The X-Ray Tube Flashcards
What is the cathode?
the negative side of an x-ray tube. Its function is to produce a thermionic cloud
What is the filament?
A small coil of thin thoriated tungsten wire. It is set in the cathode assembly within the focusing cup
Why is tungsten the material of choice for the filament?
because it has a high melting point of 3,370 degrees and it is difficult to vaporize.
Why are rhenium and molybdenum also desirable xray tube materials?
because of their high melting points. Rhenium = 3170 degrees and molybdenum is 2620 degrees.
Most x-ray tubes have dual filaments, what is this called?
Dual focus arrangements
What is the function of the filament?
to provide sufficient resistance to the flow of the electrons so that the heat produced will cause thermionic emission to occur
What is one of the primary causes of premature tube failure?
the radiographer’s habit of holding the rotor switch prior to making exposures. Every second the switch is depressed, life is removed from the filament.
What do most tube manufacturers recommend to avoid premature tube failure?
they recommend that two step exposure switches be fully
depressed in one motion
What is the focusing cup?
a shallow depression in the cathode assembly designed to house the filament
What is the purpose of the focusing cup?
it is provided with a low negative potential. As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons. this is called a space charge affect
What is the anode?
the positive side of the x-ray tube. It has three functions:
serves as a target surface for the high voltage electrons from the filament, there by becoming the source of the x-ray photons
it conducts the high voltage from the cathode back into the x-ray generator circuitry
it serves as the primary thermal conductor
What happens to electrons at the anode target surface?
high speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons
Why is a faster rotating anode better to have?
the faster it rotates, the better the heat dissipation
What are the 3 reasons why tungsten is the metal of choice for the source of x-ray photons?
high atomic number
high melting point
heat conducting ability
What happens to a rotating anode with time?
eventually some of the rotating anode will vaporize sufficient target focal track material to roughen or pit the target area
What can be done to prevent target pitting?
properly performing anode warm - up procedure
What are the repercussions of failing to follow proper x-ray warm up procedures?
It can cause the entire anode to crack if the molybdenum absorbs the heat too rapidly and exceeds its expansion capability
What is the target area?
AKA focal point, focal spot, or the focal track
the portion of the anode where the high voltage electron steam will impact
What is the actual focus spot?
the physical area of the focal track that is impacted
What is the effective focal spot?
the area of the focal spot that is projected out of the tube toward the object being radiographed
What is the line focus principle?
it is used to reduce the effective area of the focal spot
What controls the effective focal spot size?
by the size of the actual focal spot and the anode target angle
What is the most common diagnostic radiography target angle?
12 degrees
The effective focal spots vertical dimension is the one that is stated as what?
the focal spot size
