BASICS OF RADIATION AND PRODUCTION OF X-RAYS

Presented by- Dr. Dinanath Chavan First year PGT, Department of Radiodiagnosis SMCH.

Moderator- Dr. Mrinal DeyProfessor, Department of Radiodiagnosis SMCH.

Radiation

• Radiation is energy that travels through space or matter. • Two types of radiation used in diagnostic imaging are 1. electromagnetic (EM) and 2. particulate.Electromagnetic Radiation• EM radiation includes: (a) gamma rays,(b) x-rays,(c) visible light,(d) radiofrequency

• In this type, the energy is "packaged" in small units known as photons or quanta.

• Visible light, radio waves, and x-rays are different types of EM radiation.

• EM radiation has no mass, is unaffected by either electrical or magnetic fields, and has a constant speed in a given medium.

• EM radiation travels in straight lines; however, its trajectory can be altered by interaction with matter.

• EM radiation is characterized by wavelength (λ), frequency (v), and energy per photon (E)

EM RADIATION

• The other general type of radiation consists of small particles of matter moving through space at a very high velocity.

• Particle radiation differs from electromagnetic radiation in that the particles consist of matter and have mass.

• Particle radiation is generally not used as an imaging radiation because of its low tissue penetration.

• ex. Electron, alfa particles.

Particulate Radiation

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 ~ 3900 - 7500 Å.

X-rays are electromagnetic radiation of exactly the same nature as light but of very much shorter wavelength

Electromagnetic radiation is the transport of energy through space as a combination of electric and magnetic fields.

Electromagnetic ( EM ) radiation is produced by a charge ( charged particle ) being accelerated . { electrons are consider as standing waves around the nucleus and therefore do not represent acclerating charges. }

Any accelerating charge not bound to atom will emit EM radiation .

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

X-ray Production

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)

Site of discovery

Roentgen's lab where, on 8 November 1895, he noticed an extraordinary glow while investigating the behavior of light outside a shrouded cathode tube. To his astonishment, he saw the shadows of the bones of his hand when held between the tube and a fluorescent screen. Within two months he had published a carefully reasoned description of his work and the famous radiograph of his wife's hand.

The first x-ray photograph: Roentgen’s wife Bertha’s hand

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 is used.

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

CATHODE --------

MADE OF TUNGSTEN + 1%-3% THORIUM ( better emission of electrons. )

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 .

Cathode assembly of a dual-focus x-ray tube. The small filament provides a smaller focal spot and a radiograph with greater detail, provided that the patient does not move. The larger filament is used for high-intensity exposures of short duration.

Focusing Cup

1: long tungsten filament2 : short tungsten filament3 : real size cathode

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

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 (Tungeston heated >22000C)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 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

The heel effect: The heel effect is due to a portion of the x-ray beam being absorbed by the anode. This results in an x-ray beam that is less intense on the anode side and more intense on the cathode side. The heel effect is more pronounced with steeper anode angles.

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→

Factors affecting the heel effect:

1. Anode angle: the steeper the target → ↑↑ heel effect.2. FFD: ↑↑ FFD → ↓↓ heel effect "with fixed film size".3. Film size: ↓↓ film size → ↓↓ heel effect "with fixed FFD".4. Roughening of the target surface → ↓↓ X-rays output &

↑↑ the heel effect.• In radiographs of body parts of different thicknesses → the

thicker parts should be placed toward the cathode (filament) side of the x-ray tube.

• e.g. AP film of the thoracic spine → anode end over the upper thoracic spine where the body is less thick & the cathode end of the tube is over the lower thoracic spine where thicker body structures will receive the increased exposure.

• The intensity of the x-rays emitted through the heel of the target is reduced because they have a longer path to travel in the target. The diff in intensity is as much as 45%

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

W

M Shell

L Shell

Target atom

K shell

Incoming projectile electron (high energy)

Characteristic X-ray emission

K X-ray

L X-ray

Outgoing projectile electron (lower energy)

Ejected electron ionizes atom

Characteristic X-Ray Production

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

Factors affecting x- ray spectrum:-1) Effect of tube current (mA) (while others

remain constant): More mA more e- s flow from cathode to anode Change in mA is directly proportional to the change

in the amplitude of the x-ray spectrum Shape of the curve remain unchanged

The effect on the tube spectrum when the mA has been halved.

2)Effect of kV ( other factors remaining constant)

-> As kV is raised area under the curve increased

-> The position of the curve has been shifted to the right to the high energy side

-> The increase is relatively greater for high energy x-ray than for low energy x-ray

-> Characteristic curve doesn't change position

The effect on the tube spectrum when the kV has been reduced from 80 kV to 70 kV.

3)Effect of added filtration: ( other factors remaining constant)

-> Added filtration absorbs low energy x-rays and allow high energy x-rays to pass through.

-> The curve is shifted. The bremsstrahlung emission spectrum is reduced more on left than on right.

->effect of added filtration is the increase in the effective energy of the x-ray beam (high quality)

-> Characteristic curve doesn't change position

The effect on the tube spectrum when filtration has been added to the exit beam.

Super Rolatix ceramic x-ray tube

Metal casing instead of glass envelope.

Three ceramic insulators – two insulators for the two high voltage cables, and one supports the anode stem. • Allows more compact tube design.• Most common - Aluminium oxide.

Anode rotates on an axle which has bearings at each end – provides greater stability and reduce the stress on shaft.• Allows use of massive anode up to 2KG. • larger heat storage capacity. Allows higher mAs settings.

Advantages of Metal --

• less off focus Radiation .• higher tube loading. • longer tube life with high tube currents.

Cooling – better cooling due to more efficient transfer of heat to the oil through the metal enclosure, as compare to the glass enclosure. ( metal is better conductor of heat )

Super Rolatix ceramic x-ray tube

Thank you