RADT 1065 Unit 4 The X-ray tube

X-ray tube

-where electrical energy is converted into heat and x-rays

-the most important part of an x-ray machine

vacuum, source of electrons, acceleration of electrons, and deceleration of electrons

what are the 4 requirements of x-ray production?

ceiling support

-most common type of tube support

-2 sets of rails for longitudinal and transverse movement

floor to ceiling support

C-arm support

-used in interventional radiology

-used in OR during surgery and fluoroscopy

isotropically (all direction)

how do x-rays emit?

protective housing

-limits the x ray bam

-directs the x-rays to one direction

insulation against shock and heat cushion

what is the purpose of oil in an X-ray tube?

Leakage radiation

-any photons that escape from the protective housing

-less than 1 mGya/ hr at 1 m

mGya

the intensity of X-rays

electric shock and excessive exposure

what does the X-ray tubes protective housing reduce?

Glass envelope

-maintains the vacuum of the X-ray tube

-made of pyrex and can withstand heat

partially, gas

The Crookes tube was partially _________ and contained _______.

Drawbacks of the glass tube

-over time the tungsten coats vaporize from the heat and allows electrons to diverge

-the tube will fail eventually

Metal envelope

-improvement over the glass tube

-tungsten will not collect = constant electric potential

-longer life and less likely to fail

window

the thin area of the envelope where useful x ray beam leaves the tube

cold cathode tube

-crookes tube

-no thermionic emission

-partially evacuated

-relied on ionization of gases within the tube to create x rays

hot cathode tube

-coolidge tube

-total vacuum

-Improved efficiency

-heated filament= thermionic emission

thermionic emission

emitting electrons with heat

cathode (-) and anode (+)

what are the 2 major internal components of the X-ray tube?

cathode

-the negative side of the tube

-acceleration

anode

-the positive side of the tube

-deceleration

electrostatic repulsion

-unlike charges attract and like charges repel

the focusing cup is even more negative than the electrons

If all of the negative electrons are repelled from one another, how do we get the together?

space charge

the focusing cup is so negative, it forces the electrons to attract and form an electron cloud

Space Charge Effect

-once apace charge is generated, electrostatic repulsion makes it difficult to produce more electrons from the filament

saturation current

-tube current rises with increasing tube voltage to a maximum value

-all available electrons have been used, and no increase of kVp can increase number of electrons used for x-ray production

-the only way to increase electrons is to increase mA

3 functions of the anode

1. electrical conductor

2. mechanical support

3. thermal dissipater

stationary anode

-lower exposure

-dental X-ray tubes

-immobile

rotating/conventional anode

-high intensity system

-faster rotation= more heat dissipation

-3400 rpm

strator magnets

spins rotor and anode through electromagnetic induction

target

Area of the anode struck by the electrons from the cathode

tungsten and rhenium

what is the anode made of?

to withstand rotation speeds (all tungsten would be too heavy), and expansion/contraction from heat

why is rhenium added to the anode?

high atomic number, good thermal conductivity, high melting point

why is the anode made of tungsten?

induction motor

-current passes through starter magnets, causing rotor to spin, causing anode to spin

-this is used because in order to maintain a vacuum, no mechanical connect is possible

Rotoring

-anode side: strator receives current, turns rotor

-cathode side: filament receives current, space charge is created

Exposure

once kVp is applied, electrons will travel from cathode to anode at a very high speed. The electrons will hit the target at focal spot.

focal spot

where x-rays are emitted

dual focus focal spot

has a small focal spot that corresponds with the small filament, and a large focal spot that corresponds with the large filament

small focal spot

lower heat capacity, lower technique is used

large focal spot

higher heat capacity

line focus principle

-By angling the target 15-20 degrees, the effective area of the target much smaller than the actual area of electron interaction

-Effective focal spot is always smaller than the actual focal spot

Anode heel affect

x-ray photons on anode side of the tube are attenuated by the anode heel, and are therefore weaker than the cathode side photons

off focus radiation

when electrons bounce off the focal spot and photons are created. this leads to unfocused x-rays, high dose, and reduces contrast

tube failure

almost always related to heat

heat dissipation

radiation, conduction, and convection

radiographic rating chart

shows which radiographic techniques are safe for tube operation

anode cooling chart

chart than shows how long it takes the anode to cool completely

housing cooling chart

Graph showing the cooling rate of an x-ray tube housing