RADT 1065 X-ray Tube Questions

what is the most important part of the x-ray machine?

the X-ray tube

what are the 4 requirements of x-ray production?

1. vacuum

2. source of electrons

3. acceleration of electrons

4. deceleration of electrons

what is the most common type of x-ray support?

ceiling support

what type of x-ray support is used in interventional radiography, such as fluoroscopy?

C-arm

what does the protective housing of the X-ray tube do?

directs the x-ray beam, protects against shocking exposure

is a glass or metal envelope more likely to fail?

glass

what is the thin area of the envelope where the x-rays leave the tube called?

the window

what are the 2 main parts of the X-ray tube?

the cathode (-) and the anode (+)

what are the 2 main parts of the cathode side of the tube?

the filament and focusing cup

how does the filament produce electrons?

through thermionic emission (released by heat)

what is the filament made of and how is it controlled?

made of tungsten and controlled by mA

what does the small filament correspond to?

the small focal spot on the anode

what is the small filament of the cathode used for?

detail, 300 mA and below

what is the large filament of the cathode used for?

higher exposure levels, 400 mA and above

what is the function of the focusing cup?

to condense the electron cloud

where is the filament embedded?

in the focusing cup

is the focusing cup positive or negative?

extremely negative

electrostatic repulsion generates...

space charge

The focusing cup is so ______, it forces the electrons to play nice and form an __________.

negative, electron cloud

After electrostatic repulsion, it is hard for the filament to produce more electrons. What is this effect called?

space charge effect

What type of techniques are space charge limited?

techniques that are high in mA and low in kVp

A _________ occurs when all available electrons have been used.

saturation current

what is the only was to increase the number of electrons when there is a saturation current?

increasing mA

What are the 3 functions of the anode side of the tube?

1. electrical conduction

2.mechanical support

3. thermal dissipator

Describe the 2 types of anodes.

Stationary: lower exposure, dental X-ray tubes

Rotating: high intensity system (conventional)

What RPM does the rotating anode have?

3400 rpm

What are the 2 main parts of the anode?

the stator magnets and the rotor

Why are the stator magnets important?

the stator magnets are what spins the rotor

What is the anode coated with?

Tungsten Rhenium alloy

Why is the anode coated with tungsten?

it has good thermal conductivity, a high atomic number (74), and a high melting point

What is the target?

the part of the anode actually struck by the electrons

How does the electromagnetic induction motor of an X-ray tube work?

As current passes through the stator magnets, a magnetic field is generated, which turns the rotor

What happens to the anode side when rotoring?

the stator magnets receive current which turns the rotor

What happens to the cathode side when rotoring?

the filament receives current and space charge is created

After the cathode side and anode side begin to rotor, what happens?

voltage is applied

Once voltage is applied, electrons travel from _______ to ______ at a very high speed.

cathode, anode

At what speed do electrons travel from cathode to anode?

1/2 the speed of light

What happens when the electrons hit the target at the focal spot?

kinetic energy is lost and x-rays are produced

X-rays are emitted from the_________.

focal spot

Most X-ray tubes are "dual focus" and have 2 focal spots at different angles. What are these different spots called?

the small focal spot and the large focal spot

What does the small focal spot correspond with and what is it used for?

corresponds with the small filament, and has a lower heat capacity, is used for radiographs with lower technique

What does the large focal spot correspond with?

corresponds with the large filament and has a higher heat capacity

What is the effective focal spot?

the beam projected onto the patient

When the target is angled, the effective focal spot is much smaller than the actual area of electron interaction. What principle is this describing?

Line-focus principle

The line focus principle helps maintain _______ and________ the actual surface used for x-ray production.

detail, increase

A small focal spot leads to a smaller effective focal spot. This means there will be less beam divergence, leading to....

better detail

A large focal spot leads to a larger effective focal spot. This means that the heat capacity will be...

higher

The effective focal spot is ALWAYS _____ than the actual focal spot.

smaller

What is the downside to the line focus principle?

The Anode Heel Effect

The x-ray photons on the _____ side are weaker than the photons on the ______ side. This leads to the Anode Heel Effect.

anode, cathode

The anode heel effect is greater with _______ angles.

smaller

The anode heel affect can cause intensity to vary by _____% across the x-ray beam.

45%

If the CR is 100%, then the anode side will be ______% and the cathode side will be _____%

75%, 120%

On what side of the tube should you put thicker anatomy?

the cathode side

Due to the anode heel, the effective focus is always _____ on the _____side.

smaller, anode

What is it called when electrons bounce off of the focal spot and photons are created?

off-focus radiation

What are the effects of off-focus radiation?

-increased focal spot size

-adds to patient skin dose

-reduces contrast

What can limit off-focus radiation?

a metal tube envelope or the use of a fixed filtration diaphragm

Recall that ____% of electron energy is converted into heat and only ___% into x-ray photons.

99%, 1%

Almost all tube failure is related to...

heat

What is the primary way that the X-ray tube cools?

infrared radiation (anode glows)

_______ heat dissipation is through the shaft of the anode.

conduction

________ heat dissipation is through the oil within the tube.

convection

If the X-ray tube is not "warmed up" correctly, what can happen?

the anode could crack, leading to tube failure

Surface melting and pitting of the anode can be caused by...

a single excessive exposure

Excessive heating can cause ________ damage resulting in imbalanced rotation and friction.

bearing

Excessive heating of the filament (_______) produces an open filament.

high mA

Tungsten in the filament can be ________ and lead to electron arcing.

vaporized

What does the radiographic rating chart show?

safe techniques

What chart shows how long is takes for an anode to cool completely?

anode cooling chart

What does the housing cooling chart show?

how long it takes for the housing to cool completely

What formula should be used for the anode cooling chart?

HU= kVp x mA x time x rectification

Does the anode or the housing have a higher heat limit?

the housing

What rectification should you use for single face?

1.0

What rectification should you use for high frequency?

1.4