X-Ray Generator: Physics and Construction

1. Resident Physics Lectures (Year 1) • Christensen, Chapter 3 X-Ray Generators George David, MS FAAPM, FACR Associate Professor of Radiology

2. Requirements to Produce X-Rays • Filament Voltage • High Voltage anode filament filament voltage source + high voltage source

3. X-Ray Generator • Supplies electrical power to x-ray tube • high voltage between anode & cathode • filament voltage • Controls exposure timing • Turns exposure on and off • Filament heated before exposure • High voltage switched on and off anode filament filament voltage source + high voltage source

4. Voltage • Voltage from US power company • Home • 120 / 240 V • Industrial • 480 V • 240 V • Voltage required • Home • Most stuff: 120 V • AC / Dryer: 240 V • Door bell: 15 V • Computer 5 V • X-Ray • Filament: 8-12 V • High voltage: 40-150 kV (40,000 – 150,000)

5. Transformers • Magical devices that allow voltage to be changed to any desired value

6. Current Flow and Magnetic Fields • Magnetic field surrounds conductor carrying electric current • Magnetic field concentrated by coiling conductor Magnetic Field N S Magnetic Field Current Flow

7. Transformer Construction • Transformers have 2 coils of wire • coils not in electrical contact with each another • When electric current passed through one coil • magnetic field develops around first coil • second coil near enough to feel magnetic field Magnetic Field Current Flow

8. Incoming AC Power Primary Coil Secondary Coil(s) Transformer Coil Designations • primary • coil to which power is applied • secondary • coil which feels magnetic field of primary coil

9. Transformer Coils • When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil • power induced in secondary coil • no physical connection Incoming AC Power Primary Coil Secondary Coil(s)

10. NS = 40 • NP = 20 Turns Ratio Definition TR=NS / NP • number of windings of secondary coil divided by number of windings of primary coil • 40 / 20 for transformer below

11. Transformer Theory • transformers do not work with direct (unchanging) current (DC) • Current induced in secondary coil only when primary coil current / magnetic field is changing Input DC Voltage & Current No Output Voltage Voltage Time Time

12. Transformer Theory • Transformers alter both voltage & current of AC waveforms • Voltage in secondary can be > or < voltage in primary Input AC Voltage & Current Output AC Voltage & Current

13. Transformer Law • Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage NS VS ----- = ----- NP VP

14. Transformer Types • Step down Transformer • # primary coils > # secondary coils • primary voltage > secondary voltage • Step up Transformer • # primary coils < # secondary coils • primary voltage < secondary voltage

15. How Does a Transformer Magically Increase Voltage without Some Source of Power?

16. Transformer Law (cont.) • Current ratio is inverse of voltage ratio # Sec. Coils Sec. Voltage Prim. Current----------------- = --------------- = ----------------- # Prim. Coils Prim. Voltage Sec. Current NS VS IP ----- = ----- = ----- NP VP IS VPRIM X IPRIM = VSEC X ISEC

17. Transformers Power = Voltage X Current • Electrical power not changed • Current exchanged for voltage • Voltage goes up – current goes down • Voltage goes down – current goes up

18. Power • Power is rate of energy usage • Power defined as Voltage X Current • Units • Voltage => Volts • Current => Amps • Power => Watts • Voltage => Kilovolts • Current => milliamps • Power => Watts

19. Power • Power = Voltage X Current • Transformer primary power = secondary power • transformer neither creates nor consumes power PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC

20. Transformer Ratio • Ratio = Output voltage / Input voltage • Most transformers have fixed ratios • X-Ray requires variable ratios • Accommodate selection of different kVs

21. Autotransformer Taps • Only one winding • Incoming AC voltage connected across coils • primary • Output voltage proportional to # coils between taps • secondary Input NS NP Primary Secondary

22. Autotransformer • Voltage law for autotransformers same as for transformers • Variable ratio transformer • Secondary voltage adjustable by moving to a different tap • changes # secondary coils NS Input NS NP NS VS ----- = ----- NP VP Primary

23. Generator Components • control console • kVp adjust • mA adjust • time adjust • transformer • high voltage (step up) • filament • low voltage (step down) • electronics cabinet • support circuitry or mAs adjust

24. + X-ray Circuit High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer

25. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Line Incoming line voltage connected to generator through a circuit breaker. Typ. 220-240 volt AC single phase 240, 480 volt AC three phase

26. Generator Incoming Power Line Circuit Breaker Circuit Breaker • Generator connected to power line through a circuit breaker • Limits current from power line to generator • Allows generator to be disconnected from power line

27. + High Voltage Transformer Rectifier Circuit Variable ratio Timer Circuit Fixedratio Auto- trans-former Line mA regulator Filament Transformer • Auto transformer • Variable ratio transformer allowing operator to specify kVp • High voltage transformer • Boosts output of autotransformer by fixed ratio

28. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Timer • Starts & stops exposure • Turns transformer primary (low voltage) on & off

29. Exposure Timing • Manual • Operator sets time • Automatic (Phototimed) • Equipment measures radiation • Terminates exposure when designated radiation is measured

30. Primary Switching Device • silicon-controlled rectifiers (SCR’s or thyristers) • solid state • turned on by small gate voltage

31. Phototiming • Detector in front of receptor • Must be essentially invisible Grid ImageReceptor Radiation Sensor

32. Phototiming Fields • 1, 2, or 3 • fields may be selected individually or in combination • proper positioning critical

33. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Timer • Develops DC high voltage for x-ray tube

34. High Voltage Transformer • Grounded metal box filled with oil • electrical insulator • Function • Develops proper high voltage for tube • Also contains rectifier circuit

35. Rectification • allows current flow in one direction only • Transformers only work with alternating current (AC) • Rectifier changes alternating current output of high voltage transformer to direct current • DC voltage applied to tube

36. Rectifier • Also called diode • Diode is a two-component device • cathode • anode • Note symbol for diode below Cathode Anode

37. Diodes are One Way Streets for Electric Current • Allows current to flow from cathode to anode • Blocks current from flowing from anode to cathode No Yes C A C A

38. Diodes: A one-way valve for current • Allows current to flow through diode in cathode to anode direction • acts like closed switch • diodes connected like this said to be “forward biased” Load (tube) + Battery -

39. Diodes: A one-way valve for current • Does not allow current to flow through diode in anode to cathode direction • acts like open switch • diodes connected like this said to be “reversed biased” + - X

40. AC DC Rectifiers (Diodes) • converts AC to pulsating DC • Configurations • solid state sticks • many small diodes placed end to end • materials • selenium • silicon (most common today)

41. Three Phase Power Three-Phase Generators • Commercial power generally delivered as three phase • 3 phases • Each phase has same waveform but 120o apart in time Single Phase Power

42. Three Phase Power Rectification Three Phase Output Three Phase Rectification Three Phase Rectification Circuit follows highest of Three Phases

43. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer mA regulator • Circuitry for mA selection • Adjusts mA on the fly during exposure.

44. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Filament Transformer Changes AC voltage to smaller voltage required by filament (8-12 volts typical)

45. Power Storage Generators • Use batteries • Application • Remote locations • Inadequate power from power line or power line not accessible • Outlet inaccessible

46. Battery-Powered Generators • Batteries used for • x-ray • transport • Independent of power line during exposure • Disadvantages • Require charging • Heavy

47. Battery-Powered Generators • Batteries used for • x-ray • transport • Independent of power line during exposure • Disadvantages • Batteries must be charged • Batteries maintenance • Heavy • Battery’s DC converted to AC for high voltage transformer to operate

48. Medium (or high) Frequency Generators • higher frequency square wave voltage sent to primary of high voltage transformer • Conventional generators use power line’s sine wave • very efficient • transformer & generator very small • some transformers integral with x-ray tube head

49. Medium FrequencyGenerator Operation • incoming AC converted to DC DC AC First Step

50. Medium FrequencyGenerator Operation • Pulsating DC smooth to constant voltage Pulsating DC Constant DC Second Step