The X-Ray Beam

stationary anode

a nonmoving anode, usually found in dental and small portable radiography units

kilovoltage (kVp)

applied across x-ray tube at time the exposure is initiated, determines speed at which electrons in the tube current move; higher voltage produces higher quality (higher energy), higher beam penetrability and higher quantity (more efficient) x-rays

filtration

attenuates (absorbs) photons

space charge

cloud of electrons liberated from the filament during thermionic emission

X-ray tube housing

components of x-ray production enclosed in a glass enevelope;allows air to be evacuated completeley from the x-ray tube which allows the flow of electrons from cathode to anode;allows no more than mR/hr of leakage at 1 meter from source

instantaneous load tube rating chart

describes exposure limits of x-ray tubes; determines whether particular exposure will be safe to make; determines limits on kVp, mA and exposure time for safe exposure

stator

electric motor that turns the rotor at very high speeds during x-ray production

filament current

electrical current that is induced across the filament in the cathode, which heats the tungsten filament; 3-5 amps and operates at 10 V; amount depends on mA set at the control panel;

x-ray production

electrons travel from cathode to anode at half the speed of light, strike the target, transferring kinetic energy to tungsten atoms in the anode

bremsstrahlung

german word meaning braking or slowing down; projectile electron completely avoids orbital electrons of tungsten atom and travel close to nucleus; electrostatic force of nucleus causes electron to slow down, losing energy, changing direction; energy loss reappears as x-ray photon; 30-150 kVp

low-energy photons

increase patient dose and do not contribute to image formation

x-ray exposure

kVp level is applied across the tube from cathode to anode; creates potential difference, cathode becomes negatively charged, repelling electrons; anode becomes positively charged, attracting electrons; electrons strike anode, x-rays and heat is produced

keV

kiloelectron volts; measurement of x-ray energy

deadman switches

located on x-ray unit control panel; require positive pressure to be applied during entire x-ray exposure process

mAs

mA multiplied by exposure time; quantity of electrons flowing from cathode to anode -- directly proportional; no effect on x-ray quality

focusing cup

made of nickel and nearly surrounds the filament; open at one end to allow electrons to flow freely across tube from cathode to anode; has negative charge; focuses the stream of electrons

rhenium

makes up 10% of composition of rotating target

tungsten

makes up 90% of composition of rotating target; high atomic number 74 for efficient x-ray production and high melting point 3370 C

target

metal that abruptly decelerates and stops electrons in the tube currentallowing the production of x-rays;can be rotating or stationary;partr of the anode that is struck by the stream of electrons

X-ray tube

most important part of the x-ray machine;production of x-rays occurs inside the tube when high speed electrons from cathode side collide with a metal object on the anode side

dual-focus

most x-ray tubes are referred to dual-focus because they have two filaments;a large and a small filament

Cathode

negative side of tube;has two parts a fillament and a focusing cup;cathode produce electrons and focus them toward the anode

x-ray quantity

number of x-ray photons in the primary beam; higher voltage increases efficiency of x-rays; directly proportional to mA, exposure time and mAs

anode heel effect

occurs because the angle of the target;describes how the x-ray beam has a greater number of x-rays on the cathode side of the tube;intensity dimenishes toward the anode side;can be as much as a 45% difference

x-ray quality

penetrating power, energy level of the radiation; higher voltage produces higher energy x-rays with higher penetrating power

focal spot

physical area of the target that is bombarded by electrons during x-ray production

Anode

positively charged electrode;consist of a target,stator and rotor;dissapates heat from the tube

effective focal spot

refers to focal spot size as measured dierectly under the anode target

actual focal spot

refers to the size of the area on the anode target that is exposed to electrons from the tube current

rotor

rigidly connected to the target through the anode stem; causes the target to rotate rapidly and produce x-rays

filament

small coil of high melting point tingsten metal;the current heats the filament where electrons are boiled off and emitted from the filament;similiar to a light bulb;source of electrons during x-ray production;dual large and small filament

space charge effect

tendency of the space charge to not allow more electrons to be boiled off the filament

rotating anode

used more frequently;used in all applications of radigraphy;can withstand greater heat loads

thermionic emission

when tungsten filament gains enough heat, outer shell electrons of the filament are boiled off or emitted from the filament