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xray production and properties of xray radiology
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X-ray production and properties of x-ray
Dr . Sameeha Khan
X-rays were first discovered in 1895 by the German physicist William Roentgen, when using a Crookes tube
He called them ‘x’ rays, ‘x’ for ‘unknown’.
Wilhelm Conrad Roentgen (1845-1923)
The first x-ray photograph: Roentgen’s wife Bertha’s hand
Electromagnetic spectrum
Unit of measurement in x-ray region is Å and nm. 1 Å = 10-10 m, 1 nm = 10 Å = 10-9 m X-ray wavelengths are in the range 0.5 – 2.5 Å. Wavelength of visible light ~ 6000 Å.
X-rays are electromagnetic radiation of exactly the same nature as light but of very much shorter wavelength
Properties of electromagnetic radiation
• Electromagnetic radiation → wavelike fluctuation of electric and magnetic fields set up in space by oscillating electrons
Electromagnetic radiation
According to the classical theory Electromagnetic
radiation can be considered as wave motion .
According to the quantum theory electromagnetic radiation can also be
considered as a particles called photons
Wave concept of electromagnetic radiation
•All EM radiations travel with the speed of light 186000miles/sec, 3×10ˆ8 m/sec but they differ in wavelength •Wavelength (λ) – distance between 2 successive crests / trough •Frequency (ν) – number of crests /cycle per second (Hz) • (λ) wavelength ↓ (ν) frequency ↑•EM travel with the speed of light c , c=λν•Wave concept of EMR explains why radiation may be reflected , refracted, diffracted and polarized .
If each wave has length λ and ν waves pass a given point in unit time
velocity of wave is v = λ× ν
Particle concept of electromagnetic radiation
•Short EM waves like XRAYS react with matter as if they are particles rather than waves.•These particles are discrete bundles of energy and each bundle is called quantum /photon.•Photon travel at the speed of light.•Amount of energy carried by each photon depends on frequency of radiation. •If frequency doubled energy doubled .•Particle concept can explain the interaction with matter like photoelectric and Compton effect .
Energy calculated E=hν h= Planck's constant (4.13×10 ˆ-18 Kev sec )
Relationship between wavelength and energy
Relationship between wavelength and frequencyν= c/λ
c – velocity of light (~3×108 m/s) also E= hνInstead of ν
E =hc/λ ( h×c = 12.4)
• Energy of photon =ev
•X-ray measured in kilo ev , 1Kev = 1000 ev
E= 12.4/λ
Source of x-rays in radiology
History:Gas x-ray tubes Cold cathode tubes
X-ray tube
Basic elements of an X Ray source assembly
Glass enclosure
•Vacuum: to control the number and speed of the accelerated electrons independently.•Pyrex glass : connecting wires same coefficient of linear expansion as glass.
Cathode --------
•Negative terminal of the x-ray tube is called cathode or filament. •Along with filament 2 other elements : connecting wires and focusing cup
Filament made of tungsten wire 0.2 mm diameter coiled to form a vertical spiral 0.2 cm diameter and 1 cm length
When Current flows – wire heated
Absorbs thermal energy – electrons move a small distance from the surface of metal
This escape is referred to as thermionic emission
Thermionic emission
Thermionic emission
Emission of electrons resulting from the absorption of thermal energy – thermionic emission Electron cloud surrounding the filament produced by thermionic emission is termed “Edison effect”
Space charge
•Collection of negatively charged electrons in the vicinity of filament when no voltage applied btw cathode and anode – space charge •Number of electrons in space charge remain constant •Tendency of space charge to limit the emission of more electrons from the filament is called space charge effect
Filament current →filament temperature →rate of thermionic emission
Space charge cloud
Space charge cloud shield the electric field for tube voltages of 40kvp and less ( space charge limited ) , above 40kvp space charge cloud is
overcome by voltage applied
Temperature limited
Tungsten
Z # 74
MELTING POINT- 3,370 DEG. CELSIUS
1. Thin wire 2. Strong 3. High melting point 4. Less tendency to vaporize 5. Long life expectancy
Filament and focusing cup ( Nickel )
• Modern tubes have two filaments
1. Long One : higher current/lower resolution, larger exposure
2. Short One : lower current/higher resolution.
At one point only one is used
Focusing cup maintained at same negative potential as the filament .
Focusing cup
Current across tube one direction only
Mutual repulsion
↑Number of electrons
Electron stream spread out
Bombarding Large area of
anode
Prevented by focusing cup – forces the electron stream to converge on the anode in
required shape and size
Filament vaporization
•Filament vaporization – shorten the life •Not heated for too long- filament boosting circuit •Vaporized filament usually deposited on the inner surface of glass wall •Color deepens as the tube ages- bronze colored “sunburn” •Tends to increase filtration and changes the quality of beam
Anode +++++
Stationary anode Tungsten target in copper anode
Rotating anode+++
Spread the heat produced during an exposure over a large area of anode – capable of withstanding high temperature of large exposures
Anode +++ parts
1. Anode disk –tungsten •3600rpm •Beveled edge – line focus•Target area increased but effective focal size remains the same.
2. Stator3. Rotor4. Bearings - metallic
lubricants (silver )
5. Stem - molybdenum
90%tungsten W and 10 % rhenium Re- ↑resistance to surface roughening - ↑thermal capacity
Anode +++
Modification of tube to improve speed of rotation and in turn increased ability to withstand heat .
•As short as possible
•Decrease inertia
1.Stem length
•2 sets as far as possible
2.Bearings
•Decrease weight ( molybdenum + W Re alloy )
•Reduced inertia
3.weight
Focal spot
•True focal spot :Area of the tungsten target (anode) that is bombarded by electrons from the cathode.•The size and shape of focal spot is determined by the size and shape of the electron stream which hits the target.•Heat uniformly distributed on focal spot
Line focus principle
•Anode angle : defined as the angle of the target surface with respect to the central ray in the x-ray field. •Anode angle range :6°- 20° •Line focus principle - Effective focal spot size is the length and width of the focal spot projected down the central ray in the x-ray field .
Line focus principle
Foreshortening of the focal spot length
Line focus principle
effective focal length = focal length • sinq Effective focal spot<actual focal spot
Anode angle
Large focal spot = greater heat loading. Small focal spot = good radiographic detail.
Heel effect
Intensity of beam depends on the angle at which the x-rays are emitted from the focal spot
Heel effect
Intensity of exposure on anode side < cathode side
of tube
Heel effect less noticeable with large focus-film
distance
Heel effect is less with smaller films
Anode Cathode ←Intensity→
Properties of xrays
1. X-rays travel in straight lines.2. X-rays are electrically neutral 3. X-rays are Polyenergetic and heterogeneous 4. X-rays travel at the speed of light -
electromagnetic radiation 5. X-rays are highly penetrating , invisible rays.
Properties of x-rays
6. X-rays cannot be deflected by electric field or magnetic field.
7. X-rays cannot be focused by lens.8. Photographic film is blackened by X-rays.9. Fluorescent materials glow when X-rays are directed
at them.10. Produce chemical and biologic changes by ionization
and excitation. 11. Liberate minute amounts of energies while passing
through matter. 12. X-rays interact with matter produce photoelectric and
Compton effect.
Processes of x-ray generation
When high speed electrons lose energy in the target of the x-ray tube
2 processes of x-ray generation
General
Characteristic
General radiation ( Bremsstrahlung) • High speed electrons with nucleus of the tungsten atom
Characteristic radiation • High sped electrons with the electrons in the shell of tungsten atoms
Degree of deceleration
+
+
e‾
e‾
0.5%time electron comes in proximity
with nucleus
Coloumbic forces attract and decelerate the
electron
Loss of kinetic energy and change in trajectory
Bremsstrahlung ( braking radiation )
Enrgy of photon = enrgy of initial ectron – enrgy of braked electron
Energy of photon E = 12.4 /λ Energy is related to the potential difference across tube or
λmin = 12.4 / kVp
Continuous spectrum
Highest energy determined by the kVpMinimum wavelength determined by the kVp
Maximum wavelength determined by the filters used
Brems radiation- Polyenergetic
Characteristic radiation results when the Electrons bombarding the target eject electrons from the inner orbits of target atoms
Characteristic radiation
Characteristic radiation
BINDING ENERGIESOF DIFFERENT SHELL ELECTRONS
K-70 KEVL-11 KEVM-2 KEV
Characteristic radiation
L K(α)70-11= 59 keV
M K (β)70-2 = 68 keV
ML 11-2 = 9 keV
Between 80 and 150 kVp , k shell characteristic contributes to about 10 %(80kVp) to 28%(150kVp) of useful beam.
Characteristic radiation
EACH CHARACTERISTIC RADIATION ( eg. K TO L TRANSFER) IS:
Monoenergetic
THERE ARE MANY CHARACTERISTIC RADIATION
PRODUCED IN ONE ATOMTHEREFORE CHARACTERISTIC
RADIATION IS ALSO POLYENERGETIC !
Characteristic radiation
Less Polyenergetic
Typical x-ray pattern
Thank you