production of x rays & generators
DESCRIPTION
Production of x rays & generatorsTRANSCRIPT
Production of X Rays
Rakesh C A
PRODUCTION OF X-RAYS
• X-rays are produced by energy conversion when fast moving stream of electrons is suddenly decelerated in the target anode of an x-ray tube.
X-ray tube is made of Pyrex glass that encloses a vacuum containing two electrodes - anode and cathode.
X-ray tubes are 2 types
1.Stationary anode x-ray tube
2. Rotatory anode x-ray tube
X-RAY TUBE STRUCTURE
Cathode
Anode
Filters
Tube housing
Tube envelope
Forms of tube used by Roentgenin 1895–1896 for the productionof X rays.
CATHODE
• Negative terminal of the x-ray tube.
• Contains 3 elements.
1. Filament: source of electrons for the x-ray tube.
2. Connecting wires: supply voltage (10V) and amperage (3-5 A) that heat the filament.
3. Metallic focusing cup: Made up of nickel and it surrounds the filament.
FILAMENT
• Made of Tungsten wire.• Diameter is about 0.2 mm• Coiled to form vertical spiral of 0.2
cm in diameter and 1 cm or less in length.
• Filament is source of electrons.
ADVANTAGES OF TUNGSTEN AS CATHODE
• High melting point (3370o C).• Little tendency to vaporize.• Ductility & Stability• Malleability and strength• Long life expectancy.
Disadvantages:-Not an efficient electron emitting material.
TUBE CURRENT
• Is the number of electrons flowing from cathode to anode per second.
• Measured in milli amperes(mA)• The tube current is unidirectional - from cathode
to anode.
THERMIONIC EMISSION•When current flows though filament, it becomes heated up, thermal energy is taken up by electrons and they move a small distance from the surface. •Emission of electrons resulting from absorption of thermal energy is k/a thermionic emission.
EDISON EFFECT
•The electron cloud surrounding the filament, which is produced by thermionic emission.
SPACE CHARGE• Electrons emitted from
the tungsten filament form a small cloud in front of the filament. This collection of negatively charged electrons forms space charge.
SPACE CHARGE EFFECT
• Tendency of space charge to limit emission of other electrons from filament.
EQUILIBRIUM STATE
• As the electrons leave the filament, it acquires a positive charge attracting some electrons back to itself.
• Number of electrons returning to filament is equal to number of electrons being emitted.
• As a result, space charge remains constant with actual number depending on filament temperature.
METALLIC FOCUSSING CUP
• Made of nickel.• Prevents bombardment of unacceptably large
target area
• The specially designed cup cause the electron stream to converge to the target area on the anode
• It is maintained at same negative terminal as that of filament.
• In grid controlled tubes the cup (-1.5 kV) acts as a switch
MODERN DAY X-RAY TUBES
• Contain single filament/ double filaments/ sometimes 3 filaments
DOUBLE FILAMENT ARRANGEMENT:-• They are placed side by side or one above the
other.• One filament is large, other one is small.• Only one filament is used for any fixed x ray
exposure. Larger filament is used for longer exposure.
DOUBLE FILAMENTS IN FOCUSING CUP
Filament for large focal spot
Filament for small focal spot
HIGHLY SPECIALISED X-RAY TUBES
• Tube with 3 filaments
• Stereoscopic angiographic tube
In this tube, 2 focal spots are widely separated producing stereoscopic film pair when 2 films are exposed.
Used in angiography.
AUTOMATIC FILAMENT BOOSTING CURCUIT
• When x-ray is turned on and no exposure is made, (as in fluoroscopy) stand by current heats the filament at low current (5mA).
• When exposures are needed, automatic circuit will raise filament current to required value and lower it to stand by after exposure.
ANODE
• The positive terminal of tube
STATIONARY ANODE ROTATING ANODE
STATIONARY ANODE
– Made of tungsten– 2-3 mm thick.– Embedded in large mass of copper– Triangular/ rectangular shape – Anode angle = 15-200
Adv of using tungsten:- 1. High melting point2. High atomic number3. Good absorption & dissipation of heat
Tungsten should be bonded with copper of anode to further facilitate heat dissipation
STATIONARY ANODE
ROTATING ANODE X-RAY TUBE
• Consists of 1. Rotating anode
2. Anode stem
3. Stator of induction coil
4. Rotor of induction coil
5. Ball bearings
6. Safety circuit
ROTATING ANODE
• Made of tungsten or alloy of tungsten with Rhenium.
• Has beveled edge • Angle of bevel is 6 to 20 degrees• Speed of rotation is 3000rpm practically
The purpose of rotating anode is to spread the heat produced during an exposure over a large area of the anode.
ANODE STEM• Made up of molybdenum • It has high melting point
but poor conductor of heat - It protects ball bearings from un-desirable heat
• The length of the molybdenum should be as short as possible
(↑length ⇒↑inertia⇒ ↑load
on the bearings)
STATOR OF INDUCTION COIL
– Provides magnetic field necessary for induction of current.
ROTOR OF INDUCTION COIL
– The magnetic field provided by stator induces current in copper rotor
– This current provides power for rotation of anode assembly
BEARINGS
• Increases life of the tube.
• Lubricant used is silver.
• Silver is suitable in vacuum
– Ball bearings
SAFETY CIRCUIT
• There is short delay (0.5 – 1 sec) between application of force and full rotation of anode due to inertia.
• Safety circuit prevents un-necessary exposure during this delay.
HALF LIFE OF ROTATING X-RAY TUBE
• Depends on roughing and pitting of surface of anode exposed to electron beam.
• Prevented by using alloy of 90% tungsten and 10% rhenium
Why increased speed of rotation?
∀ ↑speed of rotation ⇒ ↑ ability of anode to withstand heat
MODIFICATIONS TO INCREASE SPEED OF ANODE
1.Decrease anode-stem length (↓ inertia)
2.Use of two sets of ball bearings.
3.Decrease weight of anode (↓ inertia)
- compound anode disc
- molybdenum or graphite
Compound Anode
Compound anodes contain combination of rhenium, molybdenum, and graphite being application based
GRID CONTROL X-RAY TUBE
• It contains its own switch, which allows it to be turned on and off rapidly (as in cineflurography)
• Third electrode - focusing cup – is used to control the flow of electrons from filament to target.
• Focusing cup is negative to filament. (in conventional, it is connected to the filament)
• Voltage(-1.5 kV) applied between focusing cup and filament acts as switch
Emission limited OrTemperature limited
Using resistors
FOCAL SPOT
• Area of tungsten target i.e. bombarded by electrons from the cathode.
• Large focal spot – greater heat loading
v/s• Small focal spot – better resolution
Ans:Ans: LINE FOCUS PRINCIPLELINE FOCUS PRINCIPLE
Anode is inclined so it forms an angle with the plane perpendicular to electron beam. The angle is anode angle.Effective or apparent focal spot is smaller than actual focal spot d/t anode angle.
CathodeCathodeElectronsElectrons
Apparent Focal Apparent Focal spot sizespot size
202000 AnodeAnode
(-)(-)(+)(+)
Normal anode angle is 60 –200 • Apparent focal spot ∝ sine (anode angle)• For FFD 40 inch, anode angle is usually no smaller
than 150 (heel effect)
CathodeCathodeElectronsElectrons
Apparent Focal Apparent Focal spot sizespot size
202000 AnodeAnode
(-)(-)(+)(+)
HEEL EFFECT
• Intensity of x-rays depends on the angle at which the x-rays are emitted from the focal spot.
• The intensity of beam towards anode side is less than that towards cathode side.
• Intensity of the beam towards the anode side of the tube is less because of absorption of some of the x-ray photons by the target itself.
CLINICAL IMPORTANCE
1. Used for obtaining balanced densities in radiographs of body parts of different thickness, i.e. thicker parts towards cathode
2. When FFD is increased, heel effect is reduced.
3. For smaller films, less heel effect.
USEFUL RADIATION – PROJECTED TOWARD THE PATIENT
LEAKAGE RADIATION
HOUSING
TUBE HOUSING
• Consists of metal case made up of Aluminum alloy lined on the inside by a layer of lead which protects and supports the glass x-ray tube insert.
– Tube housing provides an efficient radiation barrier where in the x-rays produced in the x-ray tube are attenuated in all the directions except at the tube port.
– Provides shielding for the high voltages required to produce x rays
• The tube housing is packed with industrial grade oil to provide electrical and thermal insulation.
GLASS ENCLOSURE
• Is a sealed evacuated tube made up of borosilicate into which are mounted the anode and cathode.
• Accelerated electrons → collide with gas molecules → secondary electrons (less speed) →wide variation in tube current and energy of x ray produced.
• The purpose of the vacuum in the modern x-ray tube
is to allow the number and speed of accelerated
electrons to be controlled independently.
• The shape and size of these x-ray tubes are specially
designed to prevent electric discharge between
electrodes.
Disadvantages of glass enclosure:
• On long term use, tungsten vaporizes and form thin coat on inner surface of glass wall of x-ray tube.
• It gives bronze colored sunburn to x-ray tube.
Results of sun burning of x-ray tube
1. It filters x-ray beam and gradually changes its quality.
2. It increases probability of arcing between glass and electrodes at higher kVp which may puncture the tube.
SOLUTION ⇒ Metallic Enclosures
CERAMIC/ METALLIC X-RAY TUBES
• Has metal encasing and 3 ceramic insulators – high voltage cables and anode stem
• Anodes up to 2000 g can be supported (conventional tubes – 700g)
CERAMIC INSULATORS
• Made of aluminum oxide. They insulate high voltage parts of x-ray tube from metal envelope.
• The use of insulators allows a more compact tube design.
ADVANTAGES OF METALLIC ENCLOSERS
• Less off focus radiation.
• Longer tube life with high tube currents.
• Higher tube loading.
• Adequate electrical safety
• Compact size
OFF FOCUS RADIATION
It is due to electron back scatter from anode interacting with metal other than the focal track and striking anode a second time to produce X-rays.
Decreased by 1.Placing collimator2.Lead diaphragm as close to X-ray tube as possible.3.Using a metal enclosure – attracts off focus radiation to the grounded metal tube.
USEFUL
OFF-FOCUS
COOLING MECHANISM OF X-RAY TUBE
• Almost all energy put into x-ray tube is converted into heat and <1% is converted into x-rays.
The process of heat dissipation:
Conduction: Through solid parts of anode. Convection: Through oil surrounding the tube. Radiation : Occurring through the vacuum of the tube which passes off the heat to glass envelope or from metallic housing through air into the atmospheric air.
PROCESS OF X-RAY GENERATION
There are three basic requirements for X-Ray production in an X-Ray tube:
1. A source of electrons (cathode)
2. A target to stop the electrons (anode)
3. A method of accelerating the electrons from the source to the target (PD maintained across the tube)
1% of the incident energy is converted to X-Rays the remainder is converted into heat
Produced by 2 different processes
GENERAL RADIATIONCHARACTERISTIC RADIATION
• Reaction of electron with nucleus of tungsten atoms, producing x-rays.
• Collision between high speed electrons and electrons in the shells of target tungsten atoms.
GENERAL RADIATION
When electron passes near the nucleus of tungsten atom, the +ve charge of the nucleus acts on the –ve charge of the electron.The electron is attracted towards the nucleus and deflected from its original direction.The electron will lose some energy and slows down when its direction changes.The kinetic energy lost by the electron directly in the form of a photon of radiation called general radiation / bremsstrahlung.
CHARACTERISTIC RADIATION
Results when electrons bombarding the target eject electrons from the inner orbits of the target atoms.Removal of an electron from a tungsten atom causes the atom to have an excess positive charge, and the atom thus becomes a positive ion
CHARACTERISTIC RADIATION
In the process of returning to its normal state the ionized atom of tungsten may get rid of excess energy in two ways.
CHARACTERISTIC RADIATION
An additional electron (auger electron) expelled by the atom and carry off the excess energy - does not produce x-rays.
CHARACTERISTIC RADIATION
An alternative way to get rid of excess energy is for the atom to emit radiation that has wave lengths with in the x-ray range.
BRAKE RADIATIONCHARACTERISTIC
RADIATION
• Only about 5 % of electrons participate
• These 5% produce most of the X-Rays
• There is no ionization
• No characteristic radiation below 70 kV
• In the 70 to 150 kv range contribution is 10-28 %
• Ionization of target atom
Intensity of X ray beams
Intensity of an x ray beam=
Number of photons x Energy of each photon
•Varies with:– Kilovoltage (proportional to kVp2)– X ray tube current – Target material
• Atomic number – quantity(number) of bremsstrahlung – quality(energy) of characteristic radiation
– Filtration
FILTRATION
• The spectrum of x-ray beam includes a wide range of energies – low to high.
• For diagnostic purposes very low intensity x-rays are not useful & increase the patient dose and decrease the quality of the image.
• Hence the x-ray beam reaching the patient is filtered so as to make it most useful for image formation without increase in patient dose.
Filtration of x-ray beam – 2 types
Inherent Filtration: Provided by the tube envelope, tube housing and the tube port.
Added Filtration: Provided by the use of Aluminum and Copper across the beam path.
First X ray made in public.
• Hand of the famed anatomist, Albert von Kölliker, made during Roentgen's initial lecture before the Würzburg Physical Medical Society on January 23, 1896.
GENERATORS
Generator
Ordinary GeneratorMechanical energy Electrical energy
X-ray generator
device that supplies electric power to the X-ray tube
•Electric supply Generator X-ray tube
•Electric supply : 220 V, 50Hz, 3 phase, AC power
Why does x ray tube require electrical energy ?
• To boil of electrons from cathode - filament circuit(10V)
•To accelerate from cathode to anode - high voltage circuit(40-150kVp)
•To regulate the length of exposure - timer circuit
(-)(+)cathode anode
Generator
Control panel
- kVp
-mA
-Exposure time
Transformer assembly
-Grounded metal box
-Low voltage filament
transformer
-High voltage transformer
-Group of rectifiers
-Oil (Insulator)
•Electric supply : 220 V, 50Hz, 3 phase, AC power
•Filament circuit - 10 V
• High voltage circuit - 40-150 kVp
Transformer
•A transformer is a device that either increases or decreases the voltage in a circuit
•When current flows in primary coil it creates a magnetic field in the core, and this induces current in the secondary coil
•Current only flows in the secondary circuit when the magnetic field is increasing or decreasing,(i.e. when there is some potential difference between the two ends of primary coil), hence requirement for AC current
v
Primary coil
Secondary coil
Switch
Laws of transformer
1. The voltage in the two circuits is proportional to the number turns in the two coils
N P = VP
NS VS
• More turns in the secondary coil - step up transformer
• Fewer turns in the secondary coil - step down transformer
2. A transformer cannot create energy. An increase in voltage must be accompanied by a corresponding decrease in current
Vp Ip = Vs Is
• The output of Transformers is an alternating current(AC)
•Electric supply : 220 V, 50Hz, 3 phase, AC power
•Filament circuit - 10V
• High voltage circuit - 40-150kVp
Step down transformer
Step up transformer
Autotransformer
Electric supply Autotransformer Generator X-ray tube
Principle of self induction
110v/55
230v/115
320v/160
230v/115
Turns of windings
Incoming power voltage
Step down transformer(filament)
- 10 V
-Turns in secondary coil less
Step up transformer (cathode to anode)
- 40-150 kV
- Turns in secondary coil > 600
Autotransformer
Alternating current
(-)(+)
X-ray tube
PROBLEM- Output of Transformers is alternating current(AC)
Alternating current
(+)
X-ray tube
PROBLEM- Output of Transformers is alternating current(AC)
(-)
SOLUTION: RECTIFY!!!
Rectification
Electric supply
Autotransformer
Generator
Rectifier
X-ray tube
•Rectification is the process of changing the alternating current into direct current
•A rectifier is the device that allows electrical current in one direction but does not allow current to flow in the other direction
AC
DC
High voltage rectifiers
• Two types– vacuum – tube type (thermionic diode tubes)
– solid state rectifier
• Modern x-ray tubes use solid state rectifier which are more reliable and have a longer life.
• Selenium was the first material and silicon is most widely used in modern days.
• The heart of a solid state rectifier is a semi conductor.
Solid state semiconductor - Rectifier
Conduction band
Forbidden gap
Valency band
Solid state semiconductor - Rectifier
Conduction bandForbidden gap
Valency bandConductor
Solid state semiconductor - Rectifier
Conduction band
Forbidden gap
Valency band
Insulator (10 eV)
Solid state semiconductor - Rectifier
Conduction band
Forbidden gap
Valency band
Semi-conductor (1 eV)
Semiconductor acts like an insulator at low temperature and acts like a conductor at room temperature
N-type semiconductors
• The material with five valence electrons is added as a impurity to silicon lattice creating a extra electron.
• Impurity- arsenic or antimony.(N – Negative)
• These are called donors.
P-type semiconductors
• In this an impurity with only three valance electrons is added to silicon crystal.
• This leaves behind a Hole (P – Positive)
• Impurity- indium, gallium and aluminum.
• These are called acceptors.
P-N junction
• It is formed when N-type and P-type crystals are joined.
• This will create a depletion layer which is opposite in polarity to the adjacent material.
• If voltage is applied across current will flow or not flow depending upon the polarity.
• Forward biasWhen the negative pole of a
battery is connected to the N-type and positive pole to the P-type.
Electrons will start flowing from negative to positive direction.
• Reverse biasWhen negative pole is
connected to P-type and positive to N-type.
Electrons will stop flowing.
•P-N diode conducts current in a forward direction only, hence it meets the definition of a rectifier.
1 cycle
1/60 sec
Two rectifiers connected in series with X-ray tube
Half wave rectification
1 PULSE
Advantage:Prevents damage to the tube in inverse cycle.
Disadvantage:Half of the power will be wasted.
1 cycle
1/60 sec
Diode bridge
Full wave rectification
2 PULSE
Seen in Modern X ray tubes.This is brought about by using 4rectifiers connected in
opposite circuits.Advantage:
Utilizes full electrical potentialPrevents tube damage.
PULSED POTENTIAL
Fluctuating voltage from 0 to its Max.
Disadvantage:
- Loss of exposure time
- Low energy X-rays
-Increased patient dose
- Tube life
1 cycle
1/60 sec
2 PULSE
Three phase generators
•Three phase generator consists of three single phase currents out of step with each other by 1200
00
1200 2400
•Supply power at near constant potential
•2000 mA, 150 KV, 1 msec
Three phase transformers
• Consists of three sets of primary and secondary windings.
• Two types of configurations:
Delta
Wye (star)
• Generally Primary – delta
Secondary – wye / delta
Six pulse six rectifier
Six solid state rectifiers are used.
Six max. positive voltages per cycle
Six pulse twelve rectifier.
Employs 12 rectifiers
Advantage:
allows a 150 kV generator to allow a wide range of voltage (-75 to +75 kV)
Twelve pulse
Secondary winding will have both delta and wye connection
Advantage:
Delta will lag the wye by 30degrees which fills the ripple
Ripple factor
•The ripple factor is the variation in the voltage across the x-ray tube expressed as the percentage of maximum value
Single phase 100%
Triple phase
Six pulse six rectifier 13.5%
Six pulse twelve rectifier 13.5%
Twelve pulse 3.5%
Advantages• Produces X rays efficiently throughout the
exposure.
• No time is spent in bombarding the X ray tube with low energy electrons.
• High tube ratings for extremely short exposure times.
• Produces radiographs with short exposure times and high repetition rates.
kVp
mA
AUTOTRANSFORMER
150 kVp to rect.
.
..
.
.
.
STEP UP
STEP DOWN
TO X-RAY TUBE FILAMENT
.
-11OV
+11OV
+75kVp
-75kVp
Transformer rating
•The rating of a transformer states the maximum safe output of its secondary winding , expressed in kilowatts
kW = kV x mA
1000
•Transformer rating is determined when the generator is under load ( 100 kVp)
Power storage generator
•Provides means of supplying power for the tube independent of an external power supply ( mobile radiographic equipment)
Battery powered generator
Electric supply
Nickel-cadmium battery
DC chopper
Transformer
Rectifier
X-ray tube
•Supplies constant kVp and mA
Storage generators
Capacitor discharge generator
Electric supply
Step up transformer
Large capacitor
X-ray tube
•KV falls during exposure at the rate of 1KV for each mA
Advantage:Small and easy to move.
Limitations:Must be charged prior to use.Cannot be used for thicker
body parts – abdomen
Limitation:Heavy and requires regular
battery maintenance.Advantages:
Stores considerable energyIndependent of power supply
Medium frequency generator
• V = f n A
Electric supply (50 Hz)
Invertor (DC chopper)
Power (6500 Hz)
Transformer
13000 Hz constant potential
• Transformer is made more compact and small
• Constant nearly ripple free voltage to the X-ray tube
•Produce X ray exposure in shortest possible time by operating X ray tube at its maximum Kilowatt rating.
•Disadvantages:Causes Focal spot bloomingReduces Tube lifeExpensive
Falling load generators
Exposure switching
•A switch is a device that turns the high voltage to the X-ray on and off
Primary switching
- Takes place in the primary coil
- All general purpose generator
- Easier and cheaper
- 1-2 millisecond
-Cannot produce repeated exposure
Silicon controlled rectifiers (thyrister)
Secondary switching
- Secondary circuit of high voltage
- Special purpose generator like in angiography , cinefluoro
- Costly
- 0.5 ms (shorter exposures)
- Repeated exposure
Triode vacuum tube
Grid controlled X-ray tubes
SOLID STATE RECTIFIERS
Exposure timers
•To control the length of an X-ray exposure
Exposure timer
Electromechanical Automatic exposure control Pulse counting timer
Photomultiplier
Ionisation chamber
Solid state
X-RAY GENERATOR-THEN
VALVE RECTIFERS
STEP UP TRANSFORMER
STEP DOWN TRANSFORMER
DUAL TUBE 800 mA HIGH FREQUENCY GENERATOR & CONTROL ASSEMBLY IN SINGLE CABINET
HT
CAPACITORS
TIMER & ANODE CONTROL
MICROPROCESSORS
X-RAY GENERATOR- NOW !
The first medical X ray takenDrs. Edwin and Gilman Frost perform the first American medical X-ray in Reed Hall, Dartmouth College on February 3, 1896
The first American medical X-ray : 14-year-old Eddie Murphy’s fractured ulna.
