Specific Absorption Rate Reduction Research Articles

Application of PINS radiofrequency pulses to reduce power deposition in RARE/turbo spin echo imaging of the human head

To explore the use of PINS radiofrequency (RF) pulses to reduce RF power deposition in multiband/simultaneous multislice imaging with the RARE/turbo spin echo (TSE) sequence at 3T and 7T. A PINS-TSE sequence was implemented and combined with blipped CAIPI to improve the reconstruction of superposed slices. Whole brain imaging of healthy volunteers was performed at both 3T and 7T using a 32-channel coil for signal reception. A considerable reduction in power deposition was achieved compared with a standard sequence of the manufacturer. At 3T, the reduction in specific absorption rate (SAR) made short pulse repetition times (TRs) possible, however, in order to obtain a good T2 contrast, it is advisable to maintain TR while extending the echo train length. At 7T, whole brain coverage with a spatial resolution of 1 × 1 × 2 mm(3) was achieved in an acquisition time of 150 s. Furthermore, it could be shown that pulse sequences that use PINS pulses do not suffer from having additional magnetization transfer contrast. PINS RF pulses combined with multiband imaging reduce SAR sufficiently to enable routine TSE imaging at 7T within clinically acceptable acquisition times. In general, the combination of multiband imaging with PINS RF pulses represents a method to reduce total RF power deposition.

Assessment of specific absorption rate reduction in human head using metamaterial

Abstract In this paper, the specific absorption rate (SAR) reduction in human head with metamaterial attachment is calculated. The finite-difference time-domain (FDTD) method has been used to evaluate the SAR in an anatomically correct model of the human head. We designed the double-negative metamaterials by placing periodic array arrangement of split ring resonators (SRRs). By properly designing and tuning the structural parameters of SRRS, the effective medium parameters can be made negative at 900 and 1800 MHz band, as presented in this paper. Experimental results show that, with presence of resonators, SAR reduction in the human head is achievable. These results can provide useful insight into the design of safety-compliant mobile communication equipment.

A New Design of Metamaterials for SAR Reduction

The purpose of this paper is to calculate the reduction of specific absorption rate (SAR) with a new design of square metamaterials (SMMs). The finite-difference time-domain (FDTD) method with lossy-Drude model is adopted in this analysis. The method of SAR reduction is discussed and the effects of location, distance, and size of metamaterials are analyzed. SMMs have achieved a 53.06% reduction of the initial SAR value for the case of 10 gm SAR. These results put forward a guideline to select various types of metamaterials with the maximum SAR reducing effect for a cellular phone.

Flexible and Compact AMC Based Antenna for Telemedicine Applications

We present a flexible, compact antenna system intended for telemedicine applications. The design is based on an M-shaped printed monopole antenna operating in the Industrial, Scientific, and Medical (ISM) 2.45 GHz band integrated with a miniaturized slotted Jerusalem Cross (JC) Artificial Magnetic Conductor (AMC) ground plane. The AMC ground plane is utilized to isolate the user's body from undesired electromagnetic radiation in addition to minimizing the antenna's impedance mismatch caused by the proximity to human tissues. Specific Absorption Rate (SAR) is analyzed using a numerical human body model (HUGO) to assess the feasibility of the proposed design. The antenna expresses 18% impedance bandwidth; moreover, the inclusion of the AMC ground plane increases the front to back ratio by 8 dB, provides 3.7 dB increase in gain, in addition to 64% reduction in SAR. Experimental and numerical results show that the radiation characteristics, impedance matching, and SAR values of the proposed design are significantly improved compared to conventional monopole and dipole antennas. Furthermore, it offers a compact and flexible solution which makes it a good candidate for the wearable telemedicine application.

Analysis of circumferential shielding as a methodto decouple radiofrequency coils for high‐field MRI

AbstractReducing the impact of mutual coupling in magnetic resonance imaging (MRI) radiofrequency (RF) coil transmit/receive arrays constitutes a serious design challenge. Recently, it has been shown that mutual coupling in such arrays can be significantly reduced by circumferentially shielding the individual array elements. For such a decoupling method to be optimized and readily incorporated into future array designs, comprehensive analysis of the shield's effect on overall element and array performance is required. This study is focused on methodically analyzing this decoupling method to determine the effect of shielding geometry on overall coil performance in high‐field MRI. To this end, experiments on a 7‐Tesla MRI system, as well as full‐wave electromagnetic simulations, were performed and performance metrics, such as Q‐factor, power efficiency, transmit power, signal‐to‐noise ratio (SNR), and specific absorption rate (SAR), were investigated for different coil/shield configurations. Differences in the geometry of a RF coil and its respective shield resulted in an increase in peak SNR of up to 40% compared to an unshielded coil. This was concurrent with a 2–4 dB reduction in the required transmit power to produce a given flip angle and a 25% reduction in peak local SAR compared to the unshielded coil. Coupling between adjacent loaded coils was strongly influenced by the coil/shield geometry and ranged from –6.5 to –22.1 dB. Based upon the analysis presented herein, example coil‐array designs are provided and have been optimized for either peripheral or whole‐brain imaging. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B:11–21, 2013

Combined acquisition technique (CAT) for high-field neuroimaging with reduced RF power

Clinical 3 T MRI systems are rapidly increasing and MRI systems with a static field of 7 T or even more have been installed. The RF power deposition is proportional to the square of the static magnetic field strength and is characterized by the specific absorption rate (SAR). Therefore, there exist defined safety limits to avoid heating of the patient. Here, we describe a hybrid method to significantly reduce the SAR compared to a turbo-spin-echo (TSE) sequence. We investigate the potential benefits of a combined acquisition technique (CAT) for high-field neuroimaging at 3 and 7 T. The TSE/EPI CAT experiments were performed on volunteers and patients and compared with standard TSE and GRASE protocols. Problems and solutions regarding T2 weighted CAT imaging are discussed. We present in vivo images with T2 and proton density contrast obtained on 3 and 7 T with significant SAR reduction (up to 60%) compared with standard TSE. Image quality is comparable to TSE but CAT shows fewer artifacts than a GRASE sequence. CAT is a promising candidate for neuroimaging at high fields up to 7 T. The SAR reduction allows one to shorten the waiting time between two excitations or to image more slices thereby reducing the overall measurement time.

Effects of Phase Difference in Dipole Phased-Array Antenna Above EBG Substrates on SAR

In this letter, we investigate the specific absorption rate (SAR) of a dipole phased-array antenna above an electromagnetic band-gap (EBG) substrate. A two-element array antenna above an EBG substrate is considered for fourth-generation mobile applications at the 3.5-GHz band. The computational results indicate that the use of an EBG substrate in a phased-array antenna can provide an SAR reduction of over 70%. The SAR reduction also depends on the phase difference of the phased-array antenna.

Analysis of the Human Head Interaction in Handset Antennas with Slotted Ground Planes

The interaction between the human head and handset antennas needs to be taken into account, since mobile phones have to be functional, and, at the same time, guarantee biological compatibility. The present research analyzed the performance of several handset antennas with different slotted ground planes in free space and also in the presence of the human head. The main objective was to compare the measured bandwidth and efficiency in both environments (free space and human interaction), and the impact on the measured SAR (Specific Absorption Rate) of such antennas as functions of the slot configuration in the ground plane and the antenna's location. Results showed that slots may be useful for increasing bandwidth and efficiency while keeping similar SAR values compared to the non-slotted ground plane. Changing the antenna's location was a good way to achieve a significant SAR reduction. In some cases, when the antenna was at the lower edge of the ground plane (down position), the SAR could be reduced by a factor of two. Impedance, efficiency, and radiation patterns were measured and analyzed with a phantom head in order to understand the human head's effect on the antenna's performance. Although all antennas suffered from changes, shorted-end slots were more robust for human interaction than open-ended slots.

Improved Handset Antenna Performance via an Electrically Extended Ground Plane

A novel and practical approach is presented providing improved antenna performance without enlarging the antenna or the ground plane. The approach electrically extends the ground plane using wire(s) that behave as surface metal extensions of the ground plane. The wire extensions can be accommodated within typical handset housing or as part of the stylish metal used on the handset’s exterior perimeter; hence don’t require enlargement of the device. Consequently, this approach avoids the costs and limitations traditionally associated with physically lengthening of a ground plane. Eight variations are presented and compared with baseline antenna performance. Both far-field patterns and near-field electromagnetic scans demonstrate that the proposed approach controls the electrical length of the ground plane and hence its chassis wavemodes, without negatively impacting the characteristics of the antenna. Improvements in performance of up to 56% in bandwidth at 900 MHz and up to 12% in efficiency with a reduction of up to 12% in the specific absorption rate (SAR) are achieved. An 8% increase in efficiency with a 1.3% improvement in bandwidth and a 20% reduction in SAR is achieved at 1880 MHz. Thus, improvements in bandwidth are achieved without compromising efficiency. Further, improvements at lower frequencies do not compromise performance at higher frequencies.

Contrast enhancement in TOF cerebral angiography at 7 T using saturation and MT pulses under SAR constraints: Impact of VERSE and sparse pulses

Cerebral three-dimensional time of flight (TOF) angiography significantly benefits from ultrahigh fields, mainly due to higher signal-to-noise ratio and to longer T(1) relaxation time of static brain tissues; however, specific absorption rate (SAR) significantly increases with B(0). Thus, additional radiofrequency pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work, we aimed at reducing SAR for each radiofrequency pulse category in a TOF sequence. We use the variable-rate selective excitation principle for the slab selective TOF excitation as well as the venous saturation radiofrequency pulses. In addition, magnetization transfer pulses are implemented by sparsely applying the pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast, and SAR reduction were investigated as a function of variable-rate selective excitation parameters and of the total number of magnetization transfer pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits.

System and SAR characterization in parallel RF transmission

The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.

The study of specific absorption rate (SAR) reduction in mobile phones using materials and metamaterials

In this paper, reducing of specific absorption rate (SAR) with materials and metamaterials attachment is investigated. The finite-difference time-domain method with lossy-Drude model is adopted in this study. The methodology of SAR reduction is addressed and the belongings of attaching position, distance, and size of ferrite sheet material properties, perfect electric conductor (PEC), and metamaterials on the SAR reduction are investigated. Materials have achieved a 47.02% reduction of the initial SAR value while metamaterials achieved a reduction of 49.21% respectively for the case of 1 g SAR. These results propose a guideline to decide assorted types of materials and metamaterials with the utmost SAR reducing effect for a phone model. Key words: Antenna, human head model, lossy-Drude model, materials, metamaterials, specific absorption rate (SAR), symmetry.

Local SAR management by RF Shimming: a simulation study with multiple human body models

Parallel transmission facilitates a relatively direct control of the RF transmit field. This is usually applied to improve the RF field homogeneity but might also allow a reduction of the specific absorption rate (SAR) to increase freedom in sequence design for high-field MRI. However, predicting the local SAR is challenging as it depends not only on the multi-channel drive but also on the individual patient. The potential of RF shimming for SAR management is investigated for a 3 T body coil with eight independent transmit elements, based on Finite-Difference Time-Domain (FDTD) simulations. To address the patient-dependency of the SAR, nine human body models were generated from volunteer MR data and used in the simulations. A novel approach to RF shimming that enforces local SAR constraints is proposed. RF shimming substantially reduced the local SAR, consistently for all volunteers. Using SAR constraints, a further SAR reduction could be achieved with only minor compromises in RF performance. Parallel transmission can become an important tool to control and manage the local SAR in the human body. The practical use of local SAR constraints is feasible with consistent results for a variety of body models.

Design of Optimized Multilayer PIFA With the EBG Structure for SAR Reduction in Mobile Applications

This paper proposes an optimized multilayer planar inverted-F antenna (PIFA) with an electromagnetic bandgap (EBG) at personal communication services band for specific absorption rate (SAR) reduction. The EBG structure, which has bandgap capability and acts as a perfect magnetic conductor surface, can suppress surface waves and reduce undesired radiation from an antenna. Thus, the EBG structure is capable of preventing electromagnetic (EM) fields being emitted in the direction of the human head. To optimize the multilayer PIFA structure for SAR reduction, the EBG structure between the printed circuit board and antenna layer is designed first. Second, a parameter study about the EBG cell numbers, height, and location of the PIFA with the EBG structure is performed. Finally, the optimized multilayer PIFA with the EBG is investigated and the S-parameters, radiation patterns, total radiated power values, and SAR values are presented. The results demonstrate the reduction of SAR values while maintaining the antenna performance. Thus, the human head can be protected from hazardous EM fields using the EBG structure.

Analysis of Electromagnetic Absorption in Mobile Phones Using Metamaterials

The reducing specific absorption rate with metamaterials attachment is investigated in this article. The finite-difference time-domain method with a lossy-Drude model is adopted in this analysis. The technique of specific absorption rate reduction is discussed, and the effects of attaching location, distance, and size of metamaterials, perfect electric conductor, and materials on the specific absorption rate reduction are investigated. Metamaterials have achieved a 42.12% reduction of the initial specific absorption rate value for the case of 1 gm specific absorption rate and 53.94% reduction for the cases of 10 gm specific absorption rate. These results suggest a guideline to choose various types of metamaterials with the maximum specific absorption rate reducing effect for a phone model.

SAR reduction in a muscle cube with metamaterial attachment

The aim of this paper is to evaluate the specific absorption rate (SAR) reduction in a muscle cube by using metamaterial. To evaluate the SAR in a realistic anatomically based model of the muscle cube, the finite-difference time-domain (FDTD) method has been utilized. The effective medium parameter is obtained to be negative at 900 MHz and 1800 MHz band by designing structural parameter of split ring resonators. The reduction is about 44.73% for 900 MHz, and about 48.27% for 1800 MHz was observed in this paper.

Electromagnetic (EM) absorption reduction in a muscle cube with metamaterial attachment

The purpose of this paper is to calculate the specific absorption rate (SAR) reduction in a muscle cube with metamaterial attachment. The finite-difference time-domain (FDTD) method has been used to evaluate the SAR in a realistic anatomically based model of the muscle cube. In this paper, we have designed the single-negative metamaterials from a periodic arrangement of split ring resonators (SRRs). By properly designing the structural parameter of the SRRs, the effective medium parameter can be tuned negative at the 900 MHz and 1800 MHz bands. Numerical results concerning the SAR values in the muscle cube in the presence of resonators exhibit significant SAR reduction. These results can provide useful information when designing safety-compliant mobile communication equipment.

Reduction of SAR for bar type handsets with metal frame

Handsets with metal frame, which are very popular recently, cause higher SAR value due to current leakage into the metal frame. We propose a new design to reduce the SAR (Specific Absorption Rate) without degrading antenna performance which is represented by TRP (Total Radiated Power). We place the contact points between PCB GND and the metal frame at the length of λ/2. This distributes the current over wider area and hence reduces SAR. Besides, we extend the PCB GND around the antenna feed to reduce the high current density in the area. Our experiment shows 32% reduction in SAR without degrading TRP.

Dipole Antenna Above EBG Substrate for Local SAR Reduction

This letter investigates the performance of a dipole antenna above an electromagnetic band-gap (EBG) substrate to realize a low specific absorption rate (SAR) antenna. The frequency band considered is 3.5 GHz, which is assigned to fourth-generation mobile communications. One of the motivations for this proposal is that the size of the EBG substrate can be small enough for a handset. The dipole antenna above the EBG substrate was found to reduce the local SAR in a cubic head model by 84% as compared to the local SAR of a perfect electric conductor. The radiation efficiency was also improved by 10%. These results were also confirmed for a realistic head model.

Specific absorption rate reduction in parallel transmission byk-space adaptive radiofrequency pulse design

The specific absorption rate (SAR) is an important safety criterion, limiting many MR protocols with respect to the achievable contrast and scan duration. Parallel transmission enables control of the radiofrequency field in space and time and hence allows for SAR management. However, a trade-off exists between radiofrequency pulse performance and SAR reduction. To overcome this problem, in this work, parallel transmit radiofrequency pulses are adapted to the position in sampling k-space. In the central k-space, highly homogeneous but SAR-intensive radiofrequency shim settings are used to achieve optimal performance and contrast. In the outer k-space, the homogeneity requirement is relaxed to reduce the average SAR of the scan. The approach was experimentally verified on phantoms and volunteers using field echo and spin echo sequences. A reduction of the SAR by 25-50% was achieved without compromising image quality.