x ray production

PHYSICS OF MEDICAL IMAGING

1

X-ray Production, X-ray Tubes and Generators

MUHAMMED ANEES.KResident Medical Physicist

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The study and use of ionizing radiation in medicine started with three important

discoveries

X rays by Wilhelm Roentgen in 1895. Natural radioactivity by Henri Becquerel in 1896. Radium-226 by Pierre and Marie Curie in 1898.

INTRODUCTION


Roentgen discovered x rays in 1895 while experimentingwith a Crookes “cold cathode” tube.

• Crookes tube is a sealed glass cylinder with two embeddedelectrodes operated with rarefied gas.• The potential difference between the two electrodes producesdischarge in the rarefied gas causing ionization of gas molecules.• Electrons (cathode rays) are accelerated toward the positiveelectrode producing x rays upon striking it.

A bit of History

Photograph of Roentgen’s apparatus

5/6/2015 4

William Roentgen discovered X-rays in 1895 &

determined they had the following properties

1. Travel in straight lines

2. Are exponentially absorbed in matter with the exponent proportional to the mass of the absorbing material

3. Darken photographic plates

4. Make shadows of absorbing material on photosensitive paper

• Roentgen was awarded the Nobel Prize in 1901

• Debate over the wave vs. particle nature of X-rays led the development of relativity and quantum mechanics

A bit of History


The first x-ray photograph:

Roentgen’s wife Bertha’s hand


X-rays are one of the

main diagnostical tools

in medicine since its

discovery by

Wilhelm Roentgen in

1895

Current estimates show

that there are approximately

650 medical and dental X-ray

examinations per

1000 patients per year

“ Internal structures of the body could bemade visible withoutthe necessity of surgery “


To heat the filamentso that electrons are given off

To connect the X-ray tube to a source of high voltageElectrons are accelerated & convert their KE into X-rayswhen impinge on the target of the X- ray tube

To produce X-rays


X-rays characteristics

• Highly penetrating, invisible rays• Electrically neutral• Travel in straight lines.• Travel with the speed of light in vaccum:

300, 000 km/sec or 186, 400 miles/sec.• Ionize matter by removing orbital electrons• Induce fluorescense in some substances. • Fluorescent screen glow after being stricken with photons.• Can't be focused by lenses nor by collimators.


X-ray Imaging System

PRINCIPAL PARTS

Operating Console High-voltage generator X-ray tube

PRIMARY FUNCTION

The system is designed to provide a large number of e- at cathode

with high kinetic energy focused to a small target at anode.


X-ray Tube Construction

Radiographic Equipment


X-ray tube Main voltage supply High tension source Kilo-voltage control Filament circuit and milli-amperage control Timer

Features of X-ray equipment

Main supply Filament CKT including mA control

kV control kV indication

High Tension source

Exposure timing and switching

mA indication

X- Ray Tube

Block Diagram of X-ray equipment


X-ray Tube


How “X-rays” are created ?

Power is sent to x-ray tube via cables

mA is sent to filament on cathode side.

Filament heats up – electrons are produced

Positive voltage (kVp) is applied to anode

Negative electrons are attracted across the tube to the positive anode.

Electrons slow down and finally come to rest

Electron beam is focused from the cathode to the anode target by the

focusing cup


Fundamentals of X ray Production

Bombardment of a thick target with energetic electrons

Electrons undergo a complex sequence of collisions & scattering process

Which results in the production of bremsstrahlung and characteristic

radiation

Bremsstrahlung Radiation

Characteristic X-rays

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X-ray Production

When Highly Energetic Electrons Interact with Matter &

Convert their Kinetic Energy into X-rays

X - rays


When the electrons from the cathode are accelerated at high voltage to the anode:

• 99% of the energy is dissipated as heat

(anode materials are selected to withstand the high temperatures they are able to withstand)

• 1% is given off as x-rays


BREMSSTRAHLUNG RADIATION

CHARACTERISTIC RADIATION

X-ray Production

Conversion of Electron Kinetic Energy into X-rays


If an incoming free electron gets close to the nucleus of a target atom

The strong electric field of the nucleus will attract the electron

Changing direction & speed of the electron

Electron looses energy which will be emitted as an X-ray photon

Energy of this photon depend on the interaction between nucleus and

electron

X-rays originating from this process are called bremsstrahlung.



A large voltage is applied between two electrodes in an evacuated envelope

The cathode is negatively charged and is the source of electrons

the anode is positively charged and is the target of electrons

These electrons travel from cathode to anode & they were accelerated

The electrical potential difference between these electrodes attain kinetic energy

Emits Electromagnetic radiation, The name given as bremsstrahlung


A part or all of its energy is dissociated from it

An electron may have one or more bremsstrahlung interactions

Bremsstrahlung photon may have any energy up to the initial energy of the electron

Bremsstrahlung Spectrum



Bremsstrahlung is a German word "Strahlung" means “radiation”

"Bremse" means “brake”


The high energy electron can also cause an electron close to the nucleus in a

metal atom to be knocked out from its place.

This vacancy is filled by an electron further out from the nucleus.

The well defined difference in binding energy,

characteristic of the material, is emitted as a monoenergetic photon.

When detected this X-ray photon gives rise to a characteristic X-ray line in the

energy spectrum.




• Characteristic radiation are emitted at discrete energies

• hv = Ek-El

• Ek and El are the electron binding energies of the K shell and the L shell


The bremsstrahlung spectrum originates in the x-ray target.

The characteristic line spectra originate in the target and

in any attenuators placed into the x-ray beam.

A typical spectrum of a clinical x-ray beam consists of: • Continuous bremsstrahlung spectrum

• Line spectra characteristic of the target material and superimposed onto the

continuous bremsstrahlung spectrum


X-ray spectra are composed of:

1. Continuous bremsstrahlung spectra

2. In most cases, discrete spectra peaks known as characteristic x-rays.keV


E max

Bremsstrahlung xrays form a continuous energy spectra. The frequency distribution is continuous and shows that the Bremsstrahlung process produces more low energy that higher energy x-rays. The average energy is approximately 1/3 of the Emax.


The Emax or the maximum energy of

the x-rays measured as (keV) is equal

to

voltage applied to the Xray tube

(kilovolt peak or kVp).

E max

For example:An applied voltage of 70 kVp produces an x-ray spectra with Emax of

70 KeV and average energy of about 23 keV.

70KeV

23keV


Filtration

It is the removal of x-rays as the beam passes through a layer of material

Added FiltrationInherent filtration of the tube

Attenuation of the x-ray beam


Includes

the thickness (l to 2 mm) metal insert at the x-ray tube port.

Glass (Si02) and aluminum effectively attenuate all x-rays in

the spectrum below approximately 15 keV:

Inherent filtration

Dedicated mammography tubes,

require beryllium (Z = 4) to improve the transmission of low-energy x-rays


Added Filtration

Added filters attenuate the low-energy x-rays in the spectrum

That’s slow-energy x-rays are absorbed by the filters instead of the patient, The radiation dose is reduced. More uniform x-ray exposure to the detector

Other common filter materials copper and plastic (e.g., acrylic)

Aluminum (Al) is the most common ‘Added Filter material ‘"Bow-tie“

filters are used in CT to reduce dose to the periphery of the patient


Adjust the size and shape of the x-ray

field emerging from the tube port

Collimators

Adjustable parallel-opposed lead

shutters define the x-ray field

Collimation of the x-ray field is identified by the collimator's shadows


X-RAY GENERATOR FUNCTION


Electrical power available to a hospital up to about 480 V

20,000 V to 150,000 V needed for x-ray production

Transformers are principal components of x-ray generators

they convert low voltage into high voltage through a process called

Electromagnetic Induction


Induction of an electrical current in a wire conductor coil

by a moving magnetic field. The direction of the current is dependent on the direction of

the magnetic field motion.

Creation of a magnetic field by the current in a conducting coil. The polarity and magnetic field strength are directly dependent on the amplitude and direction of the current


x-ray generator circuit

Rectifier circuit (AC DC )

Rectifier circuits divert the flow of electrons in the high-voltage circuit

Direct current is established from the cathode to the anode in the x-ray

tube

Despite the alternating current and voltage produced by the

transformer


To avoid back-propagation, the placement of a diode of correct orientation in

the

high-voltage circuit allows electron flow during only .one half of the AC cycle

If an alternating voltage were applied directly to the x-ray tube, Electron back-

propagation could occur and the cathode is positive with respect to the

anode.

If the anode is very hot, electrons can be released by thermionic emission, and

such electron bombardment could rapidly destroy the filament of the x-ray tube.


Single-Phase X-Ray Generator

x-ray generator uses a single-phase input line voltage source (e.g 220 V at 50 A)

Produces either a single-pulse or a two-pulse DC waveform depending on the high-voltage rectifier circuits.

bridge rectifier routes the electron flow so that the electrons always emergefrom the cathode and arrive at the anode.

The x-ray tube current for a specific filament current is nonlinear below 40 kV due to

Space Charge Effect


Single-phase x-ray generator

Single-Phase full-wave rectified

circuit shows the basiccomponents and their

locations in the primary or secondary side of the

generator.


Three-Phase X-Ray Generator

x-ray generators use a three-phase AC line source

Three voltage sources,

each with a single-phase AC waveform oriented one-third of a cycle (120 degrees)

apart from the other two (i.e., at 0, 120, and 240 degrees).

high-powered triode, tetrode, or pentode circuits on the secondary side of the circuit

control the x-ray exposure timing.

Theses witches turn the beam on and off during any phase of the applied voltage

within

extremely short times


beam on / off during any phase

of the applied voltage within

Extremelyshort times

(millisecond or better accuracy)

very short exposure

times


Timing The X-ray Exposure In Radiography

Electronic Timers

Digital timers have largely replaced electronic timersDigital timer circuits have extremely high reproducibility and microsecond accuracy.

The timer activates and terminates a switch on either the primary or the secondary

side of the x-ray tube circuit

Precision and accuracy of the x-ray exposure time depends on the type of exposure

switch employed in the x-ray system X-ray systems

Use a countdown timer to terminate the exposure in timer or exposure switch failure


Phototimers

Phototimers are often used instead of manual exposure time settings in radiography.

Measuring the actual amount of radiation incident on the image receptor

Terminating x-ray production when the proper amount is obtained.

It helps provide a consistent exposure to the image receptor by compensating for

thickness and other variations in attenuation in a particular patient and from patient to

patient


X-ray Tube: Cathode

A filament : small coil of thin thoriated tungsten wire2 mm dia. & 1 to 2 cm long.

(A trace of thorium in the tungsten wire increases the efficiency of the electronemission and prolongs the life of filament)other materials : Molybdenum , Rhenium

The emission of electron from the filament based on thermionic emission- themetal is heated to sufficient temperature (about 2500 degree Celsius) to enablethe free electrons to leave the metal surface. The number of electrons emitteddepends upon the temperature. The higher the temperature, the greater is theemission of electrons.


The amount of thermion emission increases rapidly as the emitter temperature is

raised.

The emission is markedly affected by temperature changes. Doubling the

temperature of an emitter may increase electron emission by more than 107 times.

Small changes in the work function of the emitter can produce enormous effects

on emission.

Low work function, high melting point, High mechanical strength , high emission

Efficiency

Cathode


Tungsten: Φ = 4.52 eV

High melting point (3400 0C)

Thoriated tungsten (1-2 %): Φ = 2.63 eV (Th - 3.4 eV)

Thermionic emission – (1700 0C)

Can be drawn into very thin wire, Greater mechanical strength, withstands positive

ion bombardment, Low tendency to vapourise , Long structural & emission life.

Disadvantage:

High operating temperature (>2500 0K)

High work function

Low emission efficiency.

Commonly used Thermionic Emitters


The filament is embedded in a metal shroud called the focusing cup.

All electrons accelerated from cathode to anode are electrically negative, the

beam tends to spread out due to electrostatic repulsion, and some electrons can

even miss the anode completely.

The focusing cup is negatively charged so that it condenses the electron beam to

a small area of the anode.

There can be two filaments with focusing cups, providing

different focal spots on the anode.

Focusing Cup


.

Cathode design determines the size of focal spot

Factors determining the size of focus

1. Size and shape of filament

2. Dimension of focusing cup

3. Depth at which the coil is kept in the slot

4. Electric field associated with focusing cup

Focal spot size determines amount of x-rays falling on image receptor and resolution

(definition) of image

Focusing Cup


Cathode and focusing cup


Cathode: filament + focusing cup Two electrical currents flow in an x-ray tube.

The filament current : It is the flow of electrons throughthe filament to raise its temperature and releaseelectrons.

The electrical current: It is the flow of releasedelectrons from the filament to the anode across the x-ray tube. This current, referred to as the tube current,varies from a few to several hundred milliamperes(mA).

Cathode


Space charge

•Collection of negatively charged electrons in the vicinity of filament when no voltage applied btw cathode and anode – space charge

•Number of electrons in space charge remain constant

•Tendency of space charge to limit the emission of more electrons from the filament is called space charge effect

Filament current → filament temperature → rate of thermionic emission


Space Charge Effect

The two currents (filament and tube current) are separate but interrelated. Oneof the factors that relates them is the concept of “space charge.”

At low tube voltages, electrons are released from the filament more rapidly thanthey are accelerated toward the target.

A cloud of electrons, termed the space charge, accumulates around the filament.This cloud of negative charges opposes the release of additional electrons fromthe filament until they have acquired sufficient thermal energy to overcome theforce caused by the space charge.

At higher tube voltages, space charge cloud is overcome by applied potentialdifference.


At low filament currents, a saturation voltage is reached above which the currentthrough the x-ray tube does not vary with increasing voltage.

Influence of tube voltage and filament current upon tube current

At the saturation voltage, tube current is limited by the rate at which electrons arereleased from the filament.

Above the saturation voltage, tube current can be increased only by raising the filament’stemperature in order to increase the rate of electron emission. In this situation, the tubecurrent is said to be temperature or filament emission limited.

To obtain high tube currents and x-ray energies useful for diagnosis, high filamentcurrents and voltages between 40 and 140 kV must be used.

With high filament currents and lower tube voltages, the space charge limits the tubecurrent, and hence the x-ray tube is said to be space-charge limited.


Space charge cloud

Temperature limited Space charge cloudshield the electric

field for tube voltagesof 40kvp and less

( space charge limited )above 40kvp space

charge cloud isovercome by

voltage applied


Filament current Vs Tube current

• At low tube potential (40kVp and lower), tube current will be space charge limited.

• Space charge places upper limit on tube current (space charge compensation necessaryfor change in tube current)

• At higher tube potential, tube current will be emission limited.

• Emission limited tube current cannot be increased by increase in tube potential.

• Emission limited tube current can be changed only by increasing the filament heating.


Filament current Vs Tube current


.

The anode is provided with positive potential in the x-ray tube

Function of Anode

It serves as a target surface for the highly energetic electrons thereby becoming thesource of X-rays.

Serves as the electrical Conductor. ie, receives electrons emitted by the cathode andconducts them through the tube to the connecting cables and back to the high-voltage section of the x-ray machine.

Provides mechanical support for the target. Serves as the primary thermal conductor.


Anode: TARGETTARGET: The area of electron bombardment is the place where the both

heat and X-rays are produced. SO it should be made of a metal that is

able to withstand high temperatures without melting and is efficient in

the production of X-rays.


Tungsten is chosen as efficient target material because it has

High melting point (33600 C)

High Z element (more bremsstrahlung yield)

Fairly a good conductor of heat. So heat can be passed

reasonably quickly away from the small area where it is

produced and the rise in temperature at that area is prevented

from being too great.

It does not vaporize easily. The presence of metal vapour

inside an

X-ray tube would spoil the vacuum which essential for its proper

operation

It can be machined and made smooth. Smooth anode surface

avoids attenuation of a fraction of X-ray intensity


Focal Spot

Focal spot-the area on the anode, which is bombarded by the electrons. The focal spotbecomes the source of the X-ray in an X-ray tube.

The electron beam produced from an helical filament in a rectangular slot, coversrectangular area. That is focal spot is rectangular. The x-ray tube with rectangular focalspot is described as LINE FOCUS TYPE

Focal spot size depends upon the following factors:

The size and shape of the filament,

The dimension of the focusing cup and the depth of filament in it

Electric field associated with the focusing cup

The spacing between cathode and anode .


Focal Spot

To produce radiographic images with sharp edges the size of the focal spot should besmall. Ie, smaller the focal spot size better the spatial resolution of the image.

But…

As the size of the focal spot decreases, the heating of the target is concentrated in asmall area and damages the target area of X-ray production.


Focal Spot: Line focus principle

The conflicting demands can be facilitated byangling the target.

Sloping the anode face provides a larger area forheating, while maintaining small effectivefocal spot. The principle behind this design iscalled Line-focus principle. Effective size

Actual size


Anode Angle & Effective Focal spot size

Effective focal length =Actual focal length x sin


Angled Target: advantages & disadvantagesAdvantages:

Large actual focal size: more x-ray yield

: better heat dissipation

Small effective focal size: for image sharpness

Disadvantages:

Steeper anode angle restricts the field size

Heel effect


Anode angle and useful beam


Heel Effect

Reduction in the x-ray beam intensity toward the anode side of the x-ray field

X-rays are produced isotropically at depth in the anode structure.

therefore experience more attenuation than those directed toward the cathode side of the field

Photons directed toward the anode side of the field transit a greater thickness of the anode


Heel Effect


Anode Configuration

Two types of anodes:1. Stationary : The simplest type of x-ray tube has a stationary (i.e., fixed) anode. It consists

of a tungsten insert embedded in a copper block. The copper serves a dual role: It supports the tungsten target It removes heat efficiently from the tungsten target . Heat must be conducted away

quickly before it can melt the anode.

Dental x-ray and portable x-ray machines (where high tube current and power are notrequired) use fixed anode tube x-ray tubes.


Types of Anode Rotating Anode:

For most of the diagnostic x-ray application, the rotating anode is used. The purpose ofthe rotating anode is to spread the heat produced during an exposure over a large areaof the anode and consequently higher x-ray output capabilities.

In rotating-anode tubes the entire rotating disc is the target. The rotating anode is a heavy disc mounted on a Molybdenum stem, which functions as

its support. Disc is made up of W-Rh alloy, or Tungsten followed by graphite or Molybdenum base.


Types Of Anodes


Rotating Anode

To compare the target areas of typical stationary-anode and rotating-anodex-ray tubes with 1mm focal spot

(The area on the target that is struck by electrons)

Actual area of the ST is 1 mm x 4 mm = 4 mm2.

Diameter of Rotating anode = 7 cm, r of the target area is approximately 30mm. So Total target area = 754 mm2.


Advantage of Rotating Anode

Higher tube currents and shorter exposure times are possible with rotating anode.

The rotating-anode x-ray tube allows the electron beam to interact with a much largertarget area, and therefore the heating of the anode is not confined to one small spotas in a stationary-anode tube.

Less geometric un-sharpness and movement un-sharpness in the image due to smallerfocal spot and shorter exposure timings.


•Rotated Anode x-ray tube

Conventional X-ray Tube


Conventional X-ray Tubes

•Rotated Anode x-ray tube


Target

The target is the area of the anode struck by the electrons from the cathode.

Tungsten is the material of choice for the target for three main reasons:Atomic number-tungsten’s high atomic number, 74, results in higher-efficiency x-ray production and in higher-energy x-rays.Thermal conductivity-tungsten has a thermal conductivity nearly equal to that of copper. It is therefore an efficient metal for dissipating the heat produced.High melting point-any material, if heated sufficiently, will melt and become liquid. Tungsten has a high melting point (3410 oC compared with 1083 oC for Cu) and therefore can stand up under high tube current without pitting or bubbling

Further It can be machined and made smooth which avoids attenuation of a fraction of x-ray intensity .


Protective Housing The x-ray tube is always mounted inside a lead-lined protective housing designed to

control two serious hazards

Excessive radiation exposure Electric shock

When x-rays are produced, they are emitted isotropically i.e. with equal intensity inall directions.

Only those x-rays are used which are emitted through special section of x-ray tube,called the window.

The thin window serves to allow maximum emission of x-rays with minimumabsorption in the glass envelope.

Conventional X-ray Tube: Parts


A properly designed protective housing reduces the level of leakageradiation to less than 100 mR/hr at 1 m when operated at maximumconditions.

It also provides mechanical support for the x-ray tube and protects the tubefrom damage caused by rough handling.

The protective housing around some x-rays tubes contains oil that serves asboth an electrical insulator and a thermal cushion.

Some protective housings have a cooling fan to air-cool the tube or the oil inwhich the x-ray tube is immersed.

X-ray Tube


The x-ray tube is a special kind of vacuum tube.: 20-35 cm long and 15 cm in diameter.

The glass envelope is made of Pyrex glass to enable it to withstand the tremendous heatgenerated, maintains a vacuum inside the tube.

This vacuum allows for more efficient x-ray production and longer tube life.

The tube is evacuated to pressure less than 10-7 mm Hg If tube becomes glassy, x-ray production will fall off and tube will fail. As glass envelope tube age, some tungsten vaporize and coats the inside of the glass

envelope. This alters the electric potential of the tube, allowing tube current to stray andinteract with the glass envelope; the result is arcing and tube failure.

Glass Envelope


Because of this problem, a recent improvement in tube design incorporates metal rather than glass as part or all of the envelope.

Metal envelope tubes maintain a constant electric potential between the electron of the tube current and the envelope.

Therefore they have longer life and less likely to fail.


There are two primary parts; the cathode and the anode. Each of these is called an

electrode

Any tube with two electrodes is called a diode. An x-ray tube is a special type of

diode.

Conventional X-ray Tube


Those x-rays emitted through the window are called the useful beam.

Other x-rays that escape through the protective housing are leakage radiation –notdesirable.

X-ray Tube


Conventional X-ray Tubes


Factors affecting x-ray beamQuality and Quantity


• The energy of the x-rays is determined by the voltage applied.

• The amount of x-rays is determined by the current.


Factors affecting x-ray beam quality and quantity

• Anode material

• Voltage applied (kVp)

• Tube Current (mA)

• Filters used


Different anode materials will produce different characteristic x-ray spectra and different amounts of bremsstrahlung radiation.

1. Anode material


Note that increasing the applied voltage or kVp will increase the maximal energy, the average energy and the intensity of the x-rays. Characteristic x rays do not change with a change in kVp

40keV 75keV

2. Voltage (kVp)


100 mA

200 mA

75 keV

Increasing the current (ie mA) will not change energy of the beam only the intensity

(i.e. the amount) of x-rays. The quantity of x-rays is directly proportional to the tube

current.

3. Tube current (mA)


Power Ratings

The energy per unit time that can be supplied by the x-ray generator or received by the x-ray tube during operation.

Power delivered by an electric circuit is equal to the product of the voltage and the current

Power = 100 kVp X A max for a O.I-second exposure

Maximum tube current (Amax) for 100 kVp and 0.1-second exposure time

The maximal tube current for high-powered generators can exceed I,OOOmA (I A) for short exposures


Heat Unit (HU)

simple way of expressing

the energy deposition on and dissipation from the anode of an x-ray tube

Energy (HU) = Peak Voltage (kVp) X Tube Current (mA) X Exposure time

For continuous x-ray production (fluoroscopy),

the HU/sec is defined as follows

Energy (HU) = kVp X mA


X-ray Exposure Rating charts

Operational limits of the x-ray tube for single and multiple exposures and the

permissible heat load of the anode and the tube housing.

The single-exposure chart contains the information to determine whether a

proposed

exposure is possible without causing tube damage.

Rating chart is specific to a particular x-ray tube and must not be used for other

tubes.

Charts show the limitations and allowable imaging techniques for safe

operation


A single-exposure rating chart

Multiple-exposure rating charts

Anode heat input and cooling chart

Housing Cooling Chart

X- ray exposure rating chart


Anode heat input and cooling chart


Housing Cooling Chart


Thank you for your attention!!!

x ray production

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