Proton magnetic resonance imaging (MRI) and spectroscopy (MRS) are widely used in clinical and research applications. Recent interest in X-nuclei studies highlights their ability to provide additional biochemical information, but the intrinsically low X-nuclear signal-to-noise ratio (SNR) significantly increases scan time. Simultaneous (rather than serial) acquisition of multiple nuclei can significantly reduce experiment time, but most conventional MR systems lack this capability without modifications. We present a cost-effective system that enables simultaneous multinuclear imaging and spectroscopy on conventional MR spectrometers. Our approach offers enhanced flexibility for multinuclear experiments, supports multinuclear array receive capability, and maintains phase stability in the radio frequency (RF) chain. The proposed system comprised multiple transmit and receive mixing channels and a four-channel flexible local oscillator (LO) source. By interfacing with the spectrometer, simultaneous transmit and receive at different frequencies were achieved. The performance of the system was evaluated through bench measurement and phantom multinuclear MRI and MRS experiments. Transmit and receive channel isolation of better than 30 dB was measured on the bench. Simultaneous excitation and reception of 2H and 23Na gradient echo images were acquired, as well as interleaved excitation with simultaneous reception of 1H, 2H, and 23Na FIDs. Water-suppressed 1H and 31P MRS were performed simultaneously on phantoms mimicking muscle metabolites. Results across all experiments showed no signal-to-noise ratio (SNR) loss compared to single-frequency operation. The proposed system supports multiple variations of simultaneous experiments on conventional MRI systems, demonstrating its flexibility in configuring experiments with varying numbers of nuclei (2-4), different transmit modes (simultaneous or interleaved), and supporting receive array coils of up to 16 channels, while maintaining phase stability in the RF chain without the need for retrospective correction.

Objective: Receive-only frequency translation enables MRI scanners with X-nuclear capabilities to perform simultaneous/interleaved multi-nuclear experiments. Mixing only on the receive side avoids modifying the transmit path, which often has narrow-band components. However, phase incoherence is introduced at the radio frequency mixer due to differing local oscillator frequencies between transmit and receive, necessitating phase correction. This paper presents a hardware solution for automatic phase correction during scans, eliminating the need for retrospective correction and allowing flexible scan parameter adjustments. Methods: The hardware solution detects phase changes in the system LO (local oscillator) between transmit and receive, calculates, and applies phase correction in the translator LO in real time. Programming spare TTL signals and accessing the scanner system LO are required to implement the phase correction method. Results: Phase correction accuracy was evaluated via averaged 31P spectroscopy and 23Na imaging. On top of the noise introduced by the additional mixer, the imperfect phase correction resulted in approximately 3% SNR loss at both frequencies. The corrected 23Na signal exhibited approximately an 8-degree phase standard deviation, compared to 6 degrees in the reference signal. Conclusion: The proposed hardware solution effectively corrects phase incoherence introduced by receive-only frequency translation. While minor imperfection exists, future upgrades are expected to improve the phase correction accuracy. Significance: This approach eliminates the need for retrospective phase correction when using receive-only frequency translation techniques for multi-nuclear acquisition, enabling real-time data acquisition and greater flexibility in scan parameter adjustment for simultaneous/interleaved multi-nuclear experiments.

The shortened radio frequency wavelength in high field MRI makes it challenging to create a uniform excitation pattern over a large field of view, or to achieve satisfactory transmission efficiency at a local area. Transmit arrays are one tool that can be used to create a desired excitation pattern. To be effective, it is important to be able to control the current amplitude and phase at the array elements. The control of the current may get complicated by the coil coupling in many applications. Various methods have been proposed to achieve current control, either in the presence of coupling, or by effectively decouple the array elements. These methods are applied in different subsystems in the RF transmission chain: coil; coil-amplifier interface; amplifier, etc. In this review paper, we provide an overview of the various approaches and aspects of transmit current control and decoupling.

Although MR scanners are widely available clinically, access for research and teaching can be more limited and expensive. In response a number of low-cost spectrometers, some open-source, have been reported. We have previously reported using the Digilent Analog Discovery 3 device (AD3) as a very low-cost and easily accessible MR spectrometer. Here we describe improvements to the system described last year, providing enhanced features needed for research and teaching more advanced courses. By using streaming modes, and the phase and amplitude modulation capability of the system, extended acquisition is demonstrated enabling a variety of imaging techniques, such as fast-spin echo and two-dimension spatially selective RF pulses. This work demonstrates that low-cost "lab-in-a-box" devices such as the AD3, combined with low-cost magnets, allows anyone to have the ability to explore and develop MR techniques, and should be of use in both teaching and research applications.Clinical Relevance- The very low-cost desktop MRI system provide opportunities for students of all levels to be introduced to MRI physics and pulse sequence design and be introduced to practical issues that may not be easily illustrated in simulation based teaching tools.

Characterization of RF components is important to assure that components behave as expected under given conditions. Some of the most important conditions for testing are power tolerance and slew speed. With regards to RF power tolerance, an RF component may appear properly tuned at the low power levels applied by a standard vector network analyzer but may detune at the higher power levels for transmitting RF pulses for an MR scan. Isolating and debugging this issue can be di cult when the problem only occurs at high power. Using an MR scanner to debug can be impractical and expensive. Device speed plays an important role in beam steering which requires rapid phase changes. We report a system that can characterize one or two port RF components and systems at 1000W in microsecond intervals without need for an MR scanner and can characterize S-parameter behavior in microsecond intervals. To illustrate, we made measurements that show complete and partial failures of devices at high power as well as device slew speed.

Multinuclear MRI/S offers a powerful tool for noninvasive diagnostics and disease monitoring but remains largely underutilized in part due to the intrinsically low sensitivity of X-nuclei scans. This has led to continued research interest into the optimization of multi-nuclear coil designs for double-and triple-tuned coils that can facilitate scaling into higher channel count arrays to increase sensitivity. Here we present a new design for a parallel trap network that can create a simultaneous triple-tuned coil for 1 H, 31 P and 23 Na at 4.7T. Coils were characterized both on the bench and in the scanner for the relative SNR of the different multi-tuning networks. The parallel network was shown to have a relative SNR gain of over 20% compared to a series network for 1 H imaging. X-nuclei had a decrease in SNR of less than 10% for both 31 P and 23 Na. Future work involves further optimization of inductor design and placement to improve the X-nuclei sensitivity.

Despite the proliferation of commercial MRI scanners across much of the world, there are many approaches to constructing low-cost MRI systems being reported. These respond to many needs, from teaching to research accessibility to accessible clinical health care. Most require a fairly high level of expertise to develop the actual system or use relatively expensive hardware. Here we present a minimal, but very capable MRI scanner constructed from the widely used Digilent Analog Discovery II or III devices. Very few external components are required, all available at low-cost from Amazon or other vendors. All programming is done in python, no expertise in FPGA programming or other low-level languages is required. A variety of applications are demonstrated, illustrating the capability of this minimalist system. By using widely available and relatively inexpensive components, the proposed system should make research and education in MRI available to essentially anyone anywhere in the world.

Magnetic resonance imaging instrumentation is taught at Texas A&M University through the ECEN 463 course and its graduate level equivalent. This class guides students through several labs where they design their own desktop MRI system using various hardware components and LabVIEW. Because the system uses professional grade equipment, the cost of each lab station is high. As a result, there are only four lab stations available, which limits the class to 32 students. The equipment also contains parts that have become obsolete, inhibiting the ability to maintain the system long term. This project focuses on using easily accessible and more affordable equipment for the MRI system. It can also potentially provide opportunities for remote learning, where students could work on assignments off-campus. Other projects have aimed to design low-cost MRI systems with an emphasis on clinical applications or which require advanced FPGA programming skills or pre-programmed modules. This project will develop the MRI instrumentation with updated off-the-shelf components. The current equipment will be replaced with two Analog Discovery 2 devices, which are low-cost teaching tools. It will also feature inexpensive transmit and receive chains, off-the-shelf gradient amplifiers suitable for teaching, gradient coils for signal localization, and a lighter-weight Halbach magnet. In this stage of the project, projections and images have been captured using a 0.06T permanent magnet. In addition to validating successful system operation, each lab of the course will be integrated with current materials to comply with the new equipment. Hardware and software resources will also be prepared and scaled to meet classroom needs and ensure a smooth transition. The goal of the project is to use the new system starting in the fall 2023 semester.Clinical Relevance— This project shows that low-cost equipment can be implemented into a working MRI system. The intent for this project may be educationally focused, but it shows that extremely light and low-cost systems can be created. It may be reconstructed to have a deployable system that could be used in the field.

Array coils are ubiquitous in MRI and are becoming more widely used in MR spectroscopy. Conventional PIN diode decoupling circuits require significant currents to forward bias the diodes. The approach proposed here does not require significant current and thus reduces concerns for contaminating the B 0 homogeneity with the detune current. Additionally, the proposed approach will facilitate the construction of array coils for MRI due to its simplicity.Clinical Relevance— Decoupling is critical for constructing RF coil arrays and enables rapid MR imaging.

Purpose: Multinuclear MRI/S is of increasing interest. Currently, most multinuclear receive array coils are constructed by nesting multiple single-tuned array coils or using switching elements to control the operating frequency, in which case more than one set of conventional isolation preamplifiers and associated decoupling circuits is required. These conventional configurations rapidly become complicated when greater numbers of channels or nuclei are needed. In this work, a novel coil decoupling mechanism is proposed to enable broadband decoupling for array coils with one set of preamplifiers. Methods: Instead of using conventional isolation preamplifiers, a high-input impedance preamplifier is proposed to create broadband decoupling of the array elements. A matching network consisting of a single inductor-capacitor-capacitor multi-tuned network and a wire-wound transformer was used to interface the surface coil to the high-impedance preamplifier. To validate the concept, the proposed configuration was compared to the conventional preamplifier decoupling configuration on both bench and scanner. Results: (2) The approach can provide more than 15dB decoupling over a range of 25MHz, covering the Larmor frequencies of Na-23 and H-2 at 4.7T. This multi-tuned prototype obtained 61% and 76% of the imaging SNR at H-2 and Na-23 respectively, 76 and 89% in a higher loading test phantom, when compared to the conventional single-tuned preamplifier decoupling configuration. Conclusion: With the multinuclear array operation and decoupling achieved using only one layer of array coil and preamplifiers, this work provides a simple approach of building high element-count arrays to enable accelerated imaging or SNR improvement from multiple nuclei.

The utility of interleaved odd-number leg birdcage coils is demonstrated for decoupling in double- and triple-tuned multinuclear applications. The birdcage was designed to geometrically decouple from a planar double-tuned (1H-23Na) array and from a 31P saddle coil insert to create a triple-tuned configuration. Comparisons between an actively detuned coil and a purely geometrically decoupled architecture were used to demonstrate the capabilities of the design. In particular cases, the simplicity and adaptability of the interleaved nine-leg design for multinuclear nuclear magnetic resonance (NMR) offer a straightforward alternative to the often complex and lossy designs currently available for multinuclear birdcages and volume coils.

Low-impedance preamplifier decoupling is commonly used in RF coil array construction to minimize coupling between elements through mutual impedance. The trap circuit is an essential component in preamp decoupling techniques, but becomes a limiting factor in constructing multi-tuned, multi-nuclear coil arrays. In principle, it is possible to double-tune or multi-tune the trap circuits, but will add complexity and loss. We present a broadband decoupling approach using high impedance preamplifiers. A dual-tuned prototype four-channel array using this approach which targets 2 H and 23 Na at 4.7T, has been previously constructed, evaluated and reported. Without any retuning of the array, the same setup is tested at the 23 Na and 31 P frequencies for 3T. Initial bench measurements and Chemical Shift Imaging (CSI) results are acquired and presented in this study.Clinical Relevance— This study could reduce the complexity of multi-nuclear array coil design.

PurposeMitigating coupling effects between coil elements represents a continuing challenge. Here, we present a 16-bowtie slot volume coil arranged in eight independent dual-slot modules without the use of any decoupling circuits. MethodsTwo electrically short "bowtie" slot antennas were used to form a "module." A bowtie configuration was chosen because electromagnetic modeling results show that bowtie slots exhibit improved B1+Pin$$ \frac{B_1<<^>>{+}}{\sqrt{P_{in}}} $$ efficiency when compared to thin rectangular slots. An eight-module volume coil was evaluated through electromagnetic modeling, bench tests, and MRI experiments at 4.7 T. ResultsBench tests indicate that worst-case coupling between modules did not exceed -14.5 dB. MR images demonstrate well-localized patterns about single excited modules confirming the low coupling between modules. Homogeneous MR images were acquired from a synthesized quadrature birdcage transmit mode. MRI experiments show that the RF power requirements for the proposed coil are 9.2 times more than a birdcage coil. Whereas from simulations performed to assess the proposed coil losses, the total power dissipated in the phantom was 1.1 times more for the birdcage. Simulation results at 7 T reveal an equivalent B-1(+) homogeneity when compared with an eight-dipole coil. ConclusionAlthough exhibiting higher RF power requirements, as a transmit coil when the power availability is not a restriction, the inherently low coupling between electrically short slots should enable the use of many slot elements around the imaging volume. The slot module described in this paper should be useful in the design of multi-channel transmit coils.

The use of traps as well as the use of switching circuitry to develop multinuclear coils is well established. However, it is well known that the use of traps introduces undesired losses to one or more nuclei in the structure while switching eliminates applications requiring true simultaneous imaging. As a result, our group developed a triple-tuned volume coil that solely uses geometric decoupling using only two structures. The coil demonstrated homogeneous fields with sufficient decoupling between the structures to acquire multinuclear NMR data.

OBJECTIVE:The feasibility of conducting in vivo non-localized 31P Magnetic Resonance Spectroscopy (MRS) with a 1.0T extremity scanner and the potential to increase accessibility of this important diagnostic tool for low cost applications is revisited.METHODS:This work presents a custom transmit-only quadrature birdcage, four-element receive coil array, and spectrometer interfaced to a commercial ONI 1.0T magnet for enabling multi-channel, non-1H frequency capabilities. A custom, magnetic resonance compatible plantar flexion-extension exercise device was also developed to enable exercise protocols. The coils were assessed with bench measurements and 31P phantom studies before an in vivo demonstration.RESULTS:In pulse and acquire spectroscopy of a phantom, the array was found to improve the signal-to-noise ratio (SNR) by a factor of 1.31 and reduce the linewidth by 13.9% when compared to a large loop coil of the same overall size. In vivo testing results show that two averages and a four second repetition time for a temporal resolution of eight seconds was sufficient to obtain phosphocreatine recovery values and baseline pH levels aligned with expected literature values.CONCLUSION:Initial in vivo human skeletal muscle 31P MRS allowed successful monitoring of metabolic changes during an 18-minute exercise protocol.SIGNIFICANCE:Adding an array coil and multinuclear capability to a commercial low-cost 1.0T extremity scanner enabled the observation of characteristic 31P metabolic information, such as the phosphocreatinerecovery rate and underlying baseline pH.

This paper demonstrates a rapid B1 field benchtop measurement system that is independent of an MR scanner and network analyzer. This system can be used to obtain radiofrequency (B1 field) strength distribution plots of multiple 2D slices (with an extension to 3D) of a liquid cylindrical phantom for multi-element phased arrays used in MRI. The system can be used in three modes- element, phased array, and multiple fixed point pattern measurement. These modes are demonstrated for a 7T 1H eight-channel dipole array and a corn-syrup based phantom. The system can measure complex phase and amplitude measurements from up to 8 elements in the first mode one or 8 different phase settings in the second mode at a rate of approximately 37 positions per minute, allowing a full 2D B1 mapping for 1303 points in 33.05 minutes. The scan patterns obtained using this setup are compared to the ones obtained using an HP network analyzer and simulations. This work can be extended to measure the E field, SAR and upon increasing the speed of measurement, could be used for applications such as Transmit SENSE. Clinical Relevance- This work benefits a faster and more widely accessible measurement system for phased array antennas for MRI. As phased arrays are becoming very important in MRI, the ability to assess individual element performance more rapidly and B1 shimming performance is important to aid in their further development.

Objective: Considering the reported elevation of ω-6/ω-3 fatty acid ratios in breast neoplasms, one particularly important application of 13C MRS could be in more fully understanding the breast lipidome's relationship to breast cancer incidence. However, the low natural abundance and gyromagnetic ratio of the 13C isotope lead to detection sensitivity challenges. Previous 13C MRS studies have relied on the use of small surface coils with limited field-of-view and shallow penetration depths to achieve adequate signal-to-noise ratio (SNR), and the use of receive array coils is still mostly unexplored. Methods: This work presents a unilateral breast 16-channel 13C array coil and interfacing hardware designed to retain the surface sensitivity of a single small loop coil while improving penetration depth and extending the field-of-view over the entire breast at 7T. The coil was characterized through bench measurements and phantom 13C spectroscopy experiments. Results: Bench measurements showed receive coil matching better than -17 dB and average preamplifier decoupling of 16.2 dB with no evident peak splitting. Phantom MRS studies show better than a three-fold increase in average SNR over the entirety of the breast region compared to volume coil reception alone as well as an ability for individual array elements to be used for coarse metabolite localization without the use of single-voxel or spectroscopic imaging methods. Conclusion: Our current study has shown the benefits of the array. Future in vivo lipidomics studies can be pursued. Significance: Development of the 16-channel breast array coil opens possibilities of in vivo lipidomics studies to elucidate the link between breast cancer incidence and lipid metabolics.

Purpose shimming is an important method for mitigating B1 inhomogeneity in high‐field MRI. Using independent power amplifiers for each transmit (Tx) element is the preferred method for B1 shimming but comes with a high cost. Conversely, the simplest approach to control a Tx array is by using coaxial cables of varying length in the Tx chain, but this approach is cumbersome and impractical for dynamic shimming. In this article, a system is described that enables dynamic, phase‐only, eight‐channel steering on a 7T MR scanner with only two power amplifiers.MethodsPower dividers were utilized to first split the existing two‐channel Tx signal into eight channels. Digitally controlled phase shifters on each channel were designed to provide independent phase shifts with a resolution of 22.5° (from 0°, 22.5° … 337.5°). To validate the system, an eight‐channel body dipole array was simulated and constructed for bench and 7T imaging and evaluation.ResultsThe phase conjugate steering method was employed at three different spatial positions in simulation, bench measurements, and scanner measurements—all with matching results. At the desired points, regions with homogenous were generated, indicating good Tx steering to the selected region.ConclusionThe described system can be used as a simple retrofit to existing hardware to provide phase control while avoiding the need to manually switch cables and without requiring independent power amplifiers for each channel, thus demonstrating the ability to perform dynamic shimming with increased degrees of freedom but without significantly increased hardware cost.

PurposeThis study is to investigate time‐resolved 13C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1‐13C]pyruvate in the human brain.MethodsTime‐resolved 13C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1‐13C]pyruvate. 13C MRS with low flip‐angle excitations and a multichannel 13C/1H dual‐frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice‐wise areas under the curve (AUCs) of 13C‐metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and . Linear regression analysis between brain volumes and HP signals was performed. Region‐focused pyruvate metabolism was estimated using coil‐wise 13C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45‐minutes interval.Results[1‐13C]Lactate relative to the total 13C signal (tC) was 0.21–0.24 in all slices. [13C]/tC was 0.065–0.091. Apparent conversion rate constants from pyruvate to lactate and were calculated as 0.014–0.018 s−1 and 0.0043–0.0056 s−1, respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1‐13C]pyruvate.ConclusionsDynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1‐13C]pyruvate.

OBJECTIVE:Most MRI scanners are equipped to receive signals from 1H array coils but few support multi-channel reception for other nuclei. Using receive arrays can provide significant SNR benefits, usually exploited to enable accelerated imaging, but the extension of these arrays to non-1H nuclei has received less attention because of the relative lack of broadband array receivers. Non-1H nuclei often have low sensitivity and stand to benefit greatly from the increase in SNR that arrays can provide. This paper presents a cost-effective approach for adapting standard 1H multi-channel array receivers for use with other nuclei - in this case, 13C.METHODS:A frequency translation system has been developed that uses active mixers residing at the magnet bore to convert the received signal from a non-1H array to the 1H frequency for reception by the host system receiver.RESULTS:This system has been demonstrated at 4.7T and 7T while preserving SNR and isolation. 1H decoupling, particularly important for 13C detection, can be straightforwardly accommodated.CONCLUSION:Frequency translation can convert 1H-only multi-channel receivers for use with other nuclei while maintaining SNR and channel isolation while still enabling 1H decoupling.SIGNIFICANCE:This work allows existing multi-channel MRI receivers to be adapted to receive signals from nuclei other than 1H, allowing for the use of receive arrays for in vivo multi-nuclear NMR.