1 H-13 C independently tuned radiofrequency surface coil applied for in vivo hyperpolarized MRI

Design and construction of an 8-channel transceiver coil array for rat imaging at 9.4 T

Ultra-high field (UHF) small animal magnetic resonance imaging (MRI) is a crucial tool permitting investigation of metabolic diseases and identification of imaging biomarkers suitable for clinical diagnosis and translation. Radiofrequency (RF) coils are critical components in enabling acquisition of high-quality rat abdomen MRI data. However, efficient RF coils with high-channel count, capable of sensitive and accelerated rat abdomen imaging at 9.4 T, are not available commercially. The SNR of the commonly-used 9.4 T birdcage coil is relatively weak, particularly in the peripheral area of the subject. In addition, the birdcage is not readily to perform parallel imaging due to unavailability of the required multiple channels. Consequently, the extended scanning duration may cause unnecessary hazards to the rat. In this work, an 8-channel transceiver coil array was designed and constructed to provide good image quality and large coverage for rat abdomen imaging at 9.4 T. The structure and the performance of the developed array was optimized and evaluated by numerical electromagnetic simulations and bench tests, respectively. The MR imaging experiments in phantoms and rat models were also performed on a Bruker 9.4 T preclinical MRI system to validate the feasibility of the proposed design. The coil array supports a one-dimensional acceleration factor up to R = 4, providing good parallel imaging capabilities. These results demonstrated that the proposed 8-channel transceiver coil array for rat imaging has the ability to obtain high spatial resolution of rat abdomen anatomical structure images at 9.4 T.

A 13C/31P surface coil to visualize metabolism and energetics in the rodent brain at 3 Tesla

PURPOSE

We constructed a 13C/31P surface coil at 3 T for studying cancer metabolism and bioenergetics. In a single scan session, hyperpolarized 13C-pyruvate MRS and 31P MRS was carried out for a healthy rat brain.

METHODS

All experiments were carried out at 3 Tesla. The multinuclear surface coil was designed as two coplanar loops each tuned to either the 13C or 31P operating frequency with an LCC trap on the 13C loop. A commercial volume proton coil was used for anatomical localization and B0 shimming. Single tuned coils operating at either the 13C or 31P frequency were built to evaluate the relative performance of the multinuclear coil. Coil performance metrics consisted of measuring Q factor ratio, calculating system input power using a single-pulse acquisition, and acquiring SNR and flip angle maps using 2D CSI sequences. To observe in vivo spectra, a bolus of hyperpolarized [1-13C] pyruvate was administered via tail vein. In vivo13C and endogenous 31P spectra were obtained in a single scan session using 1D slice selective acquisitions.

RESULTS

When compared with single tuned surface coils, the multinuclear coil performance showed a decrease in Q factor ratio, SNR, and transmit efficiency. Flip angle maps showed adequate flip angles within the phantom when the transmit voltage was set using an external phantom. Results show good detection of 13C labeled lactate, alanine, and bicarbonate in addition to ATP from 31P MRS.

CONCLUSIONS

The coil enables obtaining complementary information within a scan session, thus reducing the number of trials and minimizing biological variability for studies of metabolism and bioenergetics.

9.4 T double-tuned 13 C/1 H human head array using a combination of surface loops and dipole antennas

MRI at ultra-high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X-nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal-to-noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double-tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X-nucleus surface loop array with ¹ H dipoles can substantially simplify the design of a double-tuned UHF human head array coil. Recently, we developed a novel bent folded-end dipole transceiver array for human head imaging at 9.4 T. The new eight-element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16-element surface loop array. In this work, we developed, constructed and evaluated a double-tuned ¹³ C/¹ H human head 9.4 T array consisting of eight ¹³ C transceiver surface loops and eight ¹ H transceiver bent folded-end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four ¹ H traps into each ¹³ C loop. The presented double-tuned RF array coil substantially simplifies the design as compared with the common double-tuned surface loop arrays. At the same time, the coil demonstrated an improved ¹ H longitudinal coverage and good transmit efficiency.

Comparison of hyperpolarized 13 C and non-hyperpolarized deuterium MRI approaches for imaging cerebral glucose metabolism at 4.7 T

PURPOSE

The purpose of this study was to directly compare two isotopic metabolic imaging approaches, hyperpolarized (HP) ¹³ C MRI and deuterium metabolic imaging (DMI), for imaging specific closely related segments of cerebral glucose metabolism at 4.7 T.

METHODS

Comparative HP-¹³ C and DMI neuroimaging experiments were conducted consecutively in normal rats during the same scanning session. Localized conversions of [1-¹³ C]pyruvate and [6,6-² H₂ ]glucose to their respective downstream metabolic products were measured by spectroscopic imaging, using an identical 2D-CSI sequence with parameters optimized for the respective experiments. To facilitate direct comparison, a pair of substantially equivalent 2.5-cm double-tuned X/¹ H RF surface coils was developed. For improved results, multidimensional low-rank reconstruction was applied to denoise the raw DMI data.

RESULTS

Localized conversion of HP [1-¹³ C]pyruvate to [1-¹³ C]lactate, and [6,6-² H₂ ]glucose to [3,3-² H₂ ]lactate and Glx-d (glutamate and glutamine), was detected in rat brain by spectroscopic imaging at 4.7 T. The SNR and spatial resolution of HP-¹³ C MRI was superior to DMI but limited to a short time window, whereas the lengthy DMI acquisition yielded maps of not only lactate, but also Glx production, albeit with relatively poor spectral discrimination between metabolites at this field strength. Across the individual rats, there was an apparent inverse correlation between cerebral production of HP [1-¹³ C]lactate and Glx-d, along with a trend toward increased [3,3-² H₂ ]lactate.

CONCLUSION

The HP-¹³ C MRI and DMI methods are both feasible at 4.7 T and have significant potential for metabolic imaging of specific segments of glucose metabolism.

15 N-carnitine, a novel endogenous hyperpolarized MRI probe with long signal lifetime

PURPOSE

The purpose of this study was to investigate hyperpolarization and in vivo imaging of [¹⁵ N]carnitine, a novel endogenous MRI probe with long signal lifetime.

METHODS

L-[¹⁵ N]carnitine-d₉ was hyperpolarized by the method of dynamic nuclear polarization followed by rapid dissolution. The T₁ signal lifetimes were estimated in aqueous solution and in vivo following intravenous injection in rats, using a custom-built dual-tuned ¹⁵ N/¹ H RF coil at 4.7 T. ¹⁵ N chemical shift imaging and ¹⁵ N fast spin-echo images of rat abdomen were acquired 3 minutes after [¹⁵ N]carnitine injection.

RESULTS

Estimated T₁ times of [¹⁵ N]carnitine at 4.7 T were 210 seconds (in H₂ O) and 160 seconds (in vivo), with an estimated polarization level of 10%. Remarkably, the [¹⁵ N]carnitine coherence was detectable in rat abdomen for 5 minutes after injection for the nonlocalized acquisition. No downstream metabolites were detected on localized or nonlocalized ¹⁵ N spectra. Diffuse liver enhancement was detected on ¹⁵ N fast spin-echo imaging 3 minutes after injection, with mean hepatic SNR of 18 ± 5 at a spatial resolution of 4 × 4 mm.

CONCLUSION

This study showed the feasibility of hyperpolarizing and imaging the biodistribution of HP [¹⁵ N]carnitine.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Broadband and multi-resonant sensors for NMR

It has always been of considerable interest to study the nuclear magnetic resonance response of multiple nuclei simultaneously, whether these signals arise from internuclear couplings within the same molecule, or from uncoupled nuclei within sample mixtures. The literature contains numerous uncorrelated reports on techniques employed to achieve multi-nuclear NMR detection. This paper consolidates the subset of techniques in which single coil detectors are utilized, and highlights the strengths and weaknesses of each approach, at the same time pointing the way towards future developments in the field of multi-nuclear NMR. We compare the different multi-nuclear NMR techniques in terms of performance, and present a guide to NMR probe designers towards application-based optimum design. We also review the applicability of micro-coils in the context of multi-nuclear methods. Micro-coils benefit from compact geometries and exhibit lower impedance, which provide new opportunities and challenges for the NMR probe designer.

Low-Noise Active Decoupling Circuit and its Application to 13C Cryogenic RF Coils at 3 T

We analyze the loss contributions in a small, 50-mm-diameter receive-only coil for carbon-13 (¹³C) magnetic resonance imaging at 3 T for 3 different circuits, which, including active decoupling, are compared in terms of their Q-factors and signal-to-noise ratio (SNR). The results show that a circuit using unsegmented tuning and split matching capacitors can provide >20% SNR enhancement at room temperature compared with that using more traditional designs. The performance of the proposed circuit was also measured when cryogenically cooled to 105 K, and an additional 1.6-fold SNR enhancement was achieved on a phantom. The enhanced circuit performance is based on the low capacitance needed to match to 50 Ω when coil losses are low, which significantly reduces the proportion of the current flowing through the matching network and therefore minimizes this loss contribution. This effect makes this circuit particularly suitable for receive-only cryogenic coils and/or small coils for low-gamma nuclei.