Specific absorption rate (SAR) simulations for low-field (< 0.1 T) MRI systems

Objective

To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.

Methods

Electromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.

Results

Simulations of RF coil characteristics and B₁⁺ transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180° refocusing pulses of duration ~ 10 ms are used for TSE sequences does SAR need to be carefully considered.

Conclusion

This work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T_1ρ, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed.

     

Comparison of three commercially available radio frequency coils for human brain imaging at 3 Tesla

Objective To evaluate a transverse electromagnetic (TEM), a circularly polarized (CP) (birdcage), and a 12-channel phased array head coil at the clinical field strength of B ₀ = 3T in terms of signal-to-noise ratio (SNR), signal homogeneity, and maps of the effective flip angle α. Materials and methods SNR measurements were performed on low flip angle gradient echo images. In addition, flip angle maps were generated for α_nominal = 30° using the double angle method. These evaluation steps were performed on phantom and human brain data acquired with each coil. Moreover, the signal intensity variation was computed for phantom data using five different regions of interest. Results In terms of SNR, the TEM coil performs slightly better than the CP coil, but is second to the smaller 12-channel coil for human data. As expected, both the TEM and the CP coils show superior image intensity homogeneity than the 12-channel coil, and achieve larger mean effective flip angles than the combination of body and 12-channel coil with reduced radio frequency power deposition. Conclusion At 3T the benefits of TEM coil design over conventional lumped element(s) coil design start to emerge, though the phased array coil retains an advantage with respect to SNR performance.     

A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.     

Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle with respect to B₀. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 m). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at =0°, 45°, and 90°. As increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B₀ without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.     

Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T

Introduction

Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.

Methods

Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and k_T-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity.

Results

For RF shimming, the 16L array performs best. For k_T-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays.

Discussion and conclusion

For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in k_T-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by k_T-points at the expense of larger power deposition. For k_T-points, the dipole arrays seem to outperform loop coil arrays.

     

Tackling SNR at low-field: a review of hardware approaches for point-of-care systems

Objective

To review the major hardware components of low-field point-of-care MRI systems which affect the overall sensitivity.

Methods

Designs for the following components are reviewed and analyzed: magnet, RF coils, transmit/receive switches, preamplifiers, data acquisition system, and methods for grounding and mitigating electromagnetic interference.

Results

High homogeneity magnets can be produced in a variety of different designs including C- and H-shaped as well as Halbach arrays. Using Litz wire for RF coil designs enables unloaded Q values of ~ 400 to be reached, with body loss representing about 35% of the total system resistance. There are a number of different schemes to tackle issues arising from the low coil bandwidth with respect to the imaging bandwidth. Finally, the effects of good RF shielding, proper electrical grounding, and effective electromagnetic interference reduction can lead to substantial increases in image signal-to-noise ratio.

Discussion

There are many different magnet and RF coil designs in the literature, and to enable meaningful comparisons and optimizations to be performed it would be very helpful to determine a standardized set of sensitivity measures, irrespective of design.

     

Sources of variation in multi-centre brain MTR histogram studies: body-coil transmission eliminates inter-centre differences

Object: 1. Identify sources of variation affecting Magnetisation Transfer Ratio (MTR) histogram reproducibility between-centres. 2. Demonstrate complete elimination of inter-centre difference. Materials and methods: Six principle sources of variation were summarised and analysed. These are:the imager coil used for radiofrequency (RF) transmission, imager stability, the shape and other parameters describing the Magnetisation Transfer (MT) pulse, the MT sequence used (including its parameters), the image segmentation methodology, and the histogram generation technique. Transmit field nonuniformity and B1 errors are often the largest factors. PLUMB (Peak Location Uniformity in MTR histograms of the Brain) plots are a convenient way of visualising differences. Five multi-centres studies were undertaken to investigate and minimise differences. Results: Transmission using a body coil, with a close-fitting array of surface coils for reception, gave the best uniformity. Differences between two centres, having MR imagers from different manufacturers, were completely eliminated by using body coil excitation, making a small adjustment to the MT pulse flip angle, and carrying out segmentation at a single centre. Histograms and their peak location and height values were indistinguishable. Conclusions: Body coil excitation is preferred for multi-centre studies. Analysis (segmentation and histogram generation) should ideally be carried out at a single site.     

B1 field-insensitive transformers for RF-safe transmission lines

Objective: Integration of transformers into transmission lines suppresses radiofrequency (RF)-induced heating. New figure-of-eight-shaped transformer coils are compared to conventional loop transformer coils to assess their signal transmission properties and safety profile. Materials and methods: The transmission properties of figure-of-eight-shaped transformers were measured and compared to transformers with loop coils. Experiments to quantify the effect of decoupling from the B₁ field of the MR system were conducted. Temperature measurements were performed to demonstrate the effective reduction of RF-induced heating. The transformers were investigated during active tracking experiments. Results: Coupling to the B₁ field was reduced by 18 dB over conventional loop-shaped transformer coils. MR images showed a significantly reduced artifact for the figure-of-eight- shaped coils generated by local flip-angle amplification. Comparable transmission properties were seen for both transformer types. Temperature measurements showed a maximal temperature increase of 30K/3.5 K for an unsegmented/ segmented cable. With a segmented transmission line a robotic assistance system could be successfully localized using active tracking. Conclusion: The figure-of-eight-shaped transformer design reduces both RF field coupling with the MR system and artifact sizes. Anatomical structure close to the figure-of-eight-shaped transformer may be less obscured as with loop-shaped transformers if these transformers are integrated into e.g. intravascular catheters.     

Determination of appropriate RF blocking impedance for MRI surface coils and arrays

Surface and phased array receiving coils in MRI typically require that RF excitation be accomplished using the body coil. This process requires that the receiving coils contain blocking circuitry to increase the overall circuit impedance during RF excitation and withstand the electromotive force induced by the applied electromagnetic field. The aim of this study was to determine the optimal impedance range required during RF excitation based on an assessment of image quality. The experimental results are fit by an exponential model and establish criteria that can be applied for general receiver coil design.     

Investigation of complex phased array coil designs for cardiac imaging

In this study we present a method to simulate complex phased array coil designs for cardiac imaging. It is based on the combination of numerically calculatedB ₁ field vectors for each coil of the array and a noise resistance data set, which is acquired only once with a set of test coils. This technique allowed fast assessment of the SNR performance of arbitrary geometries of single coils to be used as building blocks in complex array configurations. In addition, since clinical scanners usually provide only four receiver channels, we used this method to investigate the use of hardware combiners for different array configurations, consisting of up to eight coils. Simulated array geometries resulted in up to ≈30% gain in SNR for deep cardiac structures, compared to a conventional linear four coil array. This was confirmed by phantom experiments with implemented coils     

Optimized quadrature surface coil designs

Background Quadrature surface MRI/MRS detectors comprised of circular loop and figure-8 or butterfly-shaped coils offer improved signal-to-noise-ratios (SNR) compared to single surface coils, and reduced power and specific absorption rates (SAR) when used for MRI excitation. While the radius of the optimum loop coil for performing MRI at depth d in a sample is known, the optimum geometry for figure-8 and butterfly coils is not. Materials and methods The geometries of figure-8 and square butterfly detector coils that deliver the optimum SNR are determined numerically by the electromagnetic method of moments. Figure-8 and loop detectors are then combined to create SNR-optimized quadrature detectors whose theoretical and experimental SNR performance are compared with a novel quadrature detector comprised of a strip and a loop, and with two overlapped loops optimized for the same depth at 3 T. The quadrature detection efficiency and local SAR during transmission for the three quadrature configurations are analyzed and compared. Results The SNR-optimized figure-8 detector has loop radius r ₈ ~ 0.6d, so r ₈/r ₀ ~ 1.3 in an optimized quadrature detector at 3 T. The optimized butterfly coil has side length ~ d and crossover angle of ≥ 150° at the center. Conclusions These new design rules for figure-8 and butterfly coils optimize their performance as linear and quadrature detectors. This work is supported by NIH grant R01 RR15396.     

Efficient estimation of adjoint‐variable S‐parameter sensitivities with time domain TLM

We present a novel algorithm for efficient estimation of S‐parameter sensitivities with the time‐domain transmission line modelling (TLM) method. The original electromagnetic structure is simulated using TLM to obtain the S‐parameters in the desired frequency band. For each port, an adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the S‐parameters in the desired frequency band are estimated using only the original and adjoint simulations. For a structure with N_p ports and n designable parameters, our approach requires only N_p additional simulations regardless of n. This can be easily contrasted with the 2nN_p additional simulations required by the central difference approximation. Our algorithm is illustrated through the estimation of S‐parameter sensitivities with respect to the dimensions of waveguide discontinuities. Very good match is obtained between our sensitivity estimates and those obtained using central difference approximation. Copyright © 2005 John Wiley & Sons, Ltd.     

Analytical estimation of high-frequency properties of RF micro-inductors prepared by direct-write techniques

In this work, the high-frequency characteristics of micron size, square spiral-type RF thin film air-core inductors prepared by direct-write supersonic jet deposition of laser ablated nanoparticle were estimated by a software package. The important parameters that are determined by direct-write process, such as the materials, the cross-section shape, the thickness, and the conductivity of the coil, were varied systematically to estimate the trend of the inductors’ performances. The area dimensions of RF inductors utilized and the number of coil turns were 1.3 mm?×?1.3 mm and 3, respectively. The 96 wt% Al₂O₃, SiO₂-coated Al₂O₃, and SiO₂-coated Si wafers were used as the substrate material. The high-frequency characteristics of the inductance (L) and quality factor (Q) of the utilized inductors were simulated using a Maxwell three-dimensional field simulator (HFSS 8.5), which employs the finite element method. The used inductors, which have silver metal (Ag) coils, the trapezoid shaped cross-section of the coil, the coil thickness of 30 μm, and the coil conductivity of 70% of Ag bulk value, exhibit L of 8 to 9 nH. They also exhibit a maximum Q of 35.5 near the frequency of 750 MHz and a self-resonant frequency of 5.14 GHz. The simulated high-frequency data of the inductors were agreed well with those obtained from the equivalent circuit model of the utilized inductors.     

A shell type superconducting transformer for experiments using Nb3Sn superconducting cables

A shell-type superconducting transformer was developed for experiments using Nb₃Sn superconducting cables. The designed capacity is 667 kVA (single phase), the voltage is 440/220 V, the current is 1515/3030 A and the percent impedance is 16 percent. Main features of the transformer are as follows: (1) Magnetic field in superconducting coils is decreased by increasing the number of high and low coil groups. (2) Large-scale superconducting cables are not needed when the number of high and low coil groups is increased. (3) Epoxy impregnated coils are used to withstand an electromagnetic force at 120 Hz. The Nb₃Sn basic strand was manufactured by the internal tin diffusion process. The cable consists of seven insulated subcables, and the subcable consists of seven strands. The primary (HV) coil of the transformer was excited, in which the secondary (LV) coil was shortened. The primary current reached 1618 A_rms without quenching, and the reached capacity corresponds to 712 KA. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 13–21, 1997     

Measurement of arterial input function of <Superscript>17</Superscript>O water tracer in rat carotid artery by using a region-defined (REDE) implanted vascular RF coil

A method of determining arterial input function (AIF) by continuously detecting the ¹⁷O MR signal changes of ¹⁷O-labeled water tracer in the rat carotid artery using a region-defined (REDE) implanted vascular RF coil at 9.4 Tesla is reported. This coil has a compact physical size of 1 mm inner diameter, 3 mm outer diameter and 11 mm in length. It can be readily implanted into the rat neck and wrapped around the rat carotid artery for achieving adequate MR detection sensitivity for determining AIF with minimal surgical trauma. Water phantom and in vivo MR experiments were conducted for validating the coil's performance. A signal-to-noise ratio of ~20:1 was achieved for the ¹⁷O signal acquired from naturally abundant H₂ ¹⁷O in a small amount of blood (~7 μl) inside the rat carotid artery with an acquisition time of 11 s. The REDE RF coil design electromagnetically isolates the rat carotid artery from surrounding tissues and ensures that the MR signal detected by the RF coil is only attributable to the artery blood. It also minimizes the electromagnetic coupling between the implanted RF coil and a head surface coil tuned at the same operating frequency (two-coil configuration). This configuration allowed simultaneous measurements of dynamic changes of ¹⁷O MR signal of the H₂ ¹⁷O tracer in both rat carotid artery and brain. Compared to most contemporary MR approaches, the REDE implanted RF provides a simple, accurate, and promising solution for determination of AIF in small experimental animals.     

A method of RF inhomogeneity correction in MR imaging

A direct postprocessing method for correcting RF inhomogeneity in MR imaging is proposed. First, two images with different flip-angles of ψ and 2ψ are obtained. Next, the spatial distribution maps of the sensitivity of the surface coil and theB ₁ field intensity are produced by employing those images. Finally, the correction of the MR image is achieved, dividing the original image by distribution maps of the coil sensitivity and theB ₁ field intensity. The method was applied to image obtained by a gradient echo sequence and the corrected image is presented.     

Improvement of equivalent magnetic coupling coefficient of wireless power transfer system with multiwinding transmission coil

This article proposes a novel method of improving the overall coupling coefficient of a wireless power transfer system by using a four‐winding transformer of which two pairs of windings each are connected in parallel in the primary side and secondary side. By weakening the coupling coefficients k_c between the parallel windings, the overall magnetic coupling between the primary side and secondary side of the transformer is improved. In order to confirm the validity of the proposed method, a conventional high‐k_c four‐winding transmission coil and the proposed low‐k_c four‐winding transmission coil are experimentally assessed. Then, the coils are introduced into a nonresonant inductive power transfer system utilizing a dual active bridge converter in order to confirm the correction of the input power factor. As a result, the overall magnetic coupling of the proposed four‐winding coils was found to be improved by 17% compared with the overall magnetic coupling of the conventional coil. Moreover, the input power factor for the entire load is corrected with the proposed four‐winding coil. Therefore, a low k_c is effective in improving the overall magnetic coupling.     

Self‐adjoint S‐parameter sensitivities for lossless homogeneous TLM problems

We present a novel efficient algorithm for the estimation of S‐parameter sensitivities in homogeneous and lossless transmission line modelling (TLM) problems. Our approach estimates S‐parameter adjoint‐based sensitivities without actually carrying out any adjoint simulation. By applying a transformation to the original TLM simulation we establish an isomorphism between the original and the adjoint problem. The unique properties of the TLM node in a lossless and homogeneous problem are also exploited in establishing the isomorphism. For an electromagnetic structure with N_p ports, only the N_p original simulations utilized in evaluating the S‐parameters are required to estimate their sensitivities as well. Our novel approach is illustrated through estimating S‐parameter sensitivities with respect to waveguide discontinuities. Good match is obtained between our sensitivity estimates and those calculated using finite differences at the response level. Copyright © 2005 John Wiley & Sons, Ltd.     

Development of a 400 kJ Nb3Sn superconducting magnet for an SMES system

An Nb₃Sn superconducting magnet to store 400 kJ was developed as a unit magnet for a 2.4-MJ SMES system used for stabilization studies of electrical power systems. The superconducting magnet consists of a cryostat and an Nb₃Sn coil. The dimensions of the coil are: 340 mm inner diameter, 700 mm outer diameter and 177 mm axial length. The pool-cooled coil is a stack of 20 Nb₃Sn double pancakes, and the cooling channels are aligned between pancake coils. To reduce Joule loss in electrical power converters, the maximum operating current of the coil is designed to be 350 A, which is one order of magnitude less than the operating currents of similar scale coils for pulse use. The conductor is an Nb₃Sn monolithic conductor with cross section 1.50 × 2.38 mm. For good superconducting stability and high dielectric strength of the coil, the Nb₃Sn double pancakes were wound by the react-and-wind technique. Operation of dc current to 105% (367.5 A) of the design operating current was achieved without quench. After the whole of the coil was exposed out of liquid helium, the coil did not quench under 120 A current operation for more than 2 hours. It was verified that the coil was stable for the SMES system. © 1998 Scripta Technica, Inc. Electr Eng Jpn, 121(3): 44–52, 1997     

Scaling Law of Coupling Coefficient and Coil Size in Wireless Power Transfer Design via Magnetic Coupling

Wireless Power Transfer (WPT), such as magnetic resonant coupling using a magnetic field, is being studied and discussed for a wide variety of applications. When the transmission distances are large, very large transmitters and receivers need to be considered. However, in the early stages of an investigation, it might be prohibitive to manufacture and evaluate coils of such a large size. To reduce costs and effort, a scaling law can be used to estimate the WPT efficiency of very large coils using the results of smaller coils. In this paper, a scaling law is proposed that relates the coil size to the coupling coefficient, assuming the ratio between the coil diameter and coil length remains constant. The coupling coefficient is one of the parameters that determine the maximum efficiency of magnetic field WPT. The proposed method was verified by an electromagnetic field simulator and experiment. The results of this study provide an easy method for estimating the WPT efficiency of very large coils.