Search

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. [ABSTRACT FROM AUTHOR]

Purpose: Transmit arrays for body imaging have characteristics of both volume and local transmit coils. This study evaluates two specific absorption rate (SAR) aspects, local and whole‐body SAR, of arrays for body imaging at 7 T and also for a 3 T birdcage. Methods: Simulations were performed for six antenna arrays at 7 T and one 3 T birdcage. Local SAR matrices and the whole‐body SAR matrix were computed and evaluated with random shims. A set of reduced local SAR matrices was determined by removing all matrices dominated by the whole‐body SAR matrix. Results: The results indicate that all RF transmit coils for body imaging in this study are constrained by the local SAR limit. The ratio between local and whole‐body SAR is nevertheless smaller for arrays with large FOV, as these arrays also expose a larger part of the human body. By using the whole‐body SAR matrix, the number of local SAR matrices can be reduced (e.g., 33.3% matrices remained for an 8‐channel local array and 89.7% for a 30‐channel remote array; 12.1% for the 3 T birdcage). Conclusion: For transmit antenna arrays used for body imaging at 7 T as well as for the 3 T birdcage, all evaluated cases show that the local SAR limit was reached before reaching the whole‐body SAR limit. Nevertheless, the whole‐body SAR matrix can be used to reduce the number of local SAR matrices, which is important to reduce memory and computing time for a virtual observation points (VOP) compression. This step can be included as a pre‐compression prior to a VOP compression. [ABSTRACT FROM AUTHOR]

Parallel transmission (pTX) techniques are required to tackle a number of challenges, e.g., the inhomogeneous distribution of the transmit field and elevated specific absorption rate (SAR), in ultra-high field (UHF) MR imaging. Additionally, they offer multiple degrees of freedom to create temporally- and spatially-tailored transverse magnetization. Given the increasing availability of MRI systems at 7 T and above, it is anticipated that interest in pTX applications will grow accordingly. One of the key components in MR systems capable of pTX is the design of the transmit array, as this has a major impact on performance in terms of power requirements, SAR and RF pulse design. While several reviews on pTX pulse design and the clinical applicability of UHF exist, there is currently no systematic review of pTX transmit/transceiver coils and their associated performance. In this article, we analyze transmit array concepts to determine the strengths and weaknesses of different types of design. We systematically review the different types of individual antennas employed for UHF, their combination into pTX arrays, and methods to decouple the individual elements. We also reiterate figures-of-merit (FoMs) frequently employed to describe the performance of pTX arrays and summarize published array designs in terms of these FoMs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]