Synopsis
Gradient driver high capability is needed in the magnetic
resonance imaging (MRI) for better image quality, better resolution and faster imaging.
Imaging speed and SNR require increased PSD reproduction
fidelity and higher power capability. Higher power has been possible with the
change of implementation from linear amplifiers to much lower internal losses
switched amplifiers.
Switched amplifiers consist on a power stage combining
multiple switching semiconductors, commanded with high performance digital control.
The digital controller requires precise feedback control, gradient coil model
and compensation of nonlinearities. The design has to consider efficiency for operation
cost and practical thermal management.
Purpose
The presentation will provide detailed description of the
electronics technology and technical implementation of the amplifiers currently
used on the high performance MRI systems.
Gradient
driver system overview
- Amplifier functionality in the MRI system and gradient amplifier capability impact on imaging. Figure 1 shows the gradient driver as part of the overall scanner.
- The talk will include the overall view of changes we have seen for the drivers’ requirements.
Gradient driver challenges
- The gradients on the magnetic field are proportional to the current through the gradient coils and the speed of change, or slew rate, is proportional to the voltage applied. Current high performance drivers require Volts x Amps levels of more than 2MVA.
- Electronics implementation for high power and precise driving of the gradient coils is challenging for he high power levels and the multiple types of sequences like the ones shown in Figure 2 that require combinations of high voltage and high currents.
- The non-linearity and small errors result in a lot of artifacts and less contrast. We will talk about some of them.
Gradient driver architecture
-
The gradient driver comprises the power supplies the amplifier stage with the filter and the digital control. Figure 3 shows a representation of the driver connected to the gradient coil in the scanner.
Switched power electronics amplifiers overview:
-
Fundamentals of how switched amplifiers provide the amplitude and fidelity with lower losses and higher performance.
-
The circuit architecture and selection of semiconductors is of outmost imprtance to achieve the performance needed at the power level required.
Power stage
-
Circuit topology with existing semiconductors and operation principle to achieve the functionality.
-
Power supplies for the amplifier. The circuit topology requires a set of isolated power supplies that have to be sized to deliver the power losses of the gradient coil and the electronics.
-
New power semiconductors like SiC may results in circuit simplification and lower power losses. We will talk briefly about the new devices available.
-
Filter design to avoid switching impact on gradient fields and maintain fidelity. Filters affect the driver bandwidth and reduce voltage availability for slew rate. Design issues and possible solution will be visited.
-
Figure 4 shows the key waveforms for each stage of the gradient driver: the power supply voltage delivered to the amplifier stage, the waveforms before the filter and the voltage applied to the gradient coil that will produce the gradient field.
Digital control
-
Control implementation and implemented functions needed for consistent best performance. The digital control provides a lot of possibilities for corrections to minimize the error at which the PSDs are produces by the driver.
-
The control loops are shown in Figure 5. The tuning of the feedback gain, the gradient coil model and the compensation are fundamental to achieve the accuracy and repeatability needed for the PSDs commanded to the gradient driver. We will cover some of the features that are needed for proper tuning of the control and impact on performance.
Losses and thermal management
-
The gradient system is normally the subsystem that requires the largest amount of power in the scanner. These requirements are an important part of the sizing and the cost of the system.
-
Analysis of power needs based on losses and amplitude requirements. This includes the characteristics of the semiconductors used for the driver.
-
Correct thermal management implementation is needed to achieve consistent and reliable operation. The semiconductor and some other elements can be liquid cooled, some other parts are normally air cooled and will be a source of heating of the system equipment room.
-
The design should consider the tradeoffs between the performance and losses. Correct decisions are fundamental for a practical implementation of the gradient driver.
Summary
-
We will provide description design decisions explanation for a good understanding of the challenges faced for high performance gradient driver and the benefits obtained by the correct selection.
-
The presentation will cover the details of all the elements and subsystems needed for the gradient driver. Also, a description of new technologies that make possible further improvements like new wide band semiconductors like SiC MOSFETs.
-
The digital control limitations and the design to overcome them and the tuning needed to achieve the desired performance. Detailed description of the control implementation and hardware options.
-
We expect to provide the audience with more understanding of the complexity of the driver and how can the requirements be achieved with existing and emerging new technologies for power electronics.
Acknowledgements
I want to acknowledge all the support that I have received from GE Healthcare MRI scientists and engineering teams.References
[1] Steigerwald, R et al, Proc. IEEE PESC 2000, pp. 643-647;
[2] Sabate, J., et al, Proc. IEEE PESC 2004, pp.261-266; [3] Sabate, J., et al, Proc. IEEE
IPEMC 2004, pp. 1563-1567;
[4] Watanabe, S., et al, Proc.
IEEE PESC 1999, pp.909-913; [5] Sabate, J., et al, Proc. IEEE
APEC 2004, pp.792-796;
[6] R. Wang, et al., Pittsburg, IEEE ECCE, 2014. [7] J. Sabate, et al., Proc. ISMRM. 2007. [8] R. Wang, et al., Geneva, IEEE ECCE-EPE, 2015.