Browsing by Subject "Parallel Transmission"

Now showing 1 - 3 of 3

Open Access
A digitally controlled class-e amplifier for MRI
(2016-08) Poni, Redi
Radio-frequency (RF), or B1field is used for slice selection purposes in Magnetic Resonance Imaging (MRI). In an MRI scanner thisfield is handled by the RF chain which consists on a pulse generator unit, a high power amplifier and a transmit coil. Relatively low effciency linear power amplifiers are used. These amplifiers are placed in the system room, far from the transmit coil. In this work we propose to design the amplifier and the coil as a single unit, aiming to decrease cost and complexity while improving performance. Splitting the coil into multiple elements makes possible to drive these elements with individual amplifiers in Transmit Array (TxArray) mode. Also these elements are integrated as the load network of the amplifier, in this case a high effciency Class-E amplifier. The Class-E amplifier was modified for the intended application and it was digitally controlled. For the pulse generation two dierent methods were applied. One was by controlling separately the phase and amplitude of the pulse. The other method generates simultaneously phase and amplitude by controlling the switching pattern of the amplifier. An amplifier of output power 100W with effciency up to 88% was developed. As a step toward 36 channel complete system, 2 element prototype's operation was tested in a 3T Tim Trio Siemens scanner. In overall, Class-E amplifier is shown to be a promising candidate for on-coil RF excitation in TxArray in terms of size, cost, effciency and complexity.
Open Access
A gate modulated digitally controlled modified class-E amplifier for on-coil application in MRI
(2018-03) Ashfaq, Bismillah Nasir
The switch-mode RF power amplifiers, known for their high output power capability and good efficiency, have proved valuable for on-coil applications in MRI hardware. The class-D and class-E amplifier topologies have been demonstrated to be promising candidates to replace the conventional inefficient linear RF power amplifiers used in MR hardware which are placed away from the scanner room. Conventionally, the amplitude modulation of the output waveform in such switch-mode RF power amplifier applications is achieved either by implementing an amplitude modulation block at the drain of the amplifier, or by encoding the amplitude modulation information in the phase of the carrier signal at the gate of the amplifier. Both these approaches require additional hardware, thus increasing the cost and complexity of the system. Considering the aforementioned background, a novel technique of modulating both the amplitude and frequency of the output waveform, without the need for any additional hardware other than the driver circuitry for the amplifier itself, is presented and implemented for class-E amplifier topology. At 64 MHz (1.5 T), the analytical models of the amplifier for both the switch-on and switch-off cases are first derived and implemented in software. The period of the digital carrier signal at the gate of the amplifier is then divided into k bits, where k is greater than 2. It is then noted that both the amplitude and frequency of the output waveform can be controlled by altering this digital input in a certain manner. For a typical 2 ms 1.5 T MRI RF pulse, the digital carrier bitstream would consist of k × 128000 bits. This would require testing 2k×128000 bitstream combinations to achieve the desired output waveform, requiring infeasible computational power. It is however shown that by intelligently programming the bitstream patterns for a selected number of periods, and by repeating those patterns for a chosen duration of time, the desired amplitude and frequency modulation of the output waveform can be achieved. The normalized root mean square error (NRMSE) for a 2 ms sinc pulse designed using such an approach is calculated to be 11%. The designed bitstreams are tested on hardware as well, both in bench-top and MRI experiments. Bench-top experiment results correlate well with the software predictions. The amplifier shows a peak drain efficiency of 89% at 50 W input power. The MR images obtained at 50 W input power using a 2 ms sinc pulse designed using the presented approach show no artifacts. The ultimate goal of the current research is to design a 32-channel transmit array coil for the MRI, capable of delivering a total of approximately 10 kW output power. Each amplifier element should therefore be able to deliver about 300 W output power. In this regard, further research needs to be conducted to achieve such output power level using the presented modulation approach. Nonetheless, the approach is general and can be implemented to other switch-mode RF power amplifier topologies as well. It promises to provide a performance equivalent to the other modulation approaches while reducing the overall cost and complexity of the system at the same time.
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.