Dual-Channel Stretchable, Self-Tuning, Liquid Metal Coils and Their Fabrication Techniques.

Flexible and stretchable radiofrequency coils for magnetic resonance imaging represent an emerging and rapidly growing field. The main advantage of such coil designs is their conformal nature, enabling a closer anatomical fit, patient comfort, and freedom of movement. Previously, we demonstrated a proof-of-concept single element stretchable coil design with a self-tuning smart geometry. In this work, we evaluate the feasibility of scaling this coil concept to a multi-element coil array and the associated engineering and manufacturing challenges. To this goal, we study a dual-channel coil array using full-wave simulations, bench testing, in vitro, and in vivo imaging in a 3 T scanner. We use three fabrication techniques to manufacture dual-channel receive coil arrays: (1) single-layer casting, (2) double-layer casting, and (3) direct-ink-writing. All fabricated arrays perform equally well on the bench and produce similar sensitivity maps. The direct-ink-writing method is found to be the most advantageous fabrication technique for fabrication speed, accuracy, repeatability, and total coil array thickness (0.6 mm). Bench tests show excellent frequency stability of 128 ± 0.6 MHz (0% to 30% stretch). Compared to a commercial knee coil array, the stretchable coil array is more conformal to anatomy and provides 50% improved signal-to-noise ratio in the region of interest.

Silicone-based materials with tailored MR relaxation characteristics for use in reduced coil visibility and in tissue-mimicking phantom design.

BACKGROUND: The development of materials with tailored signal intensity in MR imaging is critically important both for the reduction of signal from non-tissue hardware, as well as for the construction of tissue-mimicking phantoms. Silicone-based phantoms are becoming more popular due to their structural stability, stretchability, longer shelf life, and ease of handling, as well as for their application in dynamic imaging of physiology in motion. Moreover, silicone can be also used for the design of stretchable receive radio-frequency (RF) coils. PURPOSE: Fabrication of materials with tailored signal intensity for MRI requires knowledge of precise T1 and T2 relaxation times of the materials used. In order to increase the range of possible relaxation times, silicone materials can be doped with gadolinium (Gd). In this work, we aim to systematically evaluate relaxation properties of Gd-doped silicone material at a broad range of Gd concentrations and at three clinically relevant magnetic field strengths (1.5 T, 3 T, and 7 T). We apply the findings for rendering silicone substrates of stretchable receive RF coils less visible in MRI. Moreover, we demonstrate early stage proof-of-concept applicability in tissue-mimicking phantom development. MATERIALS AND METHODS: Ten samples of pure and Gd-doped Ecoflex silicone polymer samples were prepared with various Gd volume ratios ranging from 1:5000 to 1:10, and studied using 1.5 T and 3 T clinical and 7 T preclinical scanners. T1 and T2 relaxation times of each sample were derived by fitting the data to Bloch signal intensity equations. A receive coil made from Gd-doped Ecoflex silicone polymer was fabricated and evaluated in vitro at 3 T. RESULTS: With the addition of a Gd-based contrast agent, it is possible to significantly change T2 relaxation times of Ecoflex silicone polymer (from 213 ms to 20 ms at 1.5 T; from 135 ms to 17 ms at 3 T; and from 111.4 ms to 17.2 ms at 7 T). T1 relaxation time is less affected by the introduction of the contrast agent (changes from 608 ms to 579 ms; from 802.5 ms to 713 ms at 3 T; from 1276 ms to 979 ms at 7 T). First results also indicate that liver, pancreas, and white matter tissues can potentially be closely mimicked using this phantom preparation technique. Gd-doping reduces the appearance of the silicone-based coil substrate during the MR scan by up to 81%. CONCLUSIONS: Gd-based contrast agents can be effectively used to create Ecoflex silicone polymer-based phantoms with tailored T2 relaxation properties. The relative low cost, ease of preparation, stretchability, mechanical stability, and long shelf life of Ecoflex silicone polymer all make it a good candidate for "MR invisible" coil development and bears promise for tissue-mimicking phantom development applicability.

Caterpillar traps: A highly flexible, distributed system of toroidal cable traps.

PURPOSE: Coil arrays are connected to the main MRI system with long, shielded coaxial cables. RF coupling of these cables to the main transmit coil can cause high shield currents, which pose risks of heating and RF burns. High-blocking resonant RF traps are placed at distinct positions along cables to mitigate these currents. Traditional traps are designed to be stiff to avoid changes in their resonant frequency, hindering the overall system flexibility. Instead of using a few high-blocking traps, we propose the use of caterpillar traps-a distributed system of small, elastic traps that cover the full length of cables. METHODS: We leverage an array of resonant toroids as traps, forming a caterpillar-like structure whereby bending only impacts individual traps minimally. Benchtop measurements are used to determine the blocking of caterpillar traps and show their robustness to bending. We also compare an anterior array system cable covered with caterpillar traps to a commercial cable with B1 + and heating measurements. RESULTS: Benchtop experiments with caterpillar traps demonstrate high robustness to bending. B1 + mapping experiments of an anterior array cable show improved blocking and flexibility compared to a commercial cable. CONCLUSION: Caterpillar traps provide sufficient attenuation to shield currents while allowing cable flexibility. Our distributed design can provide high blocking efficiency at different positions and orientations, even in cases where commercial cable traps cannot.

A pathway towards a two-dimensional, bore-mounted, volume body coil concept for ultra high-field magnetic resonance imaging.

Lack of a body-sized, bore-mounted, radiofrequency (RF) body coil for ultrahigh field (UHF) magnetic resonance imaging (MRI) is one of the major drawbacks of UHF, hampering the clinical potential of the technology. Transmit field (B1 ) nonuniformity and low specific absorption rate (SAR) efficiencies in UHF MRI are two challenges to be overcome. To address these problems, and ultimately provide a pathway for the full clinical potential of the modality, we have designed and simulated two-dimensional cylindrical high-pass ladder (2D c-HPL) architectures for clinical bore-size dimensions, and demonstrated a simplified proof of concept with a head-sized prototype at 7 T. A new dispersion relation has been derived and electromagnetic simulations were used to verify coil modes. The coefficient of variation (CV) for brain, cerebellum, heart, and prostate tissues after B1 + shimming in silico is reported and compared with previous works. Three prototypes were designed in simulation: a head-sized, body-sized, and long body-sized coil. The head-sized coil showed a CV of 12.3%, a B1 + efficiency of 1.33 µT/√W, and a SAR efficiency of 2.14 µT/√(W/kg) for brain simulations. The body-sized 2D c-HPL coil was compared with same-sized transverse electromagnetic (TEM) and birdcage coils in silico with a four-port circularly polarized mode excitation. Improved B1 + uniformity (26.9%) and SAR efficiency (16% and 50% better than birdcage and TEM coils, respectively) in spherical phantoms was observed. We achieved a CV of 12.3%, 4.9%, 16.7%, and 2.8% for the brain, cerebellum, heart, and prostate, respectively. Preliminary imaging results for the head-sized coil show good agreement between simulation and experiment. Extending the 1D birdcage coil concept to 2D c-HPLs provides improved B1 + uniformity and SAR efficiency.

Characterization of a Low-Profile, Flexible, and Acoustically Transparent Receive-Only MRI Coil Array for High Sensitivity MR-Guided Focused Ultrasound.

Magnetic resonance guided focused ultrasound (MRgFUS) is a non-invasive therapeutic modality for neurodegenerative diseases that employs real-time imaging and thermometry monitoring of targeted regions. MRI is used in guidance of ultrasound treatment; however, the MR image quality in current clinical applications is poor when using the vendor built-in body coil. We present an 8-channel, ultra-thin, flexible, and acoustically transparent receive-only head coil design (FUS-Flex) to improve the signal-to-noise ratio (SNR) and thus the quality of MR images during MRgFUS procedures. Acoustic simulations/experiments exhibit transparency of the FUS-Flex coil as high as 97% at 650 kHz. Electromagnetic simulations show a SNR increase of 13× over the body coil. In vivo results show an increase of the SNR over the body coil by a factor of 7.3 with 2× acceleration (equivalent to 11× without acceleration) in the brain of a healthy volunteer, which agrees well with simulation. These preliminary results show that the use of a FUS-Flex coil in MRgFUS surgery can increase MR image quality, which could yield improved focal precision, real-time intraprocedural anatomical imaging, and real-time 3D thermometry mapping.

Stretchable self-tuning MRI receive coils based on liquid metal technology (LiquiTune).

Magnetic resonance imaging systems rely on signal detection via radiofrequency coil arrays which, ideally, need to provide both bendability and form-fitting stretchability to conform to the imaging volume. However, most commercial coils are rigid and of fixed size with a substantial mean offset distance of the coil from the anatomy, which compromises the spatial resolution and diagnostic image quality as well as patient comfort. Here, we propose a soft and stretchable receive coil concept based on liquid metal and ultra-stretchable polymer that conforms closely to a desired anatomy. Moreover, its smart geometry provides a self-tuning mechanism to maintain a stable resonance frequency over a wide range of elongation levels. Theoretical analysis and numerical simulations were experimentally confirmed and demonstrated that the proposed coil withstood the unwanted frequency detuning typically observed with other stretchable coils (0.4% for the proposed coil as compared to 4% for a comparable control coil). Moreover, the signal-to-noise ratio of the proposed coil increased by more than 60% as compared to a typical, rigid, commercial coil.

Characterization and evaluation of a flexible MRI receive coil array for radiation therapy MR treatment planning using highly decoupled RF circuits.

The growth in the use of magnetic resonance imaging (MRI) data for radiation therapy (RT) treatment planning has been facilitated by scanner hardware and software advances that have enabled RT patients to be imaged in treatment position while providing morphologic and functional assessment of tumor volumes and surrounding normal tissues. Despite these advances, manufacturers have been slow to develop radiofrequency (RF) coils that closely follow the contour of a RT patient undergoing MR imaging. Instead, relatively large form surface coil arrays have been adapted from diagnostic imaging. These arrays can be challenging to place on, and in general do not conform to the patient's body habitus, resulting in sub optimal image quality. The purpose of this study is to report on the characterization of a new flexible and highly decoupled RF coil for use in MR imaging of RT patients. Coil performance was evaluated by performing signal-to-noise ratio (SNR) and noise correlation measurements using two coil (SNR) and four coil (noise correlation) element combinations as a function of coil overlap distance and comparing these values to those obtained using conventional coil elements. In vivo testing was performed in both normal volunteers and patients using a four and 16 element RF coil. Phantom experiments demonstrate the highly decoupled nature of the new coil elements when compared to conventional RF coils, while in vivo testing demonstrate that these coils can be integrated into extremely flexible and form fitting substrates that follow the exact contour of the patient. The new coil design addresses limitations imposed by traditional surface coil arrays and have the potential to significantly impact MR imaging for both diagnostic and RT applications.

Active-passive gradient shielding for MRI acoustic noise reduction.

An important source of MRI acoustic noise-magnet cryostat warm-bore vibrations caused by eddy-current-induced forces-can be mitigated by a passive metal shield mounted on the outside of a vibration-isolated, vacuum-enclosed shielded gradient set. Finite-element (FE) calculations for a z-gradient indicate that a 2-mm-thick Cu layer wrapped on the gradient assembly can decrease mechanical power deposition in the warm bore and reduce warm-bore acoustic noise production by about 25 dB. Eliminating the conducting warm bore and other magnet parts as significant acoustic noise sources could lead to the development of truly quiet, fully functioning MRI systems with noise levels below 70 dB.