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Antenna Lab
Microwave Antennas forMicrowave Antennas forMedical ApplicationsMedical Applications
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
]