11 april, 2003 pmril autotuning interventional coils for imaging ross venook, greig scott, garry...
TRANSCRIPT
![Page 1: 11 April, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook, Greig Scott, Garry Gold & Bob Hu](https://reader035.vdocuments.us/reader035/viewer/2022062410/5697bfc71a28abf838ca7c2c/html5/thumbnails/1.jpg)
11 April, 2003PMRIL
Autotuning Interventional Coils for Imaging
Ross Venook, Greig Scott,
Garry Gold & Bob Hu
![Page 2: 11 April, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook, Greig Scott, Garry Gold & Bob Hu](https://reader035.vdocuments.us/reader035/viewer/2022062410/5697bfc71a28abf838ca7c2c/html5/thumbnails/2.jpg)
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
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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
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11 April, 2003PMRIL
Agenda
• Motivation
• Background– Autotuning method– Electronics development– Old results
• Autotuning with Signa
• Theoretical SNR analysis
• Future work
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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
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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
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 [O
hms]
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11 April, 2003PMRIL
At 63.9MHz
0
10
20
30
40
50
60R
esis
tanc
e [O
hms]
-20
-10
0
10
20
30
Rea
cta
nce
[Ohm
s]
50 55 60 65 70 75
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]
10
20
30
40
50
60
Res
ista
nce
[Ohm
s]
-20
-10
0
10
20
30
Rea
cta
nce
[Ohm
s]
0
0
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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
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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
)
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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
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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
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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
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11 April, 2003PMRIL
Initial Images (Problems)
• RF interference artifact caused by switching power supply
• Common-mode
transients affect
microcontroller,
TR switch fails
while imaging
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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….
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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)
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11 April, 2003PMRIL
Experimental Setup
Varactor-tunedCoil
Phantom
AutotuningElectronics
cable
GE Signa1.5 T
Status LEDs(not blinking)
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11 April, 2003PMRIL
Experiment #1The Pepsi Challenge
• Varactor-tuned vs. passively-tuned images Varactor-diode
Passive variablecapacitor
Both images have nominal SNR=326
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11 April, 2003PMRIL
Experiment #2:Tune-Detune-Retune
Autotune Detune (deform coil) Re-autotune
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11 April, 2003PMRIL
SNR Profiles
Noise Box Image Sample
SNR
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11 April, 2003PMRIL
Typical ResultSN
R
Autotune Detune (deform coil) Re-autotune
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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
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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
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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
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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
1
2
3
4
5
6
7
0.1 1 10
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11 April, 2003PMRIL
Preamplifier NF• Actually, NF is a surface on the complex-Z plane
Im[Z]
Re[Z]
NF
(dB
)
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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
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11 April, 2003PMRIL
Autotuning Trajectory
Im[Z
] (Ω
)
Re[Z] (Ω)0 1000
0
-500
500
Preamplifier NF Contours [dB] on Z-plane
•••
•
• •
•• •
•••
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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
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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??’
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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
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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)