Browsing by Subject "Class-E amplifier"

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    ItemOpen Access
    A 600W on-coil Class-E RF power amplifier array with dynamic phase control for 3T MRI
    (2022-09) Arslan, Abdullah Erkam
    Due to their size and cooling constraints, conventional Magnetic Resonance Imaging (MRI) places radio frequency (RF) amplifiers away from the scanner. These RF amplifiers have relatively low efficiency due to the matching of 50Ω output impedance for means of transmission with cables. Switching Class-E amplifiers on the other hand, by default need a bare RLC network as their load and thus can be directly integrated with the bare unmatched coils and reduce the cost and power losses significantly. This thesis aims to build up on the previous theses’ line of work including [1, 2, 3, 4, 5]. Instead of mitigating the symptoms, chronic problems of artifacts have been fixed by focusing on their root causes in the FPGA side of the updated design.The driver has been updated, timing problems have been resolved. FPGA design is also extended to support multichannel phase control. A dual channel imaging configuration of on-coil Class-E amplifiers with on-the-fly digital fine phase control is presented for 3T MRI. The system can control the phase with less than 2° granularity (this setting can be fine-tuned down to 0.15°). Without any mechanical intervention with the coil setup, using merely phase control, illuminated slice depth is modulated to three times its base-size during scantime. B1 field maps are also extracted for another setup. Periodically linear switching (PLS) circuit model of the Class-E amplifier is derived and computed, yielding a simulator with fast and customizable optimization capability. The PLS model is also verified by SPICE and theoretical analysis.
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    ItemOpen 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.
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    ItemOpen Access
    Highly efficient 300 W modified class-E RF amplifiers for 64 MHz transmit array system
    (2017-12) Tu Zahra, Fatima
    The conventional MRI system uses high power linear RF amplifiers which are placed away from scanner room, have low efficiency, high cost and need a cooling system. We aim to use parallel transmit system with on-coil amplifiers that has shown to be advantageous in improving B1 field homogeneity, slice selectivity and SAR reduction. In this work the on-coil class-E RF switching amplifier is suggested for MRI to decrease the cost and complexity of the current system while improving performance. The amplifier is digitally controlled and for pulse generation purpose supply modulation is used. The transmit coil acts as the load network of the amplifier so the need for matching circuit is eliminated. The amplifier has an output power of 300 W with maximum efficiency of 92%. The efficiency does not drop below 75% in 1 MHz bandwidth. The performance of amplifier at high temperature is also evaluated and it is established that at low duty cycles due to high efficiency no cooling system is required but for high duty cycle applications a cooling system might be needed for the uninterrupted operation. The ultimate goal of this research is to design a 32-channel transmit array using on-coil amplifiers, as a step forward towards this goal a prototype for two channels is designed. Our results depict that the behavior of the class-E amplifiers under coupled operations is acceptable. The dual-channel prototype was tested on Scimedix 1.5 T and no artifacts were observed in the images due to the presence of amplifiers near transmit coils inside the bore. To sum up, the class-E amplifier is proved to be a favorable candidate for on-coil applications in RF excitation due to its small size, reduced complexity and high efficiency.