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 Modulation By Britton Chance Page 389: An important use of electrical amplitude modulators is in the conversion of low frequency signals to 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: fold better thanks 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 attenuation Internal amplifier? Detect and amplify: Parametric amplifiers

Detection coil with gain Signal Voxel Rs Rs - Rn Rs = R coil + R tissue Signal Voxel Pick-up Loop Passive* - Noise of pick-up loop dominates Active - Noise of local loop dominates Local Loop *Schnall, M., et al JMR (1986)

Traditional parametric amplifier Three distinct resonance modes Three distinct current loops -R Neg

Wireless parametric amplifier Intrinsic resonance: w 10 + w 20 ~ w 30 Operating frequency : w 1 + w 2 = w 3 Frequency matching Oscillation condition M: modulation index M < M 0 to avoid oscillation Internal resonator Sample coil

A model triple frequency resonator Loaded by 1 M NaCl/D 2 O w 10 /2p= MHz, Q=93 w 20 /2p = MHz, Q=48 w 30 /2p = MHz, Q=60 Operating frequencies w 1 /2p = MHz ( 23 Na 11.7T) w 2 /2p = MHz w 3 /2p = (w 1 + w 2 ) /2p = MHz (Frequency matching)

Bandwidth v.s. Gain Double pick up loop

3D FLASH Na images Weakly coupled loops TR=100 ms, TE=3.1 ms FOV=1.9 x 1.9 x 1.9 cm 3 Matrix: 64 x 64 x 64, NEX=1

Na Images Detected at 1 H Freq w2w2 w1w1 Excite by loop 1 at w 1 Detect by loop 2 at w 2 Excite at w 1,detect at w 1 Excite by loop 1 at w 1 Detect by loop 2 at w 2

Enhanced sensitivity for weak coupling Adjust the distance separation Sensitivity reference standard (Normalized)

Wireless parametric amplifier is… Nonlinear triple frequency resonator Pump at w 3, detect at w 1 and w 2 =w 3 - w 1 Frequency matching: Q 1 Dw 1 / w 10 = Q 2 Dw 2 / w 20 Gain adjusted by pumping power (M < M 0 ) 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 (Actual gain at w 1 ) (Nominal gain at w 2 )