Rad Physics Chapter 6: X-Ray Tube

Support structureprotective housingglass (or metal) housing

Overheadfloor to ceilingc-arm

• longitudinal/latitudinal rails and telescoping column for SID changes
• rotate 360° and can angle in a 300° arc
• counterbalanced so it will not drift from side to side
• each movement is controlled by a solenoid (detente)

• rails on ceiling and floor for longitudinal positioning
• uses a telescoping arm for transverse movements
• pivots for additional manoeuvrability (but cannot angle, roll, pitch)
• rotation must be accomplished by rotating the main column

• designed to support the tube and IR which are attached to opposite ends of the c-arm
• used in head units, mobile fluoroscopy, ceiling suspended angiography and surgical units

1. control leakage and off focus radiation
2. isolate the high voltage and provide a means to cool the tube

• ideally the only x-rays that leave the tube leave at the window
• since x-rays are isotropic (emit from all directions) they can escape any area of the housing so the protective is composed of cast steel and is lead lined to absorb most of the unwanted x-ray photons

0.1% of the exposure rate (100mR/hr at 1m)

• tube housing will absorb most of the scatter radiation but some will escape the tube window and produce a ghosting next to the edge of the primary beam
• can be totally eliminated by grounded metal and ceramic envelope but is very expensive and doesn't fit most tub housings

25-30%

Radiation that is produced when scattered photons from the focal spot have enough energy to strike another object in the tube (cathode, vapourized envelope, of off target sites at the anode)

• dielectric oil between the glass envelope and the tube housing insulates the high voltage components
• oil absorbs most of the heat produced during an exposure (one end of the tube has an expandable gasket that permits the oil to expand into it as its heated)
• if the expansion is too great, a microswitch will be activated to prevent use until the tube has cooled

99% of the kinetic energy of electrons is given off as heat (IR radiation)

99% of kinetic energy of e- given off as IR heat, this heat is conducted through the rotor and glass envelope to the oil which absorbs the heat and expands into gasket and convected to the air

Glass (pyrex) tubes were very common because they provided a vacuum needed for x-ray production but eventually would reduce beam quality through vapourization of metal metal tubes are now more common because vapourization does not leave a build up of tungsten on the tube both glass and metal tubes have a window through which the primary beam escapes