UNIT 2: The 5 cm rule

in order to half the kV using the 13% rule you should multiply by

.87

in order to double the kV using the 15% rule, you should multiply by

1.15

When to use a grid

10 cm or more of tissue thickness radiographer can use discretion whenever imaging larger body parts that will result in more x-ray interactions with body tissue

When adjusting kV, density can be doubled by using the ___________ and halved by using the ____________

15%rule 13% rule

Standard mA stations

50 100 200 320 400 640 800 1200 1600 2000

Bucky Conversion factors

5:1-2X 8:1-4X 12:1-5X 16:1-6X

What changes should be made to mAs for a 5 cm increase/decrease in thickness

Increase: double decrease: half all other factors remain the same

Unusable mAs

a mAs setting that is so high that the exposure time is excessively long or a mAs setting that is too low and results in a grainy image

placement of grid

between patient and IR

kV rule of thumb

between the 60-80 kV range, a 10 kV increase/decrease will double or half the kV

For every 5 cm decrease in thicnkess, exposure factors should be

cut in half

If kV is increased, mAs should be

cut in half

baseline kV

determines a Kv that adequately penetrates the anatomical part under examination

For every 5 cm increase in thickness, exposure factors should be

doubled

If kV is decreased, mAs should be

doubled

types of grids

focused-most common parallel

If a grid is used improperly this can result in

grid cut off this happens when the bucky is not aligned tube titled against grid lines SID out of focal range

Desired mA setting

highest mA at the shortest exposure time

Purpose of a grid

improves image contrast by removing scatter radiation before it interacts with the image receptor

Exposure factors need to be ________________ when using a grid

increased

Disadvantage of using a grid

increases patient exposure

optimal kV

kV setting that is high enough to ensure proper penetration from the part under examination not to high to have adverse effects on contrast

Variable kV/ fixed mAs

mAs remains constant while kV varies with part thickness kV increases as part thickness increases baseline kv is the original kv for part under examination

fixed kV/variable mAs

mAs varies as part thickness varies x-ray quantity increases as tissue thickness increases

Advantage of fixed kV/variable mAs

maintains a constant image contrast and easier to use

aluminum interspace material

more homogenous more expensive

Bucky

moving grid check detent for accuracy

Quantum mottle

noisy,grainy, salt and pepper image resulting in loss of detail

fiber interspace material

not as homogenous less expensive most common can be affected by moisture

grid frequency

number of lead strips per in/cm

Disadvantage of fixed kV/variable mAs

patient exposure increases proportionally as tissue thickness increases

mAs

product of mA and exposure time indicates x-ray quantity

kV has an impact on 2 factors of an x-ray beam. These factors are

quantity-amount quality- penetrating ability

Grid ratio

rato of height of lead strips to the distance between them

advantage of variable kv/fixed mas

results in lower patient dose effective with pediatric patients when imaging extremeties

disadvantage of variable kv/ fixed mas

results in substantial variations in image contrast requires more precise measuring than variable mas charts

Interspace material

space between lead can be aluminum or fiber

Finding the steps in between when changing kV

take the difference between the original kV and the new kV and then divide by 5 to determine how much to add for each additional cm

To find the in between steps you should

take the original mAs and divide by 5 to determine what should be added for each cm

purpose of a grid

to clean up scatter radiation from the xray beam exiting the patient(remnant beam) results in increased contrast

Adding a grid without changing exposure factors will result in

underexposed image

Calipers

used to accurately measure the anatomical part under examination