MRI Coil Research Articles

Design and construction of new small gradient coils for mouse MRI

We represent small Z- and Y-gradient coils with higher gradient strength and slew rate for mouse MRI. The coil geometries are optimized based on predetermined conditions for the gradient strength and slew rate in terms of minimum power consumption and acceptable linearity for the variation of the magnetic fields using the particle swarm optimization algorithm in computer simulation technology. The simulation results verify that the magnitudes of the field strength and slew rate have reached up to and for the Z-gradient coil and increased to and for the Y-gradient coil, respectively. Furthermore, the optimal geometry of the Z-gradient coil derived from a simulation study was then fabricated, and its parameters were measured using a Hall sensor. The gradient strength and slew rate of the fabricated Z-gradient coil were measured at and , respectively, which have only a 9% difference from the corresponding simulated parameters.

Stretchable receive coil for 7T small animal MRI

Receive coils used in small animal MRI are rigid, inflexible surface loops that do not conform to the anatomy being imaged. The recent trend toward design of stretchable coils that are tailored to fit any anatomical curvature has been focused on human imaging. This work demonstrates the application of stretchable coils for small animal imaging at 7T. A stretchable coil measuring 3.5 × 3.5 cm was developed for acquisition of rat brain and spine images. The SNR maps of the stretchable coil were compared with those of a traditional flexible PCB coil and a commercial surface coil. Stretch and conformance testing of the coil was performed. Ex vivo images of rat brain and spine from the stretchable a coil was acquired using T1 FLASH and T2 Turbo RARE sequences. The axial phantom SNR maps showed that the stretchable coil provided 48.5% and 42.8% higher SNR than the commercial coil for T1-w and T2-w images within the defined ROI. A 33% increase in average penetration depth was observed within the ROI using the stretchable coil when compared to the commercial coil. The ex-vivo rat brain and spine images showed distinguishable anatomical details. Stretching the coil reduced the resonant frequency with reduction in SNR, while the conformance to varying sample volumes increased the resonant frequency with decreased SNR. This study also features an open-source plug-and-play system with preamplifiers that can be used to interface surface coils with the 7T Bruker scanner.

Design and application of a multimodality-compatible 1Tx/6Rx RF coil for monkey brain MRI at 7T

ObjectiveBlood-oxygen-level-dependent functional MRI allows to investigte neural activities and connectivity. While the non-human primate plays an essential role in neuroscience research, multimodal methods combining functional MRI with other neuroimaging and neuromodulation enable us to understand the brain network at multiple scales. ApproachIn this study, a tight-fitting helmet-shape receive array with a single transmit loop for anesthetized macaque brain MRI at 7T was fabricated with four openings constructed in the coil housing to accommodate multimodal devices, and the coil performance was quantitatively evaluated and compared to a commercial knee coil. In addition, experiments over three macaques with infrared neural stimulation (INS), focused ultrasound stimulation (FUS), and transcranial direct current stimulation (tDCS) were conducted. Main resultsThe RF coil showed higher transmit efficiency, comparable homogeneity, improved SNR and enlarged signal coverage over the macaque brain. Infrared neural stimulation was applied to the amygdala in deep brain region, and activations in stimulation sites and connected sites were detected, with the connectivity consistent with anatomical information. Focused ultrasound stimulation was applied to the left visual cortex, and activations were acquired along the ultrasound traveling path, with all time course curves consistent with pre-designed paradigms. The existence of transcranial direct current stimulation electrodes brought no interference to the RF system, as evidenced through high-resolution MPRAGE structure images. SignificanceThis pilot study reveals the feasibility for brain investigation at multiple spatiotemporal scales, which may advance our understanding in dynamic brain networks.

Inductively coupled, transmit-receive coils for proton MRI and X-nucleus MRI/MRS in small animals

We report several inductively coupled RF coil designs that are very easy to construct, produce high signal-to-noise ratio (SNR) and high spatial resolution while accommodating life support, anesthesia and monitoring in small animals. Inductively coupled surface coils were designed for hyperpolarized 13 C MR spectroscopic imaging (MRSI) of mouse brain, with emphases on the simplicity of the circuit design, ease of use, whole-brain coverage, and high SNR. The simplest form was a resonant loop designed to crown the mouse head for a snug fit to achieve full coverage of the brain with high sensitivity when inductively coupled to a broadband pick-up coil. Here, we demonstrated the coil's performance in hyperpolarized 13 C MRSI of a normal mouse and a glioblastoma mouse model at 4.7 T. High SNR exceeding 70:1 was obtained in the brain with good spatial resolution (1.53 mm x 1.53 mm). Similar inductively coupled loop for other X-nuclei can be made very easily in a few minutes and achieve high performance, as demonstrated in 31 P spectroscopy. Similar design concept was expanded to splitable, inductively coupled volume coils for high-resolution proton MRI of marmoset at 3T and 9.4T, to easily accommodate head restraint, vital-sign monitoring, and anesthesia delivery.

Manipulating Bandwidth of Cryogenically Cooled Receive-Only MRI Detectors

Cryogenically cooled MRI coils demonstrate extremely high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factors leading to a very narrow bandwidth. This bandwidth can be increased in a controlled manner using a preamplifier decoupling. In this work, an MRI coil design procedure is explained and an example showing over 2 times bandwidth increase in a cryogenic coil for 13C isotope imaging in 3-tesla MRI systems is demonstrated. The achieved wider bandwidth preserves the spectral components of magnetic resonance signals and makes coils less susceptive to detuning due to parasitic coupling to surrounding structures. The presented method is also useful in other applications of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> coils, for instance, in low field magnetic resonance imaging systems.

A Study of the Improvement in Signal-To-Noise Ratio Using Two Different MRI Coils in a Toe Fat Removal Test

A Study of the Improvement in Signal-To-Noise Ratio Using Two Different MRI Coils in a Toe Fat Removal Test

Investigating Local Receive Arrays in tcMRgFUS System and Their Influence by Passive Antennas: A Simulation Study.

Transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) revolutionizes non-invasive therapy by combining MRI and high-intensity focused ultrasound for precise thermal treatment. MRI scans play an essential role during tcMRgFUS treatment in that they are used to localize the target and monitor temperature. Using the body coil for MRI introduces imaging challenges, notably extremely low signal-to-noise ratio (SNR) and a distinct dark band in 3 Tesla brain images. This study explores the impact of diverse local receive coils on SNR and parallel imaging capabilities in tcMRgFUS. Simulation results underscore the significant SNR enhancement, especially with helmet-shaped coils, crucial for capturing signals from the head's top and sides. Additionally, the study delves into integrating passive antennas to address the dark band issue, revealing a combined improvement in SNR and transmit field recovery. The study demonstrates that even a coil array outside the water bath can enhance SNR. This work offers critical insights into optimizing the imaging quality, improving temperature mapping accuracy, and recovering the transmit field in tcMRgFUS technology, holding potential for refined treatment visualization, targeting precision, and real-time monitoring.

A truncated twisted solenoid RF phase gradient transmit coil for TRASE MRI

Transmit array spatial encoding (TRASE) is an MR imaging technique that achieves k-space encoding through the use of phase gradients in the RF transmit field. Without requiring B0 gradient fields, TRASE MRI can be performed using significantly cheaper bi-planar permanent magnets or Halbach arrays. For TRASE encoding with these magnets, the twisted solenoid has been demonstrated as the most efficient RF transmit coil; however, this specific geometry results in a long coil with a relatively short imaging volume. We introduce a new truncated design to increase the usable imaging volume relative to the coil length. Based on simulations of optimal parameters, a 200 mm long, 100 mm inner diameter coil pair was constructed with an imaging volume 100 mm in length and 80 mm in diameter. The coil pair was tested using an un-shimmed 2.84 MHz Halbach array. Results indicate the truncated design can create a similar imaging volume and quality to the untruncated version whilst significantly reducing the length of the coil by as much as a half.

Design of standalone wireless impedance matching (SWIM) system for RF coils in MRI

The radio frequency (RF) power transfer efficiency of transmit coils and the signal-to-noise ratio (SNR) at the receive signal chain are directly dependent on the impedance matching condition presented by a loaded coil, tuned to the Larmor frequency. Sub-optimal impedance condition of receive coils significantly reduces coil sensitivity and image quality. In this study we propose a Standalone Wireless Impedance Matching (SWIM) system for RF coils to automatically compensate for the impedance mismatch caused by the loading effect at the target frequency. SWIM uses a built-in RF generator to produce a calibration signal, measure reflected power as feedback for loading change, and determine an optimal impedance. The matching network consists of a capacitor array with micro-electromechanical system (MEMS) RF switches to electronically cycle through different input impedance conditions. Along with automatic calibration, SWIM can also perform software detuning of RF receive coils. An Android mobile application was developed for real-time reflected power monitoring and controlling the SWIM system via Bluetooth. The SWIM system can automatically calibrate an RF coil in 3 s and the saline sample SNR was improved by 24% when compared to a loaded coil without retuning. Four different tomatoes were imaged to validate the performance of SWIM.

Vacuum formed coils for MRI.

To describe a digital fabrication method used for custom MRI receive coils with vacuum forming and electroless copper plating. Our process produces intricate copper traces on curved surfaces. A three-dimensional scan of a desired anatomy is obtained and used to design coil elements. The layout is predistorted with a self built simulation of the vacuum forming process and the geometric overlaps are tested with electromagnetic simulation software. The desired coil geometry is patterned onto a polycarbonate sheet by sandblasting through a tape mask. The sandblasted areas are then catalyzed with a palladium-tin solution and vacuum formed. The catalyzed, three-dimensional part is placed into a custom built plating tank and copper plated. Electronic components are attached to the copper traces to form resonant receive coils. The methods described here are demonstrated and tested with an 8 channel visual cortex coil array. The prototype coils exhibit quality factor ratios higher than three, indicating body noise dominance. The coil array shows high signal-to-noise ratio (SNR) near the periphery of a head shaped phantom. In vivo images with up to spatial resolution were acquired on a human volunteer. This work presents the first example of vacuum formed coils with direct electroless copper plating. Our fabrication method results in coil arrays that are in close proximity to the body. This methods described here may enable the rapid development of a set of coils of different sizes for applications including longitudinal fMRI studies and MR-guided therapies.

Quadrature transceive wireless coil: Design concept and application for bilateral breast MRI at 1.5 T.

Development of a novel quadrature inductively driven transceive wireless coil for breast MRI at 1.5 T. A quadrature wireless coil (HHMM-coil) design has been developed as a combination of two linearly polarized coils: a pair of 'metasolenoid' coils (MM-coil) and a pair of Helmholtz-type coils (HH-coil). The MM-coil consisted of an array of split-loop resonators. The HH-coil design included two electrically connected flat spirals. All the wireless coils were coupled to a whole-body birdcage coil. The HHMM-coil was studied and compared to the linear coils in terms of transmit and SAR efficiencies via numerical simulations. A prototype of HHMM-coil was built and tested on a 1.5 T scanner in a phantom and healthy volunteer. We also proposed an extended design of the HHMM-coil and compared its performance to a dedicated breast array. Numerical simulations of the HHMM-coil with a female voxel model have shown more than a 2.5-fold increase in transmit efficiency and a 1.7-fold enhancement of SAR efficiency compared to the linearly polarized coils. Phantom and in vivo imaging showed good agreement with the numerical simulations. Moreover, the HHMM-coil provided good image quality, visualizing all areas of interest similar to a multichannel breast array with a 32% reduction in signal-to-noise ratio. The proposed quadrature HHMM-coil allows the -field to be significantly better focused in the region-of-interest compared to the linearly polarized coils. Thus, the HHMM-coil provides high-quality breast imaging on a 1.5 T scanner using a whole-body birdcage coil for transmit and receive.

Low-cost inductively coupled stacked wireless RF coil for MRI at 3 T.

Inductively coupled RF coils are an inexpensive and simple method to realize wireless RF coils in MRI. They are low cost and can greatly ease the MR scan setup and improve patient comfort, since they do not require bulky components such as cables, baluns, preamplifiers, and connectors. Previous works have typically used single-layer loops as wireless coils. In this work, we present a novel wireless coil, where two loops are stacked and decoupled with a shared capacitor. We found that such a stacked structure could increase the coil efficiency and SNR. Compared with the single-layer wireless coil, both electromagnetic simulation and MR experiment results demonstrate that the stacked wireless coil has a considerable SNR improvement of approximately 35%.

RF shielding designs for birdcage coils for preclinical MRI at 9.4 T

Birdcage coils are widely used in preclinical MRI as they perform well, allow for quadrature drive, and can provide a homogeneous transmit field. Unlike in larger bore scanners, an RF shield is essential to avoid strong cross-talk with gradient coils that are in close proximity. However, gradient switching induces eddy currents that heat the shield and coil and impair the temporal signal-to-noise ratio (tSNR). The motivation of this study is to investigate the performance of different designs of RF shields on a birdcage coil used for high resolution functional MRI of small primates at 9.4 T. We found the choice of materials for RF shields significantly affected ghosting and tSNR in fMRI scans. Both ultrathin foils and a slotted pattern reduce eddy currents and improve imaging quality. Our results also demonstrate that a 9-um-thick copper foil is sufficiently thin to reduce the eddy current effects for high-resolution fMRI scans and there is no need for high-cost 4-um-thick foil. For slotted shields, our results demonstrate that the number of slots should be carefully considered, and an excessive number of slots can lead to a lower SNR and tSNR. We believe the results from this study can be used as a reference to design future RF coil shields selection for preclinical scanners.

Deformation Analysis of No-Insulation REBCO Coils Considering Turn-to-Turn Contact Configuration

In recent year, there has been remarkable progress in the use of high-field rare-earth barium copper oxide (REBCO) coils in NMR, MRI, and accelerator applications. No-insulation (NI) winding techniques are adopted to achieve high thermal stability and high current density. Thus, each winding in a non-impregnated NI REBCO coil can deform separately and move freely because of the thermal strain and electromagnetic force. The additional force and stress due to a screening current has also become an issue in REBCO coils. The screening current leads to a non-uniform force distribution in REBCO tapes and forces exceeding an acceptable value. It has been reported that REBCO coils have experienced mechanical deterioration or damage when operating under high magnetic fields. However, the detailed mechanism for this damage or deterioration has not been clarified. In order to clarify the detailed mechanism of this damage to a REBCO coil, the stress and strain during the winding, cooling down, charging, and discharging processes need to be evaluated. Furthermore, in NI REBCO coils, the turn-to-turn contact configurations are very important for the current bypassing, the thermal and mechanical stability. In previous studies, we have developed the numerical simulation code on the electromagnetic behavior of screening current and mechanical behavior of the turn-to-turn contact state. This study numerically evaluated the deformation and stress in REBCO tape and the turn-to-turn contact in NI REBCO coils.

Evaluation of a novel 8-channel RX coil for speech production MRI at 0.55T.

Speech production MRI benefits from lower magnetic fields due to reduced off-resonance effects at air-tissue interfaces and from the use of dedicated receiver coils due to higher SNR and parallel imaging capability. Here we present a custom designed upper airway coil for 1H imaging at 0.55 Tesla and evaluate its performance in comparison with a vendor-provided prototype 16-channel head/neck coil. Four adult volunteers were scanned with both custom speech and prototype head-neck coils. We evaluated SNR gains of each of the coils over eleven upper airway volumes-of-interest measured relative to the integrated body coil. We evaluated parallel imaging performance of both coils by computing g-factors for SENSE reconstruction of uniform and variable density Cartesian sampling schemes with R = 2, 3, and 4. The dedicated coil shows approximately 3.5-fold SNR efficiency compared to the head-neck coil. For R = 2 and 3, both uniform and variable density samplings have g-factor values below 1.1 in the upper airway region. For R = 4, g-factor values are higher for both trajectories. The dedicated coil configuration allows for a significant SNR gain over the head-neck coil in the articulators. This, along with favorable g values, makes the coil useful in speech production MRI.

Improving protocols for whole-body magnetic resonance imaging: oncological and inflammatory applications.

Whole-body MRI is increasingly used in the evaluation of a range of oncological and non-oncological diseases in infants, children and adolescents. Technical innovation in MRI scanners, coils and sequences have enabled whole-body MRI to be performed more rapidly, offering large field-of-view imaging suitable for multifocal and multisystem disease processes in a clinically useful timeframe. Together with a lack of ionizing radiation, this makes whole-body MRI especially attractive in the pediatric population. Indications include lesion detection in cancer predisposition syndrome surveillance and in the workup of children with known malignancies, and diagnosis and monitoring of a host of infectious and non-infectious inflammatory conditions. Choosing which patients are most likely to benefit from this technology is crucial, but so is adjusting protocols to the patient and disease to optimize lesion detection. The focus of this review is on protocols and the elements impacting image acquisition in pediatric whole-body MRI. We consider the practical aspects, from scanner and coil selection to patient positioning, single-center generic and indication-specific protocols with technical parameters, motion reduction strategies and post-processing. When optimized, collectively these lead to better standardization of whole-body MRI, and when married to systematic analysis and interpretation, they can improve diagnostic accuracy.

Development of an Animal Stereotactic Device for Preclinical Research on Tumor Response After Stereotactic Radiosurgery

In gamma knife radiosurgery, the tumor response to radiation is an important predictor of clinical treatment results. Since brain tumors have different characteristics and growth patterns, depending on the type, the tumors' response to radiation are also different. Compared with various other clinical treatments, there is a dearth of research on the development of gamma knife-magnetic resonance imaging (MRI) preclinical experimental equipment. Hence, the identification of preclinical equipment necessity for experimental animals will provide meaningful data for the provision of clinical assistance to humans. A device for stereotactic radiosurgery capable of MRI in small animals was developed. The feasibility of creating a preplan by means of small animal images was then assessed. A device for stereotaxic surgery of small animals using a 48-channel MRI coil was developed using a 3 dimensional printer. Rat brain-MRI images were obtained with a 3.0T MRI scanner using a multi-channel coil. The acquired MRI images were transferred to a GammaPlan workstation to establish a preplan. To gamma rays to the targeted site on animals, a positioning device combined with a G-frame was mounted on a gamma knife. Planning of radiosurgery based on MRI images became possible with GammaPlan workstations. Preclinical experiments using small animals are possible with the use of stereotactic devices. In clinical treatment, preclinical experimental results will provide meaningful information.

Predefined magnetic field formation with parallel transmit coils in MRI

Predefined magnetic field formation with parallel transmit coils in MRI. / M. Kelenjeridze. / Nano Studies. – 2020. – # 20. – pp. 31-36. – geo. Predefined tailored field of excitation, that is granted by the parallel transmit coils, is a huge potential technology in MRI. It enables choosing the area in the field of view for localized imaging, which makes a considerable reduction of the imaging time. Fig. 3, Ref. 6.

A z-gradient array coil with a dedicated active-shielded array coil for MRI.

An array-based z-gradient coil with a set of programmable power amplifiers can outperform a conventional z-gradient coil and make it highly customizable with a broader range of tunable features. A dynamically adjustable imaging volume can be achieved using a pair of independent arrays and a modified optimization procedure based on analytic equations. Two modes of operation are provided: (a) standard mode that resembles a conventional coil; (b) advanced mode, where all performance parameters can be adjusted employing a controllable feeding mechanism. Commercial software is used to demonstrate the validity and feasibility of the proposed coil. Primary and shield array diameters are 24 and 30 cm, both of which comprise 12 bundles of 10 turns copper wires. Maximum feeding voltage/current is 250 V/100 A for all array elements. Four distinct magnetic profiles are provided: (a) conventional profile with 140 mm diameter spherical region of interest, 120 mT/m gradient, and up to 4500 T/m/s slew rate; (b) profile of 200 mT/m, 70 mm region of interest, and up to 6900 T/m/s slew rate; (c) 60 mm axially shifted 70 mm region of interest with 120 mT/m strength and 3600 T/m/s slew rate; and (d) profile of 370 mT/m, 120 mm region of interest, and 3700 T/m/s slew rate when the active shield is reverse fed. By using an active-shielded gradient array coil, the magnetic field profile of the imaging volume can be adjusted dynamically, and it can provide new features and a wide range of field profiles for diverse applications in MRI.

Improved total sensitivity estimation for multiple receive coils in MRI using ratios of first-order statistics.

Spatial variation in the sensitivity profiles of receive coils in MRI leads to spatially dependent scaling of the signal amplitude across an image. In practice, total sensitivity of the coil array is either calibrated or corrected directly by comparison to a uniform sensitivity image, fitting of coil profiles, or indirectly by constraining the reconstructed image or coil profiles. In the absence of these corrections, popular coil summation strategies are often designed to maximize the signal-to-noise ratio or optimize under-sampled encoding but not necessarily estimate the value of the signal unscaled by the coil spatial sensitivity. We use ratios of first-order statistics to approach the unscaled value of the signal at any position. Motivated by the assumption that the coil array is a sample from much larger number of possible coils, we present two approaches to scale the mean signal in all coils: (1) an argument for use of the mode of the normalized signals, and (2) using a one-dimensional analog derive an approximate expression for scaling with the ratio of the square-of-the-mean to the mean-of-the-squares. We test these approaches with simulation where idealized coil elements are arrayed around an object, and on directly acquired data with an 8-channel coil array on a uniform 13C phantom, and on Hyperpolarized 13C pyruvate brain MRI. We show improved image uniformity using the ratios of first order statistics compared to a simple homomorphic filter, noting that these approaches are more sensitive to noise. We present simple methods for correcting the spatial variation in sensitivity profiles in the context of a coil array. These methods can be used as an initial or adjunct step in data post-processing.