Imaging unit 4

Focal distance

Distance btwn the grid & convergent line or point (grid radius)

Convergent line

If points were connected along the length of the grid, they would form an imaginary line

↑ grid efficiency & ↑ mAs = ↑ pt dose

Higher grid ratio =

Thin lead strips

What materials make up a grid?

Btwn pt & grid; Used if 60 kVp or higher OR 10 cm part thickness

Where is the grid located and when is it used?

Interspace material

Radiolucent strips between the lead lines of a grid, generally made of aluminum

Reciprocating

Type of motion most used for moving grids today is?

OID

Air gap technique uses an increased ________ instead of a grid

Linear Focused

Which one of the following grids would a convergent line be formed if imaginary lines from its grid lines were drawn in space above it?

Aperture diaphragm

Flat piece of lead (diaphragm) that has a hole (aperture) in it

Collimator

Two sets of adjustable lead shutters located 3-7 in below tube

Automatic collimator

Automatically limits size & shape of primary beam to size of IR

Grid frequency

Expresses the # of lead line per unit length (inches or cm) (Range from 20-45 lines/cm or 60-100 lines/in)

Grid Ratio

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

Linear Grid

Has lead lines that run in one direction only

Parallel Grid

Has lead lines that run parallel to one another (nonfocused grid)

Focal Range

Recommended range of SIDs that can be used with a focused grid

Convergent point

If imaginary lines were drawn from each of the lead lines in a linear focused grid, lines would meet and form an imaginary point

Grid cassette

IR that has a grid permanently mounted to front surface

Wafer Grid

Stationary grid placed on top of IR

Grid cap

Contains permanently mounted grid allows IR to slide behind it

Bucky

Contains grid & holds IR

Short Dimension grid

Has lead strips running perpendicular to long axis of grid

Grid conversion factor

Used to determine the adjustment in mAs needed when changing from Using grid to non-grid (or vice versa) or changing to grids w/different grid ratios

Grid cutoff

Decrease in # of transmitted photons that reach IR be of misalignment of the grid

Off level grid cutoff ( ↓ exposure entire image)

X-ray beam angled across the lead strips

Off center grid cutoff (↓ exposure entire image)

Occurs when the CR of x-ray beam is not aligned from side to side with center of a focused grid

Off focus grid cutoff (↓ exposure @ periphery of image)

Occurs when using an SID outside of recommended focal range

Upside down focused grid cutoff (under exposure @ lateral border)

Occurs when a focused grid is placed upside down on IR; results in grid lines

Larger light field = ↑ IR exposure = ↑ Pt dose

↑ FOV = ↓ Collimation =

↓ FOV = Smaller light field = ↓ IR exposure = ↓ Pt dose

↑ Beam restriction = ↑ Collimation =

Moire effect

Artifact that can occur when a stationary grid is used during CR imaging if the grid frequency is similar to the scanning frequency (Zebra pattern)

Air gap technique

Concept that much of the scatter will miss the IR if there is an increased distance btwn the pt and the IR (increased OID) (8:1 grid)

Compton Interaction

What interaction has a primary result of scatter?

Compton interactions decrease as photon energy increases

What relationship does Compton interactions have with the energy of photons ?

• Volume: Increasing volume of tissue irradiated = increased scatter production .
• KVp: Higher kVp increases x-ray transmission & reduces it's overall absorption

List and describe the factors affect the amount of scatter ?

Higher KVp = Lower contrast = More scatter = Higher pt dose = Tighter collimation -> less scatter

Relationship btwn kVp, contrast, field size, collimation, pt dose & scatter

Circular

What shape is the primary x-ray beam

mAs

What factor must be increased in order to compensate for increased collimation since scatter radiation will be reduced?

Grid Patten

Linear pattern of the lead lines of grid (linear & crossed or cross-hatched

Controls grid motion by moving slightly back & forth in lateral direction over IR during exposure

What is the purpose of a reciprocating grid?

Grid ratio = H/D

List the formula used to determine the grid ratio

12:1 grid ratio

Which of the following would result in the greatest radiation exposure to pt?

Allow more transmitted photons to reach IR

Compared with parallel grids, focused grids:

Increase image contrast

The purpose of a grid in radiography is to

Were once required on new radiographic installations

Which is true for positive beam-limiting devices

Contrast

Which one of the following increases as collimation increases?

mAs should be increased

When making a significant increase in collimation, __________

Field size does not exceed IR size

Purpose of automatic collimation is to ensure?

Cone & Cylinder

Of beam-restricting devices, which two are most similar?

Collimator

Most effective type of beam restricting device is?

Decrease scatter radiation produced

Purpose of beam-restricting devices is to ________ by changing size and shape of primary beam