Chapter 8 RADR 1313

Which tools limit scatter radiation?

Beam-restricting devices and radiographic grids.

What results from a larger x-ray beam field size?

Increased scatter and fog, with decreased contrast.

What is the purpose of positive beam limitation?

To limit the exposure field to the image receptor size and reduce patient exposure.

Which beam-restricting device best limits unsharpness surrounding the radiographic image?

Collimator.

What shape does the unrestricted primary beam produce?

Round.

What does decreasing collimation result in?

A larger field size.

As beam restriction increases, what happens to field size and patient dose?

Both decrease.

What happens as beam restriction increases?

Less scatter, less tissue irradiated, and higher image contrast.

Which beam-restriction device can be cut to the required size?

Aperture diaphragm.

Which beam-restricting device is least effective at limiting surrounding unsharpness?

Aperture diaphragm.

What is an aperture diaphragm with an attached elongated tube called?

Cylinder.

Which beam-restricting device has two or three sets of lead shutters?

Collimator.

What is the purpose of the mirror inside a collimator?

To project a light field onto the patient.

Which devices limit the volume of tissue irradiated?

Cone and collimator.

What device uses thin lead strips and radiolucent interspaces to absorb scatter?

Radiographic grid.

Which is not a way to reduce scatter production?

Increasing the grid ratio.

What affects scatter reaching the IR without reducing scatter production?

Increasing the grid ratio.

What does grid ratio measure?

Lead-strip height divided by the distance between lead strips.

How can grid effectiveness at removing scatter be increased?

Increase lead-strip height and decrease the space between strips.

What does the number of lead strips per inch measure?

Grid frequency.

Which grid is most challenging because of increased potential grid cutoff?

Cross-hatched grid.

What results from imaging a thicker part or patient?

More Compton interactions.

Grid strips are 16 mm high and 2 mm apart. What is the grid ratio?

8:1. Grid ratio = 16 ÷ 2 = 8.

What happens to scatter removal as grid ratio increases?

Scatter removal increases.

How is the grid conversion factor (GCF), also called the Bucky factor, calculated?

mAs with grid ÷ mAs without grid.

Which grid design matches the divergence of the x-ray beam?

Focused grid.

What is the grid conversion factor for a 12:1 grid?

5×. mAs with grid = mAs without grid × 5.

What occurs when the x-ray beam is not properly aligned with grid strips?

Grid cutoff.

When should a grid generally be used?

When part thickness exceeds 10 cm; the study guide also recommends kVp above 60.

What technique may be used for certain examinations when a grid is needed but unavailable?

Air-gap technique.

What grid error causes proper exposure in the middle of the IR but marked underexposure along its sides?

Upside-down focused grid.

What is found between the lead strips in a grid?

Radiolucent material.

A knee image uses 10 mAs with an 8:1 grid. How much mAs is needed without the grid?

2.5 mAs. Without-grid mAs = 10 ÷ 4 = 2.5 mAs.

An AP abdomen uses 40 mAs with a 12:1 grid. How much mAs is needed with a 6:1 grid?

24 mAs. New mAs = 40 × (3 ÷ 5) = 24 mAs.

What grid error is also called lateral decentering?

Off-center grid error.

What results from changing an 8:1 grid to a 12:1 grid with appropriate technique adjustments?

Increased contrast, increased mAs requirement, and increased patient dose.

What is the typical range of grid frequencies in lines per centimeter?

25 to 80 lines/cm.

How is grid ratio expressed mathematically?

Height of lead strips ÷ distance between strips.

What is the typical range of grid ratios?

5:1 to 16:1.

What happens when grid ratio decreases without a change in frequency?

Lead content and scatter cleanup decrease.

What do the recommended SIDs for a focused grid describe?

Focal range.

What are grids that move during exposure called?

Reciprocating grids; they are part of the Bucky.

What is a grid with lead strips perpendicular to the grid's long axis called?

Short-dimension grid.

When does patient dose increase in relation to grids?

When a grid is used and when changing from a lower to a higher grid ratio.

What is the most common cause of grid cutoff?

Off-level misalignment.

Which type of grid cutoff occurs with both focused and parallel grids?

Off-level misalignment.

What happens to scatter production when the volume of irradiated tissue decreases?

Scatter production decreases.

According to the study guide, significant collimation requires what mAs adjustment?

An increase of 30% to 50% of the mAs.

Why does the air-gap technique work?

Scatter photons are more likely to miss the image receptor.

True or false: Digital image receptors are not sensitive to scatter radiation.

False.

True or false: Grids do not reduce scatter production.

True.

True or false: Increasing collimation reduces patient exposure, increases field size, and reduces scatter production.

False. Increasing collimation decreases field size.

True or false: As collimation decreases, exposure to the IR increases.

True, according to the study guide.

True or false: An adult knee measuring 14 cm should be radiographed without a grid.

False. The study guide recommends a grid for parts thicker than 10 cm.

True or false: The only reason to use a grid is to increase radiographic contrast.

True, according to the supplied quiz feedback; verify with your textbook.

True or false: Electronic masking is a good alternative to collimation.

False.

True or false: Newer DR units may automatically collimate based on the preselected anatomic region.

True.

True or false: Grids should be used even when the anatomic part produces little scatter.

False.

True or false: A lead shield behind the lower back during lumbar imaging can absorb exiting scatter.

True, according to the supplied quiz feedback; actual placement and technique matter.

True or false: Lead shield placement will not affect digital image processing.

False, according to the supplied answer; verify the intended concept with your course material.