GRIDS and AEDs

Question 1

Ionising radiation when it leaves the x ray tube

Answer 1

A point source, straight lines, diverges

Question 2

Grid

Answer 2

• filter out scattered radiation from patient
• image formed is with improved contrast
• positioned between patient and image receptor (Bucky)
• contains rows of fixed lead slays at spaced intervals
• radiolucent between slats
• design important so that it doesn’t show up as artefact on image
• design feature to prevent this is called moving grid

Question 3

Advantages of grid

Answer 3

• reduces scatter from patient
• improves contrast between anatomical structures
• improves diagnostic quality for techniques that include many tissue types

Question 4

Disadvantages of grid

Answer 4

• increased exposure factor needed
• increased dose
• is used incorrectly = image quietly can reduce = SID is crucial to allow alignment of angled slats

Question 5

AED stand for

Answer 5

Automatic exposure device

Question 6

AED involve

Answer 6

And exposure measuring device

Consists on ionisation chamber, capacitor, voltage sensitive switch

It determines when the exposure should be terminated by detecting when appropriate exposure on receptor has reached

MAS pre established, only need to set Kvp

Backtimer prevents overexposure

Question 7

Ionisation chambers in AED

Answer 7

More than one chamber selected, electrical energy selected as a result of combined air ionisation is averaged and delivered to capacitor

Question 8

Advantages of AED

Answer 8

• maintains quality of images

- efficient = reduce number of repeats = reduce patient dose

Question 9

Disadvantages of AED

Answer 9

• chambers cannot differentiate between transmitted and scattered radiation
• still radiographer dependent for patient positioning
• unknown pathology/ metalwork present = images of sub optimal density