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Transcript of Dosimetry
Dosimetry
Sanja Dolanski Babić March, 2010.
Ultrasound interaction with tissue
Radiation - the process of emitting energy as waves or particles, and the radiated energy
Radioactivity - the property of radionuclides of spontaneously emitting ionizing radiation
Dosimetry – assessment of radiation dose by measuring or calculating and assessment of
biological damages
What’s the difference between radiation and radioactivity?
Sources of radiation – natural and artificial
The half-life is the time taken for half of the atoms of a radioactive substance to decay.
[A]= Bq Bq = 27 x 10-12 Ci
Standard warning sign for a possible radioactive hazard
Pie chart for background radiations
Basic principle of devices for detecting and measuring of radiation
Effectivevolume of detector
Basic detector
gassliquidsolid
Measuring effects of radiation
Fragments of fission particlesX, - radiation
Electrical: el.current or impulseOpticalCalorimetricChemicalBiological
Devices for detecting and measuring of radiation:
1. Devices for track visualisation of radioactive particles – radiographic track and Wilson cloud chamber
3. Dosimeter is a small portable instrument for measuring and recording the total accumulated dose of ionizing radiation a person receives – such as a film badge, thermoluminescent or pocket dosimeter
2. Counters (detectors) – Geiger counter, scintillation detector, semiconductor detector, gamma-camera etc.
Radiation detectors are used in: nuclear physics, medicine, particle physics, astronomy and industry
Ionization Chamber (1898. Marie Curie)
- device used for two major purposes: detecting charged particles in the air (as in a smoke detector) and for detecting or measuring of ionizing radiation- device measures the electric current generated when radiation ionizes the gas in the chamber and therefore makes it electrically
conductive
- electric currents are very low: around 10-15 A - dosimetry in medicine measures very low doses X
and radiation
Principle of an ionization chamber
– higher voltage at the source of current
Geiger-Müller Counter
– imediate “chain” ionization of the gas
– electric current is 1010 times higher than primary electric current
- detects a single particle of ionizing radiation and produce an audible click for each
GM tube – simply gas detector
– GM counter is used to detect usually alpha and beta radiation
Scintillation counterScintillation counter- measures ionizing radiation
- sensor fluoresces when struck by ionizing radiation
- a sensitive photomultiplier tube measures the light from crystal
- sodium iodide scintillation counter detects gamma rays
EXPOSURE (X)– measure for ionization of the air mass of 1kg
X = X = Q/Q/m m [[XX]]=C/kg 1 R = 2.58 x 10=C/kg 1 R = 2.58 x 10-4 -4 C/kgC/kg
- defined only for electromagnetic radiations which energies are smaller than 3 MeV
RADIATION ABSORBED DOSE (D)– measure of increasing internal energy (U) in a mass of 1 kg
tissue (U) caused by absorption of ionizing radiations
D = D = U/U/m m [[DD]]=Gy=J/kg 1Gy = 100 rad=Gy=J/kg 1Gy = 100 rad
- defined for all ionizing radiations
- measuring of absorbed doses are not possible in living tissues
Radiation dose measurements and units
activity in becquerel =disintegrations per second
Radiation quality factors
alpha
beta
gamma
x-rays
neutrons
20
1
1
1
10
20 Sv
1 Sv
1 Sv
1 Sv
10 Sv
radiation factor dose equivalentof 1 gray
dose in gray = energy deposited per kg
dose equivalent in sievert = dose in gray quality factor
energy in different typesof radiation
radiationsource
dose ingray
dose in sivert
- EQUIVALENT DOSE is calculated: DDeqeq = D = D xx H H;;
H is radiation quality factorH is radiation quality factor [[DDeqeq]]= Sv= Sv ; 1 Sv = 100 rem 1 Sv = 100 rem
Radiation dose measurements and units
activity in becquerel =disintegrations per second
Radiation quality factors
alpha
beta
gamma
x-rays
neutrons
20
1
1
1
10
20 Sv
1 Sv
1 Sv
1 Sv
10 Sv
radiation factor dose equivalentof 1 gray
dose in gray = energy deposited per kg
dose equivalent in sievert = dose in gray quality factor
energy in different typesof radiation
radiationsource
dose ingray
Radiation dose measurements and units
activity in becquerel =disintegrations per second
Radiation quality factors
alpha
beta
gamma
x-rays
neutrons
20
1
1
1
10
20 Sv
1 Sv
1 Sv
1 Sv
10 Sv
radiation factor dose equivalentof 1 gray
dose in gray = energy deposited per kg
dose equivalent in sievert = dose in gray quality factor
energy in different typesof radiation
radiationsource
dose ingray
EFFECTIVE EQUIVALENT DOSE: DDeq eq xx f f- each body part has its own weight factor,
f
Whole body 1 (100%)
Ovaries, testes
0.25 (25%)
Bone marrow
0.12 (12%)
Bone surface 0.03 (3%)
Thyroid 0.03 (3%)
Lungs 0.15 (15%)
Breasts 0.12 (12%)
Other tissues
0.30 (30%)
--- DOSE SPEED – radiation doses during 1sec- very important to biological damages
Upper limit for Radiation Workers 50 mSv per year
Upper limit for general population 5 mSv per year
Visible blood changes 0.5 mSv per year
Lethal dose for 50% of individuals 500 mSv
Lethal dose for all 1 500 mSv
Radiation dose by one chest X ray examination
0.2 mSv
Radiation dose by one head CT scanning 11 mSv
Natural radiation in Croatia 2 mSv per year
Smoking 13 mSv per year
Radiation levels and their effects
0.02- 0.05 mSv 0.03 mSv
X ray examination
Hard technique
Soft technique
Effective dose (mSv) (ICRP 25) 0,03 0,14
Effective equivalent dose(mSv) (ICRP 60) 0,04 0,22
Chest X ray examination
Marija Surić, Institut za medicinska istraživanja, Zagreb
Conditions of ultrasound scanning Diagnostic- frequency range: 0,5 MHz - 20 MHz
- the time of puls: 0,2 - 2,0 s - intensity: 1 - 100 mW/cm2
- amplitudes of vibrating: do 10 nm
Therapy- frequency range : 0,8 MHz - 1 MHz
- constant wave
- intensity : 100 - 1000mW/cm2
- amplitudes of vibrating: 10 - 25 nm
have the depth of penetration at the f=0,88 MHZ around 5 cm for muscle tissue, 10 cm for fat tissue and 0,3 cm for bone
have very low apsorbtion in fat tissues (use for diagnostic), higher in muscles and the highest in bones (use for therapy)
Ultrasound waves
The sideeffects in diagnostic usage almost do not exist because the pulses are short, intensities are low and frequencies are very high.
Scanning of small structures (embrio, eye) requires higher frequencies because of the better resolution. But higher frequencies mean fast changes in local pressure what presents risk for breaking of macromolecules. Intensity must be lower.
Ultrasound waves
1. Thermical effects – are used in therapy; thermical stress possible
2. Mechanical effects – bonds in macromolecules break when amplitude of the oscillation is higher than 10 nm and the change of local pressure is few atmospheres
3. Cavitation – the appereance of air bubbles inside the tissue that is treated by ultrasound of high intensity- molecular bonds are breaking and free radicals are occuring
4. Chemical effects – breaking of chemical bonds and producing free radicals
5. Biological effects – ultrasound is fatal for bacteria, viruses and fungies