Functional sodium magnetic resonance imaging of the intact rat kidney

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.

Clinically feasible B1 field correction for multi-organ sodium imaging at 3 T

Sodium MRI allows the non-invasive quantification of intra-organ sodium concentration. RF inhomogeneity introduces uncertainty in this estimated concentration. B₁ field corrections can be used to overcome some of these limitations. However, the low signal-to-noise ratio in sodium MRI makes accurate B₁ mapping in reasonable scan times challenging. The study aims to evaluate Bloch-Siegert off-resonance (BLOSI) B₁ field correction for sodium MRI using a 3D Fermat looped, orthogonally encoded trajectories (FLORET) read-out trajectory. We propose a clinically feasible B₁ field map correction method for sodium imaging at 3 T, evaluating five healthy subjects' brain, heart blood, kidneys, and thigh muscle. We scanned the subjects twice for repeatability measures and used sodium phantoms to determine organ total sodium concentration. Conventional proton scans were compared with sodium images for organ structural integrity. The BLOSI approach based on the 3D FLORET read-out trajectory was used in B₁ field correction and 3D density-adapted radial acquisition for sodium imaging. Results indicate improvements in sodium imaging based on B₁ field correction in a clinically feasible protocol. Improvements are determined in all organs by enhanced anatomical representation, organ homogeneity, and an increase in the total sodium concentration after applying a B₁ field correction. The proposed BLOSI-based B₁ field correction using a 3D FLORET read-out trajectory is clinically feasible for sodium imaging, which is shown in the brain, heart, kidney, and thigh muscle. This supports using fast B₁ field mapping in the clinical setting.

Functional Sodium MRI Helps to Measure Corticomedullary Sodium Content in Normal and Diseased Human Kidneys

Background To the knowledge of the authors, urinary osmolarity is the only tool currently available to assess kidney corticomedullary gradient (CMG). Comparisons between CMG and urinary osmolarity and the use of modalities such as sodium MRI to evaluate renal disease in humans are lacking. Purpose To investigate the ability of sodium MRI to measure CMG dynamics compared with urinary osmolarity after water load in healthy volunteers and CMG in participants with kidney disease. Materials and Methods A prospective study was conducted from July 2020 to January 2021 in fasting healthy volunteers undergoing water load and participants with chronic kidney disease (CKD) from cardiorenal syndrome included in a clinical trial. In both groups, CMG was estimated by measuring the medulla-to-cortex signal ratio from sodium MRI at 3.0 T. A custom-built two-loop (diameter, 18 cm) butterfly radiofrequency surface coil, tuned for sodium frequency (33.786 MHz), was used to acquire renal sodium images. Two independent observers measured all sodium MRI cortical and medullary values for each region of interest to compute the intraclass correlation coefficient. Pearson correlation was performed between urinary osmolarity and CMG. Results Five participants with CKD (mean age, 77 years ± 12 [standard deviation]; all men) and 10 healthy volunteers (mean age, 42 years ± 15; six men, four women) were evaluated. A reduction was observed between baseline and peak urinary dilution time for both mean medulla-to-cortex ratios (1.55 ± 0.11 to 1.31 ± 0.09, respectively; P < .001) and mean urinary osmolarity (756 mOsm/L ± 157 to 73 mOsm/L ± 14, respectively; P < .001) in healthy volunteers. Medulla-to-cortex and corresponding urinary osmolarity were correlated in both groups (r² = 0.22; P < .001). Kidney sodium tissue content was successfully acquired in all five participants with CKD. The intraclass correlation coefficient measurement was 0.99 (P < .001). Conclusion Functional sodium MRI accurately depicted corticomedullary gradient (CMG) dynamic changes in healthy volunteers and demonstrated feasibility of CMG measurement in participants with reduced kidney function. Clinical trial registration no. NCT04170855. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Laustsen and Bøgh in this issue.

Hardware Considerations for Preclinical Magnetic Resonance of the Kidney

Magnetic resonance imaging (MRI) is a noninvasive imaging technology that offers unparalleled anatomical and functional detail, along with diagnostic sensitivity. MRI is suitable for longitudinal studies due to the lack of exposure to ionizing radiation. Before undertaking preclinical MRI investigations of the kidney, the appropriate MRI hardware should be carefully chosen to balance the competing demands of image quality, spatial resolution, and imaging speed, tailored to the specific scientific objectives of the investigation. Here we describe the equipment needed to perform renal MRI in rodents, with the aim to guide the appropriate hardware selection to meet the needs of renal MRI applications.This publication 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 chapter on hardware considerations for renal MRI in small animals is complemented by two separate publications 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.

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.

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.

Metabolic Response in Rabbit Urine to Occurrence and Relief of Unilateral Ureteral Obstruction

Ureteral obstruction will lead clinically to hydronephrosis, which may further develop into partial or complete loss of kidney function and even cause permanent histological damage. However, there is little knowledge of metabolic responses during the obstructed process and its recoverability. In this study, a complete unilateral ureteral obstruction (CUUO) model was established in the rabbit, and ¹H NMR-based metabolomic analysis of urine was used to reveal the metabolic perturbations in rabbits caused by CUUO and the metabolic recovery after the CUUO was relieved. Univariate and multivariate statistical analyses were used to identify metabolic characteristics. The gradually decreased levels of 3-hydroxykynurenine, 3-methylhistidine, creatinine, guanidoacetate, meta- and para-hydroxyphenylacetate, and phenylacetylglycine and the gradually increased levels of acetate, alanine, citrate, glycine, lactate, and methionine in urine could be regarded as potential biomarkers for the occurrence and severity of ureteral obstruction. And the reduced levels of 3-methylhistidine, creatinine, guanidoacetate, hippurate, meta-hydroxyphenylacetate, and methylguanidine and the elevated levels of 2-aminoisobutyrate, acetylcholine, citrate, lactate, lysine, valine, and α-ketoglutarate in urine compared with the obstructed level could characterize the metabolic recovery of ureteral obstruction. Our results depicted the disturbed biochemical pathways involved in ureteral obstruction and demonstrated the practicability of recovering renal functions for the patients with severe hydronephrosis in clinical practice by removing causes for obstruction.

Quantitative MRI of kidneys in renal disease

PURPOSE

To evaluate the reproducibility and utility of quantitative magnetic resonance imaging (MRI) sequences for the assessment of kidneys in young adults with normal renal function (eGFR ranged from 90 to 130 mL/min/1.73 m²) and patients with early renal disease (autosomal dominant polycystic kidney disease).

MATERIALS AND METHODS

This prospective case-control study was performed on ten normal young adults (18-30 years old) and ten age- and sex-matched patients with early renal parenchymal disease (autosomal dominant polycystic kidney disease). All subjects underwent a comprehensive kidney MRI protocol, including qualitative imaging: T1w, T2w, FIESTA, and quantitative imaging: 2D cine phase contrast of the renal arteries, and parenchymal diffusion weighted imaging (DWI), magnetization transfer imaging (MTI), blood oxygen level dependent (BOLD) imaging, and magnetic resonance elastography (MRE). The normal controls were imaged on two separate occasions ≥24 h apart (range 24-210 h) to assess reproducibility of the measurements.

RESULTS

Quantitative MR imaging sequences were found to be reproducible. The mean ± SD absolute percent difference between quantitative parameters measured ≥24 h apart were: MTI-derived ratio = 4.5 ± 3.6%, DWI-derived apparent diffusion coefficient (ADC) = 6.5 ± 3.4%, BOLD-derived R2* = 7.4 ± 5.9%, and MRE-derived tissue stiffness = 7.6 ± 3.3%. Compared with controls, the ADPKD patient's non-cystic renal parenchyma (NCRP) had statistically significant differences with regard to quantitative parenchymal measures: lower MTI percent ratios (16.3 ± 4.4 vs. 23.8 ± 1.2, p < 0.05), higher ADCs (2.46 ± 0.20 vs. 2.18 ± 0.10 × 10^-3 mm²/s, p < 0.05), lower R2*s (14.9 ± 1.7 vs. 18.1 ± 1.6 s^-1, p < 0.05), and lower tissue stiffness (3.2 ± 0.3 vs. 3.8 ± 0.5 kPa, p < 0.05).

CONCLUSION

Excellent reproducibility of the quantitative measurements was obtained in all cases. Significantly different quantitative MR parenchymal measurement parameters between ADPKD patients and normal controls were obtained by MT, DWI, BOLD, and MRE indicating the potential for detecting and following renal disease at an earlier stage than the conventional qualitative imaging techniques.

Image texture features predict renal function decline in patients with autosomal dominant polycystic kidney disease

Magnetic resonance imaging (MRI) examinations provide high-resolution information about the anatomic structure of the kidneys and are used to measure total kidney volume (TKV) in patients with Autosomal Dominant Polycystic Kidney Disease (ADPKD). Height-adjusted TKV (HtTKV) has become the gold-standard imaging biomarker for ADPKD progression at early stages of the disease when estimated glomerular filtration rate (eGFR) is still normal. However, HtTKV does not take advantage of the wealth of information provided by MRI. Here we tested whether image texture features provide additional insights into the ADPKD kidney that may be used as complementary information to existing biomarkers. A retrospective cohort of 122 patients from the Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP) study was identified who had T2-weighted MRIs and eGFR values over 70 mL/min/1.73m² at the time of their baseline scan. We computed nine distinct image texture features for each patient. The ability of each feature to predict subsequent progression to CKD stage 3A, 3B, and 30% reduction in eGFR at eight-year follow-up was assessed. A multiple linear regression model was developed incorporating age, baseline eGFR, HtTKV, and three image texture features identified by stability feature selection (Entropy, Correlation, and Energy). Including texture in a multiple linear regression model (predicting percent change in eGFR) improved Pearson correlation coefficient from -0.51 (using age, eGFR, and HtTKV) to -0.70 (adding texture). Thus, texture analysis offers an approach to refine ADPKD prognosis and should be further explored for its utility in individualized clinical decision making and outcome prediction.

Imaging Renal Urea Handling in Rats at Millimeter Resolution using Hyperpolarized Magnetic Resonance Relaxometry

In vivo spin spin relaxation time (T₂) heterogeneity of hyperpolarized [¹³C,¹⁵N₂]urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized ¹³C signal with a macromolecular relaxation agent revealed that a long-T₂ component of the [¹³C,¹⁵N₂]urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the [¹³C,¹⁵N₂]urea to be distinguished via multi-exponential analysis. The T₂ response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized [¹³C,¹⁵N₂]urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-¹³C-cyclopropane-²H₈. Large T₂ increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis. Therefore, [¹³C,¹⁵N₂]urea relaxometry is sensitive to two steps of the renal urea handling process: glomerular filtration and the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Simple motion correction and subspace denoising algorithms are presented to aid in the multi exponential data analysis. Furthermore, a T₂-edited, ultra long echo time sequence was developed for sub-2 mm³ resolution 3D encoding of urea by exploiting relaxation differences in the vascular and filtrate pools.

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.

Utilizing magnetization transfer imaging to investigate tissue remodeling in a murine model of autosomal dominant polycystic kidney disease

Purpose

Noninvasive imaging techniques that quantify renal tissue composition are needed to more accurately ascertain prognosis and monitor disease progression in polycystic kidney disease (PKD). Given the success of magnetization transfer (MT) imaging to characterize various tissue remodeling pathologies, it was tested on a murine model of autosomal dominant PKD.

Methods

C57Bl/6 Pkd1 R3277C mice at 9, 12, and 15 months were imaged with a 16.4T MR imaging system. Images were acquired without and with RF saturation in order to calculate MT ratio (MTR) maps. Following imaging, the mice were euthanized and kidney sections were analyzed for cystic and fibrotic indices, which were compared with statistical parameters of the MTR maps.

Results

The MTR‐derived mean, median, 25th percentile, skewness, and kurtosis were all closely related to indices of renal pathology, including kidney weight/body weight, cystic index, and percent of remaining parenchyma. The correlation between MTR and histology‐derived cystic and fibrotic changes was R² = 0.84 and R² = 0.70, respectively.

Conclusion

MT imaging provides a new, noninvasive means of measuring tissue remodeling PKD changes and may be better suited for characterizing renal impairment compared with conventional MR techniques. Magn Reson Med 75:1466–1473, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.

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.

Imaging of intrarenal haemodynamics and oxygen metabolism

The interruption of blood flow results in impaired oxygenation and metabolism. This can lead to electrophysiological changes, functional impairment and symptoms in quick succession. Quantitative measures of organ perfusion, perfusion reserve and tissue oxygenation are crucial to assess normal tissue metabolism and function. Magnetic resonance imaging (MRI) provides a number of quantitative methods to assess physiology in the kidney. Blood oxygenation level-dependent (BOLD) MRI provides a method for the assessment of oxygenation. Blood flow to the kidney can be assessed using phase contrast MRI. Dynamic contrast-enhanced MRI and arterial spin labelling (ASL) provide methods to assess tissue perfusion, ASL using the magnetization of endogenous water protons and thus providing a non-invasive method to assess perfusion. The application of diffusion-weighted MRI allows molecular motion in the kidney to be measured. Novel techniques can also be used to assess oxygenation in the renal arteries and veins and, combined with flow measures, provide an estimation of oxygen metabolism. Magnetic resonance imaging provides a synergy of non-invasive techniques to study renal function and the demand for these techniques is likely to be driven by the incentive to avoid the use of contrast media, to avoid radiation and to avoid complications with intervention procedures.

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].

Comparing anisotropic diffusion filters for the enhancement of sodium magnetic resonance images

The anisotropic diffusion (AND) filter, an image processing technique derived from physics, was applied to low-resolution sodium magnetic resonance imaging (MRI) to examine the possibilities of image enhancement by postprocessing. We compared six different variants of AND filters. Besides the qualitative good results on phantom measurements, quantitative analyses on MRI of human kidney yielded major improvements in noise reduction and other quality measures: the noise (i.e., the standard deviation in the image background) could be reduced to 1%-2% of its original value, while linear filters (Gaussian, Fermi, Hamming) achieved a reduction to 42%-64%. Besides that, less than 5% of structures and intensities are lost when using AND filters. Comparing the different variants, the two-dimensional and the three-dimensional AND filter outperformed the histogram-of-gradient and tensor-based AND filter. We envision that by using these AND filters, quantitative analysis of sodium MRI of kidney could be improved.

The design of a double-tuned two-port surface resonator and its application to in vivo hydrogen- and sodium-MRI

The design and construction of a two-port surface transceiver resonator for both (1)H-and (23)Na-MRI in the rodent brain at 7 T is described. Double-tuned resonators are required for accurately co-registering multi-nuclei data sets, especially when the time courses of (1)H and (23)Na signals are of interest as, for instance, when investigating the pathological progression of ischaemic stroke tissue in vivo. In the current study, a single-element two-port surface resonator was developed wherein both frequency components were measured with the same detector element but with each frequency signal routed along different output channels. This was achieved by using the null spot technique, allowing for optimal variable tuning and matching of each channel in situ within the MRI scanner. The (23)Na signal to noise ratio, measured in the ventricles of the rat brain, was increased by a factor of four compared to recent state-of-the-art rat brain studies reported in the literature. The resonator's performance was demonstrated in an in vivo rodent stroke model, where regional variations in (1)H apparent diffusion coefficient maps and the (23)Na signal were recorded in an interleaved fashion as a function of time in the acute phase of the stroke without having to exchange, re-adjust, or re-connect resonators between scans. Using the practical construction steps described in this paper, this coil design can be easily adapted for MRI of other X-nuclei, such as (17)O, (13)C, (39)K, and (43)Ca at various field strengths.

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.

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.

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.

Functional MRI of the kidney: tools for translational studies of pathophysiology of renal disease

Magnetic resonance imaging (MRI) provides exquisite anatomic detail of various organs and is capable of providing additional functional information. This combination allows for comprehensive diagnostic evaluation of pathologies such as ischemic renal disease. Noninvasive MRI techniques could facilitate translation of many studies performed in controlled animal models using technologies that are invasive to humans. Such a translation is being recognized as essential because many proposed interventions and drugs that prove efficacious in animal models fail to do so in humans. In this article, we review the state-of-the-art functional MRI technique as applied to the kidneys.

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

The clinical detection of evolving acute tubular necrosis (ATN) and differentiating it from other causes of renal failure are currently limited. The maintenance of the corticomedullary sodium gradient, an indicator of normal kidney function, is presumably lost early in the course of ATN. Herein, sodium magnetic resonance imaging (23Na MRI) was applied to study the early alteration in renal sodium distribution in rat kidneys 6 h after the induction of ATN. Three-dimensional gradient echo sodium images were recorded at 4.7 T with high spatial resolution. ATN was produced by the administration of radiologic contrast medium, combined with inhibition of nitric oxide and prostaglandin synthesis. The sodium images revealed that the sham-controlled kidney exhibited a linear increase in sodium concentration along the corticomedullary axis of 30+/-2 mmol/l/mm, resulting in an inner medulla to cortex sodium ratio of 4.3+/-0.3 (n=5). In the ATN kidney, however, the cortico-outer medullary sodium gradient was reduced by 21% (P<0.01, n=7) and the inner medulla to cortex sodium ratio was decreased by 40% (P<0.001, n=7). Small, though significant, increments in plasma creatinine at this time inversely correlated with the decline in the corticomedullary sodium gradient. Histological findings demonstrated outer medullary ATN involving 4% of medullary thick ascending limbs. Hence, 23Na MRI non-invasively quantified changes in the corticomedullary sodium gradient in the ATN kidney when morphologic tubular injury was still focal and very limited. MRI detection of corticomedullary sodium gradient abnormalities may serve to identify evolving ATN at its early phases.

Sodium magnetic resonance imaging of diuresis: spatial and kinetic response

Renal function is highly correlated with the sodium concentration gradient along the corticomedullary axis. The application of 3D high-resolution sodium magnetic resonance imaging (MRI) provided a means to quantify in vivo the spatial and temporal changes in renal tissue sodium concentration under normal and diuretic conditions. A detailed, pixel-by-pixel analysis of the intact rat kidney sodium MR images yielded a quantitative measure of the corticomedullary sodium gradient before and at early and later times after the administration of two distinct diuretic agents, furosemide and mannitol. Furosemide, a loop diuretic, induced a fivefold reduction in the cortical-outer medullary sodium gradient, whereas mannitol, an osmotic diuretic, did not affect this gradient. Both diuretics induced a 50% decrease in the sodium concentration of the inner medulla; however, mannitol exerted its effect twice as fast as furosemide with a 2.5-min exponential decay constant. These specific changes were attributed to the different mechanism of action and site of activity of each diuretic agent. Thus, high-resolution (23)Na MRI offers a unique, noninvasive tool for functional imaging of the kidney physiology.