Radiology year 4 lecture 3

Question 1

Define the intensity of the beam.

Answer 1

The amount of energy flowing per second through a unit area perpendicular to the direction of the beam.

Question 2

When does the beam stop diverging?

Answer 2

When it encounters an object with which it interacts

Question 3

What happens to the area covered and the intensity when the beam diverges?

Answer 3

Area covered increases and intensity decreases.

Question 4

What does the inverse square law state?

Answer 4

The intensity of the x-ray beam covering a given cross-sectional area is inversely proportional to the square of the distance from the source.

Question 5

What is the distance from the source to the film known as?

Answer 5

Film-focal distance

Question 6

If the film focal distance is doubled, what must mAs be multiplied by in order to keep the intensity constant?

Answer 6

4

Question 7

Why is the inverse square law important for safety?

Answer 7

The further away the radiographer stands from the primary source, the lower the intensity of the beam and thus the smaller the chance of recieving a radiation dose.

Question 8

how is the shadowgram (radiograph) generated?

Answer 8

differential absoption levels by different tissues in the body.

Question 9

What would the radiofraph look like if all body tissues absorbed xrays to the same extent?

Answer 9

No detail

Question 10

In which two ways do xrays interact with matter in diagnostic radiography?

Answer 10

Photoelectric attenuatio and Compton attenuation.

Question 11

What does the degree of attenution depend on?

Answer 11

atomic number of the tissue, physical density/ specific gravity of the tissue, thickness of the tissue.

Question 12

What happens to x-rays that reach the film?

Answer 12

blacken it

Question 13

What happens if there is more attenuation?

Answer 13

Fewer photons reach the film and thus the image is whiter.

Question 14

put fat, muscle, lead, bone, water and air in order of radiographic opacity

Answer 14

air (black), fat, water, muscle, bone, lead (bright white).

Question 15

Why is air less radiopaque than far, despite having a higher effective atomic number?

Answer 15

It is much less dense.

Question 16

Describe the photoelectric effect.

Answer 16

This is when an xray photon is completely absorbed. it collides with an inner orbital electron and ejects it from its shell, releasing heat. The ejected inner orbital electron (the photoelectron) travels a short distance in the tissue before being converted to head. An outer orbital electron drops into the available space in the inner shell, giving off a photon of characteristic radiation, which is of insufficient energy to leave the body and is thus absorbed.

Question 17

How does the photoelectric effect vary with the atomic number of the tissue?

Answer 17

Proportional to the cube of the atomic number

Question 18

How does the photoelectric effect vary with the kv?

Answer 18

It is inversely proportional to the cube of the kV

Question 19

Up to what level of exposure does the photoelectric effect predominate?

Answer 19

60kV

Question 20

What effect is responsible for the formation of the shadowgram?

Answer 20

photoelectric.

Question 21

What is the Compton effect?

Answer 21

This is a type of xray attenuation when the xray photon is incident to an outer, loosely bound electonr. The incident photon removes the elecron from the shell but does not give up all its energy in doing so. Instead, is looses some energy and changes to a londer wavelength and travels in a different direction. This leads to scattered radiation as the resulting energy may still be able to reach the film but hits it at a different direction. This leads to fogging (blackening) of the film and increases the radiation dose.

Question 22

What does the Compton effect lead to?

Answer 22

fogging of the film

Question 23

What factors do the Compton effect depend on?

Answer 23

Thickness of tissue, density and kV settings. (not atomic number).

Question 24

How does Compton attenuation change as kV increases?

Answer 24

It also increases.

Question 25

Can radiation which initially goes through the patient be scattered back?

Answer 25

yes

Question 26

how does the angle of deflection relate to the energy loss of the photon in the compton effect?

Answer 26

The greater the angle of deflection, the greater the energy loss of the photon, so the less likely it will reach the film and cause fogging.

Question 27

In what three ways can scattered radiation be reduced?

Answer 27

Collimation of the primary beam, low kV settings and compression of the patient.

Question 28

Decribe how collimation of the primary beam can reduce scatter

Answer 28

If the primary beam is collimated only to the area of interest, there will be less tissue to act as a focus for scatter production.

Question 29

What is the most accurate collimating device and why?

Answer 29

light beam diaphragm as the light and mirror components can be angled so as to cover the same area as the primary beam, so the area exposed can be directly visualised.

Question 30

How can a light beam diaphragm be checked for accuracy?

Answer 30

Eight coins, pairs placed either side of the collimation line on the film. When the film is exposed and developed. the area of the primary beam should correspond to that described by the collimarion light (limited by the junction of each pair of coins).

Question 31

How can altering the kV settings be used to reduce scatter? What are the limitations of this?

Answer 31

Setting it lower reduces scatter as photoelectric effect will predominate. The problems are that it increases the radiation dose in the patient and also depends on the area of interest and power of the xray machine. Also, if kV is reduced, mAs must be increased which may result in movement blur and may not be possible in low/ fixed mAs machines.

Question 32

Why does compression limit the compton effect?

Answer 32

As it is dependent on the thickness and density of the tissue.

Question 33

How may areas be compressed to reduce the compton effect?

Answer 33

wooden paddles or with a Bucky band (a rubber sheet buit into some xray tables which stretches across and can compress the animal).

Question 34

How can the amount of scatter reaching the film be reduced?

Answer 34

using air gap, lead-backed cassettes, intensifying screens or grids.

Question 35

What is the air gap technique and what is it used for?

Answer 35

used to reduce scatter reaching the film. It increases the object-film distance and thus the lower energy scattered radiation is unable to reach the film so does not cause fogging.

Question 36

How do lead-backed cassettes reduce scatter?

Answer 36

Absorbs the radiation, preventing it from reaching the film or patient.

Question 37

What do intensifying screens do?

Answer 37

Intensify the primary beam much more than the scattered radiation so minimise scatter

Question 38

What are potter-bucky grids and what do they do?

Answer 38

they are flat, square plates between the patient and cassette. They consist of thin stribs of xray abrosbing material (eg lead) interspaced with strips of radiiolucent material (wood, plastic etc). They are designed so that the primary beam passes straight through whilst oblique, low energy scattered radiation is absorbed by the lead.

Question 39

What two categories of grid are there?

Answer 39

stationary and moving

Question 40

What are the types of stationary grid?

Answer 40

parallel, focused,. pseudofocussed and cross-hatched.

Question 41

Describe each a parallel, focussed, pseudofocussed and cross-hatched grid

Answer 41

see sheets

Question 42

Waht design can the moving grids be? and what is the purpose of them?

Answer 42

same as the stationary but they oscillate around a centre point during exposure to blur out the grid lines.

Question 43

How can grids be classified and discuss each.

Answer 43

lines per centimeter (24-60- more is more costly but bread up the image less) grid ratio (height of the lead strips to the width of the interspace. higher ratio, more scatter absorbed) Grid factor- factor by which the mAs is increased in order to account for the primary xray photons absorbed by the grid. Depends on lines/cm and grid ratio. Can be counterproductive if too long Grid efficiency- depends on the amount of primary beam allowed thorugh relative to the scatter absorbed. Depends on lines/cm, grid ratio and thickness of the strips

Question 44

Which grid should be chosen?

Answer 44

The most expensive! of such a size to fit the largest cassette used.