1

2

Lecture 2 10-feb-2013

Radiation physics � Dental Radiography: Principles and Techniques, chapter 2 p9-p19.

We will talk about parts of radiation, properties of X-ray and X-ray machine.

Let us take a basic review of physics to understand how we can generate X-ray, and how it will interact with our body, how the image will be formed. You know that we are dealing with matters which is anything that occupy a space and has a mass , matters composed of molecules , molecules composed of atoms and each atom composed from a nucleus and orbital electrons(-ve charge) , inside the nucleus there are neutrons(no charge) and protons (+ve charge).

The number of protons = the number of electrons in atoms in the equilibrium state, the Atomic number is the number of protons , the Atomic mass number is the number of protons and neutrons, figure 2-1 ( the basic structure of the atom).

Electrons must be in shells, never in between shells or the forbidden zone, we have K, L, M, N, O, P & Q shells(fig 2-3), and in each shell we have a rule for the number of electrons.

� Number of electrons in the shell=2n^2, where n is the number of the shell. So in K we have 2e (2X1^2=2) in L we have 8e (2X2^2=8) M we have 18e (2X3^2= 18) and so on. (fig 2-4)

The K shell is the closest shell to the nucleus has the maximum binding energy for electrons. So we need high energy to remove an electron from K shell not like removing electron from outer shells which needs less energy.

� Radiation (أشعة) it emission and propagation of energy in a form of waves or particle , X-ray is one

3

type of radiation, we can deal with X-ray as waves or we can consider it as photons both are applicable.

Radiation types:

1-Electromagnatic: X-ray, radio wave, ultra violet light and infra red

2-Particulate: alpha, beta and neutrons

3-Sound and ultra sound

** So X-ray is one kind electromagnetic radiation.

**Electromagnetic means that that we have electrical field and magnetic field at the same time.

X-ray as other electromagnetic radiation travels as speed of light, gamma rays is another example of electromagnetic radiations, X-rays will be produced when electrons hit the nucleus itself.

Radioactive decay: unstable atoms undergo spontaneous disintegration to reach more stable state which can result in alpha particles, protons, beta particles, electrons and gamma rays (electromagnetic waves).

Electromagnetic radiation means that there are both electrical and magnetic fields moving together at the same time and perpendicular to each other and also to the direction of propagation, when we have a magnetic field rays will lead to electrical current and the opposite is correct.

Electromagnetic radiation can be natural or man-made, travel as waves and can be considered as photons( photons means it has no mass or weight), examples: light, ultra violet, infra red, radio waves, gamma

4

rays and X-rays ( the most important for us)

# What is unique about X-ray? How can it penetrate our body?

It has short wave length (distance between two crests), high energy (can lead to ionization) and high frequency (number of waves passing upon a specific time)

* ionization: atoms having a positive charge by losing electrons and the other having a negative charge by gaining electrons this is called ion pair production.

So when X-rays hits atoms in our body will force these atoms to lose electrons, I that will result in ionization, atoms will be having positive charges because of losing electrons, therefore the number of protons will be higher than electrons.

** so we call X-ray an ionizing radiation, in conventional machine we use x-rays but in MRI we use radio waves because they are magnetic resonance images so they considered as non-ionizing radiation.

X-ray properties:

*X-ray was discovered by Roentgen by chance and had got a noble prize in physics. 1- X-rays travel in straight lines.

2- Travels as same speed of light 6000miles/sec.

3- X-rays are invisible

4- X-rays weightless we can't see, hear or feel

5- X-rays can show us hidden objects, like the bone in our body.

6- X-rays can't be reflected, refracted or focused to a point, but it can be deflected (change the direction).

7- X-rays carry no electric charge so can't be deviated be the magnet

8- Can result in ionization (removing of electrons, ion pair production)

9- X-rays can interact with electrons in outer shells of atoms and with inner shells leading to different effects on cells.

10- X-rays can be absorbed, scattered and attenuated.

11- X-rays can result in excitation (enforce electrons to go to a higher shells)

5

12- Penetration because of its high energy, but it can't penetrate some materials depending on the density of the object, like lead so we use lead to protect patient and doctor during taking the graph, and also depending on the energy of the x-ray itself by changing the wave length.

13- X-rays have a photographic effect it has the same effect of light on films which is being black after chemical processing. If you expose film to light and you process it, it will be black, and if you expose film to X-ray and then process it, it will be black too. So they produce latent image (invisible) but after chemical processing you will get the image. After you take the radiograph you should put t in the processor in order to see the image.

14- Fluorescence, X-ray causes some substances to fluoresce to emit light radiation of longer wave length to reduce the energy dose; the main benefit is in extra oral radio graph (PRG)

films of Panoramic Radio Graph is placed inside a cassette to protect the body from high energy level in order to x-ray to penetrate the body, inside the cassette there is material that will fluoresce when x-ray hits it, the new light produced from this material will hit the film, when we change x-ray to light we are decreasing the dose.

15- X-ray can cause biological damage, somatic damage (in the body itself) and genetic damage (it will pass through generations, maybe the patient is not affected but new generations are like Chernobyl normal parents having child without hands)

Radiation is something bad not good! Whatever the small is the dose you are using it causes a genetic mutation, it can result in cancer.

Principles of x-ray production

How can we produce X-rays?

6

we need source of electrons and a target to get the photons, we need electron cloud those electron have to be accelerated to travel in high speed hitting a matter which we call the

target which is tungsten, those accelerated electrons have kinetic energy when they hit the outer shells of the target atoms 99% of the kinetic energy become heat and when they hit the inner shells the rest of it (1%) becomes X-ray, we call the inner and outer hitting of the atom's shell inner and outer interaction. Interactions of the outer shells will be responsible of heat production. X-ray produced from inner interactions of electrons to the target.

7

Essential requirements:

1- Source of electrons

2- Accelerating system by having a high difference in voltage around 70000 V, in the outlet we have 220 V in Jordan however it's not enough to generate x-ray so we need to use transformers.

3- Target or matter (tungsten) which is the target to the electron to strike.

X-ray machine (fig 2-11):

Components:

1- control panel: different from company to another but mainly it has: on/off switch, timer, exposure time selector increase or decrease time, warning light, film spread selector (D speed or E speed), kilo voltage, mille ampere, time, the body (adult or young), the area (incisors or molars), dose amount (digital film needs lower dose than normal films) it depends! Changing the time and mille ampere will affect the dose proportionally. And also the kilo voltage in the same manner but we should notice that the quality of the image will change also, so it's better to change the time and mille ampere. Increasing the kilo voltage 50% it's like doubling the mille ampere. If you increase the time, milliampere, or kilovoltage will increase the dose. However changing the kilovoltage will change the quality not only the quantity.

2- Positioning arms: support the positioning of tube head, allow for movement of the tube head and house electrical wires.

3- Tube head: heavy metal contain the tube, transformer, insulating oil and tube head accessories. (Figure 2-12) Function of this metal housing will protect the tube and will prevent unnecessary radiation to the surroundings.

8

◄ the tube head is connected to the positioning indicating device (PID) this will shape the

beam and direct the beam to area of interest

◄ The tube: target and transformers located here (figure 2-13), (figure 2-12).

It contains:

◛ cathode (-ve electrode)

◛ anode (+ve electrode) anode is connected to tungsten target

◛ glass envelope : Glass house is leaded preventing x-ray from passing, and there is an

unleaded window to allow x-ray to pass through.

◛ Tungsten filament

◛ Focusing cup focuses electrons

◛ Vacuum to prevent electron from collusions

◛tungsten target is connected to cupper's arm to dissipate heat like insolating oil; there is oil

inside the tube also.

◛ Unleaded glass window to allow X-ray to pass through.

9

Line focus principles

If we use the target 20° with the electron beam we will have high focal spot but in fact we use a tilted target to decrease the focal spot; (focal spot: area of the x-ray collection or concentration) we have to get effective focal spot to increase resolution by tilting the target.

So we can tilt the target to decrease the focal spot by that we increase the resolution.

doctor started explaining pic.s i could get those piece of information from it ! If X-ray is generated from a point this is good for us, it will increase the resolution and this can be made by tilting the target. When it is smaller it is more effective which could be achieved by tilting the target!! But we can't make this because of heat problem so the only way to get better picture and in the same time avoid the heat problem is by tilting the target not by decreasing the area.

What is the function of the Cathode and the Anode? Cathode: producing electron beam, Anode: convert electrons into x-rays

Inside the tube head we can find:

◛◛◛◛ Insolating oil.

◛◛◛◛ Tube head seal

◛◛◛◛ Aluminum disks (filtration is the main benefit allowing only the most penetrating x-rays

to pass through those having short waves and high energy, we don’t want low energy X-rays because they will not contribute to the image those low energy X-rays only increase the dose to the patient) .

� Added filtration: It will only allow high frequency, high energy X-rays to pass not long wavelength. � Inherent filtration: filtration done by components of

the x-ray tube assembly

◛◛◛◛ lead collimator (a metal lead part will allow passage

of x-ray in certain shapes(rectangular, circular, etc.) we

10

use the rectangular one because the x-ray films are rectangular this will shape the x-ray beam like the shape of the film just to cover the film by using them we can decrease divergence and scattering of the beam and increase the sharpness of the image)

X-ray travels in straight lines but in a divergent path way so by using collimator we decrease from this divergence and scattering and that will increase the sharpness of the image.

◛◛◛◛ PID or BID (position indicating device) dead line

cylindrical or rectangular extension from the metal housing, in the past they were calling it long cone (16 inch), intermediate cone (12 inch) and short cone (8 inch). They used to use one of them depending on the doctor technique, nowadays we stopped using the word cone because it's not conical shaped any more it's rectangular or cylindrical in shape.

RECALL : added filtration ( allow only high frequency high energy waves to pass) and inherent filtration (filtration by components itself)

◛◛◛◛ transforemers

you have to differentiate between components inside the tube and tube head!

Generating system

The volt is the measure of the electrical force kilo volt means 1000 volt , kilo volt peak is the crest value of voltage.

Ampere is the measurement of current, numbers of electrons, mille ampere 1/1000 ampere, mille ampere/second measure the quantity of major exposure which is the main dose. Exposure means mille ampere multiplied by second. Main dose determine the quantity of the beam. Mainly determined by milliampere.

11

So if you want to change the quantity you have to think about mille ampere and time , if you want to change the quality you have to change kilo voltage

◛◛◛◛ Trans formers.

a piece of metal surrounded by coils if the number of secondary coils more than primary it's called step up transformer this will increase the kilo voltage here we need high voltage to accelerate electrons moving from cathode to anode, and if the number of secondary coils less than primary it's called step down transformer we need it to decrease the voltage from 100 or 200 up to 5 kilo voltage to generate electrons and heating the filaments, also we need autotransformers to determine the kilo voltage peak and correct minor fluctuations in the current.

Generation of X-ray:

❍ Step down transformers will heat the filaments and generate electron cloud (thermionic

emission)

Homework: why we use tungsten not other material as filament and target ???

❍ Step up transformer will accelerate those electrons

❍ Electron hit the target and generate heat and x-rays.

Tube Current:

Alternating current (AC) or direct current (DC), which is better

Alternating current (AC) we have positive and negative cycles in -ve ones no electrical flow there is area without x-ray generation so it's not good, so we need to rectify the current, the X-ray production will be better, here is the constant potential generator is the best, more efficient, higher energy, shorter exposure of time, so we use direct current (DC) ** x-ray will not be produced without exposure ,while acceleration can occur before exposure.

12

Other requirements for X-ray machines: (Advices when you buy a machine)

safe, accurate, simple, stable, easy adequate mechanism of heat removal, not less than 7 mille ampere not less than 65 kilo voltage, small focal spot, adequate filtration, digital timer sensitive move selector, error sell diagnostic system, adequate for sell services.

sry for any mistake! there is no slides for this lec. You need to read the book also :)

صلى هللا عليه وسلم هللا صدق رسول)) وعالماَ و متعلماَ �ه اا ووم, إ� ذكر هللا , ملعوُن ما فيھا , الدنيا ملعونة((

P :D: شكرا لصبرك اسمــــــــاء

☀☁【THE HAPPY END 】☀☁

Done by :

Baraa'h Al-Salamat

Checked by Sawsan Jwaied ;)