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

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 # Prim. Coils Prim. Voltage

Transformer Law # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------- # Prim. Coils Prim Voltage NP = 1200 NS = 850 NS VS ----- = ----- NP VP 240 VAC ? V 850 ? ------ = ----- 1200 240 If VPRIM = 240 Volts then VSEC = 170 Volts

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

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

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

Autotransformer Primary 100 Input 50 80 Input 50 100 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 No Yes 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 transformer electronics cabinet kVp adjust mA adjust time adjust transformer high voltage (step up) filament low voltage (step down) electronics cabinet support circuitry or mAs adjust

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

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

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

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA regulator 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 to high voltage transformer primary Line Compensation to high voltage transformer primary Timer Circuit minor kV selector to filament transformer primary mA regulator 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 Auto- transformer Rectifier Circuit mA High Voltage Transformer

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)

Voltage applied to tube Fullwave Rectifier Voltage applied to tube (also mA waveform) First Half Cycle Second Half Cycle + - 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 & applies to x-ray tube 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 Rectified To X-Ray Tube

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 Ripple = 80 - 72 = 8 kVp OR 8 / 80 = .1 = 10% 80 kVp

Constant Potential or High Frequency Output 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

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

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

Phototiming Geometry entrance type exit type detector in front of film Sensor Grid Recptor Entrance type 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

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

High Voltage Transformer Primary switching High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA regulator 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 Rectifier Circuit Timer Circuit Secondary switching + Auto- trans-former Line mA regulator Filament Transformer High voltage switched, not filament Requirement fast smooth

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.

High Voltage Transformer Rectifier Circuit Timer Circuit + Auto- trans-former Line mA selector 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 DC AC First Step

Medium Frequency Generator Operation Pulsating DC smooth to constant voltage Pulsating DC Constant DC Second Step

Medium Frequency Generator Operation smooth DC regulated to voltage level proper for primary Constant DC Exposure-level DC Third Step

Medium Frequency Generator Operation DC converted (chopped) to AC Exposure-level DC Chopped AC Fourth Step

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