This document discusses the interaction of x-ray photons with matter and their attenuation. There are five main interaction types: coherent scattering, Compton scattering, photoelectric effect, pair production, and photodisintegration. In diagnostic medical imaging, the dominant interactions are the Compton effect and photoelectric effect. The Compton effect involves the x-ray scattering off and ejecting an outer shell electron. This produces scattered x-rays and reduces image contrast. The photoelectric effect fully absorbs the x-ray when it ejects an inner shell electron. This is the primary means of x-ray absorption and produces high quality images. The degree of x-ray attenuation is dependent on factors like energy, density, and atomic number of

1. INTERACTION & ATTENUATION
DR PRAJWITH RAI

2. INTERACTION & ATTENUATION
• Xray photons interact with matter at atomic level, either with nucleus
or orbital electrons, depending on its energy.

3. INTERACTION & ATTENUATION
These interactions are
1. Coherent scattering
2. Compton effect
3. Photoelectric effect
4. Pair production
5. Photodisintegration.

4. INTERACTION & ATTENUATION
• In diagnostic range these xrays interact with orbital electrons
causing ionization.
• Two interactions which play dominant role in forming the
radiographic image are
• Compton effect and
• Photoelectric effect.

5. COMPTON EFFECT OR COMPTON SCATTERING
• Xray photons interact with outer orbital electrons,
• ejects it from the atom,
• thus ionizing the atom.
• In this interaction xray photon loses some of its energy in ejecting the electron
(Compton or recoil electron)
• retains rest of the energy and moves in different direction (Scattered xrays).

6. COMPTON EFFECT OR COMPTON SCATTERING
• These scattered xrays produce no useful information in the radiograph , rather
they produce fog
• A uniform optical density on the radiograph that results in reduced image
contrast.
• Or in other words, this is an undesirable interaction.
• This scattered radiation from patient during fluoroscopy is the major source of
radiation exposure that radiologic technologist recieves.

7. Compton Scattering
I. Recoil
Electron
2. Scattered
Photon
3. Positive Ion
N
N
q

8. • α-1 xray energy
• α density of absorber
• does not depend on the atomic number
COMPTON EFFECT OR COMPTON SCATTERING

9. PHOTOELECTRIC EFFECT
• Xray photon interacts with inner shell electron,
• xray photon is completely absorbed, loses all its energy.
• Ejected electron (now called photoelectron) escapes with kinetic energy equal
to the difference between energy of incident xray and the blinding energy of the
electron.
• Thus this is an xray absorption interaction.

10. PHOTOELECTRIC EFFECT
• Major source of radiation exposure to the patient, as xray photons are
completely absorbed.
• Excellent quality of radiographic image as there is no scatter.

11. I. Photo
Electron
N
N
N
2. Characteristic
Radiation
3. Positive Ion
Photoelectric Effect

12. PHOTOELECTRIC EFFECT
• α-1 (xray energy)3
• α mass density of absorber.
• α (atomic number)3

13. PHOTOELECTRIC EFFECT
• Xray photon interacts with inner shell electron,
• xray photon is completely absorbed, loses all its energy.
• Ejected electron (now called photoelectron) escapes with kinetic energy equal
to the difference between energy of incident xray and the blinding energy of the
electron.
• Thus this is an xray absorption interaction.

14. Attenuation
• Attenuation is the reduction in the intensity of xray beam passing through the
matter due to
• absorption (Photoelectric reaction)
• deflection (Compton scattering).
• Xrays which are either absorbed or transmitted (Differential absorption)
eventually produce the useful radiographic image.
• Compton scattering results in fog, a generalized increased optical density not
representing the diagnostic information.

15. Differential Absorption (image quality)
• α-1 xray energy(KVP)
• α mass density of absorber.
• α atomic number

16. Factors affecting differential
absorption
Transmission
Decreases Decreases Increases
No change Increases Decreases
Increases Increases Decreases

17. Attenuation
• Radiographic contrast such as Iodine and Barium use the principle of differential
absorption due to their high atomic numbers (Iodine 53, Barium 56),
↑ image contrast and quality
↑ absorption compared to soft tissues,
↑ photoelectric reactions
high atomic numbers (Iodine 53, Barium 56),

18. Water
Compact
Bone
Sodium
Iodide
PhotoelectricCoherentCompton
100
50
0
20 60 100 20 60 100 20 60 100
keV keV keV
Percent
Interaction Between X-rays & Matter