Chapter 5. X-ray Production, X-ray Tubes, and Generators

Transcription

1 Chapter 5 X-ray Production, X-ray Tubes, and Generators

2 X-ray Generation

3 Production: Bremsstrahlung Brake Radiation Interaction between accelerated electron and nucleus of target material Radiation energy loss E k ZZ % Collisional energy loss 820, ,000 Bremsstralung radiation from energetic electron interaction Bremsstralung radiation energy distribution

4 Characteristic Radiation Interaction between accelerated electron and bounded electrons of target material Shell Tungsten (74) Molybdenum Rhodium (42) (45) K L 12.1/11.5/ /2.6/ /3.1/3.0 M 2.8~ ~ ~0.2 E K -E L = =59.3KeV Generation of Characteristic X-ray Filtered spectrum of X-ray

5 X-ray Tube Voltage: quality 20~150KeV Current: quantity Fluoroscopy 1~5mA Radiography 100~1000mA Time: quantity 1/100~ 10sec

6 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

7 X-ray Tube Cathode Emission limited

8 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

9 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

10 Anode angle & Focal Spot Size Tilting angle θ: 7~20 Effective focal width=actual focal width Effective focal length=actual focal length sin θ

11 Angle & Field Coverage

12 Focal Spot Size Variation

13 Resolution Measurement Pinhole Image Slit Image Star pattern Bar Pattern

14 Heel Effect Reduction of X-ray intensity toward anode side of the field. Great escaping thickness Position thicker body part to cathode side

15 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

16 Collimators Restrict the X-ray field into minimum area

17 X-ray Generator Components Auto Exposure Control Overload protection: by temperature sensor, by calculating KVP ma time

18 High Voltage Supply Generate High Voltage: 100V 100KV Generate High Voltage: 100V 100KV Reduce X-ray Ripple

19 Three-Phase X-ray Generator Using 3 Phase AC line Using 3-Phase AC line Reduced ripple: 13~25%

20 High Freq. Inverter Generator More efficient, More compact, Less costly to manufacture, minimized ripple Easy Control for KVP & ma Widely using

21 Automatic Exposure Control Exposure time selection Exposure time selection Using ionization sensor

22 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

23 Target Material Efficiency of bremsstralung Higher interaction with higher Z- material Quantity of bremsstralung radiation Determines characteristic ti radiation energy Quality of characteristic radiation

24 KVp Determine maximum energy in bremsstralung Quality of X-ray Increase in quantity of X-ray yphotons Exposure KVp 2

25 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

26 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

27 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

28 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

29 Cooling Chart & Heat Input For continuous usage Estimate cooling time for tube safety Anode X-ray tube housing