interaction of x-rays

Question 1

quanta

Answer 1

also known as photons, have no mass or electric charge which can travel in straight lines

Question 2

four possible fates of individual x-ray photons

Answer 2

1) penetration
2) absorption
3) scattered with energy loss
4) scattered without energy losss

Question 3

penetration

Answer 3

photon penetrates the matter without interacting with it

Question 4

absorption

Answer 4

photo interacts with the matter and is completely absorbed by depositing all of its energy into the matter

Question 5

scattered

Answer 5

photon interacts with the matter and is deflected from its original direction, depositing either part of none of its energy into the matter

Question 6

attenuation

Answer 6

reduction in beam intensity, an attempt to remove x-ray beam by measuring how many photons have been removed from beam

Question 7

attenuation processes for diagnostic imaging

Answer 7

elastic scatter, Compton scatter and photoelectric absorption

Question 8

elastic scatter

Answer 8

occurs at very low x-ray energies and involves scattering of photons without loss of energy

Question 9

process of elastic scatter

Answer 9

• energy of the incident photon is less than the energy gap; isn’t sufficient to excite the atomic electron to the next allowable orbital shell
• atomic electron is unable to absorb any of the energy of the incident photon
• no loss of photon energy, therefore elastically scattered

Question 10

inelastic scatter

Answer 10

deflection of photons with an associated loss of photon energy

Question 11

process of inelastic scatter

Answer 11

• energy of the incident photon is greater than the energy gap
• some of the energy of the incident photon is transferred to the atomic electron, raising it to the next allowable orbital shell. Atom is in an excited state. Electron will fall back to its original orbital shell, emitting radiation in the process
• photon is scattered inelastically, loss of energy

Question 12

process of COMPTON inelastic scatter

Answer 12

• energy of the incident photon is greater than the binding energy of the outer electron
• some of the energy of the photon is transferred to the outer electron as KE, causing it to recoil resulting in ionisation of the atom
• photon is scattered inelastically, increase in wavelength and decrease in frequency

Question 13

photoelectric absorption

Answer 13

inelastic collision of a photon with an orbiting electron in an atom of an absorbing material, with all the energy of the photon given up to the electron

proportional to the cube of the atomic number

Question 14

process of photoelectric absorption

Answer 14

-energy of the incident photon is greater than the energy and greater than the binding energy of the inner shell electron

• all the energy of the incident photon is transferred to the electron as KE, causing the electron to be liberated from the atom and resulting in ionisation of the atom.
• electron vacancy will be filled by an electron from another emitting radiation.

Question 15

mass attenuation coefficient

Answer 15

fraction of photons removed from a beam of unit cross-sectional area by unit mass of the attenuating medium

Question 16

area mass equation

Answer 16

thickness x density

Question 17

why are anti-scatter grids placed between the patient and the image receptor?

Answer 17

to reduce the scattered photons reaching the receptor by Compton scatter, thus improving image quality

Question 18

grids

Answer 18

made of parallel strips of high attenuating material with an interspace filled with low attenuating material

Question 19

grid ratio

Answer 19

measure of the height of the lead strip to the interspace distance, a higher grid ratio will reject scatter better, however a high dosage is usually involved

Question 20

grid frequency

Answer 20

measure of grid lines per unit distance