Collimators and Grids Flashcards
What is the use of wedge filters in x-ray filtration?
produce uniform density image, if tissue is thicker on one side (e.g. breast?)
Functions of added filtration in x-ray production?
- lowers patient dose (removes unwanted low energy x-rays)
- change beam spectrum (e.g. optimise for mammography/ paeds)
- remove useless high energy x-rays
- change density of x-rays as needed (depending on tissue to be images)
Metric to measure filtration of x-ray
Half-Value Layer
Unit for Half Value Layer
mm Aluminium
What are compensating filters used for in x-ray production?
Decrease total radiation dose by decreasing x-ray intensity to thinner areas of tissue
(e.g. trough filter for chest x-ray)
Metric(s) to measure anti-scatter grid performance?
Contrast Improvement (CIF)
Bucky Factor (BF)
What are the two basic categories of anti-scatter grids?
Stationary Grids
moving Grids
What are filtration grids normally made of?
Lead, with aluminium or plastic in between
What is ‘Grid Frequency’ in anti-scatter x-ray grids?
The number of lines/cm of grid strips.
Benefits/ negatives of high grid frequency, in x-ray filtration grids?
- less distinct grid lines,
- more radiation dose
What are the 2x2 types of anti-scatter grids (based on construction/ alignment)
Linear
- parallel
- focussed
Crossed
- parallel
- focussed
What is the downside of PARALLEL x-ray filtration grid?
cut-off of x-rays near the edge of the image (as they are coming in at an angle from the source)
What is the difference between parallel/ focused x-ray filtration grids?
Parallel grids: the grid stripes are all parallel and go straight up vertically.
Focused grids: the strips are pointed towards a focal point (where the x-rays come from),
What is the downside of FOCUSED x-ray filtration grid?
need to be careful with distance from x-ray source
What is ‘Grid Ratio’ in x-ray filtration grids?
Ratio of the height of the strips to the width of the gaps at the central line
What is ‘Contrast improvement Ratio’ in x-ray filtration grids?
Ratio of the transmission of primary radiation to the transmission of total radiation
What is ‘Grid Exposure Factor’ in x-ray filtration grids?
Ratio of total radiation without the anti-scatter grid to with the anti-scatter grid
What is ‘Strip Number’ in x-ray filtration grids?
strips/ cm
What is ‘Grid focusing distance in x-ray filtration grids?
Distance between the front of a focused grid and the line formed by the converging planes which include the attenuating lamella of the grid
What is ‘Contrast Improvement Factor’ (CIF) in x-ray filtration grids?
image contrast with/ without grid
radiographic contrast with grid) / (radiographic contrast without grid
What is ‘Bucky Factor’ (BF) in x-ray filtration grids?
increase in dose associated with the use of grid
Calculation of ‘Grid Ratio’
in x-ray filtration grids
height of grid/ thickness of interspace material
Good/ Bad of high Grid Ratio?
in x-ray filtration grids
– are more effective
– are more expensive
– deliver higher dose
What is ‘Bucky Factor’ also known as?
Grid Factor
What is ‘Selectivity’ in x-ray filtration grids?
(primary radiation after grid)
______________________
(scatter radiation after grid)
What is ‘Grid Cut-off’ in x-ray filtration grids?
undesirable absorption of primary radiation by the grid
What is the ‘Air Gap Technique’
in x-ray scatter reduction
the patient is deliberately moved away from the image receptor.
Reduces scatter
