Title: Autotuning Electronics for Varactor Tuned, Flexible Interventional RF Coils
1Autotuning Electronics for Varactor Tuned,
Flexible Interventional RF Coils
- Ross Venook, Greig Scott,
- Garry Gold, and Bob Hu
2Introduction
- Basics of Magnetic Resonance Imaging (MRI)
- 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
3The First Thing About MRI
- Bloch Equation
- ? ?B
- ? precession/Larmor frequency
- ? gyromagnetic ratio (2p42.575MHz/Tesla)
- B local magnetic field strength (Tesla)
B
Hydrogen atom spin
?
B
4The Second Thing About MRI
Before RF Excitation
RF Excitation
RF Relaxation
Tip
B
?
Transverse component
- During relaxation, the spins emit EM radiation at
- ? ?Blocal
- RF coil inductively couples this signal
5Simple Example
Bo
- Linear gradient produces frequency encoding of
spatial hydrogen atom distribution
6Other Important Points
- Signal to Noise Ratio (SNR) is the figure of
merit for MRI - SNR acts as a currency for other MRI attributes
(resolution, field of view, scan time) - Clinically-driven field
- Focus on medical problems/solutions
- Factors of two matter
- Primary advantage of MRI it is a non-invasive
imaging modality
7Why Use Interventional Coils?
- Increased signal coupling reduced noise
coupling ? better SNR
Coupled noise
Coupled signal
8SNR Comparison
9Applications Existing and Potential
- Existing
- Intravascular coils
- Endorectal coils
- Potential
- Inter-articular
- ltadd your application heregt
10Why 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
11So
- 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
12RF Coils
- RF transmitters and receivers (in MR) are
magnetic field coupling resonators that are tuned
to the Larmor frequency - Examples
- Saddle
- Surface
- Interventional
13Resonance
- Parallel RLC circuit
- Governing equation
- Familiar result
14Impedance of Resonant Circuits
15Goals Tuning and Matching
- Tuning
- Center Frequency near Larmor
- Bandwidth appropriate to application
- Matching
- Tuned impedance near 50 j0 ohms
16Complications
- Loading the coil with a sample necessarily
creates coupling (it better!) - Dynamic coupling creates dynamic tuning/matching
conditions
17Detuned
Tuned
18History
- Tuning MRI coils (Boskamp 1985)
- Automatic Tuning and Matching (Hwang and Hoult,
1998) - IV Expandable Loop Coils (Martin, et al, 1996)
19Shoulders
- Varactor Tuned Flexible Interventional Receiver
Coils (Scott and Gold, ISMRM 2001)
Cadaver Shoulder, 1.5T 3D/SPGR/20 slices 6cm FOV,
512x512
20Greigs Tunable Coil
21Basic 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 with maximization
22A Better Method Using Phase
- Zero-crossing at resonant frequency
23At 63.9MHz
24Measuring Phase Offset
25What We Tried
26Phase Comparator
Old
New
Vo
Va
Cref
_
Vo
Vb
_
AD835 250 MHz Multiplier
Filter
Vo VaVbcos(F)
27Phase Detector Results
- Multiplier Output vs. Receiver Center Frequency
- Half-wavelength Txn Line
28Phase Detector Results (cont)
29Closed 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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31Microcontroller
- Why use a microcontroller?
- Controlling reference signal generation
- Opportunity for tuning algorithms
- Atmel AT90S8515
- Serial Peripheral Interface
- Analog Comparator
- Simple
32Atmel AT90S8515
- Serial Peripheral Interface
- Analog Comparator
- Simple development platform
- STK500 Starter Kit
- CVAVR C compiler
33Reference Signal Requirements
- Accurate and stable reference signal at Larmor
frequency during tuning - Signal well above Larmor frequency during receive
mode
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35PLL 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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37Tune/Receive (TR) Switch
- Loading effects categorically harmful
- Ideal
- Complete isolation of tuning and receiving
circuitry
38Actual TR Switches
Microcontroller
Scanner
Tuning Circuit
- PIN-diodes control signal direction
- RF chokes ensure high-impedance, reduce
- loading
39Complete System
40Results
- Basic tuning functionality
- 300ms total tuning time
41Next 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