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

Current methods for hyperpolarized [1-13C]pyruvate MRI human studies

MRI with hyperpolarized (HP) 13C agents, also known as HP 13C MRI, can measure processes such as localized metabolism that is altered in numerous cancers, liver, heart, kidney diseases, and more. It has been translated into human studies during the past 10 years, with recent rapid growth in studies largely based on increasing availability of HP agent preparation methods suitable for use in humans. This paper aims to capture the current successful practices for HP MRI human studies with [1-13C]pyruvate-by far the most commonly used agent, which sits at a key metabolic junction in glycolysis. The paper is divided into four major topic areas: (1) HP 13C-pyruvate preparation; (2) MRI system setup and calibrations; (3) data acquisition and image reconstruction; and (4) data analysis and quantification. In each area, we identified the key components for a successful study, summarized both published studies and current practices, and discuss evidence gaps, strengths, and limitations. This paper is the output of the "HP 13C MRI Consensus Group" as well as the ISMRM Hyperpolarized Media MR and Hyperpolarized Methods and Equipment study groups. It further aims to provide a comprehensive reference for future consensus, building as the field continues to advance human studies with this metabolic imaging modality.

MRI Application and Challenges of Hyperpolarized Carbon-13 Pyruvate in Translational and Clinical Cardiovascular Studies: A Literature Review

Cardiovascular disease shows, or may even be caused by, changes in metabolism. Hyperpolarized magnetic resonance spectroscopy and imaging is a technique that could assess the role of different aspects of metabolism in heart disease, allowing real-time metabolic flux assessment in vivo. In this review, we introduce the main hyperpolarization techniques. Then, we summarize the use of dedicated radiofrequency 13C coils, and report a state of the art of 13C data acquisition. Finally, this review provides an overview of the pre-clinical and clinical studies on cardiac metabolism in the healthy and diseased heart. We furthermore show what advances have been made to translate this technique into the clinic in the near future and what technical challenges still remain, such as exploring other metabolic substrates.

Current Methods for Hyperpolarized [1-13C]pyruvate MRI Human Studies

MRI with hyperpolarized (HP) 13C agents, also known as HP 13C MRI, can measure processes such as localized metabolism that is altered in numerous cancers, liver, heart, kidney diseases, and more. It has been translated into human studies during the past 10 years, with recent rapid growth in studies largely based on increasing availability of hyperpolarized agent preparation methods suitable for use in humans. This paper aims to capture the current successful practices for HP MRI human studies with [1-13C]pyruvate - by far the most commonly used agent, which sits at a key metabolic junction in glycolysis. The paper is divided into four major topic areas: (1) HP 13C-pyruvate preparation, (2) MRI system setup and calibrations, (3) data acquisition and image reconstruction, and (4) data analysis and quantification. In each area, we identified the key components for a successful study, summarized both published studies and current practices, and discuss evidence gaps, strengths, and limitations. This paper is the output of the HP 13C MRI Consensus Group as well as the ISMRM Hyperpolarized Media MR and Hyperpolarized Methods & Equipment study groups. It further aims to provide a comprehensive reference for future consensus building as the field continues to advance human studies with this metabolic imaging modality.

Considerations for hyperpolarized 13 C MR at reduced field: Comparing 1.5T versus 3T

PURPOSE

In contrast to conventional MR, signal-to-noise ratio (SNR) is not linearly dependent on field strength in hyperpolarized MR, as polarization is generated outside the MR system. Moreover, field inhomogeneity-induced artifacts and other practical limitations associated with field strengths ≥ $$ \ge $$ 3T are alleviated at lower fields. The potential of hyperpolarized   13 $$ {}^{13} $$ C spectroscopy and imaging at 1.5T versus 3T is demonstrated in silico, in vitro, and in vivo for applications on clinical MR systems.

THEORY AND METHODS

Theoretical noise and SNR behavior at different field strengths are investigated based on simulations. A thorough field comparison between 1.5T and 3T is performed using thermal and hyperpolarized   13 $$ {}^{13} $$ C spectroscopy and imaging. Cardiac in vivo data is obtained in pigs using hyperpolarized [1-   13 $$ {}^{13} $$ C]pyruvate spectroscopy and imaging at 1.5T and 3T.

RESULTS

Based on theoretical considerations and simulations, the SNR of hyperpolarized MR at identical acquisition bandwidths is independent of the field strength for typical coil setups, while adaptively changing the acquisition bandwidth proportional to the static magnetic field allows for net SNR gains of up to 40% at 1.5T compared to 3T. In vitro   13 $$ {}^{13} $$ C data verified these considerations with less than 7% deviation. In vivo feasibility of hyperpolarized [1-   13 $$ {}^{13} $$ C]pyruvate dynamic metabolic spectroscopy and imaging at 1.5T is demonstrated in the pig heart with comparable SNR between 1.5T and 3T while B   0 $$ {}_0 $$ artifacts are noticeably reduced at 1.5T.

CONCLUSION

Hyperpolarized   13 $$ {}^{13} $$ C MR at lower field strengths is favorable in terms of SNR and off-resonance effects, which makes 1.5T a promising alternative to 3T, especially for clinical cardiac metabolic imaging.

RF coil design for accurate parallel imaging on 13 C MRSI using 23 Na sensitivity profiles

PURPOSE

To develop a coil-based method to obtain accurate sensitivity profiles in ¹³ C MRI at 3T from the endogenous ²³ Na. An eight-channel array is designed for ¹³ C MR acquisitions. As application examples, the array is used for two-fold accelerated acquisitions of both hyperpolarized ¹³ C metabolic imaging of pig kidneys and the human brain.

METHODS

A flexible coil array was tuned optimally for ¹³ C at 3T (32.1 MHz), with the coil coupling coefficients matched to be nearly identical at the resonance frequency of ²³ Na (33.8 MHz). This is done by enforcing a high decoupling (obtained through highly mismatched preamplifiers) and adjusting the coupling frequency response. The SNR performance is compared to reference coils.

RESULTS

The measured sensitivity profiles on a phantom showed high spatial similarity for ¹³ C and ²³ Na resonances, with average noise correlation of 9 and 11%, respectively. For acceleration factors 2, 3, and 4, the obtained maximum g-factors were 1.0, 1.1, and 2.6, respectively. The ²³ Na profiles obtained in vivo could be used successfully to perform two-fold acceleration of hyperpolarized ¹³ C 3D acquisitions of both pig kidneys and a healthy human brain.

CONCLUSION

A receive array has been developed in such a way that the ¹³ C sensitivity profiles could be accurately obtained from measurements at the ²³ Na frequency. This technique facilitates accelerated acquisitions for hyperpolarized ¹³ C imaging. The SNR performance obtained at the ¹³ C frequency, compares well to other state-of-the-art coils for the same purpose, showing slightly better superficial and central SNR.

Trehalose as an alternative to glycerol as a glassing agent for in vivo DNP MRI

PURPOSE

In dynamic nuclear polarization (DNP), the solution needs to form a glass to attain significant levels of polarization in reasonable time periods. Molecules that do not form glasses by themselves are often mixed with glass forming excipients. Although glassing agents are often essential in DNP studies, they have the potential to perturb the metabolic measurements that are being studied. Glycerol, the glassing agent of choice for in vivo DNP studies, is effective in reducing ice crystal formation during freezing, but is rapidly metabolized, potentially altering the redox and adenosine triphosphate balance of the system.

METHODS

DNP buildup curves of ¹³ C urea and alanine with OX063 in the presence of trehalose, glycerol, and other polyol excipients were measured as a function of concentration. T₁ and T_m relaxation times for OX063 in the presence of trehalose were measured by EPR.

RESULTS

Approximately 15-20 wt% trehalose gives a glass that polarizes samples more rapidly than the commonly used 60%-wt formulation of glycerol and yields similar polarization levels within clinically relevant timeframes.

CONCLUSIONS

Trehalose may be an attractive biologically inert alternative to glycerol for situations where there may be concerns about glycerol's glucogenic potential and possible alteration of the adenosine triphosphate/adenosine diphosphate and redox balance.

A smart switching system to enable automatic tuning and detuning of metamaterial resonators in MRI scans

We present a radio-frequency-activated switching system that can automatically detune a metamaterial resonator to enhance magnetic resonance imaging (MRI) performance. Local sensitivity-enhancing metamaterials typically consist of resonant components, which means that the transmitted radio frequency field is spatially inhomogeneous. The switching system shows for the first time that a metamaterial resonator can be detuned during transmission and tuned during reception using a digital circuit. This allows a resonating system to maintain homogeneous transmit field while maintaining an increased receive sensitivity. As a result, sensitivity can be enhanced without changing the system-provided specific absorption rate (SAR) models. The developed digital circuit consists of inductors sensitive to the transmit radio-frequency pulses, along with diodes acting as switches to control the resonance frequency of the resonator. We first test the automatic resonator detuning on-the-bench, and subsequently evaluate it in a 1.5 T MRI scanner using tissue-mimicking phantoms. The scan results demonstrate that the switching mechanism automatically detunes the resonator in transmit mode, while retaining its sensitivity-enhancing properties (tuned to the Larmor frequency) in receive mode. Since it does not require any connection to the MRI console, the switching system can have broad applications and could be adapted for use with other types of MRI scanners and field-enhancing resonators.

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.

Autonomous cryogenic RF receive coil for 13 C imaging of rodents at 3 T

PURPOSE

To develop an autonomous, in-bore, MR-compatible cryostat cooled with liquid nitrogen that provides full-day operation, and to demonstrate that the theoretical signal-to-noise benefit can be achieved for ¹³ C imaging at 3 T (32.13 MHz).

METHODS

The cryogenic setup uses a vacuum-insulated fiberglass cryostat, which indirectly cools a cold finger where the RF coil is attached. The cryostat was evacuated before use and had a reservoir of liquid nitrogen for full-day operation. A 30 × 40 mm² copper coil was mounted inside the cryostat with a 3-mm distance to the sample. Two examples of in vivo experiments of rat brain metabolism after a hyperpolarized [1-¹³ C]pyruvate injection are reported.

RESULTS

A coil Q-factor ratio of Q_88K /Q_290K = 550/280 was obtained, and the theoretical SNR enhancement was verified with MR measurements. We achieved a coil temperature of 88 K and a preamplifier temperature of 77 K. A 2-fold overall SNR enhancement was achieved, compared with the best case at room temperature. The thermal performance of the coil was adequate for in vivo experiments, with an autonomy of 5 hours consuming 6 L of LN₂ , extendable to over 12 hours by LN₂ refilling.

CONCLUSION

Cryogenic surface coils can be highly beneficial for ¹³ C imaging, provided that the coil-to-sample distance remains short. An autonomous, in-bore cryostat was developed that achieved the theoretical improvement in SNR.

PIN diode driver for NMR and MRI

Designing custom coils for magnetic resonance systems, such as nuclear magnetic resonance (NMR) spectrometers and magnetic resonance imaging (MRI) scanners, often entails using non-standard configurations of the transmit-receive (T/R) switch and Q-spoiling circuits. The built-in drivers of commercial NMR and MRI systems are, typically, only reconfigurable within a narrow application range (if at all). Thus, the built-in driver may not be able to properly control the custom T/R switches and Q-spoiling circuits when using custom built coils. We present a PIN diode driver which functions in both an MRI scanner and NMR spectrometer. The PIN diode driver is based on readily available discrete components and achieves switching times for the reverse and forward bias states (transmit on and off) of 2 μs and 0.4 μs respectively. Hence, this work enables a higher degree of customization of the RF switching circuits in an MR system and is potentially of interest for designers of custom coils for both NMR spectrometers and MRI scanners.

Improved Decoupling for Low Frequency MRI Arrays using Non-conventional Preamplifier Impedance

OBJECTIVE

In this study, we describe a method to improve preamplifier decoupling in low frequency MRI receive coil arrays, where sample loading is low and coils exhibit a high Q-factor.

METHODS

The method relies on the higher decoupling obtained when coils are matched to an impedance higher than 50 Ω. Preamplifiers with inductive (and low resistive) input impedance, increase even further the effectiveness of the method.

RESULTS

We show that for poorly sample loaded coils, coupling to other elements in an array is a major source of SNR degradation due to a reduction of the coil Q-factor. An 8-channel 13C array at 32 MHz for imaging of the human head has been designed following this strategy. The improved decoupling even allowed constructing the array without overlapping of neighboring coils. Parallel imaging performance is also evaluated demonstrating a better spatial encoding of the array due to its non-overlapped geometry.

CONCLUSION

The proposed design strategy for coil arrays is beneficial for low frequency coils where the coil thermal noise is dominant. The method has been demonstrated on an 8-channel array for the human head for 13C MRI at 3 T (32 MHz), with almost 2-fold SNR enhancement when compared to a traditional array of similar size and number of elements.

SIGNIFICANCE

The proposed method is of relevance for low frequency arrays, where sample loading is low, and noise correlation is high due to insufficient coil decoupling.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Cryogenic Preamplifiers for Magnetic Resonance Imaging

Pursuing the ultimate limit of detection in magnetic resonance imaging (MRI) requires cryogenics to decrease the thermal noise of the electronic circuits. As cryogenic coils for MRI are slowly emerging cryogenic preamplifiers are required to fully exploit their potential. A cryogenic preamplifier operated at 77 K is designed and implemented for C imaging at 3 T (32.13 MHz), using off-the-shelves components. The design is based on a high electron mobility transistor (ATF54143) in a common source configuration. Required auxiliary circuitry for optimal cryogenic preamplifier performance is also presented consisting of a voltage regulator (noise free supply voltage and optimal power consumption), switch, and trigger (for active detuning during transmission to protect the preamplifier). A gain of 18 dB with a noise temperature of 13.7 K is achieved. Performing imaging experiments in a 3 T scanner showed an 8% increased signal-to-noise ratio from 365 to 399 when lowering the temperature of the preamplifier from 296 to 77 K while keeping the coil at room temperature. This paper thus enables the merger of cryogenic coils and preamplifiers in the hopes of reaching the ultimate limit of detection for MRI.