Kamal Aggarwal1, Kiran Raj Joshi1, Yashar Rajavi1,2, Mazhareddin Taghivand1,2, Ada S. Y. Poon1, John M. Pauly1, and Greig Scott1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Qualcomm Atheros, San Jose, CA, United States
Synopsis
High path loss and
availability of wide bandwidth make mm-waves an ideal candidate for short
range, high data rate transmission for wireless MRI applications. The proposed
system uses a custom designed integrated chip (IC) radio that uses mm-waves (60
GHz) as the radio frequency carrier. We report link tests up-to 500 Mb/s for distances up-to 50cm in the MRI bore. The addition of time division multiplexing
(TDM) circuitry allows multiple wireless links to be created simultaneously
with minimal inter-channel interference. This leads to a highly scalable, low-power solution for wireless MRI.Introduction
In modern MRI
systems, the number of elements in receive coil arrays continues to increase
but with a commensurate increase in cable
complexity to ensure minimal inter element interference. This in turn has led
to an increased focus towards the development of wireless MRI systems
[1]-[7].
Despite rapid advances in wireless communications technology, commercial MRI
systems have largely been left out due to the unique challenges and
requirements posed by the MRI environment. Here we report our progress in
developing digital links operating at mm-wave frequencies with an RF carrier of
60GHz. The system consists of a custom designed mm-wave CMOS transceiver
[7],
discrete horn antenna, low noise amplifiers and fiber optic transceivers. The
system has been verified for data rates up to 500Mb/s and distances up to 50cm
inside the MRI tunnel including an image transfer operation consisting of a
pre-digitized MRI image.
Methods
The system block diagram and setup details are shown in
Fig. 1 and Fig. 2(a) respectively. The experiments were conducted by placing the
setup along the axis of GE 1.5 T Signa scanner as shown in Fig 2(b). The MRI
data is emulated by a standard 7-bit pseudo random bit generator (PRBS). The
clock for the PRBS generator is supplied from the MRI console room over a fiber
optic cable to minimize any interference with the MRI signals. The 60GHz transmitter is powered using a
non-magnetic battery (Powerstream). The system uses ON-OFF key (OOK) modulation
with return to zero (RZ) signaling to minimize the power consumption. The
transmitter consists of two identical TX elements each consisting of a cross-coupled NMOS voltage controlled oscillator (VCO), a class E/F2,odd power
amplifier (PA), and a high gain, on-chip
dipole antenna. The OOK modulation results in an impulse radio ultra-wide-band
(IR-UWB) waveform at the transmitter output.
The receiver
(RX) consists of a high gain mm-wave horn antenna followed by a mm-wave discrete
low noise amplifier (LNA). The output of the LNA is wirelessly coupled to the
on-chip dipole antenna of the custom receiver via WR-15 waveguide. Inside the
receiver, the on-chip dipole antenna is followed by a 3-stage transformer
coupled LNA. A passive AC-coupled self-mixer is then used to extract the OOK
modulation envelope. The mixer output is amplified by the RX baseband which
then drives a fiber optic transceiver (Firecomms). The fiber optic transceiver sends the
received baseband data to the MRI console room over plastic optical fiber (POF)
to an optical receiver. In the MRI console room, the data is
captured using a high speed sampling scope and is finally processed using
Matlab.
Results
and Discussion
The custom transceiver was fabricated using TSMC 40nm
CMOS process and is shown in Fig. 3 (a). The transmitter power consumption varies
from 1.3mW to 11.2mW as the data rate is increased from 200Mb/s to 2.0Gb/s as
shown in Fig. 3(b). Inside the MRI bore, the maximum data rate is limited to
500Mb/s because of the bandwidth limitation of the fiber optic transceiver. The
quality of the mm-wave wireless link was verified by measuring the system bit
error rate (BER). The sampling scope
memory limit of 1 million samples set a lower bound on measureable BER of 10-6.
Even after multiple readings, no bit errors were observed for data rates up-to
500Mb/s. The system
was further analyzed using a BERTScope by replacing the fiber optic transceiver
and POF with electrical amplifier and SMA cables respectively. A raw BER of
8.25x10-12 was measured for a link distance of 50cm and data rate of
2Gb/s. The corresponding eye-diagram and the BER screen image from
the BERTScope is shown in Fig. 4(a) and Fig. 4(b) respectively. As a comparison,
this is orders of magnitude better than the raw BER specification of 10-2
for a conventional 802.11n Wi-Fi system.
To verify the image transfer operation, an MRI image
was digitized and transmitted over the mm-wave wireless link. The transmitted
image was broken into nine pieces to emulate multiple image transfer scenario
in an actual MRI image sequence. The
transmitted and received image is shown in Fig. 5(a) and Fig 5(b) and no pixel
error was observed in the image transfer operation.
Conclusion
The technology now exists to have high date rate
wireless MRI systems using low-power mm-wave transmission without degrading the
image quality. But to complete this
goal, low power high dynamic range front ends and data conversion remain to be
developed. Milli-meter wave digital transmission is a critical step to the
long-term goal of wireless MRI.
Acknowledgements
TSMC
University Shuttle Program, NIH Grant support: R01EB019241, GE Healthcare.References
[1]
Y. Murakami, US Patent 5,384,536 Jan 1995 [2] E. Boskamp, US 2003/020619A1, Nov
2003 [3] G. Scott, ISMRM 2005
[4] M. J. Riffe, ISMRM, 2009 [5] J. Wei, JMR
2007[6] O. Heid, Proc. ISMRMed 2009 [7] K. Aggarwal, Proc. ISMRM, 2015.