The AAPM/RSNA Physics Tutorial...

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Abbreviations: AC alternating current, DC = direct current

Index terms: Physics . Radiography, technology

RadloGraphics 1997; 17:1533-1557

1533

The AAPM/RSNA PhysicsTutorial for Residents

X-ray Generators1This article meets the

criteria for 1. 0 credit

hour in Category 1 of

the AMA Physician’s

Recognition Award.

To obtain credit, see

the questionnaire on

pp 1527-1532.

. Understand the general

operational aspects and

functional components of

the x-ray generator.

. Be able to describe dcc-

tromagnetic induction prin-

ciples, basic electrical corn-

ponents, and operation of

transformers.

. Understand the differ-

ence between filament cur-rent and tube current, the

space charge effect, and its

impact on tube current.

S Be able to describe the

different types of x-ray gen-

erators and the conse-

quences of voltage wave-

form on x-ray quantity and

quality.

j Anthony Seibert, PhD

The x-ray generator delivers the electrical power to energize the x-ray tubeand permits the selection of x-ray energy, x-ray quantity, and exposure time.Major internal components of the generator include transformers, diodes andrectifier circuits, filament and stator circuits, timer switches, and kilovolt andmiffiampere meters. Single-phase, three-phase, high-frequency, and constant

potential generators produce different voltage waveforms (ripple) and x-raybeam spectra. Phototimer and automatic brightness control subsystems mea-

� sure radiation exposure incident on the image receptor to give instantaneous

j � feedback for optimal radiographic film densities and fluoroscopic imagebrightness, respectively. At the generator control console, the operator sets

the tube voltage, tube current, exposure time, phototimer film density, spotfilm acquisition, and fluoroscopic parameters. Selection of generator power

and options depends on the intended clinical use. X-ray tube focal spot sizeand power loading capability should be matched to the x-ray generator and

clinical imaging requirements. Single and multiple exposure rating charts aswell as anode and housing thermal characteristic charts indicate power input

� and dissipation rates specific to a generator and x-ray tube target and housing.

. INTRODUCTIONThe x-ray generator delivers the power to the x-ray tube necessary to produce x rays

in a defmed and predictable manner. The generator also provides mechanisms to se-

lect techniques appropriate for a given examination and to protect the x-ray tube and

patient from possible overload situations.

The objectives of this article are to provide an overview of the various components

that constitute the x-ray generator, to illustrate their functions, and to explain the key

roles of the x-ray generator in the production of x rays. Principles of electromagnetic

induction, generator design, electronic circuitry design, characteristics of the high-

voltage waveform, methods of exposure control, power specifications, tube rating

charts, and anode and housing thermal characteristic charts are explained. Much of

the information contained herein is attributed to several excellent references (1-6).

. Be aware of the issues re-lated to generator power,tube loading limitations,

and rating charts.

‘From the Department of Radiology, School of Medicine, University of California Davis Medical Center. FOLB-2E, 2421

45th Street, Sacramento, California 95817. From the AAPM/RSNA Physics Tutorial at the 1996 RSNA scientific assembly.

Received May 23, 1997; revision requested July 17 and received August 19; accepted August 2 1 . Address reprint re-

quests to the author.

©RSNA, 1997

1534 U Imaging & Therapeutic Technology Volume 17 Number 6

Figure 1.

Diagram shows

the intercon-

nected sub-

systems of a

modem x-ray

generator.

AEC = auto-

matic expo-

sure control,

ABC = auto-

matic bright-

ness control.

(Redrawn.

with permis-

sion, from ref-

erence 6.)

U FUNCTIONAL CHARACTERISTICSOVERVIEW

The x-ray generator is often considered as only

a high-voltage transformer circuit, but this is

only one of its many functions. The x-ray gen-

erator allows the operator to select the x-ray

energy (kilovolt peak [kVpl), x-ray quantity

(the number of x rays in the beam, which is

proportional to the tube current), focal spot

size, and exposure time. Specifically, the kilo-

volt peak applied to the x-ray tube determines

the quality (penetrability) of the resultant x-ray

beam and the subject contrast of the imaged

object. The tube current (expressed in milliam-

peres [mA]) is controlled by the generator fila-

ment circuit and determines the quantity of x

rays (photon flux or number of photons) emit-

ted by the x-ray tube. Selection of the focal

spot size is performed within the filament cir-

cuit. Large and small focal spots are selected

on the basis of the geometry and requirements

of the radiographic examination with respect

to magnification of the object and trade-offs

between tube loading and exposure times.

Length of exposure is selected with the expo-

sure timer, which is used to switch the applied

energy (ie, tube potential) to the x-ray tube on

and off. The product of tube current and expo-

sure time is expressed as milliampere seconds

(mAs). Both manual timing (usually in dura-

tions of milliseconds to seconds) and automatic

timing (after an optimal exposure to the image

receptor has occurred to achieve a proper film

density) are employed.

The x-ray generator also protects the x-ray

tube from potentially damaging overload condi-

tions. Combinations of tube potential, tube cur-

rent, and exposure time that would deliver too

much power to the focal spot are not allowed

in properly configured x-ray generator circuits.

Devices measuring the amount of power de-

posited on the x-ray tube anode in x-ray genera-

tors provide extra protection to the x-ray tube

when the heat loading limits are reached. This

safeguard is particularly important for corn-

puted tomographic scanners and high-powered

interventional angiography systems.

The modern x-ray generator is composed of

many subsystems (Fig 1). Major components

are the high-voltage power circuit, stator cir-

cuit, filament circuit, circuits for automatic ex-

posure control and automatic brightness con-

trol detectors. focal spot selector, and density

control circuits. Each component performs an

important part of the overall function of the

generator. In modern designs, the subsystems

are interfaced by microprocessor control and

closed-loop feedback signals to ensure that the

requested parameters are achieved and that the

exposure has occurred. With the control con-

sole of the generator, the technologist can

manually set the parameters or use prepro-

4

November-December 1997 Seibert U RadioGraphics U 1535

Figure 2. Diagram of a common

mammographic unit illustrates the

external shell of a mammographic

generator and the peripheral compo-

nents that are interfaced to specific

generator subsystems. Selection

switches on the control panel in-

elude focal spot size, tube potential

(kVp), milliampere and exposure

time (mAs), and automatic exposure

control (AEC) and film density ad-

justment (if nonmanual techniques

are used). (Redrawn, with permis-

sion, from reference 6.)

Induced electron Change in currentflow in conductor direction

Figure 3. Diagram illustrates the principles of

electromagnetic induction. On the right, a magnet

moving toward a wire coil creates an electromotive

force, causing electrons to flow in the wire. When

the magnet is moved away from the coil in the op-

posite direction, the magnetic field induces a similar

electromotive force in the wire but in the opposite

direction. causing the electrons to reverse flow.

grammed parameters (which are activated by

a single button) for a given examination. The

service module allows rapid diagnosis of a

problem and quick resolution to minimize

downtime (usually from a remote location).

The x-ray generator in a modern mammograph-

ic system also contains circuitry to control and

automatically adjust parameters, such as tube

potential and special filters, to acquire optimal

mammograms (Fig 2).

. ELECTROMAGNETIC INDUCTION

PRINCIPLES

The major “task” of the x-ray generator is to

provide extremely high voltages to produce x

rays of sufficient energy and quantity. Electrical

voltages available in standard circuits provide

up to approximately 480 V, which is much

lower than the 20,000- 1 50,000 V needed for

x-ray production.

High voltage is obtained with a transformer,

in which a conversion from low into high volt-

age is achieved through a process called elec-

tromagnetic induction. Electromagnetic induc-

tion is an effect that occurs with changing mag-

netic fields and alternating electric current. For

example, a stationary bar magnet is character-

ized by a static magnetic field; if one moves the

magnet toward a wire capable of electrical con-

duction, a current (or motion of electrons) will

be induced in the wire by the “changing” mag-

netic field that resulted from the motion of the

magnet (Fig 3). An “electromotive force” is

causing the electrons to flow. If one reverses

the direction of the movement of the magnet,

the direction of the current will also change

Electric Field

4/Magnetic Field


�aiiCurrent high low tow h�h

Diredion: forward forward reverse reverse

Figure 4. Diagrams illustrate the principles of electromagnetic in-

duction. A high current load in a wire produces a corresponding large

magnetic field strength (left). If the current amplitude is reduced, the

magnetic field strength is similarly reduced (center left). Changing the

direction of the current causes a ifip of the magnetic field polarity

(center right). Increasing the current results in a corresponding in-

crease in the magnetic field (right).

from the opposite electromotive force (Fig 3).

A changing magnetic field thus induces an al-

ternating current (AC) in the nearby wire. The

magnitude of the electromotive force (and thus

the amplitude of the current) is proportional to

the magnetic field strength.

Electromagnetic induction principles also

explain the association of a magnetic field with

a moving charge. For example, an electron

moving in a wire under a potential difference

creates an associated magnetic field. The mag-

nitude (strength) and polarity (direction) of the

magnetic field is proportional to the magnitude

and direction of the current: A large current in-

duces a large magnetic field and vice versa (Fig

4). if the current does not change amplitude or

direction, however, as with constant-potential

direct current (DC), the magnetic field wifi not

change in strength or direction. Because AC

voltage amplitude and polarity change sinusoi-

dally at a fixed frequency of 60 times per sec-

ond (60 Hz) with standard line voltages, the as-

sociated magnetic field strength and direction

change at the same rate (Fig 5). Thus, a chang-

ing magnetic field coexists with an AC in a

wire.

For coiled wire geometry, superimposition

of the magnetic fields from adjacent wires in-

creases the magnitude of the overall magnetic

field produced by a specific input current and

voltage. This higher magnetic field strength af-

fects the electromotive force (output voltage)

acting on other nearby wire coils. Producing

and controlling the magnetic field and the out-

put voltage is the function of the transformer.

Figure 5. Diagram depicts AC as a sinusoidally

varying electric field (voltage) that induces direc-

tional electron flow and the associated magnetic

field that sinusoidally oscillates in tandem at 60

cycles per second (60 Hz). The magnetic field is

perpendicular to the electric field in three-dimen-

sional space.

S ELECTRICAL COMPONENTS AND

CIRCUITRY

X-ray generators consist of several electrical

components, including transformers; autotrans-

formers; diodes, triodes, tetrodes, and pen-todes; rectifier circuits; and filament circuits.

. TransformersTransformers perform the task of “transform-

ing” an input voltage into an output voltage and

function according to the principles of electro-

magnetic induction. The generic transformer is

composed of two distinct, electrically insulated

wires wrapped about a common iron core (Fig

6). Input AC power (voltage and current) is

composed of an electrical field and a magnetic

field that oscillate “in phase” at a rate of 60

cycles per second (Fig 5). One wire wrapping

is called the primary winding, through which

the input power load (ie, primary voltage and

current) is passed. The other wire wrapping is

called the secondary winding, through which

the output load (ie, secondary voltage and cur-

rent) is passed.

secondary windings

Primary Secondary

r\j


Iron core: conduit forchanging magnetic field

Electrically insulated wires

Figure 6. Diagram shows a simple transformer,

which consists of an iron core and electrically insu-

lated coil windings. One set of insulated wires

forms the primary coil winding and is attached to

the input voltage source. The other set of wires

forms the secondary coil winding and is attached to

the output voltage source.

�, ‘Li

‘Er

1111Figure 7. Diagrams demonstrate three types of

simple transformers. The output voltage increases

in a step-up transformer, remains the same in an iso-

lation transformer, or decreases in a step-down

transformer. (Redrawn, with permission, from refer-

ence 6.)

The primary and secondary windings are

electrically (but not magnetically) isolated from

each other. When the AC power load is ap-

plied to the primary winding, electric current

and magnetic field strength change amplitude

and direction. As previously explained, mag-

netic field strength is proportional to the num-

ber of turns on the primary coil. The magnetic

field freely passes through electrical insulation

and permeates the iron core, which serves as a

conduit and containment for the changing mag-

netic field. The secondary winding, which is

wrapped around the same iron core, is simulta-

neously immersed in the magnetic field. An

electromotive force is induced in the second-

ary winding (referred to as secondary voltage),

and its strength is determined by the magnetic

field strength and the number of coil turns

around the iron core on the secondary circuit

(due to constructive superimposition of the

magnetic field lines). Overall, the ratio of the

number of coil turns on the primary winding to

the number of coil turns on the secondary

winding determines the magnitude of the sec-

ondary voltage relative to the primary voltage.

A key point to remember is the need for an AC

load as the input power to produce a changing

magnetic field.

The transformer law quantitatively ex-

presses the relationship of the output voltage

(V5 ) as a product of the input voltage (Vi,)times the secondary-to-primary turns ratio in

Equation (1):

(1)

where N3 = the number of coil turns on the pri-

Step-up mary side of the transformer, N5 = the number

transformer of coil turns on the secondary side of the trans-

former, 1’; = the voltage applied on the primary

coil, and V� = the voltage produced on the sec-

ondary coil. The secondary voltage can easily

Isolation be determined by multiplying the turns ratiotransformer (ie, secondary to primary turns or N/Nt) by

the known input voltage as described in Equa-

tion(1).

An ideal transformer has the same amount

Step-down of power at the output relative to the input. In

transformer typical transformers, power losses caused byheating, electrical resistance, and hysteresis

(ie, a lag of the resulting magnetization in the

iron core due to the changing magnetic force)

occur; however, for the equations given here,

an ideal transformer is assumed. Power is equal

to the product of the voltage and current, and

the power out is equal to the power in, as de-

scribed by Equation (2):

power = l/�I� = V/i. (2)

In this equation, V,� and V� = the primary and

secondary voltages, respectively, and I,, and I� =

the primary and secondary currents, respec-

lively. Thus, an increase in voltage requires a

proportional decrease in current. The trans-

former law and electromagnetic induction ef-

fects delineate transformer function.

Simple transformers are classified into one

of three groups: a step-up transformer, an isola-

tion transformer, and a step-down transformer

(Fig 7). The step-up transformer has more see-

ondary coil turns relative to primary coil turns

I 20 voltsInput

(10 turns)


Figure 8. Diagrams show two

types of autotransformers. Fixed

simple autotransformers (left) con-

veil voltage and current based on

the turns between the input and out-put leads. Switching autotransform-

ers (right) have moving contact

points that allow small voltage

changes to be increased or de-

creased relative to the input voltage.

(Redrawn, with permission, from

reference 6.)

and increases the output voltage relative to the

input voltage. The isolation transformer has the

same number of primary and secondary coil

turns so that the voltage remains the same but

the input and output voltages are electrically

isolated (transformers provide electrical isola-

tion in the other situations as well). The step-

down transformer has fewer secondary coil

turns relative to primary coil turns and de-

creases the output voltage. The type of trans-

former needed thus depends on the magnitude

of input and output voltage. With respect to

power (ie, the product of voltage and current),

the transformer actually causes a slight loss of

power because of imperfect transfer of mag-

netic field through the iron core caused by hys-

teresis effects and imperfect wire conductors.

For this discussion, an ideal transformer is one

in which the input power equals the output

power. Therefore, when the output voltage is

increased in a step-up transformer, a concomi-

tant decrease in the output current occurs.

The relationships between input voltage and

output voltage as well as input current and out-

put current can be quantitatively determined

with the transformer law. In the high-voltage

transformer, which is a step-up transformer, a

fixed turns ratio is typically on the order of ap-

proximately 500 to 1 ,000: 1 . Thus, an increase

of input voltage on the order of approximately

500- 1 ,000 times is achieved. Secondary output

voltage is selected by fine adjustment of the

primary input voltage. Secondary output cur-

rent is reduced by the same factor that the see-

ondary voltage is increased. High current on

the primary side of the transformer (-200 A) is

required to deliver a reasonable current on the

secondary side (-500 mA).

. Autotransformers

Autotransformers convert voltage and current

based on the principles of seif-induction. The

autotransformer is composed of an iron core

wrapped by a single coiled wire attached to

the input power source. Access points to the

wire coil (ie, taps) are connected to the output

wires. An AC waveform applied on the input

wire coil creates a magnetic field that perme-

ates the iron core and induces an electromo-

tive force on the coil itself, creating variable

voltage. A selectable output voltage (usually a

lower voltage) is available at the specific tap lo-

cations. The peak output voltage depends on

the overall number of coil turns in the auto-

transformer between the two input wires rela-

tive to the number of turns between the two

taps that connect the output wires.

A simple autotransformer has a fixed num-

ber of input turns and several output taps that

provide a variable output of lower voltage (Fig

8). A switching autotransformer has several

taps on both the input and output sides, which

allows greater flexibility and the ability to step

the output voltage either up or down relative

to the fixed input voltage in small increments

(Fig 8). The transformer law applies to the ati-

totransformer as well as the standard trans-

former. Primary and secondary turns are deter-

mined by the number of coil turns between the

taps of the input and output lines, respectively.Incremental voltage (or current) adjustments

can thus be performed by the autotransformer.

Autotransformers are often used to adjust

the primary voltage at the generator console in

older single- and three-phase x-ray generator

designs. This “adjusted” low voltage is then in-

put to the high-voltage transformer to achieve

the selected tube potential. Line voltage com-

pensation circuits consist of autotransformers

with mechanical sliders to compensate for any

variation in the input voltage from the power

source.

. Diodes, Triodes, Tetrodes, and

PentodesMajor electrical circuit components in the x-ray

generator include the diode (an electrical de-

vice with two electrodes), triode, tetrode, and

One-way flow of electronsthrough dlod

O Valve diode

� � (e.g., x-ray tube)

Diodes

�

Solid-state diode

P-type semiconductor: � N-type semiconductor:Excess “holes” � Excess electrons

Reverse bias: Forward bias:No electron flow Electron flow

� I +++++� � � i � I ++:� � l___J ‘� +++++

I +++++ � I �

e- � �c�:Z�:::= e�


Figure 9. Circuit diagram symbols for

valve and solid-state diodes are shown. In

each case, flow through the diode fromthe electron source to the electron target.The solid-state diode is represented witha triangle and a connecting straight verti-

cal line with the apex of the triangle,

where electrons flow in the direction

passing the vertical line and then through

the triangle. Although this symbol iscounterintuitive when one views the di-

ode as an “arrow, ‘ current has historically

been described as the flow of positivecharge (ie, opposite the electron flow di-

rection). In this context, the diode actu-ally does point in the direction of current

(flow of positive charge). Because dee-trons flow through the circuit oppositethe direction of the current, the diode (ifviewed as an arrow) points in the block-

ing direction of electron flow. Bottom

diagrams show the design and operation

of a solid-state diode N-P junction. When

a positive charge is placed on the P-type

material and negative charge on the N-

type material, a one-way flow of currentoccurs through the junction. Note the di-rectional flow of electrons through the di-

ode device from a wmbolic viewpoint.

pentode (electrical devices with three, four,

and five electrodes, respectively). A diode con-

tains an electron SOUrCe (cathode) and an elec-

tron target (anode) to permit current to flow in

only one direction along a conductor, despite

any changes in voltage polarity in the electrical

circuit. A triode contains a cathode, anode, and

a third control electrode to adjust or switch

(turn on and off) the current. Tetrodes and

pentodes contain additional control electrodes

but function similar to triodes. In general, these

devices are constructed with vacuum or gas-

filled envelopes or solid-state materials.

The valve diode, which contains an anode

and cathode within a vacuum envelope, per-

mits the flow of electrons only when the polar-

ity of the cathode is negative and the anode

positive. A pertinent example is the x-ray tube,

in which electrons flow in the circuit from the

heated cathode (the source of electrons cre-

ated by thermionic emission) to the positive

anode. When the polarity of the anode and

cathode reverses during the AC cycle, current

ceases because no electrons are available from

the anode to flow to the cathode. Thus, cur-rent is permitted only when the electron

source and target have the correct polarity and

is blocked in the other direction.

Solid-state diodes are composed of semicon-

ductors (materials such as pure silicon that

conduct electricity under certain conditions)

doped with impurities (eg, boron, indium, alu-

minum, gallium) that provide a surplus of free

electrons in N-type materials (a net negative

charge) or a shortage of electrons in P-type ma-

terials (a net positive charge). These N-typeand P-type semiconductors are joined together

in such a way as to permit electrons to pass tin-

inhibited in one direction when the potential

difference is applied negatively and positively

to the N-type and P-type materials, respec-

tively. In this situation, the electron surplus in

the N-type material and the electron shortage

in the P-type material are simultaneously in-

creased, and electrons are conducted across

the junction. If the voltage bias is reversed,

however, the electron surplus in the N-type

material and the electron shortage in the P-type

material are both decreased, which causes a

depletion layer to form at the junction and pro-

hibits electron flow within the semiconductor

materials (similar to the preceding analogy of

the x-ray tube). Design and operation of the

solid-state diode P-N junction under reverse

and forward bias are shown in Figure 9.


Figure 10. Diagrams show a

vacuum-based triode switch (left),its electronic symbol (center), andthe circuit diagram symbol for asolid-state triode device (a thyristor)

(right).

The triode is a diode with the addition of a

third electrode placed close to the cathode (Fig

10). This additional electrode, called a grid, is

situated between the cathode and anode. All

electrons must therefore pass through the grid

structure. Electrons flow freely in the triode

when the grid has no applied voltage; how-

ever, because the grid is much closer to the

cathode than the anode, a small negative volt-

age can exert a tremendous force on the elec-

trons emitted from the cathode and effectively

stop the current. Even with extremely heavy

loads applied on the secondary side of the x-

ray transformer, in a properly confIgured tri-

ode, the current can be rapidly switched sim-

ply by controlling the grid voltage by placing a

nominally small negative voltage on the grid.

A “grid-switched” x-ray tube is a notable ex-

ample of a triode; in this device, the cathode

cup is isolated from the filament structure and

negatively biased to turn the x-ray tube current

on and off with microsecond accuracy. Gas-

filled triodes that contain thermionic cathodes

are known as thyratrons. Solid-state triodes,

known as thyristors, operate with the use of a“gate” electrode at the N-P junction. These

switches control the timing of the x-ray expo-

sure under heavy power loads.

Tetrodes and pentodes are also used in

some x-ray generators. These devices have

multiple grid electrodes, which provide further

control of the current and voltage characteris-

tics. Thermionic diodes, triodes, tetrodes, and

pentodes have almost universally been re-

placed by their solid-state counterparts in mod-

em x-ray generators.

Anode

Grid __,/JCathodeelectrode

. Rectifier Circuits

Rectifier circuits are composed of two or more

diodes arranged to divert the flow of electrons

through a single path of a multipath circuit. A

bridge rectifier is an electrical circuit com-

posed of multiple diodes on the secondary side

of the high-voltage transformer in an x-ray gen-

erator. The bridge rectifier diverts electron

flow through the high-voltage circuit to ensure

correct polarity of the cathode (negative) and

anode (positive) at all times.

For example, during the first (positive) half-

cycle of the AC waveform, one side of thetransformer is maintained at one-haLf the posi-

tive peak voltage, and the other side is main-

tamed at one-half the negative peak voltage

(Fig 1 1). Electrons flow from the negative to

the positive side of the circuit. Through one

side of the bridge rectifier, the electrons pass

through only one diode, as two others actively

block their flow. Thus, the electrons are routed

through the cathode of the x-ray tube to the an-

ode and back into the bridge rectifier circuit.

At this juncture, any one of three diodes in the

rectifier circuit will allow the electrons to pass,but only the diode directly connected to the

positive pole of the transformer will conduct.

In the next half-cycle, the polarity of the trans-

former is reversed. The electrons flow from

the negative pole (opposite side of the trans-

former from the previous half-cycle), through

the bridge rectifier through only one diode, to

the x-ray tube, and back to ground (Fig 1 1). For

this half-cycle (reverse polarity), the other two

diodes pass electrons through the circuit.

Therefore, the applied voltage across the x-ray

tube always has the correct polarity, and a rec-

tified AC waveform with two lobes per cycle is

produced.

:�+

First half-cycle


X-ray tube

Second half-cycle

Figure 11. Diagram illustrates the

flow of electrons through a bridgerectifier circuit during each half-cycle of the applied voltage. Thegroup of diodes is arranged so that

the electron flow is always divertedthrough the cathode to the anode ofthe x-ray tube, independent of thechange in polarity of the high-volt-age transformer. For each half-cycle,two of the four diodes conduct(black diode symbols).

Figure 12. Diagram shows the components of a filament circuit and its relationship to thex-ray tube. A space-charge compensation circuit is present in the filament circuit to correct

for nonlinear attributes of the filament current relative to the x-ray tube current, particularly

when the generator is operated below 40 kVp.

I Filament Circuits

The filament circuit consists of a step-down

transformer connected by output lines to the

focal spot filaments in the cathode of the x-ray

tube (Fig 1 2). When voltage is applied to the x-

ray tube, a current passes through the selected

filament: This is the filament current. Electrical

resistance causes the filament to heat up and

release electrons by thermionic emission. The

number of electrons released is related to the

filament current: The higher the current, the

higher the heat and the greater the number of

electrons released. When a high voltage is ap-

plied to the x-ray tube, all free electrons re-

leased from the filament surface are acceler-

ated toward the anode. This flow of electrons

constitutes the tube current, which is the rate

of electron flow from the cathode to the an-

ode. Thus, tube current is controlled by but not

the same as filament current.

Filament current (1 - 10 A) is much greater

than tube current (1-1,000 mA), usually by a

factor of 10 or more. The resultant tube current

is proportional to the ifiament current except

when the tube is operated at 40 kVp or less.

The filament current is adjusted through a Se-

ries of resistors (or a variable rheostat) to con-

trol the heating and subsequent thermionic

emission of electrons.

- C,)� .�

C)�-.

�C-)>

0)

current

0 10 20 30 40 50 60 70 80 90 100 kVp

Applied tube voltage


Figure 13. Graph depicts the rela-

tionship of the filament current to

the tube current as a function of

tube potential. The three curves il-lustrate the nearly proportional rela-

tionship between filament current

and tube current above 40 kVp,

where emission limited output oc-curs. Below 40 kVp. the tube cur-

rent becomes space charge limited,

resulting in a reduced tube current

for a specific filament current. (Re-

drawn, with permission, from refer-

ence 6.)

Space charge is the accumulation of dcc-

trons around the filament before high voltage is

applied to the x-ray tube. Like-charge repulsion

of the electrons creates a coulombic force that

quickly equals the force of thermionic emis-

sion, creating a steady-state transfer of dcc-

trons to and from the filament surface. Space

charge has an important impact on the relation-

ship between filament current and tube cur-

rent, particularly when an x-ray tube is oper-

ated at tube potentials below 40 kVp (Fig 13).

At tube potentials above 40 kVp, which is

referred to as emission-limited operation, fila-

ment current is proportional to x-ray tube cur-

rent. Here, high, medium, or low filament cur-

rent results in a proportional high, medium, or

low tube current, essentially independent of

the applied tube potential. Filament current of

3 A and higher (�3,000 mA) is typical, whereas

the tube current ranges from about 50 to 800

mA for general diagnostic work performed

with single exposure techniques. Only minor

compensation for variations in filament and

tube current is necessary for variations in x-ray

tube voltage. At tube potentials below 40 kVp,

space charge-limited operation occurs; that is,

the accumulated electron cloud surrounding

the heated filament does not instantaneously

accelerate to the anode in the x-ray tube. In

these cases, because of the nonlinear relation-

ship between filament current and tube cur-

rent, space-charge compensation circuits are

implemented to adjust the filament current so

that an accurately known tube current can be

delivered.

Separate filament circuits (step-down trans-

formers) are directly connected on the second-

ary side of the circuit to the long and short fo-

cal spot filaments in the x-ray tube. When theoperator selects a large focal spot, the step-

down transformer connected to the long fila-

ment is energized; similarly, when a small focal

spot is selected, the short filament is ener-

gized.

. GENERATOR DESIGNSSeveral designs of x-ray generators of varying

complexity and cost are currently in use: the

single-phase generator, the three-phase genera-

tor, the constant potential generator, and thehigh-frequency generator. These generator

types all incorporate the following basic com-

ponents: (a) the high-voltage transformer (a

step-up transformer), which converts the low

voltage to a proportional high voltage; (b) the

filament transformer (a step-down trans-

former), which controls the filament current

and tube current; (c) the autotransformer (used

only in single- and three-phase generators),

which compensates for input line voltage

changes; (ii) rectifier circuits, which reroute

the flow of electrons in the high-voltage circuit

of the generator, ensuring the correct polarity

of the cathode and anode, independent of the

waveform polarity; (e) the kilovolt meter,

which indicates the voltage applied to the x-ray

tube and measures the potential difference on

the primary side of the high-voltage circuit to

minimize the hazards of high voltage; and

(f) the milliampere meter, which measures

the current through the x-ray tube on the high-

voltage side of the transformer.


Line voltagecompensation

220 VSinglePhase

� Input

4-

I

Figure 14. Circuit diagram of a single-phase full-wave-rectified generator illustrates the various major Compo-

nents of the generator. (Redrawn. with permission. from reference 6.)

. Single-Phase GeneratorsA single-phase x-ray generator with a simplified

full-wave-rectified circuit is diagrammed in Fig-

ure 14. Single phase refers to the input power

with a single-phase AC line (compared with a

three-phase input). The circuit is divided into

primary (low-voltage) and secondary (high-volt-

age) halves. On the primary side of the circuit

are the autotransformer, line voltage compen-

sator, kilovolt selector, kilovolt meter, filament

transformer, focal spot selector, and exposure

switch.For a single-phase generator, the minimum

exposure time is typically one half-cycle (1/120

second or �8 msec). This limit results from the

mechanical contact switch that is often used in

the primary circuit of such generators. When

the generator is energized, power within the

generator circuit tends to keep the mechanical

contact switch closed under the force of the

electrical load. At half-cycle, when the applied

voltage passes through zero (and the power in

the electrical circuit is 0), the switch is re-

leased under normal spring-loaded forces.

Thus, exposure times with single-phase genera-

tors are typically limited to multiples of ill 20

second (eg, 1/12 second, 10/120 second, 1/10

second I 1 2/1 20 second], 1/5 second [24/120

second]).

The secondary side of the generator is iso-

lated from the primary side by the high-voltage

and filament transformers. The filament trans-

former typically uses a relatively low voltage

and high current, on the order of 10 V and 5 A.

The closed-loop circuit of the filament trans-

former is unaffected by the high voltage exist-

ing across the x-ray tube because of its dcc-

trical isolation properties. The high-voltage

transformer produces the tube potential in a

precisely known way determined by the fixed

turns ratio on the iron core.

In most conventional general diagnostic x-

ray generators, the output of the high-voltage

transformer is “ center-tapped” to ground. Con-

sequently, the maximum potential difference

created by the high-voltage transformer is di-

vided evenly with respect to ground, where

one-half of the secondary potential difference

is positive and the other half is negative at any

point in time. Thus, for 100,000 V, +50 kV cx-

ists on one pole of the transformer and -50 kV

exists on the other pole. The overall potentialdifference between the poles is +50 kV - (-50kV) = 100 kV. Relative to ground, however, the

Cable capacitance effects


60 HzAC linevoltage input

Transformer

1\f\Low voltage

1W��Pulsating DCOutput voltage

kV

mA

mA is emission limited above 40 kVp

Figure 15. Diagrams illustrate the output voltage and current for a single-

phase, full-wave-rectified generator. The rectified AC output waveform var-

ies from 0 V to the peak voltage for each half-cycle (top). Tube current does

not directly follow applied voltage due to space charge effects below 40

kVp (lower left). Cable capacitance minimizes the wide swings in voltage

from pulse to pulse (lower right). (Redrawn, with permission, from refer-

ence 6.)

peak voltage is only one-half of the peak poten-tial difference, which minimizes the electrical

insulation safety requirements for the high-volt-

age cables. (For mammographic x-ray tubes,

center-tapping usually is not used, but these

tubes operate to a maximum of only about 50

kVp.)

The miffiampere meter directly measuresthe tube current in the secondary circuit. Be-

cause the meter is placed near the electrical

ground of the secondary circuit, electrical

safety is ensured, even though the meter is on

the high-voltage circuit.

The bridge rectifier circuit, which is corn-

posed of four diodes in a single-phase genera-

tor, ensures that electrons in the secondary cir-

cuit always flow from the cathode to the an-

ode, despite the changes in secondary voltage

polarity with time.

“Two-pulse” single-phase generator is thename often attributed to single-phase genera-

tors with a bridge rectifier. Self-rectified or half-

wave-rectified “one-pulse” single-phase genera-

tors do not use a bridge rectifier and produceonly one pulse per cycle. For nonrectified x-ray

tube circuits, electrons may propagate from

the anode to the cathode if the anode reaches

a temperature high enough to release electrons

by thermionic emission. In addition, without

the rectifier circuit in place, only one-half of

each cycle of the applied voltage will produce

x rays.In a single-phase generator, the low-voltage

AC waveform is first transformed into high-volt-

age AC, rectified in the bridge rectifier circuit,

and then applied to the x-ray tube as a pulsat-

ing DC. For a significant fraction of time, the

applied voltage is much less than the peak volt-age (Fig 1 5). During the time of low voltage, ra-

diation output is reduced because of inefficient

x-ray production and space charge-limited op-

eration occurs. In this situation, the tube out-

put varies considerably, increasing and decreas-

ing in phase with the applied pulsating DCwaveform. At tube potentials above 40 kVp,

operation is emission limited, which explains

the flat peak of the milhiampere curve in Figure15. Actual applied voltage across the x-ray tube

electrodes does not vary as much as depicted

because of cable capacitance effects (stored

charge chiefly depends on the length of the

cables). Cable capacitance creates a lag in the

applied potential difference across the x-ray

tube that is related to the delay in discharge of

the cables (Fig 15).

120#{176}phase

�1�

Composite 3 phase

240#{176}phase

Transformer

60 Hz AC line voltage input for each of 3 phases phase map:


00 (360#{176})phase � I00

c;� �o#{176}180\�270#{176}�/’�360#{176}

240#{176}

Bridge

Rectifiers �

Output voltage:

6 pulses per cycle

Figure 16. Diagrams demonstrate the output voltage for a three-phase, full-wave-rectified

generator circuit. Diagram in the upper right illustrates three phases. The availability of linesources with 1 20#{176}different phase (upper left) can be combined to produce a rectified out-

put voltage with small variations in peak to minimum voltage (lower right). (Redrawn, with

permission. from reference 6.)

Voltage ripple describes the percentage dif-

ference between the minimum and maximumvoltages divided by the maximum voltage in

the circuit: % voltage ripple = [(Vm� _

V,uaxl 100. In theory, a single-phase generator

with either one- or two-pulse output has 100%

ripple. For a single-phase one-pulse generator

(which does not produce x rays during one-half of the cycle) to deliver the same output of

radiation intensity as produced by a full-wave-

rectified (two-pulse) generator, a longer expo-

sure time is required. The actual voltage ripple

for a single-phase two-pulse generator is actu-

ally less than 100%, due to cable capacitance

effects. Single-phase generators have the high-

est ripple, which results in suboptimal perfor-

mance of the x-ray tube, leading to slightly

higher patient dose and potentially poorer im-

age quality compared with those produced by

other generators that have less voltage ripple.

. Three-Phase GeneratorsThree-phase generators use the three separate

input lines supplied by the electric company.

These input lines are identical in frequency and

voltage amplitude but differ in the phase of the

AC waveforms. Phase is characterized as the

delay between the sinusoids with respect to a

reference starting point. If a single AC wave-

form starts at the origin (0#{176}),in the first half-

cycle, it wifi reach a maximum peak at 90#{176},

pass through 0 V at 180#{176},reverse polarity,

reach a negative peak at 270#{176},return to the ori-

gin at 360#{176},and repeat. Because the repetition

of the waveform occurs over an angular distri-

bution of 360#{176},two extra waveforms withphase differences of 1 20#{176}(ie, starting at I 20#{176}

and 240#{176})will symmetrically fill the phase map

(Fig 16). Three-phase power supplies consist of

three separate AC lines with relative phase dii-

ferences of 0#{176}, 1 20#{176}, and 360#{176}. When these

lines are simultaneously combined, the overall

voltage variation in the rectified output is sub-

stantially reduced. Separate transformation of

the individual line voltages, rectification of the

high-voltage output, and combination of the

output rectified waveform results in a very low

voltage ripple. Increased efficiency in x-ray

production and higher tube output for a given

tube current is achieved, because the average

applied voltage during the exposure is very

close to the peak voltage.

[Rectifiers I


I�T�

“Delta” wiring confi

Figure 17. Circuit diagram of a three-phase six-pulse generator transformer and rectifier

circuit. Three primary and three secondary- transformers are wired together in a delta con-

figuration. Each transformer has its own line voltage source, and all three share a common

iron core. Current is routed through the bridge rectifier circuit to ensure correct I)OlafltY of

the cathode and anode within the x-ray tube for all three simultaneous line voltages. produc-

ing a total of six pulses per cycle.

A three-phase six-pulse generator is corn-

posed of a high-voltage circuit consisting of

two delta configurations of transformers, oneon the primary side and one on the secondary

side of the circuit (Fig 17). Each delta configu-

ration contains three separate yet intercon-

nected transformers, each operating indepen-

dently with line voltages of different phases. A

bridge rectifier circuit containing six diodes

(two diodes per transformer) is necessary to

ensure the correct polarity and electron flow

through the x-ray tube. The designation “six-

pulse” refers to the number of pulses per cycle

produced from the secondary delta configura-

tion. Output voltage for a six-pulse system var-

ies from the peak voltage on the order of 13%-

25% (compared with 100% variation for a

single-phase generator).

A more sophisticated three-phase 1 2-pulse

generator consists of an input delta configura-tion and two secondary groups of transformers

configured in a delta and a w�’e (Fig 18). In ef-

feet, this configuration represents two six-

pulse generators coupled to the x-ray tube that

are offset from each other by an additional 60#{176}

resulting from the wiring configuration and

phase variation produced by the wye-config-

ured transformers. The number of output

pulses per cycle is doubled, producing 12

pulses per cycle, with an applied voltage s-aria-

tion on the order of 5%- 10% of the peak volt-

age (Fig 19).

In three-phase generator designs, high-pow-

ered mode (or tetrode) circuits on the second-

ary side of the circuit are needed to control the

x-ray exposures. As explained, these switches

can turn the x-ray beam on and off during any

phase of the applied voltage within extremely

short times (millisecond or better accuracy).

Although their voltage ripple is very desirable

(3%-25%) and exposure control is fast and ac-

curate, three-phase generators require a sub-

stantial amount of hardware and electronics,

are more expensive than single-phase systems,

take longer to install, and occupy signifIcantly

more floor space.

. Constant Potential GeneratorsConstant potential generators are so named be-

cause of their ability to produce a high-voltage

waveform with essentially no voltage ripple

(essentially DC or “constant potential”). This is

the preferred waveform, because the electrons

are accelerated at an even potential difference

throughout the exposure. A higher effective

energy beam and greater tube output is ob-

tamed with a DC waveform.

Constant potential generators are even more

complex and bulky than three-phase systems.

A constant potential generator contains two

high-voltage electron tubes (triodes or tet-

AppliedTube

Voltage

resultant 12-pulse rectifiedwaveform across x-ray tube

November-December 1997 Seibert U RadioGrapbics U 1547

I Pdrnaiy

In�’�1 _

/Wye or Star

w,nng configuration

Figure 18. Circuit diagram of a

three-phase 12-pulse generator trans-

former and rectifler circuit. A see-

ond set of three secondary trans-

former windings is configured in a

s�5.e configuration and is connectedto a second bridge rectifier circuit.

The output of the delta and wye see-

ondary transformers is two six-pulse

rectified waveforms.

AnodeVoltage

CathodeVoltage

6-pulse voltage rectifiedwaveforms per electrode;note 60#{176}phase difference

between anode and cathodeFigure 19. l)iagram depicts the ef�

feet of the 12-pulse generator design

on the AC waveform. In the three-

phase 1 2-pulse generator. the two

rectifier circuits are symmetrically

placed on the anode and cathode. A

60#{176}phase shift in the waveforms is

produced by the specific delta and

wye configurations. Because the in-

dividual six-pulse outputs per recti-

fier circuit do not overlap, a total of1 2 pulses per cycle is produced (bot-

tom).

rodes) placed on the cathode and anode sides

of the secondary circuit. These tubes control

both exposure (switching the tube voltage Ofl

and off with about 20-�.tsec accuracy) and the

high voltage in the secondary circuit (with 20-

50-�.tsec adjustment capability). Voltage divid-

ers measure the actual kilovoltage, which is

compared with the reference kilovoltage in a

comparator circuit that controls the grid dcc-

trode (and thus the current and potential differ-

ence) of the triode or tetrode tubes. This

closed-loop feedback ensures extremely rapid

adjustment in kilovoltage and a nearly ripple-

free (DC) waveform. Fast response time of the

frilly electronic adjustment allows repetition

rates of 500 frames per second or more.

However, even though the constant poten-

tial generator produces the superior voltage

waveform and has the highest output power

compared with any other generator type, the

high hardware cost, high operational expense

(eg, calibrations), and immense space require-

ments have diminished its popularity for new

or replacement radiographic systems.

. High-Frequency Inverter Generators

The high-frequency inverter generator is the

current state-of-the-art choice for diagnostic ra-

diographic systems and has been available for

the past 10- 1 5 years, during which time signifi-

cant improvements have been made. Its name

is descriptive of its function: conversion of a

low-frequency, low-voltage input into a high-

frequency, low-voltage waveform that pro-

duces a high-frequency, high-voltage output

waveform.

Input voltage Rectifier Smooth

�1V’�\ /W\

IIOV 60Hz

Transformer

lnverter

“IA”

Rectifier

AC 500 - 40000 Hz

X-ray tubeSmooth


Figure 20. Diagram illustrates the

steps by which a high-frequency gen-

erator produces a high-frequency,

high-voltage AC waveform. The AC

input voltage is converted to DC. An

inverter circuit chops the DC inputinto high-frequency AC of 500-

40,000 Hz (dependent on severalfactors explained in the text), which

is applied to the high-voltage trans-

former. High-voltage conversion,

rectification, and smoothing provide

a potential difference to the x-ray

tube with minimum voltage ripple.

(Redrawn, with permission, from

reference 6.)

Either single- or three-phase input lines can

be used to produce an output voltage with

minimal ripple characteristics (the three-phase

source provides greater overall power). The

AC input power is first converted into a low-

voltage DC waveform by rectification and

smoothing. An inverter circuit “chops” the DC

into a high-frequency AC square wave, which

is input into the high-voltage transformer to

produce a high-voltage, high-frequency AC

waveform. Subsequent rectification and capaci-

tance smoothing produce a potential difference

across the x-ray tube with very small voltage

ripple (Fig 20). In function, the generator fre-

quency is variable, since it depends on the set

tube potential, the tube current, and the fre-

quency-to-voltage characteristics of the trans-

former.

In addition to the components just de-

scribed, a high-frequency generator contains

high-voltage capacitors in the secondary circuit

that produce a voltage across the x-ray tube de-

pendent on the accumulated charge within

each capacitor (Fig 2 1). The voltage-charge-ca-

pacitance relationship V = QIC describes this

process, where V = the voltage across the ca-

pacitor, Q = the charge on the capacitor, and C

= the capacitance in farads. The generator op-

crates with a closed-loop regulation circuit,whereby during an exposure, feedback circuits

monitor both the tube voltage and tube cur-

rent. Instantaneous adjustments are made to

correct for variations in kilovoltage and mill-amperes that occur during the exposure.

In the case of kilovoltage, a voltage corn-

parator or oscillator measures the difference

between the reference voltage (a calibrated

value proportional to the requested kilovolt-

age) and the actual kilovoltage measured

across the x-ray tube by a voltage divider (kilo-

volt sense circuit). Trigger pulses generated by

the comparator circuit are produced with a fre-

quency proportional to the voltage difference:

A large discrepancy between the compared

voltages produces a high trigger pulse fre-

quency, whereas no difference produces few

or no trigger pulses. For each trigger pulse, the

DC-AC inverter circuit produces an output AC

waveform, which is converted into a high-volt-

age output pulse by the transformer. Two high-

voltage capacitors symmetrically balanced on

the cathode and anode of the x-ray tube store

the charge and create a potential difference. If

the actual voltage measured by the kilovolt

sense circuit is low, the trigger pulse rate to

the inverter increases, actuating the high-volt-

age transformer, the output of which ‘dumps”charge into the high-voltage capacitors to raise

kilovoltage across the x-ray tube. When the x-

ray tube potential reaches the desired value,

the output pulse rate of the comparator circuit

settles down to an approximately constant

value and recharges the high-voltage capacitors

only when the actual tube voltage drops below

a predetermined limit.

Tube current is regulated in a manner anal-

gous to that used for kilovoltage, with a resis-

tor circuit sensing the actual miffiamperes (mil-

liampere sense voltage) and comparing that

value to a reference milliampere value (milli-

ampere reference voltage). If the milliampere

sense value is too low, the voltage comparator

increases the trigger pulse frequency, which

boosts the power to the filament to raise tern-

perature and increase thermionic emission of

electrons. The closed-loop feedback circuit

eliminates the need for space charge compen-

sation and adjusts for filament aging effects.

The design of the high-frequency inverter

generator has several advantages. Because the

input line voltage is initially converted from anAC to a DC waveform, either single- or three-

+kV/2

Typical voltage

waveform

TVThvr\

kV

ripple

100%

100%

_\ 13%-25%

� 3% . 10%

ence 6.)


Figure 21. Block diagram shows the internal components and feed-hack circuits of a general-purpose high-frequency generator. Voltage

and current are maintained by comparing the actual kilovolt and milli-

ampere values (directly measured during the exposure) to referencevalues selected at the control console. High-voltage capacitors on the

secondary circuit supply the potential difference across the x-ray tube

and are continuously charged as needed to maintain the desired volt-

age. Autotransformers and space charge compensation circuits are not

required because of the closed-loop feedback circuits maintained by

this generator.

Generatortype

Single-phase 1-pulse(self rectified)

Single-phase 2-pulse(full wave rectified)

3-phase6-pulse

3-phase12-pulse

Medium-high frequencyinverter

ConstantPotential

Figure 22. Chart illustrates the4% . 15% voltage waveforms and voltage

ripple percentages for the x-ray gen-

erators described in this article. (Re-<2% drawn, with permission, from refer-

phase input lines can be used. Closed-loop

regulation circuits, which ensure reproducible

and accurate kilovoltage and tube current, pro-

vide extremely accurate exposures and obviate

autotransformers. Output voltage ripple of

high-frequency inverter generators is kilovolt-

age dependent and is similar to that of three-

phase 1 2-pulse generators (Fig 22). Transform-

ers operating at high frequencies are more effi-

cient, more compact, and less costly to manu-

facture. Finally, most high-frequency genera-

tors have an excellent service record and are

relatively easy to repair.

Currently, the high-frequency inverter gen-

erator is the most desired type for all but a few

selected applications (eg, extremely high

power needs or extremely fast kilovoltage

switching or short [< 1 msec] exposure times).

In only rare instances is the constant potential

generator possibly a better choice.


Figure 23. Diagram of a common mammographic unit illustrates the components of the

phototimer system. Modern phototimer systems in mammography have microprocessor

controls to adjust and compensate for unusual variations in the acquisition process. (Re.

drawn. with permission. from reference 6.)

U METhODS OF EXPOSURE CONTROL

Most single-phase generators and low-power x-

ray systems use simple mechanical switches

coupled to either mechanical or electronic tim-

ers to open and close the circuit on the pri-

mary side of the high-voltage transformer. For

these applications, the minimum exposure

time is 1/1 20-second (8-msec), since the

switch, once closed, stays closed as long as

power is applied. During each half-cycle, the

voltage passes through zero with the power in

the circuit also at zero, which allows easy re-

lease of the mechanical switch. Thus, the cx-

posure times allowed occur in 1/1 20-second in-

crements.

For three-phase and constant potential gen-

erators, high-voltage triode, tetrode, or pentode

switches initiate and stop the exposure on the

secondary side of the circuit under the control

of an electronic timer (or phototimer feed-

back) that is connected to the grid electrode.

Small negative voltage biasing on the grid (eg,

1 ,000 V) produces an electric field that over-

whelms the large potential difference (eg,

100,000 V) because the grid is close to the

cathode of the thyratron (gas-filled triode) or

thyristor (solid-state triode) switch. The flow of

electrons is stopped when the grid is activated.

Because the switch can he opened and closed

independent of the power on the circuit, expo-

sure times of 1 rnsec and less are possible.

The high-frequency invei-ter generator typi-

cally uses electronic switching on the priniat-y

side of the high-voltage transformer to initiate

and stop the exposure. The high-frequency

waveform characteristics of the generator cir-

cuit enable relatively rapid switching. which al-

lows exposure times typically a� short as 2

msec.

. PhototimersPhototimers are implemented in lieu of manual

exposure settings. These devices measure the

actual exposure incident on the x-ray receptor

(eg, screen-film detector) and turn off the x

rays when the proper optical density is oh-

tamed. An accurately adjusted phototimer pro-

vides consistent film optical density and corn-

pensates for variations in thickness and attenu-

ation within the patient.

The phototimer system consists of a radia-

tion detector, amplifier, density selector, corn-

parator circuit, termination switch, and backup

tinier. X rays transmitted through the patient

instantaneously generate a small signal in an

ionization chamber (or other radiation-sensitive

device such as a solid-state detector). An ampli-

tier boosts the signal, which is fed to a voltage

comparator circuit and integrated. When the

accumulated signal equals a preselected refer-

ence value, a pulse is generated to terminate

Cassette stand Film-screen cassette

:: � � � � :� � � � . � #{149}

.�.1. 1.4�3 2 1 0+1+2+3+4

I__I__I I I I�;:� � � � � �

-�.‘‘�-

Density control setting

AEC detector locations


Figure 24. Diagram demonstrates the typical location of automatic exposure control

(AJIC) detectors in a conventional projection radiographic system. Usually, three ionization

detectors are placed in front of the screen-film detector in the positions shown. Signals from

either or all of the detectors can be selected by the automatic exposure control electronics

circuit. This arrangement allows for global or regional compensation for variations in ana-

tomic structure. which is particularly important for chest imaging. (Redrawn, with permis-

SiOfl. from reference 6.)

the exposure. A radiographic system with a

properly adjusted phototirner circuit will pro-

duce films with the desired optical density.

The reference voltage can be adjusted with a

user-selectable density selector switch on the

generator control panel to modify the optical

density in steps of±lO%-20% from the normal

density position.

In a marnmographic system, the ionization

chamber of the phototimer is positioned be-

hind the screen-film cassette (Fig 23). This

placement is necessary to avoid producing arti-

facts from the chamber on the film. For most

general diagnostic imaging, the phototimer sen-

sors are placed in front of the detector (Fig 24)

because the high kilovolt spectrum used

makes the chamber transparent to x rays.

A backup timer ensures that the x-ray expo-

sure is terminated after a predetermined time

in the event of a phototimer detector or circuit

failure. Often in mammographic examinations,

the backup timer will be activated because the

operator inappropriately selected a low tube

potential that resulted in x rays which failed to

penetrate the thick, dense breast.

. Automatic Exposure ControlsAutomatic exposure control is another name

for a l)liototimer, and its circuitrr is an integral

part of most modern x-ray generators. Its cir-

cuitry is composed of detectors that measure

the exposure incident OIl the x-ray receptor

and that provide feedback signals to the timing

switch of the generator. Predetei-niined sensi-

tivity settings are matched to the speed of the

x-ray receptor to achieve optimal-quality radio-

graphs by turning off the power to the x-ray

tube after the necessary x-ray exposure.

To compensate for different x-ray penetra-

tion through distinctly different anatomic struc-

tures, three sensors are typically used in a

chest or table cassette stand with automatic cx-

posure control capability (Fig 24). The tech-

nologist can use the exposure signal from one,

two, or three of the sensors either indepen-

dently or together, depending on the particular

application. For instance, in chest radiography,

the outside sensors are often the only detectors

activated so that the x-ray beam transmitted

through the lung areas determines the expo-

sure time. In this way, “burn-out” of the lungs

that might occur if the transmitted x-ray flux

were measured solely under the highly attenu-

ating mediastinum is prevented.

In fluoroscopic systems, automatic bright-

ness control circuitry, which is similar to auto-

matic exposure control circuitry, is used to

provide optimal kilovolt and milliampere set-

tings for dynamic image display. When auto-

matic brightness control is enabled, the overall

image brightness on the display monitor re-

mains consistent because the generator pro-

�‘ides changes in the fluoroscopic parameters

to compensate for changes in the thickness

and composition of the patient that occur with

movement during the procedure.

500 -

Example: 50 mAs necessary for exposure:mAs delivered a area under curve

mAs is equal in both situations

400 -

E

300

200

With falling load generatorMaximum variable mA x 0.08 sec

Without falling load generator:400 mA x 0.125 sec

Falling load curve

I ‘I I � � � � � I I I

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Exposure time (sec)


Figure 25. Plot demonstrates the advantage of using a falling load

generator. In the example plotted, a particular examination to be per-formed at a specific tube potential requires 50 mAs to record the

proper optical density on film, and a maximum constant tube current

of 400 mA (as determined with a tube rating chart) can be tolerated by

the focal spot. For these examination parameters, an exposure time of1 25 msec is required. With a falling load generator, however, it is de-

termined that a maximum tube current of 600 mA can be delivered, if

applied for a short duration, much less than the total exposure time.Even though the tube current continuously falls during the exposureaccording to a predetermined power load curve, a total of 50 mAs

equivalent output can be completed within an exposure time of 80

msec, or about 45 msec less than the scenario with a constant maxi-

mum tube current. The shaded bars have the same area; therefore, thesame film optical density is achieved with both techniques. (Redrawn,with permission, from reference 6.)

. Faffing Load GeneratorsThe falling load generator works with the pho-

totimer or automatic exposure control systems

to acquire an image in as short an exposure

time possible. This type of generator is pro-

grammed to deliver the maximum possible

tube current for the selected tube potential;

this maximum value is determined from the fo-

cal spot selected and the instantaneous heat

load characteristics of the x-ray tube anode.

The preprogrammed circuitry of the generator

determines the maximum power load limit of

the x-ray anode and then continuously reduces

the power as the exposure continues so that

the heat load limit of the anode is not cx-

ceeded during the exposure time (Fig 25).

Thus, use of a falling load generator allows the

optimal film density to be recorded in the

shortest possible exposure time. In addition,

shorter exposure times may help decrease im-

age blurring caused by involuntary or voluntary

patient motion.

U SELECTION OF GENERATOR POWER

AND OPTIONS

. Power Rating: Generators and Focal

Spots

The power rating of generators is determined

by the characteristics of the input voltage, the

filament transformer, and the high-voltage

transformer. The typical unit for power is the

Table 1

Power Ratings of X-ray Generators and Focal Spot Sizes

Focal Spot

Application Generator Power Rating (kW) Size (mm) Power Rating (kW)

Dental, mammography � 10 0.1-0.3 1-10

Fluoroscopy, general � 50 0.3-0.6 10-30

Angiography, interven- � 150 0.8-1.0 50-80

tional

Contant potential � 200 1 .2- 1 .5 80-125

November-December 1997 Scibert U RadioGraphics U 1553

kilowatt, which is the product of the voltage

and tube current expressed in kilovolts and

milliamperes, respectively. Strictly speaking,

the voltage is assumed to be a pure DC wave-

form with 0% ripple. For voltage waveforms

with substantial ripple, the power rating is a

product of the effective voltage (the root-mean-

square voltage [ V,.,,,,]) and the effective current.

The generator power capability is typically

benchmarked at an exposure time of 0. 1 see-

ond for the maximum milliamperage and

kilovoltage. From this definition, a 100-kW gen-

erator can therefore provide 1 ,000 mA at 100

kVp for a 0. 1-second exposure. Lower or

higher tube potentials can support higher or

lower tube current output, respectively, within

a range of 20-30 kV. For example, when the

same 100-kW generator is operated for a 0.1-

second exposure at 80 kVp, the tube current

will be about 1,200 mA; at 120 kVp, the maxi-

mum tube current will be about 800 mA.

For single-phase generators with substantial

voltage ripple, the power rating is reduced by

about 30% because of the lower effective tube

potential caused by the large variation in the

voltage. A generator operating at 100 kYp with

a single-phase waveform that is rated at 1,000

mA for 0. 1 second has a power rating of about

70 kW.

The power output of the generator must

also be matched with the power dissipation

(loadability) characteristics of the focal spot

and anode for a specific x-ray tube. A bigger fo-cal spot will have a larger power rating than a

small focal spot. Power loading of the x-ray

tube can be determined from the single expo-

sure rating chart in a manner analogous to that

described for the generator power rating.Power ratings of generators and focal spots

are specified in Table 1 . Generator power re-

quirements should be considered in terms of

the imaging application. In addition, it makes

no sense to have a very high rated generator

and then be limited by the focal spot power

rating. The appropriate equipment (x-ray gen-

erator and x-ray tube) must be specified for the

intended imaging tasks, so that efficiency of

economy, size, and utilization are maximized.

. Tube Rating Charts

Tube rating charts provide guidance regardingthe maximum allowable deposition of powerfor a single exposure, multiple exposures, and

continuous x-ray exposure for a specific x-ray

tube and x-ray generator combination. The

power limits are determined by the selected

tube potential, tube current, exposure time, fo-

cal spot size, anode rotation speed, and genera-tor type. Historically, the heat unit has been

used to indicate the power deposited on the

anode for single exposures. It is calculated as

the product of the tube potential, tube current,

and exposure time in seconds. To determinethe continuous power deposition during fluo-

roscopic procedures, the heat unit per second

is calculated as the product of tube potential

and tube current (which follows from the defi-

nition of the heat unit).

Table 2Generator Waveform Characteristics andAverage Voltage

Note.-V� = peak voltage, � = root-mean-square voltage.


The heat unit does not account for voltage

ripple and therefore represents an underesti-

mation of the power deposited on the anode

for three-phase, high-frequency, and constant

potential generators by about 35%-40%. As a

result, one must be cautious when using the

heat unit to determine the amount of power

deposited on the anode. To estimate the true

power loading, the generator waveform must

be considered. This more accurate estimate re-

quires that the heat unit be multiplied by a fac-

tor of 1 .35- 1 .40 for three-phase generators (six-

pulse vs 1 2-pulse), about 1 .40 for high-fre-

quency generators, and 1 .40 for constant

potential generators. (The average applied volt-

age of a full-wave-rectified single-phase output

is approximately 70% of the peak voltage.)

An alternative description for the power

deposition and heat loading unit is the joule,

which is defmed as the product of the root-

mean-square voltage (this value is similar to the

average applied voltage), the tube current (in

amperes), and the exposure time (in seconds).

Because the joule intrinsically takes the voltage

waveform into account, the type of generator

used is not a consideration. Table 2 shows therelationship between peak voltage and root-

mean-square voltage for the four generator de-

signs.

. Single Exposure Rating Charts

The single exposure rating chart graphically de-

scribes the maximum instantaneous power

loading capability of the x-ray tube anode for a

combination of radiographic techniques. In the

charts illustrated in Figure 26, exposure time

and tube kilovoltage are the units of the x and

y axes, respectively. Each plotted curve repre-

sents the maximum tube current allowed for a

choice of tube voltage and exposure time. (Al-

though this is the typical arrangement for a

single exposure rating chart, tube current ver-

sus exposure time for a series of kilovoltage

curves is sometimes plotted.) The rating chart

is specific for a particular tube, stator and rotor

assembly, anode rotation speed, and applied

generator waveform.

Generator Type V� as fraction of Vpeak

Single-phase 0.71

Three-phase and 0.95-0.99

high-frequency

Constant potential 1.00

In Figure 26, the single exposure rating

charts are for a large (1 .2 mm, 100 kW) and a

small (0.3 mm, 10 kW) focal spot combinations

with an anode rotation speed of 10,000 revolu-

tions per minute. Obvious differences exist in

the two charts, particularly with respect to the

maximum tube current curves, because of the

difference in instantaneous power loading

caused by the two largely different focal spot

sizes. To achieve the same overall tube output

(ie, power deposition on the anode) for a small

focal spot, the exposure time must be drasti-

caily longer. The small focal spot technique

provides minimal geometric blurring but is sus-

ceptible to image blurring because patient mo-

tion is likely during long exposure times. A

large focal spot technique will minimize mo-

tion blurring with short exposures; however, if

magnification of the subject is needed, substan-

tial geometric blurring and loss of resolution in

the resultant image will occur. Compromises in

acquisition parameters are needed to achieve

optimal image quality for the wide range of cx-

aminations encountered in diagnostic imaging.

There are several ways to use a single expo-sure rating chart to determine whether a spe-

cific combination of tube potential, tube cur-rent, and exposure time are permitted. One

method is to fmd the intersection of the re-

quested kilovoltage (horizontal line) and expo-

sure time (vertical line) values. This point,

when referenced to the milliampere curves

above and below, will indicate the maximum

tube current possible (determined by interpo-

lating between the milliampere curves) for the

requested combination of kilovoltage and cx-

posure time. Two examples in Figure 27 illus-

trate how this is accomplished.

50

Maximal exposure time, seconds

b.


100 kW; 1.2 mm focal spot; 180 Hz I 10000 RPM150

140

130

120

110

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90

80

70

60

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100 kVp700 mAI sec?

70 kVp1200 mA0.Olsec?



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Figure 26. Single exposure rating charts are shown for two focal spots of widely different sizes: 0. 3 mm (a)

and 1 .2 mm (b). These charts plot the kilovoltage (y axis) versus exposure time (x axis) for a number of tube

current values indicated by the solid lines. The maximum allowable power depends on the tube potential, cx-

posure time, focal spot size, anode rotation speed, and generator type. For a specific tube potential and expo-

sure time, one can determine the maximum tube current allowed for a single exposure without exceeding the

heat load limitations of the anode (assuming that the retained heat in the anode assembly is far below the maxi-

mum anode limit). (Redrawn, with permission, from reference 6.)

‘- c’l M) e (‘4 C’) 10 ‘- C’4 C’) U) P.. . C’l C’) SO � 0

80 �#{176}#{176}#{176}.#{176}#{243}o#{243}o#{243} I-. 0� #{149} �0 0 0

Figure 27. Use of the single exposure rating chart (for determining

instantaneous deposition of heat energy) is demonstrated for two hy-

pothetical radiographic examinations. In the first example, the opera-tor attempts to use 100 kVp with a tube current of 700 mA and 1-see-

ond exposure time. Finding the intersection of the kilovoltage and cx-

posure time (arrows) reveals that the maximum tube current allowed

is approximately 650 mA (obtained by interpolation, since the point

lies about halfway between the 600 and 700 mA curves). Thus, the at-

tempted exposure of 700 mA is not allowed. In the second example,

the operator attempts to use 70 kVp with a tube current of 1 ,200 mA

and 0.01-second exposure. As seen by the intersection points of the

kilovoltage and exposure time values, 1 ,200 mA is the maximumvalue. Thus, the exposure is allowed, since 1 ,200 mA is requested.

Time (minutes) Cooling time (minutes)

Anode Thermal Characteristics Housing Thermal Characteristics


Figure 28. Charts of anode and housing thermal characteristics are illustrated. Anode heat input

and heat dissipation rates are specific to a given x-ray tube. In the left chart, the largest y-axis value

most always indicates the maximum anode heat load limit (measured in kilo heat units). Heat dissipa-

tion is characterized by the exponentially decreasing curve, and heat input rate (heat units per see-

ond) is usually shown as a family of individual curves to determine the amount of heat accumulating

as a result of fluoroscopic exposures. The right chart illustrates how use of a heat exchanger systemcan accelerate the cooling rate of the housing. (Redrawn, with permission, from reference 6.)

In modern x-ray generators, system logic cir-

cuits prohibit any combination of tube poten-

tial, tube current, and exposure time that

would exceed the tube rating capabilities of

the x-ray tube and the chosen focal spot. Forangiography, however, because multiple expo-

sure studies are common, one must consult the

multiple exposure rating chart (containing a se-

ties of tables) to determine the ability of the

system to perform the requested examination

series without damaging the x-ray tube.

. Thermal Characteristic Charts for

X-ray Tube Anodes and HousingsHeat loading and dissipation rates for the x-ray

tube anode and x-ray tube housing must be

considered for extended imaging examinations

(eg, combined fluoroscopy and radiographicimaging for interventional angiography). The

anode and housing thermal characteristiccharts are used for this purpose (Fig 28).

In the anode chart, the x axis indicates time

in minutes, and the y axis indicates the heat

units accumulated in the anode as a result of

previous exposures or continuous fluoroscopic

input. The largest value on the y axis indicates

the maximum power load rating for the x-ray

tube. In the anode chart example, this value is

140,000 heat units. Single and multiple expo-

sures instantaneously deposit energy on the an-

ode. For a three-phase generator, the heat unit

value must be multiplied by 1 .35- 1 .40 to deter-

mine the actual number of heat units depos-

ited. During continuous fluoroscopy, the

amount of heat deposited on the anode is de-

termined by the heat units per second input.Several heat input curves corresponding to

various values of heat units per second (ie,

tube potential times tube current) are used to

estimate the total amount of heat deposited on

the anode. Many of these curves at low miii-

amperes reach a maximum when the heat in-

put rate equals the heat dissipation rate. For

situations in which the exact heat units per

second input curve is not available, interpola

This article meets the criteria for 1. 0 credIt hour in Category 1 of the AMA Physician ‘s Recognition

Award. To obtain credit, see the questionnaire on pp 1527-1532.

November-December 1997 Seibert U RadioGrapbks U 1557

tion between the two curves above and below

the desired heat units per second value is nec-

essary to estimate the accumulated heat after a

specified fluoroscopic exposure time.

The anode-cooling curve describes the rate

at which heat is reemitted from the anode. A

much steeper slope is evident at high heat

loads, indicating that much faster cooling oc-

curs with a hotter anode. For example, a tube

with 140,000 heat units on the anode will lose

50,000 heat units (to 90,000 heat units) in

about 2 minutes, whereas a tube with 90,000

heat units on the anode will lose 50,000 heat

units (to 40,000 heat units) in approximately

3’/2 minutes. The anode thermal characteristics

chart is used with single and multiple exposure

charts to determine whether a single or series

of subsequent exposures are permitted based

on the amount of heat already accumulated.

In the tube housing chart, a maximum heat

accumulation is indicated on the y axis. In this

situation, thermal expansion of the insulation

oil surrounding the x-ray tube insert occurs. Ex-

ceeding the maximum allowed level can causecatastrophic failure of the housing or x-ray

tube. Like the anode, the housing cools fasterwhen it is hotter. In some x-ray tubes, heat cx-

changers are available to cool the housing and

its components more rapidly.

Specific situations in which the anode ther-

mal characteristic chart would be used include

(a) determination of the maximum anode heat

loading value, (b) calculation of the amount of

heat deposited on the anode as a result of fluo-

roscopic and radiographic techniques, and

(c) determination of the length of time required

for cooling before initiating another imaging se-

quence on an already hot anode. For extended

use of the x-ray system, the housing thermal

characteristics chart would also be used in situ-

ation C.

U SUMMARY

The x-ray generator provides the power neces-sary to produce x rays within the x-ray tube.

The user has the ability to select and accurately

control the x-ray energy, quantity, and duration

of the exposure. Electrical components of the

x-ray generator include transformers, diodes,

triodes, tetrodes, rectifier circuits, filament cir-

cuits, compensation circuits, phototimer cir-

cuits, and kilovolt-milliampere meters.

High-frequency inverter generators are be-

., coming the universal choice for radiographicsystems, because of feedback regulation, accu-

rate kilovolt and milliampere linearity, repro-

ducibility of examination techniques, and cx-posure timing. The compact size, simple siting

requirements, use of single-phase line voltage,

and lower cost of these generators are other

beneficial attributes. Modern generators make

extensive use of microprocessors, which facili-tate the use and improve the serviceability of

the equipment, improve the accuracy of diag-

nostic examinations, and protect the x-ray tube

and patient.

. REFERENCES1 . Ammann E. X-ray generators and control cir-

cuits. In: KrestelE, ed. Imaging systems for

medical diagnostics, section 7. 1 .4. Berlin, Ger-many: Siemens Aktiengesellschaft, 1990; 284-

317.

2. Bushong SC. The x-ray machine. In: Radiologic

science for technologists, 3rd ed. St Louis, Mo:

Mosby, 1984; 107-137.

3. Gaunt DM. Mammography x-ray generators:

conventional and high-frequency designs. In:

Barnes GT, Frey GD, eds. Screen film mammog-

raphy: imaging considerations and medicalphysics responsibilities. Madison, Wis: Medical

Physics, 1991; 47-66.

4. Hardy PR. High voltage generation and expo-

sure timers. In: Hill DR, ed. Principles of diag-

nostic x-ray apparatus. London, England: Mac-Milan, 1975; 147-182.

5. Mansfield BA. Electron emission and solid statedevices. In: Hill DR, ed. Principles of diagnostic

x-ray apparatus. London, England: MacMillan,

1975; 63-112.

6. Seibert JA. Generation and control of x-rays. In:

Bushberg JT, Seibert JA, Leidholdt EM Jr, Boone

JM, eds. The essential physics of medical imag-ing. Baltimore, Md: Williams & Wilkins, 1994;

65-108.

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