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Low field MRI is safer and more cost effective than the high field MRI. One of the inherent problems of low field MRI is its low signal-to-noise ratio or sensitivity. In this work, we introduce a multimodal surface coil technique for signal excitation and reception to improve the RF magnetic field (B 1) efficiency and potentially improve MR sensitivity. The proposed multimodal surface coil consists of multiple identical resonators that are electromagnetically coupled to form a multimodal resonator. The field distribution of its lowest frequency mode is suitable for MR imaging applications. The prototype multimodal surface coils are built, and the performance is investigated and validated through numerical simulation, standard RF measurements and tests, and comparison with the conventional surface coil at low fields. Our results show that the B 1 efficiency of the multimodal surface coil outperforms that of the conventional surface coil which is known to offer the highest B 1 efficiency among all coil categories, i.e., volume coil, half-volume coil and surface coil. In addition, in low-field MRI, the required low-frequency coils often use large value capacitance to achieve the low resonant frequency which makes frequency tuning difficult. The proposed multimodal surface coil can be conveniently tuned to the required low frequency for low-field MRI with significantly reduced capacitance value, demonstrating excellent low-frequency operation capability over the conventional surface coil. [ABSTRACT FROM AUTHOR]

Purpose: There is currently a strong trend in developing RF coils that are high‐density, lightweight, and highly flexible. In addition to the resonator structure of the RF coil itself, the balun or cable trap circuit serves as another essential element in the functionality and sensitivity of RF coils. This study explores the development and application of reproducible highly miniaturized baluns in RF coil design. Methods: We introduce a novel approach to producing Bazooka baluns with printed coaxial capacitors, enabling the achievement of significant capacitance per unit length. Rigorous electromagnetic simulations and thorough hardware fabrication validate the efficacy of the proposed design across various magnetic field strengths, including 1.5 T, 3 T, and 7 T MRI systems. Results: Bench testing reveals that the proposed balun can achieve an acceptable common‐mode rejection ratio even when it is highly miniaturized. The use of printed capacitors allows for a notable reduction in balun length and ensures high reproducibility. Findings demonstrate that the proposed balun exhibits no RF field distortion even when placed close to the sample, making it suitable for flexible coils, wearable coils, and high‐density coils, particularly in high‐field MRI. Conclusion: The reproducibility inherent in the manufacturing process of printed coaxial capacitors allows for simple fabrication and ensures consistency in production. These advancements pave the way for the development of flexible coils, wearable coils, and high‐density coils. [ABSTRACT FROM AUTHOR]

In vivo phosphorus-31 (31P) magnetic resonance spectroscopy (MRS) imaging (MRSI) is an important non-invasive imaging tool for studying cerebral energy metabolism, intracellular nicotinamide adenine dinucleotide (NAD) and redox ratio, and mitochondrial function. However, it is challenging to achieve high signal-to-noise ratio (SNR) 31P MRS/MRSI results owing to low phosphorus metabolites concentration and low phosphorous gyromagnetic ratio ( γ ). Many works have demonstrated that ultrahigh field (UHF) could significantly improve the 31P-MRS SNR. However, there is a lack of studies of the 31P MRSI SNR in the 10.5 Tesla (T) human scanner. In this study, we designed and constructed a novel 31P-1H dual-frequency loop-dipole probe that can operate at both 7T and 10.5T for a quantitative comparison of 31P MRSI SNR between the two magnetic fields, taking into account the RF coil B1 fields (RF coil receive and transmit fields) and relaxation times. We found that the SNR of the 31P MRS signal is 1.5 times higher at 10.5T as compared to 7T, and the power dependence of SNR on magnetic field strength (B0) is 1.9. [ABSTRACT FROM AUTHOR]

Temporomandibular Joint Magnetic Resonance Imaging (TMJ MRI) is crucial for diagnosing temporomandibular disorders (TMDs). This study advances the use of inductively coupled wireless coils to enhance imaging quality in TMJ MRI. After investigating multiple wireless resonator configurations, including a 1-loop design with a loop diameter of 9 cm, a 2-loop design with each loop having a diameter of 7 cm, and a 3-loop design with each loop having a diameter of 5 cm, our findings indicate that the 3-loop configuration achieves the optimal signal-to-noise ratio (SNR), surpassing other wireless arrays. Bilateral deployment of wireless coils further amplifies SNR, enabling superior visualization of TMJ structures, particularly with the 3-loop design. This cost-effective and comfortable solution, featuring a detunable design, eliminates the need for system parameter adjustments. The study indicates broad adaptability across MRI platforms, enhancing TMJ imaging for routine clinical diagnostics of TMDs. [ABSTRACT FROM AUTHOR]

Recent developments in the field of radiofrequency (RF) coils for magnetic resonance imaging (MRI) offer flexible and patient-friendly solutions. Previously, we demonstrated a proof-of-concept single-element stretchable coil design based on liquid metal and a self-tuning smart geometry. In this work, we numerically analyze and experimentally study a multi-channel stretchable coil array and demonstrate its application in dynamic knee imaging. We also compare our flexible coil array to a commonly used commercial rigid coil array. Our numerical analysis shows that the proposed coil array maintains its resonance frequency (<1% variation) and sensitivity (<6%) at various stretching configurations from 0% to 30%. We experimentally demonstrate that the signal-to-noise ratio (SNR) of the acquired MRI images is improved by up to four times with the stretchable coil array due to its conformal and therefore tight-fitting nature. This stretchable array allows for dynamic knee imaging at different flexion angles, infeasible with traditional, rigid coil arrays. These findings are significant as they address the limitations of current rigid coil technology, offering a solution that enhances patient comfort and image quality, particularly in applications requiring dynamic imaging. [ABSTRACT FROM AUTHOR]

Purpose: There are 118 known elements. Nearly all of them have NMR active isotopes and at least 39 different nuclei have biological relevance. Despite this, most of today's MRI is based on only one nucleus—1H. To facilitate imaging all potential nuclei, we present a single transmit coil able to excite arbitrary nuclei in human‐scale MRI. Theory and Methods: We present a completely new type of RF coil, the Any‐nucleus Distributed Active Programmable Transmit Coil (ADAPT Coil), with fast switches integrated into the structure of the coil to allow it to operate at any relevant frequency. This coil eliminates the need for the expensive traditional RF amplifier by directly converting direct current (DC) power into RF magnetic fields with frequencies chosen by digital control signals sent to the switches. Semiconductor switch imperfections are overcome by segmenting the coil. Results: Circuit simulations demonstrated the effectiveness of the ADAPT Coil approach, and a 9 cm diameter surface ADAPT Coil was implemented. Using the ADAPT Coil, 1H, 23Na, 2H, and 13C phantom images were acquired, and 1H and 23Na ex vivo images were acquired. To excite different nuclei, only digital control signals were changed, which can be programmed in real time. Conclusion: The ADAPT Coil presents a low‐cost, scalable, and efficient method for exciting arbitrary nuclei in human‐scale MRI. This coil concept provides further opportunities for scaling, programmability, lowering coil costs, lowering dead‐time, streamlining multinuclear MRI workflows, and enabling the study of dozens of biologically relevant nuclei. [ABSTRACT FROM AUTHOR]

Radio Frequency (RF) coils are necessary for MRI machines and are critical in improving image quality. Due to numerous research efforts in RF coil development, birdcage and surface coils are the most commonly used in clinical applications to enhance medical imaging quality. This article introduces a new surface-type RF coil that is improved by its proximity to the scanned object, boosting its effectiveness. Contrary to past research that examined RF coils operating at one resonant frequency, this study presents a microstrip-based surface RF coil created for dual-frequency use at 64 MHz and 128 MHz, matching the Larmor frequencies for 1.5 T and 3 T MRI machines. The coil was carefully planned, tested, and created, with simulations including a human body model to assess the distribution of the $B_{1}$ field and signal-to-noise ratio (SNR), both essential for assessing the RF coil’s performance in the coronal plane. A tuning circuit was created and tested to maximize resonance at specific frequencies. In contrast, a matching circuit was designed to improve power transfer efficiency between the signal generator and the coil. The suggested RF coil was enhanced by incorporating these circuits, improving overall performance. Both the suggested surface-type microstrip RF coil, which includes tuning and matching circuits, effectively functions at 64 MHz and 128 MHz, as shown by both simulation and experimental results, making it a viable clinical option.

Magnetic resonance imaging (MRI) is an invaluable tool for the diagnosis and study of brain function. Central to the MRI system, the radio frequency (RF) coil plays a dual role: stimulating spins and receiving the MR signal, enabling the generation of detailed images from inside the body. Compared to using linear polarised (LP) methods, the efficiency of the RF coil can be improved by leveraging circular polarised (CP) driving methods, which operate with at least two elements and 90° phase input difference. Coil efficiency can also be improved with the use of a multi-channel phased array receive-only coils, as they provide higher SNR compared to volume coils. Generally, all elements of phased array coils operate in LP mode. However, if the LP elements are replaced by dimension-matched CP elements, the SNR of the phased array coil can be further increased. This study introduces a novel single-channel butterfly coil uniquely capable of operating in CP mode. This is achieved by operating the CP element with asymmetrical tuning and a split resonance peak. The performance of the single-channel CP butterfly coil was evaluated in simulations and MR measurements, and the results were compared to those from single-channel LP coils. The results show that the single-channel CP butterfly coil is a good solution for increasing SNR in phased array receive-only coils.

Inductively coupled radiofrequency (RF) coils are an inexpensive and simple method to realize wireless RF coils in magnetic resonance imaging (MRI), which can significantly ease the MRI scan setup and improve patient comfort because they do not require bulky components such as cables, baluns, preamplifiers, and connectors. However, volume‐type wireless coils are typically operated in transmit/receive mode because detuning such coils is much more challenging due to their complex structure and multiple resonant modes. Meanwhile, adding too many detuning circuits to a wireless coil would decrease the coil's quality factor, impair the signal‐to‐noise ratio, and increase the cost. In this work, we proposed, constructed, and tested a novel wireless volume coil based on the Litzcage design for 1.5‐T head imaging. Being an inductively coupled coil, it has a much simpler structure, resulting in a lighter weight and less bulky design. Despite its simpler structure, it exhibits comparable imaging performance with a commercial receive array, providing an alternative to conventional wired coils with a high cost and complex structure. The unique figure‐of‐8 conductor pattern within the rungs ensures that the proposed wireless Litzcage can be efficiently detuned with minimal detuning circuits. [ABSTRACT FROM AUTHOR]

The design of radiofrequency (RF) coils is crucial for ultra-high field (UHF) magnetic resonance imaging (MRI) systems. To analyze RF coils, various numerical methods, such as finite-difference time-domain (FDTD) and method of moments (MoM), are usually adopted. In this paper, we present a novel hybrid approach that combines a two-dimensional (2D) FDTD with a three-dimensional (3D) MoM to analyze MRI RF problems. In our algorithm, the MoM is utilized for calculating the coil current, and FDTD is assigned for solving the electromagnetic (EM) fields in the imaging region. The hybrid method achieves superior efficiency and acceptable accuracy than using either method individually. To validate the hybrid method, we analyze an ellipse coil loaded with a uniform phantom and a realistic human head model, with the objective of tailoring the magnetic field intensity by adding a multilayer dielectric pad (DP). The results show an improvement in the magnetic field after optimizing the DP configuration. These simulation studies indicate the potential of the new numerical method for the design and analysis of RF systems for ultra-high field applications. [ABSTRACT FROM AUTHOR]

Cerebral blood flow (CBF) is a critical physiological parameter of brain health, and it can be non-invasively measured with arterial spin labeling (ASL) MRI. In this study, we evaluated and optimized whole-brain, high-resolution ASL as an alternative to the low-resolution ASL employed in the routine assessment of CBF in both healthy participants and patients. Two high-resolution protocols (i.e., pCASL and FAIR-Q2TIPS (PASL) with 2 mm isotropic voxels) were compared to a default clinical pCASL protocol (3.4 x 3.4 x 4 mm³), all of whom had an acquisition time of ≈5 min. We assessed the impact of high-resolution acquisition on reducing partial voluming and improving sensitivity to the perfusion signal, and evaluated the effectiveness of z-deblurring on the ASL data. We compared the quality of whole-brain ASL acquired using three available head coils with differing number of receive channels (i.e., 20, 32, and 64ch). We found that using higher coil counts (32 and 64ch coils as compared to 20ch) offers improved signal-to-noise ratio (SNR) and acceleration capabilities that are beneficial for ASL imaging at 3 Tesla (3 T). The inherent reduction in partial voluming effects with higher resolution acquisitions improves the resolving power of perfusion without impacting the sensitivity. In conclusion, our results suggest that high-resolution ASL (2 to 2.5 mm isotropic voxels) has the potential to become a new standard for perfusion imaging at 3 T and increase its adoption into clinical research and cognitive neuroscience applications. [ABSTRACT FROM AUTHOR]

Pulsed electromagnetic field (PEMF) stimulation is a non-invasive therapeutic modality that has been employed in a variety of applications, including skin wound healing, tissue regeneration, blood vessel regeneration, repair of nonunion bone fractures, and dental treatment. Currently, the potential side effects of electromagnetic fields on tissues have not been comprehensively explored, despite their established therapeutic benefits. In this study, we examined the side effects of PEMF devices on whole-body tissues using simulations. The electromagnetic field simulation software CST Studio Suite was utilized, and an RF coil that could output different frequencies (100 Hz, 100 kHz, and 27 MHz) and magnetic fields (0.25 mT, 0.5, and 1 mT) was designed for the purpose of simulations. A realistic rat model, available from the CST library, was employed. Using the Debye equations according to the relevant frequency value, the electrical and thermal properties of the rat were calculated, and the simulation was carried out in two stages: EM and thermal analysis. The specific absorption rate in the tissues with EM simulation and the temperature distribution in the tissues with thermal analysis were examined. The effects created by the PEMF device at different frequencies and magnetic field intensities were presented comparatively. According to the results, the highest SAR value was observed for three different magnetic field intensities at a frequency of 27 MHz, resulting in a temperature difference of 0.7 °C. This study aims to determine the EM effects on rats in a simulated environment according to different frequencies and magnetic field values, as well as to determine whether the thermal effects created can produce a side response. By doing so, it will be possible to improve noninvasive clinical PEMF devices and provide additional information about the interaction of electromagnetic fields with tissues. [ABSTRACT FROM AUTHOR]

Cerebral blood flow (CBF) is a critical physiological parameter of brain health, and it can be non-invasively measured with arterial spin labeling (ASL) MRI. In this study, we evaluated and optimized whole-brain, high-resolution ASL as an alternative to the low-resolution ASL employed in the routine assessment of CBF in both healthy participants and patients. Two high-resolution protocols (i.e., pCASL and FAIR-Q2TIPS (PASL) with 2 mm isotropic voxels) were compared to a default clinical pCASL protocol (3.4 × 3.4 × 4 mm3), all of whom had an acquisition time of ≈ 5 min. We assessed the impact of high-resolution acquisition on reducing partial voluming and improving sensitivity to the perfusion signal, and evaluated the effectiveness of z-deblurring on the ASL data. We compared the quality of whole-brain ASL acquired using three available head coils with differing number of receive channels (i.e., 20, 32, and 64ch). We found that using higher coil counts (32 and 64ch coils as compared to 20ch) offers improved signal-to-noise ratio (SNR) and acceleration capabilities that are beneficial for ASL imaging at 3 Tesla (3 T). The inherent reduction in partial voluming effects with higher resolution acquisitions improves the resolving power of perfusion without impacting the sensitivity. In conclusion, our results suggest that high-resolution ASL (2 to 2.5 mm isotropic voxels) has the potential to become a new standard for perfusion imaging at 3 T and increase its adoption into clinical research and cognitive neuroscience applications.

Combining multimodal approaches with functional magnetic resonance imaging (fMRI) has catapulted the research on brain circuitries of non-human primates (NHPs) into a new era. However, many studies are constrained by a lack of appropriate RF coils. In this study, a single loop transmit and 16-channel receive array coil was constructed for brain imaging of macaques at 7 Tesla (7 T). The 16 receive channels were mounted on a 3D-printed helmet-shaped form closely fitting the macaque head, with fourteen openings arranged for multimodal devices around the cortical regions. Coil performance was evaluated by quantifying and comparing signal-to-noise ratio (SNR) maps, noise correlations, g-factor maps and flip-angle maps with a 28-channel commercial knee coil. The in vivo results suggested that the macaque coil has higher SNR in cortical regions and better acceleration ability in parallel imaging, which may benefit revealing mesoscale organizations in the brain. [ABSTRACT FROM AUTHOR]

Purpose: To improve intraoral transverse loop coil design for high‐resolution dental MRI. Methods: The transverse intraoral loop coil (tLoop) was modified (mtLoop) by overlapping the feed port conductors, bending the posterior section, introducing a parallel plate capacitor, optimizing the insulation thickness, and using it in receive‐only mode. In addition, an MR‐silent insulation was introduced. The performances of the mtLoop and tLoop coils were compared in terms of sensitivity, image SNR, and eddy currents using electromagnetic simulations and MRI measurements at 3T. Results: The receive‐only mode of the mtLoop increases the sensitivity at the apices of the roots, and the overlapped feed port design eliminated signal voids along the incisors. The bent posterior section with the parallel plate capacitor reduced the unwanted signal of the tongue by a factor of 2.3 in the selected region off interest and lowered the eddy currents by 10%. The proposed new coil provided higher SNR by elevenfold and 2.5‐fold at the incisors and apices of the molar roots within the selected regions of interest, respectively, in the experiments, as well as improved comfort. Optimal insulation thickness was determined as 1 mm. With the mtLoop, a (250 μm)3 isotropic resolution of the dental arch could be realized using a UTE sequence within 2 min total acquisition time. A T2‐SPACE protocol with (350 μm)2 in‐plane resolution was also demonstrated. Conclusion: The proposed new coil offers higher SNR at the incisors and apices of the molar roots, less unwanted signals from tongue, lower eddy currents, and improved patient comfort. [ABSTRACT FROM AUTHOR]

Purpose: To introduce the dipolectric antenna: a novel RF coil design for high‐field MRI using a combination of a dipole antenna with a loop‐coupled dielectric resonator antenna. Methods: Simulations in human voxel model Duke involving 8‐, 16‐, and 38‐channel dipolectric antenna arrays for brain MRI were conducted. An 8‐channel dipolectric antenna for occipital lobe MRI at 7 T was designed and constructed. The array was built of four dielectric resonator antennas (dielectric constant = 1070) and four segmented dipole antennas. In vivo MRI experiments were conducted in one subject, and the SNR performance was benchmarked against a commercial 32‐channel head coil. Results: A 38‐channel dipolectric antenna array provided the highest whole‐brain SNR (up to a 2.3‐fold SNR gain in the center of the Duke's head vs. an 8‐channel dipolectric antenna array). Dipolectric antenna arrays driven in dipole‐only mode (with dielectric resonators used as receive‐only) yielded the highest transmit performance. The constructed 8‐channel dipolectric antenna array provided up to threefold higher in vivo peripheral SNR when compared with a 32‐channel commercial head coil. Conclusion: Dipolectric antenna can be considered a promising approach to enhance SNR in human brain MRI at 7 T. This strategy can be used to develop novel multi‐channel arrays for different high‐field MRI applications. [ABSTRACT FROM AUTHOR]

PurposeTo use fiducial markers containing manganese 55 to rapidly localize carbon 13 (13 C) RF coils for correcting images for B1 variation.MethodsHollow high-density polyethylene spheres were filled with 3M sodium permanganate and affixed to a rectangular 13 C-tuned RF coil. The relative positions of the markers and coil conductors were mapped using CT. Marker positions were measured by MRI using a series of 1D projections and automated peak detection. Once the coil location was determined, coil sensitivity was estimated using a quasi-static calculation. Simulations were performed to determine the minimum number of projections required for robust localization. Phantom experiments were used to confirm the accuracy of marker localization as well as the calculated coil sensitivity. Finally, in vivo validation was performed using hyperpolarized 13 C pyruvate in a rat model.ResultsIn simulations, our algorithm was accurate in determining marker positions when at least 6 projections were used (RMSE 1.4 ± 0.9 mm). These estimates were verified in phantom experiments, where markers locations were determined with an RMS accuracy of 1.3 mm. A minimum SNR of 4 was required for automated detection to perform accurately. Computed coil sensitivity had a median error of 17% when taken over the entire measured area and 5.7% over a central region. In a rat, correction for nonuniform reception and flip angle was able to normalize the signals arising from asymmetrically positioned kidneys.ConclusionManganese 55 fiducial markers are an inexpensive and reliable method for rapidly localizing 13 C RF coils and correcting 13 C images for B1 variation without user intervention.

Objective: Low-cost low-field point-of-care MRI systems are used in many different applications. System design has correspondingly different requirements in terms of imaging field-of-view, spatial resolution and magnetic field strength. In this work an iterative framework has been created to design a cylindrical Halbach-based magnet along with integrated gradient and RF coils that most efficiently fulfil a set of user-specified imaging requirements. Methods: For efficient integration, target field methods are used for each of the main hardware components. These have not been used previously in magnet design, and a new mathematical model was derived accordingly. These methods result in a framework which can design an entire low-field MRI system within minutes using standard computing hardware. Results: Two distinct point-of-care systems are designed using the described framework, one for neuroimaging and the other for extremity imaging. Input parameters are taken from literature and the resulting systems are discussed in detail. Discussion: The framework allows the designer to optimize the different hardware components with respect to the desired imaging parameters taking into account the interdependencies between these components and thus give insight into the influence of the design choices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The design and testing of a Helmholtz prototype probe with a relatively homogeneous radiofrequency field that can be used for MRI are presented in this paper. It consists of two coaxial separately tuned ellipsoidal rings of wire in a symmetric arrangement. The developed ellipsoidal structure is tested experimentally in free space and is compared with the results obtained with an equivalent circular standard Helmholtz coil. Complete electrical modeling of the coils taking into account all couplings (inductive and capacitive) is studied. The proposed ellipsoidal Helmholtz coil provides better performance in terms of field homogeneity, efficiency, and quality factor. The width of the lobe at 10% for the ellipsoidal coil is 20% bigger. Moreover, a significant improvement in the quality factor is observed in free space. Also, the efficiency is increased by 37%. Finally, the Signal to Noise Ratio (SNR) of the ellipsoidal coil is higher than that of the circular coil. [ABSTRACT FROM AUTHOR]

Purpose: To bring metabolic imaging based on multi‐NMR toward practical use from the RF hardware perspective. Methods: A highly integrated RF coil is designed for whole‐brain MRI and MRS targeted to five nuclear species: 1H, 19F, 31P, 23Na, and 13C. Dipole antennas and closely loaded local receiver loops are combined in this setup. Results: High‐quality in vivo scan results of 1H, 31P, 23Na, and 13C on healthy volunteers have been achieved. For 1H, the transmit efficiency is 77% of a single‐tuned commercial head coil (NOVA 8‐transmit [Tx]/32‐receive [Rx]; NOVA Medical, Wilmington, MA, USA). For 31P, 110% SNR of a dual‐tuned close‐fit head‐birdcage was achieved at the center of the subject, based on MR experiments on a phantom. For 31P, 23Na, and 13C, bench measurements indicate SNR loss of 15%, 27%, and 30% compared with single‐tuned conditions. 19F performance has been proven to be similar to that of 1H through bench tests and electromagnetic simulations. Conclusion: With this device, 1H‐based anatomic images that are expected to meet clinical requirements, as well as high‐quality multi‐NMR images and spectra, can be acquired within one scan session without hardware replacement or patient repositioning, enabling morphologic and metabolic MRI within acceptable scan time. [ABSTRACT FROM AUTHOR]

Objective: To determine whether a multi-feed, loop-dipole combined approach can be used to improve performance of rectangular dielectric resonator antenna (DRA) arrays human brain for MRI at 7 T. Materials and methods: Electromagnetic field simulations in a spherical phantom and human voxel model "Duke" were conducted for different rectangular DRA geometries and dielectric constants εr. Three types of RF feed were investigated: loop-only, dipole-only and loop-dipole. Additionally, multi-channel array configurations up to 24-channels were simulated. Results: The loop-only coupling scheme provided the highest B1+ and SAR efficiency, while the loop-dipole showed the highest SNR in the center of a spherical phantom for both single- and multi-channel configurations. For Duke, 16-channel arrays outperformed an 8-channel bow-tie array with greater B1+ efficiency (1.48- to 1.54-fold), SAR efficiency (1.03- to 1.23-fold) and SNR (1.63- to 1.78). The multi-feed, loop-dipole combined approach enabled the number of channels increase to 24 with 3 channels per block. Discussion: This work provides novel insights into the rectangular DRA design for high field MRI and shows that the loop-only feed should be used instead of the dipole-only in transmit mode to achieve the highest B1+ and SAR efficiency, while the loop-dipole should be the best suited in receive mode to obtain the highest SNR in spherical samples of similar size and electrical properties as the human head. [ABSTRACT FROM AUTHOR]

Objective: Blood-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. Approach: In 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 results: The 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. Significance: This pilot study reveals the feasibility for brain investigation at multiple spatiotemporal scales, which may advance our understanding in dynamic brain networks.

Self-decoupling technology was recently proposed for radio frequency (RF) coil array designs. Here, we propose a novel geometry to reduce the peak local specific absorption rate (SAR) and improve the robustness of the self-decoupled coil. We first demonstrate that B1 is determined by the arm conductors, while the maximum E-field and local SAR are determined by the feed conductor in a self-decoupled coil. Then, we investigate how the B1, E-field, local SAR, SAR efficiency, and coil robustness change with respect to different lift-off distances for feed and mode conductors. Next, the simulation of self-decoupled coils with optimal lift-off distances on a realistic human body is performed. Finally, self-decoupled coils with optimal lift-off distances are fabricated and tested on the workbench and MRI experiments. The peak 10 g-averaged SAR of the self-decoupled coil on the human body can be reduced by 34% by elevating the feed conductor. Less coil mismatching and less resonant frequency shift with respect to loadings were observed by elevating the mode conductor. Both the simulation and experimental results show that the coils with elevated conductors can preserve the high interelement isolation, B1+ efficiency, and SNR of the original self-decoupled coils. [ABSTRACT FROM AUTHOR]

In ultra-high-field (UHF) magnetic resonance imaging (MRI) applications, the design and analysis of high-frequency radio frequency (RF) coils requires full-wave electromagnetic (EM) methods that can handle complex field-tissue interactions. Using a Huygens' equivalent surface, the Method of Moments (MoM) and the Finite-Difference Time-Domain (FDTD) algorithm can be combined to accurately model the high-frequency RF coils. In previous research, a uniform FDTD mesh structure was considered, providing a compromised solution for coil-tissue interactions. This paper proposes a hybrid FDTD/MoM algorithm with non-uniform meshes. The fine mesh domain is set at the Huygens' surface, and the other domain uses coarse meshes. The proposed algorithms are strictly validated, and their computational performance is compared against conventional methods. Results show that the new algorithm can improve the calculation efficiency without losing accuracy. Specifically, compared with the uniform FDTD method, the numerical difference between both hybrid methods remains at 3.2%. Still, the calculation time of the non-uniform grid algorithm is reduced by 64.2%, demonstrating the effectiveness of the new algorithm for modeling RF coils for UHF-MRI applications. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging (MRI) using an ultra-high magnetic field (7 Tesla) enables detailed and non-invasive studies of the function and anatomy of the human visual cortex, which is the brain region responsible for visual signal processing. However, 7T human MRI often suffers from image shading in the occipital region due to the radiofrequency (RF) wave propagation effect. Dedicated visual cortex coils, on the other hand, often lack the capability to visualize the whole brain which is necessary for image registration. We propose a novel RF coil structure in which a 2-channel transmit and receive (TRx) coil is grafted onto the frontal part of a multi-channel transmit-only/receive-only (TORO, 4Tx/14Rx) visual cortex coil. This coil was tested for high-resolution functional MRI with an in-plane resolution of 0.5 mm. The results showed that the proposed coil achieved a higher ( $\times 2.5$ ) temporal signal-to-noise ratio (tSNR) in functional imaging of the visual cortex area than that of a commercial 7T whole-head coil. The added 2-channel TRx elements allowed whole-brain edge images to be acquired, enabling successful brain segmentation and atlas registration without the need for a second scan using a whole-head coil. The proposed coil structure can be useful for high-resolution visual functional MRI at very high magnetic fields due to its sensitivity, open geometry, and compatibility with the standard image processing workflow.

The functional magnetic resonance imaging (fMRI) at ultra-high field (UHF, $\geqslant 7\text{T}$) is a powerful temporal acquisition method which promises to capture neuronal activities at submillimeter scale. But high-spatial-resolution fMRI still remains difficult, as the nuisance temporal noise which also grows with the main magnetic field strength. For decades, mainstream solutions in reducing motion-induced temporal noise include motion-correction algorithms in image post-processing as well as MR acquisition schemes in RF pulse sequence designs, however hardware-related studies have been rarely reported over the RF receive coil. In this study, we have proposed the intrinsic temporal performance model, which is specifically used for measuring coil-related intrinsic temporal SNR (tSNR*), and the intrinsic sensitivity variability and thermal noise variability have been proposed as model parameters. The intrinsic temporal performance of single-channel loops and array coils were evaluated using numerical electromagnetic simulations, and phantom experiments were designed to investigate the intrinsic thermal noise variability. It was observed that the achievable intrinsic tSNR* can be greatly lowered by ~90% even with 2 mm translational motion in the normal direction, suggesting the effect of RF receive coils in producing temporal noise. The proposed model provides a new perspective in optimizing coil designs and array coil temporal combination methods, which may offer a feasible means in achieving submillimeter resolutions at UHF. Moreover, model parameters from the intrinsic temporal performance model can be directly calculated based on single MRI acquisition, offering a practical performance metric for manufactures and customers in quality and assurance checks of RF receive coil products. [ABSTRACT FROM AUTHOR]

This study outlines the design, construction and characterization of a tailored low-cost linear birdcage (BC) resonator for magnetic resonance imaging at 7T. Typically, different BC designs found in literature are well described in theory but lack crucial information for practical realization. This is challenging, as theoretical and practical aspects often differ greatly from each other, especially in the field of high frequency technology. We propose a simple and open-source 3D printable design which results in a working BC if the instructions in this publication are followed. The aim is to open up the possibility of building a functioning BC with simple means and a budget below 750€, even for users without a great deal of expertise in MRI coil building. We demonstrate that the BC can achieve a good B1 field homogeneity using the double angle method. The proposed design is qualitatively compared to a commercially available resonator. Both perform equally well in terms of SNR and image quality.

BackgroundIt is technically challenging to design compact yet sensitive miniature catheter radio frequency (RF) coils for endovascular interventional MR imaging.MethodsIn this work, a new design method for catheter RF coils is proposed based on the coaxial transmission line resonator (TLR) technique. Due to its distributed circuit, the TLR catheter coil does not need any lumped capacitors to support its resonance, which simplifies the practical design and construction and provides a straightforward technique for designing miniature catheter-mounted imaging coils that are appropriate for interventional neurovascular procedures. The outer conductor of the TLR serves as an RF shield, which prevents electromagnetic energy loss, and improves coil Q factors. It also minimizes interaction with surrounding tissues and signal losses along the catheter coil. To investigate the technique, a prototype catheter coil was built using the proposed coaxial TLR technique and evaluated with standard RF testing and measurement methods and MR imaging experiments. Numerical simulation was carried out to assess the RF electromagnetic field behavior of the proposed TLR catheter coil and the conventional lumped-element catheter coil.ResultsThe proposed TLR catheter coil was successfully tuned to 64 MHz for proton imaging at 1.5 T. B1 fields were numerically calculated, showing improved magnetic field intensity of the TLR catheter coil over the conventional lumped-element catheter coil. MR images were acquired from a dedicated vascular phantom using the TLR catheter coil and also the system body coil. The TLR catheter coil is able to provide a significant signal-to-noise ratio (SNR) increase (a factor of 200 to 300) over its imaging volume relative to the body coil.ConclusionsCatheter imaging RF coil design using the proposed coaxial TLR technique is feasible and advantageous in endovascular interventional MR imaging applications.

Balun or trap circuits are critical components for suppressing common-mode currents flowing on the outer conductors of coaxial cables in RF coil systems for Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS). Common-mode currents affect coils’ tuning and matching, induce losses, pick up extra noise from the surrounding environment, lead to undesired cross-talk, and cause safety concerns in animal and human imaging. First proposed for microwave antenna applications, the Lattice balun has been widely used in MRI coils. It has a small footprint and can be easily integrated with coil tuning/matching circuits. However, the Lattice balun is typically a single-tuned circuit and cannot be used for multi-nuclear MRI and MRS with two RF frequencies. This work describes a dual-tuned Lattice balun design that is suitable for multi-nuclear MRI/MRS. It was first analyzed theoretically to derive component values. RF circuit simulations were then performed to validate the theoretical analysis and provide guidance for practical construction. Based on the simulation results, a dual-tuned balun circuit was built for 7T 1H/23Na MRI and bench tested. The fabricated dual-tuned balun exhibits superior performance at the Larmor frequencies of both 1H and 23Na, with less than 0.15 dB insertion loss and better than 17 dB common-mode rejection ratio at both frequencies. [ABSTRACT FROM AUTHOR]

Background Trans-cranial MR guided focused ultrasound (tcMRgFUS) ablation targets are

Baluns are crucial in MRI RF coils, essential for minimizing common-mode currents, maintaining signal-to-noise ratio, and ensuring patient safety. This paper introduces the innovative float solenoid balun, based on the renowned solenoid cable trap, and conducts a comparative analysis with the widely used float bazooka balun. Leveraging robust inductive coupling between the cable shield and float resonator, the float solenoid balun offers compact dimensions and post-installation adjustability. Through electromagnetic simulations and bench testing across static fields (1.5, 3, and 7 T), the float solenoid balun demonstrates superior common-mode rejection ratios compared to the float bazooka balun. Notably, its float design facilitates easy post-installation adjustment and eliminates the need for soldering on the cable shield, enhancing usability and reducing risks. Furthermore, the solenoid balun's compact footprint addresses the increasing demand for smaller baluns in modern MRI scanners with denser coil arrays. The float solenoid balun offers a promising solution by conserving valuable space within the RF coil, simplifying practical hardware implementation and cable routing, and accommodating more elements in RF arrays, with great potential for enhancing MRI performance., (© 2024 The Author(s). NMR in Biomedicine published by John Wiley & Sons Ltd.)

PurposeTo develop a lump-element double-tuned common-mode-differential-mode (CMDM) radiofrequency (RF) surface coil with independent frequency tuning capacity for MRS and MRI applications.MethodsThe presented design has two modes that can operate with different current paths, allowing independent frequency adjustment. The coil prototype was tested on the bench and then examined in phantom and in vivo experiments.ResultsStandard deviations of frequency and impedance fluctuations measured in one resonator, while changing the tuning capacitor of another resonator, were less than 13 kHz and 0.55 Ω. The unloaded S21 was -36 dB and -41 dB, while the unloaded Q factor was 260 and 287, for 13 C and 1 H, respectively. In vivo hyperpolarized 13 C MR spectroscopy data demonstrated the feasibility of using the CMDM coil to measure the dynamics of lactate, alanine, pyruvate and bicarbonate signal in a normal rat head along with acquiring 1 H anatomical reference images.ConclusionIndependent frequency tuning capacity was demonstrated in the presented lump-element double-tuned CMDM coil. This CMDM coil maintained intrinsically decoupled magnetic fields, which provided sufficient isolation between the two resonators. The results from in vivo experiments demonstrated high sensitivity of both the 1 H and 13 C resonators. Magn Reson Med 76:1612-1620, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

We evaluated a 16-channel loop + dipole (LD) transceiver antenna array with improved specific absorption rate (SAR) efficiency for 10.5 Tesla (T) human head imaging applications. Three different array designs with equal inner dimensions were considered: an 8-channel dipole antenna, an 8-channel loop, and a 16-channel LD antenna arrays. Signal-to-noise ratio (SNR) and B1+ efficiency (in units of μT per √W) were simulated and measured in 10.5 T magnetic resonance imaging (MRI) experiments. For the safety validation, 10 g SAR and SAR efficiency (defined as the B1+ over √ (peak 10 g SAR)) were calculated through simulation. Finally, high resolution porcine brain images were acquired with the 16-channel LD antenna array, including a fast turbo-spin echo (TSE) sequence incorporating B1 shimming techniques. Both the simulation and experiments demonstrated that the combined 16-channel LD antenna array showed similar B1+ efficiency compared to the 8-channel dipole antenna and the 8-channel loop arrays in a circular polarized (CP) mode. In a central 2 mm × 2 mm region of the phantom, however, the 16-channel LD antenna array showed an improvement in peak 10 g SAR of 27.5 % and 32.5 % over the 8-channel dipole antenna and the 8-channel loop arrays, respectively. We conclude that the proposed 16-channel head LD antenna array design is capable of achieving ~7% higher SAR efficiency at 10.5 T compared to either the 8-channel loop-only or the 8-channel dipole-only antenna arrays of the same dimensions.

Self-decoupling technology was recently proposed for radio frequency (RF) coil array designs. Here, we propose a novel geometry to reduce the peak local specific absorption rate (SAR) and improve the robustness of the self-decoupled coil. We first demonstrate that B1 is determined by the arm conductors, while the maximum E-field and local SAR are determined by the feed conductor in a self-decoupled coil. Then, we investigate how the B1, E-field, local SAR, SAR efficiency, and coil robustness change with respect to different lift-off distances for feed and mode conductors. Next, the simulation of self-decoupled coils with optimal lift-off distances on a realistic human body is performed. Finally, self-decoupled coils with optimal lift-off distances are fabricated and tested on the workbench and MRI experiments. The peak 10 g-averaged SAR of the self-decoupled coil on the human body can be reduced by 34% by elevating the feed conductor. Less coil mismatching and less resonant frequency shift with respect to loadings were observed by elevating the mode conductor. Both the simulation and experimental results show that the coils with elevated conductors can preserve the high interelement isolation, B1+ efficiency, and SNR of the original self-decoupled coils.

Background: Low-field open magnetic resonance imaging (MRI) systems, typically operating at magnetic field strengths below 1 Tesla, has greatly expanded the accessibility of MRI technology to meet a wide range of patient needs. However, the inherent challenges of low-field MRI, such as limited signal-to-noise ratios and limited availability of dedicated radiofrequency (RF) coils, have prompted the need for innovative coil designs that can improve imaging quality and diagnostic capabilities., Purpose: In response to these challenges, we introduce the coupled stack-up volume coil, a novel RF coil design that addresses the shortcomings of conventional birdcage in the context of low-field open MRI., Methods: The proposed coupled stack-up volume coil design utilizes a unique architecture that optimizes both transmit/receive efficiency and RF field homogeneity and offers the advantage of a simple design and construction, making it a practical and feasible solution for low-field MRI applications. This paper presents a comprehensive exploration of the theoretical framework, design considerations, and experimental validation of this innovative coil design., Results: We demonstrate the superior performance of the coupled stack-up volume coil in achieving 47.7% higher transmit/receive efficiency and 68% more uniform magnetic field distribution compared to traditional birdcage coils in electromagnetic simulations. Bench tests results show that the B1 field efficiency of coupled stack-up volume coil is 57.3% higher compared with that of conventional birdcage coil., Conclusions: The proposed coupled stack-up volume coil outperforms the conventional birdcage coil in terms of B1 efficiency, imaging coverage, and low-frequency operation capability. This design provides a robust and simple solution to low-field MR RF coil design.

Rodent models of spinal cord injury (SCI) have been widely used in pre-clinical studies. Injuries may occur at different levels of the lumbar and thoracic cord, and the number of segments injured and their depths may vary along the spine. It is thereby challenging to build one universal RF coil that exhibits optimal performance for all spinal cord imaging applications, especially in an animal scanner with small in-bore space and limited hardware configurations. We developed an interchangeable RF coil system for a 9.4 T small animal MRI scanner, in which the users can select an optimal coil specialized for imaging specific parts of a rat spine. We also developed the associated animal management device for immobilization and positioning. The whole system allows ease of RF coil exchange, animal fixation, and positioning, and thus reduces the animal preparation time before the MRI scan significantly. Compared to a commercial general-purpose 2-cm-diameter coil that was used in our previous studies, the specialized coil optimized for Sprague-Dawley rat lumbar spinal cord imaging exhibits up to 2.4 times SNR improvement. [ABSTRACT FROM AUTHOR]

Surgery for tongue cancer often results in a major loss in quality of life. While MRI may be used to minimise the volume of excised tissue, often the full tumour extent is missed. This tumour extent may be detected with metabolic imaging. One of the main reasons for the lack of metabolic information on tongue cancer would be the absence of an x-nuclear coil with the tongue as a focus target. Metabolic MRI through 31P MRSI is known as a powerful tool to non-invasively study elevated cell proliferation and disturbed energy metabolism in tumours. Severe magnetic field non-uniformities are inherently caused by the substantial difference in magnetic susceptibilities of tissue and air in the mouth and its environs. Despite this, the wide chemical shift dispersion of 31P could still facilitate precise detection of the cell proliferation biomarkers, phospomonoesters and diesters, as well as energy metabolites ATP, inorganic phosphate, and phosphocreatine potentially mapped over the tongue or tumour in vivo. In this study, we present the first 31P MRSI data of the human tongue in vivo from healthy volunteers and a patient with a tongue tumour at 7 T MRI using a 1H 8-channel transceiver setup placed inside a body 31P transmitter, which is able to get a uniform excitation from the tongue while providing comfortable access to the patient. In addition, a user-friendly external 31P receiver array is used to provide high sensitivity (80%) comparable to an uncomfortable inner mouth loop coil positioned on the tongue. The primary aim is the demonstration of 31P metabolite profiles in the tongue and the differences between healthy and malignant tissue. Indeed, clear elevated cell proliferation expressed as enhanced phosphomonoesters is observed in the tumour vs. the healthy part of the tongue. This can be performed within a total scan duration of 30 min, comparable to clinical scans, with a spatial resolution of 1.5 cm for the 10-min 31P MRSI scan. [ABSTRACT FROM AUTHOR]

Purpose: Design, implementation, evaluation, and application of a 32‐channel Self‐Grounded Bow‐Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. Methods: The array consists of 32 compact SGBT building blocks. Transmission field (B1+) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo B1+ efficiency mapping was conducted with actual flip angle imaging. The array's applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n = 7). Results: B1+ shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured B1+ efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5) mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R = 4 (mean signal‐to‐noise ratio (whole heart) ≥ 16.7, mean contrast‐to‐noise ratio ≥ 13.5) without impairing image quality significantly. Conclusion: The compact SGBT building block facilitates a modular high‐density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques. [ABSTRACT FROM AUTHOR]

Surgery for tongue cancer often results in a major loss in quality of life. While MRI may be used to minimise the volume of excised tissue, often the full tumour extent is missed. This tumour extent may be detected with metabolic imaging. One of the main reasons for the lack of metabolic information on tongue cancer would be the absence of an x-nuclear coil with the tongue as a focus target. Metabolic MRI through 31P MRSI is known as a powerful tool to non-invasively study elevated cell proliferation and disturbed energy metabolism in tumours. Severe magnetic field non-uniformities are inherently caused by the substantial difference in magnetic susceptibilities of tissue and air in the mouth and its environs. Despite this, the wide chemical shift dispersion of 31P could still facilitate precise detection of the cell proliferation biomarkers, phospomonoesters and diesters, as well as energy metabolites ATP, inorganic phosphate, and phosphocreatine potentially mapped over the tongue or tumour in vivo. In this study, we present the first 31P MRSI data of the human tongue in vivo from healthy volunteers and a patient with a tongue tumour at 7 T MRI using a 1H 8-channel transceiver setup placed inside a body 31P transmitter, which is able to get a uniform excitation from the tongue while providing comfortable access to the patient. In addition, a user-friendly external 31P receiver array is used to provide high sensitivity (80%) comparable to an uncomfortable inner mouth loop coil positioned on the tongue. The primary aim is the demonstration of 31P metabolite profiles in the tongue and the differences between healthy and malignant tissue. Indeed, clear elevated cell proliferation expressed as enhanced phosphomonoesters is observed in the tumour vs. the healthy part of the tongue. This can be performed within a total scan duration of 30 min, comparable to clinical scans, with a spatial resolution of 1.5 cm for the 10-min 31P MRSI scan.

In vivo diffusion-weighted magnetic resonance imaging is limited in signal-to-noise-ratio (SNR) and acquisition time, which constrains spatial resolution to the macroscale regime. Ex vivo imaging, which allows for arbitrarily long scan times, is critical for exploring human brain structure in the mesoscale regime without loss of SNR. Standard head array coils designed for patients are sub-optimal for imaging ex vivo whole brain specimens. The goal of this work was to design and construct a 48-channel ex vivo whole brain array coil for high-resolution and high b-value diffusion-weighted imaging on a 3T Connectome scanner. The coil was validated with bench measurements and characterized by imaging metrics on an agar brain phantom and an ex vivo human brain sample. The two-segment coil former was constructed for a close fit to a whole human brain, with small receive elements distributed over the entire brain. Imaging tests including SNR and G-factor maps were compared to a 64-channel head coil designed for in vivo use. There was a 2.9-fold increase in SNR in the peripheral cortex and a 1.3-fold gain in the center when compared to the 64-channel head coil. The 48-channel ex vivo whole brain coil also decreases noise amplification in highly parallel imaging, allowing acceleration factors of approximately one unit higher for a given noise amplification level. The acquired diffusion-weighted images in a whole ex vivo brain specimen demonstrate the applicability and advantage of the developed coil for high-resolution and high b-value diffusion-weighted ex vivo brain MRI studies.

Purpose: Parallel transmit technology for MRI at 7 tesla will significantly benefit from high performance transmit arrays that offer high transmit efficiency and low mutual coupling between the individual array elements. A novel dual-mode transmit array with nested array elements has been developed to support imaging the human brain in both the single-channel (sTx) and parallel-transmit (pTx) excitation modes of a 7 tesla MRI scanner. In this work, the design, implementation, validation, specific absorption rate (SAR) management, and performance of the head coil is presented.Methods: The transmit array consisted of a nested arrangement to improve decoupling between the second-neighboring elements. Two large cut-outs were introduced in the RF shield for an open-face design to reduce claustrophobia and to allow patient monitoring. A hardware interface allows the coil to be used in both the sTx and pTx modes. SAR monitoring is done with virtual observation points (VOP) derived from human body models. The transmit efficiency and coverage is compared with the commercial single-channel and parallel-transmit head coils.Results: Decoupling inductors between the second-neighboring coil elements reduced the coupling to less than −20 dB. Local SAR estimates from the electromagnetic (EM) simulations were always less than the EM-based VOPs, which in turn were always less than scanner predictions and measurements for static and dynamic pTx waveforms. In sTx mode, we demonstrate improved coverage of the brain compared to the commercial sTx coil. The transmit efficiency is within 10% of the commercial pTx coil despite the two large cut-outs in the RF shield. In pTx mode, improved signal homogeneity was shown when the Universal Pulse was used for acquisition in vivo.Conclusion: A novel head coil which includes a nested eight-channel transmit array has been presented. The large cut-outs improve patient monitoring and reduce claustrophobia. For pTx mode, the EM simulation and VOP-based SAR management provided greater flexibility to apply pTx methods without the limitations of SAR constraints. For scanning in vivo, the coil was shown to provide an improved coverage in sTx mode compared to a standard commercial head coil.

In this paper, we propose a metamaterial-inspired RF coil structure based on a multiple split-ring resonator (MSRR) for 1.5T MRI. An additional component that enables to enhance the magnetic field of RF coil is proposed. This novel component is incorporated an array of split-rings and a central feature combined with square and circular rings (SC). By aligning the component with the split-ring resonator (SRR) in a coplanar arrangement, we derive a compact MSRR equipped with the SC structure (SC MSRR). We analyze the effect of different numbers of split rings within the component and the distance from the feed coil on gain of the magnetic field. The results indicate that the proposed metamaterial-inspired structure can effectively improve the near-field properties of the coil. The proposed SC MSRR has enhanced B 1 − field strength effectively and keeps a lower specific absorption rate (SAR) level in the field of view (FOV), resulting in a significant increase in SAR efficiency and received sensitivity. Compared with the conventional SRR surface coil, the peak B 1 − field strength can be enhanced by 3.15 times in the case of using SC MSRR, the peak SAR can be reduced by 20.14 % and the peak SAR efficiency can be improved 3.60 times. In addition, the proposed 4-channel head array consisting of SC MSRRs efficiently generates a higher quality B 1 − field compared to the standard head birdcage coil. The high directivity of the metamaterial-inspired structure allows for good isolation between ports/channels. The high compactness and excellent performance of the SC MSRR prove that it can be an efficient candidate for the surface coil and high density array. This approach provides a new practical way to combine RF coils with metamaterials for MRI applications. • Metamaterial-inspired SC MSRR structure for 1.5T MRI. ∙ Compared to SRR RF coil, SC MSRR offer higher receiver sensitivity, magnetic field penetration, and SAR efficiency. ∙ The SC MSRR with high directivity and good self-decoupling characteristics between channels. ∙ The SC MSRR element and array are more compact and have superior performance compared to other MRI enhancement devices. [ABSTRACT FROM AUTHOR]

The combined use of a torso-pelvic RF array coil and endorectal RF coil is the current state-of-the-art in prostate MRI. The endorectal coil provides high detection sensitivity to acquire high-spatial resolution images and spectroscopic data, while the torso-pelvic coil provides large coverage to assess pelvic lymph nodes and pelvic bones for metastatic disease. However, the use of an endorectal coil is an invasive procedure that presents difficulties for both patients and technicians. In this study, we propose a novel non-invasive RF coil design that can provide both image signal to noise ratio and field of view coverage comparable to the combined torso-pelvic and endorectal coil configuration. A prototype coil was constructed and tested using a pelvic phantom. The results demonstrate that this new design is a viable alternative for prostate MRI.

In magnetic resonance imaging (MRI), several studies have demonstrated that the metamaterial-based structures can effectively improve the sensitivity, and thus the signal-to-noise ratio (SNR), of receiving radio-frequency (RF) coils. However, the use of metamaterials for this type of the MRI application is often limited due to the bulkiness of the metamaterial structure at RF wavelengths and a lack of frequency tunability of the final design. In this work, we propose a planar compact sub-wavelength (1- field at the depth of 30mm into a load experimentally. Even at a penetration depth as much as 60 mm (deep brain in the case of head imaging), an enhancement of 9% was observed. Moreover, the magnetic field enhancement comes with a decrease (10%) in specific absorption rate (SAR). In terms of tuning, by controlling the water level in the cavity, the proposed spiral resonator shows a wide tuning range of 35MHz, centered around 64 MHz, with high tunability sensitivity (2.4-0.75 MHz/ml or 15-4.8 MHz/mm), which is due to the fact that the tuning cavity is located between the two spirals, where the fields are highly confined.

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided. [ABSTRACT FROM AUTHOR]

Purpose: In this work, we investigated how the position of the radiofrequency (RF) shield can affect the signal‐to‐noise ratio (SNR) of a receive RF coil. Our aim was to obtain physical insight for the design of a 10.5T 32‐channel head coil, subject to the constraints on the diameter of the RF shield imposed by the head gradient coil geometry. Method: We used full‐wave numerical simulations to investigate how the SNR of an RF receive coil depends on the diameter of the RF shield at ultra‐high magnetic field (UHF) strengths (≥7T). Results: Our simulations showed that there is an SNR‐optimal RF shield size at UHF strength, whereas at low field the SNR monotonically increases with the shield diameter. For a 32‐channel head coil at 10.5T, an optimally sized RF shield could act as a cylindrical waveguide and increase the SNR in the brain by 27% compared to moving the shield as far as possible from the coil. Our results also showed that a separate transmit array between the RF shield and the receive array could considerably reduce SNR even if they are decoupled. Conclusion: At sufficiently high magnetic field strength, the design of local RF coils should be optimized together with the design of the RF shield to benefit from both near field and resonant modes. [ABSTRACT FROM AUTHOR]

In this simulation study, we describe a microstrip transmission line (MTL) array coil, designed to provide a uniform |B1| field along the spinal cord in 7 T MRI. The coil structure was designed by segmenting the spinal cord into four sections according to its shape. The geometry and properties of the MTL coils were optimized individually to obtain a uniform |B1| field distributed according to the shape of the spinal cord in each section. On combining the individually-optimized MTL coils, we present a microstrip array design that produces a highly uniform |B1| field for the whole spinal cord. Furthermore, with this design, the |B1-| field contour lines followed the shape of the spinal cord. A comparison with a standard 8-channel loop array coil shows that our design produces a more uniform |B1-| field in the target imaging area, demonstrating its potential for spinal cord imaging. [ABSTRACT FROM AUTHOR]

Objective: The purpose of the present study was to fabricate a novel RF coil exclusively for visualizing submillimeter tissue structure and probing neuronal activity in cerebral cortex over anesthetized and awake animals on 7T human scanners. Methods: A novel RF coil design has been proposed for visualizing submillimeter tissue structure and probing neuronal activity in cerebral cortex over anesthetized and awake animals on 7T human scanners: a local transmit coil was utilized to save space for auxiliary device installation; 16 receive-only loops were densely arranged over a 5 cm-diameter circular area, with a diameter of 1.3 cm for each loop. Results: In anesthetized macaque experiments, 60 μm T2*-weighted images were successfully obtained with cortical gyri and sulci exquisitely visualized; over awake macaques, bilateral activations of visual areas including V1, V2, V4, and MST were distinctly detected at 1 mm; over the cat, robust activations were recorded in areas 17 and 18 (V1 and V2) as well as in their connected area of lateral geniculate nucleus (LGN) at 0.3 mm resolution. Conclusion: The promising brain imaging results along with flexibility in various size use of the presented design can be an effective and maneuverable solution to take one step close towards mesoscale cortical-related imaging. Significance: High-spatial-resolution brain imaging over large animals by using ultra-high-field (UHF) MRI will be helpful to understand and reveal functional brain organizations and the underlying mechanism in diseases. [ABSTRACT FROM AUTHOR]