Ionizing Radiation
Radioactivity is ONE source of ionising radiation
Deposits an amount of energy in matter which is sufficient to cause the breaking of chemical bonds
Wave and particle descriptions are both used and correct
One radiation particle often deposits energy at multiple sites - either directly or via the creation of other particles
Ionization
Ionization occurs when energetic ionizing radiation strips an electron from an electrically neutral atom. The positively charged atom and the negatively charged electron are called ions.
Ionization
enables radiation to be detected;
enables radiation to be shielded;
but can cause physical damage in the human body that leads to biological effects from radiation exposure if radiation safety measures are not consistently applied.
Types of Radiation (1)
X-rays and gamma rays = photons
electrons and beta particles - negative charge
neutrons
protons - positive charge
alpha particles and heavy charged particles
Types of radioactivity
Alpha particles (Helium nuclei) - “heavy”, dual positive charge, strongly interacting with matter
Beta particles/radiation (electron) - light particle, loosely interacting, still finite range
Gamma radiation (photons)
Radioactivity
Radioactivity results from an atom with too many or too few neutrons in the nucleus.
Unstable atoms become stable by emitting energy as radiation
Different radioisotopes may emit different types of ionizing radiation.
X-ray equipment and linear accelerators generate radiation in a different way to radioactive substances but the radiation emitted by these devices is also ionizing radiation.
Radiation
Unstable nuclides emit certain types of radiation.
Alpha Particle: a nucleus that is too heavy may emit two protons and two neutrons.
Beta Particle: a nucleus with too many neutrons converts one to a proton and ejects an electron.
Gamma Radiation: (electromagnetic radiation) may accompany either alpha or beta particles from the nucleus.
Neutron radiation: a nucleus emits neutrons during fission; a light nucleus is hit by an alpha-particle and emits a neutron.
Radiation - We live with
Natural Radiation: Cosmic rays, radiation within our body, in food we eat, water we drink, house we live in, lawn, building material etc.
Human Body: K-40, Ra-226, Ra-228
e.g. a man with 70 kg wt.
140 gm of K
140 x 0.012%=
0.0168 gm of K-40
0.1 mCi of K-4
»28,000 photons emitted/min
(T1/2 of K-40 = 1.3 billion yrs)
K-40 Estimate for Lean Body Mass
Body weight = Fat + lean body mass
K-40 directly related to lean body mass
Whole body counter used
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Earth: Top 1m of 0.1 acre garden
=1200 kg of K of which K-40 =1.28 Kg
= +3.6 Kg of Th + 1 Kg Ur
mGy/yr
New Delhi 700
Bangalore 825
Bombay 424
Kerala 4000
(in narrow coastal strip)
X-Ray Tube for low and medium X-ray production
X- ray were not developed ; they were discovered quite by accident .
On November , 1895, Roentgen was working in his laboratory on his crooks tube enclosed with black photographic paper so that he could better visualize the effect of the cathode rays in the tube .
A plate coated with barium platincocyanide , a florescent material , florescent regardless of its distance from the crooks tube .He called these light as x-ray. From that time x-ray were used for diagnostic and rapidly by investigation Coolidge tube then at 1928 Bucky grid which greatly increased contrast was developed . X-ray computed tomography (CT) were developed in the 1970 s.
Anode
The anode is the positive side of x- ray tube . There are two types of anode : stationary and rotating . The anode service three functions in x-ray tube
1-it receives electrons emitted by cathod
2-the anode is electrical conductor
3-provides mechanical support to the target.
Stationary anode : the anode consist of target from tungsten-alloy-metal embedded in the copper the tungsten used for the following reasons :-
1- its height atomic weight
2- height melting point
Rotation anode
Higher tube current and short exposure time are possible with the rotation anode . The rotation –anode x-ray tube allows the electron beam to interact with a mach larger target area , and therefore the heating of a stationary –anode is not confined to one small spot as in a stationary – anode tube.
Cathode
The cathode is the negative side of x-ray tube and has two primary parts : a filament and focusing cup . An x- ray filament emits electron when it is heated . This phenomenon is known as thermionic emission. Filaments are usually made of thoriated tungsten .most diagnostics x-ray tubes have small spot in use for high resolution image .
The x-ray photon interact with inner electron, atom, and the field of nucleus
The photoelectronic effect is a photo absorption interaction . The electron remove from the atom, called a photoelectron , escapes with kinetic energy equal to the difference between the energy of the incident x-ray and the binding energy of the electron .
Compton effect
Moderate – energy x-ray , x-ray throughout the diagnostic range , can undergo an interaction with outer –shell electron that not only scatters the photon but reduce its energy and ionizes the atom as well . this interaction is called Compton effect
All x- ray beam are effected by the inherent filtration which absorbed low energy x-ray to reduce the dose.
A foils of Al were added to x-ray tube in order to absorbed low x-ray energy.
X ray beam restructures
Proper collimation of the x-ray beam has the primary effect of reducing patient dose by restricting the volume of tissue irradiated . The corresponding reduction in scatter radiation , and the fogging it produce , is a secondary benefit . Ideally , only those photons that did not interact with the patient would reach the film .
Aperture diaphragm
An aperture diaphragm is the simplest of all beam-restricting device . It is basically a lead or lead-lined metal diaphragm attached to the x-ray tube head .
Cones and cylinders
Radiograph cones and cylinder can be considered modification of the aperture diaphragm are widely used in x-ray diagnostics to reduce the patient dose ,it made of lead which absorbed scattered photon .
The X-ray beam scattered by patient body and reach the film by different angle forming fog on the film. In order to reduce the absorbed scattering Grid is inserted between the patient body and film .Which consist of slices of lead because of its height density and interspaced material . The grid is designed to transmit those x-ray whose direction is on a strait line from the source to the film .
X-ray generator (I)
It supplies the X-ray tube with : Current to heat the cathode filament Potential to accelerate electrons Automatic control of exposure (power application time) Energy supply » 1000 ´ X-ray beam energy (of which 99.9% is dissipated as thermal energy)
X-ray generator (II)
Generator characteristics have a strong influence on the contrast and sharpness of the radiographic image The motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievable Since the dose at the image plane can be expressed as:
D = k0 . Un . I . T
U: peak voltage (kV)
I: mean current (mA)
T: exposure time (ms)
n: ranging from about 1.5 to 3
X-ray generator (III)
Peak voltage value has an influence on the beam hardness
It has to be related to medical question
What is the anatomical structure to investigate?
What is the contrast level needed?
For a thorax examination : 140 - 150 kV is suitable to visualize the lung structure
While only 65 kV is necessary to see bone structure
The ripple “r” of a generator has to be as low as possible r = [(U - Umin)/U] x 100%
Tube potential wave form (I)
Conventional generators single f 1-pulse (dental and some mobile systems) single f 2-pulse (double rectification) three f 6-pulse three f 12-pulse Constant potential generators (CP) HF generators (use of DC choppers to convert 50Hz mains into voltages with frequencies in the kHz range) --> “Inverter technology
Tube potential wave form (II)
The choice of the number of pulses (I)
Single pulse : low power (<2 kW)
2-pulse : low and medium power
6-pulse : uses 3-phase mains, medium and high power (manual or automatic compensation for voltage drop)
12-pulse : uses two shifted 3-phase system, high power up to 150 kW
CP eliminates any changes of voltage or tube current
high voltage regulators can control the voltage AND switch on and off the exposure voltage can be switched on at any moment (temporal resolution)
kV ripple <2% thus providing low patient exposure
HF : combines the advantages of constant potential and conventional generator reproducibility and consistency of tube voltage
high frame rate possible
Automatic exposure control
Optimal choice of technical parameters in order to avoid repeated exposures (kV, mA)
Radiation detector behind (or in front of) the film cassette (with due correction)
Exposure is terminated when the required dose has been integrated
Compensation for kVp at a given thickness
Compensation for thickness at a given kVp
