Autotuning Electronics for Varactor Tuned, Flexible Interventional RF
Coils
Ross Venook, Greig Scott,
Garry Gold, and Bob Hu
Introduction
• 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
The First Thing About MRI
• Bloch Equation:
ω = γB• ω : precession/Larmor frequency
• γ : gyromagnetic ratio (2π•42.575MHz/Tesla)
• B : local magnetic field strength (Tesla)
z
x
y Bω
B
Hydrogen atom“spin”
z
x
y
The Second Thing About MRI
• During relaxation, the spins emit EM radiation at ω = γBlocal
• RF coil inductively couples this signal
z
x
yB ω
Before RF Excitation
z
x
y
Transversecomponent
RF Excitation“Tip”
RF Relaxation
Simple Example
• Linear gradient produces frequency encoding of spatial hydrogen atom distribution
Boz
y
x
+
By-gradient
=
Bfinal
Object
ωωo
Signal
Relaxation Signal (freq. domain)
Other 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
Why Use Interventional Coils?
• Increased signal coupling & reduced noise coupling better SNR
Coupled noise
Coupled signal
SNR Comparison
Applications: Existing and Potential
• Existing– Intravascular coils – Endorectal coils
• Potential– Inter-articular– <add your application here>
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
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
“RF Coils”
• RF transmitters and receivers (in MR) are magnetic field coupling resonators that are tuned to the Larmor frequency
• Examples:– Saddle– Surface – Interventional
3” surface coil (GE)
Resonance
• ‘Parallel RLC’ circuit
• Governing equation
• Familiar result
011
2
2
VLCdt
dV
RCdt
Vd
LCf
1
2
10
Impedance of Resonant Circuits
50 55 60 65 70 750
10
20
30
40
50
60
Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75-30
-20
-10
0
10
20
30
Frequency [MHz]
Rea
ctan
ce [
Ohm
s]
Goals: Tuning and Matching
• Tuning– Center Frequency near Larmor– Bandwidth appropriate to application
• Matching– Tuned impedance near 50 + j0 ohms
Complications
• Loading the coil with a sample necessarily creates coupling (it better!)
• Dynamic coupling creates dynamic tuning/matching conditions
TunedDetuned
History
• Tuning MRI coils (Boskamp 1985)
• Automatic Tuning and Matching (Hwang and Hoult, 1998)
• IV Expandable Loop Coils (Martin, et al, 1996)
Shoulders
• Varactor Tuned Flexible Interventional Receiver Coils (Scott and Gold, ISMRM 2001)
Cadaver Shoulder, 1.5T
3D/SPGR/20 slices
6cm FOV, 512x512
Greig’s Tunable Coil
22 or 68pFVaractor
150pF
<360nH
Flex coil
20K 20K
9 Vmanual
tune10K
C DC bias,RF isolate
75nH
Q spoil Rcv
PortC
2.5
cm ~15 cm
Pull wire
2 turns
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 with maximization
A Better Method Using Phase
• Zero-crossing at resonant frequency
50 55 60 65 70 750
10
20
30
40
50
60
Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75-30
-20
-10
0
10
20
30
Frequency [MHz]
Rea
ctan
ce [
Ohm
s]
50 55 60 65 70 750
10
20
30
40
50
60
Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75
-20
-10
0
10
20
30
Frequency [MHz]
Rea
cta
nce
[Ohm
s]
50 55 60 65 70 750
10
20
30
40
50
60
Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75
-20
-10
0
10
20
30
Frequency [MHz]
Rea
cta
nce
[Ohm
s]
50 55 60 65 70 750
10
20
30
40
50
60
Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75
-20
-10
0
10
20
30
Frequency [MHz]
Rea
cta
nce
[Ohm
s]
At 63.9MHz
Measuring Phase Offset
coil
Vo>0
Vo=0
Vo<0
Cref
Sig
nal so
urc
e Va
Vb
+_
_+
AD835250 MHzMultiplier
Vo
Vo=|Va||Vb|cos(Φ) + …
Filter
Vo ~ |Va||Vb|cos(Φ)
What We Tried
Phase Comparator
coil
CrefVa
Vb
++
_
_
AD835250 MHzMultiplier
Vo
Filter
Vo ~ |Va||Vb|cos(Φ) Vo ~ cos(Φ)
Old New
Vo
Phase Detector ResultsMultiplier Output vs. Receiver Center Frequency
Half-wavelength Txn Line
-600-500-400-300-200-100
0100200300400500
55 57 59 61 63 65 67 69
Frequency (MHz)
DC
out
put (
mV
)
Phase Detector Results (cont…)
• λ/4
• 3λ/8
• 5λ/8
-600
-500
-400
-300
-200
-100
0
55 57 59 61 63 65 67 69
Frequency (MHz)
0
100
200
300
400
500
600
700
DC
ou
t (m
v)_
__
0
100
200
300
400
500
600
Closed Loop Feedback?
• Tempting…– Simple DC negative feedback about zero-point
• …but unsuccessful– Oscillations– Railing
• Phase detection scheme probably requires a different method (?)
Microcontroller
• Why use a microcontroller?– Controlling reference signal generation– Opportunity for tuning algorithms
• Atmel AT90S8515– Serial Peripheral Interface– Analog Comparator– Simple
Atmel AT90S8515
• Serial Peripheral Interface
• Analog Comparator
• Simple development platform– STK500: Starter Kit– CVAVR: C compiler
Reference Signal Requirements
• Accurate and stable reference signal at Larmor frequency during tuning
• Signal well above Larmor frequency during receive mode
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
Tune/Receive (TR) Switch
• Loading effects categorically harmful
• Ideal
– Complete isolation of tuning and receiving circuitry
TuningCircuit
Scanner
Actual TR Switches
• PIN-diodes control signal direction• RF chokes ensure high-impedance, reduce loading
Scanner
TuningCircuit
Microcontroller
Complete System
Results
• Basic tuning functionality– 300ms total tuning time
Detuned
Retuned
Retuned
Detuned
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 λ/2 cable restriction