Detunable wireless Litzcage coil for human head MRI at 1.5 T.

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.

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%.

Improving Specific Absorption Rate Efficiency and Coil Robustness of Self-Decoupled Transmit/Receive Coils by Elevating Feed and Mode Conductors.

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.

Minimal artifact actively shimmed metallic needles in MRI.

PURPOSE: Signal voids caused by metallic needles pose visualization and monitoring challenges in many MRI applications. In this work, we explore a solution to this problem in the form of an active shim insert that fits inside a needle and corrects the field disturbance (ΔB0 ) caused by the needle outside of it. METHODS: The ΔB0 induced by a 4 mm outside-diameter titanium needle at 3T is modeled and a two-coil orthogonal shim set is designed and fabricated to shim the ΔB0 . Signal recovery around the needle is assessed in multiple orientations in a water phantom with four different pulse sequences. Phase stability around the needle is assessed in an ex-vivo porcine tissue dynamic gradient echo experiment with and without shimming. Additionally, heating of the shim insert is assessed under 8 min of continuous operation with 1A current and concurrent imaging. RESULTS: An average recovery of ~63% of lost signal around the needle across orientations is shown with active shimming with a maximum current of 1.172 A. Signal recovery and correction of the underlying ΔB0 is shown to be independent of imaging sequence. Needle-induced phase gradients outside the perceptible signal void are also minimized with active shimming. Temperature rise of up to 0.9° Celsius is noted over 8 min of continuous 1A active shimming operation. CONCLUSION: A sequence independent method for minimization of metallic needle induced signal loss using an active shim insert is presented. The method has potential benefits in a range of qualitative and quantitative interventional MRI applications.

On the design and manufacturing of miniaturized microstripline power splitters for driving multicoil transmit arrays with arbitrary ratios at 7 T.

The purpose of the current study was to implement unequal microstrip power splitters for parallel transmission at 7 T that are optimized for size and loss and that can be configured for a wide range of power ratios. The splitters will enable the use of more transmit coils without a corresponding increase in the number of transmit channels or amplifiers to control specific absorption rate, shorten RF pulses, and shim inhomogeneous RF fields. Wilkinson unequal power splitters based on a novel microstrip network design were optimized to minimize their size under 8 cm in length and 9 cm in width, enabling them to be included in coil housing or cascaded in multiple stages. Splitters were designed and constructed for a wide range of output power ratios at 298 MHz. Simulations and bench tests were performed for each ratio, and a methodology was established to adapt the designs to other ratios and frequencies. The designs and code are open source and can be reproduced as is or reconfigured. The single-stage designs achieved good matches and isolations between output ports (worst isolation -15.9 dB, worst match -15.1 dB). A two-stage cascaded (one input to four outputs) power splitter with 1:2.5, 1:10, 1:3, and 1:6 ratio outputs was constructed. The worst isolation between output ports was -19.7 dB in simulation and the worst match of the three ports was -17.8 dB. The measured ratios for one- and two-stage boards were within 10% of the theoretical ratios. The power-handling capability of the smallest trace was approximately 70 W. Power loss for the one- and two-stage boards ranged from 1% to 3% in simulation compared with 5.1% to 7.2% on the bench. It was concluded that Wilkinson unequal microstrip power splitters can be implemented with a small board size (low height) and low loss, and across a wide range of output power ratios. The splitters can be cascaded in multiple stages while maintaining the expected ratios and low loss. This will enable the construction of large fixed transmit array-compression matrices with low loss.

Hybrid-pair ratio adjustable power splitters for add-on RF shimming and array-compressed parallel transmission.

PURPOSE: A ratio adjustable power splitter (RAPS) circuit was recently proposed for add-on RF shimming and array-compressed parallel transmission. Here we propose a new RAPS circuit design based on off-the-shelf components for improved performance and manufacturability. THEORY AND METHODS: The original RAPS used a pair of home-built Wilkinson splitter and hybrid coupler connected by a pair of connectorized coaxial cables. Here we propose a new hybrid-pair RAPS (or HP-RAPS) circuit that replaces the home-built circuits with two commercially available hybrid couplers and replaces connectorized cables with interchangeable microstrip lines. We derive the relation between the desired splitting ratio and the required phase shifts for HP-RAPS and investigate how to generate arbitrary splitting ratios using paired meandering and straight lines. Several HP-RAPSs with different splitting ratios were fabricated and tested on the workbench and MRI experiments. RESULTS: The splitting ratio of an HP-RAPS circuit has a tan or cot dependence on the meandering line's additional length compared to the straight line. The fabricated HP-RAPSs exhibit accurate splitting ratios as expected (<4% deviations) and generate transmit fields that well agree with predicted fields. They also demonstrated a low insertion loss of 0.33 dB, high output isolation of -26 dB, and acceptable impedance matching of -16 dB. CONCLUSION: A novel HP-RAPS circuit was developed and implemented. It is easy-to-fabricate/reproduce with minimal expertise. It also preserves the features of the original RAPS circuit (ratio-adjustable, small footprint, etc.) with lower insertion loss.

k-Space Domain Parallel Transmit Pulse Design.

PURPOSE: To accelerate the design of (under- or oversampled) multidimensional parallel transmission pulses. METHODS: A k-space domain parallel transmission pulse design algorithm was proposed that produces a sparse matrix relating a complex-valued target excitation pattern to the pulses that produce it, and can be finely parallelized. The algorithm was applied in simulations to the design of 3D SPINS pulses for inner volume excitation in the brain at 7 Tesla. It was characterized in terms of the dependence of computation time, excitation error, and required memory on algorithm parameters, and it was compared to an iterative spatial domain pulse design method in terms of computation time, excitation error, Gibbs ringing, and ability to compensate off-resonance. RESULTS: The proposed algorithm achieved approximately 80% faster pulse design compared to the spatial domain method with the same number of parallel threads, with the tradeoff of increased excitation error and RMS RF amplitude. It reduced the memory required to store the design matrix by 99% compared to a full matrix solution. Even with a coarse design grid, the algorithm produced patterns that were free of Gibbs ringing. It was similarly sensitive to k-space undersampling as the spatial domain method, and was similarly capable of compensating for off-resonance. CONCLUSIONS: The proposed k-space domain algorithm accelerates and finely parallelizes parallel transmission pulse design, with a modest tradeoff of excitation error and RMS RF amplitude.

Designing parallel transmit head coil arrays based on radiofrequency pulse performance.

PURPOSE: A new approach to design parallel transmit (pTx) head arrays is proposed that integrates transmit radiofrequency pulse designs with electromagnetic modeling of array coil elements. THEORY AND METHODS: An approach to design pTx head arrays is proposed that finds optimal groupings of a large number of coils into a small number of channels. An algorithm is proposed to extend array-compressed parallel transmit pulse design by adding the ability to optimally select and prune coil elements, in addition to optimizing compression weights. The performance of the method is demonstrated in simulations of dynamic multislice shimming of the human brain in axial, coronal, and sagittal directions, and of reduced field-of-view excitation targeting the human occipital lobe, with simulated electromagnetic field maps from a group of 5 human head models at 7T. RESULTS: For both dynamic multislice shimming and reduced field-of-view excitation, the method successfully designed pTx arrays that simultaneously achieved in general 15% lower mean excitation errors with 20% lower SDs, along with 20% lower mean global averaged specific absorption rate and 50% lower SD than previously reported pTx head array designs. CONCLUSION: With the proposed optimal coil element selection algorithm, the array-compressed parallel transmit pulse design can be extended to design pTx transmit head arrays with joint consideration of the fields within the sample and the radiofrequency pulse. The pTx arrays from such an approach achieved higher transmit excitation accuracy, lower radiofrequency heating in subjects, and more robust performance across subjects compared with previously reported pTx head arrays with the same number of channels.

Ratio-adjustable power splitters for array-compressed parallel transmission.

PURPOSE: To implement and validate low-loss ratio-adjustable power splitters (RAPS) for array-compressed parallel transmission (acpTx). METHODS: In acpTx, a small number of transmit channels drive a larger number of transmit coils, which are connected via an array compression network that implements optimized coil-to-channel combinations. Previous networks comprised a bank of power splitters, followed by attenuators to implement the amplitudes of the compression weights for each coil, but this resulted in high power dissipation in the network. Recognizing that an acpTx network need only implement relative attenuations between outputs, a RAPS circuit was developed which combines power splitting and relative attenuation, and has low insertion loss. RAPS circuits were experimentally validated and used to build an array compression network for a one-channel-to-four-coil spiral acpTx excitation experiment. RESULTS: Bench tests showed that the RAPS circuits came within 0.05 dB of the desired output ratios, and power dissipation was approximately 0.5 dB (10%). The spiral excitation experiment showed that the ability to optimally drive four coils with a single channel reduced excitation error by 46% compared to driving one coil, without using attenuators in the array compression network. CONCLUSION: RAPS circuits enable the construction of low-loss array compression networks for parallel transmission. Magn Reson Med 79:2422-2431, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Traveling-wave meets standing-wave: A simulation study using pair-of-transverse-dipole-ring (PTDR) coils for adjustable longitudinal coverage in ultra-high field MRI.

1.At ultrahigh fields (B 0 ≥7T), it is challenging to cover a large field of view using single-row conventional RF coils (standing wave resonators) due to the limited physical dimensions. In contrast, traveling wave approaches can excite large fields of view even using a relatively simple hardware setup, but suffer from poor efficiency and high local specific absorption rate in non-imaged regions. In this study, we propose and numerically analyze a new coil which combines the concept of traveling wave and standing wave. The new coil consists of a Pair of Transverse Dipole Rings (PTDR) whose separation is adjusted according to the desired imaging coverage. The PTDR coil was validated using electromagnetic (EM) simulations in phantoms and human leg models, which showed that coverage can be as long as 60 cm. When the coverage of the PTDR coil was shortened to 20 cm to cover the knees only, it's transmit and SAR efficiencies were 84% and 37% higher than those of the 50 cm coverage, respectively.

Array-compressed parallel transmit pulse design.

PURPOSE: To design array-compressed parallel transmit radiofrequency (RF) pulses and compare them to pulses designed with existing transmit array compression strategies. THEORY AND METHODS: Array-compressed parallel RF pulse design is proposed as the joint optimization of a matrix of complex-valued compression weights that relate a full-channel physical array to a reduced-channel virtual array, along with a set of RF pulses for the virtual array. In this way, the physics of the RF pulse application determine the coil combination weights. Array-compressed pulse design algorithms are described for four parallel transmit applications: accelerated two-dimensional spiral excitation, multislice RF shimming, small-tip-angle kT -points excitation, and slice-selective spokes refocusing. Array-compressed designs are compared in simulations and an experiment to pulses designed using four existing array compression strategies. RESULTS: In all cases, array-compressed pulses achieved the lowest root-mean-square excitation error among the array compression approaches. Low errors were generally achieved without increasing root-mean-square RF amplitudes or maximum local 10-gram specific absorption rate. Leave-one-out multisubject shimming simulations demonstrated that array-compressed RF shimming can identify useful fixed coil combination weights that perform well across a population. CONCLUSION: Array-compressed pulse design jointly identifies the transmit coil array compression weights and RF pulses that perform best for a specific parallel excitation application. Magn Reson Med 76:1158-1169, 2016. © 2015 Wiley Periodicals, Inc.

Experimental implementation of array-compressed parallel transmission at 7 tesla.

PURPOSE: To implement and validate a hardware-based array-compressed parallel transmission (acpTx) system. METHODS: In array-compressed parallel transmission, a small number of transmit channels drive a larger number of transmit coils, which are connected via an array compression network that implements optimized coil-to-channel combinations. A two channel-to-eight coil array compression network was developed using power splitters, attenuators and phase shifters, and a simulation was performed to investigate the effects of coil coupling on power dissipation in a simplified network. An eight coil transmit array was constructed using induced current elimination decoupling, and the coil and network were validated in benchtop measurements, B1+ mapping scans, and an accelerated spiral excitation experiment. RESULTS: The developed attenuators came within 0.08 dB of the desired attenuations, and reflection coefficients were -22 dB or better. The simulation demonstrated that up to 3× more power was dissipated in the network when coils were poorly isolated (-9.6 dB), versus well-isolated (-31 dB). Compared to split circularly-polarized coil combinations, the additional degrees of freedom provided by the array compression network led to 54% lower squared excitation error in the spiral experiment. CONCLUSION: Array-compressed parallel transmission was successfully implemented in a hardware system. Further work is needed to develop remote network tuning and to minimize network power dissipation. Magn Reson Med 75:2545-2552, 2016. © 2016 Wiley Periodicals, Inc.

Simulation verification of SNR and parallel imaging improvements by ICE-decoupled loop array in MRI.

Transmit/receive L/C loop arrays with the induced current elimination (ICE) or magnetic wall decoupling method has shown high signal-to-noise (SNR) and excellent parallel imaging ability for MR imaging at ultrahigh fields, e.g., 7 T. In this study, we aim to numerically analyze the performance of an eight-channel ICE-decoupled loop array at 7 T. Three dimensional (3-D) electromagnetic (EM) and radiofrequency (RF) circuit co-simulation approach was employed. The values of all capacitors were obtained by optimizing the S-parameters of all coil elements. The EM simulation accurately modeled the coil structure, the phantom and the excitation. All coil elements were well matched to 50 ohm and the isolation between any two coil elements was better -15 dB. The simulated S parameters were exactly similar with the experimental results, indicating the simulation results were reliable. Compared with the conventional capacitively decoupled array, the ICE-decoupled array had higher sensitivity at the peripheral areas of the image subjects due to the shielding effect of the decoupling loops. The increased receive sensitivity resulted in an improvement of signal intensity and SNR for the ICE-decoupled array.

Eight-Channel Monopole Array Using ICE Decoupling for Human Head MR Imaging at 7 T.

Due to the unique structure of radiative coil elements, traditional decoupling methods face technical challenges in reducing the electromagnetic coupling of the radiative arrays. In this study, we aim to investigate the possibility of using the recently introduced induced current elimination (ICE) decoupling technique for cylindrical shaped radiative coil array designs. To evaluate the method, an eight-channel transmit/receive monopole array with the ICE decoupling, suitable for human head imaging at 7 T, was built and comparatively investigated. In vivo human head images were acquired and geometry factor maps were measured and calculated to evaluate the performance of the ICE-decoupled monopole array. Compared with the monopole array without decoupling methods, the ICE-decoupled monopole array had a higher signal-to-noise ratio and demonstrated improved parallel imaging ability. The experimental results indicate that the ICE decoupling method is a promising solution to addressing the coupling issue of radiative array at ultrahigh fields.

Closely-spaced double-row microstrip RF arrays for parallel MR imaging at ultrahigh fields.

Radiofrequency (RF) coil arrays with high count of elements, e.g., closely-spaced multi-row arrays, exhibit superior parallel imaging performance in MRI. However, it is technically challenging and time-consuming to build multi-row arrays due to complex coupling issues. This paper presents a novel and simple method for closely-spaced multi-row RF array designs. Induced current elimination (ICE) decoupling method has shown the capability of reducing coupling between microstrip elements from different rows. In this study, its capability for decoupling array elements from the same row was investigated and validated by bench tests, with an isolation improvement from -8.9 dB to -20.7 dB. Based on this feature, a closely-spaced double-row microstrip array with 16 elements was built at 7T. S21 between any two elements of the 16-channel closely-spaced was better than -14 dB. In addition, its feasibility and performance was validated by MRI experiments. No significant image reconstruction- related noise amplifications were observed for parallel imaging even when reduced factor (R) achieves 4. The experimental results demonstrated that the proposed design might be a simple and efficient approach in fabricating closely-spaced multi-row RF arrays.

Design and Test of Magnetic Wall Decoupling for Dipole Transmit/Receive Array for MR Imaging at the Ultrahigh Field of 7T.

Radio-frequency coil arrays using dipole antenna technique have been recently applied for ultrahigh field magnetic resonance (MR) imaging to obtain the better signal-noise-ratio (SNR) gain at the deep area of human tissues. However, the unique structure of dipole antennas makes it challenging to achieve sufficient electromagnetic decoupling among the dipole antenna elements. Currently, there is no decoupling methods proposed for dipole antenna arrays in MR imaging. The recently developed magnetic wall (MW) or induced current elimination decoupling technique has demonstrated its feasibility and robustness in designing microstrip transmission line arrays, L/C loop arrays and monopole arrays. In this study, we aim to investigate the possibility and performance of MW decoupling technique in dipole arrays for MR imaging at the ultrahigh field of 7T. To achieve this goal, a two-channel MW decoupled dipole array was designed, constructed and analyzed experimentally through bench test and MR imaging. Electromagnetic isolation between the two dipole elements was improved from about -3.6 dB (without any decoupling treatments) to -16.5 dB by using the MW decoupling method. MR images acquired from a water phantom using the MW decoupled dipole array and the geometry factor maps were measured, calculated and compared with those acquired using the dipole array without decoupling treatments. The MW decoupled dipole array demonstrated well-defined image profiles from each element and had better geometry factor over the array without decoupling treatments. The experimental results indicate that the MW decoupling technique might be a promising solution to reducing the electromagnetic coupling of dipole arrays in ultrahigh field MRI, consequently improving their performance in SNR and parallel imaging.

Hybrid monopole/loop coil array for human head MR imaging at 7T.

The monopole coil and loop coil have orthogonal radiofrequency (RF) fields and thus are intrinsically decoupled electromagnetically if they are laid out appropriately. In this study, we proposed a hybrid monopole/loop technique which could combine the advantages of both loop arrays and monopole arrays. To investigate this technique, a hybrid RF coil array containing 4 monopole channels and 4 loop channels was developed for human head MR imaging at 7T. In vivo MR imaging and g-factor results using monopole-only channels, loop-only channels and all channels of the hybrid array were acquired and evaluated. Compared with the monopole-only and loop-only channels, the proposed hybrid array has higher SNR and better parallel imaging performance. Sufficient electromagnetic decoupling and diverse RF magnetic field (B1) distributions of monopole channels and loop channels may contribute to this performance improvement. From experimental results, the hybrid monopole/loop array has low g-factor and excellent SNR at both periphery and center of the brain, which is valuable for human head imaging at ultrahigh fields.

Detunable wireless resonator arrays for TMJ MRI: A comparative study.

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.

A detunable wireless resonator insert for high-resolution TMJ MRI at 1.5 T.

MRI is essential for evaluating and diagnosing various conditions affecting the temporomandibular joint (TMJ) and surrounding structures, as it provides highly detailed images that enable healthcare professionals to assess the joints and surroundings in great detail. While commercial MRI scanners typically come equipped with basic receive coils, such as the head receive array, RF coils tailored for specialized applications like TMJ MRI must be obtained separately. Consequently, TMJ MRI scans are often conducted using the head receive array, yet this configuration proves suboptimal due to the lack of specialized coils. In this study, we introduce a simple, low-cost, and easy-to-reproduce wireless resonator insert to enhance the quality of TMJ MRI at 1.5 T. The wireless resonator shows a significant improvement in signal-to-noise ratio (SNR) and noticeably better imaging quality than the head array alone configuration in both phantom and in vivo images.

Miniature and flexible Bazooka balun for high-field MRI.

Flexible coils offer improved patient comfort and better imaging quality. However, rigid and bulky baluns in RF coils limit flexibility and manufacturing. A miniaturized and flexible balun design was proposed to address this issue. It replaced rigid components with an ultra-flexible rubber tube and a flexible coaxial capacitor. Simulations validated the concept, and bench tests confirmed its performance, including a measured common-mode rejection ratio of -15.8 dB. The flexible balun was integrated into a 4-channel coil array, evaluating impedance changes caused by the "hand effect." Compared to coils without the balun, the flexible coil with the proposed balun showed improved robustness in impedance matching and inter-element couplings. Transmit efficiency of the flexible coil with the balun was compared to coils without a balun and with a rigid, shielded cable trap. Results demonstrated that the proposed balun circuit maintained high transmit efficiency. Overall, the flexible balun design offers a promising solution for improving the flexibility and performance of RF coil arrays in MRI applications.