The X-Ray Tube

actual focal spot

the physical area of the focal track that is impacted, controlled by filament size

anode heel effect

due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side

dual-focus

a to filament arrangement within the x-ray tube

effective focal spot

the area of the focal spot that is projected out of the tube toward the patient being imaged, controlled by size of actual focal spot and the anode target agle

exit radiation

the portion of the primary beam that emerges from the examined object and interacts with the image receptor to record the radiographic image; AKA remnant radiation

focal point/spot/track

the portion of the anode where the high-voltage electron stream will hit

focal track with a rotating anode

this describes the circular path that will be hit by the electron beam

leakage radiation

any photons that escape from the housing except from the port/window; not in the useful beam

line-focus principle

describes the effect of the angulation of anode surface on the area of the effective focal spot size

off-focus radiation

occurs when x-rays are created from interactions from materials within the x-ray tube OTHER than the focal spot

primary radiation

radiation that comes directly from a source before it interacts with matter

scatter radiation

radiation that spreads out in different directions from a radiation beam when the beam interacts with matter

space charge effect

as more electrons build up around the filament, their negative charges begin to oppose the emission of more electrons

target

the portion of the anode where the high voltage electron stream will hit

tube current

flow of electrons from cathode to anode, measured in mA

The x-ray tube consists of

cathode, anode, envelope, protective housing

cathode

source of electrons, negative side

cathode function

• produce thermionic cloud

• conduct high voltage to the gap b/w cathode and anode

• focus the electron stream as it heads for the anode

cathode consists of

filaments, focusing cup, associated wiring

Which of the following is the function of the cathode?

a) To produce a focal point
b) To produce an electrical connection to the rectifier
c) To produce a thermionic cloud
d) To produce a grounding effect

to produce a thermionic cloud

filaments

small coil of thin thoriated tungsten, found in focusing cup

why is tungsten used in filaments

has a high melting point

why is thorium added to filaments

help reduce vaporization and prolong tube life

most machines have which filament

dual focus arrangemnet

the filament’s length and width affect

the ability of the x-ray tube to image fine details

when an x-ray machine is turned on it stay “preheated” by a mild current until

immediately before an exposure

during an exposure one of the filaments is heated to a level causing

thermionic emissions (electrons boiling off)

the space charge effect limits the functional mA of the tube to

1,000

tube failure can be caused by

filaments vaporizing and coating the inner surface of the tube with tungsten

rotoring the exposure switch

preps the tube for exposure and heats the filament, causes most of th filament vaportization

All of the following are preferred cathode filament
materials EXCEPT
a) Tungsten
b) Molybdenum
c) Copper
d) Rhenium

copper

the focusing cup

shallow depression in cathode assembly to house filaments, made of nickel

focusing cup function

narrow thermionic cloud as it is driven to the anode

the focusing cup has a low negative voltage because

it keeps the electrons focused and concentrated allowing for better detail

What component of the x-ray tube is the source of free
electrons?
a) Tube housing
b) Glass envelope
c) Anode
d) Cathode

cathode

the anode consists of

anode, stator, rotor

the anode

postive side

anode function

target surface for x-ray photons to be produced, maintains closed circuit pathway, primary thermal conductor

stationary anode

limited to lower power function, dental unit

rotating anode

better for heat dissipation

anode disks are comprised of

molybdenum

the focal track target material is

tungsten-rhenium alloy

tungsten is the metal of choice to source x-ray photons because

high atomic number, high melting point, heat-conducting ability

the target area

the portiong where the high-voltage electron stream hit the anode, where the x-ray photon are created

refer to the area of the focal track that is impacted by the electron beam at one time

focus, focal point, focal spot, focal track

rotating anode are designed to

increase target area and dissipate heat better

off focus radiation

contributes up to 25 percent of total primary beam, low energy/not diagnostic, produces ghosting

What component of the x-ray tube is the source of x-
rays?
a) Tube housing
b) Glass envelope
c) Anode
d) Cathode

anode

focal spot size

effective focal spot, most are dual focus

dual focus spots

small focal spot for fine detail, large focal spot for heavy tube loads

Which of the following situations will require the use
of the large filament within a dual focus x-ray tube?
a) Spinal imaging on a pediatric patient
b) Abdominal imaging on a bariatric patient
c) Chest imaging on a neonatal patient
d) Extremity imaging on a trauma patient

Abdominal imaging on a bariatric patient

photons emitted towards the anode are

more likely to be absorbed than those emitted toward the cathode

the thicker/denser part of the body should be positioned toward the

cathode end (fat cat)

The anode heel effect is defined as a variation in
which of the following?
a) Patient thickness across the x-ray field
b) Contrast resolution across the x-ray field
c) Beam quality across the x-ray field
d) Beam quantity across the x-ray field

Beam quantity across the x-ray field

the stator

induction-motor electromagnets that turns the anode, outside the vacuum of the envelope

if the stator fails the

rotor will not turn the anode

the rotor

located inside the stator and envelope, consists of copper cylinder (cuff) connected to the anode disk by molybdenum stem (anode stem)

the rotor turns when

stator is energized

the inside of the rotor contains

bearings that use silver plating as a high temperature lubricant b/w cuff and anode shaft

common anode rotation speed

3,200 - 3,600 rpm

high speed anode rotation speed

10,000 - 12,000 rpm for high-speed anodes

What is the purpose of the induction motor within the
x-ray tube?
a) To boil off electrons
b) To rotate the anode
c) To send electrons across the tube
d) To produce x-ray photons

to rotate the anode

the envelope

heat tolerant Pyrex glass or metal tube that encloses the entire cathode and anode assembly, AKA the tube

envelope functions

• support anode/cathode assemblies

• maintain vacuum

the envelope contains a window segment which is

where the primary x-ray beam exits the envelope

protective housing

supports the x-ray tube (part we see)

protective housing functions

• control leakage and scatter radiation

• isolate the high voltages

• provide mechanisms to cool tube

leakage radiation limit

100 milliroentgens per hour (mR/hr) measured at 1 meter (m)

protective housing cooling mechanisms

• cooling oil surrounds tube

• cooling fans

• water cooling in high end tube designs

X-rays can only be produced under what special conditions

• have a source of electrons

• have high voltage

• have a way to focus electrons

• have appropriate target material

• be in a vacuum

extending x-ray tube life: follow

recommended tube warm-up procedures

extending x-ray tube life: avoid

frequent and successive “boosting” or “rotoring” of tube

extending x-ray tube life: use

low mA settings when possible

extending x-ray tube life: use

low speed rotor rotation when possible

extending x-ray tube life: follow

rating charts

extending x-ray tube life: avoid

rough handling of x-ray tube head

extending x-ray tube life: listen

for unusual sounds and report to service engineer

anode warm up procedure

should be done after long idle periods, gradual heating to prevent anode cracking, maintains a strong vacuum

anode pitting

commonly occurs with extended use

anode metling

occurs when the stator fails and the rotor doesn’t turn

when does cracking of an anode usually happen

when a cold anode is hit with high kVp exposure

radiographic tube rating charts

most valuable since they give a guide regarding max technical factor combos without overloading the tube

three types of rating charts to help radiographers avoid thermal tube damage

• radiographic tube rating

• anode cooling

• housing cooling

anode cooling curves

non linear, rapid then gradually slows, calculated in terms of radiographic heat units

radiographic heat units (HU)

measure the heat generated in an x-ray tube’s anode during an exposure

radiographic heat units (HU)

kVp x mA x time x rectification constant

rectification constant

takes the electrical waveform in account

singe phase rectification constant

1.0

high frequency rectification constant

1.4

heat unit calculation being integrated into the generator software by the manufacturer prevents

unacceptable exposure combinations based upon heat loading characteristics of the x-ray tube

x-rays travel in

straight line

highly penetrating, invisible rays

x-rays

x-rays are electronically

neutral

x-rays can be produced over a

variety of energies and wavelengths (polyenergetic and heterogenous)

x-rays travel at the

speed of light

x-rays release some heat when

passing through matter

x-rays can ionize

matter

x-rays cause

fluorescence of certain crystals

x-rays cannot be focused with

a lens

x-rays affect

photographic film