Browsing by Subject "RF power"

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Open Access
A novel verse optimal RF pulse design method for parallel transmission in magnetic resonance imaging
(2009) Bayındır, Haldun Özgür
A novel radio frequency (RF) pulse design method for magnetic resonance imaging (MRI) and an improvement to an existing method that reduces specific absorption rate (SAR) in MRI are presented. The new RF pulse design method, variable rate selective excitation optimal RF pulse design method for parallel transmission (VERSEp), is developed for parallel transmission and aim of the method is to design RF pulses with lowest SAR after SAR reduction with variable rate selective excitation (VERSE) method. This is achieved by modifying the SAR optimal RF pulse deisgn method for parallel transmission. Performance of the VERSEp method is tested by comparing VERSE-SAR reduced SAR of the RF pulses designed with SAR optimal RF pulse design method and VERSE-SAR reduced SAR of the RF pulses designed using VERSEp. In the simulations, SAR reductions up to 47% are obtained. Different aspects of VERSEp are also shown with simulations. The second contribution of this work is an improvement made to an existing constrained VERSE-SAR reduction method. The existing VERSE-SAR reduction method uses a peak RF constaint for SAR reduction. In this work, peak square root power constraint is used instead of peak RF constraint in the VERSE-SAR reduction method. In the simulation results, the SAR of the RF pulses designed using the improved method were compared with SAR of the RF pulses designed using the method before improvement. SAR reductions up to 50% are obtained by using peak square root power constrained SAR reduction instead of peak RF constrained SAR reduction.
Open Access
Optimization of coupled Class-E RF power amplifiers for a transmit array system in 1.5T MRI
(2020-01) Aldemir, Muhammed Said
Radio Frequency (RF) ﬁeld is generated mostly by linear RF power ampliﬁers in Magnetic Resonance Imaging (MRI). These ampliﬁers have relatively low eﬃciency and are placed far from the transmit coil in MRI system room. Additional cooling systems and long transmission cables increase the cost of MRI hardware. Coupled Class-E on-coil RF ampliﬁers for a Transmit Array System is proposed instead of linear RF ampliﬁers. Class-E RF ampliﬁer is a nonlinear switching ampliﬁer which has 100% drain eﬃciency ideally. State-space models for single Class-E ampliﬁer and coupled Class-E ampliﬁers are derived and steady-state operation of coupled Class-E ampliﬁers is investigated. The state-space models are veriﬁed with the simulation results of the Class-E ampliﬁer. Eﬀect of coupling between channels on the output characteristics of the transmit system is observed. Instead of implementing decoupling methods, coupling is maintained and optimization of circuit parameters for coupled Class-E ampliﬁers is proposed to achieve high drain eﬃciency and high output RF power. Output power of 600 W and overall drain eﬃciency higher than 90% are achieved by two coupled Class-E ampliﬁers even with high coupling levels in the state-space model. Power and eﬃciency measurements are done with coupled ampliﬁers and MRI experiments are performed in 1.5T Scimedix MRI Scanner. In conclusion, coupled Class-E RF power ampliﬁers can be operated as on-coil ampliﬁers in a Transmit Array system with high output characteristics by optimization of circuit parameters.