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