Autotuning%20Electronics%20for%20Varactor%20Tuned,%20Flexible%20Interventional%20RF%20Coils

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Autotuning Electronics for Varactor Tuned,
Flexible Interventional RF Coils

• Ross Venook, Greig Scott,
• Garry Gold, and Bob Hu

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Introduction

• Motivation
• Why use interventional coils?
• Why is this hard?
• Background
• History
• RF coil tuning method(s)
• What we tried
• Modular electronics discussion
• Results
• Next steps

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Why Use Interventional Coils?

• Increased signal coupling reduced noise
  coupling ? better SNR

Coupled noise
Coupled signal
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Applications Existing and Potential

• Existing
• Surface coils
• Intravascular coils
• Potential
• Inter-articular
• ltadd your application heregt

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SNR Comparison
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Why Interventional Coils Are Harder to Use
Dynamic loading

• Proximity works both ways
• Closer coupling also means greater local tissue
  dependency
• Requires deployability in some applications
• Scaling works both ways
• Human-scale effects are significant
• Geometry more important

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So

• Dynamic loading conditions require dynamic tuning
  to maximize SNR advantages with interventional
  coils
• The tuning process should be automatic, and must
  add neither noise nor interference to the
  acquired signal

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RF Coils

• RF transmitters and receivers (in MR) are
  magnetic field coupling resonators that are tuned
  to the Larmor frequency
• Examples
• Saddle
• Surface
• Interventional

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Resonance

• Parallel RLC circuit
• Governing equation
• Familiar result

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Impedance of Resonant Circuits
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Goals Tuning and Matching

• Tuning
• Center Frequency near Larmor
• Bandwidth appropriate to application
• Matching
• Tuned impedance near 50 j0 ohms

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Complications

• Loading the coil with a sample necessarily
  creates coupling (it better!)
• Dynamic coupling creates dynamic tuning/matching
  conditions

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Detuned
Tuned
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History

• Tuning MRI coils (Boskamp 1985)
• Automatic Tuning and Matching (Hwang and Hoult,
  1998)
• IV Expandable Loop Coils (Martin, et al, 1996)

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Shoulders

• Varactor Tuned Flexible Interventional Receiver
  Coils (Greig and Garry, ISMRM 2000)

Cadaver Shoulder, 1.5T 3D/SPGR/20 slices 6cm FOV,
512x512
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Greigs Tunable Coil
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Basic Tuning Method

• Manually change DC bias on varactor
• Maximize magnitude response
• FID is a reasonable measure
• Drawbacks
• Requires manual iterative approach
• Maximum FID may not correspond to maximum SNR
• Feedback not effective for maximization

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A Better Method Using Phase

• Zero-crossing at resonant frequency

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At 63.9MHz
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Measuring Phase Offset
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What We Tried
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Phase Comparator
Old
New
Vo
Va
Cref
_


Vo
Vb
_
AD835 250 MHz Multiplier
Filter
Vo VaVbcos(F)
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Phase Detector Results

• Multiplier Output vs. Receiver Center Frequency
• Half-wavelength Txn Line

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Phase Detector Results (cont)

• ?/4
• 3?/8
• 5?/8

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Closed Loop Feedback?

• Tempting
• Simple DC negative feedback about zero-point
• but unsuccessful
• Oscillations
• Railing
• Phase detection scheme probably requires a
  different method (?)

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Microcontroller

• Why use a microcontroller?
• Controlling reference signal generation
• Opportunity for tuning algorithms
• Atmel AT90S8515
• Serial Peripheral Interface
• Analog Comparator
• Simple

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Atmel AT90S8515

• Serial Peripheral Interface
• Analog Comparator
• Simple development platform
• STK500 Starter Kit
• CVAVR C compiler

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Reference Signal Requirements

• Accurate and stable reference signal at Larmor
  frequency during tuning
• Signal well above Larmor frequency during receive
  mode

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PLL Synthesizer

• Phase Locked Loop
• Frequency to voltage
• Voltage-Controlled Oscillator
• Voltage to frequency
• Current Feedback Amplifier
• Tri-statable turns off signal
• Low Pass Filter
• Cleans VCO output

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TR Switches

• Loading effects categorically harmful
• Ideal
• Complete isolation of tuning and receiving
  circuitry

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Actual TR Switches
Microcontroller
Scanner
Tuning Circuit

• PIN-diodes control signal direction
• RF chokes ensure high-impedance, reduce
• loading

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Complete System
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Results

• Basic tuning functionality
• 300ms total tuning time

Retuned
Detuned
Detuned
Retuned
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Next Steps

• Get an image with autotuned receiver on 1.5T
  scanner
• SNR advantage (validation) experiments
• Minimize tuning time
• Explore VSWR bridge tuning
• Remove need for ?/2 cable restriction