Introduction

In many MRI applications devices are operated in the MRI environment which need to be supplied with electric energy. For example, in radio-frequency (RF) coils low noise amplifiers are used that have a power consumption of up to several hundred milliwatts, or ECG systems utilize power circuits to detect, amplify and transmit the electric surface potentials of the heart for ECG gating. Typically, the energy for these devices is either delivered directly via a galvanic connection to a power source (e.g., via the RF cables), or a nonmagnetic battery is integrated into the device. Direct galvanic connections can act as antennae that are prone to RF heating [1], and the multiple connections in high-density coil arrays can lead to bulky and inflexible cables [2]. Batteries on the other hand are often limited in the available power that they can deliver, and they require space in the device which is often very limited.
To overcome this limitation wireless energy harvesting has been suggested where the required power is taken from the existing electromagnetic fields in the MRI. For RF amplifiers energy harvesting has been realized by extracting energy from the RF transmit field [3,4], and also first attempts to utilize the gradient power have been suggested [5,6]. However, so far in all these realizations the harvested power was always less than a Watt. In this work we demonstrate that the gradient system of a clinical MRI alone can provide enough power on the order of several hundred Watts – for this, we constructed a simple induction coil, connected it to a coffee machine and cooked the first-ever MR-generated coffee.

Materials and Methods

Energy harvesting from a time-varying magnetic field $$$B(t) = z_0 G_z (t)$$$ can be realized using Faraday’s law of induction and a gradient $$$G_z (t)$$$:
$$U_{ind} = - \frac{d\Phi}{dt} = - N \frac{d(A(t) B(t))}{dt} = - N A \frac{d B(t)}{dt} = = - N A z_0 \frac{d G_z(t)}{dt}$$
Here, $$$U_{ind}$$$ is the voltage induced in a coil with $$$N$$$ windings and an area $$$A$$$, which is placed at the position $$$z_0$$$ away from the iso-center of the magnet. The slew rate $$$s = \frac{d G_z(t)}{dt}$$$ is limited by the maximum slew rate of the gradient system.
To demonstrate that $$$U_{ind}$$$ can be as high as typical line voltages (110-220 V), a dedicated rectangular pick-up coil with the following parameters was built from insulated copper wire: $$$N = $$$ 25 windings, $$$A = $$$ 34 cm x 34 cm = 0.12 m². In a first test this coil was placed at $$$z_0 = $$$ 25 cm in a clinical 3 T MRI system (Siemens PRISMA), which is equipped with a gradient system that offers a maximum slew rate of $$$s_{max} = $$$ 200 T/m s - with these values a $$$U_{ind}$$$ of 144 V can be realized.
For energy harvesting the coil was placed on the patient table of the MRI system at $$$z_0 = $$$ = 25 cm, and a conventional drip coffee machine (Siemens, nominal power consumption at 220 V: 1 kW) was connected to the coil (Fig. 1). Before the experiment the magnetic parts of the machine were removed (e.g., the over-temperature fuse) or replaced with non-magnetic elements (copper ground plate), and the machine was filled with water and ground espresso coffee (Intermezzo, Segafredo Zanetti S.P.A., Pianolo, Italy). Then, a gradient-only pulse sequence with a sinusoidal gradient (amplitude: 60 mT/m, frequency: 3 kHz, $$$s = $$$ 180 T/m s was executed for about 5 min until the coffee was ready.

Results and Discussion

With the simple energy harvesting setup a peak voltage of about 130 V could be created with a clinical gradient system. At the relative high gradient oscillation frequency of 3 kHz no vibration of the pick-up coil could be detected. The applied slew rate was intentionally set slightly below the attainable hardware maximum to ensure that the heating element of the coffee machine does not overheat, as the magnetic over-temperature fuse had to be removed. Even though the applied power is in this experiment about 2.9 times lower than nominal power consumption of 1 kW, substantially higher power levels could be produced than with previously described harvesting setups.
Obviously, a major drawback of this demonstration setup is its large size and the need for long cables. If the pick-up coil is used to energize a separate unit such as a TMS generator, a defibrillator or a disinfection device, such a coil setup could be integrated advantageously into the device housing.

Conclusion

High quality coffee can be prepared with and in MRI systems.

Acknowledgements

No acknowledgement found.