RADT 146 C - Unit 1: X-Ray Tube Construction

cathode

negative side of the x-ray tube

anode

positive side of the x-ray tube

thermionic emission

the process of the filament wires heating up to glow and releasing ("boiling off") electrons

cathode ray

a flow of electrons emanating from the cathode in a vacuum tube and focused into a narrow beam

x-ray production

when electrons strike the target on the anode

milliampere (mA)

controls the amount of electrons emitted at the filament by process of thermionic emissioin

more mA applied

more electrons

less mA applied

less electrons

factor(s) that control quantity and density of an x-ray

milliampere (mA) and time (s)

mAs

determines the number of electrons emitted from the filament and the rate in which these electrons (-) are attracted to the anode (+)

quantity

the amount of x-rays produced

density

overall degree of darkness of an image

kilovoltage (kV)

controls the energy (penetrating power/"force") of the x-ray beam, sending electrons from the cathode to the anode

factor(s) that control the quality and density of an x-ray

kilovoltage (kV)

contrast

difference between the darkness and brightness of an image

more kV applied

stronger x-rays

less kV applied

weaker x-rays

methods of x-ray tube support

ceiling, floor-to-ceiling, c-arm

ceiling

-most common form of external support

-freely moves in all directions: transverse, longitudinal, up, and down

floor-to-ceiling

-requires more effort to move

-multi-directional

C-arm

-nearly limitless in tube position

-robotic and fluid in motion

-angiography, fluoroscopic capabilities

protective housing

-provides mechanical support

-protects from damage caused by rough handling

-contains oil to insulate from shock and dissipate heat

-reduces leakage radiation

leakage radiation

-anything that escapes the protective housing

-non-diagnostic

-unnecessary exposure to both patient and technologist

6 ft

safe distance from x-ray tube

primary beam

also known as the "useful beam"; the 1% of rays emitted through the window and used to project the diagnostic image (x-ray)

secondary beam

99% of all the other rays scattering inside the protective housing and dissipating into heat

benefits of pyrex glass envelope

withstands large amounts of heat

benefits of metal envelope

more efficient, prevents tube arcing and tube life is better

benefits of pyrex glass or metal envelopes

vacuum creates longer tube life: less collision of electrons with gas molecules, increase x-ray production, less heat generated

parts of cathode

negative electrode, filament (2 coils of wire), dual focus, focusing cup

features of small filament

-200 mA or less

-better details

-smaller body parts

features of large filament

-300 mA or more

-larger body parts

-can withstand more heat

focusing cup

confines the electron beam to a small area of the anode

space-charge effect

prevents more electrons from being boiled off due to electrostatic repulsion; results in a cloud of electrons

parts of anode

positive electrode, target (track)

types of anodes

stationary and rotating

features of stationary anode

-does not require high tube current or high power

-can be found in dental and portable x-ray units

-composed of copper anode and tungsten target

features of rotating anode

-produce high-intensity x-ray beams in shorter time periods

-better for heat dissipation

-rotates at 3,600 to 10,000 RPM

-composed of copper anode and target track

target (track)

area that the electrons strike and are converted to x-rays

layers on rotating anode

-tungsten-rhenium

-molybdenum

-graphite

properties of tungsten

-high atomic number (74)

-better at heat dissipation

-high melting point (3410 degrees C or 6170 degrees F)