The X-Ray Tube

What is the biggest problem with x-ray production?

99% heat

1% x-ray

What are the 3 conditions required for x-ray production?

1. source of electrons

2. means of accelerating electrons

3. means of decelerating electrons

(all must occur in a vacuum)

What is the source of electrons?

heated filament

What is the means of accelerating electrons?

high voltage (kV)

What is the means of decelerating electrons?

appropriate target material

What are the 5 characteristics of the Protective Housing of the tube?

1. cast steel structure lined with lead

2. Dielectric oil; dissipates heat

3. absorbs xrays traveling in multi directions

4. Leakage radiation may not exceed 1 mGya/hr or 100 mR/hr at 1 meter

5. Helps protect operator against electrical shock

What is the purpose of dielectric oil?

serves as an electrical insulator and thermal cushion

Leakage radiation may not exceed ___

100 mR/hr at 1 meter

What is leakage radiation?

radiation that escapes the tube from somewhere other than the window

Describe the tube envelope

• vacuum sealed (allows electrons to flow w/o running into gas atoms)

• made of pyrex glass or metal

• Tube Window (area where xrays exit tube)

What is the function of the tube envelope?

to maintain the vacuum

What is the tube window?

the area where x-rays exit the tube

The X-ray tube is a ______ meaning it contains 2 charged electrodes that are positive and negative

diode

Define Electrode

conductor device that allows electricty to flow

Electrons move across the tube from ___ to ___

from cathode to anode

How fast do electrons travel from cathode to anode?

1/2 the speed of light

What is potential difference?

difference in the charge of electrodes

Is the cathode negative or positive?

negative

Is the anode negative or positive?

positive

The cathode is the source of________?

electrons

What are the 3 components of the cathode?

1. wiring

2. filament

3. focusing cup

The wiring of the tube has _____ separate circuits with distinct functions

two

Explain the difference between filament current and tube current

filament current (heat filament)

• low voltage

• occurs when x-ray machine is on

• current flowing through filament heats up when rotoring

• temp of filament determines number of electrons emitted

tube current (speed of electrons)

• high voltage

• kicks boiled off electrons across tube to anode

• determined by the kVp

The filament is the source of ___

electrons

Where is the filament located?

within the focusing cup of the cathode

What is the filament made of? Why?

made of thoriated tungsten wire because it is a good conductor, has a high melting point, and increases thermionic emission

Of the 2 filaments present, which will give the best resolution?

the smaller one

Of the 2 filaments present, which will have the highest heat load?

the smaller one

Describe the focusing cup

• a shallow depression

• houses the filament(s)

• made of nickel

What does the focusing cup do?

creates a tighter stream of electrons

-negative charge applied to the focusing cup to push those electrons out

Define thermionic emission

electrons leave the filament surface and form a cloud (electrons are “boiled off” of the filament)

The cloud of electrons form when the technologist ______ the rotor

depresses

Define space charge

a cloud of electrons

What is the space charge effect?

when no more electrons can boil off (reaches saturation)

What does it mean that the space charge effect is self-limiting?

limits tubes to max mA range (1000-1200 mA)

mA is the quantity of ______

electrons

What happens when the technologist depresses the button? (Exposure Slide)

electrons are sent across the tube toward the anode

What is the saturation current? (Exposure Slide)

increasing kVp will increase % of electrons driven across tube

What is a grid biased tube?

• addition of positive/negative potential difference (wire mesh)

• negative charge of focal cup is greater than negative charge of filament

• quickly controls the flow of e-

  • acts as a switch for very short exposures

What are the 3 components of the anode?

1. target

2. stator

3. rotor and bearings

What are the 3 functions of the anode?

1. site of energy conversion/x-ray production (stops or slows high speed electrons)

2. electrical conductor (returns high voltage to generator cicuitry)

3. thermal conductor (dissipates heat)

What is the target area of an anode?

the portion of the anode where electron stream will impact

What are the two types of anode?

stationary and rotating

Efficient X-ray production

1. high atomic #

2. heat dissipation

3. greater elasticity

4. high melting point

5. good conductor

The target on the stationary anode is made of rhenium-alloyed tungsten embedded in a ______ degree angled end of a copper rod

45

What is the benefit of a rotating anode?

improved heat dissipation and provides a larger target area

A rotating anode has a focal _____ and a focal ______

track and spot

The rotating anode disk is made up of ___ and ___. What is the function of each of these metals?

Molybdenum (to get rid of energy) and graphite (for heat dissipation)

The focal track is made of ___ to provide ___.

Rhenium-alloyed tungsten (90:10 ratio) to provide heat dissipation

Where is the stator located?

outside of the tube envelope/vacuum

What theory does the stator operate on?

Theory of Mutual Induction (stator turns rotor, which turns anode)

The stator is made up of a series of electromagnets and uses ___

electromagnetic induction motor

Where are the rotor and bearings located?

inside the tube envelope

Explain the make-up of the rotor and bearings

hollow copper cylinder/cuff attached to anode disk by the molybdenum anode stem

silver-plated ball bearings (silver serves as lubricant)

What does depressing the exposure switch halfway do?

activates anode rotation and electron boiling

What does depressing the exposure switch completely do?

initiates exposure and produces the x-ray

How should you use the exposure switch?

completely depress both buttons in one motion

What does it mean that the exposure switch is a “dead man” switch?

x-ray exposure only occurs while the switch is depressed

Describe the target angle

anode disk is beveled at the region of the focal track to allow target to be steeper/more vertical

Target angles range from ___-___ degrees

7-17 degrees

What is the most common angle of target angle?

12 degrees (smaller angle wont cover a large IR)

Define actual focal spot

physical area of track that is impacted by electrons

Define effective focal spot

area of focal spot projected out of tube

Explain the benefits of both large and small focal spots

large: better for heat dissipation

small: better for resolution/detail

Technologist selects the large or small focal spot

Explain the line focus principle

When the target angle is less than 45 degrees, the effective focal spot will be smaller than the actual focal spot

The smaller the target angle, the ___ the effective focal spot

smaller

Where is the anode heel?

lower back corner of the anode disc

What is the anode heel effect? What is it caused by?

intensity is greater at the cathode end of the tube; caused by absorption of some of the x-rays within the anode

The anode heel effect causes a variation in radiation intensity of ___ from cathode to anode

45%

Photons emitted toward the cathode are ___ (more/less) likely to be absorbed

less

What does it mean that photons emit isotropically?

they are emitted in all directions

The smaller/steeper the anode angle, the ___ (more/less) pronounced the heel effect is

more

The heel effect is more evident when using ___

• larger film/IR sizes

• shorter SID

• steeper/smaller target angle

Should the thicker part of the body (or part that is being imaged) be placed under the cathode or anode side of the x-ray tube?

cathode (think FAT CAT)

What is off-focus/extrafocal radiation? What does it cause?

photons not produced at the focal spot; causes “ghosting”

What percent of the primary beam is comprised of off-focus/extrafocal radiation?

25-30%

Explain the effects of tungsten from both filaments and anode vaporizing

• acts as filtration (accumulates along bottom of tube)

• deposits on glass (arcing/cracking of the glass)

  • most common cause of failure

  • “gassy” tube - inefficient x-ray production

• small pitting on anode

Explain the effects of tungsten vaporization

• filament becomes thinner and thinner

• eventually falls apart from heat load

• small pitting

Explain the potential failures of the ball bearings

• prolonged periods of excessive heat

  • warping

• increased friction/rough rotation

  • wobbling of the anode disk/grinding noise

• bearing failure

  • large pitting of anode

Explain the effects of excessive heat on the anode caused by several high exposures in a row

fine cracks and pits

Explain the effects of high exposures on a cold tube

anode cracking

What are some ways you can prolong tube life?

• warm up a cold tube before using

• don’t use high exposure on cold tube

• avoid a succession of high technique exposures

• fully depress exposures in one motion

• move tube with care

Explain the anode warm up procedure

series of exposures bring anode heat from room temp to tube temp (usually 3 exposures 5 seconds apart)