Transmit and receive transmission line arrays for 7 Tesla parallel imaging

Electric field and SAR reduction in high-impedance RF arrays by using high permittivity materials for 7T MR imaging

In the field of ultra-high field MR imaging, the challenges associated with higher frequencies and shorter wavelengths necessitate rigorous attention to multichannel array design. While the need for such arrays remains, and efforts to increase channel counts continue, a persistent impediment-inter-element coupling-constantly hinders development. This coupling degrades current and field distribution, introduces noise correlation between channels, and alters the frequency of array elements, affecting image quality and overall performance. The goal of optimizing ultra-high field MRI goes beyond resolving inter-element coupling and includes significant safety considerations related to the design changes required to achieve high-impedance coils. Although these coils provide excellent isolation, the higher impedance needs special design changes. However, such changes pose a significant safety risk in the form of strong electric fields across low-capacitance lumped components. This process may raise Specific Absorption Rate (SAR) values in the imaging subject, increasing power deposition and, as a result, the risk of tissue heating-related injury. To balance the requirement of inter-element decoupling with the critical need for safety, we suggest a new solution. Our method uses high-dielectric materials to efficiently reduce electric fields and SAR values in the imaging sample. This intervention tries to maintain B1 efficiency and inter-element decoupling within the existing array design, which includes high-impedance coils. Our method aims to promote the full potential of ultra-high field MRI by alleviating this critical safety concern with minimal changes to the existing array setup.

Universal modes: Calibration-free time-interleaved acquisition of modes

PURPOSE

To introduce universal modes by applying the universal pulse concept to time-interleaved acquisition of modes (TIAMO), thereby achieving calibration-freeB 1 + $$ {B}_1^{+} $$ inhomogeneity mitigation for body imaging at ultra-high fields.

METHODS

Two databases of different RF arrays were used to demonstrate the feasibility of universal modes. The first comprised 31 cardiac in vivo data sets acquired at 7T while the second consisted of 6 simulated 10.5T pelvic data sets. Subject-specific solutions and universal modes were computed and subsequently evaluated alongside predefined default modes. For the cardiac database, subdivision into subpopulations was investigated. The optimization was performed using least-squares (LS) TIAMO and acquisition modes optimized for refocused echoes (AMORE). Finally, universal modes based on simulated pelvis data were applied in vivo at 10.5T.

RESULTS

In all studied cases, the universal modes yield improvements over the predefined default modes of up to 51% (cardiac) and 30% (pelvic) in terms of median excitation error when using two modes. The subpopulation-specific cardiac solutions revealed a further improvement of universal modes at the expense of increased errors when applied outside the appropriate subpopulation. Direct application of simulation-based universal modes in vivo resulted in up to a 14% reduction in excitation error compared to default modes and up to a 34% reduction in peak 10 g local specific absorption rate (SAR) compared to subject-specific solutions.

CONCLUSIONS

Universal modes are feasible for calibration-freeB 1 + $$ {B}_1^{+} $$ inhomogeneity mitigation at ultra-high fields. In addition, simulation-based solutions can be applied directly in vivo, eliminating the need for large in vivo databases.

Dual-Tuned Coaxial-Transmission-Line RF Coils for Hyperpolarized 13C and Deuterium 2H Metabolic MRS Imaging at Ultrahigh Fields

OBJECTIVE

Information on the metabolism of tissues in healthy and diseased states plays a significant role in the detection and understanding of tumors, neurodegenerative diseases, diabetes, and other metabolic disorders. Hyperpolarized carbon-13 magnetic resonance imaging (13C-HPMRI) and deuterium metabolic imaging (2H-DMI) are two emerging X-nuclei used as practical imaging tools to investigate tissue metabolism. However due to their low gyromagnetic ratios (ɣ13C = 10.7 MHz/T; ɣ2H = 6.5 MHz/T) and natural abundance, such method required a sophisticated dual-tuned radiofrequency (RF) coil.

METHODS

Here, we report a dual-tuned coaxial transmission line (CTL) RF coil agile for metabolite information operating at 7T with independent tuning capability. The design analysis has demonstrated how both resonant frequencies can be individually controlled by simply varying the constituent of the design parameters.

RESULTS

Numerical results have demonstrated a broadband tuning range capability, covering most of the X-nucleus signal, especially the 13C and 2H spectra at 7T. Furthermore, in order to validate the feasibility of the proposed design, both dual-tuned 1H/13C and 1H/2H CTLs RF coils are fabricated using a semi-flexible RG-405 .086" coaxial cable and bench test results (scattering parameters and magnetic field efficiency/distribution) are successfully obtained.

CONCLUSION

The proposed dual-tuned RF coils reveal highly effective magnetic field obtained from both proton and heteronuclear signal which is crucial for accurate and detailed imaging.

SIGNIFICANCE

The successful development of this new dual-tuned RF coil technique would provide a tangible and efficient tool for ultrahigh field metabolic MR imaging.

Twisted pair transmission line coil - a flexible, self-decoupled and robust element for 7 T MRI

OBJECTIVE

This study evaluates the performance of a twisted pair transmission line coil as a transceive element for 7 T MRI in terms of physical flexibility, robustness to shape deformations, and interelement decoupling.

METHODS

Each coil element was created by shaping a twisted pair of wires into a circle. One wire was interrupted at the top, while the other was interrupted at the bottom, and connected to the matching circuit. Electromagnetic simulations were conducted to determine the optimal number of twists per length (in terms of B₁+ field efficiency, SAR efficiency, sensitivity to elongation, and interelement decoupling properties) and for investigating the fundamental operational principle of the coil through fields streamline visualisation. A comparison between the twisted pair coil and a conventional loop coil in terms of B₁+ fields, maxSAR₁₀g, and stability of S₁₁ when the coil was deformed was performed. Experimentally measured interelement coupling between individual elements of multichannel arrays was also investigated.

RESULTS

Increasing the number of twists per length resulted in a more physically robust coil. Poynting vector streamline visualisation showed that the twisted pair coil concentrated most of the energy in the near field. The twisted pair coil exhibited comparable B₁+ fields and improved maxSAR₁₀g to the conventional coil but demonstrated exceptional stability with respect to coil deformation and a strong self-decoupling nature when placed in an array configuration.

DISCUSSION

The findings highlight the robustness of the twisted pair coil, showcasing its stability under shape variations. This coil holds great potential as a flexible RF coil for various imaging applications using multiple-element arrays, benefiting from its inherent decoupling.

Local SAR management strategies to use two-channel RF shimming for fetal MRI at 3 T

PURPOSE

This study evaluates the imaging performance of two-channel RF-shimming for fetal MRI at 3 T using four different local specific absorption rate (SAR) management strategies.

METHODS

Due to the ambiguity of safe local SAR levels for fetal MRI, local SAR limits for RF shimming were determined based on either each individual's own SAR levels in standard imaging mode (CP mode) or the maximum SAR level observed across seven pregnant body models in CP mode. Local SAR was constrained either indirectly by further constraining the whole-body SAR (wbSAR) or directly by using subject-specific local SAR models. Each strategy was evaluated by the improvement of the transmit field efficiency (average |B1 + |) and nonuniformity (|B1 + | variation) inside the fetus compared with CP mode for the same wbSAR.

RESULTS

Constraining wbSAR when using RF shimming decreases B1 + efficiency inside the fetus compared with CP mode (by 12%-30% on average), making it inefficient for SAR management. Using subject-specific models with SAR limits based on each individual's own CP mode SAR value, B1 + efficiency and nonuniformity are improved on average by 6% and 13% across seven pregnant models. In contrast, using SAR limits based on maximum CP mode SAR values across seven models, B1 + efficiency and nonuniformity are improved by 13% and 25%, compared with the best achievable improvement without SAR constraints: 15% and 26%.

CONCLUSION

Two-channel RF-shimming can safely and significantly improve the transmit field inside the fetus when subject-specific models are used with local SAR limits based on maximum CP mode SAR levels in the pregnant population.

Improved 1 H body imaging at 10.5 T: Validation and VOP-enabled imaging in vivo with a 16-channel transceiver dipole array

PURPOSE

To increase the RF coil performance and RF management for body imaging at 10.5 T by validating and evaluating a high-density 16-channel transceiver array, implementing virtual observation points (VOPs), and demonstrating specific absorption rate (SAR) constrained imaging in vivo.

METHODS

The inaccuracy of the electromagnetic model of the array was quantified based on B1 + and SAR data. Inter-subject variability was estimated using a new approach based on the relative SAR deviation of different RF shims between human body models. The pTx performance of the 16-channel array was assessed in simulation by comparison to a previously demonstrated 10-channel array. In vivo imaging of the prostate was performed demonstrating SAR-constrained static RF shimming and acquisition modes optimized for refocused echoes (AMORE).

RESULTS

The model inaccuracy of 29% and the inter-subject variability of 85% resulted in a total safety factor of 1.91 for pelvis studies. For renal and cardiac imaging, inter-subject variabilities of 121% and 141% lead to total safety factors of 2.25 and 2.45, respectively. The shorter wavelength at 10.5 T supported the increased element density of the 16-channel array which in turn outperformed the 10-channel version for all investigated metrics. Peak 10 g local SAR reduction of more than 25% without a loss of image quality was achieved in vivo, allowing a theoretical improvement in measurement efficiency of up to 66%.

CONCLUSIONS

By validating and characterizing a 16-channel dipole transceiver array, this work demonstrates, for the first time, a VOP-enabled RF coil for human torso imaging enabling increased pTx performance at 10.5 T.

An 8-channel Tx dipole and 20-channel Rx loop coil array for MRI of the cervical spinal cord at 7 Tesla

The quality of cervical spinal cord images can be improved by the use of tailored radiofrequency (RF) coil solutions for ultrahigh field imaging; however, very few commercial and research 7-T RF coils currently exist for the spinal cord, and in particular, those with parallel transmission (pTx) capabilities. This work presents the design, testing, and validation of a pTx/Rx coil for the human neck and cervical/upper thoracic spinal cord. The pTx portion is composed of eight dipoles to ensure high homogeneity over this large region of the spinal cord. The Rx portion is made up of twenty semiadaptable overlapping loops to produce high signal-to-noise ratio (SNR) across the patient population. The coil housing is designed to facilitate patient positioning and comfort, while also being tight fitting to ensure high sensitivity. We demonstrate RF shimming capabilities to optimize B1 + uniformity, power efficiency, and/or specific absorption rate efficiency. B1 + homogeneity, SNR, and g-factor were evaluated in adult volunteers and demonstrated excellent performance from the occipital lobe down to the T4-T5 level. We compared the proposed coil with two state-of-the-art head and head/neck coils, confirming its superiority in the cervical and upper thoracic regions of the spinal cord. This coil solution therefore provides a convincing platform for producing the high image quality necessary for clinical and research scanning of the upper spinal cord.

Double cross magnetic wall decoupling for quadrature transceiver RF array coils using common-mode differential-mode resonators

In contrast to linearly polarized RF coil arrays, quadrature transceiver coil arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution, and parallel imaging performance. Owing to a reduced excitation power, a low specific absorption rate can also be obtained using quadrature RF coils. However, due to the complex nature of their structure and their electromagnetic properties, it is challenging to achieve sufficient electromagnetic decoupling while designing multichannel quadrature RF coil arrays, particularly in ultra-high fields. In this work, we proposed a double-cross magnetic wall decoupling for quadrature transceiver RF arrays and implemented the decoupling method on common-mode differential mode quadrature (CMDM) quadrature transceiver arrays at an ultrahigh field of 7 T. The proposed magnetic decoupling wall, comprised of two intrinsically decoupled loops, is used to reduce the mutual coupling between all the multi-mode currents present in the quadrature CMDM array. The decoupling network has no physical connection with the CMDMs' resonators, which provides less design constraint over size-adjustable RF arrays. To validate the feasibility of the proposed cross-magnetic decoupling wall, systematic studies on the decoupling performance based on the impedance of two intrinsic loops are numerically performed. A pair of quadrature transceiver CMDMs is constructed along with the proposed decoupling network, and their scattering matrix is characterized using a network analyzer. The measured results indicate that all the current modes from coupling are simultaneously suppressed using the proposed cross-magnetic wall. Moreover, field distribution and local specific absorption rate (SAR) are numerically obtained for a well-decoupled 8-channel quadrature knee-coil array.

A field focusing butterfly stripline detects NMR at higher signal-to-noise ratio

We present a compact tuned magnetic resonance detector that merges the conductor topology of a butterfly coil with that of a stripline, thereby increasing the magnetic field intensity B1 per unit current, which increases the detection signal-to-noise ratio for mass-limited samples by a factor of 2. The s-parameter measurements further reveal improved radiofrequency shielding through the suppression of B1 outside the coil when operated within an array of similar detectors. Simulations additionally show a sharper B1 fall-off for the butterfly stripline outside the sensitive sample region. Our design is compatible with 2D planar manufacturing procedures, such as printed circuit board technology, and surface micromachining.

Towards an integrated radiofrequency safety concept for implant carriers in MRI based on sensor-equipped implants and parallel transmission

To protect implant carriers in MRI from excessive radiofrequency (RF) heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant-related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state-of-the-art field simulations and the implant-specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal-cord implant in an eight-channel pTx body coil at 3 T . To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric (e.g., specific absorption rate [SAR]). Virtual experiments show that E -field and implant-current sensors are well suited for this purpose, while temperature sensors require some caution, and B 1 probes are inadequate. Based on an implant sensor matrix Q s , constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant-related) safety requirements are satisfied. Within this safe-excitation subspace, the solution with the best image quality in terms of B 1 + magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a 3-fold higher mean B 1 + magnitude compared with circularly polarized excitation for a maximum implant-related temperature increase ∆ T imp ≤ 1 K . To date, sensor-equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. This paper aims to demonstrate the significant benefits of such an approach and how this could impact implant-related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator's current responsibility.

High-resolution imaging of the excised porcine heart at a whole-body 7 T MRI system using an 8Tx/16Rx pTx coil

INTRODUCTION

MRI of excised hearts at ultra-high field strengths ([Formula: see text]≥7 T) can provide high-resolution, high-fidelity ground truth data for biomedical studies, imaging science, and artificial intelligence. In this study, we demonstrate the capabilities of a custom-built, multiple-element transceiver array customized for high-resolution imaging of excised hearts.

METHOD

A dedicated 16-element transceiver loop array was implemented for operation in parallel transmit (pTx) mode (8Tx/16Rx) of a clinical whole-body 7 T MRI system. The initial adjustment of the array was performed using full-wave 3D-electromagnetic simulation with subsequent final fine-tuning on the bench.

RESULTS

We report the results of testing the implemented array in tissue-mimicking liquid phantoms and excised porcine hearts. The array demonstrated high efficiency of parallel transmits characteristics enabling efficient pTX-based B₁⁺-shimming.

CONCLUSION

The receive sensitivity and parallel imaging capability of the dedicated coil were superior to that of a commercial 1Tx/32Rx head coil in both SNR and T₂*-mapping. The array was successfully tested to acquire ultra-high-resolution (0.1 × 0.1 × 0.8 mm voxel) images of post-infarction scar tissue. High-resolution (isotropic 1.6 mm³ voxel) diffusion tensor imaging-based tractography provided high-resolution information about normal myocardial fiber orientation.

Scalable and modular 8-channel transmit and 8-channel flexible receive coil array for 19 F MRI of large animals

PURPOSE

To introduce an RF coil system consisting of an 8-channel transmit (Tx) and 8-channel receive (Rx) coil arrays for ¹⁹ F MRI of large animals.

METHODS

The Tx efficiency and homogeneity of the 8-element loop coil array (loop size: 6 × 15 cm² ) were simulated for two different pig models rendered from MR images. An 8-channel Rx coil array consisting of a flexible 6-channel posterior and a 2-channel planar anterior array was designed to fit on the abdomen of an average-sized pig in supine position. Measurements were performed in a grid phantom and ex vivo on a pig model with perfluoroctylbromide (PFOB)-filled tubes inserted in the thorax.

RESULTS

Measured and simulated Tx efficiency and homogeneity for the 8-channel and 5-channel arrays were in good agreement: 1.87 ± 0.22μT/√kW versus 1.96 ± 0.29μT/√kW, and 2.29 ± 0.39μT/√kW versus 2.41 ± 0.37μT/√kW. An isolation of 38 ± 8 dB is achieved between the ¹⁹ F Tx and Rx elements, and over 30 dB between the ¹ H and ¹⁹ F elements. The PFOB-filled vials could be clearly identified within the cadaver abdomen with an SNR of 275 ± 51 for a 3D gradient-echo sequence with 2-mm isotropic resolution and 12 averages, acquired in 9:52 min:s. Performance of the Tx array was robust against phase and amplitude mismatches at the input ports.

CONCLUSIONS

A modular and scalable Tx array offers improved Tx efficiency in ¹⁹ F MRI of large animals with various sizes. Although conventional birdcage coils have superior Tx efficiency within the target region of interest, scalability of the Tx array to animal size is a major benefit. The described ¹⁹ F coil provides homogeneous excitation and high sensitivity detection in large pig models.

8-channel Tx dipole and 20-channel Rx loop coil array for MRI of the cervical spinal cord at 7 Tesla

The quality of cervical spinal cord images can be improved by the use of tailored radiofrequency coil solutions for ultra-high field imaging; however, very few commercial and research 7 Tesla radiofrequency coils currently exist for the spinal cord, and in particular those with parallel transmit capabilities. This work presents the design, testing and validation of a pTx/Rx coil for the human neck and cervical/upper-thoracic spinal cord. The pTx portion is composed of 8 dipoles to ensure high homogeneity over this large region of the spinal cord. The Rx portion is made of 20 semi-adaptable overlapping loops to produce high Signal-to-noise ratio (SNR) across the patient population. The coil housing is designed to facilitate patient positioning and comfort, while being tight fitting to ensure high sensitivity. We demonstrate RF shimming capabilities to optimize B ₁ ⁺ uniformity, power efficiency and/or specific absorption rate (SAR) efficiency. B ₁ ⁺ homogeneity, SNR and g-factor was evaluated in adult volunteers and demonstrated excellent performance from the occipital lobe down to the T4-T5 level. We compared the proposed coil with two state-of-the-art head and head/neck coils, confirming its superiority in the cervical and upper-thoracic regions of the spinal cord. This coil solution therefore provides a convincing platform for producing the high image quality necessary for clinical and research scanning of the upper spinal cord.

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.

Ultra-high field MRI: parallel-transmit arrays and RF pulse design

This paper reviews the field of multiple or parallel radiofrequency (RF) transmission for magnetic resonance imaging (MRI). Currently the use of ultra-high field (UHF) MRI at 7 tesla and above is gaining popularity, yet faces challenges with non-uniformity of the RF field and higher RF power deposition. Since its introduction in the early 2000s, parallel transmission (pTx) has been recognized as a powerful tool for accelerating spatially selective RF pulses and combating the challenges associated with RF inhomogeneity at UHF. We provide a survey of the types of dedicated RF coils used commonly for pTx and the important modeling of the coil behavior by electromagnetic (EM) field simulations. We also discuss the additional safety considerations involved with pTx such as the specific absorption rate (SAR) and how to manage them. We then describe the application of pTx with RF pulse design, including a practical guide to popular methods. Finally, we conclude with a description of the current and future prospects for pTx, particularly its potential for routine clinical use.

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

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

Hairpin RF resonators for MR imaging transceiver arrays with high inter-channel isolation and B1 efficiency at ultrahigh field 7 T

Electromagnetic decoupling among a close-fitting or high-density transceiver RF array elements is required to maintain the integrity of the magnetic flux density from individual channel for enhanced performance in detection sensitivity and parallel imaging. High-impedance RF coils have demonstrated to be a prominent design method to circumvent these coupling issues. Yet, inherent characteristics of these coils have ramification on the B₁ field efficiency and SNR. In this work, we propose a hairpin high impedance RF resonator design for highly decoupled multichannel transceiver arrays at ultrahigh magnetic fields. Due to the high impedance property of the hairpin resonators, the proposed transceiver array can provide high decoupling performance without using any dedicated decoupling circuit among the resonant elements. Because of elimination of lumped inductors in the resonator circuit, higher B₁ field efficiency in imaging subjects can be expected. In order to validate the feasibility of the proposed hairpin RF coils, systematical studies on decoupling performance, field distribution, and SNR are performed, and the results are compared with those obtained from existing high-impedance RF coil, e.g., "self-decoupled RF coil". To further investigate its performance, an 8-channel head coil array using the proposed hairpin resonators loaded with a cylindrical phantom is designed, demonstrating a 19 % increase of the B₁⁺ field intensity compared to the self-decoupled coils at 7 T. Furthermore, the characteristics of the hairpin RF coils are evaluated using a more realistic human head voxel model numerically. The proposed hairpin RF coil provides excellent decoupling performance and superior RF magnetic field efficiency compared to the "self-decoupled" high impedance coils. Bench test of a pair of fabricated hairpin coils prove to be in good accordance with numerical results.

Parallel transmission 2D RARE imaging at 7T with transmit field inhomogeneity mitigation and local SAR control

PURPOSE

We develop and test a parallel transmit (pTx) pulse design framework to mitigate transmit field inhomogeneity with control of local specific absorption rate (SAR) in 2D rapid acquisition with relaxation enhancement (RARE) imaging at 7T.

METHODS

We design large flip angle RF pulses with explicit local SAR constraints by numerical simulation of the Bloch equations. Parallel computation and analytical expressions for the Jacobian and the Hessian matrices are employed to reduce pulse design time. The refocusing-excitation "spokes" pulse pairs are designed to satisfy the Carr-Purcell-Meiboom-Gill (CPMG) condition using a combined magnitude least squares-least squares approach.

RESULTS

In a simulated dataset, the proposed approach reduced peak local SAR by up to 56% for the same level of refocusing uniformity error and reduced refocusing uniformity error by up to 59% (from 32% to 7%) for the same level of peak local SAR compared to the circularly polarized birdcage mode of the pTx array. Using explicit local SAR constraints also reduced peak local SAR by up to 46% compared to an RF peak power constrained design. The excitation and refocusing uniformity error were reduced from 20%-33% to 4%-6% in single slice phantom experiments. Phantom experiments demonstrated good agreement between the simulated excitation and refocusing uniformity profiles and experimental image shading.

CONCLUSION

PTx-designed excitation and refocusing CPMG pulse pairs can mitigate transmit field inhomogeneity in the 2D RARE sequence. Moreover, local SAR can be decreased significantly using pTx, potentially leading to better slice coverage, enabling larger flip angles or faster imaging.

The design and evaluation of single-channel loopole coils at 7T MRI

Objective. Improving the local uniformity of B 1 + field for awake monkey brain magnetic resonance imaging (MRI) at ultra-high fields while facilitating convenient placement and fixation of MRI-compatible multimodal devices for neuroscience study, can eventually advance our understanding of the primate’s brain organization. Approach. A group of single-channel RF coils including conventional loop coils and loopole coils sharing the same size and shape were designed for comparison; their performance as the transmit coil was quantitatively evaluated through a series of numerical electromagnetic (EM) simulations, and further verified by using 7T MRI over a saline phantom and a monkey in vivo. Main results. Compared to conventional loop coils, the optimized loopole coil brought up to 23.5% B 1 + uniformity improvement for monkey brain imaging in EM simulations, and this performance was further verified over monkey brain imaging at 7T in vivo. Importantly, we have systematically explored the underlying mechanism regarding the relationship between loopole coils’ current density distribution and B 1 + uniformity, observing that it can be approximated as a sinusoidal curve. Significance. The proposed loopole coil design can improve the imaging quality in awake and behaving monkeys, thus benefiting advanced brain research at UHF.

High-Density MRI RF Arrays Using Mixed Dipole Antennas and Microstrip Transmission Line Resonators

OBJECTIVE

High-density multi-coil arrays are desirable in MRI because they provide high signal-to-noise ratios (SNR), enable highly accelerated parallel imaging, and provide more uniform transmit fields at high fields. For high-density arrays such as a head array with 16 elements in a row, popular dipole antennas and microstrip transmission line (also referred to as "MTL") resonators both have severe coupling issues.

METHODS

In this work, we show that dipoles and MTLs have naturally low coupling and propose a novel array configuration in which they are interleaved. We first show the electromagnetic (EM) coupling between a single dipole and a single MTL across different separations in bench tests. Then we validate and analyze this through EM simulations. Finally, we construct a 16-channel mixed dipole and MTL array and evaluate its performance on the bench and through MRI experiments.

RESULTS

Without any decoupling treatments, the worst coupling between a dipole and an MTL was only -15.8 dB when their center-to-center distance was 4.7 cm (versus -5.4 dB for two dipole antennas and -6.0 dB for two MTL resonators). Even in a dense 16-channel mixed array, the inter-element isolation among all elements was better than -14 dB.

CONCLUSION

This study reveals, analyzes, and validates a novel finding that the popular dipole antennas and MTL resonators used in ultrahigh field MRI have naturally low coupling.

SIGNIFICANCE

These findings will simplify the construction of high-density arrays, enable new applications, and benefit imaging performance in ultrahigh field MRI.

A Monopole and Dipole Hybrid Antenna Array for Human Brain Imaging at 10.5 Tesla

In this letter, we evaluate antenna designs for ultra-high frequency and field (UHF) human brain magnetic resonance imaging (MRI) at 10.5 tesla (T). Although MRI at such UHF is expected to provide major signal-to-noise gains, the frequency of interest, 447 MHz, presents us with challenges regarding improved B₁ ⁺ efficiency, image homogeneity, specific absorption rate (SAR), and antenna element decoupling for array configurations. To address these challenges, we propose the use of both monopole and dipole antennas in a novel hybrid configuration, which we refer to as a mono-dipole hybrid antenna (MDH) array. Compared to an 8-channel dipole antenna array of the same dimensions, the 8-channel MDH array showed an improvement in decoupling between adjacent array channels, as well as ~18% higher B₁ ⁺ and SAR efficiency near the central region of the phantom based on simulation and experiment. However, the performances of the MDH and dipole antenna arrays were overall similar when evaluating a human model in terms of peak B₁ ⁺ efficiency, 10 g SAR, and SAR efficiency. Finally, the concept of an MDH array showed an advantage in improved decoupling, SAR, and B₁ ⁺ near the superior region of the brain for human brain imaging.

Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

Longitudinally orientated dipoles and microstrip antennas have both demonstrated superior results as RF transmit elements for body imaging at 7 T MRI, and are as of today the most commonly used transmit elements. In this study, the performances of the two antenna concepts were compared for use in local RF antenna arrays by numerical simulations. Antenna elements investigated are the fractionated dipole and the microstrip line with meander structures. Phantom simulations with a single antenna element were performed and evaluated with regard to specific absorption rate (SAR) efficiency in the center of the subject. Simulations of array configurations with 8 and 16 elements were performed with anatomical body models. Both antenna elements were combined with a loop coil to compare hybrid configurations. Singular value decomposition of the B₁⁺ fields, RF shimming, and calculation of the voxel-wise power and SAR efficiencies were performed in regions of interest with varying sizes to evaluate the transmit performance. The signal-to-noise ratio (SNR) was evaluated to estimate the receive performance. Simulated data show similar transmit profiles for the two antenna types in the center of the phantom (penetration depth > 20 mm). For body imaging, no considerable differences were determined for the different antenna configurations with regard to the transmit performance. Results show the advantage of 16 transmit channels compared with today's commonly used 8-channel systems (minimum RF shimming excitation error of 4.7% (4.3%) versus 2.7% (2.8%) for the 8-channel and 16-channel configurations with the microstrip antennas in a (5 cm)³ cube in the center of a male (female) body model). Highest SNR is achieved for the 16-channel configuration with fractionated dipoles. The combination of either fractionated dipoles or microstrip antennas with loop coils is more favorable with regard to the transmit performance compared with only increasing the number of elements.

A novel antisymmetric 16-element transceiver dipole antenna array for parallel transmit cardiac MRI in pigs at 7 T

To improve parallel transmit (pTx) and receive performance for cardiac MRI (cMRI) in pigs at 7 T, a dedicated transmit/receive (Tx/Rx), 16-element antisymmetric dipole antenna array, which combines L-shaped and straight dipoles, was designed, implemented, and evaluated in both cadavers and animals in vivo. Electromagnetic-field simulations were performed with the new 16-element dipole antenna array loaded with a pig thorax-shaped phantom and compared with an eight-element array of straight dipoles. The new dipole array was interfaced to a 7 T scanner in pTx mode (8Tx/16Rx). Imaging performance of the novel array was validated through MRI measurements in a pig phantom, an 85 kg pig cadaver, and two pigs in vivo (74 and 81 kg). Due to the improved decoupling between interleaved L-shaped and straight dipole elements, the 16-element dipole array fits within the same outer dimensions as an eight-element array of straight dipoles. This provides improvement of both transmit and receive characteristics and additional degrees of freedom for B1+ shimming. The antisymmetric dipole array demonstrated efficient suppression of destructive interferences in the B1+ field, with up to 25% improvement in the B1+ homogeneity achieved using static pTx-RFPA B1+ shimming in comparison with the hardware-adjusted state, which was optimized for single transmit. High-resolution (0.5 × 0.5 × 4 mm³ ) anatomical images of the heart after cardiac arrest proved good transmit and receive characteristics of the novel array design. Parallel imaging with an acceleration factor up to R = 6 was possible while maintaining a mean g factor of 1.55 within the pig heart. CINE images acquired in vivo in two pigs demonstrated SNR and parallel imaging capabilities similar to those of a reference 8Tx/16Rx dedicated loop array for cMRI in pigs.

Improved TSE imaging at ultrahigh field using nonlocalized efficiency RF shimming and acquisition modes optimized for refocused echoes (AMORE)

Purpose

To develop and evaluate a novel RF shimming optimization strategy tailored to improve the transmit efficiency in turbo spin echo imaging when performing time‐interleaved acquisition of modes (TIAMO) at ultrahigh fields.

Theory and Methods

A nonlocalized efficiency shimming cost function is proposed and extended to perform TIAMO using acquisition modes optimized for refocused echoes (AMORE). The nonlocalized efficiency shimming was demonstrated in brain and knee imaging at 7 Tesla. Phantom and in vivo torso imaging studies were performed to compare the performance between AMORE and previously proposed TIAMO mode optimizations with and without localized constraints in turbo spin echo and gradient echo acquisitions.

Results

The proposed nonlocalized efficiency RF shimming produced a circularly polarized‐like field with fewer signal dropouts in the brain and knee. For larger targets, AMORE was used and required a significantly lower transmitter voltage to produce a similar contrast to existing TIAMO mode design approaches for turbo spin echo as well as gradient echo acquisitions. In vivo, AMORE effectively reduced signal dropout in the interior torso while providing more uniform contrast with reduced transmit power. A local constraint further improved performance for a target region while maintaining performance in the larger FOV.

Conclusion

AMORE based on the presented nonlocalized efficiency shimming cost function demonstrated improved contrast and SNR uniformity as well as increased transmit efficiency for both gradient echo and turbo spin echo acquisitions.

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Resistor-free and one-board-fits-all ratio adjustable power splitter for add-on RF shimming in high field MRI

Ratio adjustable power splitter (RAPS) circuits were recently proposed for add-on RF shimming. Previous RAPSs split the input RF signal with a Wilkinson splitter or 50-Ω-terminated hybrid coupler into two branches, delay these two signals with cable/microstrip line phase shifters, and recombine them with another hybrid coupler. They require resistors to provide high output isolation and a cable/microstrip line library to realize desired splitting ratios. Here we propose a novel resistor-free RAPS circuit in which the Wilkinson splitter/50-Ω-terminated hybrid is replaced with a resistor-free T-junction splitter. A novel sliding mechanism was employed to further combine the T-junction's output arms with subsequent phase shifters and realize a one-board-fits-all design. The resistor-free RAPS was theoretically analyzed, simulated, and validated on workbench and MRI experiments. The resistor-free RAPS's splitting ratio has a tan/cot dependence on the phase/length difference between the T-junction output arms. The ratio can be continuously adjusted to any value by sliding the input arm without additional cable/microstrip libraries, largely saving time and effort when determining the best RF weights in practice. The fabricated resistor-free RAPS has a compact size, excellent input impedance matching, and a low insertion loss. Potential safety concerns caused by unwanted power dissipation on RF resistors are eliminated. The simulation and MRI experiments demonstrated that the resistor-free RAPS functions well on a widely-used Tx coil.

Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate, tissue temperature, and thermal dose

In this study, the performance of an integrated body-imaging array for 7 T with 32 radiofrequency (RF) channels under consideration of local specific absorption rate (SAR), tissue temperature, and thermal dose limits was evaluated and the imaging performance was compared with a clinical 3 T body coil. Thirty-two transmit elements were placed in three rings between the bore liner and RF shield of the gradient coil. Slice-selective RF pulse optimizations for B₁ shimming and spokes were performed for differently oriented slices in the body under consideration of realistic constraints for power and local SAR. To improve the B₁⁺ homogeneity, safety assessments based on temperature and thermal dose were performed to possibly allow for higher input power for the pulse optimization than permissible with SAR limits. The results showed that using two spokes, the 7 T array outperformed the 3 T birdcage in all the considered regions of interest. However, a significantly higher SAR or lower duty cycle at 7 T is necessary in some cases to achieve similar B₁⁺ homogeneity as at 3 T. The homogeneity in up to 50 cm-long coronal slices can particularly benefit from the high RF shim performance provided by the 32 RF channels. The thermal dose approach increases the allowable input power and the corresponding local SAR, in one example up to 100 W/kg, without limiting the exposure time necessary for an MR examination. In conclusion, the integrated antenna array at 7 T enables a clinical workflow for body imaging and comparable imaging performance to a conventional 3 T clinical body coil.

A Novel J-Shape Antenna Array for Simultaneous MR-PET or MR-SPECT Imaging

Simultaneous MR-PET/-SPECT is an emerging technology that capitalises on the invaluable advantages of both modalities, allowing access to numerous sensitive tracers and superior soft-tissue contrast alongside versatile functional imaging capabilities. However, to optimise these capabilities, concurrent acquisitions require the MRI antenna located inside the PET/SPECT field-of-view to be operated without compromising any aspects of system performance or image quality compared to the stand-alone instrumentation. Here, we report a novel gamma-radiation-transparent antenna concept. The end-fed J-shape antenna is particularly adept for hybrid ultra-high field MR-PET/-SPECT applications as it enables all highly attenuating materials to be placed outside the imaging field-of-view. Furthermore, this unique configuration also provides advantages in stand-alone MR applications by reducing the amount of coupling between the cables and the antenna elements, and by lowering the potential specific absorption rate burden. The use of this new design was experimentally verified according to the important features for both ultra-high field MRI and the 511 keV transmission scan. The reconstructed attenuation maps evidently showed much lower attenuation (  ∼ 15 %) for the proposed array when compared to the conventional dipole antenna array since there were no high-density components. In MR, it was observed that the signal-to-noise ratio from the whole volume obtained using the proposed array was comparable to that acquired by the conventional array which was also in agreement with the simulation results. The unique feature, J-shape array, would enable simultaneous MR-PET/-SPECT experiments to be conducted without unduly compromising any aspects of system performance and image quality compared to the stand-alone instrumentation.

Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple “dielectric pads” for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.

Mitigating transmit‐B 1 artifacts by predicting parallel transmission images with deep learning: A feasibility study using high‐resolution whole‐brain diffusion at 7 Tesla

Purpose

To propose a novel deep learning (DL) approach to transmit‐B₁ (B₁⁺)‐artifact mitigation without direct use of parallel transmission (pTx), by predicting pTx images from single‐channel transmission (sTx) images.

Methods

A deep encoder–decoder convolutional neural network was constructed and trained to learn the mapping from sTx to pTx images. The feasibility was demonstrated using 7 T Human‐Connectome Project (HCP)‐style diffusion MRI. The training dataset comprised images acquired on 5 healthy subjects using commercial Nova RF coils. Relevant hyperparameters were tuned with a nested cross‐validation, and the generalization performance evaluated using a regular cross‐validation.

Results

Our DL method effectively improved the image quality for sTx images by restoring the signal dropout, with quality measures (including normalized root‐mean‐square error, peak SNR, and structural similarity index measure) improved in most brain regions. The improved image quality was translated into improved performances for diffusion tensor imaging analysis; our method improved accuracy for fractional anisotropy and mean diffusivity estimations, reduced the angular errors of principal eigenvectors, and improved the fiber orientation delineation relative to sTx images. Moreover, the final DL model trained on data of all 5 subjects was successfully used to predict pTx images for unseen new subjects (randomly selected from the 7 T HCP database), effectively recovering the signal dropout and improving color‐coded fractional anisotropy maps with largely reduced noise levels.

Conclusion

The proposed DL method has potential to provide images with reduced B1⁺ artifacts in healthy subjects even when pTx resources are inaccessible on the user side.

Ultrafast B1 mapping with RF-prepared 3D FLASH acquisition: Correcting the bias due to T1 -induced k-space filtering effect

Purpose

The traditional radiofrequency (RF)‐prepared B₁ mapping technique consists of one scan with an RF preparation module for flip angle‐encoding and a second scan without this module for normalizing. To reduce the T₁‐induced k‐space filtering effect, this method is limited to 2D FLASH acquisition with a two‐parameter method. A novel 3D RF‐prepared three‐parameter method for ultrafast B₁‐mapping is proposed to correct the T₁‐induced quantification bias.

Theory

The point spread function analysis of FLASH shows that the prepared longitudinal magnetization before the FLASH acquisition and the image signal obeys a linear (not proportional) relationship. The intercept of the linear function causes the quantification bias and can be captured by a third saturated scan.

Methods

Using the 2D double‐angle method (DAM) as the reference, a 3D RF‐prepared three‐parameter protocol with 9 s duration was compared with the two‐parameter method, as well as the saturated DAM (SDAM) method, the dual refocusing echo acquisition mode (DREAM) method, and the actual flip‐angle imaging (AFI) method, for B₁ mapping of brain, breast, and abdomen with different orientations and shim settings at 3T.

Results

The 3D RF‐prepared three‐parameter method with complex‐subtraction delivered consistently lower RMS error, error mean, error standard deviation, and higher concordance correlation coefficients values than the two‐parameter method, the three‐parameter method with magnitude‐subtraction, the multi‐slice DREAM and the 3D AFI, and were close to the results of 2D or multi‐slice SDAM.

Conclusion

The proposed ultrafast 3D RF‐prepared three‐parameter method with complex‐subtraction was demonstrated with high accuracy for B₁ mapping of brain, breast, and abdomen.

Respiration induced B 1 + changes and their impact on universal and tailored 3D kT-point parallel transmission pulses for 7T cardiac imaging

PURPOSE

Human heart imaging at ultra-high fields is highly challenging because of respiratory motion-induced artefacts and spatially heterogeneous B 1 + profiles. This work demonstrates that respiration resolved 3D B 1 + -maps can be used with a dedicated tailored and universal parallel transmission (pTx) pulse design to compensate respiration related B 1 + changes in subjects performing shallow and deep breathing (SB/DB).

METHODS

Three-dimensional (3D) B 1 + -maps of the thorax were acquired in 31 subjects under SB and in 15 subjects under SB and DB. Different universal and tailored non-selective pTx pulses were designed from non-respiration resolved (NRR) and respiration resolved (RR) reconstructions of the SB/DB B 1 + -maps. The performance of all pulses was tested with RR-SB/DB B 1 + -maps. Respiration-robust tailored and universal pulses were applied in vivo in 5 subjects at 7T in 3D gradient-echo free-breathing scans.

RESULTS

All optimized pTx pulses performed well for SB. For DB, however, only the universal and the tailored respiration-robust pulses achieved homogeneous flip angles (FAs) in all subjects and across all respiration states, whereas the tailored respiration-specific pulses resulted in a higher FA variation. The respiration-robust universal pulse resulted in an average coefficient of variation in the FA maps of 12.6% compared to 8.2% achieved by tailored respiration-robust pulses. In vivo measurements at 7T demonstrate the benefits of using respiration-robust pulses for DB.

CONCLUSION

Universal and tailored respiration-robust pTx pulses based on RR B 1 + -maps are highly preferred to achieve 3D heart FA homogenization at 7T when subjects perform DB, whereas universal and tailored pulses based on NRR B 1 + -maps are sufficient when subjects perform SB.

Standardized universal pulse: A fast RF calibration approach to improve flip angle accuracy in parallel transmission

PURPOSE

In parallel transmission (pTX), subject-tailored RF pulses allow achieving excellent flip angle (FA) accuracy but often require computationally extensive online optimizations, precise characterization of the static field ( Δ B 0 ), and the transmit RF field ( B 1 + ) distributions. This costs time and requires expertise from the MR user. Universal pulses (UPs) have been proposed to reduce this burden, yet, with a penalty in FA accuracy. This study introduces the concept of standardized universal pulses (SUPs), where pulses are designed offline and adjusted to the subject through a fast online calibration scan.

METHODS

A SUP is designed offline using a so-called standardized database, wherein each B 1 + map has been normalized to a reference transmit RF field distribution. When scanning a new subject, a 3-slice B 1 + acquisition (scan time < 10  s) is performed and used to adjust the SUP to the subject through a linear transform. SUP performance was assessed at 7T with simulations by computing the FA-normalized root mean square error (FA-NRMSE) and the FA pattern stability as measured by the average and coefficient of variation of the FA across 15 control subjects, along with in vivo experiments using an MP2RAGE sequence implementing the SUP variant for the FLASH readout.

RESULTS

Adjusted SUP improved the FA-NRMSE (8.8 % for UP vs. 7.1 % for adjusted SUP). Experimentally in vivo, this translated in an improved signal homogeneity and more accurate T 1 quantification using MP2RAGE.

CONCLUSION

The proposed SUP approach improves excitation accuracy (FA-NRMSE) while preserving the same offline pulse design principle as offered by UPs.

A radially interleaved sodium and proton coil array for brain MRI at 7 T

The objective of the current study was to design and build a dual-tuned coil array for simultaneous ²³ Na/¹ H MRI of the human brain at 7 T. Quality factor, experimental B₁⁺ measurements, and electromagnetic simulations in prototypes showed that setups consisting of geometrically interleaved ¹ H and ²³ Na loops performed better than or similar to ¹ H or ²³ Na loops in isolation. Based on these preliminary findings, we built a transmit/receive eight-channel ²³ Na loop array that was geometrically interleaved with a transmit/receive eight-channel ¹ H loop array. We assessed the performance of the manufactured array with mononuclear signal-to-noise ratio (SNR) and B₁⁺ measurements, along with multinuclear magnetic resonance fingerprinting maps and images. The ²³ Na array within the developed dual-tuned device provided more than 50% gain in peripheral SNR and similar B₁⁺ uniformity and coverage as a reference birdcage coil of similar size. The ¹ H array provided good B₁⁺ uniformity in the brain, excluding the cerebellum and brain stem. The integrated ²³ Na and ¹ H arrays were used to demonstrate truly simultaneous quantitative ¹ H mapping and ²³ Na imaging.

A nine-channel transmit/receive array for spine imaging at 10.5 T: Introduction to a nonuniform dielectric substrate antenna

PURPOSE

The purpose of this study is to introduce a new antenna element with improved transmit performance, named the nonuniform dielectric substrate (NODES) antenna, for building transmit arrays at ultrahigh-field.

METHODS

We optimized a dipole antenna at 10.5 Tesla by maximizing the B 1 + -SAR efficiency in a phantom for a human spine target. The optimization parameters included permittivity variation in the substrate, substrate thickness, antenna length, and conductor geometry. We conducted electromagnetic simulations as well as phantom experiments to compare the transmit/receive performance of the proposed NODES antenna design with existing coil elements from the literature.

RESULTS

Single NODES element showed up to 18% and 30% higher B 1 + -SAR efficiency than the fractionated dipole and loop elements, respectively. The new element is substantially shorter than a commonly used dipole, which enables z-stacked array formation; it is additionally capable of providing a relatively uniform current distribution along its conductors. The nine-channel transmit/receive NODES array achieved 7.5% higher B 1 + homogeneity than a loop array with the same number of elements. Excitation with the NODES array resulted in 33% lower peak 10g-averaged SAR and required 34% lower input power than the loop array for the target anatomy of the spine.

CONCLUSION

In this study, we introduced a new RF coil element: the NODES antenna. NODES antenna outperformed the widely used loop and dipole elements and may provide improved transmit/receive performance for future ultrahigh field MRI applications.

Characterization of B1 + field variation in brain at 3 T using 385 healthy individuals across the lifespan

PURPOSE

The transmit field B 1 + at 3 T in brain affects the spatial uniformity and contrast of most image acquisitions. Here, B 1 + spatial variation in brain at 3 T is characterized in a large healthy population.

METHODS

Bloch-Siegert B 1 + maps were acquired at 3 T from 385 healthy subjects aged 5-90 years on a single MRI system. After transforming all B 1 + maps to a standard brain atlas space, region-of-interest analysis was performed, and intersubject voxel-wise coefficient of variation was calculated across the whole brain. The B 1 + variability due to age and brain size was studied separately in males and females, along with B 1 + variability due to nonideal transmit calibration.

RESULTS

The voxel-based mean coefficient of variation was 4.0% across all subjects, and the difference in B 1 + between central (left thalamus) and outer regions (left frontal gray matter) was 24.2% ± 2.3%. The least intersubject variability occurred in central regions, whereas regions toward brain edges increased markedly in variation. The B 1 + variability with age was mostly attributed to lifespan changes in CSF volume (which alters brain conductivity) and head orientation. Larger brain size correlated with more B 1 + inhomogeneity (p < .001). Varying head position and anatomy resulted in an inaccurate transmit calibration.

CONCLUSION

In standard atlas space, intersubject B 1 + variability at 3 T was relatively small in a large population aged 5-90 years. The B 1 + varied with age-related changes of CSF volume and head orientation, as well as differences in brain size and transmit calibration.

Progress in Imaging the Human Torso at the Ultrahigh Fields of 7 and 10.5 T

Especially after the launch of 7 T, the ultrahigh magnetic field (UHF) imaging community achieved critically important strides in our understanding of the physics of radiofrequency interactions in the human body, which in turn has led to solutions for the challenges posed by such UHFs. As a result, the originally obtained poor image quality has progressed to the high-quality and high-resolution images obtained at 7 T and now at 10.5 T in the human torso. Despite these tremendous advances, work still remains to further improve the image quality and fully capitalize on the potential advantages UHF has to offer.

Calibration-free pTx of the human heart at 7T via 3D universal pulses

PURPOSE

MRI at ultra-high fields in the human body is highly challenging and requires lengthy calibration times to compensate for spatially heterogeneous B 1 + profiles. This study investigates the feasibility of using pre-computed universal pulses for calibration-free homogeneous 3D flip angle distribution in the human heart at 7T.

METHODS

Twenty-two channel-wise 3D B 1 + data sets were acquired under free-breathing in 19 subjects to generate a library for an offline universal pulse (UP) design (group 1: 12 males [M] and 7 females [F], 21-66 years, 19.8-28.3 kg/m² ). Three of these subjects (2M/1F, 21-33 years, 20.8-23.6 kg/m² ) were re-scanned on different days. A 4kT-points UP optimized for the 22 channel-wise 3D B 1 + data sets in group 1 (UP22-4kT) is proposed and applied at 7T in 9 new and unseen subjects (group 2: 4M/5F, 25-56 years, 19.5-35.3 kg/m² ). Multiple tailored and universal static and dynamic parallel-transmit (pTx) pulses were designed and evaluated for different permutations of the B 1 + data sets in group 1 and 2.

RESULTS

The proposed UP22-4kT provides low B 1 + variation in all subjects, seen and unseen, without severe signal drops. Experimental data at 7T acquired with UP22-4kT shows comparable image quality as data acquired with tailored-4kT pulses and demonstrates successful calibration-free pTx of the human heart.

CONCLUSION

UP22-4kT allows for calibration-free homogeneous flip angle distributions across the human heart at 7T. Large inter-subject variations because of sex, age, and body mass index are well tolerated. The proposed universal pulse removes the need for lengthy (10-15 min) calibration scans and therefore has the potential to bring body imaging at 7T closer to the clinical application.

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.

Displacement current distribution on a high dielectric constant helmet and its effect on RF field at 10.5 T (447 MHz)

PURPOSE

Investigating the designs and effects of high dielectric constant (HDC) materials in the shape of a conformal helmet on the enhancement of RF field and reduction of specific absorption rate at 10.5 T for human brain studies.

METHODS

A continuous and a segmented four-piece HDC helmet fit to a human head inside an eight-channel fractionated-dipole array were constructed and studied with a phantom and a human head model using computer electromagnetic simulations. The simulated transmit efficiency and receive sensitivity were experimentally validated using a phantom with identical electric properties and helmet-coil configurations of the computer model. The temporal and spatial distributions of displacement currents on the HDC helmets were analyzed.

RESULTS

Using the continuous HDC helmet, simulation results in the human head model demonstrated an average transmit efficiency enhancement of 66%. A propagating displacement current was induced on the continuous helmet, leading to an inhomogeneous RF field enhancement in the brain. Using the segmented four-piece helmet design to reduce this effect, an average 55% and 57% enhancement in the transmit efficiency and SNR was achieved in human head, respectively, along with 8% and 28% reductions in average and maximum local specific absorption rate.

CONCLUSION

The HDC helmets enhanced the transmit efficiency and SNR of the dipole array coil in the human head at 10.5 T. The segmentation of the helmet to disrupt the continuity of circumscribing displacement currents in the helmet produced a more uniform distribution of the transmit field and lower specific absorption rate in the human head compared with the continuous helmet design.

Safety and imaging performance of two-channel RF shimming for fetal MRI at 3T

PURPOSE

This study investigates whether two-channel radiofrequency (RF) shimming can improve imaging without increasing specific absorption rate (SAR) for fetal MRI at 3T.

METHODS

Transmit field ( B 1 + ) average and variation in the fetus was simulated in seven numerical pregnant body models. Safety was quantified by maternal and fetal peak local SAR and fetal average SAR. The shim parameter space was divided into improved B 1 + (magnitude and homogeneity) and improved SAR regions, and an overlap where RF shimming improved both classes of metrics compared with birdcage mode was assessed. Additionally, the effect of fetal position, tissue detail, and dielectric properties on transmit field and SAR was studied.

RESULTS

A region of subject-specific RF shim parameter space improving both B 1 + and SAR metrics was found for five of the seven models. Optimizing only B 1 + metrics improved B 1 + efficiency across models by 15% on average and 28% for the best-case model. B 1 + variation improved by 26% on average and 49% for the best case. However, for these shim settings, fetal SAR increased by up to 106%. The overlap region, where both B 1 + and SAR metrics improve, showed an average B 1 + efficiency improvement of 6% on average across models and 19% for the best-case model. B 1 + variation improved by 13% on average and 40% for the best case. RFS could also decrease maternal/fetal SAR by up to 49%/58%.

CONCLUSION

RF shimming can improve imaging compared with birdcage mode without increasing fetal and maternal SAR when a patient-specific SAR model is incorporated into the shimming procedure.

Performance analysis of integrated RF microstrip transmit antenna arrays with high channel count for body imaging at 7 T

The aim of the current study was to investigate the performance of integrated RF transmit arrays with high channel count consisting of meander microstrip antennas for body imaging at 7 T and to optimize the position and number of transmit elements. RF simulations using multiring antenna arrays placed behind the bore liner were performed for realistic exposure conditions for body imaging. Simulations were performed for arrays with as few as eight elements and for arrays with high channel counts of up to 48 elements. The B₁⁺  field was evaluated regarding the degrees of freedom for RF shimming in the abdomen. Worst-case specific absorption rate (SAR_wc ), SAR overestimation in the matrix compression, the number of virtual observation points (VOPs) and SAR efficiency were evaluated. Constrained RF shimming was performed in differently oriented regions of interest in the body, and the deviation from a target B₁⁺  field was evaluated. Results show that integrated multiring arrays are able to generate homogeneous B₁⁺ field distributions for large FOVs, especially for coronal/sagittal slices, and thus enable body imaging at 7 T with a clinical workflow; however, a low duty cycle or a high SAR is required to achieve homogeneous B₁⁺  distributions and to exploit the full potential. In conclusion, integrated arrays allow for high element counts that have high degrees of freedom for the pulse optimization but also produce high SAR_wc , which reduces the SAR accuracy in the VOP compression for low-SAR protocols, leading to a potential reduction in array performance. Smaller SAR overestimations can increase SAR accuracy, but lead to a high number of VOPs, which increases the computational cost for VOP evaluation and makes online SAR monitoring or pulse optimization challenging. Arrays with interleaved rings showed the best results in the study.

Simultaneous optimization of radio frequency and gradient waveforms with exact Hessians and slew rate constraints applied to kT-points excitation

We consider an excitation pulse with piecewise constant gradient trajectories and radio frequency (RF) waveforms such that the solution of the Bloch equations without relaxation terms can be represented by rotations. Based on this analytic solution we formulate a non-linear program for finding sub-pulse durations, gradient strengths, and complex RF voltages which minimize the deviation between the achieved and desired magnetization. We develop explicit expressions for the first and second order derivatives of the objective function. We extend the non-linear program to precisely account for gradient slew rate constraints. Using an interior point solver we apply the developed theory to simultaneously optimize the positions of k_T-points, their associated RF voltages and durations.

Sub-millimetre resolution laminar fMRI using Arterial Spin Labelling in humans at 7 T

Laminar fMRI at ultra-high magnetic field strength is typically carried out using the Blood Oxygenation Level-Dependent (BOLD) contrast. Despite its unrivalled sensitivity to detecting activation, the BOLD contrast is limited in its spatial specificity due to signals stemming from intra-cortical ascending and pial veins. Alternatively, regional changes in perfusion (i.e., cerebral blood flow through tissue) are colocalised to neuronal activation, which can be non-invasively measured using Arterial Spin Labelling (ASL) MRI. In addition, ASL provides a quantitative marker of neuronal activation in terms of perfusion signal, which is simultaneously acquired along with the BOLD signal. However, ASL for laminar imaging is challenging due to the lower SNR of the perfusion signal and higher RF power deposition i.e., specific absorption rate (SAR) of ASL sequences. In the present study, we present for the first time in humans, isotropic sub-millimetre spatial resolution functional perfusion images using Flow-sensitive Alternating Inversion Recovery (FAIR) ASL with a 3D-EPI readout at 7 T. We show that robust statistical activation maps can be obtained with perfusion-weighting in a single session. We observed the characteristic BOLD amplitude increase towards the superficial laminae, and, in apparent discrepancy, the relative perfusion profile shows a decrease of the amplitude and the absolute perfusion profile a much smaller increase towards the cortical surface. Considering the draining vein effect on the BOLD signal using model-based spatial “convolution”, we show that the empirically measured perfusion and BOLD profiles are, in fact, consistent with each other. This study demonstrates that laminar perfusion fMRI in humans is feasible at 7 T and that caution must be exercised when interpreting BOLD signal laminar profiles as direct representation of the cortical distribution of neuronal activity.

Comparison of 16-Channel Asymmetric Sleeve Antenna and Dipole Antenna Transceiver Arrays at 10.5 Tesla MRI

Multi-element transmit arrays with low peak 10 g specific absorption rate (SAR) and high SAR efficiency (defined as ( [Formula: see text]SAR [Formula: see text] are essential for ultra-high field (UHF) magnetic resonance imaging (MRI) applications. Recently, the adaptation of dipole antennas used as MRI coil elements in multi-channel arrays has provided the community with a technological solution capable of producing uniform images and low SAR efficiency at these high field strengths. However, human head-sized arrays consisting of dipole elements have a practical limitation to the number of channels that can be used due to radiofrequency (RF) coupling between the antenna elements, as well as, the coaxial cables necessary to connect them. Here we suggest an asymmetric sleeve antenna as an alternative to the dipole antenna. When used in an array as MRI coil elements, the asymmetric sleeve antenna can generate reduced peak 10 g SAR and improved SAR efficiency. To demonstrate the advantages of an array consisting of our suggested design, we compared various performance metrics produced by 16-channel arrays of asymmetric sleeve antennas and dipole antennas with the same dimensions. Comparison data were produced on a phantom in electromagnetic (EM) simulations and verified with experiments at 10.5 Tesla (T). The results produced by the 16-channel asymmetric sleeve antenna array demonstrated 28 % lower peak 10 g SAR and 18.6 % higher SAR efficiency when compared to the 16-channel dipole antenna array.

A self-decoupled 32-channel receive array for human-brain MRI at 10.5 T

PURPOSE

Receive array layout, noise mitigation, and B₀ field strength are crucial contributors to SNR and parallel-imaging performance. Here, we investigate SNR and parallel-imaging gains at 10.5 T compared with 7 T using 32-channel receive arrays at both fields.

METHODS

A self-decoupled 32-channel receive array for human brain imaging at 10.5 T (10.5T-32Rx), consisting of 31 loops and one cloverleaf element, was co-designed and built in tandem with a 16-channel dual-row loop transmitter. Novel receive array design and self-decoupling techniques were implemented. Parallel imaging performance, in terms of SNR and noise amplification (g-factor), of the 10.5T-32Rx was compared with the performance of an industry-standard 32-channel receiver at 7 T (7T-32Rx) through experimental phantom measurements.

RESULTS

Compared with the 7T-32Rx, the 10.5T-32Rx provided 1.46 times the central SNR and 2.08 times the peripheral SNR. Minimum inverse g-factor value of the 10.5T-32Rx (min[1/g] = 0.56) was 51% higher than that of the 7T-32Rx (min[1/g] = 0.37) with R = 4 × 4 2D acceleration, resulting in significantly enhanced parallel-imaging performance at 10.5 T compared with 7 T. The g-factor values of 10.5 T-32 Rx were on par with those of a 64-channel receiver at 7 T (eg, 1.8 vs 1.9, respectively, with R = 4 × 4 axial acceleration).

CONCLUSION

Experimental measurements demonstrated effective self-decoupling of the receive array as well as substantial gains in SNR and parallel-imaging performance at 10.5 T compared with 7 T.

Radiofrequency coil for routine ultra-high-field imaging with an unobstructed visual field

Many neuroscience applications have adopted functional MRI as a tool to investigate the healthy and diseased brain during the completion of a task. While ultra-high-field MRI has allowed for improved contrast and signal-to-noise ratios during functional MRI studies, it remains a challenge to create local radiofrequency coils that can accommodate an unobstructed visual field and be suitable for routine use, while at the same time not compromise performance. Performance (both during transmission and reception) can be improved by using close-fitting coils; however, maintaining sensitivity over the whole brain often requires the introduction of coil elements proximal to the eyes, thereby partially occluding the subject's visual field. This study presents a 7 T head coil, with eight transmit dipoles and 32 receive loops, that is designed to remove visual obstructions from the subject's line of sight, allowing for an unencumbered view of visual stimuli, the reduction of anxiety induced from small enclosures, and the potential for eye-tracking measurements. The coil provides a practical solution for routine imaging, including a split design (anterior and posterior halves) that facilitates subject positioning, including those with impaired mobility, and the placement of devices required for patient comfort and motion reduction. The transmit and receive coils displayed no degradation of performance due to adaptions to the design topology (both mechanical and electrical) required to create an unobstructed visual field. All computer-aided design files, electromagnetic simulation models, transmit field maps and local specific absorption rate matrices are provided to promote reproduction.

Three-dimensional static and dynamic parallel transmission of the human heart at 7 T

Three-dimensional (3D) human heart imaging at ultra-high fields is highly challenging due to respiratory and cardiac motion-induced artifacts as well as spatially heterogeneous B1+ profiles. In this study, we investigate the feasibility of applying 3D flip angle (FA) homogenization targeting the whole heart via static phase-only and dynamic kT-point in vivo parallel transmission at 7 T. 3D B1+ maps of the thorax were acquired under free breathing in eight subjects to compute parallel transmission pulses that improve excitation homogeneity in the human heart. To analyze the number of kT-points required, excitation homogeneity and radiofrequency (RF) power were compared using different regions of interest in six subjects with different body mass index (BMI) values of 20-34 kg/m² for a wide range of regularization parameters. One subset of the optimized subject-specific pulses was applied in vivo on a 7 T scanner for six subjects in Cartesian 3D breath-hold scans as well as in two subjects in a radial phase-encoded 3D free-breathing scan. Across all subjects, 3-4 kT-points achieved a good tradeoff between RF power and nominal FA homogeneity. For subjects with a BMI in the normal range, the 4 kT-point pulses reliably improved the coefficient of variation by less than 10% compared with less than 25% achieved by static phase-only parallel transmission. in vivo measurements on a 7 T scanner validated the B1+ estimations and the pulse design, despite neglecting ΔB₀ in the optimizations and Bloch simulations. This study demonstrates in vivo that kT-point pTx pulses are highly suitable for mitigating nominal FA heterogeneities across the entire 3D heart volume at 7 T. Furthermore, 3-4 kT-points demonstrate a practical tradeoff between nominal FA heterogeneity mitigation and RF power.

Unshielded bent folded-end dipole 9.4 T human head transceiver array decoupled using modified passive dipoles

PURPOSE

To develop an unshielded dipole transceiver array for human head imaging at 9.4 Tesla and to improve decoupling of adjacent dipole elements, a novel array design with modified passive dipole antennas was developed, evaluated, and tested.

METHODS

The new array consisted of 8 bent folded-end dipole elements placed in a single row and surrounding the head. Adjacent elements of RF transceiver arrays are usually decoupled by introducing circuits electrically connected to elements. These methods are difficult to use for dipole arrays because of the distant location of the adjacent antennas. A recently developed decoupling technique using passive dipoles is simple and does not require any electrical connection. However, common parallel passive dipoles can produce destructive interference with the RF field of the array itself. To minimize this interference, we placed the passive dipoles perpendicularly to the active dipoles and positioned them at the ends of the array. We also evaluated the effect of different passive dipoles on the array transmit performance. Finally, we optimized the array transmit performance by varying the length of the dipole folded portion.

RESULTS

By rotating the passive dipoles 90º and moving them toward the ends of the array, we minimized the destructive interference to an acceptable level without compromising decoupling and the transmit efficiency.

CONCLUSION

While keeping the benefits of the passive dipole decoupling method, the new modified dipoles produce substantially less destructive interference with the RF field of the array than the common design. The constructed transceiver array demonstrated good decoupling and whole-brain coverage.

In vivo investigation of the multi-exponential T2 decay in human white matter at 7 T: Implications for myelin water imaging at UHF

INTRODUCTION

Multi-component T₂ mapping using a gradient- and spin-echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal-to-noise ratio benefits but face specific absorption rate limits and shortened T₂ times. This study investigates compartmental T₂ times in vivo and addresses advantages and challenges of multi-component T₂ mapping at 7 T.

METHODS

We acquired 3D multi-echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B₁⁺ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T₂ distributions to determine T₂ times that identify different water pools and assessed signal-to-noise and fit-to-noise characteristics of the signal estimation. We compared short T₂ fractions and T₂ properties of the intermediate water pool at 3 T and 7 T.

RESULTS

Flip angle mapping confirmed that EPG accurately determined the larger B₁⁺ inhomogeneity at 7 T. Multi-component T₂ analysis demonstrated shortened T₂ times at 7 T compared with 3 T. Fit-to-noise and signal-to-noise ratios were improved at 7 T but depended on B₁⁺ homogeneity. Adjusting the shortest T₂ to 8 ms and the T₂ threshold that separates different water compartments to 20 ms yielded short T₂ fractions at 7 T that conformed to 3 T data. Short T₂ fractions in myelin-rich white matter regions were lower at 7 T than at 3 T, and higher in iron-rich structures.

DISCUSSION

Adjusting the T₂ compartment boundaries was required due to the shorter T₂ relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi-channel transmission will improve T₂ measurements at 7 T.

CONCLUSION

We used a multi-echo 3D GraSE sequence to characterize the multi-exponential T₂ decay at 7 T. We adapted T₂ parameters for evaluation of the short T₂ fraction. Obtained 7 T multi-component T₂ maps were in good agreement with 3 T data.

A retrofit to enable dynamic B 1 + steering for transmit arrays without multiple amplifiers

PURPOSE

B 1 + shimming is an important method for mitigating B₁ inhomogeneity in high-field MRI. Using independent power amplifiers for each transmit (Tx) element is the preferred method for B₁ shimming but comes with a high cost. Conversely, the simplest approach to control a Tx array is by using coaxial cables of varying length in the Tx chain, but this approach is cumbersome and impractical for dynamic shimming. In this article, a system is described that enables dynamic, phase-only, eight-channel B 1 + steering on a 7T MR scanner with only two power amplifiers.

METHODS

Power dividers were utilized to first split the existing two-channel Tx signal into eight channels. Digitally controlled phase shifters on each channel were designed to provide independent phase shifts with a resolution of 22.5° (from 0°, 22.5° … 337.5°). To validate the system, an eight-channel body dipole array was simulated and constructed for bench and 7T imaging and evaluation.

RESULTS

The phase conjugate B 1 + steering method was employed at three different spatial positions in simulation, bench measurements, and scanner measurements-all with matching results. At the desired points, regions with homogenous B 1 + were generated, indicating good Tx steering to the selected region.

CONCLUSION

The described system can be used as a simple retrofit to existing hardware to provide phase control while avoiding the need to manually switch cables and without requiring independent power amplifiers for each channel, thus demonstrating the ability to perform dynamic B 1 + shimming with increased degrees of freedom but without significantly increased hardware cost.