Extra-focal rad

Question 1

What is extra-focal radiation?

Answer 1

: Extra-focal radiation, also known as off-focus radiation, is radiation that originates from areas outside the focal spot.

Question 2

What are the effects of extra-focal radiation?

Answer 2

It is undesirable as it increases patient skin dose and reduces image contrast.

Question 2

How can scattered radiation be controlled?

Answer 2

Scattered radiation can be controlled by:

• Reducing the amount of scattered radiation produced.
• Reducing the amount of scattered radiation reaching the receptor.
• Reducing the amount of scattered radiation affecting the quality of the image.

Question 3

What is the diagnostic purpose of extra-focal radiation?

Answer 3

It is of no use for diagnostic purposes.

Question 4

How can the amount of scattered radiation produced be reduced?

Answer 4

• Close control of extra-focal radiation.
• Reducing the thickness of tissue irradiated where possible.
• Careful selection of exposure factors.

Question 5

How can the area of tissue irradiated be restricted?

Answer 5

• Restricting the size of the primary beam to cover the smallest area possible consistent with the requirement of the examination.
• Using beam limiting equipment to remove extra-focal radiation effectively and provide a clean cut-off of the radiation at the borders of the field.
• Controlling the size of the primary beam with a simple metal diaphragm with a central aperture or a movable diaphragm.

Question 5

What are the methods to control extra-focal radiation?

Answer 5

• Close collimation to reduce extra-focal radiation.
• Using a diaphragm with the smallest aperture to cover the largest field size required.
• Positioning the diaphragm as close as possible to the focal spot.

Question 6

How can the thickness of tissue irradiated be reduced?

Answer 6

• Using external compression, mechanically or with compression bands.
• Positioning the patient so that their body weight moves displaceable tissue out of the beam (e.g., prone position for abdominal X-rays)

Question 7

What are the benefits of reducing the thickness of tissue irradiated?

Answer 7

• It produces less scatter within the tissue,
• Improves image quality
• Reduces the radiation dose to the patient

Question 8

How does the energy of the beam affect scattering?

Answer 8

• At low energy (below 70 kVp), photoelectric absorption is the most attenuating effect, leading to total absorption and no scattering, increasing image contrast.
• At high energy (above 70 kVp), Compton effect becomes more dominant, increasing scatter and reducing image contrast.

Question 9

What are the methods to absorb residual scatter before it reaches the film?

Answer 9

Use of anti-scatter grid interposed between the patient and the film.

Arranging an air gap between the patient and the film.

Absorbing any back scatter produced behind the film

Question 10

What is the function of an anti-scatter X-ray grid?

Answer 10

It improves the diagnostic quality of the radiograph by absorbing the greater part of radiation scattered by the patient in the direction of the image receptor.

Question 11

What is grid cut-off?

Answer 11

Grid cut-off refers to the decrease in the number of transmitted photons that reach the image receptor because of some misalignment of the grid.

Question 12

What are the types of grid cut-off?

Answer 12

Up-side down

Off-level

Off-center

Off-focus

Question 14

What is the principle behind the air gap technique?

Answer 14

The air gap technique works on the principle that radiation scatters uniformly, ensuring that most scatter misses the film.

Question 15

How can back scatter be absorbed?

Answer 15

By placing a sheet of lead or lead rubber immediately behind the film to absorb any transmitted radiation, leaving no scatter to be projected back to the film.