LECTURE 4: X-RAY TUBES 2 Flashcards Preview

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Flashcards in LECTURE 4: X-RAY TUBES 2 Deck (37):

Stationary Anode: What is the anode angle?



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?



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?



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?

1. 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?



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)