RAD 253 Chapter 7 The X-ray Tube Two primary components: cathode and anode

Tube must be supported: ceiling/floor mounted/c-arm, etc SID’s Detents – centers and SID’s (over ride exposure if not accurate)

Protective housing X-rays are produced isotropically (all directions) – protective 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 degree melting point Tungsten wire – usually 1-2 cm long and 2mm in diameter Dual filament (equals dual target size) for better detail Thermionic emission Negatively charged focusing cup

Anode (positive charge target) Most are rotating to give more surface area for heat dissipation ,000rpm rotation Angled to give larger actual BUT smaller effective focal spot Made of Tungsten, rhenium Support arm assists to rid anode of heat and is made of composite molybdnium/rhenium, tungsten, copper

Brems/Breaking Radiation

Characteristic Radiation

Other anode attributes Line-focus principle – actual/effective focal spot Heel effect Angles (smaller anode angel = LARGER heel effect) More penetrating beam on cathode side of beam due to heel effect

Heat – enemy #1!!! Tube warm up is required if one hour has lapsed since the last exposure (check PRIOR to putting patient in the room) Results in primary reason for all tube failures!

Tube rating charts Exposure tube rating charts –Variable kVp/fixed mAs: – not too practical as kVp is driven by atomic mass density of the patient or contrast agent –Variable mAs/fixed kVp: - most practical as atomic mass number is the driving force of kVp and mAs = amount of radiation produced Anode cooling Housing cooling