LECTURE 4: X-RAY TUBES 2 Flashcards
Stationary Anode: What is the anode angle?
15-20deg
Stationary Anode: What materials are used?
2mm thick tungsten, embedded in a mass of copper
Stationary Anode: Why is tungsten embedded in copper?
Copper is a good thermal conductor - meaning it is good at distributing heat
What is a thermal conductor
Distributes/dissipates heat well
Anode Angle: If it is smaller, then is the focal spot smaller or larger?
Smaller
Rotating Anode: rpm? Limitation? Actual rpm?
3000 rpm. Never really reach this speed because of technical factors; friction etc… 2500 rpm
What are the material components of a rotating anode?
Tungsten anode, molybdenum stick connecting to copper cylinder
What is the function of the molybdenum stick?
Transfers heat from anode to copper tube
What is the difference in focal spot size in a stationary vs rotating anode?
No difference - still the same, only total SA is changed
How does the anode rotate in an enclosure with no mechanical connection to the outside?
Electromagnetic induction motor is used to turn the anode: Stator and Rotor
Describe how electromagnetic induction motor works in terms of x-ray tube
Stator surrounds a rotor. Stator has coil windings that produce a magnetic field, which induces a current that provides power for the anode to rotate.
Why is tungsten chosen as a target material for x-ray tubes?
- High melting point
- High atomic number (means more e- e- interaction probable)
- Good at heat absorption and dissipation
Why is the tungsten target embedded in copper?
Copper is a better heat conductor than tungsten so it can increase the thermal capacity of anode and dissipates heat fast
Does the whole tungsten target get bombarded?
No- only a small area but we need to have a larger area because temperature of the area will rise and copper has a lower melting point so we don’t want direct interaction with copper
Is a graphite or molybdenum anode disc better? Why would you use graphite?
Molybdenum as graphite will become hotter (does not conduct heat as well we moly); but graphite may be used to reduce disc inertia
What is a “focal spot”?
The area of the TARGET from which the x-rays are emitted (angle from the anode)
If the focal spot size is smaller, how does this affect image quality?
Spatial resolution is better!
How is the size of the focal spot size controlled?
Depends on the angle of the anode
What is “Line-focus Principle”?
Focal spot - angle with anode
What is “apparent (effective) focal spot”?
The actual area that is projected onto the patient/image receptor
What is the difference between actual and apparent focal spot size?
Actual is what actually hits the anode, and apparent is what apparently hits the patient
How do you calculate apparent focal spot?
Actual focal spot x sine (angle of inclination)
As the angle of inclination decreases (focal spot size), does the effective focal spot size increase or decrease?
decrease!
Can an anode have 2 angles?
yes of course! this means there can be 2 focal spot sizes
What is NEMA and what does it stand for?
National Electrical Manufacturers Association- regulates the variances in focal spot size
What are the measurements of a slit camera?
Slit is 0.01mm wide. Placed in middle of a metal 5cm long and 1.5mm thick
Where is the slit camera positioned to take an image?
About 10cm from the focal spot; and is between x-ray tube and film (like an IR)
How is the slit camera used to determine focal spot size?
- Slit along axis of x-ray tube for length, then
2. Rotate 90deg for width
What are the standard measurements of focal spot size? and what does it increase in (increments)?
Starts at 0.3mm, up to 2.0mm, in 0.1mm increments
How does a pinhole camera work?
Same setup as slit - except the size of focal spot will show up on the image; must apply come correction factors if there is geometric magnification present
What is the Resolution Star TEst object?
Measures the effective blur size of the focal spot - tests spatial resolution
What is the Anode Heel Effect?
Reduced x-ray intensity on the ANODE SIDE of the useful beam because of absorption in the ‘heel’ of the target (anode)
Anode Heel Effect: on which side of the x-ray tube is there a reduced x-ray intensity?
ANODE SIDE
What is the special term we call the x-ray beam due to the anode heel effect?
Non-homogenous beam
How do we counteract the Anode Heel Effect? Explain
Increase FFD - so that the less intense x-ray beams do not hit the IR - leaving the ones that have same intensity to hit it
What is the typical focal spot size for radiography vs mammography?
radio is 0.6-1.2mm
mammo is 0.1-0.3mm
Know how to calculate heat unit (HU)
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