X-ray Generators: Basic concepts and Application

X-ray Generators
Presented by: Dr. Anish Dhakal
Resident
MD Radiodiagnosis, KUSMS
10th
June, 2024

Components of X-ray imaging system
(1) Operating console
(2) High-voltage generator
(3) X-ray tube

With dental and mobile x-ray imaging systems, the three components are housed very
compactly.

• The high-voltage generator may be housed in an equipment cabinet
positioned against a wall
• The high-voltage generator is always close to the x-ray tube, usually in the
examination room
• Operating console allows radiologic technologists to control the x-ray tube
current and voltage so that the useful x-ray beam is of proper quantity and
quality
• X-ray tube is located in the examination room, and the operating console is
located in an adjoining room with a protective barrier separating the two.
• X-ray generator is the device that supplies electric power to the x-ray tube

Circuits in x-ray imaging
• The primary circuit consists of the main power switch
(connected to the incoming power supply), circuit breakers, the
autotransformer, the timer circuit, and the primary side of the
step-up transformer.
• The secondary circuit consists of the secondary side of the step-
up transformer, the mA meter, a rectifier bank, and the x-ray
tube (except for the filaments).
• The filament circuit consists of a rheostat, a step-down
transformer, and the filaments.

Control panel
• It allows the operator to
select appropriate:
– kVp
– mA
– Exposure time

• Buttons:
– Preparation or stand by
button :
• Heats the filament
• Rotates the anode
• Gives green signal when
ready for exposure
– Exposure button

• All of the electric circuits that connect the meters and controls on the operating
console are at low voltage to minimize the possibility of hazardous shock
• Most x-ray imaging systems are designed to operate on 220 V power
• Unfortunately, electric power companies are not capable of providing 220 V
accurately and continuously
• Because of variations in power distribution to the hospital and in power
consumption by various sections of the hospital, the voltage provided to an x-
ray unit easily may vary by as much as 5%
• Such variation in supply voltage results in a large variation in the x-ray beam,
which is inconsistent with production of high quality images.

Line compensator
• Line compensator measures the voltage provided to the x-ray
imaging system and adjusts that voltage to precisely 220 V
• Older units required technologists to adjust the supply voltage
while observing a line voltage meter
• Today’s x-ray imaging systems have automatic line
compensation and hence have no meter

The autotransformers:
• An autotransformer is a kind
of electrical transformer where
primary and secondary shares
same common single winding.
• Working principle: Self
induction

Role of Autotransformer
• The power supplied to the x-ray imaging system is delivered first to
the autotransformer
• The voltage supplied from the autotransformer to the high-voltage
transformer is variable but controlled
• The auto transformer takes incoming line voltage and generates a
magnetic field which then self-induces voltage across the length of
the transformer.
• The longer the distance between two points, the higher the
voltage generated

Function of auto-transformers:
1.Provides voltage for the x-ray tube filament circuit
2.Provides voltage for the primary of the high-voltage
transformer
3.Provides a convenient location for the kVp meter

• It is much safer and easier to control a low voltage and then increase it than to
increase a low voltage to the kilovolt level and then control its magnitude.
• The autotransformer has a single winding and is designed to supply a precise
voltage to the filament circuit and to the high-voltage circuit of the x-ray imaging
system
• Autotransformers are difficult to build for very high voltages. And the voltage
required by the X-ray tube could be anywhere from 10 kV to 100 kV. For the very
high voltages needed at, its much easier to build a fixed step-up transformer.
• But we still need to be able to control the voltage at so you use the
autotransformer to adjust how much voltage gets to the high voltage step up
transformer

Adjustment of the kVp
• Some older x-ray operating consoles have adjustment controls labeled major
kVp and minor kVp
• By selecting a combination of these controls, radiologic technologists can
provide (fine tune) precisely the required kilovolt peak
• Low voltage from the autotransformer becomes the input to the high-voltage
step-up transformer that increases the voltage to the chosen kilovolt peak
• Appropriate connections can be selected with an adjustment knob, a push
button, or a touch screen.
• On most operating consoles, the kVp meter registers, even though no exposure
is being made and the circuit has no current. This type of meter is known as a
prereading kVp meter. It allows the voltage to be monitored before an
exposure.

Control of mA
• X-ray tube current is controlled through a separate circuit called the filament
circuit. Connections on the autotransformer provide voltage for the filament
circuit.
• Filament temperature is in turn controlled by the filament current, which is
measured in amperes (A)
• As filament current increases, the filament becomes hotter, and more electrons
are released by thermionic emission.
• Filaments normally operate at currents of 3 to 6 A
• Falling load generator constitutes an exception. In a falling load generator the
exposure begins at maximum mA, and the mA drops as the anode heats. The
result is minimum exposure time.

Other components in the console
• kVp meter, or the pre-reading voltmeter: Reads voltage, not kVp. Measures
the electrical potential of the x-ray tube, or the kilovoltage that will be flowing
through the tube once the exposure is made. Indirectly measures kVp applied
to primary windings of high tension transformer when exposure will begin.
Gives information before kV is actually applied to x-ray tube. Voltage absorbed
in overcoming resistance is lost to x-ray tube called kilovoltage drop.
• Exposure switch, or the timing circuit: Used to complete the x-ray exposure. It
regulates the length of the exposure, and it’s where the tech starts and the
timer stops the exposure.
• Ammeter, or the mA selector: Selects the tube current to heat the filament

Transformers assembly
• It is a grounded metal box filled with oil where oil is a insulator and prevents sparkling.
• Contains low voltage transformer for filament circuit
• Contains high voltage transformer and some rectifiers for high voltage circuit

Transformer
• A transformer is a device that changes (transforms)
and alternating potential difference (voltage) from
one value to another value be it smaller or greater
using the principle of electromagnetic induction.
• Core: Laminated.
• Efficacy: <100% (appear as heat due to eddy current)

Physics of transformers:
• Based on the principle of Electromagnetic induction.
• When alternating current flows through the primary coil, it creates a
magnetic field within the core, and this magnetic field induces a current
(emf) in the secondary coil
• Current only flows in the secondary circuit when the magnetic field is
increasing or decreasing. No current flows while the magnetic field is
stable.
• For this reason, steady direct current (like that from a battery) in the
primary coil cannot be used to produce a continuous current through the
secondary coil

Laws of transformers:
1. The voltage In the two circuits is proportional to the number
of turns in the two coils.
–Np/Ns = Vp/Vs
– Np = number of turns in the primary coil
– Ns= number of turns in the secondary coil
– Vp = voltage in the primary circuit
– Vs = voltage in the secondary circuit

Laws of transformers:
2. The second law of transformers is simply a restatement of the law of the
conservation of energy. A transformer cannot create energy. An increase
in voltage must be accompanied by a corresponding decrease in current.
The product of the voltage and current in the two circuits must be equal.
– Pp = Ps
– And since, power (P) = Voltage(V)*current (I)
– Vp*Ip = Vs*Is
• Vp = voltage in the primary coil
• Ip = current in the primary coil
• Vs = voltage in the secondary coil
• Is = current in the secondary coil

Types of transformers:
On the basis of windings:
I. Auto transformers
II. Step up transformers
III. Step down transformers
On the basis of core construction:
IV. Core Type
V. Shell Type
VI. Crossed (H) Type

Step up transformers:
• A transformer that increases the
voltage from primary to
secondary (more secondary
winding turns than primary
winding turns) is called a step-up
transformer
• Increase voltage, decrease
current

Step down transformers:
• A transformer that decreases the
voltage from primary to secondary (less
secondary winding turns than primary
winding turns) is called a step-down
transformer
• Decreases voltage, increases current
• The high voltage transformer is a step-
up transformer, that is, the secondary
voltage is higher than the primary
voltage

Rectification:
• Rectification is the process of changing alternating current
into direct current and the device that produces the change
is called a rectifier.
• The electronic device that allows current flow in only one
direction is a rectifier
• Rectification is the process of converting AC to DC

Why rectification?
• An x-ray tube requires a direct current (DC), that is, electron
flow in only one direction.
• Therefore, some means must be provided for converting AC to
DC

Rectifiers
• A rectifier is a device that allows an electrical current to flow in
one direction but does not allow current to flow in the other
direction.
– Types:
• Vacuum tube type (obsolete)
• Solid-state rectifiers:
– Smaller, reliable, longer life
– Selenium
– Silicon

Half wave rectification:
• The inverse voltage is removed from the supply to the x-ray tube by rectification.
• Voltage is not allowed to swing negatively during the negative half of its cycle.
• During the positive portion of the AC waveform, the rectifier allows electric
current to pass through the x-ray tube.
• During the negative portion of the AC waveform, however, the rectifier does not
conduct, and thus no electric current is allowed.
• In some portable and dental x-ray imaging systems, the x-ray tube serves as the
vacuum tube rectifier. Such a system is said to be self-rectified.
• Half of the available electrical cycle is not utilized to produce x-rays so exposure
times must be twice as long as they would be if the whole cycle were utilized.
Repeated or prolonged exposures heat the anode and can destroy the filament.

Full wave rectification:
• Consists at least 4 diodes (Bridge rectifier)
• Negative half-cycle corresponding to the inverse voltage is
reversed so that the anode is always positive.
• The output voltage across the x-ray tube is positive
• No gaps in the output waveform
• Exposure time for any given technique is cut in half

Semiconductors:
• Heart of rectifiers
• Made of silicon
• At absolute zero temperature, all electrons are in valence
band. Thus behaves as insulator.
• At room temperature, some of the electrons are thermally
raised to the conductions band. Thus, behaves as
conductor.
• Require 1.1 eV to raise electrons to conduction band

Why silicon as rectifier?
• Resist reverse voltage of about 1000V (10-20 times higher then
selenium).
• Withstand temp. up to 392 deg C (selenium- 266 deg C)

Types of semiconductors:
• N-type semiconductor
• P-type semiconductor

N-type semiconductor:
• Silicon (4 valence electron) +
Impurity (5 valence electron)
• Impurity: Arsenic, Antimony
• 1 atom of impurity in 10^7 atoms of
silicon
• Electrons are 0.05 eV below the
bottom of conduction band
• Donor impurity as it donates
negative free electron

P-type semiconductor:
• Silicon (4 valence electron) +
Impurity (3 valence electron)
• Impurity: Boron, gallium, aluminum
• Formation of holes (positive particle)
(absence of electron)
• Holes are 0.08 eV above the top of
valance band

P-N junction:
• Device formed by P-N
junction are Diodes.
• Solid state rectifiers are
diodes.

How diode acts as rectifier?
• If polarity of applied volt is opposite
that of junction, electrons will flow
and so will the current
• If polarity of applied volt is same as
that of junction, the junction
potential will increase and no current
will flow.

Types of generators:
A.Three phase generators
B.Power storage generators
a. Capacitor discharge generators
b. Battery powered generators
C.Medium frequency generators
D.Falling load generators

A. Three phase generators
• It produces an almost constant
potential difference across x-ray tube.
• No deep valleys between pulses.
• Uses three phase transformers.
• Types:
– Six pulse, six rectifier
– Six pulse, twelve rectifiers
– Twelve pulse

Three phase transformers
• Has three sets of primary and
secondary windings.
• The 3 sections of copper windings in
primary or secondary in following
configurations:
– Delta
– Wye (or star)
• Generally primary windings are of
delta and secondary more often wye
or both.

Ripple factor:
• It is the variation in the voltage across the x-ray
tube expressed as % of maximum value.
• For example:
– RF for single phase circuit is 100% because voltage
goes from zero to max in a single cycle
– If RF=13.5%, it means that at 100kV, the voltage
fluctuates between 86.5 and 100kV.

B. Power storage generators
a. Capacitor discharge generators:
– Capacitor is device to store charge or electron
– The rectified voltage is used to charge a large capacitor
– Charged just before use.
– Then it is discharged in the x-ray tube.
– Capacitor discharge provides high mA for short exposure time.

B. Power storage generators
b. Battery powered generators:
– Contains nickel-cadmium batteries, fully charged in 12 hours.
– Completely independent to external power supply
– DC from battery is fed to DC chopper
– Produces 500 Hz pulsed DC
– The high voltage output from secondary transformer is rectified and
supplied to x-ray tube as 1000 pulse wave.

C. Medium frequency generators:
• Latest and advanced generators.
• Based on Faraday’s law:
– “Induced voltage is proportional to the rate of change of magnetic flux.”
– V ~ fnA
• V = output voltage
• f = frequency
• n = number of windings
• A = core cross-sectional area
• Basic principle: In a transformer, the voltage induced in the secondary coil is proportional
to the rate of change of current in the primary coil.
• High-frequency current to produce an almost constant potential voltage
• Nearly 0% ripple
• Advantages:
– Ripple free voltage regardless of input power
– No special voltage regulators required
– Small size of the generator

D. Falling load generators
• Automatically starts the
exposure at highest mA
for selected kVp
• Drops it during the
exposure depending
upon the maximum head
loading capacity of the
tube
• Works with photo timer
or AEC systems

Filament Circuit
• X-ray tube current is controlled through a
separate circuit called filament circuit.
• Components:
1. Filament transformer
2. mA selector
3. Space-charge compensator
4. Frequency compensator
5. Voltage stabilizer
6. AC source

mA selector
• Regulates the mA required for filament circuit (ultimately controls tube
current)
• Altered by altering no. of electrons emitted from the heated filament
• No of electrons emitted altered by altering temperature to which filament is
raised
• mA used for the exposure can be controlled by altering the heat of the
filament.
• Small changes in the heat of the filament can result in large changes in the no
of emitted electrons and hence in the tube current.
• So alterations in filament heat give a very sensitive control for the current
passed by the X-ray tube.

• Filament current is provided by secondary winding of filament
transformer so it is controlled by altering power used in secondary side of
filament transformer.
• Control set by technologist is in primary circuit of filament transformer.
• A stepdown transformer (also called the filament circuit transformer),
which decreases the voltage and in turn increases the amperes going to
the filament.
• Connected to the step-down transformer is the focal spot selector, in
where the focal spot is the area of the target that the x-ray is emitted
from.

Exposure timers:
• To control the length of x-ray exposures.
• Types:
1) Mechanical timers (obsolete)
2) Electronic timers
3) Pulse counting timers on basis of periodic occurring events
4)Automatic exposure control (photo timers)

Automatic exposure control (photo-timers)
• Designed to eliminate human errors.
• Requires devices that can detect radiation and in response
small current.
• 3 such devices: (location in front or back of the cassette)
i. Photomultipliers detectors
ii. Ionization chambers containing aluminium or lead foils with gas in
between
iii. Solid state detectors using on junctions

Photomultipliers detectors
• Most common type
• It is made of lucite, i.e., can transmit light
• Coated with phosphor (can emit light
when irradiated, which are proportional
to intensity of x-rays that reach phosphor)
• Lucite transmit this light to photocathode,
where it is converted to electric current.
• This current is used to charge a capacitor
• When capacitor is reached to
predetermined charge, it is used to bias
the Exposure Switching.
• This terminates the exposure.

Exposure switching:
• Switch is the device that turns the high voltage to the x-ray
tube on and off.
• Based on location, 2 types:
1) Primary switching
2) Secondary switching

1) Primary switching:
• Present in the primary circuit.
• 3 types:
i. Electromechanical contractors (obsolete)
ii. Thyratrons (obsolete)
iii.Silicon controlled rectifiers or Thyristers

Silicon controlled rectifiers (SCR)
• Turned on and off by logic signal
• Cathode, anode, gate, 3 junctions
(NP, PN, NP)
• Negative at cathode and positive at
anode
• Logic signal (positive voltage
through the gate)
• A small positive pulse (the logic
signal) to the gate causes a large
current to flow through the
thyrister

2) Secondary switching:
• Is used for unit design in rapid, repetitive exposure or
extremely short exposures.
• High voltage side or secondary circuit.
• 2 types of secondary switches used:
– Triode vacuum tubes
– Grid controlled x-ray tubes

Primary switching Secondary switching
1. Most commonly used Only used for special purposes
(eg. Angiography, cinefluorography)
2. Easier Difficult
3. Cheaper Costlier
4. Can produce short exposures (1-2ms) Even shorter exposures if triode tubes
used (0.05ms)
5. Low repetition rate High repetition rate

• References
• Christensen's Physics of Diagnostic Radiology, 4th Edition
• Radiologic Science For Technologists Physics Biology And
Protection by Stewart C.
Bushong 12 th ed

X-ray Generators: Basic concepts and Application

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