Autotuning Electronics for Varactor Tuned, Flexible Interventional RF Coils Ross Venook, Greig Scott, Garry Gold, and Bob Hu

Introduction Motivation –Why use interventional coils? –Why is this hard? Background –History –RF coil tuning method(s) What we tried –Modular electronics discussion Results Next steps

Why Use Interventional Coils? Increased signal coupling & reduced noise coupling  better SNR Coupled noise Coupled signal

Applications: Existing and Potential Existing –Surface coils –Intravascular coils Potential –Inter-articular –

SNR Comparison

Why Interventional Coils Are Harder to Use: Dynamic loading Proximity works both ways –Closer coupling also means greater local tissue dependency –Requires deployability in some applications Scaling works both ways –Human-scale effects are significant –Geometry more important

So… Dynamic loading conditions require dynamic tuning to maximize SNR advantages with interventional coils The tuning process should be automatic, and must add neither noise nor interference to the acquired signal

“RF Coils” RF transmitters and receivers (in MR) are magnetic field coupling resonators that are tuned to the Larmor frequency Examples: –Saddle –Surface –Interventional

Resonance ‘Parallel RLC’ circuit Governing equation Familiar result

Impedance of Resonant Circuits

Goals: Tuning and Matching Tuning –Center Frequency near Larmor –Bandwidth appropriate to application Matching –Tuned impedance near 50 + j0 ohms

Complications Loading the coil with a sample necessarily creates coupling (it better!) Dynamic coupling creates dynamic tuning/matching conditions

TunedDetuned

History Tuning MRI coils (Boskamp 1985) Automatic Tuning and Matching (Hwang and Hoult, 1998) IV Expandable Loop Coils (Martin, et al, 1996)

Shoulders Varactor Tuned Flexible Interventional Receiver Coils (Greig and Garry, ISMRM 2000) Cadaver Shoulder, 1.5T 3D/SPGR/20 slices 6cm FOV, 512x512

Greig’s Tunable Coil 22 or 68pF Varactor 150pF <360nH Flex coil 20K 9 V manual tune 10K C  DC bias, RF isolate 75nH Q spoil Rcv Port C  2.5 cm ~15 cm Pull wire 2 turns

Basic Tuning Method Manually change DC bias on varactor Maximize magnitude response –FID is a reasonable measure Drawbacks: Requires manual iterative approach Maximum FID may not correspond to maximum SNR Feedback not effective for maximization

A Better Method Using Phase Zero-crossing at resonant frequency Frequency [MHz] Resistance [Ohms] Frequency [MHz] Reactance [Ohms]

At 63.9MHz

Measuring Phase Offset coil  Vo>0  Vo=0  Vo<0 Cref Signal source Va Vb + + _ _ AD MHz Multiplier Vo Vo=|Va||Vb|cos(ωt) + … Filter Vo ~ |Va||Vb|cos(Φ)

What We Tried

Phase Comparator coil Cref Va Vb + + _ _ AD MHz Multiplier Vo Filter Vo ~ |Va||Vb|cos(Φ) Vo ~ cos(Φ) Old New Vo

Phase Detector Results Multiplier Output vs. Receiver Center Frequency Half-wavelength Txn Line

Phase Detector Results (cont…) λ/4 3λ/8 5λ/8

Closed Loop Feedback? Tempting… –Simple DC negative feedback about zero-point …but unsuccessful –Oscillations –Railing Phase detection scheme probably requires a different method (?)

Microcontroller Why use a microcontroller? –Controlling reference signal generation –Opportunity for tuning algorithms Atmel AT90S8515 –Serial Peripheral Interface –Analog Comparator –Simple

Atmel AT90S8515 Serial Peripheral Interface Analog Comparator Simple development platform –STK500: Starter Kit –CVAVR: C compiler

Reference Signal Requirements Accurate and stable reference signal at Larmor frequency during tuning Signal well above Larmor frequency during receive mode

PLL Synthesizer Phase Locked Loop –Frequency to voltage Voltage-Controlled Oscillator –Voltage to frequency Current Feedback Amplifier –“Tri-statable:” turns off signal Low Pass Filter –Cleans VCO output

TR Switches Loading effects categorically harmful Ideal –Complete isolation of tuning and receiving circuitry Tuning Circuit Scanner

Actual TR Switches PIN-diodes control signal direction RF chokes ensure high-impedance, reduce loading Scanner Tuning Circuit Microcontroller

Complete System

Results Basic tuning functionality –300ms total tuning time Detuned Retuned

Next Steps Get an image with autotuned receiver on 1.5T scanner SNR advantage (validation) experiments Minimize tuning time Explore VSWR bridge tuning –Remove need for λ/2 cable restriction