pacificon talk

Antennas in Magnetic Resonance Imaging

John M. PaulyAG6WH

Stanford University

http://www.stanford.edu/~pauly

Why MRI?

K. Pauly, G. Gold Stanford Rad 220

MRI Contrast

Angiography Musculo-skeletalK. Pauly, G. Gold

Stanford Rad 220

MRI Applications

5

•Coronary Artery•3T Scanner•10 Images/s

Real Time MRI

6

Reed Busse, PhD, GE ASL West

35 week old fetus with nodularity along ventricles and failure of development of the corpus callosum

Reed Busse, PhD, GE ASL West

Fetal MRI

How Does MRI Work?

µ

Magnetic Moment

1H Atom

µ

µ

µ

µ

µ

µ

µ

µ

µ

µ

B0

M0

Polarization

No Polarization Polarized

MRI ScannerFigure3:Left)MEDUSAgradientandRFcontrollersystem.Right)EquipmentrackcontainingaMEDUSAsystemtogetherwithRFvectormodulatorsandRFamplifierssupporting8-channelparallelRFtransmit.

Figure4:ProposedInstrument:GEDiscoveryMR7503T,awhole-bodyMRIsystem.

Inadditiontotheimprovementsofferedbytheproposedinstrument,theDepartmentofElectricalEngineer-ing,asdescribedintheattachedletter,iscommitting$200,000forsiteupgradesandinstallationoftheproposedinstrument,includingnewRFshieldingtoallowinstallationofastandardpatienttableandmagnethousing.

Insummary,theMR750isthemostadvanced3TMRIsystemonthemarketandprovidesastate-of-the-artplatformfordevelopment.WeexpecttheinstrumentandsiteupgradestodramaticallyimprovetheresearchcapabilitiesoftheMRSRLsystem,aswellasincreasevolunteercomfortandacceptance,withoutdetractingfromthestrengthsofourpastprograminallowingforsubstantivesystemmodificationandintegrationwithprototypeequipment.SimilarInstrumentsAnumberofother3TMRIsystemsareavailableoncampus,yetnonearesufficientlyavailableorappropriatefortheproposedresearchbeingconductedattheMRSRL.Table1describesthesesystemsaslabeledinthemapshowninFig.5.Locations2and3areimagingcentersthatcontainresearch3TMRIsystemsthatserveawidebodyofdifferentusers.Bothofthesecentersoperatewithsubstantialsupportstaffandchargescanfeesontheorderof$400perhourforcostrecovery.TheMRSRLisauniquefacilityinthe

MRI Scanner

Excitation

ω0 = γB0

K. Pauly, G. Gold Stanford Rad 220

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Spatial Encoding

ω(x) = γ(Gx+B0)

K. Pauly, G. Gold Stanford Rad 220

K. Pauly, G. GoldStanford Rad 220

Spectrum

TimeSignal

Piano Analogy

Body

ReceiveSignal

ω(x) = γGx

s(t)

S(ω)

ω

ReceiveSpectrum

Spatial Encoding

Transmit Antennas

MRI Scanner

Transmit Coil

Hayes, et al. JMR 1985

Schematic Field Pattern Experiment

Birdcage Modes

One Drive Port : Linear Polarization

RF

Bei Zhang, ISMRM 2016

Birdcage Coil

Two Drive Ports : Circular Polarization

RFIRFQ

Rotation

Birdcage Modes

Bei Zhang, ISMRM 2016

Amp

Amp

Amp

Amp

Amp

RF Source Low powerRF Amps Body Coil

Amp

RF Source High powerRF Amp Body Coil

Parallel Transmit

Single High Power Transmit ChannelMultiple Low Power Transmit Channels

Parallel Transmit B1 ShimmingRF Linearization

Pascal Stang, Greig Scott, Stanford

4ch Tx Coil with sensors

Coil current sensor

Secondary Winding on Inner Layers

Primary Winding on

Outer Layers

(a)

RF Current Sensor

Linearize Entire Transmit Chain

Receive Antennas

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Receive Coils

Zhang et al, MRM 2015

Array Receive Coils

Coil layout

Receive Coils

Zhang et al, MRM 2015Array Receive Coils

Circuit for an Individual Element

Receive Coils

Zhang et al, MRM 2015Array Receive Coils

Individual Coil Sensitivities

Image courtesy: Siemens

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Current Receive Coils

Cables and Baluns Constraining Arrays

Wearable Wireless Coil Arrays

Software Defined Multi-Channel

Receiver

Wireless PowerPilot Tone

Q-spoilClock

In-Bore propagationLow Power Digital

Transponders

Wearable Wireless Coil Arrays

Wireless Power

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“Tesla apparatus”, Secor, 1921

Near-Field Power

Batteriescouldlimitscantime

Needabout100mWperchannel

Ourgoalistodeliver10W

MinimalinteractioninsidetheMRIbore

Wireless Power For Array Coils

K. Byron, et al. ISMRM 2016, Stanford

Wireless Power Transfer System

DiodeRectifierPickup

Coil

PrimaryCoil

10MHzλ/4FilterAntennaTunerPowerAmplifier

StorageCapacitor

Filter

RFSwitch

50Ω

Signal source

64MHzλ/4

Controlledby:1) Medusa

module2) Triggerdelayed

fromtheTXexciterunblanksignal

RF Gated System

Drivepoweriscycledonandoff

at1Hz

Wireless Powering 11W Light

NotTransmitting0.0215rms

Harvesting1WRFGated0.0221rms

Harvestingestimated5WRFGated0.0219rms

Areausedtomeasurebackgroundnoisevoltage

Noise Measurements

Low Power Electronics

Possible Design: Swaddle Coil

Transponders handleseveral local coils

0 50 100 150 200 250.0001

.001

.01

.1

1.0

K Sp

ace

Ampl

itude

500kHz image bandwidth, so 1 Msps or 500 Ksps I/q 16-20bits output, 100 mW/ch.

K-space compressible by ~25% because of significant bit zeros. 50% data acquisition duty cycle.

Leads to 6-8 Mbps per coil with prior decimation, compression, buffering.

MRI Signal Characteristics

LNA Pipeline 20-80 Msps

Downconvert

decimate20 Mbps transmitter

90-110mW

LNA 18-bit SAR 1Msps

18 Mbps transmitter

10-20mW60-100mW

Poly filter

Low IF image reject

Demodulate to IF

Direct to Digital

Receiver Architectures

Synchronization

LNA A/D

IF

MRI requires great frequency stability

Where do we get the IF signal?

Synchronization

LNA A/D

Put a carrier on the 10 MHzwireless power signal

Use a frequency multiplierto get the frequency youwant

Problem: wireless power coilimpedance changes with load

Wireless Power

Synchronization

LNA A/D

IFTransmit at 1 GHz

Divide down to thefrequency you want

Problem: phase shifts dueto propagation

~ 1 GHz

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Synchronization

1 GHz signal transmittedfrom the magnet bore

Demodulated with theoriginal signal

Variation is due to subject motion

Synchronization

LNA A/D

IFUse a local oscillator forthe IF

Transmit it out, and trackits frequency

Fix the data up later

~LO

Synchronization

LNA A/D

IF

Track the transmit RF

Very frequency stable

Very high SNR

Problem: people do strangethings with RF waveformsTransmit RF Coil

Wireless Data

RF coils

Low Power, mm-Wave Transmitter

mm-Wave Receiver

Wireless Data Transmission

Image: IEEE SpectrumKamal Aggarwal, Ada Poon

Stanford

802.11ac

“older” BCM4334 in iPhone 5s

BCM4354, 2x2 MIMO 867 Mb/s in 80 MHz channel (2-stream)

802.11ac (5 GHz)

UWB 3-5GHzPulse-Link Cwave

GEN 1: 1Gb/s

802.11ad 60GHz (WiGig) 4.6 Gb/s SC, 7Gb/s OFDM

Tensorcom < 250mW Wilocity, WIL6300, 200-300mW

WirelessHD (60 GHz)10-28Gb/s

Silicon Image, UltraGig-6400, 500mW

Commercial High Speed Links

802.11ad 60GHz (WiGig) 4.6 Gb/s SC, 7Gb/s OFDM

Tensorcom < 250mW Wilocity, WIL6300, 200-300mW

WirelessHD (60 GHz)10-28Gb/s

Silicon Image, UltraGig-6400, 500mW

• Power consumption < 15mW

• Distance < 50cm

• Simpler Modulation

• OOK

Commercial High Speed Links

60 GHz Radio

Aggarwal, Taghivand, Rajavi, and Poon, Stanford

60 GHz Radio

0.010.1

110

10 100 1000

TX P

ower

(mW

)

Data Rate (Mb/s)

10cm

2 Gb/s at 10 mW

Die Photo Block Diagram -2.7 dBi Antenna Gain

FR4 PCB

On-chip Dipole

Thinned to 100µm from 600µm

Max gain: 1.6dBi

On Chip Antenna Gain

Metal Reflector at lambda/4

FR4 PCB

λ/4 On-chip Dipole Thinned to 100µm

Max gain: 5.2dBi

Add Metal Reflector

Metal Reflector

FR4 Dielectric lens

FR4 PCB

λ/2

λ/4 On-chip Dipole Thinned to 100µm

Max gain: 9.1dBi

Add Dielectic Lens

Flexible Coils Printed on Fabric

Printed Electronics

J. Corea, B. Lechene, A. Flynn, A.C. Arias, M. Lustig UCB

Printed Electronics

Flexible Receive Array

Two Four-Channel Printed CoilsEight channel printed coil array (left)

Twelve Channel Commercial Array Coil (right)

Receive Array Performance

Eight Channel Printed Coil

Twelve Channel CommercialArray Coil

4-Channel Neonate Coil

Four Channel NeonateSwaddle Coil

4D Flow Images5X acceleration

Conclusions

Conclusions

LotsofantennasinanMRIsystem

LotsofborrowedHamRadiotechnology

•MikiLustig,KK6MRI(UCB)

•AnaClaudiaArias(UCB)•JoesphCorea,KK6KKE(UCB)•AnitaFlynn,KI6LO(UCB)•BalthazarLechene(UCB)

•ThomasGrafendorfer(GE)

•FraserRobb(GE)

•GreigScott(Stanford)•PascalStang,KF6PJZ(Stanford)•ShreyasVasanawala(Stanford)•JosephCheng,KK6QYY(Stanford)•JonathanLu,KK6QZH(Stanford)•KellyByron(Stanford)•SawsonTaheri,KG6NUB(Stanford)•KamalAggarwall(Stanford)

•AdaPoon(Stanford)•AndreasPort(ETH) Grant Support: R01EB008108, P41EB015891,

P01CA159992, R01EB009690, R01EB019241, and GE Healthcare

Thanks!

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