Chapter 13 - Screens Flashcards
What is an intensifying screen?
a device that converts the energy of the x-ray beam into visible light
What are the 4 layers of a screen?
base
reflective layer
phosphor
protective coating
What is the function of the base layer?
support for phosphor layer
What is the function of the reflective layer?
redirects the light to the film
The reflective layer is composed of
a shiny substance
What is the function of the phosphor?
convert the energy of the x-ray beam to visible light
What is the active substance in the phosphor?
rare earth crystals
What is luminescence?
any material that emits light in response to some outside stimulation
What are the 2 types of luminescence?
fluorescence & phosphorescence
Which type of luminescence is visible light only while the phosphor is stimulated?
flourescense
Which type of luminescence is NOT desirable?
phosphorescence
What quantitates the efficiency of conversion of x-rays to light?
screen speed
What is a measure of the ability of a system to image two separate objects and visually distinguish one from the other
resolution
factors such as fast screen speed, large phosphor crystal size, thick phosphor, high kvp will result in what type of resolution?
low resolution
what is the undesirable fluctuation in optical density
image noise
what is the mottled or noisy appearance of the film due to the number of photons exposing it
quantum mottle
what is the ability to produce an accurate and clean image
spatial rasolution
What is the holder that contains the film and intensifying screen
cassettes
as screen speed increases, density ________
increases
adding screens or increasing screen speeds, ________ contrast
increases
increasing screen speed, _______ detail
decreases
as screen speed increases, patient dose _______
decreases
what is the formula for screen speed conversion?
old mAs * new RS
—————————- = new mAs
old RS
radiation exiting the window of the tube
primary radiation
radiation produced after primary interacts with patient
secondary radiation
radiation exiting the tube housing
leakage radiation
radiation exiting the patient
remnant radiation
how does kvp affect scatter?
higher kvp, more scatter
How does collimation affect scatter?
smaller collimation, less scatter
How does patient size affect scatter?
larger patient, more scatter
How does tissue density affect scatter?
more dense, less scatter
What is the feature of radiographic collimators that automatically adjusts the radiation field to the size of the image receptor?
positive beam limitation (PBL)
What is the removal of low energy x-rays from the useful beam with aluminum or another metal?
filtration
Increasing filtration does what to patient dose?
decreases!
