1
Q
Ionising Radiation
A
- Ionizing radiation is capable to penetrate deep in the matter, to produce ionization of the atoms, to break chemical bonds and to cause harm in living matter.
- Ionization occurs when an electron is being broken off/ejected from the atomic orbit, leading to a pair of opposite charged ions.
2
Q
Direct IR
A
- includes charged particles (α, β, e, e+, p) which ionize atoms by collision
- During the collision electric interaction appears between particle passing and electrons of the matter.
3
Q
Indirect IR
A
- Indirect ionizing radiations are photons with high energy – gamma, roentgen and one type of particle - neutrons
- They do not possess electric charge and interact with atoms by their electromagnetic fiel
- As electrons are ejected from atoms, further electrons produce secondary ionization by means of direct ionization.
4
Q
Effectiveness of ionising
A
- depends on mass of ionizing particle and the atom characteristics of the matter
- During the collision, ionizing particles use up a part of their energy to break off an electron (ionization losses Ecollision) and another part for generation of breaking radiation (radiation losses Erad.)
- during deceleration of incoming electron: kinetic energy is converted to photon
5
Q
IR characterised by quantities
A
- Linear ionization Nl – the number of ion pairs produced by the given type ionizing particles during their passage through the unit path in matter medium
- Linear energy transmission (LET) – the locally transmitted energy by particles per unit path.
- Range ( R ) – the average depth in a certain substance at which the intensity of radiation decrease to half.
6
Q
Interactions between indirect ionising radiation and matter
A
- Photon radiations interacts with the matter by several processes:
Elastic scattering - no energy loss
Photoelectric absorption
Compton effect (scattering) Pair production
7
Q
Photoelectric effect
A
- occurs when a photon passes through the matter media
- Photon interacts with K-shell or L-shell electron which results in ejection of electron
- During this the entire energy of the photon is absorbed by the electron - atom is ionized
- The required minimal photon energy for production of PhE is estimated by following equation
hv = I + Ek
- The probability for appearance of a photo-effect depends on the energy of the photon incoming and on the type of matter/atom number
8
Q
Probability for the appearance of a photo-effect
A
- The quantity which represents this probability is called linear attenuation coefficient.
Linear attenuation coefficient:
τ ~ ρ.Z4 / (hv)3
Z = atomic number
p = density
9
Q
Compton scattering
A
- Compton scattering occurs when a photon interacts with free electron or with weak bonded electron from the outer shells of an atom
- Photon transfers a part of energy to the electron (The electron broken off assimilates this energy) and continues propagation with altered direction and energy
- The probability of appearance of CS is evaluated by linear coefficient of CS:
σ ~ ρ Z / hv
10
Q
Transformation of electromagnetic energy into mass
A
- High energy photons propagating through the matter media can produce this
- high energy photon interacts with electric field of atom particles – nucleus or electron
- The result of this interaction is appearance of pair electron/positron (the positron is antiparticle of electron; it is characterized by the same mass but positive charge)
- The entire energy of the photon is used up for production of the pair. - Photon does not exist more. But positron produced interacts quickly with electron from the first atom encountered and provokes the reverse process.
11
Q
Probability for transformation of electromagnetic energy into mass
A
- The probability for occurrence of this phenomenon is estimated by linear attenuation coefficient for pair production:
Χ ~ Z2 ln(hv)
12
Q
Total attenuation of photonic radiation
A
- determined quantitatively by the decrease of radiant flux of the ionising photons ψ
ψ=ψ0 e–μd
(μ = τ + σ + Χ)
13
Q
Half-value-layer HVL
A
- The quantity called Half-value-layer HVL is used for determination of absorbing ability of a given substance
- This quantity is measured as a distance of propagation for which the photon flux decrease to 50% in terms of the initial value.
14
Q
Roentgen radiation
A
- X rays are photons with high energy
- Roentgen range includes wavelength from 3 pm to 10 nm. (The minimal energy required for ionizing of the matter/atom is 35 eV. therefore X rays are ionizing radiation)
- For medical applications, X-rays used within 100 pm to x pm wavelength
15
Q
Main natural sources of X-rays
A
- cosmic rays (emitted from stars, pulsars, galaxy)
- X rays are emitted during radioactive decay of some substances as thulium - the process is called K-catch
16
Q
How are X-rays produced
A
- For medical and technical uses X-rays are produced by means of cathode tube
- Working scheme: thermo-cathode is heated up by electric current passed through separated electric circuit (low voltage electric supply), electrons acquire energy sufficient to be ejected from the metal lattice and under the action of high voltage are accelerated toward anode, electron beam hits the anode and provokes emission of X rays.
17
Q
Characteristic Roentgen radiation
A
- CR arises as a result of electron conversions after a strike of accelerated electron with electron from atom of the anode metal
– incoming electron breaks off an electron from inner electron shell. The vacancy is occupied by higher energy electron from outer shell
- This conversion is supported with an emission of a roentgen photon.
- Due to the strong determination of energy conversion, the spectrum of this radiation type is characteristic for the given substance (in mentioned process – material of the anode).
18
Q
Breaking Roentgen radiation
A
- is emission of photons from decelerating electrons entering into anode (These electrons experience decelerating effect of atomic electric field)
- Emission is consequence of transformation of kinetic energy of the passing electron into an electromagnetic energy
- The spectrum is continuous because of random distribution of energy.
19
Q
Determination of properties of x-rays
A
- The properties of emitted X-ray radiation are determined by accelerating voltage applied Ua and electric current strength passing through the roentgen tube
- A main parameter of each roentgen spectrum is the maximal energy of photons emitted – it means which is the minimal wavelength produced by tube. It depends on the voltage applied in reversed ratio.
20
Q
How to achieve appropriate X rays photon beam
A
- Variations of the voltage
- Electrons accelerated acquire kinetic energy of
E = e.U
where e is electron charge, U is voltage applied between cathode and anode
- Enhancement of U results on left shift of breaking radiation spectrum. - It means that the photon beam emitted consist of more powerful x rays, respectively the minimal wavelength decrease.
- The increase of current I involves X- photons intensity which is indicated on the spectrum graph as increase of the height of the spectrum
- Alteration of current does not influence the energy characteristics of the radiation (lambda minimal remains unchanged).
21
Q
X-ray image formation
A
- X-ray beam is collimated toward the object being investigated
- photons penetrate into the tissues and interact with different structures in different ways (compton, photo-effect)
- part of the radiation passes through the body and reaches the registration device
22
Q
How is contrast obtained in X-ray imaging?
A
- obtained due to the zonal distribution of attenuation
- intensity of absorption depends on density of biological matter and its effective Z (higher, absorb more)
- when soft tissues examined, contrast substance used
e. g. for blood vessels, water solution of iodine compounds, barium compounds for gastrointestinal tract
23
Q
Roentgenography
A
- image obtained on film-plate by transformation of photon energy to chemical one
- photo-effect and compton’s are processes which induce attenuation of X-beam
24
Q
Roentgenoscopy
A
- image obtained on fluorescent screen
- image brightness on the screen is proportional to the intensity of roentgen rays reaching it
- method is supported with higher radiation dose on the patient compared to the roentgenography
25
Digital subtraction angiography
- exclude bones, dense soft tissue in order to investigate blood vessels
- roentgenography of area examined is carried out without a contrast introduction - only bones visible
- examination of given area after introduction of contrast medium
- forming digital image by means of computer subtraction
Medical Physics
flashcards
Decks in class (10)
# Cards
