Dosimetry Flashcards
The gas ionization chamber
- Measures the radiation dose by measuring the charges produced between the two charged plates in the capacitor.
- Charges move to oppositely charged electrodes, potential difference detected and represents a current pulse.
- Size of the current is proportional to the energy of the incident radiation.
Thermoluminescent dosimetry
- Ionizing radiation excites electron from valence band.
- After electron relax, light is emitted.
- Number of photons is proportional to the dose.
The Geiger-Müller counter
- Based on a gas ionization chamber.
- Voltage much higher.
- Every radiation will form an avalanche of secondary electrons which form maximum current.
- Sensitive to low energy radiation, due to high voltage.
- Counts the number of particles that interact with the device.
Semiconductor detectors in dosimetry
- Uses a diode connected in reverse bias, no current flowing in circuit.
- Ionizing particle causes electron-hole pair in depletion region.
- Current flows. Current is evidence of presence of radiation.
Physical, chemical and biological phases of radiation effects
Physical: Ionization
Chemical: Free radicals
Biological: DNA damage
The absorbed dose
- Amount of energy absorbed per mass. (J/Kg) (Gy)
- Difficult to measure since even a lethal dose of 6Gy corresponds to an unnoticeable temperature change.
Converting exposure in air to absorbed dose in tissue
1 C/Kg (Exposure) = 34 Gy in air
The stochastic radiation effect
- Radiation damage that can occur due to absorbed dose.
- Valid for damage that can occur at low levels of absorption.
- Random, proportional to increasing dose.
The exposure
- Measures positive charge produced in air with certain mass by ionization. (C/Kg)
- Can be measured by ionization chamber.
Weighting factors in dosimetry
1) Type of radiation - Different radiation cause biological damage in different severity.
2) Which tissue exposed - Different tissue have different sensitivity to radiation.
The deterministic radiation effect
- Probability of radiation damage increases abruptly over a threshold dose.
- Severity of damage above threshold is proportional to the dose.
The equivalent dose
- Different types of radiation influences tissues in different severity associated with a particular dose. (J/Kg) (Sievert Sv)
- Wr = Radiation weighing factor, how many times greater effect with certain type of radiation compared to gamma radiation.
ALARA-principle
ALARA - As Low As Reasonably Achievable. Reducing exposure to the minimal by:
- Minimal time near source.
- Distance from source should be maximal.
- Person dealing with radioactive material should wear protective shield.
- Source must not cause any deterministic effect.
The direct and indirect effects of ionizing radiations
- Direct: Biologically important macromolecules are damaged (DNA).
- Indirect: Radiation ionizes water first, causing free radicals (OH.) which combine to form H2O2, chemically attacking DNA.
The effective dose
- Measures the absorbance dose, taking into account sum of radiation types and weighing factors, also type of organ and its weighing factor (probability of stochastic damage)
- Sum of all tissue weighing factors is 1.
- If radiation effects more than one organ, effective doses should be summed. (Sv)
Typical dose values and dose limits
- Lethal dose is 4 - 6 Gy
- Max. Limit for body is 20mSv/year, for skin and limbs is (500mSv/year)
- Background radiation: 2.4mSv/year
- X-ray image: 0.2 - 1 mSv
- CT scan: 2 - 8 mSv
The dose rate
Ratio of dose and time of irradiation (Gy/h) (Sv/h)
Information obtained by isotope diagnostics
- Organ function: Metabolism, how fast it absorbs and expels the tracker.
- Circulation in the organ if tracker placed in blood.
- Air circulation in organ if tracker is inhaled.
Principles of selecting the isotope for diagnostics according to half- life
- Its half-life should match the biological half-life.
- Needs to be as short as possible.
Principles of selecting the isotope for diagnostics according to radiation type and energy.
- Gamma radiation isotopes are used frequently due to long effective range, low absorbance in body tissue, and high energy to be easily detectable.
- Energy of gamma should be high enough to penetrate body and be detected, but not too high as it will be less effectively absorbed by detector. (More damaging to patient)
- Should be absorbed to some extent for contrast image.
- Activity needs to be high enough to detect sufficient photons.
Parts and function of Tc-generator
Machine which produces gamma radiation isotope from parent isotope with a relatively long half-life. Parts:
- Lead container
- Saline container
- Generator columb
- Tc container
Cost-benefit principle in isotope diagnostics
If the risk of not having examination is higher than that caused by the radiation exposure, the procedure can be done.
Definition of radiopharmaceutical
Chemical agent/drug having radioactivity. (Labeled with radioisotope for diagnostic and therapeutic purposes)
Determination of the biological half-life of an organ
Biological transport rate and elimination from target organ. 1/Teff = 1/Tphys + 1/Tbiol
