Sodium renal imaging in mice at high magnetic fields

Kidney medullary sodium chloride concentrations induce neutrophil and monocyte extracellular DNA traps that defend against pyelonephritis in vivo

Urinary tract infections are common. Here, we delineate a role of extracellular DNA trap (ET) formation in kidney antibacterial defense and determine mechanisms of their formation in the hyperosmotic environment of the kidney medulla. ET of granulocytic and monocytic origin were present in the kidneys of patients with pyelonephritis along with systemically elevated citrullinated histone levels. Inhibition of the transcription coregulatory, peptidylarginine deaminase 4 (PAD4), required for ET formation, prevented kidney ET formation and promoted pyelonephritis in mice. ETs predominantly accumulated in the kidney medulla. The role of medullary sodium chloride and urea concentrations in ET formation was then investigated. Medullary-range sodium chloride, but not urea, dose-, time- and PAD4-dependently induced ET formation even in the absence of other stimuli. Moderately elevated sodium chloride promoted myeloid cell apoptosis. Sodium gluconate also promoted cell death, proposing a role for sodium ions in this process. Sodium chloride induced myeloid cell calcium influx. Calcium ion-free media or -chelation reduced sodium chloride-induced apoptosis and ET formation while bacterial lipopolysaccharide amplified it. Autologous serum improved bacterial killing in the presence of sodium chloride-induced ET. Depletion of the kidney sodium chloride gradient by loop diuretic therapy diminished kidney medullary ET formation and increased pyelonephritis severity. Thus, our data demonstrate that ETs may protect the kidney against ascending uropathogenic E. coli and delineate kidney medullary range sodium chloride concentrations as novel inducers of programmed myeloid cell death.

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.

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.

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.