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Published byBlaze Short Modified over 8 years ago
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Rad T 290 Generators
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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
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Incoming Power Line affects generator performance –diameter of wire –length or wire –other devices sharing branch circuit Resistance of power line wires can reduce generator voltage during exposure affecting power available to x-ray tube calibration
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Line Voltage Compensation Incoming voltage can vary during day Generators need to correct for changes in line voltage power line fluctuations affect calibration Incoming Power Line Generator Circuit Breaker
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Line Line Voltage Compensation Compensation may be Automatic »Almost everything today Manual »user must make adjustment Line Compensation
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Line Auto- trans- former High Voltage Transformer Rectifier Circuit Timer Circuit + High voltage Transformer has fixed ratio Autotransformer has variable ratio Autotransformer needed to provide variable kilovoltage to tube Autotransformer Filament Transformer mA regulator
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Autotransformer Line Compensation Timer Circuit to high voltage transformer primary to filament transformer primary mA regulator major kV selector minor kV selector Autotransformer does line compensation & kVp selection
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Generator Voltages Input line voltage single or three phase 115 - 480 Volts AC Autotransformer provides variable voltage to primary of high voltage transformer 11 Power Line Auto Transformer High Voltage Transformer Timer Circuit
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High Voltage Circuit Supplies high voltage for x-ray tube Step-up transformer primary from autotransformer secondary to rectifier circuit mA monitored at center grounded point of secondary Auto- transformer Rectifier Circuit mA High Voltage Transformer
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Grounded metal box filled with oil electrical insulator Function increases or decreases alternating voltage Also contains rectifier circuit changes alternating current into direct current
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Fullwave Rectifier Four diodes 120 pulses/second exposure times half of halfwave circuit Secondary of High Voltage Transformer Voltage applied to tube (also mA waveform)
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Fullwave Rectifier + - X X First Half CycleSecond Half Cycle Voltage applied to tube (also mA waveform) X X + -
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Full-Wave Rectification Rectifiers Four diode “bridge” configuration used with single phase both + & - half cycle of high tension transformer used efficient circuit reverses negative half cycle & applies to x-ray tube Applied to X-ray Tube Output of High Tension Transformer Tube
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Pulsed Radiation single phase input power results in pulsed radiation Disadvantages intensity only significant when voltage is near peak low voltage heats target and produces low-energy photons »absorbed in tube, filter, or patient can contribute to dose Applied to X-ray Tube Radiation Waveform
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Three-Phase Generators Commercial power generally delivered as 3 phase phases 120 o apart Single Phase PowerThree Phase Power
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Three-Phase Generators Rectifier circuit Inverts negative voltage sends highest of 3 phases to x-ray tube To X-Ray TubeInput 3 Phase Voltage Rectified
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Three-Phase Generators much higher tube ratings than single phase more efficient than single phase shorter exposures lower exposure Three Phase OutputSingle Phase Power
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3 Generator Circuits pulses number of peaks per 1/60 second (16.6 msec) power line cycle windings 3 primary coils (one for each phase) 3 or 6 secondary »with 6 secondaries, 2 secondary coils induced per primary Three Phase Output
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Three Phase Transforming 3 coils can be hooked up in 2 ways Delta Wye
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3-phase generator Primary windings generally delta Secondary windings may be delta or wye Primary Secondary
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3 Phase Generator 6-Pulse Twelve Rectifier 1 delta primary 2 wye secondaries »6 secondary windings two diodes per winding 13.5% ripple Three Phase Output Ripple Primary Secondary
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3 Phase Generator 12-Pulse Twelve Rectifier 1 delta primary 2 secondaries, 1 wye, 1 secondary »30 o phase difference between secondaries »6 secondary windings 2 diodes per winding 3.5% ripple Three Phase Output Ripple Primary Secondary
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Line Auto- trans- former High Voltage Transformer Rectifier Circuit Timer Circuit + Circuitry for mA selection Adjusts mA on the fly during exposure. mA regulator Filament Transformer mA regulator
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Line Auto- trans- former High Voltage Transformer Rectifier Circuit Timer Circuit + Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical) Filament Transformer mA selector
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mA selection Line Compensation to filament transformer primary mA stabilizer Allows selection from available discrete mA stations. Applies correct voltage to primary of filament transformer. 10 mA 25 mA 50 mA 100 mA 200 mA 300 mA 400 mA
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mA Stabilization During Exposure On first trigger mA regulator supplies anticipated voltage to filament transformer primary mA monitored during exposure Corrections made to filament voltage during exposure as necessary if mA low, filament voltage boosted if mA high, filament voltage lowered
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1 vs. 3 Generators Typical home & small business power inexpensive transformer windings 1 primary coil 1 secondary coil u Industrial power u expensive u transformer windings 3 primary coils one for each phase 6 secondary coils »2 secondary coils induced per primary) 11 33
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1 vs. 3 Generators 100% ripple 8 ms minimum exp. Time 1/120th second lower output intensity puts less heat in tube for same technique u 4-13% ripple higher average kVp slightly less patient exposure u <=1 ms minimum exp. time u higher output intensity u puts more heat in tube 11 33
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Exposure Time Control electronic, measuring »time (crystal) »power line pulses phototimingautomatic (phototiming) terminates exposure based on radiation received by receptor
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Phototiming Geometry entrance type detector in front of film detector must be essentially invisible exit type detector behind film obsolete except for mammography »detector visible because of high contrast image Exit type Sensor Grid Film Entrance type Sensor
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Phototiming Radiation Detectors ionization chambers solid-state detectors
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Ionization Chambers Almost always entrance type Notes thin parallel aluminum plates are electrodes »voltage applied between plates »collect ions produced by radiation in air between electrodes collected ions produce electric current + - + Photon -
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Solid State Detectors PN semiconductor junction generates current when struck by radiation small fast response little beam attenuation Photon Electric Current
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Phototiming Fields 1, 2, or 3 fields may be selected individually or in combination proper positioning critical
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Phototiming Notes must be calibrated for particular film-screen system some generators allow selection from several preset film/screen combinations
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Phototiming Notes phototimer must correct for rate response kVp response of » film/screen system »phototiming sensor Higher kVp beam more penetrating »Less attenuated by phototimer detector safety exposure limited to 600 mAs if phototimer does not terminate exposure (2000 mAs for < 50 kV)
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