Autotuning Electronics for Varactor Tuned, Flexible Interventional RF Coils

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

2. Introduction • Basics of Magnetic Resonance Imaging (MRI) • 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

3. z y x The First Thing About MRI • Bloch Equation: ω = γB • ω : precession/Larmor frequency • γ : gyromagnetic ratio (2π•42.575MHz/Tesla) • B : local magnetic field strength (Tesla) Hydrogen atom “spin” B ω B

4. z z z y y y x x x The Second Thing About MRI Before RF Excitation RF Excitation RF Relaxation • During relaxation, the spins emit EM radiation at ω = γBlocal • RF coil inductively couples this signal “Tip” B ω Transverse component

5. Bfinal By-gradient z + = y x Signal ωo ω Object Relaxation Signal (freq. domain) Simple Example Bo • Linear gradient produces frequency encoding of spatial hydrogen atom distribution

6. Other Important Points • Signal to Noise Ratio (SNR) is the figure of merit for MRI • SNR acts as a currency for other MRI attributes (resolution, field of view, scan time) • Clinically-driven field • Focus on medical problems/solutions • Factors of two matter • Primary advantage of MRI: it is a non-invasive imaging modality

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

8. SNR Comparison

9. Applications: Existing and Potential • Existing • Intravascular coils • Endorectal coils • Potential • Inter-articular • <add your application here>

10. 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

11. 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

12. 3” surface coil (GE) “RF Coils” • RF transmitters and receivers (in MR) are magnetic field coupling resonators that are tuned to the Larmor frequency • Examples: • Saddle • Surface • Interventional

13. Resonance • ‘Parallel RLC’ circuit • Governing equation • Familiar result

14. 60 30 50 20 40 10 Reactance [Ohms] Resistance [Ohms] 30 0 20 -10 10 -20 0 -30 50 55 60 65 70 75 50 55 60 65 70 75 Frequency [MHz] Frequency [MHz] Impedance of Resonant Circuits

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

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

17. Tuned Detuned

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

19. Shoulders • Varactor Tuned Flexible Interventional Receiver Coils (Scott and Gold, ISMRM 2001) Cadaver Shoulder, 1.5T 3D/SPGR/20 slices 6cm FOV, 512x512

20. manual tune 9 V 10K DC bias, RF isolate 150pF 20K 20K 75nH Flex coil 22 or 68pF Rcv Q spoil <360nH Varactor Port C C 8 8 Pull wire ~15 cm 2.5 cm 2 turns Greig’s Tunable Coil

21. 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 with maximization

22. 60 30 50 20 40 10 Reactance [Ohms] Resistance [Ohms] 30 0 20 -10 10 -20 0 -30 50 55 60 65 70 75 50 55 60 65 70 75 Frequency [MHz] Frequency [MHz] A Better Method Using Phase • Zero-crossing at resonant frequency

23. 60 50 40 Resistance [Ohms] Reactance [Ohms] 30 20 10 30 30 30 0 50 55 60 65 70 75 50 55 60 65 70 75 Frequency [MHz] Frequency [MHz] 20 20 20 60 50 10 10 10 40 Resistance [Ohms] Reactance [Ohms] 30 0 0 0 20 10 -10 -10 -10 0 50 55 60 65 70 75 50 55 60 65 70 75 Frequency [MHz] Frequency [MHz] -20 -20 -20 60 50 40 Resistance [Ohms] Reactance [Ohms] 30 20 10 0 50 55 60 65 70 75 50 55 60 65 70 75 Frequency [MHz] Frequency [MHz] At 63.9MHz

24. F->0 Vo>0 Va Cref _ + + F->90 Vo=0 Signal source Vo Vb _ AD835 250 MHz Multiplier Filter coil F->180 Vo<0 Vo=|Va||Vb|cos(Φ) + … Vo ~ |Va||Vb|cos(Φ) Measuring Phase Offset

25. What We Tried

26. coil Vo ~ cos(Φ) Phase Comparator Old New Vo Va Cref _ + + Vo Vb _ AD835 250 MHz Multiplier Filter Vo ~ |Va||Vb|cos(Φ)

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

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

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

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

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

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

35. 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

37. Tuning Circuit Scanner Tune/Receive (TR) Switch • Loading effects categorically harmful • Ideal • Complete isolation of tuning and receiving circuitry

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

39. Complete System

40. Retuned Detuned Detuned Retuned Results • Basic tuning functionality • 300ms total tuning time

41. 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