physics chapter 5 Flashcards
Know what the tube housing is
Lead lined metal structure that serves as an electrical insulator and thermal cushion for the tube itself
Where is the X-ray tube situated
In a protective housing
Describe the design of the tube housing
Incorporates an oil bath and cooling fans to help dissipate heat away from the tube, protecting it from thermal damage
Know why the tube is immersed in an oil bath
Draw heat away from the tube
Know the purpose of the cooling fans
Circulate air around the assembly which also helps dissipate heat
Know another role of the tube housing
Absorb most of the photons traveling in directions other than toward the patient
Define leakage radiation
Photons produced in the X-ray tube that are traveling in directions other than toward the patient
What are 2 precautionary things to remember about the tube housing
⚫️with extended on times the housing becomes hot
⚫️the high voltage cables are not handles
Describe the general purpose X-ray tube
An electronic vacuum tube that consists of an anode, cathode, and an induction motor all encased in a glass or metal enclosure(envelope)
Know the positive and negative ends of the X-ray tube
Anode-positive
Cathode-negative
Know what the cathode consists of
Focusing cup and filament with it’s supporting wires
Know what the main purpose of the X-ray tube enclosure is
To maintain a vacuum within the tube
Be able to describe “sun tanning” of the X-ray tube
As the tube age vaporized transfer from the filament deposits on the inside of glass causing bronze discoloration of the glass. This causes problems with arching and damage.
Know the purpose of the metal envelope
Provides a constant electric potential between the electron stream from the cathode and the enclosure thereby avoiding the arching problem and extending tube life
Know why the target window is made of a thinned metal
It reduces the amount of absorption by the enclosure
Know what the anode provides
The target for electron interaction to produce X-rays and is an electrical and thermal conductor
Know what the purpose of the anode is
Dissipate heat
Know the 2 designs of the anode
⚫️stationary anode
⚫️rotating anode
Know why it’s called a stationary anode
The target doesn’t move
What is the disadvantage of the stationary anode
The electrons always hit the same small target area, heat builds up rapidly and can damage the tube
Know which anode is commonly used in today’s xray machines
Rotating anode
Know what the rotating anode consist of
A rotating disc made of molybdenum as core material costed with tungsten and mounted on a copper shaft with a molybdenum core
Know why cooper is used as part of the shaft on a rotating anode
It has an excellent thermal and electrical conductive properties
Know why molybdenum is used as the disc base on a rotating anode
It has a low thermal conductivity which slows migration of heat into the rotor bearings (minimizing heat damage) and it is light but strong making it easier to rotate the anode
Know why the target material (coating) is made of tungsten
It has a very high melting point (3400 degrees C and 6152 degrees F) and it’s thermal conductivity is almost equal to that of copper
➡️has a high atomic number improving efficiency of xray production
Know how the rotating anode is rotated
Using an induction motor
Know the two major parts of an induction motor
Stator and rotor
Know what the stator is made of
Electromagnets arranged in pairs around the rotor
Know what the rotor is made of
Iron core surrounded by coils and located in the center of the stator but within the enclosure
Know how the rotor is operated
Mutual induction
What is the purpose of the rotating anode
To spread the tremendous heat produced during xray production over a larger surface
Know the line focus principle
States that by angling the face of the anode target a large actual focal spot size can be maintained and a small effective focal spot size can be created
Know what the actual focal spot is
Area actually bombarded with filament electrons
Know what the effective focal spot is
The xray beam area as seen from the prospective of the patient
Know how the effective focal spot will be when the angle of the target is less than 45 degrees
Smaller than the actual
Know the range of the target angles for general purpose tubes
7-18
Know what the most common angle is for the general purpose tube
12
Be able to explain the anode heel effect
The angle causes the intensity of the xray beam to be less on the anode side because the “heel” of the target in in the path of the beam. Meaning, that the X-rays on the anode side must first penetrate a portion of the target before exiting the tube. Some do not have the energy to do so and are absorbed in the target heel, reducing the intensity on the anode side.
