Management Unit 3

What is scatter

adds unwanted exposure (fog)

How do we decrease scatter?

Grids

beam-restricting devices

Compression of the body part

What are beam-restricting devices?

collimation, filtters

What relationship does scatter have to contrast?

inverse

more scatter=more grays=decrease contrast

Post-exposure electronic masking:

Cropping the image to look prettier (illegal)

Aperture Diaphragms

Flat piece of lead that goes directly below tube window

What is the max collimator misalignment allowed

±2%

What is a positive beam-limiting device (PBL)

Automatically collimates to the size and shape of the IR to ensure the collimation fits on the IR

DR automatically restricts the beam

When collimating how does the mAs compensate

increase mAs 25-50%

When are grids used?

when the atomic part is 10cm or more than 60 kVp is used

What is the range of grid ratios?

4:1 to 16:1

What does the label on the grid tell you?

interspace material (aluminum)

Grid frequency

Grid ratio

Grid size

SID ranges

What are the two types of grid patterns?

linear (most popular)

cross-hatched (tube cannot be angled)

What are the two types of grid focus?

parallel grid

Focused grid

Parallel grid focus

Use most commonly in fluoro and mobile imaging

Lines are parallel to one another

Best at long SID

Focus Grid Focus

matches the angle of the divergent of the primary beam

Allows more transmitted photons to reach the IR

What is the convergent point?

in imaginary line drawn from the window to the lead lines

What does a slightly movable grid due to the x-ray exposure?

blur out the grid lines

What is focal distance?

Distance between the grid and the convergent point

What is the focal range used for?

recommended range of SIDS that can be used with the focused grid

Wafer Grid

placed on top of the IR

Grid cassette

permanently mounted to the surface of the IR

Grid cap

Slides behind permanently mounted grid

What is a long dimension type of grid

runs parallel along the long access of the grid

What is the short dimension type of grid?

lead runs perpendicular to the long access of the grid

When using a grid what else must you do?

increase mAs

Grid conversion factor:

mAs1/mAs = GCF1/ GCF2

Grid Cutoff

Not aligned side to side with the center of the focused grid

Transmitted photons do not reach IR because of misalignment with the grid

What is the most common type of grid cutoff?

off-level grid cutoff

What is Off-level Grid Cutoff

When the x-ray tube is angled, making it not aligned with the grid decreasing the exposure to the IR

What is off-focus grade cutoff?

 using an SID outside of the focal range, causing a loss in exposure to the IR

Equation for calculating grid ratio:

height/distance= Grid ratio

What determines the amount of energy scatter radiation has exiting the patient?

kVp

The projected shape of a unrestricted primary beam is?

circular

When making a significant increase in collimation, what changes in your technique?

mAs increases

What should increase as collimation increases?

contrast (more black-and-white)

What grid would a convergent line be formed from its imaginary lines?

linear focused

What type of motion is most often used for moving grids today?

Reciprocating

What is an advantage of not using a grid and having scatter correction software?

improves spatial resolution

What would not decrease patient radiation exposure using DR?

using a crosshatched grid