brit and the rad lab at mit radiation laboratory series: documented developments from the rad lab...
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Brit and the Rad Lab at MIT
Radiation Laboratory Series:Documented developments from the Rad Lab
Volume 19 (copyright 1949): Waveforms- edited by B. Chance et al
Chapter 11: Electrical Amplitude ModulationBy Britton Chance
Page 389: An important use of electrical amplitude modulators is in the conversion of low frequency signalsto modulated carriers, with a-c amplification….
Page 418: 11-5 Variable-capacitance modulators
An End in Sight to S/N Improvements???
Sensitivity has increased fast: 20-50 fold betterthanks to arrays and high field magnets.
Typcial preamps are too big: Parametric amplifiers??
To Keep Increasing S/N We Have to Get Inside the Subject
Cable losses and heating during excitation pulse
Signal attenuationInternal amplifier?
Detect and amplify: Parametric amplifiers
Detection coil with gain
Signal Voxel
+ -
Rs Rs - Rn
Rs = Rcoil + Rtissue
Signal Voxel
+ -
Pick-up Loop Pick-up Loop
Passive* - Noise of pick-up loop dominates
Active - Noise of local loop dominates
Local Loop Local Loop
*Schnall, M., et al JMR 61 161 (1986)
Traditional parametric amplifier
Three distinct resonance modes
Three distinct current loops
-RNeg
Wireless parametric amplifier
1 1 10 2 2 20
10 20
2 ( ) 2 ( )Q Q
Intrinsic resonance: w10 + w20 ~ w30
Operating frequency: w1 + w2 = w3
Frequency matching
2 20 1 2 1M QQ
Oscillation condition
2 2 3( ) (1 2 cos )C t C M t
M: modulation index
M < M0 to avoid oscillation
Internal resonatorSample coil
0Gain f M M
A model triple frequency resonator
Loaded by 1 M NaCl/D2O
w10 /2p= 131.7 MHz, Q=93
w20 /2p = 498.7 MHz, Q=48
w30 /2p = 628.4 MHz, Q=60
Operating frequencies
w1 /2p = 132.1 MHz (23Na 11.7T)
w2 /2p = 501.8 MHz
w3 /2p = (w1 + w2) /2p
= 633.9 MHz1 1 10 2 2 20
10 20
2 ( ) 2 ( )Q Q
(Frequency matching)
Bandwidth v.s. Gain
Double pick up loop
3D FLASH Na images
Weakly coupled loops
TR=100 ms, TE=3.1 msFOV=1.9 x 1.9 x 1.9 cm3
Matrix: 64 x 64 x 64, NEX=1
Na Images Detected at 1H Freq
w2
w1
Excite by loop 1 at w1
Detect by loop 2 at w2
Excite at w1,detect at w1
Excite by loop 1 at w1
Detect by loop 2 at w2
Enhanced sensitivity for weak coupling
Adjust the distance separation
Sensitivity reference standard
couplenorm
direct
SNRSNR
SNR (Normalized)
Wireless parametric amplifier is…
• Nonlinear triple frequency resonator• Pump at w3, detect at w1 and w2=w3- w1
• Frequency matching: Q1Dw1/w10=Q2Dw2/w20
• Gain adjusted by pumping power (M < M0)• The gain limited by required bandwidth• Enhanced sensitivity of implantable coil
• A 3mm version is now working……
Noise factor v.s. Gain
Sensitivity reference standard
21
norm
NfSNR
2
,2'2
,1
Amp
NoAmp
HG
H
2
,11
,1
Amp
NoAmp
HG
H
(Actual gain at w1)
(Nominal gain at w2)
couplenorm
direct
SNRSNR
SNR