introduction to x-ray equipment operation

7/27/2019 Introduction to X-Ray Equipment Operation

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APPENDIX E

X-ray equipment operationIntroduction to X-ray equipment operat ionA i mThe aim is t o provide an overall view of current X-rayequipment design and operation. This information isintended to enhance the maintenance and repairssections of this workbook, by providing a detailedexamination of equipment operation requirements. Inaddition, to provide some of the technical knowledgerequired by an electrician, or electronics technician,assisting in repairing the equipment.

ObjectWhen carrying out routine maintenance, and inparticulav, diagnosing incorrect equipment operation,a good knowledge of how equipment operates isrequired.The material in this appendix is intended both as arevision of equipment operation, and to provide spe-cific information of equipment internal operation.Thisincludes operational sequence of events,and th e inte r-nal tests and checks carried o ut by th e equipment.Thisis also an intro du ctio n to X-ray systems fo r an ele ctri-cian or electronics technician, who may be asked toassist in the event of a problem.The first three partshave been provided as the background for thisintroduction.

Contents

PART I THE P R O D U C T I O N O FX-RAYS

Contentsa. The X-ray tubeb. Bremsstrahlung radiationc. Characteristic radiationd. X-ray propertiese. Filtersf. Specification of min imu m filtra tiong. The inverse square law

a.The X-ray tubeThe X-ray tube consists of an anode and cathodeinside an evacuated glass envelope. The ca tho de is afilament, which when made very hot, emits electrons.When a high voltage supply is placed between thecathode and anode, the electrons from the cathodestrilte the anode, releasing X-rays. See Fig E-1. Thereare two main types of X-ray radiation generated:Bremsstrahlung (braking radiation) and characteristicradiation.

A t i i g h voltage s u p p l yI?Part 1. Production of X-rays 205Part 2. The X-ray tube 208Part 3. High voltag e gene ration 213Part 4. The X-ray generator contro l uni t 21 8Part 5. The high-tension cable 23 2Part 6. X-ray coll ima tor 23 3Part 7, X-ray tube suspension 235Part 8. The grid and Po tter Buclg 236Part 9. Tomography 23 9Part 10. The fluoroscopy table 240 Fig E-I. The X-ray tubePart 11.The automatic film processor 243

This is an extract from the WHO book "X-ray equipment maintenance andrepairs workbook for radiographers & radiological technologists" by Ian R

McClelland.

Copyright: World Health Organization 2004. Reproduced with permission


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X-RAY EQUIPMENT MAINTENANCE AN D REPAIRS WORKBOOK206

b. Bremsstrahlung radiotion shell, giving up it s energy as an X-ray pho ton.This hasWhen an electron passes close to the nucleus of ananode atom, it is deflected, and its speed or energyreduced. A t th e same time, an X-ray pho ton is pro-duced, which has an energy level equal to t ha t lost bythe electron. See Fig E-2. Peak X-ray energy, expressedin 'electron-volts' or 'IteV', occurs only when an elec-tron strikes the nucleus, giving up all its energy imme-diately.The electron will continue t o pass thro ugh th eanode atoms, and produce further X-ray photons.However, abo ut 99.5% of th e electron energy is lost ingenerating heat.

. ,.-Fig E-2. Bremsstrahlung radiation

c. Characteristic radiationThis occurs when an incoming electron collides withan electron in the inner'l('shell.To replace the missingelectron, an electron moves from the 'L' shell to the I(

a predominant energy of 591teV. See Fig E-3.There are other transitions, notably from the 'M '

shell to the 'I(' shell (67.2 keW and 'N ' shell to the 'I('shell (69keVI.The above energy levels are specific fortungsten, and are known as 'Characteristic radiation'.

Note. To eject an electron from the K shell, theincoming e lectron requires energy gveater than 70kV,which is the binding energy of th e I( shell electron tothe nucleus of a tungsten atom. Below 70lkV, radiationis entirely due to Bremsstrahlung.At 80 kV, character-istic radiation is about lo%, and a t 150 kV is about28% of the tot al usable X-ray beam.

Fig E-3. Characteristic radiationd. X-ray propertiesX-ray beam quality and quantity depends on threemain factors.-The IkV app lied between anode and cathode-Filtration to remove low energy X-rays.-The amoun t of electron emission fro m th e cathode,which affects quantity only.-The fil m focus distance (FFD). Radiat ion is reduced

by the inverse square law.

Fol l in output i s due

I I I I I I25 50 75 100 125 150

X- r o y p h o t o n energy. (keV)Fig E-4. illustration of relative kV output, for three values of kV


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APP EN DIX E. X-RAY EQUIPMEN T OPERATION207

e. FiltersX-ray photons below -40lteV have litt le pene tratingpower in standard diagnostic X-ray procedures, andonly contribute to unwanted radiation of the patient.To remove these lower energy X-rays, added filte rs areplaced in theX-ray beam.The filter material is normallymade of pure aluminium. For special applicationsfilters made of different materials may be used.Theseare called 'I< Edge' filters. An example of this is anX-ray tube used in mammography, which may have amolybdenum filter.

Where it is desired to make most use of low lteVradia tion, some collima tors have a removable filter.Thishas a safety switch, so that if the filter is removed,X-ray generation is not permitted above a specifiedkV level.

f: Specification of m inimu m filtrationMost countries specify a minimum filtration that willbe used for diagnostic X-ray.The to tal filtra tion is thecombination of the X-ray tube glass, the mirror in acollimatov, plus the added f ilte r in the X-ray beam.Toensure the minimum required filtration is obtained,tables are provided for measurement purposes.

Typical half value layers are provided in tableE-1. The actua l specification may diffe r in somecountries.How to measure the half-value layer

At a specified kV, a radiation meter measures theradiation from the X-ray tube. Added aluminiumfiltra tion is placed in the beam. The amount ofaluminium to reduce the beam by 50% is called thehalf-value layer.Referring to table E-1, a t 1 00 kV, this should requ irea t least 2.7mm of aluminium.I f the specified value of aluminium reduces radia-tion by more than 50%, t ota l filtration is insuffi-cient, so more perm anent alum inium must beplaced in the X-ray beam.

Table E-I. Minimum half value layer, atdifferent kV levelsX-ray tube voltage Minimum permissible firs t(IcV) HALF-VALUE LAYER (mm Al)

g.The inverse square lawThe quantity of X-rays available for a given areadepends on the distance from the X-ray tube. For agiven distance, the X-ray beam may cover an area of10 x 10cm . I f we double the distance the same beamwill now cover an area of 20 x 20cm, in other words,fou r times the previous area. Howevev, the radia tionavailable for each 10 x 10 cm section is now only onequarter its previous value. See Fig E-5.

L e iWhen the distance fromth e focal spat is doubled,th e available radiation inth e same area is onequar ter its prevous value.Fig E-5. Illustration of the inverse square law


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X-RAY EQUIPMENT MAINTE NANC E AN D REPAIRS WORKBOOK208

PART 2 T H E X-RAY TUBE motor. Special ball bearings are required, designed towithstand t he heat from the anode. A stator windingis placed over the anode end of the X-ray tube, t o form

Contents the energising section of the motor. See Fig E-7.a. The stat ionary anode X-ray tubeb. The rotating anode X-ray tubec. The X-ray tube housingd. The X-ray tube focal spote. Anode anglef. Maximu m anode heat inpu tg. Anode rota tion speedsh. E ffect of rotation speed on outputi. Anode heat and cooling timej. The X-ray tu be fila men tk. Filament focusi. Grid controlled X-ray tu be

a. The stationary anode X-ray tubeThis is usually found in portable X-ray generators, or indental units. The anode is a small insert of tungsten,inside a large copper support. The copper is to helpadsorb the he at produced.As a general rule focal spotsare larger than for the rotating anode type,as the heatproduced is in a very small area.

b. The rotating anode X-ray tubeBy rotating the anode, the heat produced is spreadaround a wide area.This allows time for heat to beabsorbed into th e body of th e anode.As a result, m uchsmaller focal spots may be used, together with anincrease in output.

Rotation is achieved by attach ing a copper cylinderto the anode. This forms the 'rotor' of an induction

c. The X-ray tube housingThe housing is lead lined, so that radiation only exitsvia the p ort in fro nt of the focal spot. This po rt isusually a truncated plastic cone, extending from thesurface of the housing close to the X-ray tube glass.This reduces the absorption of X-rays due to the oil.Oil provides the required high voltage insulation, andserves to conduct the heat from the anode and statorwinding to the outside surface. A bellows is providedt o allow the oil to expand as it becomes h0t.A thermalsafety switch is fitted to ensure protection againstexcessive housing heat. I n some cases, t his may be amicro switch, operated when the bellows expandsbeyond its operating lim it. See Fig E-9.

d. The X-ray tube focal spotBy focussing a vertical beam of electrons, onto theanode, which has a specific angle, an effective smallarea o f X-rays results.This is ltnown as the 'focal spot',and the method of generation as the 'iine focusprinciple'.

As indicated in Fig E-10, this effective focal spotbecomes enlarged as the useful beam is projectedtowards the cathode end of t he X-ray tube. While th espot will become smaller towards the anode side, apoint is reached where X-ray generation rapidlybecomes less. This is ltnown as the 'heel effect'. SeeFig E-11.

Gloss bulbi kRotat ing ano de X- ray tube

Fig E-7. Anode and motor for a rotating anode X-ray tube


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APP END IX E. X-RAY E QUIPMENT OPERATION209

T U ~ ~ousing is~ i ii l led. X - ra y t u b e a n d h o u s i n g .T h i s p r o v i d e r h i g h vol tagei n s u io t i o n, a n d c o n d u c t s X-rayh e o i f rom t he X - r a y t u b eo n d staior\ t u b e - /"b"k w i n d i n gBellows. A l l o ws

w h e n h e a t e dT h e r m o s a f e t y

h e o i sensor)A n o d e S u p p o r t

Ca t h o d eRe c e p t a c l e L A " , , ,ReceptacleFig E-9. The X-ray tube and housing

X-ray tube anode

X-ray tube cathode W

d Actual focal spotFilament insidefocus cup n

Ch a n g e o f e f f e c ti v e f o c a l s p o towoy from p e i p e n d i c u l o r t o t h eo n o d e

The wid th 'W' d e p e n d s ont h e f i l a m e n t d i a m e t e r a n dd e s i g n o f t h e focal c u p .

Cathode Anode 1 1Projecied focal spot

Fig E-10. Formation of the focal spot

e. Anode angle 100-+ 80The wider the anode angle, the greater will be the film

coverage at a spe c~ f~cistance. However, to maintain P 600the same focal spot size, the length 'l' f the electron 40beam must be reduced.This results in a smaller area $ 20to dissipate the immediate heat, so the maximum xoutpu t of the tube has to be reduced. See Fig E-10. r r O 20 1 6 12 8 4 4 8 12 1 6 20

A common angle for an over-table tube is 12? An t n o d e C a t h o d e -R a d i a t i o n c e n t r eunder-table tube in a fluoroscopy table may have anangle of 16 g.W ith a 12F: angle, ra d~ at ionmay cover a Fig E-l I . Relative radiation output for twoanode angles35 x 35cm film a t a FFD of 100cm,while a 16O lng le


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X-RAY EQUIPMENT MAINTENANCE AND REPAIRS WORKBOOK2 0

would cover the same film at a distance of 65cm.Fig E-11 indicates the relative radiation out put fo rtwo common anode angles. The rapid fall off to t heanode side is due to heel affect.f i Maximum anode heat inputThe maximum heat input for the X-ray tube anode isdetermined by:

The anode material.Anode rotation speed.Anode diameter.Focal spot size.The kV waveform. (Single-phase, or three-phase)

An X-ray tube anode load capacity is rated as thenumber of kilowatts for an exposure time of 0.1second.This is calculated fro m the rating chart for aspecified mode of operation. For example,In Fig E- lla ,the ~ ro du c t f mA and kV a t 0.1 second is 381tW.

g. Anode rotation speedsThere are two anode rotat ion speeds in use, low speedand high speed.These depend on the power main supplyfrequency. High speed was originally obtained fromstatic frequency-triplers, which generate the thir d ha r-monic of the mainsfrequency. Later high-speed systemsuse solid-sta te inverters,so high speed is now usuallyatthe higher 10800frequency, even wit h a 50Hz supply.

With the simple form of induction motor used torotate the anode, there will always be some slip, so theanode does not reach the full possible speed. Thenominal speed that may be reached is indicated inbracltets in table E-2.

Table E-2. Common anode speeds.Thespeed shown in brackets is the actualobtained speed, versus the theoreticalmaximum speedFrequency Low (Low High (High

speed speed) speed speed)5 0 Hz 300 0 (-2850) 90 00 (-8700)

h. Effect of rotation speed on outputHigh-speed operation is of maximum benefit for sho rtexposure times. (The generator should also sufficientoutput, to take advantage of high-speed anode rota-tion.) I n Fig E-12b two load lines are indicated, onefo r high-speed, and one for low-speed operation.Whilethis example is fo r 1 00K v operation, a similar result isobtained for other load factors.

i. Anode heat and cooling timeA stationary anode X-ray tube can have the coppersection of the anode extended outside the glasscontainev, and into the oil. This allows direct con-duction of anode heat. This is not possible for arotating anode, and heat is dissipated by directradiation from the anode disk. Depending on anodediameter and thickness, this can take a long timetime.

A typical cooling char t is provided in Fig E-13, andthe formulas f or calculation of the heat unit providedin table E-3.

Maximum exposure t ime (Seconds )Fig E-I2a. A typical anode-rating chart


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APPE NDIX E. X-RAY EQUIPMENT OPERATION21 1

1.:j

Fig E-I 2b. High-speed operation allows an increased anode load

30000

X 250mm0 2000*0,

5 1 5 03+z 100x0,rn2 50Q

0 0 1 2 3 4 5 6 7 8 9 10 1 1 12 13 1415T i m e in minutesFig E-13. A typical chart to indicate the rise in anode heat versus the cooling time

Table E-3. Formulas used for anode heat-unit calculationkV waveform Per exposure ContinuousSingle phase,full wave operat ion. H U = k V x m A x s HUIs = IkV x mAThree phase, full wave operation. HU = IkV x mA x s x 1.35 HUIs = kV x mA x 1.35Medium or high frequency inverter. HU = kV x mA x s x 1.35 HUIs = IkV x mA x 1.35

j.The X-ray tube filament the maximum usable emission from the filament.Fig EL14 indicates the non-linear characteristic of th eTo emit electrons, the filament must be brought to a filament.white heat temperature.As the temperature increases, When the kV is increased, electron emission froma point is reached where, despite further increases in the filament to the anode also increases.This is com-temperature, only a small increase in emission results. monly known as the 'Space charge' effect. As anI n this area tun gsten evaporation also increases, example, Fig-14 shows the change of mA th at cangreatly reducing the filame nt life. This determines take place as kV is increased, In this example, with a


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X-RAY EQUIPMENT MAINTENANCE AND REPAIRSWORKBOOK2 2

6t-

LYIX

3.5 4.0 4.5 5 0 5.5Filament current (Amps)Fig E-14. A typical filament emission chart

filament current of 5.OA, at 40kV the emission is160mA, and increases to 3 25mA at 80 kV.To lkeep mAconstant, as kV is changed, the generator controlmust change the filam ent current.This is called 'spacecharge compensation'.

k. Filament focusTo enable a ti gh t beam of electrons to th e anode, thefilam ent is placed inside a 'focus cup'.The focus cupis connected directly to the common centre point ofthe cathode.

Normally the t wo filamen ts are placed side by side,and angled,so t o strike the same anode position.Somedesigns instead have the filaments placed end. Thisallows formation of two separate tracks on the anode.These traclts can have separate angles to suit therequired application.There is, however, a problem withtwo separate tracks, as exact alignment of the colli-ma tor to bot h tracks is not possible.

I.Grid controlled X-ray tubeIn this design, the focus cup is brought out to aseparate connection. By applying a strong negativevoltage between the focus cup and the filament,electron emission is suppressed.

Wit h this change of connection, the ca thode cup isnow referred to as a 'grid'. Grid control allows controlof the X-ray exposure, while high voltage is continu-ously applied between anode and cathode. I n opera-tion, the grid is lkept negative with respect to thefilament, unt il an exposure is required. During an expo-sure, the negative voltage is removed, permittingemission fro m the filament.To terminat e th e exposurethe grid is again made negative in respect to thefilament.

Grid control may be used where rapid precise expo-sures are required, such as i n special procedure rooms.However the most common use of grid control is incapac itor discharge mobiles.

F i n e a n db r o a df i l a m e n t f o rc o m m o no n o d e f o c u st r a c kEnd v iew k F i l a m e n ta l i g n m e n t f o rtw o s e p e ra tef o c u s t r a c k so n the a n o d eFig E-15. Two versions of filament design forthe cathode


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APPE NDIX E. X-RAY EQUIPMENT OPERATION213

PART 3 HIGHVOLTAGEGENERATION

Contentsa. Single-phase, self rectifiedb. Single-phase, full-wave rectifiedc. Three-phase generatorsd. Three-phase 'Six Pulse' generatore. Three-phase 'Twelve Pulse' gen eratorf. The 'Constant pote ntial' generatorg. High-frequency generatorsh. The capacitor discharge (CD) mobile

' Inverse m

The high-tension winding is 'centre tapped', so thatboth anode and cathode have equal voltage appliedabove ground potential.

Single phase self rectified systems are normallyfound in small portable X-ray generators, or may beused in dental units. Efficiency is low, and long expo-sure times will be required.

6. Single-phase, full-wave reaifiedFuii wave rectifica tion results in b oth h alf cycies of t heac voltage used for X-ray production. There is nodanger of back-fire, as no negative voitage is appliedto the anode. Much higher ou tp ut is now avaiiabie. Fuiiwave rectification is used on systems ranging fromportable, dental, mobile, and up to heavy duty fixedinstallations. While self re ctifie d generators may havea maximum output of 10-15mA, full wave rectifiedunits have been produced wit h up to 800 mA output.

mA M e t e r+V o l t a g e b e t w e e n- 1 a n o d e a n dc a t h o d e .

Fig E-17. Single-phase self-rectifiedgenerator

a. Single-phase, self rectifiedThe X-ray tub e can also be considered as a rectifiev, inthat electrons emitted from the cathode filamenttravel t o th e positive anode. I f th e anode is negativein respect to the cathode, no electron flow occurs.

However, in case the anode is very hot, electronemission can also occur fro m th e anode, in which caseelectron flow can exist fro m th e anode to the cathode.This is called 'back-fire', and wou ld damage th e fila-ment.To prevent this, an external diode and resistor isfitte d to the primary of the HT transformer.The effectis to greatly reduce the avaiiabie high voitage on thenegative ha lf cycie.lb is is called 'inverse suppression'.

E f f e c t i v e voltage accrassth e X-ray t ubeFig E-18. Single-phase full-wave generator

The high-tension winding is centre tapped, wi th th ecentre position connected t o ground. This ensuresanode and cathode voltages are equally balancedabove ground potential.

As th e curren t in the tra nsforme r winding is AC, anadditional rectifier is required for the mA meter (nor-mally mounted on the control front panel). Exposuretimes are in multiples of the power main supply fre-quency. For a 50Hz supply, exposure time calculationis simple. See table E-4.

W it h a 6 0Hz suppiy, each pulse is 8.3 millisecondswide. So some generators may indicate exposure time sbelow 0.1 second as a nu mber of pulses, ra ther thana set time.


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X-RAY EQUIPMENT MAIN TENAN CE A N D REPAIRS WORKBOOK214

Table E-4. Indication of exposure time for a single-phase, 50 Hz generator50 Hz supply 1 0 milliseconds 0.01 second 0.05 second 0.2 second

fo r each 'pulse' exposure = 1 ulse. exposure = 5 pulses exposure = 20 pulses

c. Three-phase generatorsBy operating with three-phase power supply, severaladvantages occur:

The peak power demand per phase is reduced, withthe input power equally shared between all threephases.Rather tha n pulsed hig h voltage, the X-ray tub e nowhas continuous voltage supplied, so ra diation fo r agiven ItV and mA is considerably greater.This resultsin shorter exposure times for a given setting, whilethe radiation absorbed by the patient is alsoreduced.Shorter exposure times, down to 0.003 seconds, areavailable. Exposure time calculation for 6OHZ ismore accurate.The X-ray tube has higher anode load capacity forshort exposure times, although for long exposuretimes this will be less.Three phase generators have typical outputs of500mA up to -1200mA.

d. Three-phase 'Six Pulse' generator

I n the example shown below, t he secondary wind-ings are bo th delta configuration.The two isolated setsof windings and rectifier systems allow independentvoltage supply to both anode and cathode. By con-necting the common centre point to ground, bothanode and cathode are equally balanced aboveground.

See Fig E-19.

e. Three-phase 'Twelve Pulse'generatorWith the twelve-pulse generator, one winding is con-figured delta, and th e ot her star. The voltage pealtsbetween these two winding s have a 3 0 degree phase-shift, so that a peak of the rectified output from thedelta winding w il l coincide with a trough fro m t he starrectified outpu t.This result in twelve joined togeth erpulses for each cycle.

The overall ripple -factor is considerably improved, t oa possible 3.5%. This improved ripple factor allowshigher effective radiation output for a set ItV, com-pared to six-pulse generators. With special exposurecon tac tor systems, exposure times down t o 0.001seconds have been achieved. Conventional exposureThis system uses an ide ntical style of winding fo r bo th contactors however, have the same exaosure time

the anode and cathode side. The windings may be limitation s of t he six-pulse systems.configured 'star' or 'delta'. The system obtains its See Fig E-20.name due to the six joined together pulses that aregenerated each cycle. The 'ripple factor' for six-pulseis -13%.

T h r e e phose' S i x P u l s e ' qenerator "S ix Pulse" waveformAnode+ I7-+5 --:ID==Ca t hode 50kVIc a A o o d e and cathodewave f o rm s or e in ohoss

The ideal woveform 1s more o f f en i ike t h i s

Fig E-19. Three-phase, six-pulse generator


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APPENDIX E. X-RAY EQUIPMENT OPERATION 12 5 i

Three-phase Twelve-Pulsegen era tor Twelve-Pulse wav efo rmAnode

Iv-+5Ii- Anode and cathode

~ o v e l o r m s r e phose shifted

The ideol wove form Is more of ten l i k e this

1 (Anode and cathodecomb ined ) \Fig E-20. Three-phase twelve-pulse generator

f The 'Constant aotential' penerator voltage.This in turn is fed into the primaly winding, of-With this generator, there is NO ripple factor, and thevoltage applied t o theX -ray tub e is pure DC.To achievethis, the output of a conventional six-pulse generatoris smoothed by high voltage capacitors. The highvoitage is then passed through a pair of high voltagetet rode valves.These serve to con trol the exposure,andregulate the actua l high voitage supplied t o t he X-raytube.

To achieve good regulation, the high voltageobtained from the generator is set about 5 0kV higherthan actually required. During the exposure, thetetrodes control the voltage at the required level tothe X-ray tube. Constant-voltage generators wereused fo r special proce dure rooms,and CTscanners.Theconstruction and maintenance of these systems isexpensive. They have been largely replaced by high-frequency inverte r systems. However, they are still inuse for providing a very accurate X-ray calibrationstandard.

g. High-frequency generatorsThese are sometimes ltnown as 'medium frequency'generators, depending on the maximum frequency ofthe inverter.

Generally, if maximum frequency is below -20 kHz,the generator is called 'medium frequency'. Currenthigh-frequency generators can operate u p to 1 0 0 kHz,altho ugh most systems will op erate below 50 kHz.

Inside the high-frequency generatov, th e AC mainspower is rectified, and smoothed by a large valuecapacitor, to become a DC voltage supply.The'inverter'converts the DC voltage baclc into a high-frequency AC

the high-tension transformer.High-frequency generators have many advantages

over conventional generators, op erating a t 50 o r 60H zpower ma in frequency.

The high-tension transformer now uses ferriteinstead of an iron core, with an increase inefficiency.The required inductance of the tran sform er windingis reduced, resulting in a big drop of copper resis-tive loss, again improving efficiency.Transformer manufacturing costs are reduced.High-voltage output is tightly regulated, so normalchanges in power main voltages have no affect onthe exposure.The high-voltage waveform is simiiar to between an~d ea i six-pulse t o twelve-pulse generator f or amedium-frequency system.A high-frequency gener-ator waveform has less ripple, in many cases lessthan 2%. However, final rip ple depends on othe rdesiqn considerations.High-voltage production is highly consistent, withlittle variation in residual IcV riuule. (Unlilte threephase systems, this can suffer distortion of the ItVwaveform.)Used in a mobile system, the inverter may operatedirectly from storage batteries, or else from largecapacitors charged via the power point. I n bo ththese cases, ItV waveform remains similar t o largefixed installations.While earlier medium frequency systems had highdeveiopment costs, present high-frequency sys-tems are more cost effective than conventionalgenerators.


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X-RAY EQUIPMENT MAIN TEN ANC E AND REPAIRS WORKBOOK216

3 p h a s e SCR "Br idge" Capacitorsi nver ter prov~de+ + +-- -- --/..-. L

P o w e r 'ON'C o n t a c t o r' ~ o n i o c t o r p e r a t e sa f t e r c a p a c i t o r sare f u l l y c h a r g e d m A W T o k" 9''-Control

Fig E-Zla. Diagram to illustrate the principle of a high-frequency generator

On initial power up, a resistor limits the chargingcurrent of the capacito rs. This is necessaty, asothetwise with the capacitors discharged; it wouldbe equivalent to placing a short circuit on theoutput of th e rectifiers.After the capacitors are charged, another contac-to r shorts out the resistors.The system is now readyfor operation.The energy stored in the capacitors supplies thehigh peak curre nt required by the inverter.The inverter illustrated is an SCR 'bridge' inverter.The out pu t of this inverter is coupled via a resonantcircuit to the primary of t he HT transformeu,The capacitor 'C', and the inductance 'L', togetherwith the inductance of the transformer windingform a series resonant tuned circuit. The resonantcircuit has two functions.-As the pulse rate of the inverter increases towards

resonance, th e energy each pulse produces in th eHT transformer secondary also increases. Thisallows avery wide range of con trol.

-The resonant cir cui t has a 'flywheel affect', soth at on the reverse half cycle,the back EMF att e-mpts to reverse the current in the pair of SCRsthat produced the initial pulse.This causes thatpair to switch off. (The other pair will produce thenext pulse, bu t this tim e in the opposite direction)

The high-tension transformer is operated similar toa single-phase generator, with two exceptions.-For medium-frequency generators, added capac-

itors to provide waveform smoothing. For manyhigh-frequency generators howeveu, th e inherentcapacitance of the HT cables provides therequired smoothing, without added capacitors.

-A bu ilt in resistive voltage divider provides meas-urement of th e h igh voltage during th e exposure.This rneasuvement is compared to a referencevoltage equivalent to that for the required kV.If there is any difference, the inverter controlcircuit changes the pulse rate to correct theerror. This is called 'closed loop' or 'feedback'regulation.

'High F r e q u e n c y ' generator. ' H i g h F r e q u e n c y ' gene ra t o r .~ ~ ~ i c a l w i t h t w o t r a n s f o r m e r s T y p ic a l arrangement w i t h tw o

and 'voltage-doubler' rectification. t r a n s f o r m e r s , and b r i d g e r e c t i f i e r s

-ig E-2lb. Two versions of high voltage generation, used with a high-frequency system


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APPENDIX E. X-RAY EQUIPMENT OPERATION2 7

h. The capacitor discharpe (CD) mobile tube. (This term is used to describe conduction wit hThe capacitor-discharge or'CD'generator obtains highvoltage for a n exposure directly from a p air of capac-itors. These are charged to the required IkV beforemalting an exposure. As th e kV fo r an exposure isapplied to the X-ray tube prior t o an exposure, a 'gridcontrolled' X-ray tube is fitted. A negative voltageapplied between the 'grid', or focus-cup, and the fila-ment. This prevents an exposure until the negativevoltage is removed.

Although there is a slow capacitor charging time,the capacitor can rapidly discharge through the X-raytube,with peal< mA currents up to 50 0mA.A ctual peakmA depends on the X-ray tu be used, not th e capa citorsystem. During an exposure, the charge on the capac-itor drops by 1 V per mAs.Operation

The high voltage capacitors are charged prior topreparation for an exposure.This may take u p to aminute depending on the kV setting required. Aresistor in series wi th th e transformer primary lim itsthe charging current, allowing operation from astandard power point.The CD mobile has two capacitors, connected inseries,with th e common po int connected to ground.This ensures the high-voltage to anode and catho deof the X-ray tube is equally balanced above groundpotential. The capacitors are usually each of twomicrofarads capacity, and as they are connected inseries, make up a total value of one microfarad.The transformer secondary and rectifiers are con-nected to the capacitors to form a'voltage doubler'.On the positive half cycle D l conducts, charging C1.On the negative half cycle, conduction is via D2,charging C2.The charge on C l and C2 add togeth erto produce the total kV available for an exposure.See Fig E-22.The resistors R1 and R2 provide a voltage meas-urement for the charging circuit, and for the kVmeter.At t he s tart of an exposure, the mAs time r operatesa high voltage relay. This removes the negativevoltage applied between grid and cathode. At endof the exposure, the relay stops operating, andthe negative voltage is once more applied to thegrid.Once the capacitors are charged to the required ItV,the charge will slowly drop, partly due to the con-duction of t he ItV measurement resistors,and partlydue to a small 'dark current' current of the X-ray

a cold cathode filament.) As a result, when the IkVdrops a small amount, the charging circuit willagain operate,'topping up' the charge on the capac-itors.Topping up is disabled during an exposure, andrecharging occurs only after the charge button isagain pressed.Due to d ark-curren t, a very small emission of X-rayswill be produced once the capacitors are charged.To prevent external radiation,the collimato r is fit tedwi th a mo to r or solenoid operated lead shutter.Thisshutter bloclts all radiation, and is only opened justprior to a radiographic exposure, or at start ofpreparation for an exposure. Sometimes aftercharging the capacitors, a reset to a lower kV maybe required.This is performed by a low mA expo-sure. During this time, the collimator lead shutterremains closed.The CD mobile on preparation will operate anoderotation and filament boost as for a standardgenerator. The filam ent however does not havepre-heating, as this would increase leakage curren tthrough the X-ray tube during standby.Control by time and mA selection is not practical,as the starting mA depends on kV selected,and fallsduring the exposure as kV drops. For this reasondirect measurement of mA to operate a mAs timeris required.

High vol tage generat ion forCapacitor Discharge (CD) mobile

Grid bias controlh (Controis actualchargeI 1 X-ray exposure )controlI kV rnefer IOn +ve half cycles, Dl conducts,charging C1. O n -ve half cycles, D 2conducts, charging C2. Thiscontinues until capacitors orecharged to the required kV.Fig E-22. The capacitor discharge generator


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X-RAY EQUIPMENT MAINTENANCE AND REPAIRS WORKBOOK2 8

Relation between kV and mAsAs sown in Fig E-23, a 30mA s exposure wil l cause th ekV to reduce by 30 kV.As th e qua ntity of radiatio n fr oman X-ray tu be is controlle d as much by kV as mAs, larg emAs exposures are not practical. For example, if theabove example were fo r 40mAs, the last 10m As of theexposure would be from 60 to 50kV, and have littleeffect.

R e l o t i o n b e t w e e n kV a n d m A sf o r o CD m o b i l eI 30

I Residua l kVI 1 I capacitors.I I9Capacitor X- raycharging Exposure

Fig E-23. Relation between kV and mAs, CDmobile

PART 4 THE X-RAY GENERATORCONTROL UNIT

Contentsa. X-ray control functionsb. High-voltage control and load compensationc. m A con trold. X-ray radiographic timere. Au tom atic exposure con tro l (AEC)f. Fluoroscopy timersg. Exposure con tac torsh. X-ray tube anode rotationi. X-ray tube load calculationj. Operation sequence controlI(. Fault de tec tion an d safety systems

a. X-ray control functionsThe X-ray control provides the followingfunctions for radiography

Radiographic IkV selection.High-voltage load compensation. (For different mAoutputs)Mains-voltage regulation. (May not be required formost high-frequency generators.)X-ray tu be filamen t heating and space-charge com-pensation for each X-ray tube focal spot, and mAstation selection.Selection of required mA output.Selection of X-ray tu be foca l spot. I n some systems,this is automatically linked to the required mAposition.Exposure timer. For single and three phase systems,the tim er must be synchronized to the mains powersupply.Exposure contactor, to connect the HT generator t othe preselected primaly voltage.Anode rotation control (or starter). Some systemsallow for operator selection of low or high speed.X-ray tube safety calculation. Basic requirement isanode load a nd maxim um kV. Calculations may alsoinclude maximum filament heating, stored heatin the anode, and a safety factor .f or m ultipleexposures.Technique selection of external equipment. Eg, tableBuclq, vertical Buclq, tomography, etc.Operation sequence timing and control. Eg, afterthe preparation time delay, X-ray exposure requestis sent t o the Buclq; signal returned from th e B u c bstarts the exposure.


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Safety provision for operator errov, radiation 'ON'warning light etc.System fau lt detection, bo th prior or during an X-ray exposure.

The following additional functions areprovided for fluoroscopy

Fluoroscopy kV selectionAutomatic fluoroscopy-kV control. (May be anoption)Fluoroscopy mA control. Depending on the design,this may be not available for the operator. Insteadthe level of mA may be controlled directly by thefluoroscopy kV selection.Fluoroscopy exposure timer. Depending on systemdesign, this may either stop exposures, or just soundan alarm;after a maximum accumulated time (nor-mally five minutes) has explred.

The control may have these optional featuresAutomatic exposure control, or 'AEC'. Often knownas 'photo timer', and sometimes by th e Siemenstitle of 'Iontomat' or Philips title of 'Amplimat'.TheAEC measures th e qu anti ty of radiation as it entersa cassette.This measurement is used to control theexposure time.Anatomical programmed radiography or 'APR'.APRIS a system of p rese t exposures, depending on th earea of the body to be examined. Current systems,with microprocessor controls, allow a high degree

APPENDIX E. X-RAY EQUIPMENT OPERATION219

of flexibility, and may be treated as a pre-programmed exposure memory system.

b. High-voltage control and load compensationAdjustment of high voltage for conventional systems isby preselection of the primary voltage.This voltage issent to the primary of the high-tension transformer,when an exposure is made.This preselection of p rimaryvoltage must allow fo r voltage drop in th e generatortransformer, as well as the power mains voltage fallingwhen under load. As we change the selection of mA,this also changes the amoun t of voltage drop th at willoccur.To compensate,as we increase the mA selection,so we must also increase the primary voltage to lkeepth e previous IkV selected correct.Fig E-24 illustrates the re lation between kV, mA,and primary-voltage fo r a single-phase 400m A gener-ator. Example: I f 80 kV a t 200 mA were required, thevoltage for this exposure would be preset at 114V.However, if 400mA were required instead, then thevoltage would be increased to 134V.

I n heexample shown in Fig E-25,asimple metho d isshown to achieve load compensation.This method maybe used fo r a portable,or mobile,)(-ray genera tor.A line voltage ad justmen t switch allows compensa-tio n fo r differen t inpu t voltages. The switch isadjusted until the voltmeter is on a calibrationmark. I f he m eter is not set to this marl


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X-RAY EQUIPMENT MA INTEN ANCE A N D REPAIRS WORKBOOK220

kV s e le c ti o n a n d l o a d c o r n p e n s o t i o nf o r a p o r t a b l e X - r a y u n i t .kV selection SC R Exposureswitch^ j - c o n P c t o r----

1020

Line voltagead justment Par t of mA High tensioni r o n s f o r m e r

meter'Fig E-25. High-voltage selection, and load compensation, for a portable X-ray generator

The ItV selection switch is set to the requiredItV.On selection of the required mA position, a sectionof the mA selector switch selects the required loadcompensation voltage.This method may be also used fo r larger fixed instal-lations. More complex compe nsation is the n appliedt o allow fo r m ains supply impedance etc.

Another method is to indicate the preselected primalyvoltage on a voltmeter directly calibrated in ltV.Thesesystems often have two ItV selection switches, one forcoarse settings of about lOltV, and the other for finesettings of 1 V. (Although a large multi-step switchmay be used instead.)

On selection of a different mA station, the meterwill either increase or decrease its indicated kV,depending on the change of mA. By resetting the ItVselection switches so the meter again reads the re-quired ItV, load compensation is achieved.

A similar method to the above is a scale calibratedin kV, and a pointer moved by the kV selection ltnob.On selection of each mA station, a different kV scaleis brought into view on the control panel. Cke mAselection switch also selects the required load com-pensation, as shown in Fig E-25.)

Many X-ray controls,especially three phase versions,have motorized 'servo' controlled selection of kV, andautomatic line voltage compensation.These often usegraphite rollers moving a long a 'step-less' transfo rmerwinding. (Eg, the roller passes along individual turns onthe outside of th e transformer, malting direct co ntactwith each turn as it moves.)Servo systems measure the voltage as the rollerspass along the transformer, and compare the voltageobtained t o a required value.This value is the required

ItV to be generated, plus an addition al voltage fo r loadcompensation. For automatic line voltage regulation,the obtained voltage is simply compared to a fixedreference voltage. I n b oth cases, when th e req uiredvoltage is obtained the servo motor is stopped. Onpreparation or on malting an exposure, these motorsare locked out to prevent movement when the mainssupply voltage drops.

High frequency generators con trol ItV by comp aringdirectly the a ctual ItV across th e X-ray tube wi th a ref-erence voltage set by the required kV. For example, ifth e op erator selected 801tV, the reference voltage maybe 8V. On start of the X-ray exposure, the voltage atfirst on the X-ray tube will be OkV.Vew rapidly, it willapproach 801tV, at which point the measured voltagefrom the generator will match the reference voltageof 8V. However, as the requ ired kV becomes close to801tV, the inverter will reduce its output, so that as801tV is actually reached, inverter ou tpu t is regulatedto maintain 801tV precisely.

High-frequency generators are not affected bymains supply voltage drop during an exposure, due t othe self-regulating closed-loop mode of operation.However, some systems do require automatic mainsvoltage regulation, as well as correct mA calibration,to ensure ItV generated at the start of the exposure iscorrect, and does not 'overshoot'.

Fluoroscopy control by comparison to radiographiccontro l is m uch simplev, as no load comp ensation isrequired. On older systems this may be via a switchselecting l O k V steps, or by a sliding co nta ct on acircular 'step-less' transformer (sometimes called a'variac').

Some generators may employ electronic control ofItV, using the properties of the SCR radiographic con-


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APPE NDIX E. X-RAY EQUIPMENT OPERATION22

tactor. By this method the effective voltage to thetransformer primaty is controlled by changing thetiming pulses to th e SCR contactor. I n effect this is ahigh power version of a lamp dimmer.

With high-frequency systems, control is similar toradiographic output; however in some systems theresonant frequency of the inverter system may beraised, to reduce audible noise.

Automatic fluoroscopic ItV may be obtained byhaving a motor drive or else by direct control ofthe SCR or inverter as previously mentioned. Thecontrol signal may come directly from a TV camera,or else via a photomultiplier sampling the lightdirected to the n/ camera. Automatic fluoroscopicIkV control is used to optimise the light level into thecamera, as well as avoiding excessive radiation to thepatient.

The control of X-ray tube emission, expressed in mil-liamperes or 'mA' requires consideration of severalfactors. I n particular, the level of filamen t heating toobtain t he required emission, and the aff ect of gener-ated ItV on actu al emission.

The following requirements need to be considered.Filament current to obtain the required mA emis-sion level.Modify the filament current as the set kV, beforean exposure, is selected.This is to ensure emissionis constant over the range of available kV, andis called 'space charge' compensation. See FigE-27.Provide a level of 'pre heating's0 the filament willquickly reach the required temperature during radi-ographic preparation.The fila ment may be preset t ohalf the radiographic current in stand-by mode, orin some systems, adjusted t o th e p oint where emis-sion would just occur. (-l.OmA) Additional boostmay be applied for quick heating during prepara-tion . This is called 'flash' boost. Howevev, somesystems only provide pre-heating for a fluoroscopytube, and rely on longer preparation time for theover-table tube. See Fig E-28.The power supply for fil ament heating m ust be wellregulated, so that a drop in power mains voltagedoes not a ffe ct heating level. Earlier systems useda 'Constant voltage' or 'Ferro resonant' transforme rfor this purpose. Later systems use electronic regu-lation, which precisely measures and controls thecurrent through the filament. This is done bymonitoring either the current through the filament

transformer primaty winding, (constant current),or the voltage across the same winding, (constantvoltage).Provide protection so the filament is not over-heated. On earlier generators, no protection wasprovided. Later systems included protecti on in X-raytube overload calculation. Present microprocessorcontrolled systems can have elaborate protectioncircuits.

a The mA control has a safety system to preventan exposure, in case filamen t heating is inco rrect.For example, if the filament has become discon-nected due to a faulty high-tension cable, or incase the filament is brolten. In this case, the rewould be no lo ad on the high-tension transformer,and the generated kV could become dangerouslyhigh.Compensation for drop in mA output during expo-sure may be provided. As electrons are attractedaway from the filament, the filament temperaturefalls a small amount.This effect is more noticed asthe filament reaches the non-linear section of itsoperation. Many systems now provide feedback orclosed-loop compensation for this effect. By sam-pling the mA generated during an exposure, com-paring it to a reference level set for each mAstation, a correction factor is applied to filamentheating. Some microprocessor systems use thistechnique t o automatically re-calibrate th e filamentcontrol, by memorizing the final required value offilament heating.

I l l us tra t ion o f t ime requ i red to heatan X- ray tube f i l ament .I

Time ( in seconds) to reach s tabletempera ture ; ( A f t e r c o m m e n c e m e n t o fp repara t ion fo r exposure . )

Fig 4 6 . Comparison of filament heatingtime. Wi th and without pre-heat


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X-RAY EQUIPMENT MAINTE NANC E AN D REPAIRS WORKBOOK222

Change o f m A , c aus ed by Change o f f i l am en t hea t i ngo c h a n g e o f kV , f i l amen t w i th a c hange o f kV , m A

hea t i ng k ep i c ons tan t. i s now k ep t c ons tan t .

200 a,100ma m A

40 60 80 100 120 140 40 60 80 100 120 140k V k VFig E-27. These two graphs illustrate the need to modify filament heating, as kV is changed

To pre-selected

introduction to x-ray equipment operation

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