Chapter 5 X-ray Production, X-ray Tubes, and Generators
X-ray Generation
Production: Bremsstrahlung Brake Radiation Interaction between accelerated electron and nucleus of target material Radiation energy loss E k ZZ 90 74 0.009 1% Collisional energy loss 820,000 820,000 Bremsstralung radiation from energetic electron interaction Bremsstralung radiation energy distribution
Characteristic Radiation Interaction between accelerated electron and bounded electrons of target material Shell Tungsten (74) Molybdenum Rhodium (42) (45) K 69.5 20.0 23.3 L 12.1/11.5/10.2 2.8/2.6/2.5 3.4/3.1/3.0 M 2.8~1.9 0.5~0.4 0.6~0.2 E K -E L =69.5-10.2=59.3KeV Generation of Characteristic X-ray Filtered spectrum of X-ray
X-ray Tube Voltage: quality 20~150KeV Current: quantity Fluoroscopy 1~5mA Radiography 100~1000mA Time: quantity 1/100~ 10sec
X-ray Tube Cathode Generate thermal electron Generate thermal electron Filament Current: 10V, ~7A Tube Current: ~1500mA Determine by filament current & KVP Space charge limited/emission limited Determine Focal spot size Small/ Large Filament Biased Cup Current: 10V, ~7A
X-ray Tube Cathode Emission limited
Anode Metal target with positive potential Tungsten + copper X-ray generation Mostly by heat: more than 90% Tungsten: Z=74 high h melting point, high h atomic number Molybdenum(42), Rhodium(45):Mammography
Rotating Anode To spread out heat Rotor/stator 3000~10,000 rpm Increase spot area 300 times Heat by IR radiation To tube insert and surrounding oil bath
Anode angle & Focal Spot Size Tilting angle θ: 7~20 Effective focal width=actual focal width Effective focal length=actual focal length sin θ
Angle & Field Coverage
Focal Spot Size Variation
Resolution Measurement Pinhole Image Slit Image Star pattern Bar Pattern
Heel Effect Reduction of X-ray intensity toward anode side of the field. Great escaping thickness Position thicker body part to cathode side
Filtration Housing: Block X-ray radiation by lead Except X-ray out port Intrinsic filtration: less than 15KeV By housing metal, Glass, Oil Added filtration: To reduce patient dose Blocking out lower X-ray not contributing imaging Compensation filtration Trough filter: thinner for mediastinum More uniform exposure to detector
Collimators Restrict the X-ray field into minimum area
X-ray Generator Components Auto Exposure Control Overload protection: by temperature sensor, by calculating KVP ma time
High Voltage Supply Generate High Voltage: 100V 100KV Generate High Voltage: 100V 100KV Reduce X-ray Ripple
Three-Phase X-ray Generator Using 3 Phase AC line Using 3-Phase AC line Reduced ripple: 13~25%
High Freq. Inverter Generator More efficient, More compact, Less costly to manufacture, minimized ripple Easy Control for KVP & ma Widely using
Automatic Exposure Control Exposure time selection Exposure time selection Using ionization sensor
Factors affecting X-ray Characteristics i of generated X-ray Quality & Quantity 1. Target Material 2. Voltage 3. Current 4. Exposure time 5. Beam filtration 6. Generator waveform
Target Material Efficiency of bremsstralung Higher interaction with higher Z- material Quantity of bremsstralung radiation Determines characteristic ti radiation energy Quality of characteristic radiation
KVp Determine maximum energy in bremsstralung Quality of X-ray Increase in quantity of X-ray yphotons Exposure KVp 2
ma & exposure time Tube current: ma Current from cathode to anode Exposure ma: quantity control Exposure time Quantity control like tube current Tube current exposure time mas
Filtration & Ripple Beam filtration Reduce photon number (quantity) Shift average energy to higher value (quality) Voltage ripple Lower ripple shift average energy to higher value Affect quality & quantity of X-ray beam
Power Rating & Heat Loading Power rating Power = kvp A max Power = 100kVp 600mA=60kW Power limit on cathode filaments Heat Loading Energy(HU) = kvp A max exposure time Energy(HU) = 80kVp 250mA 0.1sec=2000HU For large HU tube Large focal spot, high anode rotation speed, massive anode, large heat sink, large anode diameter
X-ray Exposure Rating Chart To protect tube damage: Single/multiple exposure Maximum allowable tube current 3000rpm 10000rpm 0.3mm 03 focal spot 1.2mm focal spot
Cooling Chart & Heat Input For continuous usage Estimate cooling time for tube safety Anode X-ray tube housing