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Abstract:
Purpose/Introduction: Recent studies have demonstrated the high potential of local, multi-coil shim setups to mitigate B0 field inhomogeneities through dynamic and global shimming[1][2]. For this approach, each coil needs an individually controllable current driver. The drivers must be able to supply a stable and reproducible current with a minimal power dissipation. Here, we demonstrate a 32-channel, low-cost, modular current driver that can source/sink up to 5A per channel and 120A in total to drive a wide range of inductive loads.
Subjects and Methods: The amplifier core is mostly designed based on the Howland current source[3]. The circuit (Fig. 1a) is programmable during runtime by the user, which makes it adaptable for different inductive loads. Each channel comprises a pair of OPA549 high-current op-amps to push and pull the current to the load. A current sense resistor of 0.1X is series with the output to make simultaneous current feedback control and real-time monitoring possible. The power supply for each amplifier can be adjusted through
a custom-made GUI. The user can also set an offset for each channel output to compensate for any current drift. The input for the amplifier is generated by a PXIe–6738 unit, a high-density analog output module, and first passes through a PID controller. The parameters of the PID controller are adjustable through the GUI to obtain the best performance for different loads. We packed two drivers into one module (Fig. 1b) that can be readily mounted/dismounted in an
amplifier rack.
Results: We used our amplifier to drive a home-built 16-channel multi-coil shim setup [1] inside a 9.4 T MR scanner. We applied a range of currents from -2A to +2A and in reverse order with a step size of 0.25A and acquired a B0 map to evaluate the drivers’ output characteristics. The drivers’ output linearity and hysteresis (Table 1) were evaluated by fitting a linear model to the measured B0 maps.
After adjusting the PID parameters, a rise time of 50us was achieved by measuring the step response with a load of 128lH (Fig. 2a) while the coil was in the scanner’s bore. Enabling the driver and running a gradient-echo sequence resulted in a maximum current disturbance of 40 mA (Fig. 2b). No artifacts could be detected in the acquired images when the drivers were connected to the coil.
Figure 2 Driver performance test; a) a rise time of 50us was
achieved with a load of 200 lH; b) maximum current disturbance of 40 mA resulted when running GRE sequence
Discussion/Conclusion: A 32-channel flexible modular multi-coil driver was constructed and evaluated successfully. Several safety regulations (thermal, current limit, voltage limit, etc.) are considered in design with adjustable margins.
