image acquisition chapter 12

Scatter is primarily the result of which interaction

compton

patient absorbed dose is primarily the result of which interaction

photoelectric

Factors that affect the amount of scatter

volume of tissue, kvp, collimation

an increase in the amount of tissue thickness =

increase in scatter produced

increase in exposed field size=

increase in scatter produced

increase in kvp=

increase in scatter produced

methods to decrease scatter are

increase collimation, increase grid ratio, increase OID, and increase in lead shielding

an increase in collimation =

decrease in field size exposed

xray collimator=

beam restricting device

As collimation increases=

scatter decreases= increase in contrast

A decrease in scatter equals less image fog and off focus radiation which then equals

increased radiographic contrast

As collimation increases what happens to exposure factors

they increase

an increase in collimation= a ___ in scatter= ___ in mAs to compensate ( ___ EI Value)

decrease; increase; decrease

Aperture diaphragms

flat piece of lead with hole in it; placed below xray tube window

Cones and cylinders

aperture diaphragm with extended flange attached to it; great for sinuses

collimators

has 2-3 sets of shutters located 3-7'' below the tube. Rectangular or square FOV; light within the tube housing reflected by inside mirror to demonstrate exposure field

Where are the beam restricting devices located

in the housing unit

Automatic collimator

mechanically adjusts FOV to size of IR

When were grids invented

1913

Who were grids invented by

Gustave Bucky

How thick are grids

approximately ¼ inch thick

What is a grid composed of

thin lead strips with radiolucent interspaces (AI)

An increase in grid ratio = an ___ in mAs = ___ patient dose

increase; increase

What are grids typically used for

body parts 10 cm (4 inches) or thicker; or for procedures using more than 60 kVp

Where is the AEC

after the grid

ossilating grid means

it moves. used to remove grid lines

Grids absorb scatter, so ___in grid ratio = ___ in image contrast

increase; increase

what is Grid frequency

how close together the lead strips may be; number of lead strips per unit length (line pairs)

what is Grid ratio

ratio of the height of lead strips to the distance between them

What is the formula for grid ratio

height (h)/distance (d)

what is the formula for grid frequency

1/t+d

What is focal range determined by

geometry of lead strips

what is grid pattern

linear or crossed.

What does a linear grid pattern allow for

angling; must angle with grid lines

What is grid focus

parallel or focused

What is a parallel grid used for

fluoro and mobile

Focused grids are used for

angled lines to match beam divergence

What is focal distance/range

recommended range of SID’s that can be used with focused grid

What happens if the grid is placed upside down on top of IR

Only photons in the middle of the beam can reach IR

what is grid cut off

Grid/IR is not perpendicular to the CR

when is grid line artifact seen

digital imaging

what are the types of stationary grids

grid cap, wafter grid, grid cassette

What is a reciprocating grid

grid moves back and forth during exposure (called potter-bucky diaphragm)

what are final scatter reduction methods

air gap technique and shielding

Which of the following will decrease the amount of scatter radiation produced

Decreasing the xray field size

Which of the following is not a type of beam restricting device

lead shield

As collimation decreases, the quantity of scatter radiation decreases. TRUE OR FALSE

false

What type of beam restricting device provides the most flexibility in adjusting the xray field size

collimator

what describes the number of lead lines per unit length

grid frequency

A radiographic image was created using a 12:1 grid and 70 kVp at 10 mAs. What exposure technique change would maintain a similar exposure to the image receptor when converting to a 6:1 grid ratio?

70kVp at 6mAs

What type of grid has lead sttrips running parallel to the long axis of the grid

long dimension

Angling the xray tube along the long axis of a crossed grid would result in

grid cutoff

for the air gap technique to be effective in reducing scatter radiation reaching the image receptor, what must be increased

OID

With exposure technique compensation, ____ the grid ratio will ____ patient radiation exposure.

increasing, increase