1. 15 July, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook 1, Garry Gold 2, Bob Hu 3 & Greig Scott 1 1.Department of Electrical Engineering, Stanford University 2.Department of Radiology, Stanford University 3.Palo Alto Medical Foundation, Palo Alto, CA

2. 15 July, 2003 PMRIL Motivation for Flexible Coils Small coil within an area of interest vs. exterior surface coil gives higher SNR, better resolution Catheter-based RF coils Examples –Interarticular –Intravascular –Transrectal

3. 15 July, 2003 PMRIL Technical Consequences of Flexible Coils: Motivation for Automatic Tuning Changing the shape or size of an RF loop changes its resonant frequency –Coil is an inductor with a tuning capacitor SNR depends on coil tuning (matching) Manual tuning is time consuming and difficult 1cm 2cm

4. 15 July, 2003 PMRIL Preamplifier Noise Factor (F) is a function of the source impedance it sees: –Noise matching occurs when Z COIL = Z OPT Coax losses degrade SNR with power mismatch –Especially important in interventional applications due to need for small-diameter (lossier) coax SNR Analysis: How does SNR depend on Tuning/Matching?

5. 15 July, 2003 PMRIL Varactor-tunable RF Coil DC Tuning Bias Signal Flex Coil Varactor To Preamp

6. 15 July, 2003 PMRIL Tuning Voltage vs. Resonant Frequency Resonant Frequency [MHz] Varactor Reverse Bias [V]

7. 15 July, 2003 PMRIL Autotuning System Use λ/2 cable to measure coil Z remotely Tune for resonance (zero imaginary part) –Vary varactor voltage with micro-controller Frequency Synthesizer Micro- Controller Phase Detector Scanner Pre-amp Tune/Receive Switch Varactor Tuning Voltage RF Coil λ/2

8. 15 July, 2003 PMRIL Detuned Retuned Tuned Tuning to Larmor Frequency Frequency (MHz) Resistance (Ω) 61626364656667 10 20 30 40 50 60

9. 15 July, 2003 PMRIL Experimental Setup Varactor-tuned Coil Phantom Autotuning Electronics λ/2 cable GE Signa 1.5 T Status LEDs

10. 15 July, 2003 PMRIL Experiment #1 Varactor-tuned Coil Passively-tuned Coil Images have same SNR +/-2% SPGR, 12cm 2, 256x256,7.2/34ms=TE/TR,30º

11. 15 July, 2003 PMRIL Experiment #2: Tune-Detune-Retune AutotuneDetune (deform coil)Re-autotune 1cm 2cm

12. 15 July, 2003 PMRIL Experiment #2: Windowed Up Tune-Detune-Retune AutotuneDetune (deform coil)Re-autotune

13. 15 July, 2003 PMRIL SNR Profiles Noise Box Image Sample SNR

14. 15 July, 2003 PMRIL Typical Result SNR AutotuneDetune (deform coil)Re-autotune

15. 15 July, 2003 PMRIL Quantitative Comparison Low-loss coax 2.2 MHz detuning –Theory predicts 40% SNR loss –We measure 30% SNR loss Lossy vs. less-lossy, small-diameter coax +/- 2 MHz detuning –Theory predicts 15-20% additional SNR degradation –We measure 10-15% additional SNR degradation 0.5mm 0.8mm

16. 15 July, 2003 PMRIL Future Work Clinical –Scan cadaver shoulders, compare with surface coils Practical –0.5T/21MHz version –Test speed limits New directions –Automatic matching

17. 15 July, 2003 PMRIL Acknowledgements GE Medical Systems NIH R33 EB 00777 NIH R01 EB 00346 NIH 5R24CA092862-03 Whitaker Foundation