRI measures acquired at different timepoints. This review aims to provide a comprehensive description and interpretation of current studies (both preclinical and clinical) in which MRI has been used to assess AKI, and discuss future directions in the field. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 3.

Renal Artery Coil Embolization as an Endovascular Approach for Establishing a Rabbit Model of Chronic Kidney Disease

PURPOSE

To investigate the safety and feasibility of renal artery coil embolization for establishing chronic kidney disease (CKD) in rabbits.

MATERIALS AND METHODS

Ten male adult New Zealand rabbits underwent renal artery coil embolization. Initially, the main renal artery on 1 side was completely embolized, followed by embolization of approximately two-thirds of the primary branches of the contralateral renal artery 1 week later. Four rabbits were randomly chosen for sacrifice at 4 weeks after embolization, whereas the remaining 6 were sacrificed at 8 weeks after embolization. The assessment encompassed the animals' general condition, angiography, biochemical parameters, inflammatory markers, and histopathological examination of the kidneys and hearts.

RESULTS

Four weeks after embolization, serum creatinine level showed a substantial increase (2.4 mg/dL [SD ± 0.6]; P = .009 vs baseline), with a subsequent 4.12-fold elevation at 8 weeks after embolization (4.9 mg/dL [SD ± 1.4]; P < .001 vs baseline). Additionally, considerable increases in serum blood urea nitrogen, calcium, and potassium ions were observed at 8 weeks after embolization (58.3 mg/dL [SD ± 19.0]; P < .001 vs baseline; 23.1 mg/dL [SD ± 4.4]; P < .001 vs baseline; and 6.3 mEq/L [SD ± 0.7]; P < .001 vs baseline, respectively). The completely embolized kidney exhibited notable atrophy, severe fibrosis, and cortical calcification, whereas the contralateral partially embolized kidney displayed compensatory hypertrophy, along with glomerulosclerosis, tubular dilation, tubular casts, and interstitial fibrosis.

CONCLUSIONS

Renal artery coil embolization proved to be effective and safe for establishing a CKD model in rabbits.

Multi-Parametric MRI for Evaluating Variations in Renal Structure, Function, and Endogenous Metabolites in an Animal Model With Acute Kidney Injury Induced by Ischemia Reperfusion

BACKGROUND

Ischemia reperfusion injury (IRI)-induced acute kidney injury (AKI) may occur after renal ischemic injury. There is a lack of an accurate and comprehensive detection technique for IRI-AKI.

PURPOSE

To longitudinally evaluate IRI-AKI in rats by renal structure, function, and metabolites using multi-parametric MRI (mpMRI).

STUDY TYPE

Prospective.

ANIMAL MODEL

Forty-eight rats undergoing IRI-AKI.

FIELD STRENGTH/SEQUENCE

7-T, T1 mapping, and arterial spin labeling (ASL): echo planar imaging (EPI) sequence; blood oxygen level-dependent (BOLD): gradient recalled echo (GRE) sequence; T2 mapping, quantitative magnetization transfer (qMT), and chemical exchange saturation transfer (CEST): rapid acquisition with relaxation enhancement (RARE) sequence.

ASSESSMENT

The mpMRI for IRI-AKI was conducted at 0 (control), 1, 3, 7, 14, and 28 days, all included eight rats. The longitudinal mpMRI signal of manually outlined cortex, outer stripe of the outer medulla (OSOM), inner stripe of the outer medulla, and medulla plus pelvis were calculated and compared, their diagnosis performance for IRI-AKI also been evaluated.

STATISTICAL TESTS

Pearson correlations analysis for correlation between mpMRI signal and renal injury, unpaired t-tests for comparing the signal changes, and receiver operating characteristics (ROC) analysis was used to identify most sensitive indicator of mpMRI. A P-value <0.05 was considered statistically significant.

RESULTS

Compared with control kidneys, the T1 and T2 values of the cortex and medulla in IRI kidneys increased and reached their highest values on day 14, and the kidneys also showed the most severe edema and segments blurred. The RBF in the cortex and OSOM showed a significant decline after day 3. The BOLD signal in the OSOM largest increased on day 28. The cortical PSR and the amine-CEST both decreased with IRI-AKI progression, and amine-CEST achieved the highest AUC for the diagnosis (0.899).

DATA CONCLUSION

Multi-parametric MRI may show comprehensive variations in IRI-AKI, and amine-CEST may exhibit the highest accuracy for diagnosis of IRI-AKI.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY: Stage 2.

Evaluation of renal tubular function by multiparametric functional MRI in early diabetes

Purpose To evaluate the tubular function in an alloxan-induced type 1 diabetes mellitus (DM) rabbit model measured by renal oxygenation (R2*), oxygen extraction fraction (OEF), and renal blood flow (RBF) using blood oxygenation level dependent, asymmetric spin echo, and arterial spin labeling MRI. Methods Twenty-six rabbits were randomized into the 3-day DM group (n = 13) and the 7-day DM group (n = 13). We performed pairs of multiparametric MRIs (before and after furosemide injection) at baseline and 3/7 days post-DM, and scored pathological kidney injury. We performed statistical analyses using non-parametric, chi-square, and Spearman correlation tests. Results At baseline, medullary R2* significantly decreased by 24.97% and 16.74% in the outer and inner stripes of the outer medulla (OS and IS, p = 0.006 and 0.003, respectively) after furosemide administration. While the corresponding OEF decreased by 15.91% for OS and 16.67% for IS (both p = 0.003), and no significant change in medullary RBF was observed (p > 0.05). In the 3-day DM group, the decrease of medullary R2* and OEF post-furosemide became unremarkable, suggesting tubular dysfunction. We noticed similar changes in the 7-day DM group. Correlation analysis showed pathological tubular injury score significantly correlated with medullary ∆R2* (post-furosemide - pre-furosemide difference, r = 0.82 for OS and 0.82 for IS) and ∆OEF (r = 0.82 for OS and 0.82 for IS) (p < 0.001, respectively). Conclusion: The combination of medullary OEF and R2* in response to furosemide could detect renal tubular dysfunction in early DM.

Mitochondrial targeted ROS Scavenger based on nitroxide for Treatment and MRI imaging of Acute Kidney Injury

INTRODUCTION

Overproduction of reactive oxygen species (ROS) during oxidative stress is a hallmark of acute kidney injure (AKI), which induced the damage to the renal cells and mitochondrial injury.

PURPOSE

In this contribution, we prepared mitochondrial targeted nitroxide, which linked 3-carboxy-2,2,5,5-tetramethylpyrrolidine 1-oxyl (carboxy-PROXYL) with (2-aminoethyl)triphenylphosphonium bromide (TPP), named TPP-PROXYL to eliminate the ROS in situ and image the oxidative stress reaction by MRI.

METHODS

2,7-Dichlorodihydrofluorescein diacetate (DCFH-DA) staining, mitochondrial membrane potential assay (JC-1) staining and transmission electron microscope (TEM) experiments were processed to verify that TPP-PROXYL could target mitochondria, scavenge the ROS, and prevent damage to mitochondria in live cells. Contrast enhanced MRI also been used to monitor these redox reaction in AKI model.

RESULTS

TPP-PROXYL demonstrated excellent ROS T₁-weighted magnetic resonance imaging (MRI) enhancement in vitro and in vivo, with r₁ value about 0.190 mM^-1·s^-1. In vivo AKI treatment experiments proved that TPP-PROXYL could improve the survival rate of mice and inhibit kidney damage. Moreover, the great ROS scavenging capability and the renal damage reduction during AKI treatment of TPP-PROXYL was verified via MR imaging technology.

CONCLUSION

Collectively, this research provides TPP-PROXYL would serve as a powerful platform to realize ROS scavenging, treatment and MR imaging of AKI.

Dynamic R2' Imaging can Be a Biomarker for Diagnosing and Staging Early Acute Kidney Injury in Animals

Background: Early diagnosis of acute kidney injury (AKI) is essential in clinical settings. None of the current biomarkers are widely applied. The combination of pulse-shifting multi-echo asymmetric spin-echo sequence (psMASE) and a modified hemodynamic response imaging (HRI) technique is promising. The purpose of this study was to evaluate the feasibility of psMASE combined with HRI in detecting early ischemic AKI in animal models of different severities.

Methods: Twenty rabbits were divided into four groups (mild, moderate, and severe AKI and control groups). Transarterial embolization with different doses of microspheres was performed to establish AKI animal models of different severities. The 3T psMASE and HRI scans of kidneys were conducted. The R2^*, R2, and R2' during room air and gas stimulation were acquired and the difference of R2' (dR2') was evaluated in different AKI groups.

Results: The values were not different in R2^* and R2 during room air and in R2^* and R2, and R2' during gas stimulation. The value of R2' was significantly different during room air (P = 0.014), but the difference was only found between control and moderate/severe AKI groups (P = 0.032 and 0.022). The values of dR2' were different among groups (P < 0.0001) and differences between every two groups except comparison of moderate and severe AKI groups were significant (P < 0.01).

Conclusion: The dR2' imaging acquired by a combination of renal psMASE and HRI technique can serve as a potential quantitative biomarker for early detection and staging of AKI.

Clinical and experimental approaches for imaging of acute kidney injury

Complex molecular cell dynamics in acute kidney injury and its heterogeneous etiologies in patient populations in clinical settings have revealed the potential advantages and disadvantages of emerging novel damage biomarkers. Imaging techniques have been developed over the past decade to further our understanding about diseased organs, including the kidneys. Understanding the compositional, structural, and functional changes in damaged kidneys via several imaging modalities would enable a more comprehensive analysis of acute kidney injury, including its risks, diagnosis, and prognosis. This review summarizes recent imaging studies for acute kidney injury and discusses their potential utility in clinical settings.

Noninvasive Evaluation of Renal Hypoxia by Multiparametric Functional MRI in Early Diabetic Kidney Disease

BACKGROUND

Renal hypoxia, which caused by a mismatch between oxygen delivery and oxygen demand, may be the primary pathophysiological pathway driving diabetic kidney disease (DKD). Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) could detect hypoxia, but can be limited in distinguishing increased oxygen consumption or decreased blood supply.

PURPOSE

To explore multiparametric functional MRI in evaluating mechanism of the hypoxia changes in early stage of DKD.

STUDY TYPE

Prospective.

ANIMAL MODEL

Thirty-five New Zealand White rabbits were divided into control group (n = 5) and alloxan-induced diabetes mellitus (DM) groups (DM3 group: n = 15, DM7 group: n = 15).

FIELD STRENGTH/SEQUENCE

3 T MRI/BOLD, arterial spin labeling (ASL), and asymmetric spin-echo (ASE).

ASSESSMENT

The renal oxygenation level (R2*), renal blood flow (RBF), and oxygen extraction fraction (OEF) were evaluated by BOLD, ASL, and ASE MRI, respectively. The regions of interest were manually drawn including cortex, outer stripes of outer medulla (OS), and inner stripes of outer medulla (IS).

STATISTICAL TESTS

Analysis of variance, independent-sample t-test, and paired-sample t-test were applied for comparisons among groups, between groups, and within the same group. P < 0.05 was considered statistically significant.

RESULTS

All renal regions of DM3 group at Day 3 after DM induction showed significantly higher R2* and OEF values compared to baseline. The RBF values showed no statistically significant difference (P = 0.62, 0.76, 0.09 in cortex, OS, and IS, respectively). For DM7 group at Day 7, R2*, OEF, and RBF values showed no statistically significant difference compared to baseline (P = 0.06, 0.05, 0.06 of R2*; 0.70, 0.64, 0.68 of OEF; and 0.33, 0.58, 0.48 of RBF in cortex, OS, and IS, respectively).

DATA CONCLUSION

BOLD MRI could detect renal hypoxia in early stage of DKD rabbit model, which was mainly revealed by increased oxygen consumption, but not affected by renal blood flow change.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 1.

In Vivo Assessment of Metabolic Abnormality in Alport Syndrome Using Hyperpolarized [1-13C] Pyruvate MR Spectroscopic Imaging

Alport Syndrome (AS) is a genetic disorder characterized by impaired kidney function. The development of a noninvasive tool for early diagnosis and monitoring of renal function during disease progression is of clinical importance. Hyperpolarized ¹³C MRI is an emerging technique that enables non-invasive, real-time measurement of in vivo metabolism. This study aimed to investigate the feasibility of using this technique for assessing changes in renal metabolism in the mouse model of AS. Mice with AS demonstrated a significant reduction in the level of lactate from 4- to 7-week-old, while the levels of lactate were unchanged in the control mice over time. This reduction in lactate production in the AS group accompanied a significant increase of PEPCK expression levels, indicating that the disease progression in AS triggered the gluconeogenic pathway and might have resulted in a decreased lactate pool size and a subsequent reduction in pyruvate-to-lactate conversion. Additional metabolic imaging parameters, including the level of lactate and pyruvate, were found to be different between the AS and control groups. These preliminary results suggest that hyperpolarized ¹³C MRI might provide a potential noninvasive tool for the characterization of disease progression in AS.

Potential role of imaging for assessing acute pancreatitis-induced acute kidney injury

Acute kidney injury (AKI) is a common complication of acute pancreatitis (AP) that is associated with increased mortality. Conventional assessment of AKI is based on changes in serum creatinine concentration and urinary output. However, these examinations have limited accuracy and sensitivity for the diagnosis of early-stage AKI. This review summarizes current evidence on the use of advanced imaging approaches and artificial intelligence (AI) for the early prediction and diagnosis of AKI in patients with AP. CT scores, CT post-processing technology, Doppler ultrasound, and AI technology provide increasingly valuable information for the diagnosis of AP-induced AKI. Magnetic resonance imaging (MRI) also has potential for the evaluation of AP-induced AKI. For the accurate diagnosis of early-stage AP-induced AKI, more studies are needed that use these new techniques and that use AI in combination with advanced imaging technologies.