30 years of sodium/X-nuclei magnetic resonance imaging

In principle, all nuclei with nonzero spin can be employed for magnetic resonance imaging (MRI). Special scanner hardware and MR sequences are required to select the nucleus-specific frequency and to enable imaging with “sufficient” signal-to-noise ratio. This Special Issue starts with an overview of different nuclei that can be used for MRI today, followed by a review article about techniques required for imaging of quadrupolar nuclei with short relaxation times. Sequence developments to improve image quality and applications on different organs and diseases are presented for different nuclei (²³Na, ³⁵Cl, ¹⁷O, and ¹⁹F), with a focus on imaging at natural abundance.     

ECG-gated <Superscript>23</Superscript>Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na⁺–K⁺ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, ²³Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T₂ decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence ²³Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm³. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial ²³Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.     

Continuous-wave NMR imaging of solids

Current pulsed nuclear magnetic resonance methods of imaging samples such as solids with short spin-spin relaxation times are restricted to use with T₂ values longer than approximately 10 µs. In the present study a method of imaging ultra-short T² samples using continuous-wave, swept-field NMR is presented that, in principle, will be able to overcome this restriction. The technique is identical to that used in continuous-wave electron paramagnetic resonance imaging of paramagnetic species and involves irradiating the sample continuously with a radiofrequency excitation in the presence of a strong stationary magnetic field gradient. When the main magnetic field is swept over a suitable range, the variation of the NMR absorption signal with applied magnetic field yields a one-dimensional projection of the object under study along the gradient direction. Two- or three-dimensional image data sets may be reconstructed from projections that are obtained by applying the gradient in different directions. Signal-to-noise ratio can be improved by modulating the magnetic field and employing a lock-in amplifier to recover signal variations at the audio modulation frequency. Preliminary experiments were performed using a 7 Tesla magnet and a 300 MHz continuous-wave radiofrequency bridge with lock-in detection. The apparatus is described and the results of pilot experiments that employed vulcanized rubber samples are presented. The ability of the technique to detect short T² samples was demonstrated by the presence of a background signal from the Perspex former of the birdcage resonator used for signal reception.     

Signal losses in diffusion preparation: Comparison between spin-echo, stimulated echo and SEASON

Diffusion-weighted magnetic resonance imaging and spectroscopy commonly apply a spin-echo or stimulated echo preparation including sensitizing field gradients. The article reports on a systematic numerical approach to an optimum diffusion preparation considering undesired signal losses caused by relaxation. A large range of possible applications on whole-body units and animal scanners is covered. Instructions for an optimized type and timing of the diffusion preparation are provided for the readership, based on the desired diffusion weighting (b-value), the available maximum field gradient amplitudes, the RF pulse durations and gradient ramp times, and the relaxation characteristics of the specimen (or tissue) of interest. In addition, a new type of diffusion preparation named SEASON (simultaneous Spin-Echo And Stimulated echO preparatioN) is introduced and compared with spin-echo and stimulated echo diffusion preparation. It is demonstrated that spin-echo preparation is superior to stimulated echo preparation in all cases with T₂ ≈ T₁ and in all cases with relatively low diffusion weighting resulting in short duration of diffusion sensitizing gradients δ « T₂. For tissues with T₂ « T₁ (as musculature or red bone marrow) stimulated echo preparation becomes superior to spin-echo preparation for high ratios b/A² (b-value indicates diffusion weighting,A is the maximum gradient amplitude). The new SEASON technique allows a higher yield in signal intensity compared to spin-echo or stimulated echo preparations in clinically relevant cases. © 1998 Elsevier Science B.V. All rights reserved.     

Exploring and enhancing relaxation-based sodium MRI contrast

Object

Sodium MRI is typically concerned with measuring tissue sodium concentration. This requires the minimization of relaxation weighting. However, ²³Na relaxation may itself be interesting to explore, given an underlying mechanism (i.e. the electric-quadrupole-moment–electric-field-gradient interaction) that differs from ¹H. A new sodium sequence was developed to enhance ²³Na relaxation contrast without decreasing signal-to-noise ratio.

Materials and Methods

The new sequence, labeled Projection Acquisition in the steady-state with Coherent MAgNetization (PACMAN), uses gradient refocusing of transverse magnetization following readout, a short repetition time, and a long radiofrequency excitation pulse. It was developed using simulation, verified in model environments (saline and agar), and evaluated in the brain of three healthy adult volunteers.

Results

Projection Acquisition in the steady-state with Coherent MAgNetization generates a large positive contrast-to-noise ratio (CNR) between saline and agar, matching simulation-based design. In addition to enhanced CNR between cerebral spinal fluid and brain tissue in vivo, PACMAN develops substantial contrast between gray and white matter. Further simulation shows that PACMAN has a ln(T _2f/T ₁) contrast dependence (where T _2f is the fast component of ²³Na T ₂), as well as residual quadrupole interaction dependence.

Conclusion

The relaxation dependence of PACMAN sodium MRI may provide contrast related to macromolecular tissue structure.     

Quantification and localization of contrast agents using delta relaxation enhanced magnetic resonance at 1.5?T

Object

Delta relaxation enhanced magnetic resonance (dreMR) is a new imaging technique based on the idea of cycling the magnetic field B ₀ during an imaging sequence. The method determines the field dependency of the relaxation rate (relaxation dispersion dR ₁/dB). This quantity is of particular interest in contrast agent imaging because the parameter can be used to determine contrast agent concentrations and increases the ability to localize the contrast agent.

Materials and methods

In this paper dreMR imaging was implemented on a clinical 1.5?T MR scanner combining conventional MR imaging with fast field-cycling. Two improvements to dreMR theory are presented describing the quantification of contrast agent concentrations from dreMR data and a correction for field-cycling with finite ramp times.

Results

Experiments demonstrate the use of the extended theory and show the measurement of contrast agent concentrations with the dreMR method. A second experiment performs localization of a contrast agent with a significant improvement in comparison to conventional imaging.

Conclusion

dreMR imaging has been extended by a method to quantify contrast agent concentrations and improved for field-cycling with finite ramp times. Robust localization of contrast agents using dreMR imaging has been performed in a sample where conventional imaging delivers inconclusive results.     

In vivo chlorine and sodium MRI of rat brain at 21.1 T

Object

MR imaging of low-gamma nuclei at the ultrahigh magnetic field of 21.1 T provides a new opportunity for understanding a variety of biological processes. Among these, chlorine and sodium are attracting attention for their involvement in brain function and cancer development.

Materials and methods

MRI of ³⁵Cl and ²³Na were performed and relaxation times were measured in vivo in normal rat (n = 3) and in rat with glioma (n = 3) at 21.1 T. The concentrations of both nuclei were evaluated using the center-out back-projection method.

Results

T ₁ relaxation curve of chlorine in normal rat head was fitted by bi-exponential function (T _1a = 4.8 ms (0.7) T _1b = 24.4 ± 7 ms (0.3) and compared with sodium (T ₁ = 41.4 ms). Free induction decays (FID) of chlorine and sodium in vivo were bi-exponential with similar rapidly decaying components of $ T_{{2{\text{a}}}}^{*} = 0.4 $  ms and $ T_{{2{\text{a}}}}^{*} = 0.53 $  ms, respectively. Effects of small acquisition matrix and bi-exponential FIDs were assessed for quantification of chlorine (33.2 mM) and sodium (44.4 mM) in rat brain.

Conclusion

The study modeled a dramatic effect of the bi-exponential decay on MRI results. The revealed increased chlorine concentration in glioma (~1.5 times) relative to a normal brain correlates with the hypothesis asserting the importance of chlorine for tumor progression.     

Sodium ion distribution in the vitreous body

We have studied the nuclear magnetic resonance (NMR) relaxation behavior, and thus the dynamic properties, of the sodium ion in the vitreous body at different temperatures. The²³Na NMR spectrum exhibits a resonance, the intensity of which accounts for an ion visibility of 100%. The²³Na longitudinal and transverse relaxation times, at all temperatures but the highest, present two components, suggesting that the sodium ions are present in two states of different mobility, whose populations are in slow exchange on the NMR time scale. The correlation times and quadrupole coupling constants for the two sodium pools have been derived. The faster relaxation of a fraction of the vitreal sodium has tentatively been ascribed to the influence of the macromolecular framework of the vitreous body. The reported information may be of use for the understanding of the diagnostic applications of²³Na magnetic resonance imaging of the ocular structures.     

Fast three-dimensional sodium imaging of human brain

A three-dimensional sodium imaging technique with a minimum echo time of 0.9 ms is described in a 2.0 Tesla whole-body system. The relaxation behaviour in vivo of sodium was analysed: a lastT ₂ ^* relaxation component between 1.2 and 1.6 ms and a slowT ₂ ^* relaxation component between 7.1 ms and 8.4 ms were quantified in brain tissue of three volunteers. Three-dimensional sodium images of the human brain were acquired in 8.5 min with a resolution of 4.7 × 4.7 × 10 mm (0.2 cc voxel size) and a signal-to-noise ratio of 20 in brain tissue and 30 in cerebrospinal fluid.     

MRI of the lung using hyperpolarized <Superscript>3</Superscript>He at very low magnetic field (3 mT)

Optical pumping of ³He produces large (hyper) nuclear-spin polarizations independent of the magnetic resonance imaging (MRI) field strength. This allows lung MRI to be performed at reduced fields with many associated benefits, such as lower tissue susceptibility gradients and decreased power absorption rates. Here we present results of 2D imaging as well as accurate 1D gas diffusion mapping of the human lung using ³He at very low field (3 mT). Furthermore, measurements of transverse relaxation in zero applied gradient are shown to accurately track pulmonary O₂ partial pressure, opening the way for novel imaging sequences.     

3D magnetic resonance fingerprinting on a low-field 50 mT point-of-care system prototype: evaluation of muscle and lipid relaxation time mapping and comparison with standard techniques

Objective

To implement magnetic resonance fingerprinting (MRF) on a permanent magnet 50 mT low-field system deployable as a future point-of-care (POC) unit and explore the quality of the parameter maps.

Materials and methods

3D MRF was implemented on a custom-built Halbach array using a slab-selective spoiled steady-state free precession sequence with 3D Cartesian readout. Undersampled scans were acquired with different MRF flip angle patterns and reconstructed using matrix completion and matched to the simulated dictionary, taking excitation profile and coil ringing into account. MRF relaxation times were compared to that of inversion recovery (IR) and multi-echo spin echo (MESE) experiments in phantom and in vivo. Furthermore, B₀ inhomogeneities were encoded in the MRF sequence using an alternating TE pattern, and the estimated map was used to correct for image distortions in the MRF images using a model-based reconstruction.

Results

Phantom relaxation times measured with an optimized MRF sequence for low field were in better agreement with reference techniques than for a standard MRF sequence. In vivo muscle relaxation times measured with MRF were longer than those obtained with an IR sequence (T₁: 182 ± 21.5 vs 168 ± 9.89 ms) and with an MESE sequence (T₂: 69.8 ± 19.7 vs 46.1 ± 9.65 ms). In vivo lipid MRF relaxation times were also longer compared with IR (T₁: 165 ± 15.1 ms vs 127 ± 8.28 ms) and with MESE (T₂: 160 ± 15.0 ms vs 124 ± 4.27 ms). Integrated ΔB₀ estimation and correction resulted in parameter maps with reduced distortions.

Discussion

It is possible to measure volumetric relaxation times with MRF at 2.5 × 2.5 × 3.0 mm³ resolution in a 13 min scan time on a 50 mT permanent magnet system. The measured MRF relaxation times are longer compared to those measured with reference techniques, especially for T₂. This discrepancy can potentially be addressed by hardware, reconstruction and sequence design, but long-term reproducibility needs to be further improved.

     

Chlorine and sodium chemical shift imaging during acute stroke in a rat model at 9.4 Tesla

Object

A triple-resonant coil setup with an ¹H linear resonator and a double-tuned ²³Na/³⁵Cl surface coil was used to study the evolution of T ₂ ^* and M ₀ for ³⁵Cl and ²³Na in a rat stroke model during the acute phase at 9.4 Tesla.

Materials and methods

In vivo measurements were performed 1.5–7 h after onset of stroke (n = 2), ten days after onset (n = 1) and on a healthy control rat by a chemical shift imaging sequence. Measurement times were 15 min (²³Na) and 57 min (³⁵Cl).

Results

The relaxation times ten days after onset [T ₂ ^*  = 14.3 ± 1.8 ms (²³Na) and 6.0 ± 1.3 ms (³⁵Cl)] are clearly prolonged in comparison to a healthy rat [T ₂ ^*  = 4.8 ± 0.6 ms (²³Na) and 2.1 ± 0.3 ms (³⁵Cl)] and the acute phase [T ₂ ^*  = 5.6 ± 0.2 ms (²³Na) and 1.9 ± 0.1 ms (³⁵Cl)].

Conclusion

M ₀ in the infarcted region clearly rises later and slower for chlorine than for sodium. To the best of our knowledge, these are the first combined proton, sodium, and chlorine measurements in an animal stroke model during the acute phase.     

Possibilities and limitations for high resolution small animal MRI on a clinical whole-body 3T scanner

Object

To investigate the potential of a clinical 3 T scanner to perform MRI of small rodents.

Materials and methods

Different dedicated small animal coils and several imaging sequences were evaluated to optimize image quality with respect to SNR, contrast and spatial resolution. As an application, optimal grey-white-matter contrast and resolution were investigated for rats. Furthermore, manganese-enhanced MRI was applied in mice with unilateral crush injury of the optic nerve to investigate coil performance on topographic mapping of the visual projection.

Results

Differences in SNR and CNR up to factor 3 and more were observed between the investigated coils. The best grey-white matter contrast was achieved with a high resolution 3D T ₂-weighted TSE (SPACE) sequence. Delineation of the retino-tectal projection and detection of defined visual pathway damage on the level of the optic nerve could be achieved by using a T ₁-weighted, 3D gradient echo sequence with isotropic resolution of (0.2?mm)³.

Conclusions

Experimental studies in small rodents requiring high spatial resolution can be performed by using a clinical 3 T scanner with appropriate dedicated coils.     

Dual-contrast single breath-hold 3D abdominal MR imaging

Object: Multiple contrasts are often helpful for a comprehensive diagnosis. In 3D abdominal MRI, breath-hold techniques are preferred for single contrast acquisitions to avoid respiratory artifacts. In this paper, highly accelerated parallel MRI is used to acquire large 3D abdominal volumes with two different contrasts within a single breath-hold. Material and methods: In vivo studies have been performed on six healthy volunteers, combining T ₁- and T ₂-weighted, gradient- or spin-echo based scans, as well as water/fat resolved imaging in a single breath-hold. These 3D scans were acquired with an acceleration factor of six, using a prototype 32-element receive array. Results: The presented approach was tested successfully on all volunteers. The whole liver area was covered by a FOV of 350 × 250 × 200 mm³ for all scans with reasonable spatial resolution. Arbitrary scan protocols generating different contrasts have been shown to be combinable in this single breath-hold approach. Good spatial correspondence with negligible spatial offset was achieved for all different scan combinations acquired in overall breath-hold times between 15 and 25 s. Conclusion: Enabled by highly parallel imaging technology, this study demonstrates the technical feasibility and the promising image quality of single breath-hold dual contrast MRI.     

A comparison of emulsifiers for the formation of oil-in-water emulsions: stability of the emulsions within 9 h after production and MR signal properties

Objective

To provide a basis for the selection of suitable emulsifiers in oil-in-water emulsions used as tissue analogs for MRI experiments. Three different emulsifiers were investigated with regard to their ability to stabilize tissue-like oil-in-water emulsions. Furthermore, MR signal properties of the emulsifiers themselves and influences on relaxation times and ADC values of the aqueous phase were investigated.

Materials and methods

Polysorbate 60, sodium dodecyl sulfate (SDS) and soy lecithin were used as emulsifiers. MR characteristics of emulsifiers were assessed in aqueous solutions and their function as a stabilizer was examined in oil-in-water emulsions of varying fat content (10, 20, 30, 40, 50%). Stability and homogeneity of the oil-in-water emulsions were evaluated with a delay of 3 h and 9 h after preparation using T₁ mapping and visual control. Signal properties of the emulsifiers were investigated by ¹H-MRS in aqueous emulsifier solutions. Relaxometry and diffusion weighted MRI (DWI) were performed to investigate the effect of various emulsifier concentrations on relaxation times (T₁ and T₂) and ADC values of aqueous solutions.

Results

Emulsions stabilized by polysorbate 60 or soy lecithin were stable and homogeneous across all tested fat fractions. In contrast, emulsions with SDS showed a significantly lower stability and homogeneity. Recorded T₁ maps revealed marked creaming of oil droplets in almost all of the emulsions with SDS. The spectral analysis showed several additional signals for polysorbate and SDS. However, lecithin remained invisible in ¹H-MRS. Relaxometry and DWI revealed different influences of the emulsifiers on water: Polysorbate and SDS showed only minor effects on relaxation times and ADC values of aqueous solutions, whereas lecithin showed a strong decrease in both relaxation times (r_1,lecithin = 0.11 wt.%⁻¹ s⁻¹, r_2,lecithin = 0.57 wt.%⁻¹ s⁻¹) and ADC value (Δ(ADC)_lecithin =  − 0.18 × 10^–3 mm²/s⋅wt.%) with increasing concentration.

Conclusion

Lecithin is suggested as the preferred emulsifier of oil-in-water emulsions in MRI as it shows a high stabilizing ability and remains invisible in MRI experiments. In addition, lecithin is suitable as an alternative means of adjusting relaxation times and ADC values of water.

     

Magnetization transfer by simple sequences of rectangular pulses

On-resonant radio frequency (RF) sequences composed of a train of short rectangular pulses of the same kind were optimized in order to obtain selective saturation of protons with short transverse relaxation times for magnetization transfer purposes. It is demonstrated that the sequences regarded allow a good adaptation to different requirements for magnetization transfer examinations on whole-body imagers. The sequences presented here provide relatively strong saturation of protons with very short transverse relaxation timesT ₂50 µs, whereas signals from protons with longT ₂ to be recorded are hardly influenced in a broad frequency range. The sequences are especially advantageous for applications in pulse files with limited numbers of support points.     

Burst imaging

The acquisition time of common fast imaging techniques is limited by the switching times of the magnetic field gradients necessary to encode the RF signal for the spatial coordinates. We introduce a method by which multiple spin echoes are generated using a burst of short RF pulses. Spatial encoding can be introduced into the echotrain using very few gradient switching steps. Acquisition times as short as 40 ms for a 64 × 128 image matrix can thus be achieved on a whole body system using a conventional gradient system with a gradient amplitude of 10 mT/m and 1 ms switching time. Different possibilities to introduce slice selection into the basically non-slice selective experiment are presented which also allow to manipulate the image contrast. Quantitative measurements of T₁- and T₂-relaxation rates as well as diffusion and perfusion constants can thus be performed within a few seconds.     

Evaluation of a mobile NMR sensor for determining skin layers and locally estimating the T2eff relaxation time in the lower arm

Object

The nuclear magnetic resonance (NMR) mobile-universal-surface-explorer (MOUSE) was evaluated in a pilot study to determine its ability to detect physiological changes in human skin caused by physical or pharmacological interventions.

Materials and methods

The left lower arm skin thicknesses of ten male subjects were measured five times using a Profile NMR-MOUSE^? (¹H, 19?MHz) before and after a venous occlusion manoeuvre. In five of the subjects, the T_2eff relaxation times were derived from a bi-exponential fitting and were determined in the dermis and subcutis before and after applying a salve containing capsaicin.

Results

The dermis (including the epidermis) showed rather homogeneous signal amplitudes. The subcutis was characterised by higher and more variable amplitudes. The full-skin thickness values were affirmed by ultrasound imaging. The NMR profiles did not show significant skin swelling due to venous occlusion. In the dermis, capsaicin caused significant (p?<?0.05) decreases in both components of T _2eff (100?±?19?ms?C19?±?10?ms; 9.5?±?0.5?ms?C7.2?±?1.6?ms). In the subcutis, the T _2eff was not affected.

Conclusion

In principle, NMR-MOUSE profiles are capable of detecting skin structure. However, precise measurements are jeopardised by poor reproducibility, long acquisition times, and incompatibility between the geometries of the sensitive area of the instrument and the non-planar structure of the skin. In the dermis, T _2eff contrast could be used to detect the changes in tissue composition caused by inflammatory reactions.     

Structural disorder in BZN-based pyrochlores

Careful structural investigations have been carried out on the Bi_1.5ZnNb_1.5O₇ based dielectric system (of A ₂ B ₂O₇ pyrochlore structure type) in an attempt to understand the origin and tolerance of relaxor behavior in such materials. A highly structured, characteristic diffuse intensity distribution was observed in electron diffraction patterns, which arises from static disordering caused by local short range ordering of Bi and Zn ions on the pyrochlore A sites and associated structural relaxation of the O’A ₂ sub-structure. This structural disordering is not affected by B site substitution of the Nb⁵⁺ ions by Sn⁴⁺ or Ti⁴⁺ ions. The result is of significance for optimizing the dielectric properties of bismuth-based advanced ceramics.