Microwave Antennas for Medical Applications

Transcription

1 Seoul, Korea 6 Sept. 27 Microwave Antennas for Medical Applications IEEE AP-S Distinguished Lecturer Koichi ITO Department of Medical System Engineering Chiba University, Japan k-ito@ieee.org

2 Medical applications of antennas Information transmission - Wireless capsule endoscopy -RFID/ Implantable monitor Diagnosis -MRI/ fmri - Microwave CT / Radiometry Treatment - Thermal therapy - Microwave knife

3 Background Implantable devices used in wireless communications An antenna is an important element for implantable devices. Requirements for the antenna are as follows: Safety Miniaturization Mechanically robustness High radiation efficiency Low SAR (Specific Absorption Rate) It is necessary to design an antenna properly for implantable devices.

4 Implantable monitoring system Link budget in sickroom Transmit power Human body Frequency Transmit power EIRP Tx antenna gain Distance Bit rate Path loss Link C/N Required C/N 2.45 GHz dbw dbw -24. dbi 4 m 7 kbps 5.77 db 5.79 dbhz 5.55 dbhz Input power (25 μw) External antenna (reception) Sickroom Radiation efficiency (.35 %) Communication possible for gain > -24 dbi Implanted antenna (transmission)

5 H-shaped cavity slot antenna and human arm Empty (Air) Cavity Dielectric (ε r = 2.17) z θ x φ y H-shaped slot 5.2 (8.4) Uniform (2-layer) Unit [mm] Operating frequency:2.45 GHz Human model analyzed: between the shoulder and the elbow

6 Numerical calculation model (2-layer) z θ Dielectric (ε r = 2.17) y x φ Antenna (Direction 1) 2) Antenna (Direction 1) 2) Global view Skin:ε r = 38.1 σ = 1.16 S/m Fat:ε r = 5.28 σ =.1 S/m Muscle:ε r = σ = 1.74 S/m Side view Unit [mm]

7 Radiation characteristics (2-layer human model) 45 Direction 1( E φ ) Direction 2( E θ ) θ θ dbi dbi [dbi] Human model Radiation efficiency [%] Uniform xz-plane layer.33 Maximum gain : dbi (Direction 1) and dbi (Direction 2) Gain > -24 dbi : 14 deg (Direction 1) and 16 deg (Direction 2)

8 SAR (Specific Absorption Rate) distribution Input power: 25 μw y [mm] xy-plane x [mm] Peak SAR value :.63 W/kg Antenna 1 z [mm] yz-plane y [mm] Even peak SAR value satisfies the standard value of ANSI (1.6 W/kg) and ARIB (2. W/kg) SAR [W/kg]

9 Measurement of the implanted antenna Dielectric (ε r = 2.17) Inner conductor Coaxial cable Calculation model Original experiment model 1. The antenna dimension is changed 2.5 times as realistic antenna for easy fabrication Scaled experiment model

10 Measurement with phantom Network analyzer Composition of the 2/3- muscle equivalent phantom Material Deionized water Ager Polyethylene powder Sodium chloride TX Dehydroacetic acid sodium salt Amount [g] System of measurement

11 S 11 and radiation patterns Calc θ Meas θ Calc Meas -3-4 S 11 [db] [dbi] Frequency [GHz] Peak gain Calc: dbi, Meas: dbi 1

12 SAR distribution.2 Calc Meas Observation line SAR [W/kg] x [mm] Measurement: Thermographic method (Input power: 2W, Radiation time: 1s) 11

13 Block diagram of MRI system Static magnetic field generator (superconductive coil or permanent magnet) RF coil = Antenna Tuner Receiver (Detector and amplifier for NMR signal) Switch Transmitter (Generator and amplifier for RF pulse)

14 TEM coil for high H-field MRI system Unit: mm 8 elements TEM coil Analytical model of MoM

15 H-field distributions (27 MHz) y [mm] Feeding point y [mm] Feeding point y [mm] x [mm] 13 Feeding points y [mm] z [mm] Feeding points Two feeding points (quadrature excitation) generates uniform H-field H-field [db]

16 Calculation model (bird cage coil) RF shield Capacitor x Bird cage coil z y 74 6 z x y 7 RF shield 126 Feeding point Operating frequency: 64 MHz (for 1.5 T MRI system)

17 Realistic woman models Model A Non-pregnant model Model B Pregnant model Pregnant woman in early period 7-month pregnant woman model Realistic high-resolution whole-body voxel model of a Japanese female* Model A + fetus model *T. Nagaoka et al., Physics in Medicine and Biology, vol. 49, pp. 1-15, 24.

18 FDTD calculation model Calculation conditions SAR 77 σ ρ 2 = E [W/kg] Unit: [mm] E: Electric field (r.m.s.) [V/m] σ : Conductivity of the media [S/m] ρ : Density of the media [kg/m 3 ] Cell size FDTD space Frequency Absorbing boundary condition mm MHz PML (8 layers)

19 B-field (magnetic flux density) -4 Model A: non-pregnant model Model B: pregnant model y [mm] Magnetic flux density [db] x [mm] x [mm] Almost uniform distributions are observed.

20 SAR distributions (model A) Sagittal plane 8 6 Coronal plane Above z [mm] SAR [W/kg] Coil -8 RF-Shield y [mm] x [mm] 4 Normalized by 1. W radiation

21 SAR distributions (model B) Sagittal plane 8 6 Coronal plane Above z [mm] SAR [W/kg] Coil -8 RF-Shield y [mm] x [mm] 4 Normalized by 1. W radiation

22 Hyperthermia o C Decrease of the ratio of survival cancer cells Little damage to normal cells Effect of radiotherapy or chemotherapy enhanced Heating schemes Internal heating (interstitial / intracavitary) External heating

23 Radio Frequency Ablation (RFA) Microwave Coagulation Therapy (MCT) o C 6 o C > Ablation / Coagulation Treatment time: a few minutes o C Hyperthermia Treatment time: 3-6 minutes Target: small size cancer (< 3 cm) (e.g. hepatocellular carcinoma) Merits: minimally invasive short time heating

24 Major energy sources for heating $ Electromagnetic wave - RF (Radio Frequency) heating < 1MHz - Microwave heating > 1MHz $ Hot sources ( Thermal conductivity) - Ferromagnetic seed - Hot water, hot needle, etc. $ Ultrasound - HIFU (High-Intensity Focused Ultrasound) $ Others - Laser, infrared, etc.

25 Microwave heating Interstitial applicator Non-invasive applicator Tumor Tumor Human body Internal heating Human body External heating

26 Microwave external heating system Data recorder Control circuit Thermometer Cooling water Microwave generator Matching circuit Applicator Bolus Thermo sensor Without surface cooling Temp. With surface cooling Temp. Depth Depth

27 Minimally invasive microwave thermal therapies Interstitial microwave hyperthermia Microwave coagulation therapy Microwave catheter ablation Thermal treatment of BPH (BPH: Benign prostatic hypertrophy) Treatment of cancer Treatment of cardiac disease

28 Microwave interstitial heating system Microwave generator Power divider Control unit Thermometer Thin coaxial antennas Data recorder Tumor Heating for deep seated or large-volumed tumor Combined with interstitial radiation therapy

29 Combined therapy Interstitial microwave hyperthermia Interstitial radiation therapy Thin antenna Common catheter Radiation source Tumor Tumor Synergistic effect of two treatments Treatment of unradiocurable tumor

30 Coaxial-slot antenna Feeding point (2.45 GHz) [cm] [inch] 14 Air Biological tissue Coaxial cable Catheter Slots Short-circuit Longitudinal cross section of the antenna tip Unit : [mm]

31 Procedure of calculations Construction of calculation model FDTD method Electric field: E SAR σ ρ 2 = E [W/kg] Calculation for SAR distribution SAR distribution Bioheat transfer equation ρ c T t = κ 2 T ρ ρ b c b F ( T T ) + ρ SAR b Calculation for temperature distribution Temperature distribution: T

32 Calculation model of the antenna Feeding point.5 mm Antenna.5 mm Transverse cross section Air x Biological tissue y z 1. mm Calculated region Longitudinal cross section

33 SAR distributions of array applicator 16 4 y z 2 2 x Air Muscle ε r =47. σ =2.21 S/m Obs. plane (i) Obs. plane (ii) Obs. plane (i) Obs. plane (ii) z [mm] y [mm] x [mm] High SAR Low Unit: [mm] Each element fed in-phase and with the same amplitude

34 Temperature distributions x Air Muscle Obs. plane (i) Obs. plane (ii) z [mm] y [mm] o C 42 o C above Temperature [ o C] 4 y 63 z Obs. plane (i) Obs. plane (ii) 63 Unit: [mm] x [mm] Total net input power: 2. W Blood flow rate: m 3 /kg s Initial temperature: 37. o C Heating time: 3 s

35 Actual treatment with 4 antennas Thermo sensors Ichikawa General Hospital, Tokyo Dental College, Chiba, Japan (September 22, 23)

36 Details of the treatment Power off Antenna insertion depth (2) (1) or 3 25 (3) 2 Temperature [ o C] (2) (3) (1) (3) (2) (1) 42 o C Depth of temperature measurement point Unit: [mm] Time [min] Total net input power: 18. W

37 TARGET: Bile duct carcinoma Endoscope Endoscope Coaxial-slot antenna Liver Antenna Forceps channel Antenna Target Common bile duct Duodenum Stomach Pancreas Papilla of Vater Tip of endoscope Non-invasive treatment

38 Structure of the coaxial-slot antenna Feeding point 2.45 GHz Heat shrink tube φ 1.8 mm Slot Short-circuit Flexible coaxial cable 1. mm 2 m 1. mm Entire length approx. 2 m Tip of the antenna Heat shrink tube Prototype antenna Flexible and long structure

39 Simple calculation model Antenna Target Inferior vena cava Hepatic portal vein Inferior vena cava φ 27 Hepatic portal vein φ y z Muscle Bile duct φ 5 x Bile Coaxial-slot antenna Stricture Unit: [mm] Cooling effect due to two blood vessels is considered.

40 Calculated temperature distributions Observation plane 1 y [mm] -2 2 above 55 Conditions: Net input power:1. W Heating time: 3 s Initial temperature: 37 o C Observation plane 1 Observation plane 2 Observation plane 2 z [mm] Inferior vena cava Hepatic portal vein 2-2 x [mm] Temperature [ o C] The bile duct can be treated.

41 Simple experimental model for temperature distribution measurement Target Bile duct Inferior vena cava Hepatic portal vein Hepatic portal vein φ 2 mm Coaxial-slot antenna Observation plane Antenna Muscle

42 A simple dynamic phantom for temperature distribution measurement Antenna Saline solution 15 Coaxial-slot antenna Cylindrical hole Slot Observation plane (a) 3-D view Rubber sheet z y 75 x 4 5 Cylindrical hole Rubber sheet Unit : mm (b) Cross-section - Cooling effect is realized by flowing the saline solution. - Water flow can be controlled by the hole in the rubber sheet.

43 y [mm] Comparison of the effect of blood vessel With blood flow Without blood flow 4 3 Antenna Antenna ΔT = 5 o C ΔT = 5 o C Temperature rise [ o C] x [mm] x [mm] Heating region is observed under the cooling effect.

44 Realistic calculation model Liver Gallbladder Stomach 6 6 Bile duct 6 Duodenum Small intestine z y x Bile duct Unit: [mm] Realistic human model from Brooks Air Force Laboratories Organs around the bile duct are considered.

45 Calculated temperature distributions Temperature [ o C] above Net input power:5. W Heating time: 6 s Initial temperature: 37 o C y [mm] Small intestine Large intestine Blood vessel Bile duct 42 o C Muscle Blood vessel x [mm] Bile duct can be heated under the real conditions. Fat Pancreas z [mm] Bile duct Large intestine Small intestine Bile duct Blood vessel 42 o C Pancreas Fat x [mm]