Transcript 354 Chapt. 7 X

354 Chapt. 6 X-ray Tube
• TWO Primary components – cathode and
anode
• Tube must be supported: Ceiling/floor
mounted/C-arm, etc.
– SID’s
– Detents (center and SID’s)
– Over ride timer for AEC
Protective tube housing
• X-rays are produced “isotropically” (in all
directions) – tube housing allows only those
directed at the “window” to escape
• Leakage radiation = less than 100 mR/hr at 1
m in any direction other than the window
Filament (thoriated tungsten)
• 3410 F degree melting point
• Tungsten wire – usually 1-2 cm long and 2mm
in diameter
• Dual filament (equals dual target size)
• Thermionic emission
• Electron cloud (space charge affect)
• NEGATIVELY charged focusing cup
• NEGATIVE charged side of x-ray tube
Anode
• Most are rotating discs to give MORE surface
area for heat dissipation
• 3400-10,000 rpm rotation
• Angled to give LARGER actual BUT SMALLER
effective F.S.
• Composite – Tungsten/rhenium
• Support arm assembly assists to rid anode of
heat and is made of molybdnium/rhenium,
tungsten and copper
Brems/Breaking Radiation
Characteristic Radiation
Other anode attributes
• Line-focus principle – actual/effective focal
spot
• Heel effect
• Angles (smaller anode angle = LARGER heel
effect
• More penetrating beam on CATHODE side due
to heel effect
Line-focus principle/heel effect
“HEAT” enemy #1!
• Tube warm up is required if ONE hour or more
has lapsed since the last exposure – CHECK
PRIOR TO PLACING PATIENT IN THE ROOM/ON
THE TABLE!
• PRIMARY reason for tube failure!
Tube rating charts
• Variable kVp/fixed mAs – not too practical as
kVp is driven by atomic mass density or the
patient or contrast agent
• Variable mAs/fixed OPTIMUM kVp = most
practical as atomic mass number is the
“driving force” of kVp and mAs = the amount
of radiation produced to make the OD
Tube rating chart
Cooling Chart
Tube Cooling Processes