X-ray beam Flashcards
The size of the focal spot is not altered with a rotating anode, but the
actual physical area of the target bombarded by electrons is constantly changing, causing a greater area—a focal track—to be exposed to electrons.
Because of the larger area of the target being bombarded during an exposure
the rotating anode is able to withstand higher heat loads produced by greater exposure factors
Rotating anode x-ray tubes are used in all applications in radiography, whereas stationary
anode tubes are limited to studies of small anatomic structures such as teeth.
Rotating anodes can withstand higher heat loads than stationary anodes because
the rotation causes a greater physical area, or focal track, to be exposed to electrons.
The components necessary for x-ray production are housed in
a glass or metal envelope
Metal envelopes are more commonly used because of
their superior electrical properties.
A disadvantage of a glass envelope x-ray tube is
that tungsten evaporated from the filament during exposure can be deposited upon the inside of the glass, especially in the middle portion of the envelope. This evaporation can affect the flow of electrons and cause the tube to fail. Replacing this section of glass with metal prevents these problems and extends the tube life.
An additional advantage of a metal envelope is
the reduction of off-focus radiation.
Off-focus radiation occurs when projectile electrons are
reflected and x-rays are produced from outside the focal spot.
The metal tube envelope can collect
electrons and conduct them away from the anode.
The envelope allows air to be completely evacuated from the x-ray tube, which in turn allow
an efficient flow of electrons from cathode to anode.
The envelope serves two additional functions: i
provides some insulation from electrical shock that may occur because the cathode and anode contain electrical charges and it dissipates heat in the tube by conducting it to the insulating oil surrounding the envelope
The purpose of insulating oil is to
provide more insulation from electrical shock and to help dissipate heat away from the tube.
All of these components are surrounded by metal tube housing on all sides except for
a port, or window, which allows the primary beam to exit the tubE
What does the radiographer observes and handles when moving the X-ray tube
It is the metal tube housing
Leakage radiation refers to
any x-rays, other than the primary beam, that escape the tube housing.
The tube housing is required to allow a leakage radiation of no more than
100 mR/hr to escape when measured at a distance of 1 m from the source
Production of X-rays
As electrons strike the target, their kinetic energy is transferred to the tungsten atoms in the anode to produce x-rays.
Two types of interactions produce x-ray photons
Brems and characteristic
Bremsstrahlung interactions occur when
a projectile electron completely avoids the orbital electrons of a tungsten atom and travels very close to its nucleus.
What range is brems interaction
30-150kVp
100% of the x-ray beam results from bremsstrahlung interactions.
Below 70 kVp (with a tungsten target),
approximately 85% of the beam results from bremsstrahlung interactions.
Above 70 kVp,
Characteristic interactions are produced when
a projectile electron interacts with an electron from the inner shell (K-shell) of a tungsten atom.