Detection of evolving acute tubular necrosis with renal 23Na MRI: studies in rats

Daytime Variation in Kidney Perfusion, Oxygenation, and Sodium Concentration Assessed by Multiparametric MRI in Healthy Volunteers

BACKGROUND

MRI can provide information on kidney structure, perfusion, and oxygenation. Furthermore, it allows for the assessment of kidney sodium concentrations and handling, allowing multiparametric evaluation of kidney physiology. Multiparametric MRI is promising for establishing prognosis and monitoring treatment responses in kidney diseases, but its intraindividual variation during the day is unresolved.

PURPOSE

To investigate the variation in multiparametric MRI measurements from the morning to the evening.

STUDY TYPE

Prospective.

POPULATION

Ten healthy volunteers, aged 29 ± 5 without history of kidney disease.

FIELD STRENGTH/SEQUENCE

3 T/T1 mapping, blood-oxygen level dependent imaging, arterial spin labeling perfusion imaging, diffusion weighted imaging, and sodium imaging.

ASSESSMENT

A multiparametric MRI protocol, yielding T1, R2*, ADC, renal blood flow and renal sodium levels, was acquired in the morning, noon, and evening. The participants were fasting prior to the first examination. Urine biochemical analyses were performed to complement MRI data. The cortex and medulla were analyzed separately in a semi-automatic fashion, and gradients of total sodium concentration (TSC) and R2* gradients were calculated from outer cortex to inner medulla.

STATISTICAL TEST

Analyses of variance and mixed-effects models to estimate differences from time of day. Coefficients of variation to assess variability within and between participants. A P-value <0.05 was considered statistically significant.

RESULTS

The coefficients of variation varied from 5% to 18% for proton-based parametric sequences, while it was 38% for TSC over a day.

DATA CONCLUSION

Multiparametric MRI is stable over the day. The coefficients of variation over a day were lower for proton multiparametric MRI, but higher for sodium MRI.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

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.

Recent Advances in Sodium Magnetic Resonance Imaging and Its Future Role in Kidney Disease

Sodium imbalance is a hallmark of chronic kidney disease (CKD). Excess tissue sodium in CKD is associated with hypertension, inflammation, and cardiorenal disease. Sodium magnetic resonance imaging (23Na MRI) has been increasingly utilized in CKD clinical trials especially in the past few years. These studies have demonstrated the association of excess sodium tissue accumulation with declining renal function across whole CKD spectrum (early- to end-stage), biomarkers of systemic inflammation, and cardiovascular dysfunction. In this article, we review recent advances of 23Na MRI in CKD and discuss its future role with a focus on the skin, the heart, and the kidney itself.

Sodium Magnetic Resonance Imaging Shows Impairment of the Counter-current Multiplication System in Diabetic Mice Kidney

Key Points

23Na MRI allows us to noninvasively assess sodium distribution. We propose the utility of 23Na MRI for evaluating functional changes in diabetic kidney disease and not as a marker reflecting structural damage. 23Na MRI may be an early marker for structures beyond the glomeruli, enabling prompt intervention with novel efficacious tubule-targeting therapies.

Background

Sodium magnetic resonance imaging can noninvasively assess sodium distribution, specifically sodium concentration in the countercurrent multiplication system in the kidney, which forms a sodium concentration gradient from the cortex to the medulla, enabling efficient water reabsorption. This study aimed to investigate whether sodium magnetic resonance imaging can detect changes in sodium concentrations under normal conditions in mice and in disease models, such as a mouse model with diabetes mellitus.

Methods

We performed sodium and proton nuclear magnetic resonance imaging using a 9.4-T vertical standard-bore superconducting magnet.

Results

A condition of deep anesthesia, with widened breath intervals, or furosemide administration in 6-week-old C57BL/6JJcl mice showed a decrease in both tissue sodium concentrations in the medulla and sodium concentration gradients from the cortex to the medulla. Furthermore, sodium magnetic resonance imaging revealed reductions in the sodium concentration in the medulla and in the gradient from the cortex to the medulla in BKS.Cg-Leprdb+/+ Leprdb/Jcl mice at very early type 2 diabetes mellitus stages compared with corresponding control BKS.Cg-m+/m+/Jcl mice.

Conclusions

The kidneys of BKS.Cg-Leprdb+/+ Leprdb/Jcl mice aged 6 weeks showed impairments in the countercurrent multiplication system. We propose the utility of 23Na MRI for evaluating functional changes in diabetic kidney disease and not as a marker that reflects structural damage. Thus, 23Na MRI may be a potentially very early marker for structures beyond the glomerulus; this may prompt intervention with novel efficacious tubule-targeting therapies.

Fibrosis imaging with multiparametric proton and sodium MRI in pig injury models

Chronic kidney disease (CKD) is common and has huge implications for health and mortality. It is aggravated by intrarenal fibrosis, but the assessment of fibrosis is limited to kidney biopsies, which carry a risk of complications and sampling errors. This calls for a noninvasive modality for diagnosing and staging intrarenal fibrosis. The current, exploratory study evaluates a multiparametric MRI protocol including sodium imaging (²³ Na-MRI) to determine the opportunities within this modality to assess kidney injury as a surrogate endpoint of fibrosis. The study includes 43 pigs exposed to ischemia-reperfusion injury (IRI) or unilateral ureteral obstruction (UUO), or serving as healthy controls. Fibrosis was determined using gene expression analysis of collagen. The medulla/cortex ratio of ²³ Na-MRI decreased in the injured kidney in the IRI pigs, but not in the UUO pigs (p = 0.0180, p = 0.0754). To assess the combination of MRI parameters in estimating fibrosis, we created a linear regression model consisting of the cortical apparent diffusion coefficient, ΔR2*, ΔT1, the ²³ Na medulla/cortex ratio, and plasma creatinine (R²  = 0.8009, p = 0.0117). The ²³ Na medulla/cortex ratio only slightly improved the fibrosis prediction model, leaving ²³ Na-MRI in an ambiguous place for evaluation of intrarenal fibrosis. Use of multiparametric MRI in combination with plasma creatinine shows potential for the estimation of fibrosis in human kidney disease, but more translational and clinical work is warranted before MRI can contribute to earlier diagnosis and evaluation of treatment for acute kidney injury and CKD.

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.

MRI Mapping of Renal T1: Basic Concept

In renal MRI, measurement of the T₁ relaxation time of water molecules may provide a valuable biomarker for a variety of pathological conditions. Due to its sensitivity to the tissue microenvironment, T₁ has gained substantial interest for noninvasive imaging of renal pathology, including inflammation and fibrosis. In this chapter, we will discuss the basic concept of T₁ mapping and different T₁ measurement techniques and we will provide an overview of emerging preclinical applications of T₁ for imaging of kidney disease.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Sodium (23Na) MRI of the Kidney: Basic Concept

The handling of sodium by the renal system is a key indicator of renal function. Alterations in the corticomedullary distribution of sodium are considered important indicators of pathology in renal diseases. The derangement of sodium handling can be noninvasively imaged using sodium magnetic resonance imaging (²³Na MRI), with data analysis allowing for the assessment of the corticomedullary sodium gradient. Here we introduce sodium imaging, describe the existing methods, and give an overview of preclinical sodium imaging applications to illustrate the utility and applicability of this technique for measuring renal sodium handling.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Visualization of sodium dynamics in the kidney by magnetic resonance imaging in a multi-site study

Sodium magnetic resonance imaging (MRI) is a powerful, non-invasive technique to assess sodium distribution within the kidney. Here we undertook pre-clinical and clinical studies to quantify the corticomedullary sodium gradient in healthy individuals and in a porcine model of diuresis. The results demonstrated that sodium MRI could detect spatial differences in sodium biodistribution across the kidney. The sodium gradient of the kidney changed significantly after diuresis in the pig model and was independent of blood electrolyte measurements. Thus, rapid sodium MRI can be used to dynamically quantify sodium biodistribution in the porcine and human kidney.

Numerical and Workbench Design of 2.35 T Double-Tuned (¹H/²³Na) Nested RF Birdcage Coils Suitable for Animal Size MRI

The birdcage Radio Frequency (RF) coil is one of the most used configurations in Magnetic Resonance Imaging (MRI) scanners for the detection of the proton (¹H) signal over a large homogeneous volume. More recently, birdcage RF coils have been successfully used also in the field of X-nuclei MRI, where the signal of a second nucleus (e.g. ¹³C, ²³Na, ³¹P, and many others) needs to be detected with high sensitivity and spatial homogeneity. To this purpose several technical solutions have been adopted to design Double Tuned (DT) volume RF coils, including the recent configuration of the nested birdcage RF coils. One of the main problems in the design of DT RF coils is the decoupling between the ¹H and X channels, and a number of solutions have been adopted over the years. In this work, based on numerical and workbench methods, we report the decoupling optimization of DT (¹H/²³Na) nested RF birdcage coils suitable for 2.35 T MRI scanners encompassing an inner Low-Pass (LP) birdcage used for X-nuclei, an outer High-Pass (HP) birdcage for ¹H and an external cylindrical RF shield. We show that a suitable geometrical selection of the two coaxial RF birdcage coils (relative angular orientation, diameters and lengths) and RF shield (diameter, length) allows a significant decoupling optimization. We also provide valuable information about the RF B₁⁺ field homogeneity and efficiency. Our approach was validated both with numerical simulations and workbench testing using DT nested RF coil prototypes.

Oxygenation of the Transplanted Kidney

Although kidney oxygen tensions are heterogenous, and mostly below renal vein level, the nephron is highly dependent on aerobic metabolism for active tubular transport. This renders the kidney particularly susceptible to hypoxia, which is considered a main characteristic and driver of acute and chronic kidney injury, albeit the evidence supporting this assumption is not entirely conclusive. Kidney transplants are exposed to several conditions that may interfere with the balance between oxygen supply and consumption, and enhance hypoxia and hypoxic injury. These include conditions leading to and resulting from brain death of kidney donors, ischemia and reperfusion during organ donation, storage and transplantation, postoperative vascular complications, vasoconstriction induced by immunosuppression, and impaired perfusion resulting from interstitial edema, inflammation, and fibrosis. Acute graft injury, the immediate consequence of hypoxia and reperfusion, results in delayed graft function and increased risk of chronic graft failure. Although current strategies to alleviate hypoxic/ischemic graft injury focus on limiting injury (eg, by reducing cold and warm ischemia times), experimental evidence suggests that preconditioning through local or remote ischemia, or activation of the hypoxia-inducible factor pathway, can decrease hypoxic injury. In combination with ex vivo machine perfusion such approaches hold significant promise for improving transplantation outcomes.

Detection of acute kidney injury with hyperpolarized [13 C, 15 N]Urea and multiexponential relaxation modeling

PURPOSE

To assess the utility of Laplacian fitting to describe the differences in hyperpolarized [¹³ C, ¹⁵ N]urea T₂ relaxation in ischemic and healthy rodent kidneys.

METHODS

Six rats with unilateral renal ischemia were investigated. [¹³ C, ¹⁵ N]Urea T₂ mapping was undertaken with a radial fast spin echo method, with subsequent postprocessing performed with regularized Laplacian fitting.

RESULTS

Simulations showed that Laplacian fitting was stable down to a signal-to-noise ratio of 20. In vivo results showed a significant increase in the mono- and decrease in biexponential pools in ischemia reperfusion injury kidneys, in comparison to healthy (14 ± 10% versus 4 ± 2%, 85 ± 10% versus 95 ± 3%; P < .05).

CONCLUSION

We demonstrate, for the first time, the differences in multiexponential behavior of [¹³ C, ¹⁵ N]urea between the healthy and ischemic rodent kidney. The distribution of relaxation pools were found to be both visually and numerically significantly different. The ability to improve the information level in hyperpolarized MR, by using the relaxation contrast mechanisms is an appealing option, that can easily be adopted in large animals and even in clinical studies in the near future.

Cardiorenal sodium MRI in small rodents using a quadrature birdcage volume resonator at 9.4 T

OBJECTIVE

Design, implementation, evaluation and application of a quadrature birdcage radiofrequency (RF) resonator tailored for renal and cardiac sodium (²³Na) magnetic resonance imaging (MRI) in rats at 9.4 T.

MATERIALS AND METHODS

A low pass birdcage resonator (16 rungs, d_in = 62 mm) was developed. The transmission field (B₁⁺) was examined with EMF simulations. The scattering parameter (S-parameter) and the quality factor (Q-factor) were measured. For experimental validation B₁⁺-field maps were acquired with the double-angle method. In vivo sodium imaging of the heart (spatial resolution: (1 × 1 × 5) mm³) and kidney (spatial resolution: (1 × 1 × 10) mm³) was performed with a FLASH technique.

RESULTS

The RF resonator exhibits RF characteristics, transmission field homogeneity and penetration that afford ²³Na MR in vivo imaging of the kidney and heart at 9.4 T. For the renal cortex and medulla a SNRs of 8 and 13 were obtained and a SNRs of 14 and 15 were observed for the left and right ventricle.

DISCUSSION

These initial results obtained in vivo in rats using the quadrature birdcage volume RF resonator for ²³Na MRI permit dedicated studies on experimental models of cardiac and renal diseases, which would contribute to translational research of the cardiorenal syndrome.

Noninvasive imaging of renal urea handling by CEST-MRI

PURPOSE

Renal function is characterized by concentration of urea for removal in urine. We tested urea as a CEST-MRI contrast agent for measurement of the concentrating capacity of distinct renal anatomical regions.

METHODS

The CEST contrast of urea was examined using phantoms with different concentrations and pH levels. Ten C57BL/6J mice were scanned twice at 7 T, once following intraperitoneal injection of 2M 150 µL urea and separately following an identical volume of saline. Kidneys were segmented into regions encompassing the cortex, outer medulla, and inner medulla and papilla to monitor spatially varying urea concentration. Z-spectra were acquired before and 20 minutes after injection, with dynamic scanning of urea handling performed in between via serial acquisition of CEST images acquired following saturation at +1 ppm.

RESULTS

Phantom experiments revealed concentration and pH-dependent CEST contrast of urea that was both acid- and base-catalyzed. Z-spectra acquired before injection showed significantly higher CEST contrast in the inner medulla and papilla (2.3% ± 1.9%) compared with the cortex (0.15% ± 0.75%, P = .011) and outer medulla (0.12% ± 0.58%, P = .008). Urea infusion increased CEST contrast in the inner medulla and papilla by 2.1% ± 1.9% (absolute), whereas saline infusion decreased CEST contrast by -0.5% ± 2.0% (absolute, P = .028 versus urea). Dynamic scanning revealed that thermal drift and diuretic status are confounding factors.

CONCLUSION

Urea CEST has a potential of monitoring renal function by capturing the spatially varying urea concentrating ability of the kidneys.

Why Have Detection, Understanding and Management of Kidney Hypoxic Injury Lagged Behind those for the Heart?

The outcome of patients with acute myocardial infarction (AMI) has dramatically improved over recent decades, thanks to early detection and prompt interventions to restore coronary blood flow. In contrast, the prognosis of patients with hypoxic acute kidney injury (AKI) remained unchanged over the years. Delayed diagnosis of AKI is a major reason for this discrepancy, reflecting the lack of symptoms and diagnostic tools indicating at real time altered renal microcirculation, oxygenation, functional derangement and tissue injury. New tools addressing these deficiencies, such as biomarkers of tissue damage are yet far less distinctive than myocardial biomarkers and advanced functional renal imaging technologies are non-available in the clinical practice. Moreover, our understanding of pathogenic mechanisms likely suffers from conceptual errors, generated by the extensive use of the wrong animal model, namely warm ischemia and reperfusion. This model parallels mechanistically type I AMI, which properly represents the rare conditions leading to renal infarcts, whereas common scenarios leading to hypoxic AKI parallel physiologically type II AMI, with tissue hypoxic damage generated by altered oxygen supply/demand equilibrium. Better understanding the pathogenesis of hypoxic AKI and its management requires a more extensive use of models of type II-rather than type I hypoxic AKI.

Unilateral nephrectomy diminishes ischemic acute kidney injury through enhanced perfusion and reduced pro-inflammatory and pro-fibrotic responses

While unilateral nephrectomy (UNx) is suggested to protect against ischemia-reperfusion injury (IRI) in the remaining kidney, the mechanisms underlying this protection remain to be elucidated. In this study, functional MRI was employed in a renal IRI rat model to reveal global and regional changes in renal filtration, perfusion, oxygenation and sodium handling, and microarray and pathway analyses were conducted to identify protective molecular mechanisms. Wistar rats were randomized to either UNx or sham UNx immediately prior to 37 minutes of unilateral renal artery clamping or sham operation under sevoflurane anesthesia. MRI was performed 24 hours after reperfusion. Blood and renal tissue were harvested. RNA was isolated for microarray analysis and QPCR validation of gene expression results. The perfusion (T1 value) was significantly enhanced in the medulla of the post-ischemic kidney following UNx. UNx decreased the expression of fibrogenic genes, i.a. Col1a1, Fn1 and Tgfb1 in the post-ischemic kidney. This was associated with a marked decrease in markers of activated myofibroblasts (Acta2/α-Sma and Cdh11) and macrophages (Ccr2). This was most likely facilitated by down-regulation of Pdgfra, thus inhibiting pericyte-myofibroblast differentiation, chemokine production (Ccl2/Mcp1) and macrophage infiltration. UNx reduced ischemic histopathologic injury. UNx may exert renoprotective effects against IRI through increased perfusion in the renal medulla and alleviation of the acute pro-inflammatory and pro-fibrotic responses possibly through decreased myofibroblast activation. The identified pathways involved may serve as potential therapeutic targets and should be taken into account in experimental models of IRI.

Multiparametric Renal Magnetic Resonance Imaging: Validation, Interventions, and Alterations in Chronic Kidney Disease

Background: This paper outlines a multiparametric renal MRI acquisition and analysis protocol to allow non-invasive assessment of hemodynamics (renal artery blood flow and perfusion), oxygenation (BOLD T₂^*), and microstructure (diffusion, T₁ mapping). Methods: We use our multiparametric renal MRI protocol to provide (1) a comprehensive set of MRI parameters [renal artery and vein blood flow, perfusion, T₁, T₂^*, diffusion (ADC, D, D^*, f_p), and total kidney volume] in a large cohort of healthy participants (127 participants with mean age of 41 ± 19 years) and show the MR field strength (1.5 T vs. 3 T) dependence of T₁ and T₂^* relaxation times; (2) the repeatability of multiparametric MRI measures in 11 healthy participants; (3) changes in MRI measures in response to hypercapnic and hyperoxic modulations in six healthy participants; and (4) pilot data showing the application of the multiparametric protocol in 11 patients with Chronic Kidney Disease (CKD). Results: Baseline measures were in-line with literature values, and as expected, T₁-values were longer at 3 T compared with 1.5 T, with increased T₁ corticomedullary differentiation at 3 T. Conversely, T₂^* was longer at 1.5 T. Inter-scan coefficients of variation (CoVs) of T₁ mapping and ADC were very good at <2.9%. Intra class correlations (ICCs) were high for cortex perfusion (0.801), cortex and medulla T₁ (0.848 and 0.997 using SE-EPI), and renal artery flow (0.844). In response to hypercapnia, a decrease in cortex T₂^* was observed, whilst no significant effect of hyperoxia on T₂^* was found. In CKD patients, renal artery and vein blood flow, and renal perfusion was lower than for healthy participants. Renal cortex and medulla T₁ was significantly higher in CKD patients compared to healthy participants, with corticomedullary T₁ differentiation reduced in CKD patients compared to healthy participants. No significant difference was found in renal T₂^*. Conclusions: Multiparametric MRI is a powerful technique for the assessment of changes in structure, hemodynamics, and oxygenation in a single scan session. This protocol provides the potential to assess the pathophysiological mechanisms in various etiologies of renal disease, and to assess the efficacy of drug treatments.

Innovative Perspective: Gadolinium-Free Magnetic Resonance Imaging in Long-Term Follow-Up after Kidney Transplantation

Since the mid-1980s magnetic resonance imaging (MRI) has been investigated as a non- or minimally invasive tool to probe kidney allograft function. Despite this long-standing interest, MRI still plays a subordinate role in daily practice of transplantation nephrology. With the introduction of new functional MRI techniques, administration of exogenous gadolinium-based contrast agents has often become unnecessary and true non-invasive assessment of allograft function has become possible. This raises the question why application of MRI in the follow-up of kidney transplantation remains restricted, despite promising results. Current literature on kidney allograft MRI is mainly focused on assessment of (sub) acute kidney injury after transplantation. The aim of this review is to survey whether MRI can provide valuable diagnostic information beyond 1 year after kidney transplantation from a mechanistic point of view. The driving force behind chronic allograft nephropathy is believed to be chronic hypoxia. Based on this, techniques that visualize kidney perfusion and oxygenation, scarring, and parenchymal inflammation deserve special interest. We propose that functional MRI mechanistically provides tools for diagnostic work-up in long-term follow-up of kidney allografts.

Hyperpolarized 13 C,15 N2 -Urea MRI for assessment of the urea gradient in the porcine kidney

PURPOSE

A decline in cortico-medullary osmolality gradient of the kidney may serve as an early indicator of pathological disruption of the tubular reabsorption process. The purpose of this study was to investigate the feasibility of hyperpolarized ¹³ C,¹⁵ N₂ -urea MRI as a biomarker of renal function in healthy porcine kidneys resembling the human physiology.

METHODS

Five healthy female Danish domestic pigs (weight 30 kg) were scanned at 3 Tesla (T) using a ¹³ C 3D balanced steady-state MR pulse sequence following injection of hyperpolarized ¹³ C,¹⁵ N₂ -urea via a femoral vein catheter. Images were acquired at different time points after urea injection, and following treatment with furosemide.

RESULTS

A gradient in cortico-medullary urea was observed with an intramedullary accumulation 75 s after injection of hyperpolarized ¹³ C,¹⁵ N₂ -urea, whereas images acquired at earlier time points postinjection were dominated by cortical perfusion. Furosemide treatment resulted in an increased urea accumulation in the cortical space, leading to a reduction of the medullary-to-cortical signal ratio of 49%.

CONCLUSION

This study demonstrates that hyperpolarized ¹³ C,¹⁵ N₂ -urea MRI is capable of identifying the intrarenal accumulation of urea and can differentiate acute renal functional states in multipapillary kidneys, highlighting the potential for human translation. Magn Reson Med 76:1895-1899, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

Quantitative sodium MRI of kidney

One of the main tasks of the human kidneys is to maintain the homeostasis of the body's fluid and electrolyte balance by filtration of the plasma and excretion of the end products. Herein, the regulation of extracellular sodium in the kidney is of particular importance. Sodium MRI ((23)Na MRI) allows for the absolute quantification of the tissue sodium concentration (TSC) and thereby provides a direct link between TSC and tissue viability. Renal (23)Na MRI can provide new insights into physiological tissue function and viability thought to differ from the information obtained by standard (1)H MRI. Sodium imaging has the potential to become an independent surrogate biomarker not only for renal imaging, but also for oncology indications. However, this technique is now on the threshold of clinical implementation. Numerous, initial pre-clinical and clinical studies have already outlined the potential of this technique; however, future studies need to be extended to larger patient groups to show the diagnostic outcome. In conclusion, (23)Na MRI is seen as a powerful technique with the option to establish a non-invasive renal biomarker for tissue viability, but is still a long way from real clinical implementation.

Quantitative sodium MR imaging of native versus transplanted kidneys using a dual-tuned proton/sodium (1H/ 23Na) coil: initial experience

OBJECTIVES

To compare sodium ((23)Na) characteristics between native and transplanted kidneys using dual-tuned proton ((1)H)/sodium MRI.

METHODS

Six healthy volunteers and six renal transplant patients (3 normal function, 3 acute allograft rejection) were included. Proton/sodium MRI was obtained at 3 T using a dual-tuned coil. Signal to noise ratio (SNR), sodium concentration ([(23)Na]) and cortico-medullary sodium gradient (CMSG) were measured. Reproducibility of [(23)Na] measurement was also tested. SNR, [(23)Na] and CMSG of the native and transplanted kidneys were compared.

RESULTS

Proton and sodium images of kidneys were successfully acquired. SNR and [(23)Na] measurements of the native kidneys were reproducible at two different sessions. [(23)Na] and CMSG of the transplanted kidneys was significantly lower than those of the native kidneys: 153.5 ± 11.9 vs. 192.9 ± 9.6 mM (P = 0.002) and 8.9 ± 1.5 vs. 10.5 ± 0.9 mM/mm (P = 0.041), respectively. [(23)Na] and CMSG of the transplanted kidneys with normal function vs. acute rejection were not statistically different.

CONCLUSIONS

Sodium quantification of kidneys was reliably performed using proton/sodium MRI. [(23)Na] and CMSG of the transplanted kidneys were lower than those of the native kidneys, but without a statistically significant difference between patients with or without renal allograft rejection.

KEY POINTS

Dual-tuned proton/sodium RF coil enables co-registered proton and sodium MRI. Structural and sodium biochemical property can be acquired by dual-tuned proton/sodium MRI. Sodium and sodium gradient of kidneys can be measured by dual-tuned MRI. Sodium concentration was lower in transplanted kidneys than in native kidneys. Sodium gradient of transplanted kidneys was lower than for native kidneys.

Assessment of the renal corticomedullary (23)Na gradient using isotropic data sets

RATIONALE AND OBJECTIVES

(23)Na magnetic resonance imaging is a promising technique for the noninvasive imaging of renal function. Past investigations of the renal corticomedullary [(23)Na] gradient have relied on imaging only in the coronal plane and on cumbersome calculations of [(23)Na], which require the use of external phantoms. The aim of this study is therefore two-fold: to use an isotropic three-dimensional data set to compare coronal measurements of renal [(23)Na] relative to measurements obtained in planes along the corticomedullary gradients and to investigate cerebrospinal fluid (CSF) (23)Na signal as an internal reference standard, obviating the need for time-intensive [(23)Na] calculations.

MATERIALS AND METHODS

Nominal isotropic three-dimensional (23)Na MRI data sets were obtained in 14 healthy volunteers before and after a water load. Images were reconstructed in the coronal plane and in planes angled along the direction of the corticomedullary sodium gradients. [(23)Na] values and values of the corticomedullary [(23)Na] gradient were measured by placement of a linear region of interest along corticomedullary gradients in both the coronal/nonangled [(23)Na(non-ang)] and the angled [(23)Na(ang)] image reconstructions. CSF [(23)Na] was also acquired at multiple levels. Ratios of renal (23)Na and CSF (23)Na signal were calculated to construct a semiquantitative parameter, [(23)NaCSF]. Results of water stimulation as measured by [(23)NaCSF] and [(23)Na(ang)] were then compared.

RESULTS

Mean values of [(23)Na(ang)] were statistically significantly greater than those of [(23)Na(non-ang)] (P < .0001), although these values were linearly correlated (R = 0.553, P < .0001) and exhibited similar extents of decreases in absolute terms (P = .2) and in terms of the corticomedullary gradient following the water load. CSF [(23)Na] did not statistically significantly differ at any level after the water load (P > .5) but tended to increase in the cranial direction (P < .001). [(23)NaCSF] measures demonstrated analogous statistical properties to [(23)Na(ang)] before and after the water load.

CONCLUSIONS

Assessment of renal corticomedullary [(23)Na] gradients using isotropic data sets with image reconstructions along the gradients is likely more accurate than measurements in the coronal plane. Because CSF [(23)Na] differs based on anatomic levels, such measures are useful as an internal reference only if region of interest placement is consistent. With this caveat in mind, normalization of renal to CSF (23)Na signal provides a feasible, less cumbersome alternative to [(23)Na] calculations in intraindividual studies.

Quantitative in vivo 23Na MR imaging of the healthy human kidney: determination of physiological ranges at 3.0T with comparison to DWI and BOLD

OBJECTIVES

The purpose of this prospective study was to assess the normal physiologic ranges of the renal corticomedullary 23Na-concentration ([23Na]) gradient at 3.0T in healthy volunteers. The corticomedullary [23Na] gradient was correlated with other functional MR imaging parameters--blood oxygenation level dependent (BOLD) and diffusion-weighted imaging (DWI)--and to individual and physiologic parameters--age, gender, estimated glomerular filtration rate (eGFR), body mass index (BMI), and blood serum sodium concentration ([23Na]serum).

METHODS AND MATERIALS

50 healthy volunteers (30 m, 20 w; mean age: 29.2 years) were included in this IRB-approved study, without a specific a priori preparation in regard to water or food intake. For 23Na-imaging a 3D density adapted, radial gradient echo (GRE)-sequence (spatial resolution=5×5×5 mm3) was used in combination with a dedicated 23Na-coil and 23Na-reference phantoms. [23Na] values of the corticomedullary [23Na] gradient were measured by placement of a linear region of interest (20×1 mm2) from the renal cortex in the direction of the renal medulla. By using external standard reference phantoms, [23Na] was calculated in mmol/L of wet tissue volume (mmol/l WTV). Axial diffusion-weighted images (spatial resolution=1.7×1.7×5.0 mm3) and 2D GRE BOLD images (spatial resolution=1.2×1.2×4.0 mm3) were acquired. Mean values±standard deviations for [23Na], apparent diffusion coefficient (ADC) values, and R2* values were computed for each volunteer. The corticomedullary 23Na-concentration gradient (in mmol/l/mm) was calculated along the area of linear concentration increase from the cortex in the direction of the medulla. Correlations between the [23Na] and DWI, BOLD, and the physiologic parameters were assessed with Pearson correlation coefficients.

RESULTS

The mean corticomedullary [23Na] for all healthy volunteers increased from the renal cortex (58±17 mmol/l WTV) in the direction of the medulla (99±18 mmol/l WTV). The inter-individual differences ranged from respective cortical and medullary values of 27 and 63 mmol/L WTV to 126 and 187 mmol/L WTV. No statistically significant differences in renal [23Na] were found based on differences in individual or physiologic parameters (age, gender, [23Na]serum, BMI, GFR). No ADC or R2* gradients were identified, and [23Na] did not correlate with these parameters.

CONCLUSION

Renal corticomedullary [23Na] values increase from the cortex in the direction of the medullary pyramid, demonstrating wide inter-individual ranges and no significant correlations with age, gender, [23Na]serum, BMI, GFR, ADC, or R2* values. For future clinical evaluations, an approach relying on renal stimulation (e.g. pharmacologically induced diuresis) may be applicable to account for wide inter-individual ranges of normal [23Na].

Assessment of renal function after conformal radiotherapy and intensity-modulated radiotherapy by functional 1H-MRI and 23Na-MRI

PURPOSE

Adjuvant radiochemotherapy (RCHT) improves survival of patients with locally advanced gastric cancer. Conventional three-dimensional conformal radiotherapy (3D-CRT) results in ablative doses to a significant amount of the left kidney, while image-guided intensity-modulated radiotherapy (IG-IMRT) provides excellent target coverage with improved kidney sparing. Few long-term results on IMRT for gastric cancer, however, have been published. Functional magnetic resonance imaging (fMRI) at 3.0 T including blood oxygenation-level dependent (BOLD) imaging, diffusion-weighted imaging (DWI) and, for the first time, (23)Na imaging was used to evaluate renal status after radiotherapy with 3D-CRT or IG-IMRT.

PATIENTS AND METHODS

Four disease-free patients (2 after 3D-CRT and 2 after IMRT; FU for all patients > 5 years) were included in this feasibility study. Morphological sequences, axial DWI images, 2D-gradient echo (GRE)-BOLD images, and (23)Na images were acquired. Mean values/standard deviations for ((23)Na), the apparent diffusion coefficient (ADC), and R2* values were calculated for the upper/middle/lower parts of both kidneys. Corticomedullary (23)Na-concentration gradients were determined.

RESULTS

Surprisingly, IG-IMRT patients showed no morphological alterations and no statistically significant differences of ADC and R2* values in all renal parts. Values for mean corticomedullary (23)Na-concentration matched those for healthy volunteers. Results were similar in 3D-CRT patients, except for the cranial part of the left kidney. This was atrophic and presented significantly reduced functional parameters (p = 0.001-p = 0.033). Reduced ADC values indicated reduced cell density and reduced extracellular space. Cortical and medullary R2* values of the left cranial kidney in the 3D-CRT group were higher, indicating more deoxygenated hemoglobin due to reduced blood flow/oxygenation. ((23)Na) of the renal cranial parts in the 3D-CRT group was significantly reduced, while the expected corticomedullary (23)Na-concentration gradient was partially conserved.

CONCLUSIONS

Functional MRI can assess postradiotherapeutic renal changes. As expected, marked morphological/functional effects were observed in high-dose areas (3D-CRT), while, unexpectedly, no alteration in kidney function was observed in IG-IMRT patients, supporting the hypothesis that reducing total/fractional dose to the renal parenchyma by IMRT is clinically beneficial.

Dual energy CT monitoring of the renal corticomedullary sodium gradient in swine

OBJECTIVE

To evaluate the feasibility of dual-energy CT (DECT) for monitoring dynamic changes in the renal corticomedullary sodium gradient in swine.

MATERIAL AND METHODS

This study was approved by our Institutional Animal Care and Use Committee. Four water-restricted pigs were CT-scanned at 80 and 140 kVp at baseline and at 5 min intervals for 30 min during saline or furosemide diuresis. The renal cortical and medullary CT numbers were recorded. A DECT basis material decomposition method was used to quantify renal cortical and medullary sodium concentrations and medulla-to-cortex sodium ratios at each time point based on the measured CT numbers. The sodium concentrations and medulla-to-cortex sodium ratios were compared between baseline and at 30 min diuresis using paired Student t-tests. The medulla-to-cortex sodium ratios were considered to reflect the corticomedullary sodium gradient.

RESULTS

At baseline prior to saline diuresis, the mean medullary and cortical sodium concentrations were 103.8±8.7 and 65.3±1.7 mmol/l, respectively, corresponding to a medulla-to-cortex sodium ratio of 1.59. At 30 min of saline diuresis, the medullary and cortical sodium concentrations decreased to 72.3±1.0 and 56.0±1.4 mmol/l, respectively, corresponding to a significantly reduced medulla-to-cortex sodium ratio of 1.29 (P<0.05). At baseline prior to furosemide diuresis, the mean medullary and cortical sodium concentrations were 110.5±3.6 and 66.7±4.1 mmol/l, respectively, corresponding to a medulla-to-cortex sodium ratio of 1.66. At 30 min of furosemide diuresis, the medullary and cortical sodium concentrations decreased to 68.5±0.3 and 58.9±4.0 mmol/l, respectively, corresponding to a significantly reduced medulla-to-cortex sodium ratio of 1.16 (P<0.05). One of the 4 pigs developed acute tubular necrosis likely related to prolonged hypoxia during intubation prior to the furosemide diuresis experiment. The medulla-to-cortex sodium ratio for this pig, which was excluded from the mean medulla-to-cortex ratio above, was 1.07 at baseline and 1.15 at 30 min following the administration of furosemide.

CONCLUSION

DECT monitoring of dynamic changes in the renal corticomedullary sodium gradient after physiologic challenges is feasible in swine.

Early monitoring of acute tubular necrosis in the rat kidney by 23Na-MRI

Reabsorption of water and other molecules is dependent on the corticomedullary sodium concentration gradient in the kidney. During the early course of acute tubular necrosis (ATN), this gradient is altered. Therefore, 23Na magnetic resonance imaging (MRI) was used to study the alterations in renal sodium distribution in the rat kidney during ischemia and reperfusion (IR) injury, which induces ATN. In-magnet ischemia was induced for 0 (control), 10, 20, 30 or 50 min in Wistar rats. 23Na images were collected every 10 min during baseline, ischemia, and 60-min reperfusion periods. T1 and T2 relaxation times were measured by both 23Na-MRI and -MRS on a separate cohort of animals during ischemia and reperfusion for correction of relaxation-related tissue sodium concentration (TSC). A marked decrease was observed in the medulla and cortex 23Na-MRI signal intensity (SI) during the early evolution of ATN caused by IR injury, with the sodium reabsorption function of the kidney being irreversibly damaged after 50 min of ischemia. Sodium relaxation time characteristics were similar in the medulla and cortex of normal kidney, but significantly decreased with IR. The changes in relaxation times in both compartments were identical; thus the medulla-to-cortex sodium SI ratio represents the TSC ratio of both compartments. The extent of IR damage observed with histological examination correlated with the 23Na-MRI data. 23Na-MRI has great potential for noninvasive, clinical diagnosis of evolving ATN in the setup of acute renal failure and in differentiating ATN from other causes of renal failure where tubular function is maintained.

Sodium MRI of a human transplanted kidney

RATIONALE AND OBJECTIVES

Sodium magnetic resonance imaging (MRI) of the kidneys has been used to spatially map areas of sodium-concentrating activity and to quantify the corticomedullary sodium gradient in various physiologic and pathophysiologic conditions. In this case study, sodium MRI of a clinically well-functioning transplanted kidney was performed to determine whether its sodium gradient could be detected and quantified using this method.

MATERIALS AND METHODS

Sodium MRI was performed on a 3T scanner with a commercial rectangular sodium surface coil placed on the lower abdomen over the palpable transplanted kidney. A three-dimensional gradient echo sequence, modified for multinuclear imaging, was applied to acquire (23)Na images.

RESULTS

Five main renal pyramids within the medulla were detected, and the corticomedullary sodium gradient was quantified in each renal pyramid by both region of interest-based and pixel-by-pixel analyses, resulting in a mean medulla/cortex signal-to-noise ratio of 1.8 +/- 0.1 (n = 5) and a mean linear increase slope of 1.1 +/- 0.1 relative arbitrary units per mm (n = 5).

CONCLUSIONS

The feasibility and usability of (23)Na MRI of a human renal allograft was demonstrated. Further studies are required to determine the clinical significance of this technique in the follow-up of patients after renal transplantation.

Acute tubular necrosis is a syndrome of physiologic and pathologic dissociation

Acute tubular necrosis (ATN) is a syndrome of intrinsic renal failure secondary to ischemic or toxic insults. The histopathologic findings of ATN are inconstant. When present, they are limited to the tubulo-interstitium and often subtle despite profound dysfunction. Experimental models of ATN in healthy animals commonly use single insults that result in extensive injury, circumstances that do not parallel the human situation. Recently, there has been a shift to more clinically relevant models using an acute insult superimposed on predisposing factors. This review discusses the complex hemodynamic interrelationships of hypoxia, tubular injury, and altered glomerular filtration, suggesting new ways to understand the pathophysiology of ATN.

Perspectives on body MR imaging at ultrahigh field

As investigators consider approaching the challenge of MR imaging at field strengths above 3T, do they follow the same paradigm, and continue to work around the same problems they have encountered thus far at 3T, or do they explore other ways of answering the clinical questions more effectively and more comprehensively? The most immediate problems of imaging at ultrahigh field strength are not unfamiliar, as many of them are still pressing issues at 3T: radiofrequency coils, B1 homogeneity, specific absorption rate, safety, B0 field homogeneity, alterations in tissue contrast, and chemical shift. In this article, these issues are briefly reviewed in terms of how they may affect image quality at field strengths beyond 3T. The authors propose various approaches to overcoming the challenges, and discuss potential applications of ultrahigh field MR imaging as it applies to specific abdominal, pelvic, peripheral vascular, and breast imaging protocols.

Renal parenchymal hypoxia, hypoxia adaptation, and the pathogenesis of radiocontrast nephropathy

BACKGROUND AND OBJECTIVES

Renal parenchymal Po(2) declines after the administration of iodinated radiocontrast agents, reaching critically low levels of approximately 10 mmHg in medullary structures.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this review, the causes of renal parenchymal hypoxia and its potential role in the pathogenesis of contrast nephropathy are appraised.

RESULTS

Commonly associated predisposing factors are associated with a propensity to enhance renal hypoxia. Indeed, animal models of radiocontrast nephropathy require the induction of such predisposing factors, mimicking clinical scenarios that lead to contrast nephropathy in high-risk individuals. In these models, in association with medullary hypoxic damage, a transient local cellular hypoxia response is noted, initiated at least in part by hypoxia-inducible factors. Some predisposing conditions that are distinguished by chronically aggravated medullary hypoxia, such as tubulointerstitial disease and diabetes, are characterized by a priori upregulation of hypoxia-inducible factors, which seems to confer tolerance against radiocontrast-related hypoxic tubular damage. Renal dysfunction under such circumstances likely reflects to some extent altered intrarenal hemodynamics, rather than acute tubular injury.

CONCLUSIONS

Real-time, noninvasive novel methods may help to differentiate between evolving tubular damage and altered hemodynamics and in the design of appropriate preventive interventions.

Sodium renal imaging in mice at high magnetic fields

This work presents the first sodium MRI functional renal study on a mouse model. The tissue sodium concentration was monitored during induced diuresis with furosemide. By using density-weighted chemical shift imaging (DWCSI) at high field strength a temporal resolution of less than 5 min for three dimensional (3D) data sets with high spatial resolution was achieved. A maximum increase of 20% in the cortex and a decrease of 45% of the original signal strength in the medulla were observed. These findings correspond well with experiments conducted on much larger rodent models.

Sodium MRI of the human kidney at 3 Tesla

The sodium concentration gradient in the kidney (from the cortex to the medulla) serves to regulate fluid homeostasis and is tightly coupled to renal function. It was previously shown that renal function and pathophysiology can be characterized in rat kidneys by measuring the sodium gradient with (23)Na MRI. This study demonstrates for the first time the ability of (23)Na MRI to map the distribution of sodium in the human kidney and to quantify the corticomedullary sodium gradient. The study was performed on a 3T Signa LX scanner (GE) using an in-house-built quadrature surface coil. (23)Na images of volunteers were acquired using a 3D coronal gradient-echo sequence at a spatial resolution of 0.3 x 0.3 x 1.5 cm(3) in a 25-min scan time. The signal intensity (relative to the noise) increased linearly from the cortex to each of the medullae with a mean slope of 1.6 +/- 0.2 in relative arbitrary units per mm (Rel.u./mm, N = 6) and then decreased, as expected, toward the renal pelvis. Water deprivation (12 hr) induced a significant increase of 25% (P < 0.05) in this gradient. Based on these results, we suggest that sodium MRI can serve as a valuable noninvasive method for functional imaging of the human kidney.

RENAL PARENCHYMAL OXYGENATION AND HYPOXIA ADAPTATION IN ACUTE KIDNEY INJURY

The pathogenesis of acute kidney injury (AKI), formally termed acute tubular necrosis, is complex and, phenotypically, may range from functional dysregulation without overt morphological features to literal tubular destruction. Hypoxia results from imbalanced oxygen supply and consumption. Increasing evidence supports the view that regional renal hypoxia occurs in AKI irrespective of the underlying condition, even under circumstances basically believed to reflect 'direct' tubulotoxicity. However, at present, it is remains unclear whether hypoxia per se or, rather, re-oxygenation (possibly through reactive oxygen species) causes AKI. Data regarding renal hypoxia in the clinical situation of AKI are lacking and our current concepts regarding renal oxygenation during acute renal failure are presumptive and largely derived from experimental studies. There is robust experimental evidence that AKI is often associated with altered intrarenal microcirculation and oxygenation. Furthermore, renal parenchymal oxygen deprivation seems to participate in the pathogenesis of experimental AKI, induced by exogenous nephrotoxins (such as contrast media, non-steroidal anti-inflammatory drugs or amphotericin), sepsis, pigment and obstructive nephropathies. Sub-lethal cellular hypoxia engenders adaptational responses through hypoxia-inducible factors (HIF). Forthcoming technologies to modulate the HIF system form a novel potential therapeutic approach for AKI.