Xray Production, Tube And Beam Filtration Flashcards
The high energy collision of free electrons that were generated at the cathode/filament with the target on the anode results in what two things?
- heat and lots of it
- X-rays
Why is tungsten used as the filament wire?
it has lots of electrons to release and a very high melting point so it is durable
As the tungsten filament is heated, what happens?
the outer shell electrons speed up and move farther from the nucleus and become shared with adjacent atoms. This is called thermionic emission. Then they are flung out of the atom and form an electron cloud
Where is the filament located in the X-ray tube?
at the negatively charged end, the cathode
what is the target in an X-ray tube?
a slanted, smooth metal surface at the opposite end of the tube from the filament, at which the electrons will be aimed
The target is made of what metals?
It is embedded with a plate of tungsten and the remained is molybdenum and copper which help facilitate hear dissipation
What accelerates the electron cloud (aka space charge) toward the target?
the - to + flow of electricity created by the high voltage electrical circuit connecting the two ends of the tube
When are X-rays produced?
when the fast moving electrons strike the target, their kinetic energy is converted to heat (99%) and X-rays (1%)
There are two types of electron interactions at the target that produce X-rays. What are they?
- Bremsstrahlung “braking rays” are formed by sudden slowing of electrons that converts kinetic energy to other forms
- characteristic radiation is formed within target atoms as a results of electron to electron collision
Which type of electron interactions at the target produce the majority of the X-ray beam?
bremsstrahlung (85%)
As incoming electron nears the nucleus of a target atom, it slows and changes course. Its kinetic energy is released as a new X-ray photon. The “braking” will increase, the closer to the nucleus the incoming electron is and the ______ the energy of the resultant photon will be.
higher
What produces a photon of characteristic radiation?
an inbound electron will cause a K-shell electron to be ejected and an L-shell electron will move to the K-shell. When it moves, it will release an X-ray photon that is equivalent in energy to the difference in binding energy between the two shells
What is the function of X-ray tube housing?
- protect tube
- absorb radiation
- provide mounting for tube attachments such as collimator
Most modern general purpose X-ray tube are ______..
“dual focus”
What is a “dual focus” X-ray tube?
it has 2 filaments, 2 focusing cups and 2 overlapping focal spots on the anode
In a dual focus X-ray tube, why does it have two focal spots on the anode?
- one is large for larger body parts and greater output
- one is small for lesser output but smaller beam and enhanced image detail
What is the function of the focusing cup in an X-ray tube?
the focusing cup is negatively charged and will repel electrons, thus focusing them on a small target cell (“focal spot”). Without the focusing cup, the electron stream spreads like a cone beyond target area
Focal spot sizes and filament sizes are associated with what?
mA of circuits. more mA = larger filament and focal spot
Where is the focusing cup located in an X-ray tube?
surrounding the filament (focusing cup is usually made of molybdenum)
The target face on an X-ray anode is bevels so that the target area is on a _____ slant.
10-20 degree
What is the composition of a typical anode?
tungsten target imbedded in molybdenum and copper disc to conduct heat away from target
What is the focal spot in an X-ray tube?
the area on the target where the electron stream is focused. Dual focus tubes have 1 large and 1 small
What is the effective focal spot?
the vertical projection of the actual/true focal spot. the electron stream is horizontal and the actual focal spot is slanted
In terms of actual focal spot, what size is better and why?
bigger is better because the larger surface area improves heat capacity and durability
In terms of effective focal spot, what size is better and why?
smaller is better because is means more “point source” for X-ray beam and therefore more image sharpness
The size of the effective focal spot determines image sharpness (smaller effective focal spot = sharper). The relative size of the effective focal spot is determined by what two things?
- target (anode) angle, steeper angle = smaller effective focal spot = sharper image
- actual focal spot size, smaller actual focal spot = smaller effective focal spot = sharper image
The anode angle is usually what.
12-15 degrees. the steeper the angle, the greater the differential between actual and effective focal spots (i.e. smaller effect focal spot and therefore sharper image)
Why can we not steepen the angle of the anode indefinitely in an attempt to have a smaller effective focal spot and therefore a sharper image?
- maximum field size limits: node angle determines the margin of field because it extends from the same angle. So the steeper the angle, the smaller the field size.
- anode heel effect
What is the anode heel effect?
X- rays are formed within the target material of the anode and absorbed by the target as they exit. The sloping of the target face results in uneven absorption of the primary beam. More sloping = more uneven absorption
How do you compensate for the anode heel effect with patient positioning?
place the thinner portion of the body part toward the anode end of the tube
What degree of difference between one end of the part being X-rayed to the other end matters when considering the anode heel effect?
15-20%
When does SID matter when considering the anode heel effect?
shorter SIDs require more of maximum beam width, therefore they enhance heel effect visibility
How does anode angle affect the heel effect?
smaller anode angle = greater heel effect
Anode heel effect evens density for AP thoracic spine when anode is toward what part of the patient?
the patient’s head, thinner part
What is the correct use of the anode heel effect?
- thinner body part placed at the anode side where beam is weaker
- thicker body part placed at the cathode side where beam is stronger
- if the technique is calculated to penetrate the thicker side, the weaker end of the beam will expose the thinner end
What is the incorrect use of the anode heel effect?
- if the technique is calculated based on the body part thickness through the central beam. this will result in thinner portion of body part being over penetrated and therefore image will be too dark and the thicker portion will be under penetrated and appear to light
What is a collimator used for?
Allows us to control the size of the field
How do you change the size of the exposure field?
change the collimator size
What are the legal requirements for collimators?
- must have rectangular collimator
- must indicate field dimensions at standard distance
- must have light that delineates exposure field accurately to within 2% of the SID and indicates center of field
- must collimate to area of clinical interest - field may not be larger than film
radiation exposure is proportional to what?
mA and exposure time
What is the function of milliamperage?
it controls rate of flow of electrons across tube and X-ray production i.e the number of electrons flowing from the filament to the target each second
Radiation exposure is a function of what?
mA and time (s)
the overall darkness of an X-ray image
image density
image density is proportional to what?
mAs, therefore doubling mAs with double density
How is exposure time controlled?
by a timer in X-ray circuit
mAs = _______, mA = ________
milliampere-seconds
milliamperages
What does kVp control?
x-ray penetration, speed of electrons to the target from the cathode, hence responsible for the energy (wavelength) of photons and the resultant X-ray beam
What is the primary controller of radiographic contrast but also has a non-linear (logarithmic) relationship with film density?
kVp
What is the 15% rule?
raising the kVp by 15% lowers the contract, but doubles the image density
to compensate, when the kVp goes up 15%, the mAs must be cut in half to compensate for the density problem created by greater beam penetration
Where is the filter placed?
between the tube housing port and the patient
What is the function of the filter?
remove the long wavelength radiation from the primary beam to only allow productive radiation (radiation with sufficient energy) to pass through, lower patient dose SIGNIFICANTLY
Why do lower energy photons not contribute to image production
because they will be absorbed by the subject
Why do lower energy photons not contribute to image production
because they will be absorbed by the subject
because lower energy photons do not contribute to image production, what does filtration do?
it results in a higher quality beam (fewer photons but with higher average energy) that will contribute to image production, therefore creating less exposure to the patient
What does increasing filtration do to the average beam energy?
raises it
filtration is measure in _______
equivalents of millimeters of aluminum (mm Al equiv)
What is half value layer?
the thickness of a material (aluminum in radiology) that, when an X-ray beam is passed through it, will reduce the intensity of radiation at the output side by one half
half value layer is primary controlled/affected by what?
beam energy i.e. kVp
raising the beam energy raises the HVL
NOTE: increasing the number of photons does not affect the HVL
total filtration =
inherent filtration + added filtration
All X-ray machine capable of operating above 70 kVp are required to have at least _______ mm Al equiv total filtration permanently installed
2.5
Because 99% of X-ray tube energy is converted to heat, what are some of the effects of overheating the tube?
- anode face melts = unsharp image
- gas from hot components compromises vacuum = erratic exposures
- a heavy exposure on a cold tube could crack the anode
- tube breaks, compromising tube oil seal = destroyed tube
What are some tube designs that minimize heat problems?
- rotating anode
- anode disc material (tungsten, molybdenum, graphite to dissipate heat)
- stem conducts heat to a copper mass surrounding the rotor mechanism
- oil insulation in tube
- control circuits to prevent exposures in excess of tube capacity
tube rating charts (specific to tube type) are designed to depict what>
the maximum safe exposures capable with any given tube
greater total tube heat load is possible with ______
lower mA, because when exposure times are lengthened it allows for cooling of the tube during exposure
using a tube rating chart, how do you know when it is a safe exposure.
When kVp and exposure time fall to the left of the line for the given MA on the chart, it is safe
in general, should not exceed 80% of tube load
At what capacity should tube load be run at in order to prolong tube life?
80%
What are cooling charts
it plots cooling rate after exposure using heat units
i.e it tells you how long you have to wait to go from a given number of heat units to another number of heat units
For a high frequency generator, what is the formula for heat units?
mA x time x kVp x 1.45 = HU
