Browsing by Subject "Transmit array system"

Now showing 1 - 3 of 3

Open Access
Driving mutually coupled coils using an array of class-E amplifiers
(2022-09) Arghiani, Ziba
Linear radiofrequency (RF) power amplifiers are commonly used in magnetic resonance imaging (MRI) to generate radiofrequency field (B1). These low-efficiency amplifiers require cooling systems and lengthy transmission cabling, increasing the MRI hardware cost. An array of on-coil class-E amplifiers is pro-posed for the transmit system to mitigate these problems. Class-E amplifiers are switching type amplifiers capable of achieving 100% power efficiency theoretically. In this dissertation, a state-space model is proposed to simplify interpreting and analysis, and provide insight into the system of on-coil class-E amplifiers. This model can also be employed to find high power and efficiency modes of ampli-fiers’ operation. Using the state-space model, phase delays are adjusted between gate signals of the amplifiers to obtain high power and efficiency even in the presence of highly coupled coils. In addition, a sampled-data system generated from the state-space model is proposed to analyze the system’s transient behav-ior by investigating the time constant of the system. To validate the state-space model, a single class-E amplifier and two class-E amplifiers with coupled coils are simulated in LTSPICE. Moreover, novel hardware is designed for the class-E amplifier resulting in a cost-efficient design. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed model.
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.
Open Access
Reverse polarized inductive coupling to transmit and receive radiofrequency coil arrays
(Wiley, 2011-06-07) Celik, H.; Atalar, Ergin
In this study, the reverse polarization method is implemented using transmit and receive arrays to improve the visibility of the interventional devices. Linearly polarized signal sources-inductively and receptively coupled radiofrequency coils-are used in the experimental setups to demonstrate the ability of the method to separate these sources from a forward polarized anatomy signal. Two different applications of the reverse polarization method are presented here: (a) catheter tracking and (b) fiducial marker visualization, in both of which transmit and receive arrays are used. The performance of the reverse polarization method was further tested with phantom and volunteer studies, and the results proved the feasibility of this method with transmit and receive arrays.