PRESENTATION BY: Dr. Fahad Shafi P.G 1ST Year

Deptt of Radiodiagnosis

Physical interaction of x ray with matter

Dual characteristics of X-raysX-rays belong to a group of radiation called

electromagnetic radiation . Electromagnetic radiation has dual

characteristic, comprises of both WaveParticle

Wave concept : Propagated through space in the form of waves. Waves of all types have associated wavelength and frequency

Particle concept of EM radiation: Short EM waves such as X-rays predominantly react with matter as if they were particles rather than waves.. These particles are actually discrete bundles of energy and each of these bundles is called a quantum or photon.

Particle concept used to describe interaction between radiation & matter

METHODS OF INTERACTIONS

Photons : absorbed / scattered.

Attenuation : Reduction of intensity. Difference in attenuation gives the radiographic image.

Absorbed : completely removed from the x-ray beam & cease to exist.

Scattered : Random course. No useful information. No image only darkness. Adds noise to the system. Film quality affected : “film fog”.

About 1% of the x rays that strike a patient's body emerge from the body to produce the final image. The radiographic image is formed on a radiographic plate that is similar to the film of a camera.

Remaining 99% of the x-rays --- Scattered / Absorbed.

ATOMIC STRUCTUREX-ray photons may interact either with orbital

electrons or with the nucleus. In the diagnostic energy range, the interactions are always with orbital electrons.

The molecular bonding energies ,however are too small to influence the type and number of interactions .

The most important factor is the atomic make up of a tissue and not its molecular structure.

Atomic structure

K shell : 2 electrons L shell : 8 electrons Each shell has a specific binding energy & closer the shell is

to the nucleus, the tighter it is bound to the nucleus. The electrons in the outermost shell are loosely bound to the nucleus & are hence called “free electrons”.

Basic structure of an ATOM : PROTON ( +ve charge )

An atom is made up of NUCLEUS NEUTRON ( neutral ) ORBITAL ELECTRONS ( -ve charge ) ORBITS / SHELLS ( K, L, M, N etc. )

Energy value of electronic shells is also determined by the atomic number of the atom.

K-shell electron are more tightly bound in elements of high atomic number. Pb : 88keV while Ca : 4keV.

Electrons in the K -shell are at a lower energy level than electrons in the L-shell. If we consider the outermost electrons as free ,than inner shell electrons are in energy debt. The energy debt is greatest when they are close to nucleus in an element with a high atomic number.

BASIC INTERACTIONS BETWEEN X-RAYS AND MATTER

There are 12 mechanism, out of which five basic ways in which an x-ray photon may interact with matter.

These are :Broadly classified on the basis of-

A: PHOTON SCATTERING:

- COHERENT SCATTERING

- COMPTON SCATTERING

B: PHOTON DISAPPEARANCE

- PHOTOELECTRIC EFFECT

- PAIR PRODUCTION

-PHOTODISINTEGRATION

1. COHERENT SCATTERING

Radiation undergoes Only Change in direction. No change in wavelength

thats why sometime called “ unmodified scattering”

Coherent scattering of X-rays is an interaction of the wavetype in which the X-ray is deflected.

Coherent Scattering occurs mainly at low energies.

It is of two types :Both type described in terms of “ wave Particle

Interaction” ( also called “ Classical scattering”)

Thomson scattering : Single electron involved in the interaction.

Rayleigh scattering : Co-operative interaction of all the electrons.

1. COHERENT SCATTERING

What happens in coherent scattering ? Low energy radiation encounters electrons

Electrons are set into vibration Vibrating electron, emits radiation. Atom returns to its undisturbed state

Fig : Rayleigh scattering

1. COHERENT SCATTERING

No ionization --- why??? because, no energy transfer. Only change of direction.

Only effect is to change direction of incident photon.

Less than 5%. Not important in diagnostic radiology. Produces scattered radiation but of negligible quantity.

2. PHOTOELECTRIC EFFECT

What happens in Photoelectric effect ?

An incident PHOTON encounters a K shell electron and ejects it from the orbit

The photon disappears, giving up ( nearly) all its energy to the electron

The electron ( now free of its energy debt) flies off into space as

a photoelectron carrying the excess energy as kinetic energy.

The K shell electron void filled immediately by another electron

and hence the excess energy is released as CHARACTERISTIC RADIATION.

The atom is ionised.

PHOTOELECTRIC EFFECT

Percentage of photoelectric reactions

Radiation energy(keV)

Water Compact bone

Sodium iodide

20 65 89 94

60 7 31 95

100 2 9 88

Characteristic radiationHow does this happen ?

After the electron has been ejected, the atom is left with a void in the K shell & an excess of energy equivalent to the binding energy.

This state of the atom is highly unstable & to achieve a low energy stable state ( as all physical systems seek the lowest possible energy state ) an electron immediately drops in to fill the void.

As the electron drops into the K shell, it gives up its excess energy in the form of an x-ray photon. The amount of energy released is characteristic of each element & hence the radiation produced is called Characteristic radiation.

2. PHOTOELECTRIC EFFECT

Thus the Photoelectric effect yields three end products :

Characteristic radiation A -ve ion ( photoelectron ) A+ve ion (atom deficient in one electron )

2. PHOTOELECTRIC EFFECT

Probability of occurrence : The incident photon energy > binding energy of the

electron.

Photon energy similar to electron binding energy

Photoelectric effect 1

(energy)³

The probability of a reaction increases sharply as the atomic no. increases

Photoelectric effect (atomic no.)³

Low atomic number : interaction mostly at the K shell.

High atomic number : interaction mostly at L and M shell.

In summary, Photoelectric reactions are most likely to occur with low energy photons and elements with high atomic numbers provided the photons have sufficient energy to overcome the forces binding the electrons in their cells.

For eg : I2

K shell :33.2keVL-shell : 4.9keVM shell 0.6 Kev.

From L-shell to K-shell a 28.3 kev(33.2-4.9=28.3) keV photon is released.

The void in the L-shell is then filled with a photon from the M shell with the production of a ( 4.9-0.6 KeV)4.3 keV photon.

K-shell electron binding energies of elements important in diagnostic radiology

Atom Atomic number K-shell binding

energy(keV)

CalciumIodineBariumTungstenLead

2053567482

4.0433.237.469.588.O

2. PHOTOELECTRIC EFFECT : Applications in diagnostic radiology :

Advantages :

Excellent radiographic images :

No scatter radiation.

Enhances natural tissue contrast. Depends on 3rd power of the atomic no., so it magnifies the difference in tissues composed of different elements, such as bone & soft tissue

Lower energy photons : total absorption. Dominant upto 500 keV.

Disadvantage:

Maximum radiation exposure.

All the energy is absorbed by the patient whereas in other reactions only part of the incident photon’s energy is absorbed.

3. COMPTON EFFECT

The Compton effect occurs when the incident x-ray photon with relatively high energy ejects an electron from an atom and a x-ray photon of lower energy is scattered from the atom.

The reaction produces an ion pair

A +ve atomA –ve electron ( recoil

electron )

COMPTON SCATTERING

Almost all the scatter radiation that we encountered In diagnostic radiology comes from Compton Scattering

Kinetic energy of recoil electron

Energy of photon distributed

Retained by the deflected photon.

Two factors determine the amount of energy the photon transmits : The initial energy of the photon. Its angle of deflection.

1.Initial energy :- Higher the energy more difficult to deflect.High energy : Travel straight retaining most of the energy. Low energy : Most scatter back at angle of 180º

2. Angle of deflection :- Greater the angle, lesser the energy trasmitted. With a direct hit, maximum energy is transferred to the recoil electron. The photon retains some energy & deflects back along its original path at an angle of 180º.

3. COMPTON EFFECT

ENERGY OF COMPTON SCATTERED PHOTONS

· The change in wavelength of a scattered photon is calculated as :Δλ = 0.024 ( 1 – cos θ ) ,

where Δλ = change in wavelength θ = angle of photon deflection

3. COMPTON EFFECTProbability of occurence :

It depends on :-

Total number of electrons : It further depends on density and number of electrons per gram of the absorber. All elements contain approx. the same no. of electrons per gram, regardless of their atomic no. Therefore the no. of Compton reactions is independent of the atomic no. of the absorber.

Energy of the radiation : The no. of reactions gradually diminishes as photon energy increases, so that a high energy photon is more likely to pass through the body than a low energy photon.

Disadvantages of Compton reaction :

Scatter radiation : Almost all the scatter radiation that we encounter in diagnostic Radiology comes from Compton scattering. In the diagnostic energy range, the photon retains most of its original energy. This creates a serious problem, because photons that are scattered at narrow angles have an excellent chance of reaching an x-ray film & producing fog.

Exceedingly difficult to remove – ► cannot be removed by filters because they are too

energetic. ► cannot be removed by grids because of narrow angles of

deflection.

It is also a major safety hazard. Even after 90˚ deflection most of its original energy is retained. Scatter radiation as energetic as the primary radiation.

Safety hazard for the radiologist, personnel and the patient.

Exceedingly difficult to remove – ► cannot be removed by filters because they are too energetic. ► cannot be removed by grids because of narrow angles of deflection.

4. PAIR PRODUCTION

No importance in diagnostic radiology.

What happens in Pair production ?A high energy photon interacts with the nucleus of an atom.

The photon disappears & its energy is converted into matter in the form

of two particles An electron A positron (particle with same mass as electron, but with

+ve charge.)

Mass of one electron is 0.51 MeV.2 electron masses are produced.So the interaction cannot take place with photon energy less

than 1.02 MeV.

4. PAIR PRODUCTION

Positron annihilation. What happens to the Positron ?

Slowly moving Positron combines with a free electron to produce two photons of radiation. 2 mass units are converted, giving a total energy of 1.022 MeV. To conserve momentum, two photons each with 0.511 MeV energy are ejected in opposite direction.

5. PHOTODISINTEGRATION

A photon with extremely high energy ( 7-15 MeV), interacts directly with the nucleus of an atom.May eject a neutron, proton or on rare occasions even an alpha particle.

No diagnostic importance. We rarely use radiation >150 KeV in diagnostic radiology.

What happens in Photodisintegration ? A high energy photon encounters the nucleus of an atom.

Part of the nucleus which may be a neutron, a proton, an alpha particle or a cluster of particles, is ejected.

RELATIVE FREQUENCY OF BASIC INTERACTIONS

ØCoherent scattering : About 5% . Minor role throughout the diagnostic energy range.

ØCompton scattering : Dominant interaction in water.Water is used to represent tissues with low atomic nos. such as air, fat and muscle.

ØPhotoelectric reaction : usually seen in the contrast agents because of their high atomic numbers.

Bone is intermediate between water & the contrast agents. At low energies, Photoelectric reactions are more common, while at high energies, Compton scattering is dominant.

RELATIVE FREQUENCY OF BASIC INTERACTIONS

X-ray photon energy

Photoelectric

absorption %Compton

scatter %Pair production%

10 keV 95 5 0

25 keV (Mammography

50 50 0

60 keV (Diagnostic)

7 93 0

150 keV 0 100 0

4 MeV 0 94 6

10 MeV (Therapy)

0 77 23

24 MeV 0 50 50

SUMMARYOnly two interactions are important in diagnostic radiology, the Photoelectric

effect & Compton scattering.Ø The Photoelectric effect

is the predominant interaction with low energy radiation & high atomic no. absorbers.

It generates no significant scatter radiation & produces high contrast in the x-ray image.

But, unfortunately it exposes the patient to a great deal of radiation.Ø Compton scattering is the most common interaction at higher diagnostic

energies. responsible for almost all scatter radiation.

radiographic image contrast is less compared to

photoelectric effect.Ø Coherent scattering is numerically unimportant.Ø Pair production & Photodisintegration occur at energies above

the useful energy range.

THANK YOU