Antenna Lab

Microwave Antennas forMicrowave Antennas forMedical ApplicationsMedical Applications

k-ito@ieee.org

Koichi ITODepartment of Medical System Engineering

Chiba University, Japan

Seoul, Korea 6 Sept. 2007

IEEE AP-S Distinguished Lecturer

Antenna Lab

• Information transmission- Wireless capsule endoscopy- RFID / Implantable monitor

• Diagnosis- MRI / fMRI- Microwave CT / Radiometry

• Treatment- Thermal therapy- Microwave knife

Medical applications of antennas

Safety

Miniaturization

Mechanically robustness

High radiation efficiency

Low SAR (Specific Absorption Rate)

Implantable devices used in wireless communications

An antenna is an important element for implantable devices. Requirements for the antenna are as follows:

It is necessary to design an antenna properly for implantable devices.

Background

Frequency 2.45 GHzTransmit power -71.25 dBWEIRP -98.75 dBWTx antenna gain -24.0 dBiDistance 4 mBit rate 7 kbpsPath loss 50.77 dBLink C/N0 50.79 dBHzRequired C/N0 50.55 dBHz

Implanted antenna (transmission)

External antenna (reception)

Implantable monitoring system

Communication possible for gain > -24 dBi

Input power (25 μW)

Human body

Radiation efficiency (0.35 %)

Transmit power

Link budget in sickroom

Sickroom

x

z

φ

θ

y

2.8

Dielectric (εr = 2.17)

5.2 (8.4)3.2

Unit [mm]

H-shaped cavity slot antenna and human arm

Human model analyzed: between the shoulder and the elbow

Operating frequency：2.45 GHz

Empty (Air)

Uniform (2-layer)

Cavity

H-shaped slot

Skin：εr = 38.01 σ

= 1.16 S/mFat：εr = 5.28 σ

= 0.10 S/m

Muscle：εr = 52.73 σ

= 1.74 S/m

70

300

4Antenna (Direction 1) Antenna

(Direction 2)44

54

Unit [mm]

Antenna (Direction 1)

70

Antenna (Direction 2)

Numerical calculation model (2-layer)

Side view

Global view

x

z

φ

θ

y

Dielectric (εr = 2.17)

2.8

8.43.2

Direction 1( Eφ

)Direction 2( Eθ

)

Radiation characteristics (2-layer human model)

Maximum gain : -18.2 dBi (Direction 1) and -19.2 dBi (Direction 2)Gain > -24 dBi : 104 deg (Direction 1) and 160 deg (Direction 2)

0

4545

90 90

135135

180

-10

[dBi]

-20-30-40-50

θθ

160 °104 °

xz-plane

-18.2 dBi-19.2 dBi

Human model

Radiation efficiency [%]

Uniform 0.39

2-layer 0.33

y [m

m]

-5

0

10

5

-100-5 5-10 10

0.7

0

0.50.40.30.20.1

0.6

SA

R [W

/kg]

Antenna

0-5 5-10 10

Peak SAR value : 0.63 W/kgEven peak SAR value satisfies the standard value of ANSI

(1.6 W/kg) and ARIB (2.0 W/kg)

SAR (Specific Absorption Rate) distribution

xy-plane yz-plane

y [mm]

z [m

m]

-5

0

10

5

-10

x [mm]

Input power: 25 μW

Measurement of the implanted antenna

Dielectric (εr = 2.17)

2.8

5.2 3.2

Coaxial cable

Calculation model Original experiment model

Inner conductor

4.0

4.01.6

Scaled experiment model

10.0

10.0 4.0

The antenna dimension is changed 2.5 times as realistic antenna for easy fabrication

Measurement with phantom

0.3Dehydroacetic acid sodium salt4.7TX - 1512.4Sodium chloride

156.1Polyethylene powder16.1Ager

520.4Deionized waterAmount [g]Material

0.3Dehydroacetic acid sodium salt4.7TX - 1512.4Sodium chloride

156.1Polyethylene powder16.1Ager

520.4Deionized waterAmount [g]Material

System of measurement

Composition of the 2/3- muscle equivalent phantom

Network analyzer

0

4545

90 90

135135

180

θθ-10

[dBi]

-20

-30

-40

-50

Calc Meas

Peak gainCalc: -21.9 dBi, Meas: -18.6 dBi

10

0

-15

S11

[dB

] -5

Frequency [GHz]1.00.5 1.5

-10

-202.00

MeasCalc

S11 and radiation patterns

11

Observation line

0.02

0.005

0.015

0.010

0

SA

R [W

/kg]

x [mm]0-10 -5-15-20-25 20 2515105

MeasCalc

SAR distribution

Measurement: Thermographic method(Input power: 20W, Radiation time: 10s)

Tuner

Switch

Receiver(Detector and amplifier for NMR signal)

Transmitter(Generator and amplifier for RF pulse)

Static magnetic field generator(superconductive coil or permanent magnet)

RF coil = Antenna

Block diagram of MRI system

Unit: mm

180

240

207

8 elements TEM coil

TEM coil for high H-field MRI system

Analytical model of MoM

-10

-20

-30

-40

-50

H-fi

eld

[dB

]

0

H-field distributions (270 MHz)

-130

0

-65

65

130

y[m

m]

Feedingpoint

x

[mm]130650-65-130

-130

0

-65

65

130

y[m

m]

Feedingpoints

y[m

m]

130

65

0

-65

-130

y[m

m]

130

65

0

-65

-130

0 75 150-75-150z [mm]

Feedingpoint

Feedingpoints

“Two feeding points (quadrature excitation)”

generates uniform H-field

Calculation model (bird cage coil)

Operating frequency: 64 MHz (for 1.5 T MRI system)

x

yz

Capacitor

Feeding point

RF shield 1260

700

600

740

Bird cage coil

RF shield

x

y

z

Realistic woman models

Realistic high-resolution whole-body voxel modelof a Japanese female*

Pregnant modelModel BModel A

Non-pregnant model

Model A + fetus model

*T. Nagaoka et al., Physics in Medicine and Biology, vol. 49, pp. 1-15, 2004.

7-month pregnantwoman model

Pregnant womanin early period

FDTD calculation model

770

770

1670

Unit: [mm]

E: Electric field (r.m.s.) [V/m]σ : Conductivity of the media [S/m]ρ : Density of the media [kg/m3]

2SAR Eρσ

= [W/kg]

Calculation conditions

Cell size

FDTD space

Frequency 64 MHz

Absorbing boundarycondition PML (8 layers)

mm222 ××

835385385 ××

B-field (magnetic flux density)

Almost uniform distributions are observed.

4002000-200-400x

[mm]

400

200

0

-200

-400

y[m

m]

-50

-30

-20

-10

-40

0

Mag

netic

flux

den

sity

[dB

]

4002000-200-400x

[mm]

Model A: non-pregnant model Model B: pregnant model

SAR distributions (model A)

4002000-200-400y

[mm]

0

-200

600

z[m

m]

-800

400

800

200

-400

-600 Coil

RF-Shield

4002000-200-400x

[mm]

Normalized by1.0 W radiation

0.00

0.02

0.04

0.06

0.08Above

SA

R [W

/kg]

Sagittalplane

Coronalplane

SAR distributions (model B)

4002000-200-400y

[mm]

z[m

m]

0

-200

600

-800

400

800

200

-400

-600 Coil

RF-Shield

4002000-200-400x

[mm]

Normalized by1.0 W radiation

0.00

0.02

0.04

0.06

0.08Above

SA

R [W

/kg]

Sagittalplane

Coronalplane

Antenna Lab

Hyperthermia

Decrease of the ratioof survival cancer cells

Effect of radiotherapy or chemotherapy enhanced

oC

Heating schemesInternal heating (interstitial / intracavitary)External heating

Little damageto normal cells

Antenna Lab

42 - 45 oC Hyperthermia

60 oC > Ablation / CoagulationTreatment time: a few minutes

Treatment time: 30 - 60 minutes

oC

Target: small size cancer (< 3 cm) (e.g. hepatocellular carcinoma)

Merits: minimally invasiveshort time heating

Radio Frequency Ablation (RFA)Microwave Coagulation Therapy (MCT)

Antenna Lab

$ Electromagnetic wave- RF (Radio Frequency) heating < 100MHz- Microwave heating > 100MHz

$ Hot sources ( Thermal conductivity)- Ferromagnetic seed- Hot water, hot needle, etc.

$ Ultrasound- HIFU (High-Intensity Focused Ultrasound)

$ Others- Laser, infrared, etc.

Major energy sources for heating

Antenna Lab

Human body

Tumor

Interstitialapplicator

Microwave heating

Non-invasiveapplicator

Tumor

Human body

Internal heating External heating

Antenna Lab

Tem

p.

Depth

Withoutsurface cooling

Microwave external heating system

Microwavegenerator

Matchingcircuit

Controlcircuit Thermometer

Coolingwater

Datarecorder

Applicator

Bolus

Thermosensor

Withsurface cooling

Tem

p.Depth

Antenna Lab

Minimally invasivemicrowave thermal therapies

Microwave catheter ablationThermal treatment of BPH(BPH: Benign prostatic hypertrophy)

Interstitial microwave hyperthermiaMicrowave coagulation therapy

Treatment of cancer

Treatment of cardiac disease

Antenna Lab

Microwavegenerator

Controlunit

Tumor

Thin coaxial antennas

Data recorder

Thermometer

Powerdivider

Heating for deep seated or large-volumed tumorCombined with interstitial radiation therapy

Microwave interstitial heating system

Antenna Lab

• Synergistic effect of two treatments• Treatment of unradiocurable tumor

Tumor

Common catheter

Interstitial microwavehyperthermia

Thin antenna

Interstitial radiationtherapy

Tumor

Radiation source

Combined therapy

Antenna Lab

[cm] [inch]Feedingpoint (2.45 GHz)

140

Coaxial cableCatheter

Slots

Short-circuit

1.21.8

AirBiologicaltissue

Longitudinal cross section of the antenna tip

1.0

1.0

10.0

20.0

Unit : [mm]

Coaxial-slot antenna

Antenna Lab

Procedure of calculationsConstruction of calculation model

Electric field: E

SAR distribution

Temperature distribution: T

FDTD method

2SAR Eρσ

= [W/kg]

Bioheat transfer equation

( ) SAR2 ⋅+−−∇=∂∂ ρTTFcρρTκ

tTcρ bbb

Calculation forSAR distribution

Calculation fortemperature distribution

Antenna Lab

1.0 mm

Longitudinalcross section

Air

Feedingpoint

Calculated region

x

y zBiologicaltissue

Antenna

0.05 mm

0.05 mm

Calculation model of the antenna

Transverse cross section

Antenna Lab

SAR distributions of array applicator

0 10 20 30-10-20-30x

[mm]

0102030

-10-20-30

y[m

m]

010203040

z[m

m]

Obs.plane (i)

Obs.plane (ii)

SAR

High

Low

Each element fed in-phaseand with the same amplitude

Air

Obs. plane (i)

Unit: [mm]6363

40

160x

20

y z Obs. plane (ii)

Muscleεr

=47.0σ =2.21 S/m

20

Antenna Lab

Temperature distributions

0 10 20 30-10-20-30x

[mm]

0102030

-10-20-30

y[m

m]

010203040

z[m

m]

Obs.plane (i)

Obs.plane (ii)

Air

Obs. plane (i)

Unit: [mm]6363

40

160x

20

y z Obs. plane (ii)

Muscle

20

38

4042

444648

50above

Tem

pera

ture

[o C]

42oC

42oC

Total net input power: 20.0 WBlood flow rate: 8.33×10-6 m3/kg⋅sInitial temperature: 37.0 oCHeating time: 300 s

Antenna Lab

Actual treatment with 4 antennas

Thermo sensors

(September 22, 2003)Ichikawa General Hospital, Tokyo Dental College, Chiba, Japan

Antenna Lab

Details of the treatment

Antennainsertion depth

27

23 25

27

Depth of temperaturemeasurement point

1515

2 or 3

(1)(2)

(3)

18

20

Unit: [mm]

42 oC

30

35

40Te

mpe

ratu

re [o

C]

0 10 20 30

(2)

Time [min]

45

50

(2)

(3)

(1)

(1)

Power off

(3)

Total net input power: 18.0 W

Non-invasive treatment

TARGET: Bile duct carcinoma

Endoscope

Coaxial-slot antenna

Liver

Target

Common bile duct

Pancreas

Papilla of Vater

Stomach

Duodenum

— Endoscope —

Tip of endoscope

Forceps channel

Antenna

Antenna

Structure of the coaxial-slot antenna

Flexible and long structure

φ 1.8 mm

Feeding point2.45 GHz

Flexible coaxial cable

Short-circuit

2 m 10.0 mm

1.0 mm

Slot

Heat shrink tube

Heat shrink tube

Entire length approx. 2 m

— Prototype antenna —

Tip of the antenna

Antenna Lab

Antenna

TargetHepatic portalvein

Inferior venacava

Bile duct φ 5

Muscle

BileCoaxial-slotantenna

62

160

62

xy z

Stricture

Infe

rior v

ena

cava

φ 2

7

Hep

atic

por

tal v

ein

φ17

Unit: [mm]

Simple calculation model

Cooling effect due to two blood vessels is considered.

Antenna Lab

55

37

42 Tem

pera

ture

[oC

]50

45

abovey[m

m]

0

20

-20

z[m

m]

160

120

80

100

60

140

0 20-20x

[mm]

Infe

rior v

ena

cava

Hep

atic

por

tal v

einObservation

plane 1

Observationplane 2

Calculated temperature distributionsObservationplane 1

Observationplane 2

The bile duct can be treated.

Conditions:Net input power：10.0 WHeating time: 300 sInitial temperature: 37 oC

Antenna Lab

Simple experimental model fortemperature distribution measurement

Antenna

Target Hepatic portalvein

Inferior venacava

Bile duct

Muscle

Coaxial-slotantenna

Hep

atic

por

tal v

ein

φ20

mm

Observationplane

Antenna Lab

xz

y

20

Unit : mm15

075

75

Observation plane

Rubber sheet

Coaxial-slot antenna

150Saline solution

5

4

Slot

(b) Cross-section

Cylindrical hole

(a) 3-D viewRubber sheet

Cylindrical hole

Antenna

- Cooling effect is realized by flowing the saline solution.- Water flow can be controlled by the hole in the rubber sheet.

A simple dynamic phantom for temperature distribution measurement

Comparison of the effect of blood vessely

[mm

]

-40

-20

0

20

40Antenna

ΔT = 5 oC

0 20-20-40 40x [mm]

30

20

10

0

Tem

pera

ture

rise

[oC

]Antenna

ΔT = 5 oC

0 20-20-40 40x [mm]

With blood flow Without blood flow

Heating region is observed under the cooling effect.

Antenna Lab

Realistic calculation model

Realistic human model from Brooks Air Force Laboratories

Small intestine

LiverStomach

Duodenum

Gallbladder

Bile duct

x

yz

Organs around the bile duct are considered.

Bile duct

6060

60

Unit: [mm]

Antenna Lab

Calculated temperature distributions

Bile duct can be heated under the real conditions.x

[mm]0-30 30-15 15

y[m

m]

0

30

-30

15

-15

Smallintestine

Bloodvessel

Bloodvessel

Muscle

FatBileduct

PancreasLargeintestine

42 oC

Net input power：5.0 WHeating time: 600 sInitial temperature: 37 oC

60

40

20

0

z[m

m]

x

[mm]0-30 30-15 15

Bile ductBloodvessel

Smallintestine

Largeintestine

Pancreas

Bileduct Fat

42 oC

37

42

above60

Tem

pera

ture

[oC

]