X-nuclei imaging: Current state, technical challenges, and future directions.

1 H imaging is concerned with contrast generation among anatomically distinct soft tissues. X-nuclei imaging, on the other hand, aims to reveal the underlying changes in the physiological processes on a cellular level. Advanced clinical MR hardware systems improved 1 H image quality and simultaneously enabled X-nuclei imaging. Adaptation of 1 H methods and optimization of both sequence design and postprocessing protocols launched X-nuclei imaging past feasibility studies and into clinical studies. This review outlines the current state of X-nuclei MRI, with the focus on 23 Na, 35 Cl, 39 K, and 17 O. Currently, various aspects of technical challenges limit the possibilities of clinical X-nuclei MRI applications. To address these challenges, quintessential physical and technical concepts behind different applications are presented, and the advantages and drawbacks are delineated. The working process for methods such as quantification and multiquantum imaging is shown step-by-step. Clinical examples are provided to underline the potential value of X-nuclei imaging in multifaceted areas of application. In conclusion, the scope of the latest technical advance is outlined, and suggestions to overcome the most fundamental hurdles on the way into clinical routine by leveraging the full potential of X-nuclei imaging are presented. Level of Evidence: 1 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2020;51:355-376.

Feasibility study of a double resonant 8-channel 1H/ 8-channel 23Na receive-only head coil at 3 Tesla.

Sodium (23Na) magnetic resonance imaging (MRI), especially brain applications are increasingly interesting since sodium MRI can provide additional information about tissue viability and vitality. In order to include sodium MRI in the clinical routine, a single RF setup is preferable which provides high sodium sensitivity and full proton performance in terms of signal-to-noise ratio (SNR) and parallel imaging performance. The aim of this work was to evaluate the feasibility of a double resonant receive (Rx) coil array for proton and sodium head MRI. The coil was designed to provide high sodium SNR and full proton performance comparable to commercial coils which are optimized for sodium MRI or for proton MRI, respectively. A measurement setup was built which comprised an 8-channel Rx degenerate Birdcage for sodium imaging and an 8-channel Rx array for proton imaging. The performance of the coil was evaluated against commercial sodium and proton coils using phantom and in-vivo measurements of two healthy volunteers.

Feasibility study of a double resonant (1H/23Na) abdominal RF setup at 3T.

Sodium magnetic resonance imaging (MRI) of the human abdomen is of increasing clinical interest for e.g. kidney, intervertebral disks, prostate and tumor monitoring examinations in the abdomen. To overcome the low MR sensitivity of sodium, optimal radio frequency (RF) structures should be used. A common approach is to combine a volumetric transmit coil for homogeneous excitation with an array of sensitive receive coils adapted to the human shape. Additionally, proton imaging is required to match the physiological sodium images to the morphological proton images. In this work, we demonstrated the feasibility of a double resonant proton/sodium RF setup for abdominal MRI at 3T, providing a high sodium sensitivity. After extensive simulations, a 16-channel sodium receive array was built and used in combination with a volumetric sodium transmit coil. Additionally, a local proton coil was included in the setup for anatomical localizations. The setup was investigated using electromagnetic field simulations, phantom measurements and final in-vivo measurements of a healthy volunteer. A 3 to 6-fold sensitivity improvement of the sodium receive array compared to the volumetric sodium coil was achieved using the phantom simulations and measurements. Safety assessments of the local proton transmit/receive coil were performed using specific absorption rate simulations. Finally, the feasibility of such a setup was proven by in-vivo measurements.

23 Na Triple-quantum signal of in vitro human liver cells, liposomes, and nanoparticles: Cell viability assessment vs. separation of intra- and extracellular signal.

BACKGROUND: Triple-quantum (TQ) filtered sequences have become more popular in sodium MR due to the increased usage of scanners with field strengths exceeding 3T. Disagreement as to whether TQ signal can provide separation of intra- and extracellular compartments persists. PURPOSE: To provide insight into TQ signal behavior on a cellular level. STUDY TYPE: Prospective. PHANTOM/SPECIMEN: Cell-phantoms in the form of liposomes, encapsulated 0 mM, 145 mM, 154 mM Na+ in a double-lipid membrane similar to cells. Poly(lactic-co-glycolic acid) nanoparticles encapsulated 154 mM Na+ within a single-layer membrane structure. Two microcavity chips with each 6 × 106 human HEP G2 liver cells were measured in an MR-compatible bioreactor. FIELD STRENGTH/SEQUENCE: Spectroscopic TQ sequence with time proportional phase-increments at 9.4T. ASSESSMENT: The TQ signal of viable, dead cells, and cell-phantoms was assessed by a fit in the time domain and by the amplitude in the frequency domain. STATISTICAL TESTS: The noise variance (σ) was evaluated to express the deviation of the measured TQ signal amplitude from noise. RESULTS: TQ signal >20σ was found for liposomes encapsulating sodium ions. Liposomal encapsulation of 0 mM Na+ and 154 mM Na+ encapsulation in the nanoparticles resulted in <2σ TQ signal. Cells under normal perfusion resulted in >9σ TQ signal. Compared with TQ signal under normal perfusion, a 56% lower TQ signal of was observed (25σ) during perfusion stop. TQ signal returned to 92% of the initial signal after reperfusion. DATA CONCLUSION: Our measurements indicate that TQ signal in liposomes was observed due to the trapping of ions within the double-lipid membrane rather than from the intraliposomal space. Transfer to the cell results suggests that TQ signal was observed from motion restriction equivalent to trapping. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2019;50:435-444.

Risk assessment of copper-containing contraceptives: the impact for women with implanted intrauterine devices during clinical MRI and CT examinations.

OBJECTIVES: To assess the risks for implant users with copper-containing intrauterine devices (IUDs) during MR and CT examinations. METHODS: A tissue-mimicking phantom suitable for all experiments within this study was developed. Seven different types of copper IUDs were evaluated. Heating and dislocation of each IUD were investigated at two clinically relevant positions in 1.5 T and 3 T MR scanners. Artifacts in the field of view caused by each tested IUD were determined for clinical MR and CT imaging. RESULTS: No significant heating of any tested IUD was detected during MR measurements. The temperature increase was less than 0.6 K for all IUDs. Neither angular deflection nor translation of any IUD was detected. Artifacts in MR images were limited to the very vicinity of the IUDs except for one IUD containing a steel-visualizing element. Streaking artifacts in CT were severe (up to 75.5%) in the slices including the IUD. CONCLUSION: No significant risk possibly harming the patient was determined during this phantom study, deeming MR examinations safe for women with an implanted copper IUD. Image quality was more impaired for CT than for MR imaging and needs careful consideration during diagnosis. KEY POINTS: • Risk assessment of copper-containing IUDs with regard to heating, dislocation, and artifacts during MR and CT imaging. • Neither significant heating nor dislocation was determined in MR; image quality was more impaired for CT than for MR imaging and needs careful consideration during diagnosis. • The tested IUDs pose no additional risks for implant users during MR and CT examinations.

Evaluation of stacked resonators to enhance the performance of a surface receive-only array for prostate MRI at 3 Tesla.

Prostate MRI is an important tool to diagnose and characterize cancer. High local sensitivity and good parallel imaging performance are of paramount importance for diagnostic quality and efficiency. The purpose of this work was to evaluate stacked resonators as part of a surface receiver array for prostate MRI at 3 Tesla. A base array of 6-channels consisting of a flexible anterior and a rigid posterior part were built each with three loop coils. A pair of stacked resonators was added concentrically to the center loops (anterior and posterior) of the base array. The evaluated stacked resonators were butterflies, composites and dipoles which yielded a total of three 8-channel arrays. The arrays were compared using noise correlations and single-channel signal-to-noise ratio maps in a phantom. Combined signal-to-noise ratio maps and parallel imaging performances were measured and compared in vivo in 6 healthy volunteers. The results were compared to the base and a commercial array. The SNR values in the prostate yielded by all the arrays were not statistically different using fully sampled k-space. However, significant differences were found in the parallel imaging performance of the arrays. More specifically, up to 88% geometric factor reduction was found compared to the commercial array and up to 83% reduction compared to the base array using butterfly coils. Thus, signal-to-noise ratio improvements were observed with stacked resonators when using parallel imaging. The use of stacked elements, in particular butterfly coils, can improve the performance of a base array consisting solely of single loops when using parallel imaging. We expect prostate MRI at 3 Tesla to benefit from using combinations of single loops and stacked resonators.

Evaluating the effects of receive-only arrays in specific absorption rate simulations at 3 and 7 T.

Specific absorption rate (SAR) simulations are performed for most clinical and research transmit coil configurations. Such simulations allow the determination of limits in transmit power for patient safety. Different human models and coil configurations have been previously investigated using these simulations. However, only a few works have accounted for the effect of the receive (Rx) arrays in the SAR calculations and they have used very specialized setups or simplified detuning modeling of the Rx elements. In this work, we performed electromagnetic simulations using a clinical alike setup for whole-body scans at 3 T and head scans at 7 T. SAR simulations are performed for both setups with and without Rx arrays. A difference below 10% percent was found for max SAR. The maximum difference for the mean SAR values of the 3 T setups remained within 8% and within 15% of the 7 T setup.

Predicting Magnetostimulation Thresholds in the Peripheral Nervous System using Realistic Body Models.

Rapid switching of applied magnetic fields in the kilohertz frequency range in the human body induces electric fields powerful enough to cause Peripheral Nerve Stimulation (PNS). PNS has become one of the main constraints on the use of high gradient fields for fast imaging with the latest MRI gradient technology. In recent MRI gradients, the applied fields are powerful enough that PNS limits their application in fast imaging sequences like echo-planar imaging. Application of Magnetic Particle Imaging (MPI) to humans is similarly PNS constrained. Despite its role as a major constraint, PNS considerations are only indirectly incorporated in the coil design process, mainly through using the size of the linear region as a proxy for PNS thresholds or by conducting human experiments after constructing coil prototypes. We present for the first time, a framework to simulate PNS thresholds for realistic coil geometries to directly address PNS in the design process. Our PNS model consists of an accurate body model for electromagnetic field simulations, an atlas of peripheral nerves, and a neurodynamic model to predict the nerve responses to imposed electric fields. With this model, we were able to reproduce measured PNS thresholds of two leg/arm solenoid coils with good agreement.

Tracking protein function with sodium multi quantum spectroscopy in a 3D-tissue culture based on microcavity arrays.

The aim of this study was to observe the effects of strophanthin induced inhibition of the Na-/K-ATPase in liver cells using a magnetic resonance (MR) compatible bioreactor. A microcavity array with a high density three-dimensional cell culture served as a functional magnetic resonance imaging (MRI) phantom for sodium multi quantum (MQ) spectroscopy. Direct contrast enhanced (DCE) MRI revealed the homogenous distribution of biochemical substances inside the bioreactor. NMR experiments using advanced bioreactors have advantages with respect to having full control over a variety of physiological parameters such as temperature, gas composition and fluid flow. Simultaneous detection of single quantum (SQ) and triple quantum (TQ) MR signals improves accuracy and was achieved by application of a pulse sequence with a time proportional phase increment (TQTPPI). The time course of the Na-/K-ATPase inhibition in the cell culture was demonstrated by the corresponding alterations of sodium TQ/SQ MR signals.

Partially orthogonal resonators for magnetic resonance imaging.

Resonators for signal reception in magnetic resonance are traditionally planar to restrict coil material and avoid coil losses. Here, we present a novel concept to model resonators partially in a plane with maximum sensitivity to the magnetic resonance signal and partially in an orthogonal plane with reduced signal sensitivity. Thus, properties of individual elements in coil arrays can be modified to optimize physical planar space and increase the sensitivity of the overall array. A particular case of the concept is implemented to decrease H-field destructive interferences in planar concentric in-phase arrays. An increase in signal to noise ratio of approximately 20% was achieved with two resonators placed over approximately the same planar area compared to common approaches at a target depth of 10 cm at 3 Tesla. Improved parallel imaging performance of this configuration is also demonstrated. The concept can be further used to increase coil density.

Sodium-23 MRI of whole spine at 3 Tesla using a 5-channel receive-only phased-array and a whole-body transmit resonator.

Sodium magnetic resonance imaging ((23)Na MRI) is a unique and non-invasive imaging technique which provides important information on cellular level about the tissue of the human body. Several applications for (23)Na MRI were investigated with regard to the examination of the tissue viability and functionality for example in the brain, the heart or the breast. The (23)Na MRI technique can also be integrated as a potential monitoring instrument after radiotherapy or chemotherapy. The main contribution in this work was the adaptation of (23)Na MRI for spine imaging, which can provide essential information on the integrity of the intervertebral disks with respect to the early detection of disk degeneration. In this work, a transmit-only receive-only dual resonator system was designed and developed to cover the whole human spine using (23)Na MRI and increase the receive sensitivity. The resonator system consisted of an already presented (23)Na whole-body resonator and a newly developed 5-channel receive-only phased-array. The resonator system was first validated using bench top and phantom measurements. A threefold SNR improvement at the depth of the spine (∼7cm) over the whole-body resonator was achieved using the spine array. (23)Na MR measurements of the human spine using the transmit-only receive-only resonator system were performed on a healthy volunteer within an acquisition time of 10minutes. A density adapted 3D radial sequence was chosen with 6mm isotropic resolution, 49ms repetition time and a short echo time of 540µs. Furthermore, it was possible to quantify the tissue sodium concentration in the intervertebral discs in the lumbar region (120ms repetition time) using this setup.