Charlotte R Sappo1,2,3, Michele N Martin3, Sheng Shen4,5, Neha Koonjoo4,6, Anthony B Kos3, William A Grissom1,2, Matthew S Rosen4,6, and Karl F Stupic3
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, 2Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States, 3National Institute of Standards and Technology, Boulder, CO, United States, 4Massachusetts General Hospital, A.A. Martinos Center for Biomedical Imaging, Boston, MA, United States, 5Electrical Theory and New Technology, Chongqing University, Chongqing, China, 6Physics, Harvard University, Cambridge, MA, United States
Synopsis
Low-field MRI is of
increasing interest due to its low cost, improved safety, portability, and low power
requirements. A transmit/receive switch is an essential piece of
hardware used to dynamically connect a coil to either the transmitter or receiver. Current low field
systems typically use passive crossed-diode TR switches. For RF excitation with
small flip angles, there may not be sufficient forward bias voltage to turn on
the diodes in passive switches, which limits fast imaging and MR fingerprinting. In this study we evaluate alternative active T/R switches for low power experiments and compare
them, at 6.7 and 30 mT.
Purpose
Very low field MRI (<100mT) [1] is of
increasing interest due to its low cost, improved safety, portability, and low power
requirements [2,3,4]. A TR (transmit/receive) switch is an essential piece of
hardware used to dynamically connect a coil to either the transmitter or receiver
[5]. At higher frequencies, positive-intrinsic-negative
(PIN) diodes are effective switches and widely used. However, at lower
frequencies their carrier lifetime is problematic [6,7], so current low field
systems typically use passive crossed-diode TR switches. For RF excitation with
small flip angles, there may not be sufficient forward bias voltage to turn on
the diodes in passive switches, which limits fast imaging and MR fingerprinting
[8]. In addition, silicon diodes apply amplitude-dependent phase shifts when
insufficiently powered. In this study we evaluate alternative active
mechanical and electrical TR switches for low power experiments and compare
them to a classic passive TR switch, at 6.7 and 30 mT.Methods
Switch Characteristics:
Fig 1 lists the TR switches we evaluated and
compares them in terms of the most important TR switch characteristics: insertion
loss/on-resistance, power handling, isolation, and switching speed. In this
work we evaluated a conventional passive TR switch, a low voltage MEMS device
(ADGM1304), a complementary metal-oxide
semiconductor (CMOS) switch (ADG1412), and a commercial electrical switch
(Minicircuits). Micro-electromechanical
switches (MEMS) have high isolation because the leads are physically
disconnected, but have a slower switching speed because a plate must be moved
from one position to another. We
did the same experiment with the MEMS switch in both a single and a parallel
state to test the improved isolation and power performance. In comparison, electrical switches have high
switching speed and low insertion loss but can suffer from frequency limits and
power handling issues.
NMR Experiments:
F1 (RF
power) calibrations and loopback tests were performed and evaluated at 6.7 mT (288
kHz) and 30 mT (1.28 MHz) using a variable field, BFM-OC (Resonance Research Inc,
Billerica, MA, USA) electromagnet, a Tecmag Redstone (Houston, TX,USA), and a
1kW Tomco RF amplifier (Stepney, South Australia). Shown in Fig 2a, we used a 50-turn
2 layer litz wire-wound solenoid (ID=10mm, Q =24) changing the tune/match board
for each frequency. The overall system design is shown in Fig 2b.Results and Discussion
As shown in Fig 3 and Fig 4, the MEMS and CMOS switches outperformed the
conventional passive TR switch with regards to low flip angle calibrations,
power loss, and phase dependencies. Using low powers, 47dB and 53dB
attenuations (~7/30 mWatts), the nutation curves show the 90 degree flip angle
keeping the power constant. The passive TR switch uses much more power to get
to 90 degrees while the active TR switches reach 90 degrees within a fraction
of that time. The plots clearly show that the passive switch doesn’t perform
well until reaching a power threshold, seen by zero values at low amplitudes in
the nutation curves. Both 30mW and 7mW powers are used to clearly display the
power needed to reach the 90 degree flip and then the following flip angles to
monitor the amplitude of the 270 in comparison to the 90 (Fig3 47dB plots). For
lower frequencies, the amplitude of the passive switch at 270 degrees is much
lower than the 90 degree amplitude. The commercial active TR switch performs
similarly to the MEMS and CMOS solutions at 30mT but falls short at 6.7mT where
it has more loss than the parallel MEMS (41,370 compared to 33,530 for signal
magnitude) (Fig 3a). Fig 4 demonstrates
that none of the active TR switches have a phase dependency compared to the
passive switch which shows a phase roll near the diode threshold. The loopback
tests were performed for all switches at 6.7 and 30mT validating the
consistency through the kHz and low MHz frequency regime. Conclusion
We have demonstrated the benefits of using an active TR switch with
either a MEMS device or CMOS instead of the conventional passive TR switch. We
have also compared these solutions to a commercial active TR solution to show
better power handling and less loss. Both the MEMS and CMOS have improved
nutation curves at low flip angles and overall require less power than a
passive TR switch. Additionally, when using a parallel MEMS solution, the
insertion loss is lower than a commercial active TR solution. Acknowledgements
This work was
supported by NIH Grant U01 EB 025162, NIH Grant R01 EB 016695, and the ARPA-E
ROOTS (Rhizosphere Observations
Optimizing Terrestrial Sequestration) project.References
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