11 April, 2003 PMRIL Autotuning Interventional Coils for Imaging Ross Venook, Greig Scott, Garry Gold & Bob Hu

11 April, 2003 PMRIL Motivation for Automatic Tuning: Clinical Realities Interventional applications = uncontrolled variables –Flexible coils are clinically desirable –Stuff is inhomogeneous and uncooperative inside the body (even an unconscious subject has moving things under the surface) Specific interventional applications –‘deployable’ RF coil –Range of motion studies

11 April, 2003 PMRIL Motivation for Automatic Tuning: Technical Consequences RF coils are basically inductive loops with a tuning capacitance –Changing the shape or size of the loop changes the inductance (and hence its tuned peak), but manual adjustment of capacitors is slow Closer coupling between the coil and the load means increased coil dynamics SNR depends on coil tuning (matching) Goal: Create an automatic tuning device to quickly and easily optimize coil SNR

11 April, 2003 PMRIL Agenda Motivation Background –Autotuning method –Electronics development –Old results Autotuning with Signa Theoretical SNR analysis Future work

11 April, 2003 PMRIL Background I: ‘Autotuning’ Autotuning is the process by which a tuned coil’s center frequency is adjusted without manual effort (push-of-a-button) Many methods/topologies exist which can accomplish the task of tuning (and matching) a resonant circuit –Probe topology important –Tuning topology important

11 April, 2003 PMRIL Background II: Autotuning Method Tuned elements have a complex impedance ‘Resonance frequency’ is defined by zero imaginary impedance

11 April, 2003 PMRIL At 63.9MHz

11 April, 2003 PMRIL Electronics: Varactor-tunable RF Coil 75nH 22 or 68pF Varactor Q spoil Signal Port <360nH C  9 V 20K 150pF 10K C  DC Tuning Bias Signal Flex Coil DC Tuning Bias

11 April, 2003 PMRIL Electronics: Reactance Detection Phase comparator outputs a DC voltage that is a function of probe reactance

11 April, 2003 PMRIL Electronics: Microcontroller Atmel 90S8515 microcontroller operates the state machine via SPI (serial peripheral interface) Frequency Synthesizer Micro- Controller Phase Detector Scanner Pre-amp Tune/Receive Switch Varactor Tuning Voltage RF Coil

11 April, 2003 PMRIL Retuned Frequency (MHz) Resistance (Ω) Detuned Punchline

11 April, 2003 PMRIL Agenda Motivation Background Autotuning with Signa –Initial experiments: RFI, and other problems –Improved autotuner –Initial experiments (for real, this time) –SNR increase observed (!) Theoretical SNR analysis Future work

11 April, 2003 PMRIL Initial Images (Problems) RF interference artifact caused by switching power supply Common-mode transients affect microcontroller, TR switch fails while imaging

11 April, 2003 PMRIL Solution Replace switching supply and 3V Li battery (very cool, non-ferrous)… …with simple voltage regulators and 9V alkalines (not cool, very ferrous) and….

11 April, 2003 PMRIL …goodness results Reliable images with autotuner (Greig at Leipzig) High peak SNR (>300) ‘No’ RFI, some PE ghosting Windowed-down Windowed-up (SPGR, TE/TR = 7.2ms/34ms, 4mm slice, 12x12cm² FOV, 30° flip)

11 April, 2003 PMRIL Experimental Setup Varactor-tuned Coil Phantom Autotuning Electronics  cable GE Signa 1.5 T Status LEDs (not blinking)

11 April, 2003 PMRIL Experiment #1 The Pepsi Challenge Varactor-tuned vs. passively- tuned images Varactor-diode Passive variable capacitor Both images have nominal SNR=326

11 April, 2003 PMRIL Experiment #2: Tune-Detune-Retune AutotuneDetune (deform coil)Re-autotune

11 April, 2003 PMRIL SNR Profiles Noise Box Image Sample SNR

11 April, 2003 PMRIL Typical Result SNR AutotuneDetune (deform coil)Re-autotune

11 April, 2003 PMRIL Agenda Motivation Background Autotuning with Signa Theoretical SNR analysis –Noise Figure and noise circles –Coil -> preamplifier –Coil -> txn line -> preamplifier –An interesting result Future work

11 April, 2003 PMRIL Noise Figure All practical devices have NF>0dB (F>1) Convenient and sensible metric Noise Figure is in dB, Noise Factor is not

11 April, 2003 PMRIL Why We Have Preamplifiers Friis Equation Preamplifier NF dominates system NF (for moderate G 1 ) –Worry about preamp NF F 1, G 1 F 2, G 2 F N, G N … SNR IN SNR OUT F i = Noise Factor of ith stage G i = Gain of ith stage

11 April, 2003 PMRIL Preamplifier NF Depends on impedance match, R opt set by device R/R opt Noise Figure [dB] F min = 1.05, 1.07, 1, 1.2,

11 April, 2003 PMRIL Preamplifier NF Actually, NF is a surface on the complex-Z plane Im[Z] Re[Z] NF (dB)

11 April, 2003 PMRIL Noise Circles Preamplifier NF level sets are circles in the complex impedance plane Minimum NF occurs at 50 + j0 Ω for this example Im[Z] (Ω) Re[Z] (Ω) Preamplifier NF Contours [dB] on Z-plane

11 April, 2003 PMRIL Autotuning Trajectory Im[Z] (Ω) Re[Z] (Ω) Preamplifier NF Contours [dB] on Z-plane

11 April, 2003 PMRIL Lossy Transmission Lines Coaxial transmission lines connect the coil to the tuner, and the tuner to the scanner –Impedance transformation changes Z at preamp –Resistive loss adds noise Interventional devices require small-diameter coax –Greater loss (dB/m) –Imperfect impedance transformation –Together, these warp the system noise circles and trajectories for a given coil

11 April, 2003 PMRIL Lossy Transmission Lines (cont…) Evaluate impedance ‘mismatch’ by measuring reflected power from a transition –Perfect match has no reflections Statement: ‘lossier cable can improve your match because it lowers the reflected power’ Hmmm…. Counter: ‘what about SNR? Isn’t the baby being thrown out with the bathwater??’

11 April, 2003 PMRIL Interesting Result Indeed, lossy transmission line will always have worse SNR at its output than at its input But, if we have a preamplifier waiting for us on the other end, perhaps the impedance transformation can improve system SNR Conclusion: –It’s something to watch out for Noise Figure [dB] R/R opt

11 April, 2003 PMRIL Future Work Loose ends –Controlled analysis of tuning vs. SNR with data Theoretical –To tune, or not to tune (and how to decide) Clinical –Scan cadaver shoulders Practical –0.5T/21MHz version –Test speed limits New directions –Automatic matching (perhaps a successful topology to borrow from CW-EPR)