Unit 12 X-Ray and Its Medical Applications

8/14/2019 Unit 12 X-Ray and Its Medical Applications

1/33

Unit XII

Ionizing radiation


2/33

Ionizing radiation causes ionization in matter.

Radiation above10 eV is ionizing radiation.

high energy electromagnetic radiation (energeticphotons), including x-rays and gamma rays,

energetic particles of matter (alpha rays,

neutrons, beta rays, electrons, etc), coming from

radioactivity of some radioactive nuclei.

What is ionizing radiations?


3/33

For electromagnetic radiation:


4/33

X-rays in medicine

X-rays imagingX-rays therapy

X-rays imaging: bone images, mammography

X-ray therapy: x-rays are used to kill canceroustumors


5/33

Observation of bone Mammography


6/33

Rotating beam X-rays therapy


7/33

The interaction of x-rays - matters

The production of x-rays The use of X-rays in imaging The use of X-rays in therapy

Contents


8/33

The interaction of x-rays with matter

When x-rays pass through a medium, its intensity isreduced by interacting with that matter .

This is called attenuation.

There are four mechanisms of attenuation:

Simple scatterPhotoelectric effect

Compton scatterPair production


9/33

The interaction of x-rays: simple scatter

low energy photon scattering process (


10/33

The interaction of x-rays: the photoelectric effect

A high energy electron falls into the low orbit to take the

place of the electron that has been ejected,

during this process, a lower energy photon is emitted.

An x-ray photon collideswith an atom and transfer

all its energy to an inner

orbital electron.

The photon disappears. The electron is ejected. The lost of electrons

causes the ionization of

the atom.

(photon energy 1-100 keV)


11/33

The interaction of x-rays: Compton scatter

Part of the photon energy is transferred to the ejected electron the photon leaves in a different direction with a less energy

a photon collides with an

atom interaction of x-ray photon

with the outer electrons an electron is ejected from the

atom causes the ionization of the

atom.

(photon energy 100-5000 keV)


12/33

The photoelectric effectversus Compton scatter

interaction of x-ray

photon with the inner

orbital electrons

incident photondisappears

low energy photon

emitted due to electron

jump causing ionization

interaction of x-ray

photon with the outer

electrons

incident photon isscattered

low energy photon due to

scattering of the incident

photon causing ionization

The photoelectric effect Compton scatter


13/33

The attenuation of x-rays

When x-rays pass through a medium, the transmittedintensity is reduced due to interaction, this is called

attenuation.

X-ray


14/33

The linear attenuation coefficients

: the linear attenuation coefficient, unit m-

1. depends on

1. incident photon energy

2. nature of the medium.

I

Io

x

x

oeII

=


15/33


The attenuation coefficient is related to the proton number

(Z-number).


16/33

The photoelectric effect

x

oeII

=1

1

xoeII 22

=

Io

Z1x

Io

Z2 x

212121 IIZZ >


17/33

Compton Scatter

x

oeII

=1

1

x

oeII 22

=

Io

Z1x

Io

Z2 x

212121 but IIZZ ==>


18/33


x

oeII =

The half-value thicknessx1/2 : thickness forI=Io/2

2/1

2

1 xooeII

=

2/12 xLn = 693.02

2/1 ==Ln

x

ability of a medium to stop the x-raysSmallerx1/2, stronger attenuation

The half-value thicknessx1/2


19/33


20/33

The Production of x-ray: x-rays tube

I= Vray tube-xofon)(comsumptiPower

ray tube-xofpower

beamray-xofpowerray tube-xofefficiency =


21/33

Thefmax , corresponding to a collision where the

electron losses all its initial kinetic energyKo, can be

obtained from :

h

eVf =max

e: charge of an electron

V: x-ray tube voltage

h = 6.6310-34 J.s (Planks constant)

min

maxenergyphotonray-xmaximum

hchfeV ===


22/33

The X-rays tube is evacuated to high vacuum level,

Electrons are produced at the heated cathode and

accelerated toward anode under a high electricpotential,

The movement of electrons from cathode to the target

forms a tune current, When the electrons hit the anode target, less than 1 %

of incident energy is converted into X-rays photon

energy,

The reminder of the energy is converted into thermalenergy and the anode target get heated,

To prevent the target from overheating, water or oil

circulation is made around the target to remove the

heat through convection


23/33

The x-rays spectrum

The variation of x-rays intensity as a

function of x-rays photons energy

Continuous spectrum

Characteristic

x-ray peaks


24/33

Two features arise from the x-rays spectrum : one is

the continuous spectrum, another is two sharp peaks,

they are due to different mechanisms, For the continuous spectrum , an electron is

accelerated under a potential difference V. It will gain

a kinetic energy equals eV. When the electron hits the target, it undergoes a range

of decelerations. the kinetic energy of the electron is transferred to a

nearby atom it passes by, x-ray photons of a range of energies are emitted from

the tube.


25/33

Thefmax , corresponding to a collision where the

electron losses all its initial kinetic energyKo, can be

obtained from :

h

eVf =max

e: charge of an electron

V: x-ray tube voltage

h = 6.6310-34 J.s (Planks constant)

min

maxenergyphotonray-xmaximum

hchfeV ===


26/33

For the two sharp peaks labeled as characteristic peaks, they are related to

following processes: Some electrons penetrate into the atoms of the target materials,

Some inner orbit electrons are ejected due to energetic incident electrons Outer electrons jump to the vacancies formed due to the evacuation of the

inner electrons. X-ray photons are emitted.

These x-ray photons have

a specific energy which is

determined by thedifference between energy

levels of the electron

before and after jumping.Various sequences of

emissions, grouped as K-

lines and L-lines ,

correspond to different

types of electron transition.


27/33

The principle of X-ray imaging relies upon individual parts ofan absorbing medium attenuating the incident X-ray beam

differently. The typical x-ray machine uses a voltage of 80-

100keV. This produces 30-50keV photons. The dominant

attenuating mechanism at this range is photoelectric effect.

The amount of attenuation that occurs in a medium dependsprincipally :

the thickness of the medium through which the X-

rays pass, the atomic number of the atoms in the medium, the energy of the X-ray photons incident on the

medium.

The x-rays imaging


28/33

the attenuation coefficient is Z3 dependent, somedium of different Z number will results in

very different absorption behaviour of x-ray.

Soft tissue will attenuated very differently than

bone due to their large difference of Z-numbers. a good contrast can be created between bone

and soft tissue. ForCompton scatter, the attenuation coefficient

is independent of Z number, so this mechanism

does not contribute to the contrast between

different components of the tissue.

Thephotoelectric effectis the principal mechanism

involved in x-ray imaging.


29/33

While essentially an X- ray photograph is a 'shadow

photograph', there are shades of grey between the black

and white, formed where the X- ray beam has been

partially attenuated in the medium.

h h


30/33

The x-rays therapy

affect the function of molecules such as DNAcause cell deathcells in the process of dividing are easier to be

damaged by x-rays cancerous cells divide faster than healthy cells, a dose

of x-ray will kill more cancerous cells than healthycells.

When X- rays interact with matter, ionization is resulted.

Radiotherapy is the use of ionizing radiation to treat disease.

The principle of the x-ray therapy is based on the fact that

cancer cells are most susceptible to damage by x-rays.


31/33

When x-rays are used for radiotherapy, the photon

energies are in the range 0.5-5 MeV.This is the energy range for Compton scattering

Attenuation is independent of the Z-number of theabsorbing medium in this energy range.

The Compton scatteris the principal mechanism

involved in x-ray therapy.

If lower photon energy is used, photoelectric effect isdominant, bone will absorb much more x-ray energy

than surrounding tissues, more damage will be done to

the bone than to the tumour to be treated.


32/33

The tumour is accurately located and then X-rays are

aimed at the tumour from different directions: multiple

beam therapy

The X-rays tube is rotated about the patient with thetumor at the centre of the rotation: rotating beam

therapy

In both case the purpose is that the tumor receive larger dose.


33/33

Biological effect of ionizing radiation

Ionizing radiation can directly damage DNA, RNA and enzymes.

However ionizing radiation can also ionize water to form H+ and

Documents

Unit 12 X-Ray and Its Medical Applications