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X-RAY

GENERATORS

X-RAY

GENERATORSPRESENTED BY

DR OSAWE AUSTINE

RADIOLOGY DEPT. AKTH

12TH JAN 2011

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OUTLINE

OUTLINE

Introduction Components of x-ray generator

x-ray circuit design Types of x-ray generators

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INTRODUCTION

INTRODUCTION

X-ray generators deliver the power to thex-ray tube necessary to produce x-raysin a defined and predictable manner.

It also provides mechanisms to selecttechniques appropriate for a givenexamination via kv and ma selectors.

It possesses mechanisms to protect thex-ray tube and patient from possibleoverload and over exposure situationsrespectively via exposure control

devices

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Why need x-ray generator ?

Why need x-ray generator ?

Its necessity becomes glaring,considering the very high voltage (20-150kv) requirements of x-ray equipmentsfor proper operation which cannot be

catered for by the 240volts mains supplyfrom our power substations.

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X-RAY GENERATORCOMPONENTS

X-RAY GENERATORCOMPONENTS

Basic components of x-ray

generators include;1. TRANSFORMERS

2. RECTIFIERS

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What are transformers?Why need them?

What are transformers?Why need them?

Transformers perform the task oftransforming an input voltage into an outputvoltage.

They employ the principle of electromagneticinduction (E.M) in their operation.

Their importance lie in the fact that they

serve as means of easy transmission ofelectricity from point of production to site ofutilization and ease of conversion from oneform to another.

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E.M induction principleE.M induction principle

E.M induction is an effect that occurswith changing magnetic fields andalternating electric current.

This principle is very well illustrated bymoving a bar magnet towards astationary loop of wire capable ofelectrical conduction.

Current driven by an electromotiveforce is induced in the wire loopfollowing motion of the bar magnet asa result of changing magnetic field

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As the magnet moves in the oppositedirection, away from the loops of wire, the

induced current flows in the oppositedirection.

The magnitude of the induced voltage is

proportional to the magnetic field strength. Electromagnetic induction is a reciprocal

process in which electric current (flow of

electrons) produces a magnetic fieldwhose magnitude and polarity areproportional to the magnitude anddirection of the current

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Diagram illustrating how changingmagnetic field induces electron flowDiagram illustrating how changing

magnetic field induces electron flow

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BASICTRANSFORMER

BASICTRANSFORMER

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The windings of wire connected toavailable energy source as shown in

diagram above is called the primarywindings while the loop of wire from whichthe modified electrical energy is derived iscalled the secondary windings.

The primary and secondary windings areelectrically (but not magnetically)separated by insulated wire.

When current flows through the primarycoil, it creates a magnetic field within thecore which induces current in the

secondary coil.

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Current only flows through thesecondary windings with changing

magnetic field, hence the need for analternating current to produce thisrequired changing magnetic field with

subsequent continuous current flowthrough the secondary coil.

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Laws of transformerLaws of transformer 2 simple laws govern the behavior of

transformers The voltages in both the primary and

secondary circuits are proportional to thenumber of turns in the 2 coils. i.e.

Np = VpNs Vs

A transformer with more number ofsecondary windings increases the voltage in

the secondary circuit and is appropriatelyreferred to as a step up transformer, whileone with fewer turns in the secondarywindings with respect to the primarydecreases the voltage and is referred to as astep down transformer.

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The second law is simply a restatement ofthe law of conservation of energy. A

transformer cannot create energy. Anincrease in voltage must be accompaniedby a corresponding decrease in current.

VpIp = VsIs

But the expression VI = power. Thus,power in primary turn equals power insecondary turn.

The wire in the transformer must be largeenough to carry the current withoutoverheating. As a result, high voltagetransformers are both large and heavy.

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The AutotransformerThe Autotransformer

The voltage supplied to the X-ray roomconnects to the XRG through anautotransformer in most cases.

Its main function is providing voltage to both

filament and high voltage circuits.

It consists of a single coil of wire wrappedaround an iron core. It has fixed number of

turns, two lines on the input side and has lineson the output side.

The primary and secondary turns are thenumber of coil turns between the taps of the

input and output lines respectively.

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AutotransformerAutotransformer

The autotransformer operates on the principleof self induction, whereas the standardtransformer operates on the principle of mutualinduction.

Keeping in mind that voltage is proportional tothe number of turns, selecting (counting) thenumber of turns enables one to tap theappropriate voltage to different circuits.

e.g. an autotransformer with 115 turns and230V as incoming power supplyselecting 55 turns 110V

selecting 60 turns 120V

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AutotransformerAutotransformer

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RECTIFIERSRECTIFIERS

Important components of XRG Mainly used to achieve rectification, which

is the process of changing alternating intodirect current.

Rectifiers are incorporated into the X-raycircuit in series with the X-ray tube.

High voltage rectifiers can be of thevacuum tube type (often calledthermionic diode tube) or they can be ofsolid state composition.

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RectifiersRectifiers

Solid state rectifiers are preferred becausethey are smaller, more reliable, and have alonger life.

Conversion to direct current is importantbecause if an alternating voltage wereapplied directly to the X-ray tube, electronback-propagation could occur during theportion of the cycle when the cathode ispositive with respect to the anode. Suchelectrons would further heat the filament

and reduce its lifespan.

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(a) Solid State Rectifiers(a) Solid State Rectifiers The heart of a solid state rectifier is a

semiconductor (which is usually a piece ofcrystalline silicon).

Semiconductor is a material whose electricalconductivity is less than that of a metallicconductor but greater than that of an insulator.

The crystal in the diode is doped with traceamounts of impurity, which serves to increase(forward bias) and reduce it to a very low levelwhen the voltage is applied in the oppositepolarity (reverse bias).

The direction of electron flow is opposite that ofcurrent flow in an electric current.

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electrons vs current flowelectrons vs current flow

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(b) Vacuum Tube Diode(b) Vacuum Tube Diode

Diodes (having two electrodes i.e. cathodeand anode) are devices that allow currentflow in only one direction and hence canbe used as rectifiers.

A pertinent example is x-ray tube itself inwhich electrons flow from the heatedfilament (cathode) to the positive anode.

No X-ray production is achieved if thepolarity of the voltage reverses.

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RectificationRectification

Process of changing AC into DC. Mainly divided into half wave and full waverectification.

Half wave rectification: Solid state(semiconductor) diodes or vacuum tubediode used for rectification allow thecurrent to flow only during half cycles.

Thus, only half of the electric wave is usedto produce X-rays. The wave form is calledhalf wave rectification.

When the x-ray tube itself serves as a

rectifier, the circuit is called self rectified.

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RectificationRectification

Self rectification has two disadvantages:

Half of the available electrical cycle is wasted, soexposure times must to twice as long as they wouldbe if the whole cycle were utilized.

Repeated or prolonged exposures heat the anode, itmay become hot enough to emit electrons andproduce current during the inverse half cycle which

will bombard and destroy the filament.

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Rectification.Rectification.

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RectificationRectification

Full Wave Rectification: Modern XRGemploy full-wave rectification, whichutilizes the full potential of the electricalsupply.

Both halves of the alternating voltagesare used to produce xrays, so the x-rayoutput per unit time is twice as large as itis with half rectification.

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XRG CIRCUIT DESIGNSXRG CIRCUIT DESIGNS

(a) Filament circuit

The filament circuit regulates current flow through thefilament of the x-ray tube.

The filament is a coiled tungsten wire that emits

electrons when it is heated by this current flow(thermionic emission).

The power needed to heat this filament to thenecessary high temperature is not much, a current flow

of 3 to 5A with an applied voltage of about 10V aretypical values.

The power to heat this filament is provided by a smallstep-down transformer called the filament transformer.

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Filament circuitFilament circuit

The filament is connected directly to the secondarywinding of this transformer. The primary winding ofthe filament transformer obtains its voltage bytapping off an appropriate number of turns from theautotransformer.

This voltage will be around 100-220V across theprimary winding. To reduce this to the desired 10Vrange, the step-down transformer in the filamentcircuit has approximately 10 to 20 times as many

turns of wire in the primary as in the secondary coil. The x-ray tube, of course, has a very high voltage

across it. This makes it necessary to provide highvoltage insulation b/w the secondary and primary

windings of the filament transformer.

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Filament circuitFilament circuit

Precise control of filament current is criticalbecause a small variation in filament currentresults in a large variation in x-ray tube current.

e.g. a change in filament voltage of about 5%will result in a 20-30% change in x-ray tubecurrent.

Addition of resistors connected in series allowsthe control of the filament current by stabilizingthe voltage.

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HIGH VOLTAGE CIRCUITHIGH VOLTAGE CIRCUIT

The kilovoltage applied across an x-ray tube

determines the maximum energy and hence thepenetrating power of the x-ray photon.

The circuit has 2 transformers, an autotransformer anda step-up transformer.

The autotransformer is actually the Kvp selector and islocated on the control panel.

The voltage across the primary coil of the step uptransformer can be varied by selecting the appropriatenumber of turns in the auto transformer.

The step-up (high voltage) transformer has many moreturns in the in the secondary than primary windingsand increases voltage by a factor of about 600. Thisnecessitates maximum insulation by immersion in oil inthe transformer assembly.

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TYPES of XRGTYPES of XRG

Single phase XRG Three phase XRG

Constant potential XRG

High-frequency inverter XRG

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Single phase XRGSingle phase XRG

A single phase x-ray generator utilizes a singlephase AC supply as input.

These generators employ full wave rectification,

which utilizes the full potential of the electricalsupply. Hence, the x-ray output per unit time is twiceas large as that of HWR.

The system uses a minimum of four rectifiersarranged in a specific orientation to allow the flow ofelectrons from the cathode to the anode of the x-raytube throughout the AC cycle.

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FULL WAVE RECTIFIED CIRCUITFULL WAVE RECTIFIED CIRCUIT

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Single phaseSingle phase Thus, the four rectifiers produce pulsating DC

through the x-ray tube. The voltage across thetube fluctuates from zero to maximum.

Most of the x-ray pulse are generated during thepeak value of the applied voltage. Low energy

x-ray are produced in between the pulses whichhave low penetrating power and raise patientdose.

Hence, there is a need of a constant potentialcircuits, which can give better x-ray output withmore penetration. To achieve this, a morecomplex circuit design are used.

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Three phase XRGThree phase XRG

This use a phase ACline source-threevoltage sources, eachwith a single-phase ACwave form orientedone-third of a cycle(120 degrees) apartfrom the other two (i.e.at 0,120, and 240degrees).

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3 PHASE XRG.3 PHASE XRG.

Three separate high-voltage transformers arewired in a delta configuration. A bridgerectifier on the high voltage side in each circuitproduces two pulse per cycle for each line,

resulting in six pulse per cycle for a three-phase six-pulse generator.

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Delta configurationDelta configuration

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3-phase3-phase

Adding another group of three transformers ina wye configuration with additional bridgerectifiers provide a doubling of the number ofpulses, or 12 pulse per cycle in the three-phase 12-pulse generator.

They deliver a more constant voltage to the x-ray tube and can produce very short exposuretimes.

They are however, more expensive thansingle-phase systems, are more difficult toinstall, have a large amount of bulky hardwareand electronics and occupy significant floor

space.

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Constant potential XRGConstant potential XRG A constant potential generator provides a nearly

constant voltage to the x-ray tube. It uses a 3-phase AC voltage as input, connected to

the high voltage transformer.

Vacuum tubes such as triodes or tetrodes are used to

control the KV and exposure time, on the high voltageside of the transformer.

These vacuum tubes provide extremely fast KVregulation and exposure timing, so that fast outputwaveform is obtained.

This generator also gives higher average x-ray energy,with shortest exposure time (1ms). But thesegenerators are bulky, high cost and involve inefficientpower consumption.

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High Frequency InverterGenerator

High Frequency InverterGenerator

This is the contemporary state of the art choice fordiagnostic x-ray system and has replaced the othersin most demanding radiologic applications e.g.interventional angiography.

Its name describes its function, whereby a highfrequency alternating waveform (up to 50,000Hz) isused for efficient transformation of low to highvoltage. Subsequent rectification and voltage

smoothing produce a nearly constant output voltage

The basic principle involved is that, in a transformer,the voltage induced in the secondary coil isproportional to the rate of change of current in the

primary coil.

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High freqHigh freq

For a given transformer, V & FnA Where f is the frequency

n is the number of turns

A is the area of the core

. If the frequency is increased, then then or A must be decreased in order tomaintain a constant voltage.

. It follows that such generators canexist in a very small size, convenientfor portable units.

Hi h f

Hi h f

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High freqHigh freq The function of this generator has the following

steps:

the incoming power supply is standard 60Hz current

the current is rectified and smoothed

this direct current is then fed to a device called inverter (often called achopper), which converts the smoothed DC into a chopped DC with afrequency of about 6500Hz.

This 6500Hz chopped DC supplies the primary of a step-uptransformer which steps-up the voltage.

The high-voltage 6500Hz output of the transformer is rectified to

produce 13,000 high voltage pulses per second, and then smoothedby filters before being applied to the x-ray tube.

This arrangement ensures constant, nearly ripple-free voltage to the x-ray tube regardless of the inputower.

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Voltage rippleVoltage ripple This is defined as the difference between the

peak voltage and the minimum voltage, dividedby the peak voltage and multiplied by 100

% voltage ripple = Vmax-Vminx 100

Vmax With a single-phase circuit the ripple factor is

100% because the voltage goes from zero to a

maximum value with each cycle.

Ri l

Ri l

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RippleRipple Three phase 6 pulse and

12-phase generators

have a voltage ripple of3% to 25%.

Constant potentialgenerators have an

extremely low ripple(approximately 2%),which is essentiallyconsidered DC voltage.

High frequency

generators have a ripplethat is kVp and mAdependant, typicallysimilar to a three-phasegenerator ranging from

4% to 15%.

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Transformer ratingTransformer rating

The rating of a transformer states the maximumsafe output of its secondary winding. If the ratingis exceeded, the transformer may overheat andburn out its insulation and windings.

The rating is expressed as the maximum safeoutput of its secondary winding in kilowatts.

For 3-phase generators, kilowatt rating is givenby kW = kV * mA.

Rating of a 3-phase generator operating at100kV and 500mA is 50kW.This would then betermed a 50kW generator.

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FUSES & SWITCHESFUSES & SWITCHES

The purpose of a fuse is to safeguard electrical

equipment from the effect of abnormally high current.Excessive current is damaging to any equipment,which is not designed to carry it. When current passesthrough a resistor, heat is produced, which may be

enough to melt the resistor and thus break the circuit.It is on this principle that a fuse operates.

A fuse is simply a metal resistor or wire connected inseries with the equipment. When the current in the

circuit exceeds the rated value of the fuse, thetemperature of the wire become high enough to melt itand fuse burns out and opens the circuit.

Fuse ratings are expressed in amperes and it is

available with wire of different thickness.

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Switches and Circuit BreakersSwitches and Circuit Breakers

Switch is a device, which allows, electriccurrent to flow in a circuit or preventcurrent from flowing.There are many categories of switches:

The main supply switch

The on/off switch for the generator

Selector switches on the control table

The exposure switch

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EXPOSURE TIMERSEXPOSURE TIMERS

They control the length of an x-rayexposure. There are four basic typesnamely;

Mechanical Electronic

Phototimers

Pulse counting timers.

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Mechanical TimersMechanical Timers

Are mainly used in single phase generatorcircuits. In this a timer circuit energizes anelectromagnet, which is connected to thecontactors.

These contactors close the circuit on the lowvoltage side of the transformer.

When the selected time is over, theelectromagnet is deenergized so that thecontactors will turn off the applied voltage tothe x-ray tube.

The accuracy and reproducibility of thesetimers are poor and are rarely used today.

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Electronic TimersElectronic Timers

The length of the x-ray exposure is determinedby the time required to charge a capacitorthrough a resistor.

The principle of operation is that, when the

exposure button starts exposure, it also startscharging a capacitor. The exposure time isterminated when the capacitor is charged to aspecific value necessary to turn on associated

electronic circuit. It is also subject to human errors.

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Pulse counting timersPulse counting timers

They use the technology of voltage pulsecounting to control the time of short exposuretechniques.

In this, high frequencies are generated by the

oscillation in a quartz crystal. The frequency of oscillation is determined by

the size, orientation and mode of oscillation

of the crystal. Then the voltage pulse of highfrequency is counted to measure time.

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Photo Timers (AutomaticExposure Control)

Photo Timers (AutomaticExposure Control)

They terminate exposure when the voltagereceptor )file/screed) has received a preselected x-ray exposure.,

They use radiation detectors that measurethe radiation reaching the receptor.

In the event of photo timer detector or

circuit failure, a back-up timer safetydevice terminates x-ray exposure afterpreset time.

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The EndThe End

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