SAR Simulations & Safety Fiedler, Thomas M.; Ladd, Mark E.; Bitz, Andreas K.
NeuroImage (Orlando, Fla.),
March 2018, 2018-03-00, 20180301, Letnik:
168
Journal Article
Recenzirano
At ultra-high fields, the assessment of radiofrequency (RF) safety presents several new challenges compared to low-field systems. Multi-channel RF transmit coils in combination with parallel transmit ...techniques produce time-dependent and spatially varying power loss densities in the tissue. Further, in ultra-high-field systems, localized field effects can be more pronounced due to a transition from the quasi stationary to the electromagnetic field regime. Consequently, local information on the RF field is required for reliable RF safety assessment as well as for monitoring of RF exposure during MR examinations. Numerical RF and thermal simulations for realistic exposure scenarios with anatomical body models are currently the only practical way to obtain the requisite local information on magnetic and electric field distributions as well as tissue temperature.
In this article, safety regulations and the fundamental characteristics of RF field distributions in ultra-high-field systems are reviewed. Numerical methods for computation of RF fields as well as typical requirements for the analysis of realistic multi-channel RF exposure scenarios including anatomical body models are highlighted. In recent years, computation of the local tissue temperature has become of increasing interest, since a more accurate safety assessment is expected because temperature is directly related to tissue damage. Regarding thermal simulation, bio-heat transfer models and approaches for taking into account the physiological response of the human body to RF exposure are discussed. In addition, suitable methods are presented to validate calculated RF and thermal results with measurements. Finally, the concept of generalized simulation-based specific absorption rate (SAR) matrix models is discussed. These models can be incorporated into local SAR monitoring in multi-channel MR systems and allow the design of RF pulses under constraints for local SAR.
Abstract Purpose This study proposes faster virtual observation point (VOP) compression as well as post‐processing algorithms for specific absorption rate (SAR) matrix compression. Furthermore, it ...shows the relation between the number of channels and the computational burden for VOP‐based SAR calculation. Methods The proposed new algorithms combine the respective benefits of two different criteria for determining upper boundedness of SAR matrices by the VOPs. Comparisons of the old and new algorithms are performed for head coil arrays with various channel counts. The new post‐processing algorithm is used to post‐process the VOP sets of nine arrays, and the number of VOPs for a fixed median relative overestimation is compared. Results The new algorithms are faster than the old algorithms by a factor of two to more than 10. The compression efficiency (number of VOPs relative to initial number of SAR matrices) is identical. For a fixed median relative overestimation, the number of VOPs increases logarithmically with the number of RF coil channels when post‐processing is applied. Conclusion The new algorithms are much faster than previous algorithms. Post‐processing is very beneficial for online SAR supervision of MRI systems with high channel counts, since for a given number of VOPs the relative SAR overestimation can be lowered.
A 32-channel parallel transmit (pTx) add-on for 7 Tesla whole-body imaging is presented. First results are shown for phantom and in-vivo imaging.
The add-on system consists of a large number of ...hardware components, including modulators, amplifiers, SAR supervision, peripheral devices, a control computer, and an integrated 32-channel transmit/receive body array. B1+ maps in a phantom as well as B1+ maps and structural images in large volunteers are acquired to demonstrate the functionality of the system. EM simulations are used to ensure safe operation.
Good agreement between simulation and experiment is shown. Phantom and in-vivo acquisitions show a field of view of up to 50 cm in z-direction. Selective excitation with 100 kHz sampling rate is possible. The add-on system does not affect the quality of the original single-channel system.
The presented 32-channel parallel transmit system shows promising performance for ultra-high field whole-body imaging.
In this research study a novel functionally graded metal syntactic foam (FG-MSF) was manufactured using expanded perlite and activated carbon particles. A tailored arrangement of these fillers was ...infiltrated with ZA27 alloy in a single-step process. The structure of the FG-MSF contained two individual layers: ZA27/expanded perlite (EP-MSF) and ZA27/activated carbon (AC-MSF) syntactic foam. The density of these FG-MSFs varied between 2.11 and 2.15 g·cm−3. Microstructural studies confirmed that no relevant chemical reaction occurred within the foam, in particular in the vicinity of the particle-matrix interfaces. The mechanical properties of the produced FG-MSF were evaluated using quasi-static compression testing. The results showed that the deformation mechanism of the FG-MSF is a mixed mode and varies between the two different filler layers. The energy absorption of the FG-MSF sample was increased compared to uniform syntactic foams containing only a single particle filler.
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•A novel functionally graded metal matrix syntactic foam (FG-MSF) was manufactured.•Two different particles were used in a tailored arrangement to produce FG-MSF.•The density of the FG-MSFs is between the densities of uniform MSF.•The mechanical properties of the FG-MSF are governed by the contained layers.•FG-MSF showed a higher energy absorption in comparison to uniform MSFs.
Purpose
Local specific absorption rate (SAR) compression algorithms are essential for enabling online SAR monitoring in parallel transmission. A better compression resulting in a lower number of ...virtual observation points improves speed of SAR calculation for online supervision and pulse design.
Method
An iterative expansion of an existing algorithm presented by Lee et al is proposed in this work. The original algorithm is used within a loop, making use of the virtual observation points from the previous iteration as the starting subvolume, while decreasing the overestimation with each iteration. This algorithm is evaluated on the SAR matrices of three different simulated arrays.
Result
The number of virtual observation points is approximately halved with the new algorithm, while at the same time the compression time is reduced with speed‐up factors of up to 2.5.
Conclusion
The new algorithm improves the original algorithm in terms of compression rate and speed.
Over the past two decades, a large number of metallic foams have been developed. In recent years research on this multi-functional material class has further intensified. However, despite their ...unique properties only a limited number of large-scale applications have emerged. One important reason for this sluggish uptake is their high cost. Many cellular metals require expensive raw materials, complex manufacturing procedures, or a combination thereof. Some attempts have been made to decrease costs by introducing novel foams based on cheaper components and new manufacturing procedures. However, this has often yielded materials with unreliable properties that inhibit utilization of their full potential. The resulting balance between cost and performance of cellular metals is probed in this editorial, which attempts to consider cost not in absolute figures, but in relation to performance. To approach such a distinction, an alternative classification of cellular metals is suggested which centers on structural aspects and the effort of realizing them. The range thus covered extends from fully stochastic foams to cellular structures designed-to-purpose.
Purpose
Compression of local specific absorption rate (SAR) matrices is essential for enabling SAR monitoring and efficient pulse calculation in parallel transmission. Improvements in compression ...result in lower error margin and/or lower number of virtual observation points (VOPs). The purpose of this work is to introduce two algorithms for post‐processing of already compressed VOP sets. One calculates individual overestimation matrices for the VOPs to reduce overestimation, the other identifies redundant VOPs.
Methods
The first algorithm was evaluated for VOP sets calculated for three different transmit arrays with either 8 or 16 channels. For each array, two different overestimation matrices were used to generate the VOP sets. Each post‐processed VOP set was evaluated using one million random excitation vectors and the results compared to the VOP set before post‐processing. The second algorithm was evaluated by utilizing the same random excitation vectors and comparing the results after removal of the redundant VOPs with the results before removal to verify that these were identical.
Results
The first algorithm reduced the mean overestimation by up to four fifths compared to the original set, while keeping the number of VOPs constant. The second algorithm decreased the number of VOPs generated by a compression with Eichfelder and Gebhardt’s algorithm by more than 40% in 40% of the investigated cases and by more than 20% in 73% of the investigated cases.
Conclusion
Two post‐processing algorithms are presented that enhance previously compressed VOP sets by improving the accuracy per number of VOPs.
The number of shark attacks resulting in fatalities and severe injuries has increased steadily over recent years. This is mainly attributed to a growing population participating in ocean sports such ...as swimming, diving, and surfing. To mitigate the severity of shark attacks, the current study presents a novel fibre-reinforced composite for bite protection. This material is intended for integration into neoprene wetsuits, e.g., in the form of protective pads. A suitable material must be able to withstand significant bite forces, which are concentrated within a small contact area at the tips of the shark teeth. At the same time, the material should not hinder the complex motion sequences of aquatic sports. To this end, a novel fibre-reinforced composite was created by integrating Kevlar fibres into an elastic matrix. Uni-axial testing using shark teeth replicas was conducted on a specially designed test rig to quantify the effectiveness of the novel protective material.
The aim of this study was to investigate the potential application of metal foams in shell-tube recuperators. A356 aluminium foam was cast around the internal and external surfaces of a thin-walled ...copper tube to enhance heat transfer between separated water streams at different temperatures. The results demonstrated that the aluminium foam drastically increased heat transfer efficiency due to its large volumetric surface area and high thermal conductivity. In the shell-tube foam recuperators, a maximum heat transfer efficiency of 48.1% was observed, compared to only 12.2% for a single copper tube without metal foam. The pressure drop across the external foam increased with the flow rate, from an average value of 1.19 kPa at 1.0 L/min to 7.36 kPa at 3.0 L/min. These findings suggest that metal foams have great potential for use in shell-tube recuperators, which could significantly improve the efficiency of heat transfer in various industrial and engineering applications.
Purpose
This study aims to find a relation between the number of channels and the computational burden for specific absorption rate (SAR) calculation using virtual observation point‐based SAR ...compression.
Methods
Eleven different arrays of rectangular loops covering a cylinder of fixed size around the head of an anatomically correct voxel model were simulated. The resulting Q‐matrices were compressed with 2 different compression algorithms, with the overestimation fixed to a certain fraction of worst‐case SAR, median SAR, or minimum SAR. The latter 2 were calculated from 1e6 normalized random excitation vectors.
Results
The number of virtual observation points increased with the number of channels to the power of 2.3–3.7, depending on the compression algorithm when holding the relative error fixed. Together with the increase in the size of the Q‐matrices (and therefore the size of the virtual observation points), the total increase in computational burden with the number of channels was to the power of 4.3–5.7.
Conclusion
The computational cost emphasizes the need to use the best possible compression algorithms when moving to high channel counts.
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