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Resident Physics Lectures Christensen, Chapter 3 X-Ray Generators George David Associate Professor of Radiology Transformer Construction Transformers have 2 coils of wire no electrical contact between coils 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 Transformer Coil Designations Incoming AC Power primary coil to which power is applied secondary coil which feels magnetic field of primary coil Primary Coil Secondary Coil(s) Transformer Coils When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power is induced in secondary coil no physical connection Incoming AC Power Primary Coil Secondary Coil(s) Turns Ratio Definition number of windings of secondary coil divided by number of windings of primary coil 850 / 1200 for transformer below NP = 1200 NS = 850 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 Transformer Law Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------# Prim. Coils Prim. Voltage Transformer Law # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------# Prim. Coils Prim Voltage NS VS ----- = ----NP VP 240 VAC ?V NP = 1200 NS = 850 850 ? ------ = ----1200 240 If VPRIM = 240 Volts then VSEC = 170 Volts Transformer Types Step down Transformer # primary coils > # secondary coils primary voltage > secondary voltage Step up Transformer • # primary coils < # secondary coils • primary voltage < secondary voltage Autotransformer Taps Only one winding incoming AC voltage connected across coils primary Output voltage Input proportional to # coils between taps NP NS secondary Primary Secondary Autotransformer Voltage law for autotransformers same as for transformers Secondary voltage adjustable by moving Input to a different tap NP changes # secondary coils NS NS VS ----- = ----NP VP Primary NS Autotransformer Input 100 80 50 20 Primary Rectification Changes alternating current output of high voltage transformer to direct current allows current flow in one direction only x-ray tube is a rectifier because current will not flow from anode to cathode no source of free electrons at anode Diodes are One Way Streets for Electric Current Allows current to flow cathode to anode Blocks current from flowing anode to cathode C A C A 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 High voltage switched on and off Filament heated before exposure Generator Components control console kVp adjust mA adjust or mAs adjust time adjust transformer high voltage (step up) filament low voltage (step down) electronics cabinet support circuitry X-ray Circuit Timer Circuit Autotransformer mA selector Rectifier Circuit + Line High Voltage Transformer Filament Transformer Timer Circuit Rectifier Circuit Autotransformer mA selector + Line High Voltage Transformer 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 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 Incoming Power Line Generator Circuit Breaker Timer Circuit Autotransformer mA regulator Rectifier Circuit + Line High Voltage Transformer Filament Transformer Autotransformer •High voltage Transformer has fixed ratio •Autotransformer has variable ratio •Autotransformer needed to provide variable kilovoltage to tube Autotransformer major kV selector Timer Circuit Line minor kV selector to high voltage transformer primary to filament transformer primary mA regulator Line Compensation Autotransformer does line compensation & kVp selection 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 Autotransformer High Voltage Transformer mA Rectifier Circuit High Voltage Transformer Grounded metal box filled with oil electrical insulator Function increases or decreases alternating voltage Also contains rectifier circuit changes alternating current into direct current Halfwave Rectifier Circuit + First Half Cycle: Diodes closed Voltage applied to tube Tube current (mA) results - - X - + Second Half Cycle: Diodes open No voltage applied to tube No tube current (mA) 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) Fullwave Rectifier Voltage applied to tube (also mA waveform) First Half Cycle + Second Half Cycle - X X X X + 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 & Tube applies to x-ray tube Output of High Tension Transformer Applied to X-ray Tube 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 Three-Phase Generators Commercial power generally delivered as 3 phase phases 120o apart Single Phase Power Three Phase Power Three-Phase Generators Rectifier circuit Inverts negative voltage sends highest of 3 phases to x-ray tube Input 3 Phase Voltage To X-Ray Tube Rectified Three-Phase Generators much higher tube ratings than single phase more efficient than single phase shorter exposures lower exposure Single Phase Power Three Phase Output Ripple variation of kilovoltage from maximum usually expressed as percentage of maximum kV Ripple Ripple Example 80 kVp 72 kVp Ripple = 80 - 72 = 8 kVp OR 8 / 80 = .1 = 10% Ripple Typical Values single phase always 100 % (kV ranges from zero to maximum) three phase 4-13% constant potential 0% Medium / high frequency very low; approx 0. Single Phase Output Three Phase Output Constant Potential or High Frequency Output Timer Circuit Autotransformer mA regulator Rectifier Circuit + Line High Voltage Transformer Filament Transformer Timer •Starts & stops exposure •Turns transformer primary (low voltage) on & off Exposure Timing Manual Operator sets time Automatic (Phototimed) Equipment measures exposure Terminates exposure when designated exposure is measured Phototiming Geometry entrance type detector in front of film detector must be essentially invisible exit type detector behind film obsolete except for film mammography detector visible because of high contrast image Grid Recptor Entrance type Sensor Exit type Sensor Ionization Chambers thin parallel aluminum plates are electrodes voltage applied between plates radiation ionizes air ions collected in air between electrodes collected ions produce electric current Photon + + - Solid State Detectors PN semiconductor junction generates current when struck by radiation small fast response little beam attenuation Photon Electric Current Phototiming Fields 1, 2, or 3 fields may be selected individually or in combination proper positioning critical Primary switching Timer Circuit Rectifier Circuit Autotransformer mA regulator + Line High Voltage Transformer Filament Transformer •Switching (timing) almost always done using low voltage •High voltage switching used only in high end applications •Angio •Cardiac Cath Primary Switch Types solid state silicon-controlled rectifiers (SCR’s or thyristers) turned on by voltage pulse Can only be turned off by interrupting current through it High Voltage Transformer Secondary switching Autotransformer mA regulator + Line Filament Transformer High voltage switched, not filament Requirement fast smooth Rectifier Circuit Timer Circuit Timer Circuit Autotransformer mA regulator Rectifier Circuit + Line High Voltage Transformer Filament Transformer mA regulator •Circuitry for mA selection •Adjusts mA on the fly during exposure. Timer Circuit Rectifier Circuit Autotransformer mA selector + Line High Voltage Transformer Filament Transformer Filament Transformer Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical) Power Storage Generators Application Remote locations Inadequate power from power line Outlet inaccessible Types Battery-powered generators Capacitor discharge generators 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 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 Medium Frequency Generator Operation incoming AC converted to DC AC DC Medium Frequency Generator Operation Pulsating DC smooth to constant voltage Pulsating DC Constant DC Medium Frequency Generator Operation smooth DC regulated to voltage level proper for primary Constant DC Exposure-level DC Medium Frequency Generator Operation DC converted (chopped) to AC Exposure-level DC Chopped AC Medium Frequency Generator Operation AC sent to transformer primary normal transforming & rectification high voltage smoothing Medium Frequency Generators Advantages conducive to computer control immune to power line fluctuations low ripple small size of electronics & transformer small enough to spin on CT gantry Today’s trend in generators