Autotuning%20Interventional%20Coils%20for%20Imaging

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Autotuning Interventional Coils for Imaging
Ross Venook1, Garry Gold2, Bob Hu3 Greig Scott1

1. Department of Electrical Engineering, Stanford
   University
2. Department of Radiology, Stanford University
3. Palo Alto Medical Foundation, Palo Alto, CA

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Motivation for Flexible Coils

• Small coil within an area of interest vs.
  exterior surface coil gives higher SNR, better
  resolution
• Catheter-based RF coils
• Examples
• Interarticular
• Intravascular
• Transrectal

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Technical Consequences of Flexible Coils
Motivation for Automatic Tuning

• Changing the shape or size of an RF loop changes
  its resonant frequency
• Coil is an inductor with a tuning capacitor
• SNR depends on coil tuning (matching)
• Manual tuning is time consuming and difficult

1cm
2cm
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SNR Analysis How does SNR depend on
Tuning/Matching?

• Preamplifier Noise Factor (F) is a function of
  the source impedance it sees
• Noise matching occurs when ZCOIL ZOPT
• Coax losses degrade SNR with power mismatch
• Especially important in interventional
  applications due to need for small-diameter
  (lossier) coax

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Varactor-tunable RF Coil
DC Tuning Bias
Signal
Flex Coil
To Preamp
Varactor
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Tuning Voltage vs. Resonant Frequency
Varactor Reverse Bias V
Resonant Frequency MHz
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Autotuning System

• Use ?/2 cable to measure coil Z remotely
• Tune for resonance (zero imaginary part)
• Vary varactor voltage with micro-controller

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Tuning to Larmor Frequency
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Experimental Setup
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Experiment 1
Passively-tuned Coil
Varactor-tuned Coil

• Images have same SNR /-2
• SPGR, 12cm2, 256x256,7.2/34msTE/TR,30º

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Experiment 2Tune-Detune-Retune
1cm
2cm
2cm
Autotune
Detune (deform coil)
Re-autotune
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Experiment 2 Windowed UpTune-Detune-Retune
Autotune
Detune (deform coil)
Re-autotune
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SNR Profiles
SNR
Noise Box
Image Sample
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Typical Result
Autotune
Detune (deform coil)
Re-autotune
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Quantitative Comparison

• Low-loss coax
• 2.2 MHz detuning
• Theory predicts 40 SNR loss
• We measure 30 SNR loss
• Lossy vs. less-lossy, small-diameter coax
• /- 2 MHz detuning
• Theory predicts 15-20 additional SNR degradation
• We measure 10-15 additional SNR degradation

0.5mm
0.8mm
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Future Work

• Clinical
• Scan cadaver shoulders, compare with surface
  coils
• Practical
• 0.5T/21MHz version
• Test speed limits
• New directions
• Automatic matching

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Acknowledgements

• GE Medical Systems
• NIH R33 EB 00777
• NIH R01 EB 00346
• NIH 5R24CA092862-03
• Whitaker Foundation