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.

Sodium (23Na) MRI of the Kidney: Experimental Protocol.

Sodium handling is a key physiological hallmark of renal function. Alterations are generally considered a pathophysiologic event associated with kidney injury, with disturbances in the corticomedullary sodium gradient being indicative of a number of conditions. This experimental protocol review describes the individual steps needed to perform 23Na MRI; allowing accurate monitoring of the renal sodium distribution in a step-by-step experimental protocol for rodents.This chapter is based upon work from the PARENCHIMA COST Action, 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 experimental protocol chapter is complemented by two separate chapters describing the basic concept and data analysis.

Analysis Protocol for Renal Sodium (23Na) MR Imaging.

The signal acquired in sodium (23Na) MR imaging is proportional to the concentration of sodium in a voxel, and it is possible to convert between the two using external calibration phantoms. Postprocessing, and subsequent analysis, of sodium renal images is a simple task that can be performed with readily available software. Here we describe the process of conversion between sodium signal and concentration, estimation of the corticomedullary sodium gradient and the procedure used for quadrupolar relaxation analysis.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 analysis protocol chapter is complemented by two separate chapters describing the basic concept and experimental procedure.

Imaging Salt Uptake Dynamics in Plants Using PET.

Soil salinity is a global environmental challenge for crop production. Understanding the uptake and transport properties of salt in plants is crucial to evaluate their potential for growth in high salinity soils and as a basis for engineering varieties with increased salt tolerance. Positron emission tomography (PET), traditionally used in medical and animal imaging applications for assessing and quantifying the dynamic bio-distribution of molecular species, has the potential to provide useful measurements of salt transport dynamics in an intact plant. Here we report on the feasibility of studying the dynamic transport of 22Na in millet using PET. Twenty-four green foxtail (Setaria viridis L. Beauv.) plants, 12 of each of two different accessions, were incubated in a growth solution containing 22Na+ ions and imaged at 5 time points over a 2-week period using a high-resolution small animal PET scanner. The reconstructed PET images showed clear evidence of sodium transport throughout the whole plant over time. Quantitative region-of-interest analysis of the PET data confirmed a strong correlation between total 22Na activity in the plants and time. Our results showed consistent salt transport dynamics within plants of the same variety and important differences between the accessions. These differences were corroborated by independent measurement of Na+ content and expression of the NHX transcript, a gene implicated in sodium transport. Our results demonstrate that PET can be used to quantitatively evaluate the transport of sodium in plants over time and, potentially, to discern differing salt-tolerance properties between plant varieties. In this paper, we also address the practical radiation safety aspects of working with 22Na in the context of plant imaging and describe a robust pipeline for handling and incubating plants. We conclude that PET is a promising and practical candidate technology to complement more traditional salt analysis methods and provide insights into systems-level salt transport mechanisms in intact plants.

Tissue sodium content in patients with type 2 diabetes mellitus.

BACKGROUND: Tissue sodium content by 23Na magnetic resonance imaging (MRI) has been found to be increased in arterial hypertension. We analyzed whether tissue sodium content is increased in patients with type-2 diabetes (T2DM). METHODS: Patients with T2DM were compared to those with primary hypertension. Patients with T2DM were off any antidiabetic and hypertensive patients off any antihypertensive therapy for at least 4 weeks. Skin and muscle sodium content was assessed non-invasively with a 3.0 T clinical MRI system (Magnetom Verio, Siemens Health Care, Erlangen, Germany) in each patient. RESULTS: In patients with T2DM (N = 59) we observed significantly greater muscle sodium content (diabetes: 20.6 ±â€¯3.5 vs hypertension: 16.3 ±â€¯2.5 mmol/l, p < 0.001) and skin sodium content (diabetes: 24.5 ±â€¯7.2 vs hypertension: 20.6 ±â€¯5.7 mmol/l, p = 0.01) than in those with primary hypertension (N = 33). When potential confounders (age, body mass index, gender, systolic and diastolic blood pressure, estimated glomerular filtration rate) were entered in a covariance analysis, both skin sodium content (p = 0.037) and muscle sodium content (p < 0.001) were still clearly elevated. CONCLUSION: Patients with T2DM have greater skin and muscle sodium content. These are the first known data to demonstrate increased tissue sodium content in patients with T2DM, measured by 23Na magnetic resonance imaging. Since tissue sodium content is related to organ damage, therapeutic intervention should aim at reducing tissue sodium content.

Probing the microscopic environment of 23 Na ions in brain tissue by MRI: On the accuracy of different sampling schemes for the determination of rapid, biexponential T2* decay at low signal-to-noise ratio.

PURPOSE: To investigate and to reduce influences on the determination of the short and long apparent transverse relaxation times ( T2,s*, T2,l*) of 23 Na in vivo with respect to signal sampling. METHODS: The accuracy of T2* determination was analyzed in simulations for five different sampling schemes. The influence of noise in the parameter fit was investigated for three different models. A dedicated sampling scheme was developed for brain parenchyma by numerically optimizing the parameter estimation. This scheme was compared in vivo to linear sampling at 7T. RESULTS: For the considered sampling schemes, T2,s* / T2,l* exhibit an average bias of 3% / 4% with a variation of 25% / 15% based on simulations with previously published T2* values. The accuracy could be improved with the optimized sampling scheme by strongly averaging the earliest sample. A fitting model with constant noise floor can increase accuracy while additional fitting of a noise term is only beneficial in case of sampling until late echo time > 80 ms. T2* values in white matter were determined to be T2,s* = 5.1 ± 0.8 / 4.2 ± 0.4 ms and T2,l* = 35.7 ± 2.4 / 34.4 ± 1.5 ms using linear/optimized sampling. CONCLUSION: Voxel-wise T2* determination of 23 Na is feasible in vivo. However, sampling and fitting methods have to be chosen carefully to retrieve accurate results. Magn Reson Med 80:571-584, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Partial volume correction for in vivo (23)Na-MRI data of the human brain.

The concentration of sodium is a functional cell parameter and absolute quantification can be interesting for diagnostical purposes. The accuracy of sodium magnetic resonance imaging ((23)Na-MRI) is strongly biased by partial volume effects (PVEs). Hence our purpose was to establish a partial volume correction (PVC) method for (23)Na-MRI. The existing geometric transfer matrix (GTM) correction method was transferred from positron emission tomography (PET) to (23)Na-MRI and tested in a phantom study. Different parameters, as well as accuracy of registration and segmentation were evaluated prior to first in vivo measurements. In vivo sodium data-sets of the human brain were obtained at B0=7T with a nominal spatial resolution of (3mm)(3) using a density adapted radial pulse sequence. A volunteer study with four healthy subjects was performed to measure partial volume (PV) corrected tissue sodium concentration (TSC) which was verified by means of an intrinsic correction control. In the phantom study the PVC algorithm yielded a good correction performance and reduced the discrepancy between the measured sodium concentration value and the expected value in the smallest compartments of the phantom by 11% to a mean PVE induced discrepancy of 5.7% after correction. The corrected in vivo data showed a reduction of PVE bias for the investigated compartments for all volunteers, resulting in a mean reduction of discrepancy between two separate CSF compartments from 36% to 7.6%. The absolute TSC for two separate CSF compartments (sulci, lateral ventricles), gray and white brain matter after correction were 129±8mmol/L, 138±4mmol/L, 48±1mmol/L and 43±3mmol/L, respectively. The applied PVC algorithm reduces the PV-bias in quantitative (23)Na-MRI. Accurate, high-resolution anatomical data is required to enable appropriate PVC. The algorithm and segmentation approach is robust and leads to reproducible results.

Unshielded asymmetric transmit-only and endorectal receive-only radiofrequency coil for (23) Na MRI of the prostate at 3 tesla.

BACKGROUND: To develop and optimize radiofrequency (RF) hardware for the detection of endogenous sodium ((23) Na) by 3.0 Tesla (T) MRI in the human prostate. METHODS: A transmit-only receive-only (TORO) RF system of resonators consisting of an unshielded, asymmetric, quadrature birdcage (transmit), and an endorectal (ER), linear, surface (receive) coil were developed and tested on a 3T MRI scanner. Two different ER receivers were constructed; a single-tuned ((23) Na) and a dual-tuned ((1) H/(23) Na). Both receivers were evaluated by the measurements of signal-to-noise ratio (SNR) and B1 homogeneity. For tissue sodium concentration (TSC) quantification, vials containing known sodium concentrations were incorporated into the ER. The system was used to measure the prostate TSC of three men (age 55 ± 5 years) with biopsy-proven prostate cancer. RESULTS: B1 field inhomogeneity of the asymmetric transmitter was estimated to be less than 5%. The mean SNR measured in a region of interest within the prostate using the single-tuned ER coil was 54.0 ± 4.6. The mean TSC in the central gland was 60.2 ± 5.7 mmol/L and in the peripheral gland was 70.5 ± 9.0 mmol/L. CONCLUSION: A TORO system was developed and optimized for (23) Na MRI of the human prostate which showed good sensitivity throughout the prostate for quantitative measurement of TSC.

Sodium 3D COncentration MApping (COMA 3D) using (23)Na and proton MRI.

Functional changes of sodium 3D MRI signals were converted into millimolar concentration changes using an open-source fully automated MATLAB toolbox. These concentration changes are visualized via 3D sodium concentration maps, and they are overlaid over conventional 3D proton images to provide high-resolution co-registration for easy correlation of functional changes to anatomical regions. Nearly 5000/h concentration maps were generated on a personal computer (ca. 2012) using 21.1T 3D sodium MRI brain images of live rats with spatial resolution of 0.8×0.8×0.8 mm(3) and imaging matrices of 60×60×60. The produced concentration maps allowed for non-invasive quantitative measurement of in vivo sodium concentration in the normal rat brain as a functional response to migraine-like conditions. The presented work can also be applied to sodium-associated changes in migraine, cancer, and other metabolic abnormalities that can be sensed by molecular imaging. The MATLAB toolbox allows for automated image analysis of the 3D images acquired on the Bruker platform and can be extended to other imaging platforms. The resulting images are presented in a form of series of 2D slices in all three dimensions in native MATLAB and PDF formats. The following is provided: (a) MATLAB source code for image processing, (b) the detailed processing procedures, (c) description of the code and all sub-routines, (d) example data sets of initial and processed data. The toolbox can be downloaded at: http://www.vuiis.vanderbilt.edu/~truongm/COMA3D/.

Optic nerve: separating compartments based on 23Na TQF spectra and TQF-diffusion anisotropy.

We present a triple quantum filtered (TQF) sodium spectroscopy study of an excised bovine optic nerve. By choosing proper experimental parameters, this technique allowed us to independently observe the satellite transitions originating from the various compartments in the tissue. TQF-based diffusion experiments provided further characterization of the compartments in terms of their geometry. As a result, the peak that exhibited the smallest residual quadrupolar splitting, and the largest diffusion anisotropy was assigned to axons. Two other pairs of satellite peaks were assigned to extra-cellular compartments on the basis of either the size of their quadrupolar splitting or the diffusion properties.

23Na magnetic resonance imaging-determined tissue sodium in healthy subjects and hypertensive patients.

High dietary salt intake is associated with hypertension; the prevalence of salt-sensitive hypertension increases with age. We hypothesized that tissue Na(+) might accumulate in hypertensive patients and that aging might be accompanied by Na(+) deposition in tissue. We implemented (23)Na magnetic resonance imaging to measure Na(+) content of soft tissues in vivo earlier, but had not studied essential hypertension. We report on a cohort of 56 healthy control men and women, and 57 men and women with essential hypertension. The ages ranged from 22 to 90 years. (23)Na magnetic resonance imaging measurements were made at the level of the calf. We observed age-dependent increases in Na(+) content in muscle in men, whereas muscle Na(+) content did not change with age in women. We estimated water content with conventional MRI and found no age-related increases in muscle water in men, despite remarkable Na(+) accumulation, indicating water-free Na(+) storage in muscle. With increasing age, there was Na(+) deposition in the skin in both women and men; however, skin Na(+) content remained lower in women. Similarly, this sex difference was found in skin water content, which was lower in women than in men. In contrast to muscle, increasing Na(+) content was paralleled with increasing skin water content. When controlled for age, we found that patients with refractory hypertension had increased tissue Na(+) content, compared with normotensive controls. These observations suggest that (23)Na magnetic resonance imaging could have utility in assessing the role of tissue Na(+) storage for cardiovascular morbidity and mortality in longitudinal studies.

Imaging of tumor viability in lung cancer: initial results using 23Na-MRI.

PURPOSE: 23Na-MRI has been proposed as a potential imaging biomarker for the assessment of tumor viability and the evaluation of therapy response but has not yet been evaluated in patients with lung cancer. We aimed to assess the feasibility of 23Na-MRI in patients with lung cancer. MATERIALS AND METHODS: Three patients with stage IV adenocarcinoma of the lung were examined on a clinical 3 Tesla MRI system (Magnetom TimTrio, Siemens Healthcare, Erlangen, Germany). Feasibility of 23Na-MRI images was proven by comparison and fusion of 23Na-MRI with 1H-MR, CT and FDG-PET-CT images. 23Na signal intensities (SI) of tumor and cerebrospinal fluid (CSF) of the spinal canal were measured and the SI ratio in tumor and CSF was calculated. One chemonaive patient was examined before and after the initiation of combination therapy (Carboplatin, Gemcitabin, Cetuximab). RESULTS: All 23Na-MRI examinations were successfully completed and were of diagnostic quality. Fusion of 23Na-MRI images with 1H-MRI, CT and FDG-PET-CT was feasible in all patients and showed differences in solid and necrotic tumor areas. The mean tumor SI and the tumor/CSF SI ratio were 13.3 ± 1.8 × 103 and 0.83 ± 0.14, respectively. In necrotic tumors, as suggested by central non-FDG-avid areas, the mean tumor SI and the tumor/CSF ratio were 19.4 × 103 and 1.10, respectively. CONCLUSION: 23Na-MRI is feasible in patients with lung cancer and could provide valuable functional molecular information regarding tumor viability, and potentially treatment response.

Discrimination of intra- and extracellular 23Na+ signals in yeast cell suspensions using longitudinal magnetic resonance relaxography.

This study tested the ability of MR relaxography (MRR) to discriminate intra- (Nai+) and extracellular (Nae+)23Na+ signals using their longitudinal relaxation time constant (T1) values. Na+-loaded yeast cell (Saccharomyces cerevisiae) suspensions were investigated. Two types of compartmental 23Na+T1 differences were examined: a selective Nae+T1 decrease induced by an extracellular relaxation reagent (RRe), GdDOTP5-; and, an intrinsic T1 difference. Parallel studies using the established method of 23Na MRS with an extracellular shift reagent (SRe), TmDOTP5-, were used to validate the MRR measurements. With 12.8 mM RRe, the 23Nae+T1 was 2.4 ms and the 23Nai+T1 was 9.5 ms (9.4 T, 24 degrees C). The Na+ amounts and spontaneous efflux rate constants were found to be identical within experimental error whether measured by MRR/RRe or by MRS/SRe. Without RRe, the Na+-loaded yeast cell suspension 23Na MR signal exhibited two T1 values, 9.1 (+/-0.3) ms and 32.7 (+/-2.3) ms, assigned to 23Nai+ and 23Nae+, respectively. The Nai+ content measured was lower, 0.88 (+/-0.06); while Nae+ was higher, 1.43 (+/-0.12) compared with MRS/SRe measures on the same samples. However, the measured efflux rate constant was identical. T1 MRR potentially may be used for Nai+ determination in vivo and Na+ flux measurements; with RRe for animal studies and without RRe for humans.

Subcellular imaging of RNA distribution and DNA replication in single mammalian cells with SIMS: the localization of heat shock induced RNA in relation to the distribution of intranuclear bound calcium.

The subcellular localization of RNA for understanding transcriptional activity by using RNA precursors, like 5-bromouridine (BrU), generally requires chemical fixation and staining of cells with monoclonal antibody for imaging BrU-containing RNA in individual cells. Although effective for RNA localization, the native chemical composition of diffusible ions and molecules is destroyed in this approach and one cannot study their spatial relationship with RNA localization sites in this sample type. This work presents a novel secondary ion mass spectrometry approach in cryogenically prepared cells, which allows the same cell imaging of RNA (and/or replicating DNA) distribution in relation to intracellular chemical composition. The heat shock treatment of HeLa cells was used as a model system because the transcription of heat shock genes is activated during heat shock while other transcriptional activities of the cell are suppressed. The HeLa cells were heat-shocked for 1 h at 42 degrees C in presence of 100 muM BrU and/or 100 microM IdU (5-iododeoxyuridine). Following the heat shock treatments, the cells were cryogenically prepared with our sandwich freeze-fracture method and freeze-dried prior to secondary ion mass spectrometry analysis. A CAMECA IMS 3f secondary ion mass spectrometry ion microscope (CAMECA, Paris, France) capable of producing elemental (isotopic) distributions with a spatial resolution of 500 nm was used in the study. Secondary ion mass spectrometry analysis of fractured freeze-dried HeLa cells revealed well-preserved intracellular (39)K and (23)Na concentrations in heat-shocked cells. Both DNA replication and RNA distribution (total RNA) were imaged directly in the same cell by secondary ion mass spectrometry imaging of masses (127)I (from IdU) and (81)Br (from BrU), respectively. Surprisingly, the nucleus of heat-shocked cells contained spatially resolved regions with elevated levels of bound calcium (approximately 0.75 mM total calcium instead of 0.50 mM total calcium in the nucleoplasm). These regions spatially correlated with depleted levels of BrU-RNA in (81)Br secondary ion mass spectrometry images. The remainder of intranuclear regions displayed the presence of BrU-RNA with heterogeneous distribution. These observations indicate that calcium in its bound form may play a fundamental role in processes such as transcription and/or processing and storage of RNA. The shape of intranuclear regions with elevated levels of bound calcium resembled the heat shock induced nuclear bodies in HeLa cells. The analysis of cryogenically prepared frozen freeze-dried cells provides an ideal sample type for further understanding of the role of bound calcium in transcription of genes under physiological and pathological conditions.

The application of 23Na double-quantum-filter (DQF) NMR spectroscopy for the study of spinal disc degeneration.

Degenerative disc disease is an irreversible process that leads to a loss of mechanical integrity and back pain in millions of people. In this report, (23)Na double-quantum-filtered (DQF) NMR spectroscopy is used to study disc tissues in two stages of degeneration. Initial results indicate that the (23)Na DQF signal may be useful for determining the degree of degeneration. The spectral analysis reveals the presence of sodium environments with different residual quadrupolar couplings and T(2) relaxation times that we attribute to different regions, or compartments, corresponding to different biochemical regions in the tissue. In general it is found that there are compartments with no residual quadrupolar couplings, compartments with moderate couplings (200 to 1000 Hz), and compartments with couplings ranging from 1500 to 3000 Hz. The results indicate that (23)Na DQF NMR spectroscopy provides a probe of the degenerative state of the intervertebral disc tissues, and might hold potential as a novel diagnostic method for detection of disc degeneration.

Detection limit of measurement of pharmaceuticals labeled with short-lived isotopes in HPLC with flow-through gamma-counter.

This paper proposes a method for estimating the detection limit, which is defined as 3.3 times the standard deviation (S.D.) of blank measurements under the situations where the repetition of measurement is difficult or impossible because of a short half-life of radioactivity. The FUMI theory, which can estimate an S.D. value without repetition in various instrumental analyses, is adopted and proved here to be available in a radio-HPLC system as well. (99m)Tc-ECD (T(1/2)=360.6 min) that is a lipophilic compound for the diagnosis of regional brain perfusion is taken as an example.

Periodic ab initio calculation of nuclear quadrupole parameters as an assignment tool in solid-state NMR spectroscopy: applications to 23Na NMR spectra of crystalline materials.

Periodic ab initio HF calculations using the CRYSTAL code have been used to calculate (23)Na NMR quadrupole parameters for a wide range of crystalline sodium compounds including Na(3)OCl. An approach is developed that can be used routinely as an alternative to point-charge modelling schemes for the assignment of distinct lines in (23)Na NMR spectra to specific crystallographic sodium sites. The calculations are based on standard 3-21 G and 6-21 G molecular basis sets and in each case the same modified basis set for sodium is used for all compounds. The general approach is extendable to other quadrupolar nuclei. For the 3-21 G calculations a 1:1 linear correlation between experimental and calculated values of C(Q)((23)Na) is obtained. The 6-21 G calculations, including the addition of d-polarisation functions, give better accuracy in the calculation of eta((23)Na). The sensitivity of eta((23)Na) to hydrogen atom location is shown to be useful in testing the reported hydrogen-bonded structure of Na(2)HPO(4).

23Na MRI accurately measures fixed charge density in articular cartilage.

One of the initiating steps of osteoarthritis is the loss of proteoglycan (PG) molecules from the cartilage matrix. One method for assessing cartilage integrity, therefore, is to measure the PG content or fixed charge density (FCD) of cartilage. This report shows the feasibility of calculating FCD by (23)Na MRI and introduces MRI protocols for human studies, in vivo. (23)Na MRI was used to measure the sodium concentration inside bovine patellar cartilage. The sodium concentration was then converted to FCD (mM) by considering ideal Donnan equilibrium. These FCD measurements were compared to FCD measurements obtained through standard dimethylmethylene blue PG assays. There was a high correlation (slope = 0.89, r(2) = 0.81) between the FCD measurements obtained by (23)Na MRI and those obtained by the PG assays. These methods were then employed in quantifying the FCD of articular cartilage of human volunteers in vivo. Two imaging protocols were compared: one using a birdcage coil, the other using a transmit/receive surface coil. Both methodologies gave similar results, with the average sodium concentration of normal human patellar cartilage ranging from approximately 240 to 260 mM. This corresponds to FCDs of -158 mM to -182 mM.