A Comparison of Ansoft HFSS and CST Microwave Studio Simulation Software for Multi-channel Coil Design and SAR Estimation at 7T MRI

Purpose Parallel transmission techniques in MRI have the potential to improve the image quality near metal implants at 3 T. However, current testing of implants only evaluates the risk of radiofrequency (RF) heating in phantoms in circularly polarized mode. We investigate the influence of changing the transmission settings in a 2‐channel body coil on the peak temperature near 2 CoCrMo hip prostheses, using adaptive specific absorption rate (SAR) as an estimate of RF heating. Methods Adaptive SAR is a SAR averaging method that is optimized to correlate with thermal simulations and limit the temperature to 39°C near hip implants. The simulated peak temperature was compared when using whole‐body SAR, SAR10g, and adaptive SAR as a constraint for the maximum allowed input power. Adaptive SAR was used as a fast estimate of temperature to evaluate the trade‐off between good image quality and low heating near the hip implants. Electromagnetic simulations were validated by simulating and measuring B1 maps and electric fields in a phantom at 3 T. Results Simulations and measurements showed excellent agreement. Limiting whole‐body SAR to 2 W/kg and SAR10g to 10 W/kg resulted in temperatures up to 49.3°C and 40.7°C near the hip implants after 30 minutes of RF exposure, respectively. Predictions based on adaptive SAR limited the temperature to 39°C, and allowed to improve the B1 field distribution while preventing peak temperatures near the hip implants. Conclusion Significant RF heating can occur at 3 T near hip implants when parallel transmission is used. Adaptive SAR can be integrated in RF shimming algorithms to improve the uniformity and reduce heating.

Section: Resultssupporting

confidence: 92%

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

Destruel

Fuentes

Weber

et al. 2019

“…The system modeled was a 3 Tesla (T) Philips Achieva MRI scanner fitted with an eight‐channel body transmit coil as described in Vernickel et al . Simulations were performed using the time domain Finite Integration Technique of CST Microwave Studio (CST AG, Darmstadt, Germany) with a 47 ns Gaussian pulse used for excitation and −50 dB energy decay used for the convergence criterion of the 3D EM simulation . Simulations were run in a frequency range from 50 to 200 MHz.…”

Section: Methodsmentioning

confidence: 99%

“…The full coil model has 128 ports of which 120 represent lumped capacitors, with 13 per channel and 16 in the “decoupling ring” structure (see Figure ). Replacing all lumped elements with ports was also found to speed up the simulations in practice by making the coil non resonant, and, therefore, converge to a steady state for each port excitation far more quickly within the simulation .…”

Section: Methodsmentioning

confidence: 99%

Comparison between simulated decoupling regimes for specific absorption rate prediction in parallel transmit MRI

Beqiri

Hand

Hajnal

et al. 2014

Purpose: The use of EM modelling is critical for SAR characterisation in parallel transmission MRI. RF arrays that include decoupling networks can be difficult to characterise accurately in simulation. A practical method of simplifying modelling is to exclude the decoupling networks and model each transmit element in isolation. Results from this type of model can be related to a real device by applying 'active decoupling' to the real device to suppress residual coupling when in use. Here we compare this approach with a full model that includes decoupling networks.Methods: EM simulations for a variety of adult male voxel models placed within an 8-channel TEM array tuned for 3T operation were run with and without decoupling networks included. The resulting EM fields and SAR estimates were compared using basic normalisation, and simulated active decoupling. Results:Modelling the transmit elements independently leads to variations which have significantly different SAR estimates of ~20% on average compared with the full model if not normalised appropriately. After "active decoupling", SAR was still generally seen to be overestimated by ~7% with independent channel modelling; despite having similar B 1 + fields distributions. Conclusion:Modelling transmission elements independently may lead to substantially incorrect SAR estimates if the corresponding MRI system is not run in an analogous manner.

“…With the computing capabilities and procedures mentioned above, implementation of this algorithm at this increased resolution still is doable in under a minute, but for a lower number of k T ‐points pulses (e.g., 5) and iterations (e.g., 7). Nevertheless, it was shown by Kozlov and Turner that lower resolutions could possibly lead to SAR overestimations, which would put lower resolution results on the safe side; however, the opposite behavior has also been reported . Finally, additional speed‐up could possibly be gained by using graphics processing units .…”

Section: Discussionmentioning

confidence: 99%

Radiofrequency pulse design in parallel transmission under strict temperature constraints

Boulant

Massire

Amadon

et al. 2013