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The X-Ray Circuit

The X-Ray Circuit. Primary Circuit. 1) Main Switch Location – Between AC source and primary of step-up transformer Purpose – Completes external circuit to x-ray machine. 2) Fuses – Protects machine from overloaded circuit. 3) Line Voltage Compensator

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The X-Ray Circuit

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  1. The X-Ray Circuit

  2. Primary Circuit 1) Main Switch Location – Between AC source and primary of step-up transformer Purpose – Completes external circuit to x-ray machine 2) Fuses – Protects machine from overloaded circuit 3) Line Voltage Compensator Location – Within primary circuit & attached to primary of autotransformer Purpose – Maintains constant voltage in primary circuit 4) Autotransformer Location – Between the AC source and primary of the step-up transformer Purpose – Allows control of kVp by varying voltage to primary of step-up transformer Principle of operation – Self-induction

  3. The X-Ray Circuit LOCATION FOR LINE VOLTAGE COMPENSATOR

  4. Primary Circuit 5) Pre-Reading kilovoltmeter Location – Between autotransformer and primary of step-up trans. Purpose – Indirectly measures kVp selected/adjustment of line v. Principle of operation – Connected to circuit in parallel & works on motor principle 6) Exposure Switch Location – Exposure switch is between autotransformer & primary of step-up transformer Purpose – Manually closes circuit between autotransformer & step-up transformer Connected in “series” Special feature – “Deadman” switch

  5. Primary Circuit 7) Exposure Timer Location – Between autotransformer & primary of step-up trans. Purpose – Terminates exposure at proper time by opening circuit between autotransformer & step-up transformer Types of Exposure Timers: 1) Mechanical Timer 2) Electronic Timer 3) mAs Meter 4) Automatic Exposure Control (AEC) 5) Back-up Timer

  6. Electronic Timer

  7. AEC Exposure Timers

  8. Phototimer Operation

  9. AEC Exposure Timers

  10. Back-up Timer Purpose – Stops exposure in case AEC fails - Prevents overexposure to patient and tube overloads - May be set automatically by machine or manually on some equipment Setting Manual Back-up Time - Divide mAs/mA - Time must be at least 1.5 times expected exposure time or 150% of required mAs value for manual setting - mAs is limited to 600 mAs for exposures over 50 kVp Example: An AEC calls for 200 mA at .5 S exposure, what back-up timer setting should be used? .5 X 1.5 = .75 S back-up time If setting back-up timer using mAs 200 mA X .5 S = 100 mAs X 1.5 = 150 mAs back-up mAs

  11. Primary Circuit 8) Filament Circuit – Supplies heating current to the filament. - Supplies 3 – 5 amps at 6 – 10 volts - This process is controlled by mA button This circuit also consists of: mA Selector Location – Connected in series between the autotransformer and step-down transformer Purpose – Regulates amperage to filament circuit that ultimately controls tube current. - May use rheostat (variable resistance), choke coil (self-inductance) or high frequency circuit or saturable reactor (application of DC to iron core to primary, creating impedance)

  12. mA Selector in Filament Circuit mA selector

  13. 8) Filament Circuit (continued) – Also contains: Filament Stabilizer – Corrects for variation in line voltage Space Charge Compensator – Maintains filament current for different kVp selections. Filament Ammeter – Measures filament current. 9) Primary Windings – Step-up transformer

  14. Secondary Circuit 1) Secondary Coil of Transformer Principle of operation – Mutual induction Step-up transformer – Steps up voltage to tube, drives electrons from cathode to anode Step-down transformer – Steps voltage down and steps up amperage to filament of tube 2) mA Meter – Measures average tube current Principle of operation – Motor principle Location – Connected in series to the secondary of step-up transformer (includes connection to ground to protect operator from being electrocuted) mAs meter is used for very short exposures

  15. 3) Rectifier Purpose – converts AC to DC to prevent reverse bias Location – Between secondary of step-up transformer and x-ray tube 4) Cables to x-ray tube – Conducts high voltage between rectifier and x-ray tube

  16. The X-Ray Cables Shock Hazard Minimized in Three Ways: 1) Insulation 2) Wire sheath that is grounded 3) Secondary of high voltage transformer is grounded at its midpoint to minimize amount of insulation needed

  17. The X-Ray Cables Consist of 3 Conductors: Cathode end of cable – All 3 conductors attach to filament (attach to the 2 filament wires) Other end of wire connects to secondary of transformer and filament circuit Anode end of cable – One wire attaches to anode At the other end of the cable, all 3 conductors in the cable attach to a single conductor that attaches to the secondary of the transformer

  18. The Secondary Circuit

  19. The X-Ray Circuit

  20. The Control Panel Varies by machine, but may include some of the components below.

  21. Three Phase Generator Circuits Consist of 3 single phase currents running 120° out of phase with each other. 3 Ǿ may be rectified to provide with 6 pulses using 6 rectifiers, 6 pulses with 12 rectifiers or 12 pulses with 12 rectifiers (3 Ǿ, 6 p = 13% ripple, 3 Ǿ 12 p = 3% ripple

  22. Three Phase Generator Circuits • To work properly must have 3 primary & secondary windings in • transformer (one for each current) • Must have 3 autotransformers (one for each current) • Primary windings must be in delta configuration • Secondary may be arranged in delta or star (wye) configuration

  23. Advantages and Disadvantages of 3Ǿ Vs 1Ǿ Power Generation • Disadvantages: • 1) Possible power surges – • Current never reaches 0 potential • Circuit cannot be opened or closed at zero potential • 2) Less image contrast • Due to higher effective kVp generated • Advantages: • 1) Higher tube rating with short exposures • More mA can be applied during short exposure time • 2) Nearly constant potential (less ripple) • - 13% for 3 Ǿ, 6 pulse • - 4% for 3 Ǿ, 12 pulse • 3) Higher effective kVp 1Ǿ 3Ǿ • x mAs 2/3 (6 pulse) • 4) Higher mAs x mAs 1/2 (12 pulse) • x kVp - 12%

  24. Conversion Factors When Changing From 1Ǿ to 3Ǿ 1Ǿ 3Ǿ x mAs 2/3 (6 pulse) x mAs 1/2 (12 pulse) x kVp - 12% Example: If 30 mAs is required for a single phase exposure, how much mAs will be required for the same density on the image with a 3 phase, 6 pulse generator? 30 X 2/3 = 20 mAs Example: If 100 kVp were used on an x-ray machine with single phase generation, how much should be used on a three phase machine for the same density? 100 X .12 = 12 100 – 12 = 88 kVp

  25. High Frequency Generation Changes 60 Hz to high frequency current for even less ripple! Operational Steps of the High Frequency Generator 1) 1Ǿ or 3Ǿ AC current is supplied to machine 2) A DC chopper converts the AC wave to a high frequency DC wave that is less subject to line voltage fluctuations 3) An inverter converts the DC waveform to a high frequency AC wave that can be used by the transformer 4) Voltage from the secondary side of the transformer is then changed to DC for application to the tube, rectified and smoothed

  26. Operation of a High Frequency Generator High Freq Inverter DC Chopper

  27. Advantages of High Frequency Generators • Smaller size • Shorter exposure times available • High kVp and mA can be used with short exposure times • Very little ripple (1% ripple) • Less variation in line voltage • kVp can be more easily calibrated and controlled • Real-time monitoring of kV, mA & exposure time • Error detection circuitry

  28. Power Rating of X-ray Generators and Circuits Rated in kW (typically 30 – 80 kW) for x-ray machines 1 Watt = Energy expenditure of 1 joule For DC P = IV P (Watts) = Power I = Current intensity V = Voltage Since high frequency generators produce a nearly constant electrical waveform the same formula for DC can be applied: P = mA X kV

  29. Generator Problems A high frequency generator uses 100 mA at 80 kV for an exposure. How much energy was consumed to produce this exposure? What is the maximum power rating for an x-ray machine when the maximum mA for 100 kV is 300 mA. Find the maximum power rating if the maximum exposure factors for a particular x-ray machine are 800 mA at 70 kVp.

  30. Falling Load Generators A generator that automatically starts the exposure at the highest mA for a selected kVp curve and drops it during the exposure based on maximum heat loading capacity of the tube. 1) A microprocessor automatically drops mA in small steps based on the selected kVp curve. 2) Tube operates at near maximum rating to produce optimal mAs at each point on kVp curve. Two Types of Technique Selection: 1) One-knob selection - R.T. sets kVp (microprocessor sets mAs) 2) Two-knob selection – R.T. sets both kVp & mAs (microprocessor controls exposure time by the mA it selects)

  31. Falling Load Generators Advantages 1) Reduction of exposure time when using high mA 2) Simplifies technique selection by R.T. 3) Takes advantage of maximum tube loading capacity Disadvantages 1) Takes control away from R.T. in choosing technical factors

  32. Mobile X-Ray Units • 1) Battery Powered Mobile Units • Uses nine, 12 Volt DC batteries connected in series – Powers • mobile unit and x-ray tube (recharged by 110 V AC) • Circuit Operation • a. Inverter changes DC from battery to 1kHz AC • for transformer use • b) Step-up transformer increases voltage • c) Rectification system changes AC to DC for tube • operation • d) Microprocessor control of kVp and mAs for improved • accuracy • Uses rotating anode • Nominal focal spot size of .75 m.m. (varies by manufacturer) • Allows selection of kVp & mAs (no exp. time selection)

  33. Mobile X-Ray Units • 2) Capacitor Discharge Unit • Older type of mobile unit • Operates by charging a capacitor immediately prior to exposure to operate x-ray tube (does not drive unit) Operation: a) kVp/mAs values are chosen b) A charger button is pressed immediately prior to exposure to charge the capacitor c) Exposure switch is depressed to start exposure d) Exposure is terminated via a grid-controlled (triode) x-ray tube

  34. Capacitor Discharge Diagram

  35. Grid-Controlled X-ray Tubes • Act as switches to start and stop exposure • Grid is negatively charged focusing cup insulated from filament • 1 – 2 kVp is applied to cup to break mA current in tube Operation • Exposure is started by removing the negative charge from the • grid • Exposure is terminated by restoring the negative charge Advantage – Allows precise control of short exposures.

  36. Grid-controlled X-Ray Tube

  37. Wavetail Cutoff With Capacitor Discharge Units • Process of stopping capacitor discharge at a pre-set point on a discharge curve.

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