x ray machine vaibhav

X-ray production

Presented by DR VAIBHAV SALUNKE 1st YEAR PG OMDR Guided by DR JYOTSNA PATEL

on the night of 8th nov.1895 by prof.wilhelm conrad roentgen who was a professor of physics at the university of wurtzberg,germany,accidently discovered x-rays. He was experimenting with hittrof-crookstube for cathode rays,when he

Wilhelm Conrad Roentgen (1845-1923)

Observed greenish glow emanating from barium platinocynide screen kept at a distance. when he interpose his hand unintentionally between the tube and screen he could detect the image of his bones in the shadow. He at once realized the significance of this finding and established the property of penetration. he then covered the tube with black paper and florescent persisted which further confirms the property of penetration.

Roentgen has discovered these new rays and as anything unknown in maths is disignated by symbol X called then X-rays

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 10-0.01 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 → can be defined as propagation of wave like energy (witout mass) through space or matter .

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 = 1.24)

• Energy of photon =ev

•X-ray measured in kilo ev , 1Kev = 1000 ev

E= 1.24/λ

X-Ray Machine

The heart of x ray machine is made of x ray tube and its power supply.

X-ray tube

Basic elements of an X Ray source assembly

Glass tube :- the x ray tube is lead berrylium glass in which

are mounted cathode and anode assembly. The x ray is evacuated because 1. presence of air will

obstruct the flow electron and reduce there kinetic energy

2. prsence of oxygen will oxdized the tungsten fillment.

Lead beryllium glass surrounded by lead shield except that area of port lead shield is used

because 1. it absorb all the radiation except the useful beam 2. provide earthing to the x ray tube to prevent chances of electric shock 3.prevent

physical damage to the tube

Negative Cathode

Principally composed of 2 parts 1. filament 2.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

Tungsten

1. high atomic no. (Z=74) more electrons are given out i.e high thermionic emission 2. high melting point(3380 c ) so that filament should not melt 3. low vapor pressure at high temperature so that it will not vaporize 4. Thin wire 5. Long life expectancy

Molybdenum focusing cup :-function of molybdenum focusing cup is to restrict the size of electron cloud and also serve as cathode and help in repelling electrons (because of negative charge )

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”

Anode :- anode is made up of tungsten target embedded in a copper stem .tungsten is preffered as a target material because it satisfy the requirement of ideal target material i.e. 1. high atomic number z=74 which gives rise to increase interactions between incoming electrons and tungsten atoms thereby generating more number of x ray photons 2.high melting point (3380c) so that anode does not melt, despite the heat produced there 3. low vapour pressure at high temperature so that tungsten does not evaporate such vapours if present are likely to line the tube and give rise to excessive filtration & also acts as source of radiation giving rise to stray radiation .

Why tungsten mounted on copper stem :- tungsten does not have good thermal conductivity hence it is mounted on copper stem which is good conducter of heat the heat generated at anode dissipated by : 1 conduction: through copper stem . 2.convection: through oil surrounding the tube 3.Radiation: through the radiator device attached to copper stem

Roatating Anode Anode is in the form of

copper disc 7.5 – 10 cm in diameter focal spot is in the form of 0.6 mm ring incorporated in copper disc During exposure the copper disc is rotated so that at each time different focal spot get bombarded by electrons .

Line focus principle

The actual focal spot size is 1X 3mm however the anode is angled at 20 degree to x ray beam this causes the effective focal spot to be 1X1 mm . So that rectangular focal spot appeare as square this is done to facilitate the following :-1. heat distribution across larger surface at the anode at the same time. 2.sharp image because because rectangular focal spot appears as square when viewed in the direction of the central ray (smaller the focal spot sharper the image )

Line focus principle

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. diagnostic properties A. penetration B. Photographic properties C. Fluorescence 2.Biological properties A . Direct effect B . Indirect effect 3. Thermal properties

Properties of x-rays 4. Chemical properties X rays are ionising raditation ,they can cause alteration

in the chemical through which they pass .fecl 2 to fecl3 and H2O to H+ and OH-.

5. Properties of physics 1. X rays are invisble electromagnetic radiations.2. Travel at speed of light i.e. 3*10 8 m/s. 3. They can not be focused.4. They obey laws of polarization and interference. 5. They satisfy the “inverse square law” i.e. Intensity

of x ray beam at a given point varies inversly as a square of the distance of that point from the source of radiation(focus spot).

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

Bremsstrahlung ( braking radiation )

Brems radiation- Polyenergetic

Characteristic raditation

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 !

Typical x-ray pattern

Circuit of x tube :- it is divided into two parts : 1 filament circuit (to produce electrons by heating filament )2 . Cathode anode circuit (to provide the potential difference of 65000 volts) 1. filament circuit uses a step down transformer because only 10v is required to heat the fillment whereas supply is 220 v2. cathode – anode circuit has a step down transformer because voltage has to be stepped up 220v to 65000v 3. autotransformer which has a kvp selectors is used to regulate kvp a certain degree and it has a single coil which is divided by single pointer

Rectification :- Is the conversion of alternating current (AC) to direct

current direct current ( DC) during the first phase of AC cycle cathode is negatively charged and anode is positevely charged electrons hit against the anode and x rays are produced but when current changes in direction during second phase catode becomes positively charged therefore electrons present at anode (because the thermionic emission secondary to heat produced ) travel backwards & hit against the fillament , thereby causing burnout of the filament such a phenomenon takes place in general x ray machine.

In dental x ray machine however the amount of heat produced at the anode does not give rise to excess of electrons as result when current changes its direction there are no electrons at the anode to travel back to cathode and hence dental x ray tube is called as self rectifying

Thank you

References

ORAL RADIOLOGY . Principles and interpretation white and pharroh 5th edition

Fundamental of physics of radiology –meridith , massey Textbook of dental and maxillofacial radiology : freny r

karjodkar O’ briene RC the nature and generation of x ray dental

radiography Internet : www.pubmed .org www.plmer .edu www. Radiology.org Faculty .mu.edu.sa