Multiple slot modules for high field magnetic resonance imaging array coils.

PURPOSE: Mitigating 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. METHODS: Two 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 B 1 + P in $$ \frac{B_1^{+}}{\sqrt{P_{in}}} $$ 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. RESULTS: Bench 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 B1 + homogeneity when compared with an eight-dipole coil. CONCLUSION: Although 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.

Wideband receive-coil array design using high-impedance amplifiers for broadband decoupling.

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 23 Na and 2 H at 4.7T. This multi-tuned prototype obtained 61% and 76% of the imaging SNR at 2 H and 23 Na 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.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans.

PURPOSE: This study is to investigate time-resolved 13 C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-13 C]pyruvate in the human brain. METHODS: Time-resolved 13 C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-13 C]pyruvate. 13 C MRS with low flip-angle excitations and a multichannel 13 C/1 H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of 13 C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise 13 C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval. RESULTS: [1-13 C]Lactate relative to the total 13 C signal (tC) was 0.21-0.24 in all slices. [13 C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- 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 [1-13 C]pyruvate. CONCLUSIONS: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-13 C]pyruvate.

A retrofit to enable dynamic B1+ steering for transmit arrays without multiple amplifiers.

PURPOSE: B1+ 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 B1+ steering on a 7T MR scanner with only two power amplifiers. METHODS: Power 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. RESULTS: The phase conjugate B1+ 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 B1+ were generated, indicating good Tx steering to the selected region. CONCLUSION: The 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 B1+ shimming with increased degrees of freedom but without significantly increased hardware cost.

A 32-channel receive array coil for bilateral breast imaging and spectroscopy at 7T.

PURPOSE: This work describes the construction and evaluation of a bilateral 32-channel receive array for breast imaging at 7T. METHODS: The receive array consisted of 32 receive coils, placed on two 3D-printed hemispherical formers. Each side of the receive array consisted of 16 receive loops, each loop having a corresponding detachable board with match/tune capacitors, active detuning circuitry, and a balun. Coil performance was evaluated on homogeneous canola oil phantoms using a Philips Achieva 7T system. Array coil performance was compared with a bilateral forced current excitation volume coil in transmit/receive mode and with a previously reported 16-channel unilateral coil with a similar design. RESULTS: The 32-channel array had an increase in average SNR throughout both phantoms by a factor of five as compared with the volume coil, with SNR increases up to 10 times along the periphery and three times in the center. Noise measurements showed low interelement noise correlation (average: 5.4%; maximum: 16.8%). Geometry factor maps were acquired for various acceleration factors and showed mean geometry factors <1.2, for combined acceleration factors of up to six. CONCLUSIONS: The improvements achieved demonstrate the clear potential for use in dynamic contrast-enhanced or diffusion-weighted MR studies, while maintaining diagnostically relevant spatial and temporal resolutions.

Chemical Modifications of Porous Shape Memory Polymers for Enhanced X-ray and MRI Visibility.

The goal of this work was to develop a shape memory polymer (SMP) foam with visibility under both X-ray and magnetic resonance imaging (MRI) modalities. A porous polymeric material with these properties is desirable in medical device development for applications requiring thermoresponsive tissue scaffolds with clinical imaging capabilities. Dual modality visibility was achieved by chemically incorporating monomers with X-ray visible iodine-motifs and MRI visible monomers with gadolinium content. Physical and thermomechanical characterization showed the effect of increased gadopentetic acid (GPA) on shape memory behavior. Multiple compositions showed brightening effects in pilot, T1-weighted MR imaging. There was a correlation between the polymeric density and X-ray visibility on expanded and compressed SMP foams. Additionally, extractions and indirect cytocompatibility studies were performed to address toxicity concerns of gadolinium-based contrast agents (GBCAs). This material platform has the potential to be used in a variety of medical devices.

Trap Design and Construction for High-Power Multinuclear Magnetic Resonance Experiments.

Performing multinuclear experiments requires one or more radiofrequency (RF) coils operating at both the proton and second-nucleus frequencies; however, inductive coupling between coils must be mitigated to retain proton sensitivity and coil tuning stability. The inclusion of trap circuits simplifies placement of multinuclear RF coils while maintaining inter-element isolation. Of the commonly investigated non-proton nuclei, perhaps the most technically demanding is carbon-13, particularly when applying a proton decoupling scheme to improve the resulting spectra. This work presents experimental data for trap circuits withstanding high-power broadband proton decoupling of carbon-13 at 7 T. The advantages and challenges of building trap circuits with various inductor and capacitor components are discussed. Multiple trap designs are evaluated on the bench and utilized on an RF coil at 7 T to detect broadband proton-decoupled carbon-13 spectra from a lipid phantom. A particular trap design, built from a coaxial stub inductor and high-voltage ceramic chip capacitors, is highlighted owing to both its performance and adaptability for planar array coil elements with diverse spatial orientations.

Are comparisons of patient experiences across hospitals fair? A study in Veterans Health Administration hospitals.

BACKGROUND: Surveys are increasingly used to assess patient experiences with health care. Comparisons of hospital scores based on patient experience surveys should be adjusted for patient characteristics that might affect survey results. Such characteristics are commonly drawn from patient surveys that collect little, if any, clinical information. Consequently some hospitals, especially those treating particularly complex patients, have been concerned that standard adjustment methods do not adequately reflect the challenges of treating their patients. OBJECTIVES: To compare scores for different types of hospitals after making adjustments using only survey-reported patient characteristics and using more complete clinical and hospital information. RESEARCH DESIGN: We used clinical and survey data from a national sample of 1858 veterans hospitalized for an initial acute myocardial infarction (AMI) in a Department of Veterans Affairs (VA) medical center during fiscal years 2003 and 2004. We used VA administrative data to characterize hospitals. The survey asked patients about their experiences with hospital care. The clinical data included 14 measures abstracted from medical records that are predictive of survival after an AMI. RESULTS: Comparisons of scores across hospitals adjusted only for patient-reported health status and sociodemographic characteristics were similar to those that also adjusted for patient clinical characteristics; the Spearman rank-order correlations between the 2 sets of adjusted scores were >0.97 across 9 dimensions of inpatient experience. CONCLUSIONS: This study did not support concerns that measures of patient care experiences are unfair because commonly used models do not adjust adequately for potentially confounding patient clinical characteristics.

Quadrature transmit coil for breast imaging at 7 tesla using forced current excitation for improved homogeneity.

PURPOSE: To demonstrate the use of forced current excitation (FCE) to create homogeneous excitation of the breast at 7 tesla, insensitive to the effects of asymmetries in the electrical environment. MATERIALS AND METHODS: FCE was implemented on two breast coils: one for quadrature (1) H imaging and one for proton-decoupled (13) C spectroscopy. Both were a Helmholtz-saddle combination, with the saddle tuned to 298 MHz for imaging and 75 MHz for spectroscopy. Bench measurements were acquired to demonstrate the ability to force equal currents on elements in the presence of asymmetric loading to improve homogeneity. Modeling and temperature measurements were conducted per safety protocol. B1 mapping, imaging, and proton-decoupled (13) C spectroscopy were demonstrated in vivo. RESULTS: Using FCE to ensure balanced currents on elements enabled straightforward tuning and maintaining of isolation between quadrature elements of the coil. Modeling and bench measurements confirmed homogeneity of the field, which resulted in images with excellent fat suppression and in broadband proton-decoupled carbon-13 spectra. CONCLUSION: FCE is a straightforward approach to ensure equal currents on multiple coil elements and a homogeneous excitation field, insensitive to the effects of asymmetries in the electrical environment. This enabled effective breast imaging and proton-decoupled carbon-13 spectroscopy at 7T.

A Review of Current Control and Decoupling Methods for MRI Transmit Arrays.

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.

A Novel Voltage Controlled Decoupling Method for Transmit Coils in MRI.

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 B0 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.

A Four-Channel Broadband MRI Receive Array Coil.

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 2H 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 23Na and 31P 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.

Low-cost MRI System for Teaching.

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.

A parallel imaging approach to wide-field MR microscopy.

Magnetic resonance microscopy, suggested in the earliest papers on MRI, has always been limited by the low signal-to-noise ratio resulting from the small voxel size. Magnetic resonance microscopy has largely been enabled by the use of microcoils that provide the signal-to-noise ratio improvement required to overcome this limitation. Concomitant with the small coils is a small field-of-view, which limits the use of magnetic resonance microscopy as a histological tool or for imaging large regions in general. This article describes initial results in wide field-of-view magnetic resonance microscopy using a large array of narrow, parallel coils, which provides a signal-to-noise ratio enhancement as well as the ability to use parallel imaging techniques. Comparison images made between a volume coil and the proposed technique demonstrate reductions in imaging time of more than 100 with no loss in signal-to-noise ratio or resolution.

Rapid MR Scanner Independent B1 Field Measurement System for Phased Arrays.

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.

Assessing the Feasibility of Dynamic 31P Spectroscopy for Metabolic Studies With a 1.0T Extremity Scanner.

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.

A Frequency Translation System for Multi-Channel, Multi-Nuclear MR Spectroscopy.

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.

A cross-sectional, multi-year examination of rural and urban Veterans Administration users: 2002-2006.

We compared demographic profiles across two rural-urban classification schemes to determine whether rural-urban disparities in health status persisted among Veterans Administration (VA) users over time. Using demographic and SF-12 survey data collected from 2002 to 2006, we conducted serial cross-sectional analyses of demographic variables and health status for veterans residing in VA- and rural-urban commuting area (RUCA)-defined rural-urban groups. VA and RUCA definitions yielded similar results for the "urban" population; however, VA- and RUCA-defined "rural" categories represent dissimilar populations. Compared to earlier years, the VA user population in 2006 was younger, more educated, wealthier, and more likely to be employed and privately insured. For all years and using both VA and RUCA rural-urban definitions, physical component summary (PCS) scores were lower but mental component summary (MCS) scores were slightly higher for more rural compared to urban veterans. Anticipating and meeting the needs of rural VA users will require accurate identification of those who lack access to services and therefore defining "rural" appropriately.

A Review of Non-1H RF Receive Arrays in Magnetic Resonance Imaging and Spectroscopy.

It is now common practice to use radiofrequency (RF) coils to increase the signal-to-noise ratio (SNR) in 1H magnetic resonance imaging and spectroscopy experiments. Use of array coils for non-1H experiments, however, has been historically more limited despite the fact that these nuclei suffer inherently lower sensitivity and could benefit greatly from an increased SNR. Recent advancements in receiver technology and increased support from scanner manufacturers have now opened greater options for the use of array coils for non-1H magnetic resonance experiments. This paper reviews the research in adopting array coil technology with an emphasis on studies of the most commonly studied non-1H nuclei including 31P, 13C, 23Na, and 19F. These nuclei offer complementary information to 1H imaging and spectroscopy and have proven themselves important in the study of numerous disease processes. While recent work with non-1H array coils has shown promising results, the technology is not yet widely utilized and should see substantial developments in the coming years.

Multi-Tuned Cable Traps for Multinuclear MRI and MRS.

OBJECTIVE: The method of pole-insertion for multi-tuning cable traps was studied for multinuclear MRI and MRS applications. METHODS: Relative efficiency of the different cable trap modes was studied as component values were varied and at four different magnetic field strengths. In all cases, efficiencies were compared to equivalent single-tuned designs. RESULTS: The multi-tuned traps were able to block shield currents at multiple frequencies with only slightly degraded efficiencies as compared to their single-tuned counterparts. As in double-tuned coil design, the cable trap effectiveness at each frequency was found to be highly dependent on the trap inductor value with larger trap inductances leading to worse efficiency at the lower frequency but better efficiency at the higher frequency. This relationship held at all field strengths examined. CONCLUSION: This work presents design guidelines for the double-tuning method that are useful when designing RF coils for multinuclear studies. The design takes up less space than using two single-tuned cable traps mounted in series as is commonly done. Triple-tuned and "floating" designs were also demonstrated as proofs-of-concept for a single field strength and showed great promise to prove similarly useful in future studies. SIGNIFICANCE: For many applications such as when using high-density array coils, finding a space-efficient solution to eliminate common-mode currents could be of significant benefit. This multi-tuned approach provides space efficiency at a small cost in trapping efficiency.