LECTURE 4PRINCIPLE OF IMAGE FORMATION

KAMARUL AMIN BIN ABDULLAH

Lesson Objectives

At the end of the lesson, student should able to:

Define attenuation

Explain interactions between x-rays and matter in

the following :

a) coherent (elastic) scatter

b) photoelectric absorption

c) Compton (inelastic) scatter

d) pair production

Introduction

Interaction of x-ray photons with matter

When a beam of x-rays passes through matter, its

intensity is reduced: attenuated

x-rays are attenuated by either absorption or

scatter

Some x-ray photons are transmitted

Attenuation of x-ray photons

absorbed photons

scattered photons

transmitted photons

X-ray Tube

Interaction of x-ray photons with matter

Attenuation increases with:

A. physical density

B. atomic number

C. electron density

Attenuation decreases with beam energy

Inverse Square Law

Reduction in intensity of a diverging x-ray beam is

also due to beam geometry the inverse square law:

The intensity of a beam of x-rays, from a point source, is

inversely proportional to the square of the distance

from the source, in a vacuum

Inverse Square Law

example

If the output of an x-ray tube is measured as

0.2 mGy at 100 cm, what will be the

approximate output at 3 m?

Absorption

Some x-ray photons are absorbed in matter: their

energy is transferred and the photon disappears

The energy deposited per unit mass of matter is

called absorbed dose

1 joule per kg = 1 Gray (Gy)

The energy deposited as absorbed dose causes

chemical changes due to ionisation which may

result in biological effects

Scattering

Some photons are scattered:

deflected from their original

path

At high photon energies,

scatter is in the forward

direction and contributes to

absorbed dose

At low photon energies,

scatter occurs in all

directions and may

contribute to loss of image

quality by increasing the

overall density of the film

Scattered x-ray photons

contribute to exposure of

staff, patient and public

Scattering

Exponential Attenuation

When a beam of x-rays passes through matter, its

intensity is reduced i.e. attenuated, due to

absorption and scatter

The quantity of x-rays removed depends on the x-

ray photon energy and the density of the matter

Interaction Processes

coherent (elastic) scatter

photoelectric absorption

Compton (inelastic) scatter

pair production

Coherent (elastic) scatter

This type of attenuation process

occurs when the energy of

photons in the beam is small

compared with electron binding

energies in the atoms of the

attenuating medium.

This type of attenuation process

therefore occurs with low

energy radiation and is

sometimes referred to as

"elastic" scattering

This process is not important at

the X-ray energies normally used

in radiography.

Photoelectric absorption

This process occurs when the energy of the incoming photon in

the X-ray beam is equal or not much greater than the electron

binding energy

The photon transfers all it’s energy to an electron

The electron is ejected from the atom (photoelectron)

True absorption has taken place

This process leaves an electron vacancy, which is filled by electron

transition from a higher energy level

There is also the emission of characteristic radiation

The photon energy of characteristic radiation is generally low,

because the elements interacted with in the body have low

proton numbers and low binding energies.

Compton scatter

If an X-ray photon has an energy very much

greater than the binding energy of the electron

with which it interacts, Compton scattering can

take place.

As the photon energy is so great compared with

the electrons binding energy, the electron recoils

from the collision and is ejected at speed from the

atom.

The amount of energy transferred to the electron

increases with the angle of scatter

Pair production

This type of absorption process can only take place

when the incoming photon energy exceeds 1.02 MeV

As the photon passes the nucleus of an atom in the

attenuating material, it experiences the strong electric

forces around the nucleus caused by the positive

charges on the protons there.

As a result, the photon undergoes a dramatic change

of state - its energy is transformed into matter in the

form of two minute particles, hence the given term

pair production.

Summary: Interaction of x-ray photons

with matter

When a beam of x-rays passes through

matter, its intensity is reduced: attenuated

x-rays are attenuated by either absorption or

scatter

attenuation increases with density of matter,

atomic number and electron density

interaction process depends on x-ray photon

energy

Next lesson…

Radiographic Processing