Autotuning Electronics for Varactor Tuned, Flexible Interventional RF

Coils

Ross Venook, Greig Scott,

Garry Gold, and Bob Hu

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

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)

z

x

y Bω

B

Hydrogen atom“spin”

z

x

y

The Second Thing About MRI

• During relaxation, the spins emit EM radiation at ω = γBlocal

• RF coil inductively couples this signal

z

x

yB ω

Before RF Excitation

z

x

y

Transversecomponent

RF Excitation“Tip”

RF Relaxation

Simple Example

• Linear gradient produces frequency encoding of spatial hydrogen atom distribution

Boz

y

x

+

By-gradient

=

Bfinal

Object

ωωo

Signal

Relaxation Signal (freq. domain)

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

Why Use Interventional Coils?

• Increased signal coupling & reduced noise coupling better SNR

Coupled noise

Coupled signal

SNR Comparison

Applications: Existing and Potential

• Existing– Intravascular coils – Endorectal coils

• Potential– Inter-articular– <add your application here>

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

3” surface coil (GE)

Resonance

• ‘Parallel RLC’ circuit

• Governing equation

• Familiar result

011

2

2

VLCdt

dV

RCdt

Vd

LCf

1

2

10

Impedance of Resonant Circuits

50 55 60 65 70 750

10

20

30

40

50

60

Frequency [MHz]

Res

ista

nce

[Ohm

s]

50 55 60 65 70 75-30

-20

-10

0

10

20

30

Frequency [MHz]

Rea

ctan

ce [

Ohm

s]

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 (Scott and Gold, ISMRM 2001)

Cadaver Shoulder, 1.5T

3D/SPGR/20 slices

6cm FOV, 512x512

Greig’s Tunable Coil

22 or 68pFVaractor

150pF

<360nH

Flex coil

20K 20K

9 Vmanual

tune10K

C DC bias,RF isolate

75nH

Q spoil Rcv

PortC

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 with maximization

A Better Method Using Phase

• Zero-crossing at resonant frequency

50 55 60 65 70 750

10

20

30

40

50

60

Frequency [MHz]

Res

ista

nce

[Ohm

s]

50 55 60 65 70 75-30

-20

-10

0

10

20

30

Frequency [MHz]

Rea

ctan

ce [

Ohm

s]

50 55 60 65 70 750

10

20

30

40

50

60

Frequency [MHz]

Res

ista

nce

[Ohm

s]

50 55 60 65 70 75

-20

-10

0

10

20

30

Frequency [MHz]

Rea

cta

nce

[Ohm

s]

50 55 60 65 70 750

10

20

30

40

50

60

Frequency [MHz]

Res

ista

nce

[Ohm

s]

50 55 60 65 70 75

-20

-10

0

10

20

30

Frequency [MHz]

Rea

cta

nce

[Ohm

s]

50 55 60 65 70 750

10

20

30

40

50

60

Frequency [MHz]

Res

ista

nce

[Ohm

s]

50 55 60 65 70 75

-20

-10

0

10

20

30

Frequency [MHz]

Rea

cta

nce

[Ohm

s]

At 63.9MHz

Measuring Phase Offset

coil

Vo>0

Vo=0

Vo<0

Cref

Sig

nal so

urc

e Va

Vb

+_

_+

AD835250 MHzMultiplier

Vo

Vo=|Va||Vb|cos(Φ) + …

Filter

Vo ~ |Va||Vb|cos(Φ)

What We Tried

Phase Comparator

coil

CrefVa

Vb

++

_

_

AD835250 MHzMultiplier

Vo

Filter

Vo ~ |Va||Vb|cos(Φ) Vo ~ cos(Φ)

Old New

Vo

Phase Detector ResultsMultiplier Output vs. Receiver Center Frequency

Half-wavelength Txn Line

-600-500-400-300-200-100

0100200300400500

55 57 59 61 63 65 67 69

Frequency (MHz)

DC

out

put (

mV

)

Phase Detector Results (cont…)

• λ/4

• 3λ/8

• 5λ/8

-600

-500

-400

-300

-200

-100

0

55 57 59 61 63 65 67 69

Frequency (MHz)

0

100

200

300

400

500

600

700

DC

ou

t (m

v)_

__

0

100

200

300

400

500

600

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

Tune/Receive (TR) Switch

• Loading effects categorically harmful

• Ideal

– Complete isolation of tuning and receiving circuitry

TuningCircuit

Scanner

Actual TR Switches

• PIN-diodes control signal direction• RF chokes ensure high-impedance, reduce loading

Scanner

TuningCircuit

Microcontroller

Complete System

Results

• Basic tuning functionality– 300ms total tuning time

Detuned

Retuned

Retuned

Detuned

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