11 April, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook, Greig Scott, Garry...
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Transcript of 11 April, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook, Greig Scott, Garry...
11 April, 2003PMRIL
Autotuning Interventional Coils for Imaging
Ross Venook, Greig Scott,
Garry Gold & Bob Hu
11 April, 2003PMRIL
Motivation for Automatic Tuning: Clinical Realities
• Interventional applications = uncontrolled variables– Flexible coils are clinically desirable
– Stuff is inhomogeneous and uncooperative inside the body (even an unconscious subject has moving things under the surface)
• Specific interventional applications– ‘deployable’ RF coil
– Range of motion studies
11 April, 2003PMRIL
Motivation for Automatic Tuning: Technical Consequences
• RF coils are basically inductive loops with a tuning capacitance– Changing the shape or size of the loop changes the
inductance (and hence its tuned peak), but manual adjustment of capacitors is slow
• Closer coupling between the coil and the load means increased coil dynamics
• SNR depends on coil tuning (matching)
Goal: Create an automatic tuning device to quickly and easily optimize coil SNR
11 April, 2003PMRIL
Agenda
• Motivation
• Background– Autotuning method– Electronics development– Old results
• Autotuning with Signa
• Theoretical SNR analysis
• Future work
11 April, 2003PMRIL
Background I: ‘Autotuning’
• Autotuning is the process by which a tuned coil’s center frequency is adjusted without manual effort (push-of-a-button)
• Many methods/topologies exist which can accomplish the task of tuning (and matching) a resonant circuit– Probe topology important
– Tuning topology important
11 April, 2003PMRIL
Background II: Autotuning Method
• Tuned elements have a complex impedance• ‘Resonance frequency’ is defined by zero
imaginary impedance
50 55 60 65 70 750
10
20
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50
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Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75-30
-20
-10
0
10
20
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Frequency [MHz]
Rea
ctan
ce [O
hms]
11 April, 2003PMRIL
At 63.9MHz
0
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60R
esis
tanc
e [O
hms]
-20
-10
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Rea
cta
nce
[Ohm
s]
50 55 60 65 70 75
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Frequency [MHz]
Res
ista
nce
[Ohm
s]
50 55 60 65 70 75
-20
-10
0
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Frequency [MHz]
Rea
cta
nce
[Ohm
s]
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Res
ista
nce
[Ohm
s]
-20
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cta
nce
[Ohm
s]
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11 April, 2003PMRIL
Electronics:Varactor-tunable RF Coil
75nH
22 or 68pFVaractor
Q spoil SignalPort<360nH
C
9 V
20K 20K150pF
10K
C
DC Tuning Bias
Signal
Flex CoilDC Tuning Bias
11 April, 2003PMRIL
Electronics:Reactance Detection
• Phase comparator outputs a DC voltage that is a function of probe reactance
-600-400-200
0200400600
55 57 59 61 63 65 67 69
Frequency (MHz)
DC
out
put (
mV
)
11 April, 2003PMRIL
Electronics: Microcontroller
• Atmel 90S8515 microcontroller operates the state machine via SPI (serial peripheral interface)
FrequencySynthesizer
Micro-Controller
PhaseDetector
Scanner Pre-amp
Tune/ReceiveSwitch
VaractorTuningVoltage
RF Coil
11 April, 2003PMRIL
Retuned
Retuned
Frequency (MHz)
Res
ista
nce
(Ω)
61 62 63 64 65 66 67
10
20
30
40
50
60
DetunedDetuned
Punchline
11 April, 2003PMRIL
Agenda• Motivation• Background• Autotuning with Signa
– Initial experiments: RFI, and other problems– Improved autotuner– Initial experiments (for real, this time)– SNR increase observed (!)
• Theoretical SNR analysis• Future work
11 April, 2003PMRIL
Initial Images (Problems)
• RF interference artifact caused by switching power supply
• Common-mode
transients affect
microcontroller,
TR switch fails
while imaging
11 April, 2003PMRIL
Solution• Replace switching supply and 3V Li battery (very
cool, non-ferrous)…
• …with simple voltage regulators and 9V alkalines (not cool, very ferrous) and….
11 April, 2003PMRIL
…goodness results
• Reliable images with autotuner (Greig at Leipzig)
High peak SNR (>300) ‘No’ RFI, some PE ghosting
Windowed-down Windowed-up
(SPGR, TE/TR = 7.2ms/34ms, 4mm slice, 12x12cm² FOV, 30° flip)
11 April, 2003PMRIL
Experimental Setup
Varactor-tunedCoil
Phantom
AutotuningElectronics
cable
GE Signa1.5 T
Status LEDs(not blinking)
11 April, 2003PMRIL
Experiment #1The Pepsi Challenge
• Varactor-tuned vs. passively-tuned images Varactor-diode
Passive variablecapacitor
Both images have nominal SNR=326
11 April, 2003PMRIL
Experiment #2:Tune-Detune-Retune
Autotune Detune (deform coil) Re-autotune
11 April, 2003PMRIL
SNR Profiles
Noise Box Image Sample
SNR
11 April, 2003PMRIL
Typical ResultSN
R
Autotune Detune (deform coil) Re-autotune
11 April, 2003PMRIL
Agenda
• Motivation• Background• Autotuning with Signa• Theoretical SNR analysis
– Noise Figure and noise circles– Coil -> preamplifier– Coil -> txn line -> preamplifier– An interesting result
• Future work
11 April, 2003PMRIL
Noise Figure
• All practical devices have NF>0dB (F>1)• Convenient and sensible metric• Noise Figure is in dB, Noise Factor is not
10log( )
IN
OUT
SNRF
SNR
NF F
11 April, 2003PMRIL
Why We Have Preamplifiers
• Friis Equation
• Preamplifier NF dominates system NF (for moderate G1)
– Worry about preamp NF
F1, G1 F2, G2 FN, GN…SNRIN SNROUT
Fi = Noise Factor of ith stage Gi = Gain of ith stage
321 1
1 1 21
1 11IN NN
OUT nn
SNR F FFF F
SNR G G G G
11 April, 2003PMRIL
Preamplifier NF• Depends on impedance match, Ropt set by device
R/Ropt
Noi
se F
igur
e [d
B]
Fmin = 1.05, 1.07, 1, 1.2, 1.5
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7
0.1 1 10
11 April, 2003PMRIL
Preamplifier NF• Actually, NF is a surface on the complex-Z plane
Im[Z]
Re[Z]
NF
(dB
)
11 April, 2003PMRIL
Noise Circles
• Preamplifier NF level sets are circles in the complex impedance plane
• Minimum NF occurs at 50 + j0 Ω for this example
Im[Z
] (Ω
)
Re[Z] (Ω)0 1000
0
-500
500
Preamplifier NF Contours [dB] on Z-plane
11 April, 2003PMRIL
Autotuning Trajectory
Im[Z
] (Ω
)
Re[Z] (Ω)0 1000
0
-500
500
Preamplifier NF Contours [dB] on Z-plane
•••
•
• •
•• •
•••
11 April, 2003PMRIL
Lossy Transmission Lines
• Coaxial transmission lines connect the coil to the tuner, and the tuner to the scanner– Impedance transformation changes Z at preamp
– Resistive loss adds noise
• Interventional devices require small-diameter coax– Greater loss (dB/m)
– Imperfect impedance transformation
– Together, these warp the system noise circles and trajectories for a given coil
11 April, 2003PMRIL
Lossy Transmission Lines (cont…)
• Evaluate impedance ‘mismatch’ by measuring reflected power from a transition– Perfect match has no reflections
• Statement: ‘lossier cable can improve your match because it lowers the reflected power’
• Hmmm….• Counter: ‘what about SNR? Isn’t the baby being
thrown out with the bathwater??’
11 April, 2003PMRIL
Interesting Result
• Indeed, lossy transmission line will always have worse SNR at its output than at its input
• But, if we have a preamplifier waiting for us on the other end, perhaps the impedance transformation can improve system SNR
• Conclusion:– It’s something to watch out for
Noi
se F
igur
e [d
B]
1
2
3
4
5
6
7
0.1 1 10
R/Ropt
11 April, 2003PMRIL
Future Work
• Loose ends– Controlled analysis of tuning vs. SNR with data
• Theoretical– To tune, or not to tune (and how to decide)
• Clinical– Scan cadaver shoulders
• Practical– 0.5T/21MHz version– Test speed limits
• New directions– Automatic matching (perhaps a successful topology to borrow
from CW-EPR)