Seminar 6. Interaction of electromagnetic radiation with biological systems

Transcription

1 Seminar 6 Interaction of electromagnetic radiation with biological systems Ionizing and non-ionizing electromagnetic radiation tissue absorption. Overview of biological effects. Forms and characteristics of lasers. Application of lasers in medicine. Biological effects of ionizing radiation. Dosimetric quantities and units. Radiation measurement. Personnel exposure and protection. S6 1

2 Physical background Generation of electromagnetic (EM) radiation deacceleration of electric charge energy emission Faraday s (electromagnetic induction) law AC electric field always produces a magnetic field, and AC magnetic field always produces an electric field S6 2

3 Remark: Electricity and magnetism are related phenomena In some cases electric or magnetic fields dominate EM radiation propagates in vacuum An interaction with EM field is possible without direct contact with the field source S6 3

4 Quantitative description of EM radiation energy transport may be described as a series of waves or as a flux of photons Wave wavelength, frequency c λ = ν = ν c λ Photon energy, frequency E = h*ν Practical formula photon energy wave length h = 6.62*10-34 J*s Planck s constant c = 3*10 8 m/s speed of light, 1 ev = 1.6*10-19 J 1240 E(eV) = λ(nm) Example Sem6/1 λ = ~( ) nm E ( ) ev S6 4

5 EM radiation spectrum many sources (nucleus star) of the EM radiation Non-ionizing electromagnetic radiation energy too small to induce atom ionization (~10 ev ~120 nm) Remark: Ionization is material-dependent does not exist one value of energy (frequency, wavelength) S6 5

6 1 st region (Long waves)/radio/(microwaves) EM radiation source in the 1 st region antenna Generator power supply which supplies the antenna with AC electrical potential (V = V0*sin[2π*ν*t] ν frequency) or AC current (I = I0*sin[2π*ν*t]) Antenna a device to emit the power delivered by the generator Different types of antennas are in common use the construction of the antenna to maximize emission of the radiation Two basic types of transmitting antenna 1) Dipole antenna 2) Loop antenna Dipole/loop antenna alternating electric/magnetic field is induced in the vicinity of the antenna S6 6

7 The antenna produces radiation with properties that vary depending on the distance from the antenna 3 regions are distinguished in the vicinity of an antenna 1) Near (induction) field 2) Intermediate field 3) Far field S6 7

8 1) Near field the region closest to the antenna electric or magnetic fields dominates outer limit of the near field D n = λ 2π Description of the EM field E or B 2) Intermediate field the region starts at the end of the near field 3) Far field starts at the end of the intermediate field EM radiation electric and magnetic fields are symmetrical EM radiation is described by the energy flux onset of the far field DF = ~10*λ Description energy flux (W/m 2 ) S6 8

9 Strength of the EM fields Source Frequency Intensity Natural background EM smog Lamp (100 W, 30 cm) Whole range ( ) V/m (30 70) µt 50 Hz ~150 V/m Home appliances 50 Hz 60 Hz max 250 V/m ~10 µt High-voltage cable 50 Hz 60 Hz ~10 kv/m Microwave oven 2.45 GHz max 50 W/m 2 RF transmission < 300 MHz max 1 W/m 2 Radar GHz max 100 W/m 2 Portable phone 1800 MHz max 1 W/m 2 Magneto-therapy max 100 Hz max ~10 mt Short wave diathermy Microwave diathermy 27 MHz 2 kv/m (in air) 2.45 GHz 2 kv/m (in air) S6 9

10 1st region Biological effect depends on the strength and frequency of the EM radiation A) Low frequency (< tens khz) effects Electro-therapy 1) Bellow 0.1 Hz (practically DC current) galvano-therapy iontophoresis (ion migration) local anaesthetics and certain therapeutic drugs through the skin 2) Frequency < 100 Hz neural stimulation (rheobase - threshold current) and muscle spasm triangular and rectangular pulse diadynamic therapy (Bernard currents) sinusoidal pulses transcutaneous electrical nerve stimulation (TENS) 3) Frequency ~4 khz (Nemec currents) interference in the tissues of two medium frequency AC currents S6 10

11 4) Magneto-therapy and magneto-stimulation frequency < 100 Hz, intensity < ~10 mt Magneto-therapy Remark: Cause-to-effect relationship for low frequencies EM field is an open question S6 11

12 B) High frequency (> tens khz) heating of the tissue dominates Quantitative description EM field induces ionic current Ohm s law for frequency < ~1 MHz Quantitative description of the heating of the tissue power deposited in the tissue P = U*I = R*I 2 Remark: In case of point electrode the electric current is additionally modified due to geometry Sphere surface = 4π*r 2 Electric current decreases I ~ 1/r 2 Power deposited in the tissue P = R*I 2 ~ 1/r 4 S6 12

13 Electro-surgery is the application of a high-frequency electric current to cut, coagulate or desiccate tissue Electro-surgery is performed using an electrosurgical generator and a hand-piece including one or several electrodes, sometimes referred to as an RF knife S6 13

14 RF ablation of liver (kidney, lung) tumor S6 14

15 S6 15

16 Thermo-ablation (radio-frequency ablation RFA) high temperature (up to ~90 0 C) cancer therapy Generator (460 khz) induces flow of electric current heating of the tissue Current density is the biggest close to the electrode biggest heating Remark: Power deposited in the tissue P = R*I 2 ~ 1/r 4 S6 16

17 At frequency > ~1 MHz we are dealing with fast oscillations of ions (polar molecules) to describe heating of the tissue the absorption law may be used P(x) = P(0)*exp( - b*x) P(x) power density (W/m 2 ) at depth x P(0) initial power density b absorption coefficient x thickness of the medium Penetration depth (PD) thickness = 2/b b*x = 2 P(PD) P(0) = exp( 2) = Remark: 86.5% energy is absorbed in the thickness equal to PD Penetration depth (PD) for 2.45 GHz (microwave oven) Material PD (cm) Muscle 1.7 Fat 8.1 S6 17

18 Sometimes heating of the tissue is described using specific absorption rate (SAR) T SAR = c* t c specific heat (J/K*kg) T increase of the temperature (K) t inspection time (s) [SAR] = W/kg S6 18

19 Therapeutic application of high frequency EM fields Thermal destruction of tissue depends on temperature and time 60 0 C for 30 min kill all cells Diathermy works by heating body tissue to elevated temperature bellow the killing effect 1. Short wave diathermy (ν = 27 MHz, λ = m, E = 2 kv/m) volume heating 2. Microwave diathermy (ν = 2.45 GHz, λ = 12.2 cm, E = 2 kv/m) surface heating Short wave diathermy unit (Dn = 1.76 m near field limit) S6 19

20 Microwave diathermy (Dn = 1.9 cm near field limit)) Remark: Permanent control of skin temperature is necessary S6 20

21 Radiation source in the 2 nd region IR/VIS/UV laser Light Amplification by Stimulated Emission of Radiation Forms and characteristics of lasers 1) Active medium wavelength Remark: Active medium Wavelength (nm) Ruby 694 CO Nd:YAG 1060 Ho:YAG 2090 He-Ne 633 Ionic (Ar +, Kr + ) Semiconductor (diode) (gallium-arsenide GaAs) Excimer (XeF, XeCl, KrF noble gas + chloride) CO2 CO2 + N2 + H2 + He S6 21

22 2) Emitted power (energy/time) 1 mj in one short pulse (10-9 s) power = 10 6 W in the pulse average power = 10-3 W 3) Power density (power/cm 2 ) energy flux Natural collimation only photons parallel to the axis of the laser are produced S6 22

23 Application of lasers in medicine Biophysical background absorption of laser radiation (UV/VIS/IR) macroscopic description I(x) = I0*exp( -µ*x) Absorption for different tissues Penetration depth (PD) in the soft tissue Laser PD (mm) Wavelength (nm) CO Ar Nd:YAG KrF O Remark: KrF laser photo-ablation constitutes the mechanism of interaction S6 23

24 Microscopic description different effects 1) Photochemical interaction and bio-stimulation laser radiation can induce chemical effects and reactions within macromolecules in tissue empirical observation Photodynamic therapy Photo-sensitizer hemato-porphyrin derivative (HpD) remains inactive until irradiation S6 24

25 2) Thermal interaction (surgery) S6 25

26 Photo-ablation of corneal tissue S6 26

27 Cornea surgery (LASIK - laser in situ keratomileusis) S6 27

28 4) Laser lithotripsy is a surgical procedure to remove a kidney stone, ureteral stone or bladder stone The location of the calculi Ho:YAG laser (λ = 2090 nm) the light energy of the laser is transported through a flexible light guide to the stone Ho:YAG laser pulses create a shockwave that causes fragmentation of stones the stone residues are flushed out utilizing the endoscopes rinsing fluid S6 28

29 Ionizing EM radiation interaction with matter macroscopic description I(x) = I(0)*exp( -µ*x) = I(0)*exp( -(µ/ρ)*(ρ*x)) µ linear absorption coefficient [1/cm] x = thickness [cm] µ/ρ mass absorption coefficient [cm 2 /g] ρ*x areal density = thickness*density [g/cm 2 ] Remark: µ contains both a probability of interaction with one "centre" and a number of "centres" in the unit volume µ/ρ contains only a probability of interaction with one "centre" S6 29

30 Dosimetric quantities and units Dosimetry is the scientific methodology used to quantify the energy absorbed in human tissues as a result of the irradiation with the use of the ionizing radiation as well as to estimate the biological effect of the irradiation Source of ionizing radiation source emits energy in form of photons or particles interaction with a biological object produces finally absorption of energy within the object quantitative description of interaction of the radiation with the object is based on the amount of energy deposited in the object Interaction of the ionizing radiation with biological object three step process: physical interaction quantitative description bio-chemical reactions" biological effects S6 30

31 First step physical interaction Description of the radiation source exposure to radiation absorption in air (reference object) exposure unit roentgen (R) Roentgen is the quantity of radiation which will release an electric charge 2.58*10-4 C in 1 kilogram of dry air 1 R = 2.58*10-4 C/kg Energy deposited in the tissue the radiation dose Absorbed dose (D) is measured in terms of the energy absorbed per unit mass of tissue unit gray (Gy) 1 Gy = 1 J/(kg of tissue) Remark: The dose and the exposure are determined experimentally S6 31

32 Different types of the ionizing radiation different mechanisms of energy transfer different severity of bio-chemical and biological effects correction for 2 nd and 3 rd steps Correction of the dose to include different mechanisms of the interaction of different types of the ionizing radiation Dose equivalent (H) is that dose which gives the same risk of damage to health whatever the type of radiation unit sievert (Sv) Dose equivalent (Sv) = (absorbed dose (Gy))*factor The factor is called the radiation weighting factor and depends on the type of radiation (X-rays and γ-rays = 1) Remark: Sv = J/(kg of tissue) i.e. the same as Gy S6 32

33 Effective dose equivalent (HE) sensitivity of various organs to the ionizing radiation is different the effective dose equivalent represents the whole-body dose equivalent with the same risk of damage as irradiation of individual organs unit of HE = Sv HE = i w i H i Tissue/Organ wi Gonads 0.20 Bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Brest 0.05 Liver 0.05 Esophagus 0.05 Thyroid 0.05 Skin 0.01 Bone surface 0.01 Remainder 0.05 Total 1.00 S6 33

34 Natural radiation background Effective doses equivalent (HE) from different sources (1 year) natural radiation Source of radiation HE (msv) Cosmic radiation ~0.5 Natural radioactive material in ~0.5 ground (e.g. U) Natural radioactive material in the ~0.5 body (e.g. 40 K) Inhaled (Rn) ~1.0 Medical (e.g. chest radiograph) ~0.5 Total 1 year HE ~3.0 Basic exposure recommendation (national regulations) effective dose equivalent (HE) Polish regulations Dose classification Maximum dose to the general population in 1 year Maximum dose to people who work with radiation in 1 year Maximum dose to people who work with radiation 5 year average 50 Lethal dose LD 30 (50% of population will die after 30 days) HE (msv) S6 34

35 Remark: Dose equivalent (H) for the specific organs or tissues Organ/Tissue H (msv) Eyes 150 Skin 500 Hands 500 S6 35

36 Radiation measurement Total dose = (dose rate)*(exposure time) Dose Dose rate = Time The relation is valid for absorbed dose and dose equivalent Absorbed dose rate [Gy/h, mgy/h, µgy/h] Dose equivalent rate [Sv/h, msv/h, µsv/h] Standard instrument for determination of the dose rate is ionization chamber (radiometer) Dose rate Two metal electrodes DC power supply (~100 V) gas inside the chamber (mixture) ionization electric charge produces by the radiation in the unit time is measured calibration absorbed dose rate (mgy/h, µgy/h) or dose equivalent rate (msv/h, µsv/h) disadvantage is the volume of the chamber (~1 cm 3 ) S6 36

37 Professional contact with the ionizing radiation personal radiometer (dosimeter) Film dosimeter (film badge) Photographic film absorption of radiation induces identical effect as absorption of light (photography) developing darkness of the film is proportional to the dose absorbed in the film calibration procedure the exposure to radiation may be determined personnel control Film badge is used to monitor personnel over the extended period of time (1 3 months) S6 37

38 Personal dosimeter thermo-luminescence effect Ring dosimeter hand dose Whole body dosimeter S6 38

39 Personal dosimeter direct-reading dosimeter S6 39

40 Personnel exposure and protection Diagnostic or therapeutic irradiations irradiated area is limited to a part of human body personnel irradiation due to scatter radiation side effect personnel exposure General rule ALARA Factors affecting personnel exposure 1) Shielding depends on type of the radiation Lead apron ( ) mm Pb S6 40

41 Protective glasses 2) Distance from the source of the radiation exposure decreases with distance (inverse-square effect) Constant Exposure rate (mr/h) = 2 d d distance constant depends on many factors S6 41