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Introduction: The purpose of this study was to demonstrate the feasibility and efficiency of cardiac MR at 3 Tesla using local four-channel RF coil transmission and benchmark it against large volume body RF coil excitation., Methods: Electromagnetic field simulations are conducted to detail RF power deposition, transmission field uniformity and efficiency for local and body RF coil transmission. For both excitation regimes transmission field maps are acquired in a human torso phantom. For each transmission regime flip angle distributions and blood-myocardium contrast are examined in a volunteer study of 12 subjects. The feasibility of the local transceiver RF coil array for cardiac chamber quantification at 3 Tesla is demonstrated., Results: Our simulations and experiments demonstrate that cardiac MR at 3 Tesla using four-channel surface RF coil transmission is competitive versus current clinical CMR practice of large volume body RF coil transmission. The efficiency advantage of the 4TX/4RX setup facilitates shorter repetition times governed by local SAR limits versus body RF coil transmission at whole-body SAR limit. No statistically significant difference was found for cardiac chamber quantification derived with body RF coil versus four-channel surface RF coil transmission. Our simulation also show that the body RF coil exceeds local SAR limits by a factor of ~2 when driven at maximum applicable input power to reach the whole-body SAR limit., Conclusion: Pursuing local surface RF coil arrays for transmission in cardiac MR is a conceptually appealing alternative to body RF coil transmission, especially for patients with implants., Competing Interests: Andre Kuehne and Jan Rieger are employees of MRI.TOOLS GmbH, Berlin, Germany. Thoralf Niendorf is founder and CEO of MRI.TOOLS GmbH, Berlin. Germany. The authors confirm that this commercial affiliation does not alter their adherence to PLOS ONE policies on sharing data and materials. Matthias Dieringer is currently employed by Siemens Healthcare, Berlin, Germany His contributions to this work, however, happened when he was employed at Charite', University Medicine, Berlin, Germany. Therefore, Siemens did not provide funding or play a role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors confirm that this commercial affiliation does not alter their adherence to PLOS ONE policies on sharing data and materials. Antonino Cassara is currently employed by the IT'IS Foundation, Zurich, Switzerland. His contributions to this work, however, happened when he was employed at PTB, Berlin, Germany. Therefore, IT'IS did not provide funding or play a role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors confirm that this commercial affiliation does not alter their adherence to PLOS ONE policies on sharing data and materials.

Aim: To evaluate potential scan time reduction in (23)Na-Magnetic Resonance Imaging with the chemical shift imaging sequence (CSI) using undersampled data of high-quality datasets, reconstructed with an iterative constrained reconstruction, compared to reduced resolution or reduced signal-to-noise ratio., Materials and Methods: CSI (23)Na-images were retrospectively undersampled and reconstructed with a constrained reconstruction scheme. The results were compared to conventional methods of scan time reduction. The constrained reconstruction scheme used a phase constraint and a finite object support, which was extracted from a spatially registered (1)H-image acquired with a double-tuned coil. The methods were evaluated using numerical simulations, phantom images and in-vivo images of a healthy volunteer and a patient who suffered from cerebral ischemic stroke., Results: The constrained reconstruction scheme showed improved image quality compared to a decreased number of averages, images with decreased resolution or circular undersampling with weighted averaging for any undersampling factor. Brain images of a stroke patient, which were reconstructed from three-fold undersampled k-space data, resulted in only minor differences from the original image (normalized root means square error < 12%) and an almost identical delineation of the stroke region (mismatch < 6%)., Conclusion: The acquisition of undersampled (23)Na-CSI images enables up to three-fold scan time reduction with improved image quality compared to conventional methods of scan time saving., (Copyright © 2014. Published by Elsevier GmbH.)

A novel setup for (23)Na MRI, which allows bilateral imaging of the breast, is presented. For this purpose a figure-eight receive-only (23)Na surface coil was developed. For our experiments on three samples with NaCl solutions of different sodium concentrations and two female subjects we used an asymmetric birdcage coil in transmit mode and the developed surface coil for receiving the signal at 3T. Imaging of the samples showed the applicability of the employed normalization method for measuring the distribution of sodium concentration. In a sample of concentration [Na(+)]=51mM we achieved SNR=70 at a nominal isotropic resolution of 2,5mm (TR=66ms, TE=0,6ms, TA=20min). Furthermore we showed that by means of this setup it is possible to quantify the sodium concentration in breast tissue (TSC) of a female subject with an accuracy of 23% (TR=150ms, TE=0,5ms, TA=45min)., (Copyright © 2013. Published by Elsevier GmbH.)

Purpose: To develop a sodium-MRI ((23) Na-MRI) method for bilateral renal sodium concentration (RSC) measurements in rat kidneys at 9.4 Tesla (T)., Materials and Methods: To simultaneously achieve high B1 -field homogeneity and high receive sensitivity a dual resonator system composed of a double-tuned linearly polarized (1) H/(23) Na volume resonator and a newly developed two-element (23) Na receive array was used. In conjunction with three-dimensional (3D) ultra-short Time-to-Echo sequence a quantification accuracy of ± 10% was achieved for a nominal spatial resolution of (1 × 1 × 4) mm(3) in 10 min acquisition time. The technique was applied to study the RSC in six kidneys before and after furosemide-induced diuresis., Results: The loop diuretic agent induced an increase of cortical RSC by 22% from 86 ± 16 mM to 105 ± 18 mM (P = 0.02), whereas the RSC in the inner medulla decreased by 38% from 213 ± 24 mM to 132 ± 25 mM (P = 0.8×10(-4) ). The RSC changes measured in this study agreed well with the qualitative sodium signal intensity variations reported elsewhere., Conclusion: Furosemide-induced diuresis has been investigated accurately with herein presented quantitative (23) Na-MRI technique. In the future, RSC quantification could allow for defining pathological and nonpathological RSC ranges to assess sodium concentration changes, e.g., induced by drugs., (Copyright © 2013 Wiley Periodicals, Inc., a Wiley company.)

A double-tuned ²³Na/¹H resonator system was developed to record multinuclear MR image data during and after transient cerebral ischemia. ¹H-diffusion-, (¹H perfusion, ¹H T₂-, ¹H arterial blood flow- and ²³Na spin density-weighted images were then acquired at three time points in a rodent stroke model: (I) during 90 min artery occlusion, (II) directly after arterial reperfusion and (III) one day after arterial reperfusion. Normal ²³Na was detected in hypoperfused stroke tissue which exhibited a low ¹H apparent diffusion coefficient (ADC) and no changes in ¹H T₂ relaxation time during transient ischemia, while ²³Na increased and ADC values recovered to normal values directly after arterial reperfusion. For the first time, a similar imaging protocol was set-up on a clinical 3T MRI site in conjunction with a commercial double-tuned ¹H/²³Na birdcage resonator avoiding a time-consuming exchange of resonators or MRI systems. Multinuclear ²³Na/¹H MRI data sets were obtained from one stroke patient during both the acute and non-acute stroke phases with an aquisition time of 22 min. The lesion exhibiting low ADC was found to be larger compared to the lesion with high ²³Na at 9 h after symptom onset. It is hoped that the presented pilot data demonstrate that fast multinuclear ²³Na/¹H MRI preclinical and clinical protocols can enable a better understanding of how temporal and regional MRI parameter changes link to pathophysiological variations in ischemic stroke tissue.

Sodium magnetic resonance imaging (²³Na MRI) is a non-invasive technique which allows spatial resolution of the tissue sodium concentration (TSC) in the human body. TSC measurements could potentially serve to monitor early treatment success of chemotherapy on patients who suffer from whole body metastases. Yet, the acquisition of whole body sodium (²³Na) images has been hampered so far by the lack of large resonators and the extremely low signal-to-noise ratio (SNR) achieved with existing resonator systems. In this study, a ²³Na resonator was constructed for whole body ²³Na MRI at 3T comprising of a 16-leg, asymmetrical birdcage structure with 34 cm height, 47.5 cm width and 50 cm length. The resonator was driven in quadrature mode and could be used either as a transceiver resonator or, since active decoupling was included, as a transmit-only resonator in conjunction with a receive-only (RO) surface resonator. The relative B₁-field profile was simulated and measured on phantoms, and 3D whole body ²³Na MRI data of a healthy male volunteer were acquired in five segments with a nominal isotropic resolution of (6 × 6 × 6) mm³ and a 10 min acquisition time per scan. The measured SNR values in the ²³Na-MR images varied from 9 ± 2 in calf muscle, 15 ± 2 in brain tissue, 23 ± 2 in the prostate and up to 42 ± 5 in the vertebral discs. Arms, legs, knees and hands could also be resolved with applied resonator and short time-to-echo (TE) (0.5 ms) radial sequence. Up to fivefold SNR improvement was achieved through combining the birdcage with local RO surface coil. In conclusion, ²³Na MRI of the entire human body provides sub-cm spatial resolution, which allows resolution of all major human body parts with a scan time of less than 60 min.

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., (Copyright © 2012 Elsevier Inc. All rights reserved.)

A technique for noninvasively quantifying the concentration of sodium ((23) Na) ions was applied to the study of ischemic stroke. (23) Na-magnetic resonance imaging techniques have shown considerable potential for measuring subtle changes in ischemic tissue, although studies to date have suffered primarily from poor signal/noise ratio. In this study, accurate quantification of tissue sodium concentration (TSC) was achieved in (23) Na images with voxel sizes of 1.2 μL acquired in 10 min. The evolution of TSC was investigated from 0.5 to 8 h in focal cortical and subcortical ischemic tissue following permanent middle cerebral artery occlusion in the rat (n = 5). Infarct volumes determined from TSC measurements correlated significantly with histology (P = 0.0006). A delayed linear model was fitted to the TSC time course data in each voxel, which revealed that the TSC increase was more immediate (0.2 ± 0.1 h delay time) in subcortical ischemic tissue, whereas it was delayed by 1.6 ± 0.5 h in ischemic cortex (P = 0.0002). No significant differences (P = 0.5) were measured between TSC slope rates in cortical (10.2 ± 1.1 mM/h) and subcortical (9.7 ± 1.1 mM/h) ischemic tissue. The data suggest that any TSC increase measured in ischemic tissue indicates infarction (core) and regions exhibiting a delay to TSC increase indicate potentially salvageable tissue (penumbra)., (Copyright © 2011 Wiley Periodicals, Inc.)

Purpose: To estimate changes in the (23)Na density and in the (23)Na relaxation time T(2) * in the anatomically small murine brain after stroke., Materials and Methods: Three-dimensional acquisition weighted chemical shift imaging at a resolution of 0.6 × 0.6 × 1.2 mm(3) was used for sodium imaging and relaxation parameter mapping. In vivo measurements of the mouse brain (n = 4) were performed 24 hours after stroke, induced by microinjection of purified murine thrombin into the right middle cerebral artery. The measurement time was 14 minutes in one mouse and 65 minutes in the other three. An exponential fit estimation of the free induction decay was calculated for each voxel enabling the reconstruction of locally resolved relaxation parameter maps., Results: The infarcted areas showed an increase in sodium density between 160% and 250%, while the T(2) * relaxation time increased by 5%-72% compared to unaffected contralateral brain tissue., Conclusion: (23)Na chemical shift imaging at a resolution of 0.6 × 0.6 × 1.2 mm(3) enabled sodium imaging of the anatomical small mouse brain and the acquired data allowed calculating relaxation parameter maps and hence a more exact evaluation of sodium signal changes after stroke., (Copyright © 2011 Wiley-Liss, Inc.)

A method for quantifying the tissue sodium concentration (TSC) in the rat brain from ²³Na-MR images was developed. TSC is known to change in a variety of common human diseases and holds considerable potential to contribute to their study; however, its accurate measurement in small laboratory animals has been hindered by the extremely low signal to noise ratio (SNR) in ²³Na images. To address this, the design, construction and characterization of a double-tuned ¹H/²³Na dual resonator system for ¹H-guided quantitative ²³Na-MRI are described. This system comprises an SNR-optimized surface detector coil for ²³Na image acquisition, and a volume resonator producing a highly homogeneous B₁ field (<5% inhomogeneity) for the Na channel across the rat head. The resonators incorporated channel-independent balanced matching and tuning capabilities with active decoupling circuitry at the ²³Na resonance frequency. A quantification accuracy of TSC of <10 mM was achieved in Na-images with 1.2 µl voxel resolution acquired in 10 min. The potential of the quantification technique was demonstrated in an in vivo experiment of a rat model of cerebral stroke, where the evolution of the TSC was successfully monitored for 8 h after the stroke was induced.

Background: Diffusion magnetic resonance imaging (MRI) is the current-state-of-the-art technique to clinically investigate acute (0-24 h) ischemic stroke tissue. However, reduced apparent diffusion coefficient (ADC) —considered a marker of tissue damage—was observed to reverse spontaneously during the subacute stroke phase (24-72 h) which means that low ADC cannot be used to reflect the damaged tissue after 24 h in experimental and clinical studies. One reason for the change in ADC is that ADC values drop with cytotoxic edema (acute phase) and rise when vasogenic edema begins (subacute phase). Recently, combined 1H- and 23Na-MRI was proposed as a more accurate approach to improve delineation between reversible (penumbra) and irreversible ischemic injury (core). The aim of this study was to investigate the effects of reperfusion on the ADC and the sodium MRI signal after experimental ischemic stroke in rats in well-defined areas of different viability levels of the cerebral lesion, i.e. core and penumbra as defined via perfusion and histology. Transient middle cerebral artery occlusion was induced in male rats by using the intraluminal filament technique. MRI sodium, perfusion and diffusion measurement was recorded before reperfusion, shortly after reperfusion and 24 h after reperfusion. The animals were reperfused after 90 min of ischemia. Results: Sodium signal in core did not change before reperfusion, increased after reperfusion while sodium signal in penumbra was significantly reduced before reperfusion, but showed no changes after reperfusion compared to control. The ADC was significantly decreased in core tissue at all three time points compared to contralateral side. This decrease recovered above commonly applied viability thresholds in the core after 24 h. Conclusions: Reduced sodium-MRI signal in conjunction with reduced ADC can serve as a viability marker for penumbra detection and complement hydrogen diffusion- and perfusion-MRI in order to facilitate time-independent assessment of tissue fate and cellular bioenergetics failure in stroke patients. [ABSTRACT FROM AUTHOR]

A double-tuned 23Na/¹H resonator system was developed to record multinuclear MR image data during and after transient cerebral ischemia. ¹H-diffusion-, ¹H perfusion, ¹H T2-, ¹H arterial blood flow- and 23Na spin density-weighted images were then acquired at three time points in a rodent stroke model: (I) during 90 min artery occlusion, (II) directly after arterial reperfusion and (III) one day after arterial reperfusion. Normal 23Na was detected in hypoperfused stroke tissue which exhibited a low ¹H apparent diffusion coefficient (ADC) and no changes in ¹H T2 relaxation time during transient ischemia, while 23Na increased and ADC values recovered to normal values directly after arterial reperfusion. For the first time, a similar imaging protocol was set-up on a clinical 3T MRI site in conjunction with a commercial double-tuned ¹H/23Na birdcage resonator avoiding a time-consuming exchange of resonators or MRI systems. Multinuclear 23Na/¹H MRI data sets were obtained from one stroke patient during both the acute and non-acute stroke phases with an aquisition time of 22 min. The lesion exhibiting low ADC was found to be larger compared to the lesion with high 23Na at 9 h after symptom onset. It is hoped that the presented pilot data demonstrate that fast multinuclear 23Na/¹HMRI preclinical and clinical protocols can enable a better understanding of how temporal and regional MRI parameter changes link to pathophysiological variations in ischemic stroke tissue. [ABSTRACT FROM AUTHOR]