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
