Resident Physics Lectures

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Christensen, Chapter 3

X-Ray Generators

George DavidAssociate 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

Current Flow

Magnetic Field

Transformer Coil Designations

primarycoil to which power is

appliedsecondary

coil which feels magnetic field of primary coil

PrimaryCoil

SecondaryCoil(s)

IncomingAC Power

Transformer Coils

When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power is induced in secondary coilno physical connection

PrimaryCoil

SecondaryCoil(s)

IncomingAC Power

Turns RatioTurns Ratio Definition

number of windings of secondary coil divided by number of windings of primary coil850 / 1200 for transformer below

NP = 1200 NS = 850

Transformer TheoryTransformers alter both voltage &

current of AC waveformsVoltage in secondary can be > or <

voltage in primary

Input AC Voltage &

Current

Output AC Voltage &

Current

Transformer LawVoltage 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

NP = 1200 NS = 850

If VPRIM = 240 Volts then VSEC = 170 Volts

850 ?------ = -----1200 240

NS VS ----- = ----- NP VP

240VAC ? V

Transformer TypesStep down Transformer

# primary coils > # secondary coilsprimary voltage > secondary voltage

Step up Transformer• # primary coils < # secondary coils

• primary voltage < secondary voltage

Autotransformer

Only one windingincoming AC

voltage connected across coilsprimary

Output voltage proportional to # coils between tapssecondary

Primary

Input NS

Secondary

AutotransformerVoltage law for

autotransformers same as for transformers

Secondary voltage adjustable by moving to a different tap changes # secondary coils NS

Primary

Input NS

NS VS ----- = ----- NP VP

Autotransformer

Primary

100Input 50

Primary

100Input 50

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 cathodeno source of free electrons at anode

Diodes are One Way Streets for Electric Current

Allows current to flow cathode to anodeBlocks current from flowing anode to cathode

C A C A

X-Ray Generator

Supplies electrical power to x-ray tube

high voltage between anode & cathodefilament voltage

Controls exposure timingTurns exposure on and offHigh voltage switched on and off

Filament heated before exposure

Generator Componentscontrol console

kVp adjustmA adjusttime adjust

transformerhigh voltage (step up)filament

low voltage (step down)

electronics cabinetsupport circuitry

or mAs adjust

LineAuto-trans-former

High Voltage

Transformer

Filament Transformer

Rectifier Circuit

TimerCircuit

mA selector

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

Incoming line voltage connected to generator through a circuit breaker.circuit breaker.Typ. 220-240 volt AC single phase240, 480 volt AC three phase

mA selector

Circuit BreakerGenerator connected to power line through

a circuit breakerLimits current from power line to

generator Allows generator to be disconnected from

power line

Incoming Power Line

Generator

CircuitBreaker

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

•High voltage Transformer has fixed ratio•Autotransformer has variable ratio•Autotransformer needed to provide variable kilovoltage to tube

Autotransformer

mA regulator

LineCompensation

TimerCircuit

to high voltage transformer

primary

to filament transformer

primarymA

regulator

major kV selector

minor kV selector

Autotransformer does line compensation & kVp selection

High Voltage CircuitSupplies high voltage for x-ray tubeStep-up transformer

primary from autotransformersecondary to rectifier circuitmA monitored at center grounded point of

secondary

Auto-transformer

RectifierCircuitmA

High Voltage Transformer

Grounded metal boxfilled with oil

electrical insulatorFunction

increases or decreases alternating voltage

Also contains rectifier circuit

changes alternating current into direct current

Halfwave Rectifier Circuit

X Second Half Cycle:Diodes openNo voltage applied to tubeNo tube current (mA)

First Half Cycle:Diodes closedVoltage applied to tubeTube current (mA) results

Fullwave RectifierFour diodes120 pulses/secondexposure times half of halfwave circuit

Secondary of High Voltage Transformer

Voltage applied to tube(also mA waveform)

Fullwave Rectifier

First Half Cycle Second Half Cycle

Voltage applied to tube(also mA waveform)

Full-Wave RectificationRectifiers

Four diode “bridge” configuration used with single phase

both + & - half cycle of high tension transformer usedefficientcircuit reverses negative half cycle

& applies to x-ray tube

Applied to X-ray TubeOutput of High Tension Transformer

Pulsed Radiationsingle 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 GeneratorsRectifier circuit

Inverts negative voltagesends highest of 3 phases to x-ray tube

To X-Ray Tube

Input 3 Phase VoltageRectified

Three-Phase Generatorsmuch higher tube ratings than single

phasemore efficient than single phase

shorter exposureslower exposure

Three Phase OutputSingle Phase Power

Ripple

variation of kilovoltage from maximum

usually expressed as percentage of maximum kV

Ripple

Ripple Example

Ripple = 80 - 72 = 8 kVpOR

8 / 80 = .1 = 10%

80 kVp

72 kVp

Ripple Typical Values

single phasealways 100 % (kV ranges

from zero to maximum)three phase

4-13%constant potential

0 %Medium / high frequency

very low; approx 0.

Three Phase Output

Single Phase Output

Constant Potential or High Frequency Output

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

•Starts & stops exposure•Turns transformer primary (low voltage) on & off

mA regulator

Manual

Operator sets time

Automatic (Phototimed)

Equipment measures

exposure

Terminates exposure

when designated exposure

is measured

Phototiming Geometryentrance type

detector in front of filmdetector must be

essentially invisibleexit type

detector behind filmobsolete except for

film mammography detector visible

because of high contrast image

Exit typeSensor

RecptorEntrance typeSensor

Ionization Chambersthin parallel aluminum

plates are electrodesvoltage applied

between platesradiation ionizes airions collected in air

between electrodescollected ions produce

electric current

+Photon

Solid State Detectors

PN semiconductor junction generates current when struck by radiation

smallfast responselittle beam attenuation

Photon Electric Current

Phototiming Fields1, 2, or 3fields may be selected individually or in

combinationproper positioning critical

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

•Switching (timing) almost always done using low voltage•High voltage switching used only in high end applications

•Angio•Cardiac Cath

mA regulator

Primary switching

Primary Switch Typessolid statesilicon-

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

mA regulator

Secondary switching

TimerCircuit

High voltage switched, not filament

Requirementfastsmooth

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

•Circuitry for mA selection•Adjusts mA on the fly during exposure.

mA regulator

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical)

mA selector

Power Storage Generators

ApplicationRemote locationsInadequate power from power lineOutlet inaccessible

TypesBattery-powered generatorsCapacitor discharge generators

Battery-Powered Generators

Batteries used forx-raytransport

Independent of power line during exposure

DisadvantagesBatteries must be chargedBatteries maintenanceHeavy

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 transformerConventional generators use power line’s

sine wave

very efficienttransformer & generator very smallsome transformers integral with x-ray tube

Medium FrequencyGenerator Operation

incoming AC converted to DC

Pulsating DC smooth to constant voltage

Constant DCPulsating DC

smooth DC regulated to voltage level proper for primary

Exposure-level DCConstant DC

DC converted (chopped) to AC

Chopped ACExposure-level DC

AC sent to transformer primary

normal transforming & rectification

high voltage smoothing

Medium Frequency Generators

Advantagesconducive to computer controlimmune to power line fluctuationslow ripplesmall size of electronics & transformer

small enough to spin on CT gantry

Today’s trend in generators

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