Unit 2 Exam Flashcards
What is applied to the cathode side of the tube during an exposure?
A large negative potential.
What is the process of producing an electron cloud referred to as?
Thermionic emission
Two reasons why Tungsten is an ideal target material
High Melting Point
High Z number (74)
Why are the tungsten filaments coated in thorium?
Thoriated tungsten burns off 2% more electrons.
More efficient thermionic emission process.
What does the acronym mAs stand for?
Milliampere Seconds
How do you convert seconds into milliseconds?
Move the decimal 3 places to the right
How do you convert milliseconds into seconds?
Move the decimal 3 places to the left
What happens to the patient’s dose if you cut the mA in half?
Patient dose is cut in half
What happens to the patient dose if you double the exposure time?
Patient does is doubled.
What is the definition of exposure time?
The amount of time potential difference (voltage) is applied to the tube.
Amount of time the patient is exposed to x-rays.
One reason why a shorter exposure time would be desired?
Motion Artifacts (image blur)
Moving children
List one reason why a longer exposure time would be desired.
Lateral T Spine
Reciprocity Law
Exposure to the IR will remain constant with adjustments to exposure time or mA if mAs remains constant.
AFS
Actual Focal Spot
Actual area of the target where incoming electrons hit.
What is the location of x-ray production?
AFS
EFS
Effective Focal Spot
Spot on patient where x-rays hit
99.8% of the energy created at the anode target is converted into?
Heat
What is the term that describes the way the beam travels after being created at the Anode?
Isotropic
Inherent Filtration
Filtration within the tube housing.
0.5mm Al equivalent
Glass
Added Filtration
Filtration outside the tube housing.
2.0mm Al equivalent
Mirror - 1.0mm Al
Al Filter - 1.0mm
Beam-Limiting Device
Variable-Aperture Collimator
Most common device
Beam - Restrictor Examples
Aperture Diaphragm
Cones/Cylinders
Off Focus Radiation
X-rays produced outside of the focal track
Leakage Radiation
X-ray photons escape the lead housing when the housing is compromised.
3 requirements for the production of x-rays
- Source of electrons
- Way to accelerate electrons
- Target
4 requirements for efficient x-ray production
- Source of electrons
- Way to accelerate electrons
- Vacuum
- Target
What is the controlling factor for beam quality?
kVp
What is the controlling factor for beam quantity?
mA
What material is the focusing cup made out of?
Nickel
Which electrode has a negative charge?
Cathode
Which electrode has a positive charge?
Anode
The cathode has a ____ negative potential during exposure.
The cathode has a large negative potential during exposure.
The focusing cup has a ____ negative charge in order to keep the electrons tightly bound.
The focusing cup has a low negative charge in order to keep the electrons tightly bound.
Thermionic Emission
Burning off of electrons due to the heating of a small coil of thoriated tungsten wire.
Why is the X-ray tube made of Pyrex?
To withstand high temperatures.
What is the x-ray tube surrounded by?
Oil Bath
What is the purpose of the oil bath?
To dissipate heat that is generated during x-ray production.
Most common type of x-ray tube failure?
Tungsten Filament Vaporization
What does the Cathode house?
The two filaments and the focusing cup.
What is the small filament (focal spot) used for?
Smaller body parts.
Greater spatial resolution
What is the large filament (focal spot) used for?
Larger body parts.
Needs more electrons that can be turned into x-ray photons.
What term describes when the filament is heated up to be “white hot”?
Incandescent
Why are the filaments coated in thoriated tungsten?
Thorium makes for a more efficient thermionic emission process by burning off 2% more electrons.
Why is the focusing cup made of Nickel?
Nickel has a high heat capacity
What type of charge does the focusing cup have?
Negative
What does the focusing cup house?
The filaments
What is the role of the focusing cup?
Focuses electrons that are produced by thermionic emission.
The focusing cup and the electrons both have a negative charge. Because of this, they _____ each other.
Repel
What do electrons form after thermionic emission?
Thermionic Cloud or Space Charge
What focuses the thermionic cloud/space charge?
Focusing cup
Electrons travel from the cathode to the anode by ____ _____
Kinetic energy
What is kinetic energy converted into once the electrons hit the anode?
X-rays
What percentage of kinetic energy from the electrons is turned into heat?
99.8%
What percentage of kinetic energy from the electrons is turned into useful x-rays?
<1%
Is x-ray production considered efficient or inefficient?
Inefficient
Dual Focus?
2 filaments within the tube
mAs controls…?
Quantity/Intensity of x-rays in the primary beam.
Formula for mAs
mA x S = mAs
What is mAs?
The product of exposure time and x-ray tube current.
What is the tube current?
The flow of electrons from the cathode to the anode.
Does mAs have influence on penetrability or quality of the primary beam?
No. kVp.
mAs and dose are ______
Proportional
mA (milliamperage) is directly proportional to the ____ _______
mA (milliamperage) is directly proportional to the tube current.
If mA is increased, what does that do to the electrons?
It increases the amount of electrons that are able to cross the tube to reach the anode.
What is exposure time?
The amount of time that potential difference (voltage) is applied to the x-ray tube.
Amount of time the patient is exposed to x-rays.
What units do we measure exposure time in?
Seconds or Milliseconds
Exams typically require the ______ exposure time possible.
Shortest
Why is a shorter exposure time desirable?
Eliminates motion artifacts
(Patient Movement and Image Blur)
Lateral T-spine requires a longer exposure time, why?
Motion of the ribs will blur.
Exposure time is ________ proportional to the number of electrons crossing the tube, and the number of x-rays generated.
Directly
Exposure time is also directly proportional to ______
Dose
Changes in the length of time that a tube is energized (exposure time) will affect ….
The total number of electrons crossing the tube.
As a filament is heated over and over again, ________ ________ will create a film on the inside of the glass tube.
Vaporized Tungsten
Once the deposits of tungsten coat the glass, what does it “make”?
A new target
Electrons strike at the tungsten deposits on the glass tube, causing a ____.
Gas/Gassy Tube
What is the effect of a gassy tube?
The tube is no longer a vacuum.
Once exposure is engaged, a ______ ________ is applied to the anode.
Large voltage
The anode always emits what type of radiation?
Infrared
Potential Energy
Energy at rest
What possesses potential energy?
Space Charge/Thermionic Cloud
Kinetic Energy
Energy in motion
Electrons have kinetic energy once …..
A large voltage is applied to the anode
What is the target made out of?
A tungsten-coated disk, or a combination of tungsten and rhenium.
What stops or slows down the electrons?
Target
When are x-rays produced at the target?
When electrons are stopped or slowed down.
What is the area of the target that is hit by incoming electrons?
Actual Focal Track (AFS)
What is the melting point of Tungsten (W)?
6,192 degrees F
Tungsten’s thermal conductivity is nearly equal to what other material?
Copper
What is Radiation (heat)?
The transfer of heat by the emission of infrared radiation.
Most of the heat is dissipated by _____ during exposure.
Radiation
Which location of the tube is radiation?
Glowing hot anode
What is Conduction?
The transfer of heat from one area of an object to another.
Which locations are conduction occurring?
From the Anode, to the Rotor, to the Glass.
What is Convection?
The transfer of heat by the movement of a heated substance from one place to another.
What raises the temperature of the oil bath?
The heated glass enclosure
Where is convection occurring?
From the Glass to the Oil Bath to the Tube Housing to the Room Air.
How do you extend the life on an anode?
By rotating it.
How fast will the anode spin during exposures?
3400 RPM to 10,000 RPM
The heat created by electrons crashing into the ___ ____ will spread around the entire edge of the disk.
Focal Track
The faster the anode rotates, the more ___ the tube can tolerate.
Heat
What happens to the anode when it stops spinning?
Pitted Anode
What does a pitted anode result in?
Image blur
Why doesn’t pitting anode happen anymore?
The tubes have built-in safety devices that sense if the target is not spinning.
Will an exposure be made if a (modern) anode is pitted?
No
What does the term isotropic describe?
The way that the beam travels after being created at the anode (all directions)
What is the lead housing?
It surrounds the x-ray tube except at the tube port
What is the tube port?
A hole cut into the lead housing that allows x-rays to pass towards the patient.
What is filtration for?
It removes low energy photons that only serve to increase patient dose (they go skin deep).
Any x-ray tube that is capable of _______ is required to have 2.5mm Al equivalent, by law.
70 kVp or more
What are the types of filtration?
Inherent and Added
What is the aperture diaphragm?
The simplest beam-restricting device.
What material is the aperture diaphragm made of?
Lead or a lead-lined metal
Where does the aperture diaphragm go?
It attaches to the tube.
The opening of the aperture diaphragm is designed to cover less than _____
The size of the image receptor.
What are cones and cylinders?
Beam-Restrictors
Modifications of aperture diaphragms.
What are cones and cylinders made of?
Metal
What do cones and cylinders do?
Their distal end restricts the beam to the size and shape of the aperture modification.
What is a variable-aperture collimator?
Beam-Limiting device
Most common
The variable-aperture collimator has two stages of ______ ______.
Shuttering blades
What does the First-Stage shuttering blade do?
Controls off-focus radiation.
The first-stage shuttering blade has multiple collimator blades that protrude from the ______ into the _____
The first-stage shuttering blade has multiple collimator blades that protrude from the top of the collimator into the x-ray tube housing.
What does the second-stage shuttering blade do?
Allows for a square or rectangular field.
Works in pairs and are independently controlled.
What are the second-stage shuttering blade leaves made out of?
Lead that is 3mm thick
What is off-focus radiation?
X-rays produced outside of the focal track.
What is positive beam-limiting?
When the IR is loaded into the bucky tray, sensors in the collimator limit the beam size to the size and orientation of the IR.
Where does inherent filtration take place?
Inside the tube housing.
What is the value of inherent filtration?
0.5mm Al equivalent
What part of the tube is the inherent filtration?
Glass
Where does added filtration take place?
Outside of the tube housing.
What is the value of added filtration?
2.0mm Al equivalent
What are the two parts of added filtration?
Reflective surface of the mirror (Variable-aperture collimator)
Al filter that is placed between the tube housing and the collimator.
What is the value of total filtration?
2.5mm Al Equivalent
Criteria for Efficient X-ray Production
- Source of electrons
- Way to accelerate electrons
- Vacuum
- Target
