Kelly Byron1, Pascal Stang2, Shreyas Vasanawala3, John Pauly1, and Greig Scott1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Procyon Engineering, San Jose, CA, United States, 3Radiology, Stanford University, Stanford, CA, United States
Synopsis
If
wireless patient coils could be realized, they would reduce setup time and reduce worry about the liability of the connectors on coils. Battery powering these coils would limit scan
time, so it is desirable to use wireless power transfer (WPT), which uses inductively
coupled resonant coils to transmit and receive power at a particular frequency.
With RF gating, imaging
while receiving up to 11W is demonstrated and shown to have minimal impact on
image quality. Introduction
If
wireless patient coils could be realized, they would reduce setup time, improve
workflow, and reduce worry about the liability of the connectors on coils. Battery powering these coils would limit scan
time, so it is desirable to use wireless power transfer (WPT), which uses inductively
coupled resonant coils to transmit and receive power at a particular frequency. Wireless coils would require about 100mW per
channel, so our goal is to deliver about 10W of power wirelessly. The WPT system shown in Figure 1, which is a
modified version of the system proposed in
1 , is able to deliver
power inside an MRI bore with minimal RF interactions through the use of RF
gating. We present both MRI and
bench-top tests detailing the capabilities and limits of this system.
Methods
We constructed a 45x30cm
drive loop incorporating 64 MHz traps and tuned for series resonance at 10
MHz. Power harvesting was accomplished by
a one turn, 20.3cm diameter flexible pickup loop, tuned to 10MHz with a
combination of series and parallel capacitors, in order to change both the real
and imaginary part of the impedance2. In order to maintain the required power level
while gating the WPT system off during the readout time interval a 10mF storage
capacitor was added at the output of the rectifier. Gating was accomplished by adding a RF switch
at the input of the power amplifier on the transmit side. The switch was controlled by a signal coming
from a Medusa module for the head coil images and was trigger delayed from the
TX exciter unblank signal for the body coil images.
Results
The
tuned pickup loop induces an impedance on the drive loop that is much higher
than the parasitic resistance of the drive loop. This should result in a high efficiency, with
most of the power being transferred to the pickup loop. However, this induced impedance is only a few
ohms, so the long cables connecting the drive loop to the power amplifier
outside the scan room can become a dominant loss in the system. As a result, we designed a quarter-wavelength
block to immediately up-convert the coupled coil impedance, increasing our
system efficiency from 11.5% to 47.3% in bench-top tests. In the MRI bore the efficiency drops to
33.5%, due to further increasing the cable length and due to the proximity of
the large drive loop to the shield inside the bore, which decreased the
inductance of the coil causing a shift in the tuning. This efficiency is sufficient to do a high
power demonstration, wirelessly powering an 11W light bulb which turns on and
off as we cycle the drive power on and off at 1Hz, as shown in the video of Figure
2. Figure 3 shows a video of the 11W
bulb being wirelessly powered while imaging, with an initial charging time for
the storage capacitor and then turning the WPT system off during the MR receive
window. The light bulb stays lit while
the power is being cycled on and off because the storage capacitor is able to
continuously deliver power, however it does dim slightly. MRI testing with a loaded head coil
controlled by a Medusa module is shown in Figure 4. With no harvesting, transmit powers of up to
about 5W had a minimal impact on image quality (4b), however, continuous
harvesting of only 1W causes a large enough increase in background noise that
the image is wiped out (4c). With the
addition of RF gating, there is almost no change in the background noise when
harvesting 1W to a resistor load (4d) and only a slight increase powering an
11W light bulb (4e and 4f). Further MRI
testing using the whole body coil of the scanner and a 5in receive surface coil
is shown in Figure 5. With RF gating,
there is only a slight increase in background noise while harvesting power (5b
and 5c).
Discussion & Conclusions
With RF gating, imaging
with a Medusa module while receiving up to 11W is demonstrated and shown to
have minimal impact on image quality.
Imaging with the MRI gradients and body coil is also shown to have
little impact on images, demonstrating that our system does not significantly
couple to the MRI coils. Efficiency is
significantly improved by adding a quarter-wavelength block to immediately
up-convert the impedance of the coupled coils.
Future work adding a voltage regulator and changing the geometry of the
coils could potentially improve efficiency further.
Acknowledgements
We
would like to thank GE Healthcare for their research support. This project is supported by grants R01EB008108, P01CA159992, and R01EB019241. References
1.
K. Byron, P. Stang, S. Vasanawala, J. Pauly, and G. Scott, RF Gated Wireless
Power Transfer System. Proc. Intl. Sco. Mag. Reson. Med. 23 (2015).
2.
Z. N. Low, Chinga, R.A., R. Tseng, and J. Lin, Design and Test of a High-Power
High-Efficiency Loosely Coupled Planar Wireless Power Transfer System. Industrial Electronics, IEEE Transactions on
, vol.56, no.5, pp.1801-1812 (May 2009).