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A scalable, MR compatible temperature measurement and control system
David Otto Brunner1, Jonas Reber2, Simon Gross2, and Klaas Paul Pruessmann2

1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 2University and ETH Zurich, Zurich, Switzerland

Synopsis

Tight temperature control of various devices is required to guarantee highly stable experimental conditions. E.g. B0 field shifts induced by heating of shim irons or gain changes induced in preamplifiers can be avoided by oven controlling the critical components of the device. Furthermore, a recently presented approach of active shimming with controlled magnetic materials requires tight temperature control of a large count of devices in parallel in the bore. For such applications, we present a 14 channel, low power, MR compatible temperature measurement and control system which can be parallelized for a scalable channel count.

Purpose

Temperature drifts are a major source of measurement instabilities in RF devices, coils, magnets and passive shimming units and are typically hard to calibrate. Therefore, tight temperature control of various devices is often required for achieving a high degree of reproducibility. Such oven controlling could suppress B0 field drifts induced by heating up shim irons as well as temperature induced signal alterations in RF chains and coils. Additionally, a recently demonstrated approach for local shimming using temperature controlled magnetic materials [1] requires tight temperature control of a large count of particles. In order to keep the cable count and the interferences low, the control system has to be located in the bore. Therefore a high channel count, MR compatible modular thermostat system is required. In particular, when temperature controlled components, such as the mentioned magnetic materials, have to be integrated into local coil arrays the thermostat is required to have a small form factor, low weight, low power consumption and must connect via a low count of cables. In this work we present a modular, parallelizable, MR compatible, miniature thermostat system with scalable channel count comprising means for high fidelity reading of temperature sensors, power efficient control of heating currents and a digitally programmable control loop.

Methods

The system is composed of small (62x60 mm) PCBs (Fig.1) that can be controlled fully in parallel via a single supply line and a RS485 bus requiring only minimum wiring connections (Fig.2). Each board offers 24-bit readings of 14 thermocouples (RTD), a 4 MHz controller for flexible implementation of the actual control loop function and 8-bit pulse-width-modulation (PWM) control of the heating power of 16 heaters in parallel with maximally 2.4W each. Each board hosts supply voltage regulation for all components offering a high grade of interference rejection on the supply line as required for in-bore operation. Each board has isolated line drivers for the communication bus and an USB2.0 connection for debugging purposes. The reading of the temperature sensors [2] is implemented using two 24-bit sigma-delta converter with integrated precision current sources (ADS1248, TI, Dallas, TX, USA). The 7 thermocouples attached to one ADC unit are sequentially connected via the integrated switches and have individual filters for blocking RF interferences. Both converters can be configured and operated fully in parallel. The resistance of the RTD is measured in a ratiometric fashion in order to suppress externally induced voltages. The control of the heating current is implemented using a 16-channel LED low side PWM driver (TLC59116, TI, Dallas, TX, USA). The device integrates the generation of the modulation frequency, the constant current sources and the switches in a minimal form factor reducing emission of electromagnetic interferences. The outputs of the controller are filtered. The control loop is a remotely parameterizable proportional-integral-differential controller (PID) with integral term clamping implemented on an 8-bit microcontroller (MCU, ATmega32U4, Atmel, San Jose, CA, USA). The peripheral devices are connected via different bus systems in order to provide largely parallel data transfers.

Results

The noise of the RTD reader using a Pt500 thermistor (IRTD=1mA) was 0.0034°K/√Hz. The loop roundtrip time for 1 channel was below 15ms, for 14 below 60ms. Fig.3 shows an example of regulating the temperature of a device that is briefly exposed to an airstream as present in the bore in comparison to the unregulated case. It is seen that temperature drifts are majorly suppressed. Fig.4 compares levelling of a shim unit from room temperature to 57°C with and without feedback loop exhibiting a speed advantage of more than 4 times using active regulation. The applied heating current shows that the regulator makes use of the full power in order to achieve a fast temperature rise initially.

Conclusion

Precision measurements and control of the temperature of small components is realized MRI compatibly even for large channel counts regarding form factor, cabling, power consumption and its magnetic moment. Accurate temperature measurements give valuable information on system components in bore. Furthermore, thigh control of shim irons can either be used for stabilizing the main magnetic field or for adaptive control for shimming purposes. Active temperature control effectively stabilizes the temperature of a device and provides a several times faster settling. The latter is required in particular for adjusting shim settings using controllable magnetic materials [1] in order to keep preparation times low. In conjunction with the small form factor, the power efficient design and the low count of required cabling these systems can be directly employed for control of such on-coil shimming units.

Acknowledgements

No acknowledgement found.

References

[1] Brunner DO et al. Magnetic Pebbles – Materials with Controllable Magnetism for Compact, Low-Power Shim Units, Proc ISMRM 2016, Singapore, p.0492

[2] Haefner J et al, Development of a cost-effective and MRI compatible temperature measurement system, Biomed Tech 47 Suppl 1 Pt 2 , 2002, p. 664-667

Figures

Figure 1: A) Block schematic. The thermocouples (RTD) are ratiometrically read by 2 parallel 24-bit converters (∑-Δ-ADC) with integrated precision current sources (IRTD). The ADCs can be switched (MUX) to 7 differential ports. The current for the heating is power efficiently regulated using a pulse-width modulation (PWM). All inputs and outputs are filtered against interference with the MRI scanner (EMI). For control and communication, a microcontroller unit (MCU) is mounted on each board running 14 proportional-integral-differential (PID) loops. Supply voltages for digital (VDD) and analogue components (VDAs) are generated on board. B) Photo of the PCB.

Figure 2: System overview. The individual thermostat boards providing 14 channels each are wired in parallel to a single supply line and an isolated, differential serial bus. For demonstration, shimming units with controllable magnetic materials are connected to the thermostat. Each unit consists of a 2x2x2mm3 NiCu alloy heated by standard surface mount resistors. The temperature is measured via a Pt500 thermocouple.

Figure 3: Example of temperature stabilization using closed-loop temperature control. In the test, the device was exposed to an airstream. Without regulation (blue curve) the device significantly cools down and warms up again. Using active regulation (red curve) the device was constantly held at 40°C irrespective of its environment controlling the heating as depicted below. However, a slightly elevated noise floor on the temperature curve results from the feedback amplification.

Figure 4: Top: Comparison of stepping the temperature of a shimming unit using a fixed current and waiting for settling in the thermal equilibrium (blue line) or using active closed loop control (red line). The close loop approach settles into a 1° threshold after 40s while the open loop approach requires at least 170s. Bottom: Depiction of the applied current in % of the maximum heating current. As seen, the closed-loop design employs the full power of the system in order to ramp-up the temperature and regulates to the target value.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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