X RAYTUBE

X RAY GENERATORS AND X RAY TUBE CIRCUIT Transformers Laws of transformers Autotransformer Filament circuit High voltage circuit Rectification: Type of rectifiers and types of rectification Types of generators: Three phase, power storage,

battery powered, medium frequency. Transformer rating Exposure switching Falling load generators Exposure timers

PRODUCTION OF X RAYS ( X RAY TUBE )DIAGNOSTIC X RAY TUBESGlass enclosureCathode, Line focus principleAnode, rotating anode.Grid controlled x ray tubesSaturation voltageHeel effect

TUBE RATING CHARTSANODE COOLING CHART

  

DIAGNOSTIC RADIOGRAPHIC EQUIPMENT

All diagnostic x-ray equipment have three basic components:– the x-ray tube– the operation console– the high-voltage generator (discussed later)

TRANSFORMER

THE CONTROL PANEL

Three primary controls:– kVp, quality–mA, quantity– Time (s), quantity • Auxiliary controls(anatomical programsand AEC controls)

MAIN X-RAY CIRCUIT

• Two divisions to the main x-ray circuit:

Primary or control console section• Incoming current• Exposure switch• Autotransformer• Primary winding of the step-up transformer

Secondary or high voltage section• Secondary step-up transformer• Full-wave rectification circuits• Wiring leading to & from the x-ray tube

MAIN X-RAY CIRCUIT

Primay

Secondary

FILAMENT X-RAY CIRCUIT• mA Selector– is a Rheostat (variable

resistor)– Adjusts resistance and is represented by the mA

stations on the control panel

• Filament step-down transformer, 10 V, 3-5 Ma.

– Responsible for changingamps into milliamps.

RECTIFICATION

• Process of converting alternating current(AC) to direct current (DC)

• Required to ensure electron flow in one direction – from cathode to anode

– Half-wave– Two recitfiers increase

heat load capacity and

protect the x-ray tube

RECTIFIER

RECTIFICATION

– Full-wave

– Four rectifiers create a routing system sending electrons through the x-ray tube the same way every time, in effect creating DC

CHARACTERISTICS OF INCOMINGLINE POWER

• Incoming line power may be 110 or 220 volts and 60 Hz.• The usual voltage taken by the equipment is 210-220v.• May need an additional transformer to

stabilize incoming voltage

CHARACTERISTICS OF INCOMING LINE CURRENT

• Phasing– Single-phase– Three-phase, six pulse– Three-phase, twelve pulse• High Frequency

SINGLE-PHASE POWER

• Allows the potential of the main current to drop down to zero with every change of the current flow.

• It has a single wave form.

THREE-PHASE POWER

Has three waves of power flowing at evenlyspaced intervals from each other:– One wave is starting before the previous wave is

depleted

– The overallWave form neverreaches zero

THREE PHASE UNITS Need to increase pulse repetition rate to deliver high x

ray flux in a short period of time Three phase units: voltage between any pair of 3

wires

HOW PHASES AFFECT GENERATOR OUTPUT

• Single phase mode– the voltage always drops down to zero– 100% ripple

HOW PHASES AFFECT GENERATOR OUTPUT

• Three phase– Individual voltages drop to zero but there isalways an overlap of wave pulses.–When wave pulses are rectified, the averagevalue never drops to zero.

•Makes x-ray production more efficient• Easier on the equipment

HOW PHASES AFFECT GENERATOR OUTPUT

• Three-phase, 6-pulse– produces a 13% voltage ripple– voltage supply to x-ray tube never falls below

87% of maximum value.

Three-phase, 12-pulse– 4% voltage ripple isproduced– value of the voltagenever falls below 96%of maximum value.

POWERED GENERATORCAPACITOR DISCHARGE MOBILE UNITS

• A capacitor builds up a charge when thecircuit is closed (when exposure button ispushed)• When pre-selected charge is reached, thecapacitor completes the circuit & sends thecharge to the x-ray tube.• Disadvantage - x-ray production falls offthroughout exposure (end kV is approx. 1 kVper mAs lower than starting kVp)

POWERED GENERATOR BATTERY-OPERATED MOBILE UNITS

• A nickel-cadmium battery supplies thenecessary charge to produce quality x-rays• Production is of higher quality– They obtain higher rms* voltage– No possibility of leakage– Combined with High Freq. Technology– Mobile was first to benefit from High Freq.

FALLING-LOAD GENERATORS

• Used to provide the highest mA settings at theshortest time possible.– The operator selects mAs setting not the time for a

selected kvp only.– The computer automatically calibrates the time ofexposure (allowing consistently shorter exposures)– The mA is therefore controlled by the falling-loadgenerator.– The mA starts at the highest possible setting and“falls” throughout the exposure.

• This causes the kV to fluctuate slightly throughoutexposure.• Can shorten tube life due to constant use of high mA

FALLING LOAD GENERATOR

AUTOMATIC EXPOSURE CONTROL (AEC)TIMERS

Photomultiplier (old type)– Uses a fluorescent screen & converts the

lightproduced by the screen to an electrical charge

by photocathode , multiplied by dinodes.–When a pre-selected charge is reached, thephotomultiplier terminates the exposure.–Must be located behind the film!

AUTOMATIC EXPOSURE CONTROL(AEC) TIMERS:

• Ionization chamber– Uses radiolucent material located in front of the film– As x-rays pass through cell, they ionize the cell, whenpre-set ionization level is reached signal is generated toterminate exposure.– Cells must be calibrated to a particular film/screencombination when installed– The body part in front of the cell determines how long

it takes for the pre-set ionization level to be reached. Collimation is important otherwise termination can

occur before proper exposure is made.

PROBLEMS WITH MINIMUM REACTION TIME• The time needed for the AEC & generatorto terminate an exposure, also calledresponse time– Short exposure times must be long enough toget a reading from the AEC & to the generator.

• It is easy for the machine to overexpose the radiograph

• Fast film & screen speeds also contribute to this problem.

• This was a problem with older units and is less of a factor today

BACKUP TIMERS WITH AECS

A safety device used to terminate the exposure if the AEC fails to do so

As a rule, the backup timer cannot exceed the tube limit & it should be set at 150% of the expected manual exposure mAs

MANUAL TIMERS

Synchronous timer – uses a synchronous motor

that turns a shaft at 60 rps (times are a subdivision

of this 1/60, 1/20, 1/30). Electronic timer – most sophisticated and most

accurate, based on time required to charge acapacitor through a variable resistor. Accurate

down to 1ms. mAs timer – used with falling load generators,

monitors product of mA and time (tube current)and is the only timer located in the secondary

circuit.

X-RAY TUBE DESIGN

Filament is heated, releasing electrons via thermionic emission (Vf ~ 10V, If ~ 4A, resulting in T>2000oC)

X rays are produced by high-speed electrons bombarding the target

Typically < 1% of energy is converted to x rays; the rest is heat

Vaccum

X-RAY TUBE CURRENT

Electron cloud near the filament creates space charge region, opposing the release of additional electrons

Increase in tube voltage increases tube current; limited by filament emission

High filament currents and tube voltage of 40 to 140kV must be used

X-RAY TUBE: POWER SOURCE

• The source of electrical power is usually ac (easier to transmit through power lines)

• X-ray tubes are designed to operate at a single polarity: positive anode, negative cathode

• Need to manipulate available power source (suppress or rectify wrong polarity)

• The highest x-ray production efficiency can be achieved at a constant potential

ALTERNATING CURRENTS AND VOLTAGES

TUBE HOUSING

TUBE HOUSING

Made of cast steel & is usually lead-lined Provides for absorption of most off-focus

radiation.

Purposes:– Controls leakage & off-focus radiation– Isolates high voltages– Helps to cool the tube

GLASS ENVELOPE

Surrounds entire cathode & anode assemblies except for the stator

Made of several layers of Pyrex w/ varying densities

– Glass is fitted to the metal of the anode & cathode ends

–Must be airtight to maintain a good vacuum

GLASS ENVELOPE

A target window is constructed in the glass envelope to allow less scatter & attenuation of the photons

– In most tubes - simply a thinner “cut” of glass

– In mammography - a special metallic beryllium window prevents attenuation of lower energy photons

CathodeThe cathode is the negative end of the x-ray tube.– Made up of the filament(s) and a focusing cup.Filament Most x-ray tubes have a dual filament cathode

assembly - also known as dual focus– The two filaments sit parallel to each other in the

focusing cup & share a common ground wire.–Most filament coils are 7-15mm long , 1-2mm wide, 0.1-0.2mm thick

Filaments must be able to:– Boil off electrons (thermionic emission)–Withstand great amounts of heat No vapourisation Filament materials– Tungsten - most widely used material• High boiling point (3,370° C)• It is difficult to vaporize– Rhenium (3,170° C)–Molybdenum (2,620° C)

Vaporization occurs over time–When the particles vaporize (turn into a gaseous

form), they solidify on the glass of the x-ray tube, called sun-burning or sun-tanning of tube.

• Reduce the x-ray output of the tube• destroy the vacuum integrity of the tube, leads

toarcing and ultimately tube failure• Thorium (a radioactive metallic element) is

added to the filament material to make the tube last longer.

Automatic filament boosting circuit, increases current only for exposure.

FILAMENT DEPOSIT

Focusing Cup• The focusing cup helps control electron cloud– The electrons repel each other & want to spread out. The focusing cup forces the electrons to form a small stream as they move toward the target material–Made of nickel– Has a low negative charge

GRID-CONTROLLED FOCUSING CUPS

Some x-ray procedures require exposures be taken at quick intervals.

• Grid-controlled focusing cups have a variable charge applied to the focusing cup that acts as an exposure switch

–When the tube is activated, the charge increases & decreases rapidly

– Short bursts of electrons flowing to the target.

GRID-CONTROLLED FOCUSING CUPS

• May be found in:– portable capacitor discharge units– digital subtraction angiography– digital radiography– Cineradiography

Anode The Anode is the part of the x-ray tube where

accelerated electrons move to after kV is applied to the tube.

• Two types:– Stationary anode (old type) - just a tungstenbutton imbedded in copper bar.– Rotating anode consists of a molybdenum

disk(target) rotated by an induction motor. Should be larger than required for cooling. Distortion by expansion on the surface

ROTATING ANODE

ROTATING ANODE STATOR AND ROTOR

Consists of two main parts:• Stator– Rests just outside of the glass tube– Made up of a series of electromagnets equally

spaced around the neck of the tube• Designed to energize opposing pairs, in

sequence, so that they induce the rotation of the rotor.

• Rotor– Located within the glass tube– Made up of copper bars & soft iron around a

molybdenum shaft.mutual induction. Heat should be radiated

ROTATING ANODE STATOR AND ROTOR

• When the rotor is rotating at the desired level, the x-ray exposure may be completed.• Most revolve at 3400 revolutions per minute (rpm) minimum.•Inertia control• safety circuit: prevents exposurebefore full speed.•Molybdenum stem•By rotating the anode we spread the generated heat over a larger surface area allowing greater technique loads

ANODE TARGET CHARACTERISTICS

• Anode target - the point on the anode where the electrons strike

• Tungsten – rhenium alloy is the most common material and is plated onto the surface of the molybdenum disk

• Tungsten has:– High atomic number (74)– High thermal conductivity level– High melting point• Rhenium added to increase thermal capacity

and tensile strength

THE LINE-FOCUS PRINCIPLE

• Actual focal spot - the area of the target material being bombarded by electrons from the filament.

• Effective focal spot - the imaginary geometric line that can be drawn based on the actual focal spot size vs. the angle of the anode.

• Best described by the angle of the anode– the smaller the angle of the anode, the smaller

the effective focal spot size (any angle <45 results in the effective FS being smaller than the actual FS)

– 12 degree target angle most common because it is the minimum that will cover a 14x17 at 40”

When are large focal spot sizes favored : A large focal spot is favored when a short exposure time is the priority

What are small focal spot sizes favored A small focal spot is preferred : when spatial resolution is a priority.

THE LINE-FOCUS PRINCIPLE CONT.

THE ANODE HEEL EFFECT

Caused by the angle of the anode vs. the intensity of the electrons striking it.

• X-rays exiting the target on the anode side have to traverse the “heel” of the anode

– Photons directed toward the cathode end do not have to travel through as much of the anode because of the angle of the target so more make it out

– Those directed toward the anode end must travel through more material so more are absorbed

– Results in the beam being of lower intensity on the anode side.

The Anode Heel Effect• As much as 20% more

photons at the cathode end of the tube & as little as 25% fewer photons at the anode end of the tube.

Most noticeable with:– Small focal spot– Large field

PRODUCTION OF OFF-FOCUS RADIATION

• Radiation produced from x-ray photons or electrons that have reflected off the anode

• These x-rays or electrons can strike a number of things in the tube and produce scatter photons:

• Because they are not produced in the focal track they are “off-focus” and while most are absorbed by the housing, some make it out of the tube and degrade the radiographic image.

EXTENDING TUBE LIFE• Practical methods• Tube rating charts– Determines if a technique is safe– Used to test overload protection circuits• Calculating heat units and using cooling charts.

PRACTICAL METHODS

The life of the tube is under your control!• Proper warming extends tube life• Avoid repeated exposures close to tube load limit• Do not hold the rotor switch unnecessarily

Rating of diagnostic tubes •Focal spot loading determines the maximum

permissible exposure: there is a maximum power that can be tolerated before target starts melting (T melting=3400oC for tungsten)

•Anode cooling and housing cooling rates determine the number of exposures that may be given in a sequence

TUBE RATING CHARTS

• Rules for use– Select the correct chart– Plot the point using technical factors– ANYTHING ON OR ABOVE THE GIVEN mA LINE

IS UNSAFE

TUBE RATING CHARTS

CALCULATING HEAT UNITS (HU)

kV x mA x time (s) x Cr x # of exposuresThe heat unit rectification constants (Cr) are:–1 φ 2 pulse (full wave) = 1.00–3 φ 6 pulse = 1.35–3 φ 12 pulse = 1.41–High frequency = 1.45An anode cooling curve based on the tube’s

rating chart must be used when calculating multiple exposures.

CALCULATING HEAT UNITS (HU)

If 10 exposures of 80 kVp, 200 mA & 0.43 s. is made on a high frequency unit, how many heat units (hu) are produced?

kV x mA x time (s) x Cr x # of exposures 80kVp x 200mA x 0.43 sec x 1.45 x 10 =

99,760 hu If the anode is at its maximum how long

must we wait before making the exposures?

ANODE COOLING CHART

REFERENCES  Christensen’s physics of diagnostic radiology, fourth

edition.Thomas s. Curry, james E Dowdy, Robert C Murry.

  Oral Radiology principles and interpretation, 6th

edition, Stuart C. White, Michael j. Pharoah.  Bushberg et. al. The essential physics of medical

imaging, 2nd edition.  Text: H.E Johns and J.R. Cunningham, The physics of

radiology, 4th ed.

THANK YOU