Which two types of imaging are based on the absorption of x-rays?
Radiography and CT
What is another name for x-ray?
photon (can be used interchangeably)
What are x-rays?
type of electromagnetic radiation
what are some properties of x-rays? (6 of them)
No charge or mass
Cannot be felt
Travel at speed of light
Penetrate all matter to some degree
Cause ionizations if energy high enough
what is ionization?
loss of an electron
why can ionization in DNA be dangerous
can lead to cell death and mutation (carinogenesis)
T/F: veterinarians are unelievably careless about x-ray safety
True (we must implement change)
T/F these are good examples of radiation safety
what are some consequences of bad radiation safety
violation of state regulations
T/F if you get pregnant you can still receive some radiation dose
True. 10% of radiation during gestation, must notify employer in writing of pregnancy
what does ALARA stand for?
As Low As Resonably Achievable
how is ALARA implemented
Ways to reduce time exposed to x-rays
rotate personnel in room
avoid repeat examinations
minimize patient holding
T/F increasing the distance between personnel and radiation source reduces exposure significantly
True (inverse square law is key!)
what is the inverse square law?
Intensity of radiation (x rays/unit area) decreases with the square of the distance from the source
i.e double the distance reduces the x-ray intensity to 1/4th (1/2)^2; tripling the distance reduces the intensity to 1/9th (1/3)^2
If the distance between the film and the x-ray source decreases from 40” to 30”, how much does radiation intensity at the film change?
what does shielding protect from?
scattered ratiation (won't protect against primary radiation)
what causes scatter?
patient, floor, table, etc
T/F scatter degrades the image and increases personnel dose
How can scatter be reduced?
minimize the size of the beam (collimation)
what is a radiograph?
A picture of the pattern of x rays emerging from the patient
Differential absorption needed to create an image
when are x-rays produced?
when high speed electrons strike metal (occurs in an x ray tube)
T/F the anode has a negative charge (-) and the cathode has a postive charge (+)
the anode is positive (+) and the cathode is negative (-)
how does the filament (-) work?
current causes heating and electrons "boil" off
T/F the number of x rays produced is proportional to current (mA) and time current is "on" (s)
True (this is your mAs)
T/F doubling the mAs double the number of x rays
T/F the longer the time (s) the more electrons are produced
T/F the target and the filament are made of tungsten
T/F the voltage between target-filament makes target positive to attract the negatively charged electrons
what is kVp
what does increasing kVp do?
greater voltage difference
electrons travel faster
have more kinetic energy
create higher energy x rays
how do we keep the target from melting?
tungsten has a high melting point
target rotates to dissipate heat
what are the mAs and kVp setting important for avoiding
over or underexposure
this image is over/underexposed?
underexposed (either kVp or mAs is too low)
this image is under/overexposed/perfect?
overexposed (either kVp or mAs is too high)
what does it mean when your radiograph is too light?
too few x-rays hit the detector (underexposed)
fix: increase mAs (more x rays) or increase kVp (higher energy)
T/F many combinations of mAs and kVp lead to suitable exposure
whether kVp or mAs is changed will effect the ______
what is high contrast?
low kVp, high mAs
x rays are less penetrable, few shades of grey
what is low contrast?
high kVp, low mAs
x rays are more penetrable, absorption is more uniform, many shades of grey
what mAs/kVp should be used in the thorax?
high kVp, low mAs
air provide contrast-lungs!!
what mAs/kVp should be used in abdomen
low kVp, high mAs
fat provides the contrast-subject contrast needs to be maximized by technique (some animals fatter than others)
"when you're fat you have a high body mAs index"
what is needed to make an image
absorption and scatter
what are some problems caused by scatter
increases personnel dose
why are grids used?
"intercepts" scatter from patient before it reaches film
when are grids generally used?
when the patient is more than 10 cm thick
which setting needs to be changed when using a grid
need 2-3x more photons due to absorption of primary beam by lead
where should the grid be positioned?
between the patient and the cassette
what are the 2 methods for recording the x ray image
analog and digital
what are the components of analog image recording system?
x ray film
what does the intensifying screen do?
converts x ray photons to light photons
one x ray photons= many light photons
light photons expose the x ray film (sensitive to light)
>90% of blackness due to light vs x rays
advantages of intensifying screens
100x reduction in mAs
reduced movement artifact
reduced patient exposure
reduced personnel exposure
increased tube life