The Generator, the X-ray Tube, X-ray Production, Controlling Factors (mAs, kVp) Flashcards
What are the three basic steps to making x-rays?
- Generate electrons (mAs)
- Accelerate electrons (move in tube from cathode to anode)–(kVp)
- Moving electrons generate x-ray photons and hit metal target
What does the generator make?
makes kVp; causes electrons to move in tube (cathode to anode)
The generator makes kVp, but what will a variation in voltage cause?
a variation in photon number and energy—> contrast and density changes–> and increased patient dose
If we increase kVp, how does that effect the dose to the patient?
with increase the pose to the patient
What is the purpose of the generator?
create voltage difference between anode and cathode
and
supplies the current for a filament circuit
What are the two circuits that are working together, but independently for taking x-rays?
- X-ray generator–>Filament circuit
2. X-ray tube –> High-voltage circuit, aka tube circuit
What circuit controls generation of electrons at cathode? What charge will this be?
Filament circuit
negative charge
What circuit controls the flow of electrons from cathode to anode and makes the anode very positive to attract the electrons?
High-voltage circuit
What they of current must we use for taking an x-ray? Why?
DC (direct current)
want electrons to flow only in one direction from cathode to anode
What is a device that limits electron flow to one direction called?
diode
What type of photons are produced when kVp is low? How are these absorbed by the patient?
low-energy photons
are absorbed by patient’s tissue –> and cause cell damage**
What type of generators have the lowest dose and shortest image time?
High-frequency generators
Will a reduced exposure time increase or decrease the image quality? Why?
increase the image quality –> b/c less chance patient moves
Will a high or low frequency generator produce the least dose and fastest image time?
high frequency generator
What is the purpose of the x-ray tube?
generate x-ray photon beam with quantity and quality desired by the operator
What is the difference of an x-ray tube (anode type) or Stationary and Rotating?
Stationary–> cannot cool itself
Rotating–> spinning disc, so can cool (reduces heat)**
What is the electron source in the x-ray tube?
the cathode
What are the two things that make up the Cathode/ the cathode contains?
- Tungsten filament
2. Focusing cup
What material is the filament of the cathode made of? What is the anode made up of?
Tungsten
Tungsten
What does the Tungsten Filament of the cathode contain?
- Rhenium (a chemical)
- Small focal spot
- Large focal spot
What material is the Focusing cap of the cathode made up of? Is the focusing cup neg. or pos.?
Nickel
neg.
What is the process called where the filament current causes tungsten to heat up and the high heat causes outer electrons to be released?
(aka how does the cathode make electrons?)
thermionic emission**
making electrons from metal
What will the generated electrons that are freed form?
form a cloud around filament –> will spread out due to being negative and repelling each other
What charge is the focusing cap? What does this allow for?
negatively charge nickel–> forces electron cloud to stay together
smaller cloud = smaller focal spot
What controls the cathode? What controls the andoe?
cathode–> controlled by mAs
anode–> pos. attraction controlled by kVp
What three interactions happen when the target (anode) interacts with the electrons?
- Outer Shell Electron
- Inner Shell Electron
- Atomic Nucleus
What occurs when the Outer Shell Electron is hit on the anode end? What is the result?
excited but no ionization–> results in HEAT production
99% of interactions
What occurs when the Inner Shell Electron is hit on the anode end? What is the result?
ejected and replaced with outer shell electron
results in X-RAY PRODUCTION (characteristic radiation)
.15% of interactions
What occurs when the Atomic Nucleus is hit on the anode end? What is the result?
electron slows and is redirected
results in X-RAY PRODUCTION (Bremsstrahiung radiation)
.85% of interactions
What are the 3 interactions that can occur at the anode, and what are the results?
- Outer Shell Electron–> HEAT 99%
- Inner Shell Electron–> x-ray/characteristic radiation .15%
- Atomic Nucleus –> x-ray/Bremsstrahlung radiation .85%
What is the only way to produce useful x-ray photons? At what kVp will no useful photons be produced?
by ejecting the innermost electrons (Characteristic Photons)
a kVp less than 70
What are the 7 Sequence of Events for producing x-rays?
- prep button pushed
- filament circuit turned on and electron cloud begins to form
- rotor begins spinning up
- exposure button pushed
- kVp applied across tube
- neg. charged cathode repels e- toward very attractive pos. charged anode
- electrons interact with anode producing x-rays
What x-ray is produced from the interaction of the inner shell electron?
characteristic x-ray
~.15%
What is the interaction that causes an electron to slow down and give off an x-ray?
Bremsstrahlung
~.85%
What is the majority of what is produced from the x-ray tube?
HEAT ~99% of what is made in the tube
What percentage of actually x-rays are made from the x-ray tube?
1%
~.85% Bremsstrahlung
~.15% Characteristic
What is Exposure Time (S)?
length of time (in seconds) that anode target will be bombarded with electrons and therefore the duration of x-ray production
What determines the total amount of radiation produced from taking an x-ray?
exposure time (s)
and
the amount of x-ray produced per second (mA)
What is the measure of total quantity of x-ray produced?
mAs
mA x s = mAs
T/F. The same results will be had with high mA and low time or low mA and high time.
true
When possible, what is the preferred usage of mA and time exposure?
high mA and low exposure time
If we had a low mAs, how would the film appear? What if the mAs was high?
low mAs = underexposed/white
high mAs = overexposed/black
Is a higher or lower frequency produced with a high kVp?
a higher frequency, which will decrease exposure to patient, but will also make film darker
If we are looking at a film and focusing on where bones overlap and we cannot see through it, what does this mean? What do we need to do?
film is UNDERexposed
need to increase mAs by doubling it
Ex: have 50 mAs–> take new x-ray at 100 mAs
If we are looking at a film and cannot see soft tissue, what does this mean? What do we need to do?
film is OVERexposed
need to reduce mAs by 1/2
Ex: taken at 50 mAs–> need to take new one at 25 mAs
