Bushong Chapter 6; The X-Ray Tube

• The external structure of the x-ray tube consists of these three parts

  the support structure, the protective housing, and the glass or metal enclosure.
• The internal structures of the x-ray tube are the...

  anode and cathode
• X-ray tubes used in computed tomography (CT) and interventional radiology generally have a much ___ life.

  shorter
• Three methods of supporting an x-ray tube

  Ceiling support. Floor support. C-arm support.
• Which support is the most common?

  ceiling support
• the ceiling support system allows for which type(s) of travel?

  longitudinal and transverse travel of the x-ray tube.
• When the x-ray tube is centered above the examination table at the standard SID, the x-ray tube is in which position?

  preferred detent position
• Some ceiling-supported x-ray tubes have a single control that removes all locks, allowing the tube to ___.

  float
• When the ceiling supported x-ray tube is in the "float"mechanism, how far should it be moved?


  no farther than 1 m (about 3 feet)
• In a floor-to-ceiling support system the x-ray tube...


  slides up and down the column as the column rotates.
• C-arm support system are most commonly used in which diagnostic imaging field?

  interventional radiology
• C-arm support systems are ____ mounted.

  ceiling
• variations of the c-arm support system

  L-arm or U-arm
• When x-rays are produced, they are emitted ___, that is with equal intensity in all directions.

  isotropically
• We use only x-rays emitted through the special section of the x-ray tube called the...

  window
• the X-rays emitted through the window are called the ___________

  useful beam
• Protective housing reduces the intensity of leakage radiation to less than

  1 mGya/ hr
• X-rays that escape through the protective housing are called...

  leakage radiation
• Protective housing guards against excessive radiation exposure and...

  electric shock
• The protective housing also provides ___ ___ for the x-ray tube and protects the tube from damage caused by rough handling.

  mechanical support
• The protective housing around some X-ray tubes contains ___ that serves as both an ___ against electric shock and as a ___ ___ to dissipate heat.

  oil, insulator, thermal cushion
• Some protective housings have a cooling fan to....

  cool the tube or the oil in which the x-ray tube is immersed.
• A bellows-like device allows the oil to expand when heated. If the expansion is too great, a microswitch is activated, so...

  the tube cannot be used until it cools.
• An x-ray tube is an electronic vacuum tube with components contained within a ___ or ___ enclosure.

  glass, metal
• The glass enclosure is made of ___ glass to enable it to withstand the tremendous heat generated.

  Pyrex
• What happens when there is gas in the enclosure of the x-ray tube?

  the electron flow from cathode to anode is reduced, fewer x-rays are produced, and more heat is generated.
• why is a metal enclosure better than a glass one?

  metal enclosure tube maintains a constant electric potential between the electrons of the tube current and the enclosure. they have a longer tube life and are less likely to fail.
• A Dual-filament cathode is designed to provide focal spots of ___ mm and ___ mm

  0.5, 1.5
• The filament is approximately ___ mm in diameter

  2
• The filament emits electrons when it is


  heated
• When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are "boiled off" and ejected from the filament. This phenomenon is known as...

  thermionic emission
• What is the filament made of

  tungsten
• Why do we use tungsten, compared to other metals?

  tungsten has a high boiling point and it isn't likely to vaporize
• Thorium is added to the filament to prolong tube life and enhance efficiency. What percentage of the filament is thorium?

  1-2%
• What is the most common cause of tube failure?

  Tungsten vaporization with deposition on the inside of the glass enclosure
• The filament is embedded in a metal shroud called the

  focusing cup
• the focusing cup is ___ charged

  negatively
• The effectiveness of the focusing cup is determined by its...

  size, shape, charge, as well as the size, shape and position of the filament
• what does the term "grid" refer to?

  an element in the tube that acts as the switch. In a grid-controlled x-ray tube, the focusing cup is the grid and therefore the exposure switch.
• When the filament current is high enough for thermionic emission, a ___ increase in filament current results in a ___increase in x-ray tube current.

  small, large