Ch. 13 - Xray Tubes

Thermionic Emission

electrons are boiled off the cathode

As you increase kVp, what happens to the electrons?

the speed of the electrons increases

What is tube current measured in?

mA

Vacuum Tube

glass tube with all air removed from it

Window

where the x-ray photons exit the tube, thinner area of glass

Degassing

pyrex tube is baked and expels air

Why is a vacuum tube important?

provides and unobstructed path for electron to travel and slows down burnout of the filament

What makes up the cathode assembly?

filament, wires, and focusing cup

Focusing Cup

concave metal cup that surrounds the filament (negatively charged), its job is to focus the electrons and prevents them from flaring out

What does the small filament contain?

two wires, one common wire and one wire that is only connected to small filament

What does the large filament contain?

two wires, one common wire and one wire that is only connected to large filament

What happens are the tube ages?

tungsten evaporates from the filament and causes the filament to become thinner which results in a great resistance and more heat; need to have less current

When tungsten is evaporated it will condense to...

the window where tungsten with absorb x-ray photons and the wall where spark over occurs and punctures the tube

What are the two different types of anodes and what are they used for?

stationary anode is used in dentistry and the rotating anode is used in diagnostic x-ray

What properties make tungsten a good anode material?

high melting point, ability to conduct heat, and high atomic number

What is the area of the target that is bombarded with electrons called?

focal spot

What are some facts associated with the small focal spot?

produced by the small filament, have to keep techniques low, provides better image detail

What are some facts associated with the large focal spot?

produced by the large filament, can use larger techniques selection, provides less image detail

What is the line focus principle?

the anode is angled down towards the image receptor causing the effective focal spot to be smaller than the actual focal spot

How is the target angle related to the effective focal spot size?

as target angle decreases, effective focal spot size decreases

How is effective focal spot size related to image detail?

the smaller effective focal spot size, the greater image detail

How is the effective focal spot size related to direction of projection?

the effective focal spot size is smaller when projected towards the anode and larger when projected away from the anode (or toward the cathode)

What is used to rotate the anode?

induction motor

How do rotating anodes display the line focus principle?

beveled edge

Space Charge Effect

electrons has like charges which will repel, so some electrons will be forced back into the filament during thermionic emission because there is not room for all electrons burned off the filament all at once

Space Charge Compensator

keeps mA constant when kVp changes

What are the two types of vaporized tungsten?

filament and tube

What percent reduction in the diameter of filament will caused it to break, ending tube life?

10%

Filament Booster Circuit

keep the filament current just below thermionic emission levels while rotor is getting up to speed

Tube Rating Chart

tells the maximum exposures (techniques) we can use, measured in seconds

Anode Cooling Curve

guid to not melt the anode, measured in minutes

Thermal Capacity

maximum number of heat units our target can hold without melting

What is the equation for single phase thermal capacity?

HU = kVp x mA x time

What is the equation for three phase six pulse thermal capacity?

HU = kVp x mA x time x 1.35

What is the equation for three phase twelve pulse thermal capacity?

HU = kVp x mA x time x 1.41

What is the equation for high frequency thermal capacity?

HU = kVp x mA x time x 1.45

What temperature does the tube housing need to be kept below?

90 degrees celcius and 200 degrees farhenheit

How is tube housing cooled?

oil conducts heat from anode and spreads it out, fan to cool it, and forced circulating of the oil

How can you extend tube life?

don't rotor for excessive amount of time, exposures shouldn't exceed tube rating chart, perform manufactured suggested warm up, provide adequate cooling of the housing, and rapid sequence exposures must comply with anode cooling curve