X-RAY GENERATORS AND TRANSFORMERS DR.SHWETA SHENDEY

X-RAY GENERATOR Normally generator means electrical generator, which convert mechanical energy into electric energy But X RAY GENERATOR is a device that supplies electric power to the x-ray tube An x ray generator begins with a source of electrical energy. Most radiology departments have a three phase power available in range of 208 to 230 V. X-ray generator just modifies this energy to meet the needs of x-ray tube.

An x-ray tube requires electrical energy for 2 purposes 1. To boil electrons from the filament 2. To accelerate these electrons from cathode to anode The x-ray generator has a circuit for each of these functions: 1. Filament circuit 2. High voltage circuit 3. Timer circuit- for regulating the length of x-ray exposure

High Voltage Transformer X-RAY CIRCUIT High Voltage Transformer Rectifier Timer Circuit + Auto- trans-former Line mA selector Filament Transformer

X-ray generator has 2 components: CONTROL PANEL TRANSFORMER ASSEBLY 1. Control panel / console- To select kVp, mA and exposure time for a particular radiographic examination Two exposure buttons: One button- readies the x ray tube for exposure by heating the filament and rotating the anode. Second button- starts the exposure. The timing mechanism terminates the exposure.

CONSOLE

2. Transformer assembly : - a grounded metal box filled with oil. Contains - low voltage transformer (filament circuit) - high voltage transformer (high voltage - a group of rectifiers. circuit ) Contains circuits with potential difference as high as 150000V. So immersed in oil, which acts as a insulator.

TRANSFORMER ASSEMBLY

TRANSFORMER Defn: a device that either increases or decreases the voltage in a circuit. Incoming Power supply- 230 V, 60 hz AC. Filament heating requires- 10 V Electron acceleration requires- b/w 40000 and 150000 V So transformers are used to change the voltage of incoming power supply to appropriate levels.

Design : Two wire coils (copper) wrapped around a closed core. TRANSFORMER Design : Two wire coils (copper) wrapped around a closed core.

Core- made of tightly clamped thin sheets of special iron alloys separated from each other by thin insulating layers- to prevent eddy currents. Primary circuit Secondary circuit

WORKING OF TRANSFORMER PRINCIPLE: When current flows through the primary coil , it creates a magnetic field within the core, and this magnetic field induces a current in the secondary coil. Current flows through the secondary coil only when the magnetic field is changing (increasing/ decreasing ). No current flows when the magnetic field in core is in a steady state.

+ - So steady DC (from a battery) cannot be used . AC is used for transformers because it is produced by a potential difference that changes continuously in magnitude and polarity. So it produces a continuously changing magnetic field in the core. AC in primary coil AC in secondary coil + -

NP VP ----- = ----- NS VS LAWS OF TRANSFORMERS 1. The voltage in two circuits is proportional to the number of turns in the two coils. Np, Ns = No. of turns in primary and secondary coil Vp, Vs = Voltage in primary and secondary coil NP VP ----- = ----- NS VS

Transformer Types Step up Transformer No. primary coils < No. secondary coils Primary voltage < Secondary voltage Step down Transformer No primary coils > No secondary coils Primary voltage > Secondary voltage

LAWS OF TRANSFORMERS Vp Ip =Vs Is 2. The product of voltage and current in two circuits must be equal. Vp Ip =Vs Is Vp, Vs - voltage in primary and secondary coil Ip, Is - current in primary and secondary coil Restatement of law of conservation of energy. A transformer cannot create energy. An increase in voltage must be accompanied by a corresponding decrease in current.  

Ex: Vp=100 V, Vs=30000 Ip=30A, Is= ? Vp Ip =Vs Is 100 x 30 = 30000 x Is Is= 0.1 A So, step-up transformer- increases voltage and decreases the current Step-down transformer- decreases voltage and increases the current.

AUTOTRANSFORMER Voltage supplied to the x-ray room is connected to the x-ray generator through an autotransformer. Functions: 1. Supplies voltage for x-ray tube filament circuit 2. Provides voltage for primary of the high voltage transformer. 3.Provides a convenient location for kVp meter. Consists of SINGLE WINDING wound on a laminated closed core. Works on the principle of SELF INDUCTION

High Voltage Transformer X-ray Circuit High Voltage Transformer Rectifier Timer Circuit + Auto- trans-former Line mA selector Filament Transformer

AUTOTRANSFORMER

Thus, within a very limited range autotransformer can function as a step up or step down transformer.

X-RAY CIRCUITS Two basic circuits in diagnostic x-ray unit 1. Filament circuit- regulates the current flow through filament of x-ray tube. -It contains a step-down transformer - FILAMENT TRANSFORMER - provides power to heat the filament. - Current flow of 3-5 A and voltage of 10 V- sufficient to heat the filament to necessary high temperature to cause emission of electrons (thermionic emission).

FILAMENT CIRCUIT

FILAMENT CIRCUIT Precise control of filament heating is critical because a small variation in filament current results in large variation in x-ray tube current. EX: 5% change in filament voltage causes 20- 30 % change in x-ray tube current. Done by adding resistors in the filament circuit.

2. HIGH VOLTAGE (cathode- anode) CIRCUIT : Contains two transformers- Auto-transformer ( kVp selector) - located in control panel - can be adjusted in steps from 40 to 150 kVp Step-up( high voltage ) transformer- -increases voltage by a factor of approx.600. -potential difference across secondary coil as high as 150,000 V – so immersed in oil.

HIGH-VOLTAGE CIRCUIT

Two meters are incorporated into high voltage circuit. 1. Voltmeter – measures potential difference across the tube (kVp) and placed in circuit between autotransformer and step-up transformer 2. Ammeter- measures actual current flow across tube( mA) and its connections are in secondary coil of high voltage transformer to record current flow accurately. Though both are located in control panel , their connections are in high voltage circuit.

RECTIFICATION: Process of changing AC DC Device that produces the change - RECTIFIER. RECTIFIERS: Allow current to flow in only one direction. High voltage rectifiers can be- 1. Vacuum-tube type- obsolete now 2. Solid-state type- Present in most of modern day generators Heart of a solid state rectifier is a semiconductor (usually a piece of crystalline SILICON).

SEMICONDUCTORS P-type silicon semiconductor Impurity-Indium/Gallium/Aluminum N-type silicon semiconductor Impurity- Arsenic/Antimony

SEMICONDUCTORS The process of forming a PN junction (diode) Forward bias of a PN diode

SOLID STATE RECTIFIER (diode)

The circuit for self-rectification HALF WAVE RECTIFICATION- The circuit for self-rectification

SELF RECTIFICATION - when x-ray tube itself acts as rectifier. Two disadvantages of self rectification- 1. Half of available electrical cycle not utilized to produce x-rays- so exposure time is doubled. 2. Repeated / prolonged exposures heat the anode- become hot enough to emit electrons- bombard the filament and destroy it. This is not seen with rectifiers - so, they protect x-ray tube from the full potential of inverse cycle.

HALFWAVE RECTIFIER CIRCUIT

FULL WAVE RECTIFICATION

Disadvantage of pulsed radiation (single phase generators) Single phase input power results in pulsed radiation Considerable portion of exposure time is lost while the voltage is in the valley between two pulses

Disadvantages intensity of x-rays produced is significant only when voltage is near peak low voltage heats target and produces low-energy x-rays- absorbed in patient- raise patient dose

TYPES OF GENERATORS: THREE PHASE GENERATORS Commercial power generally delivered as 3 phase with phases 120o apart Has three waves of power flowing at evenly spaced intervals from each other: – One wave is starting before the previous wave is depleted – The overall waveform never reaches zero

Three basic types of 3 phase generators -Six pulse , six rectifier -Six pulse, twelve rectifier -Twelve pulse

3 Phase Generator Circuits Pulses number of peaks per 1/60 second power line cycle of output current. Windings 3 primary coils (one for each phase) 3 or 6 secondary coils

THREE PHASE TRANSFORMERS Three sets of primary and secondary copper windings Windings arranged in two configurations- DELTA or WYE (STAR)

Output voltage has same max Output voltage has same max. value but 30degree shift in phase between the two. Generally , primary windings –delta secondary windings- often wye or both

Six pulse six rectifier one primary delta one secondary wye six rectifiers When rectified, there will be 6 positive maximum voltages per cycle- hence “six pulse”

Six pulse , six rectifier generator Transformer

Has a fixed potential to ground- an advantage over 6 rectifier circuit 6-Pulse Twelve Rectifier 1 delta primary 2 wye secondaries 12 rectifiers Has a fixed potential to ground- an advantage over 6 rectifier circuit This simplifies insulating requirements of generator.

Six pulse , twelve rectifier

Twelve pulse generators

Similar to 6 pulse 12 rectifier transformer but, secondary has a Delta and wye connection. So output of delta lags behind wye by 30 degrees - output of one winding will fill in the ripple of other- resulting in 12 pulse output.

Advantages of 3 phase generators over 1 phase. Produce a nearly constant potential with very low ripple factor Produce x-rays efficiently throughout exposure (1 phase- pulsed radiation) Decrease time for exposure Higher tube rating for extremely short x-ray procedures- excelllent for angiography

RIPPLE FACTOR Variation in the voltage across the x ray tube expressed as a percentage of maximum value. Ripple factor (%) = (Vmax –V min) X 100 Vmax

Example: 80 kVp 72 kVp Ripple = 80 - 72 x 100 = 10% 80

POWER STORAGE GENERATORS Used in mobile x-ray machines. Provide a means of supplying power for the x-ray tube independent of an external power supply. Two types- 1. Capacitor discharge generators 2. Battery powered generators

Capacitor discharge generators Capacitor is electrical device for storing charge/ electrons. 110V power fed to step up transformer- output rectified- used to charge large capacitor/ bank of capacitors. Once charged, capacitor can be discharged through x-ray tube.

Capacitor- discharge generators Advantage Small Easy to move Each exposure starts at the same kVp, even if line voltage is independable Disadvantage Limitation to its mAs output(30~50 mAs) kV fall during the exposure(1kV/1mAs)- so can’t be used for thick body parts like abdomen. Capacitor must be charged immediately prior to each exposure –as charge rapidly leaks away from capacitor

BATTERY POWERED GENERATORS Uses large capacity Nickel-cadmium battries. DC current of battery not supplied directly to Xray tube. DC Chopper used

Battery powered generators Advantage Store considerable energy to generate x-rays- can store upto 10000 mAs. Make exposures independent of a power supply Supply a constant output of kV and mA throughout the exposure Disadvantage Heavy Requires regular battery maintenance

MEDIUM FREQUENCY GENERATORS Newer type generators. Principle: In a transformer, the voltage induced in secondary coil is proportional to the rate of change of current in the primary coil. It converts a 60 Hz power line frequency to upto 6500Hz before it is fed into primary coil of transformer.

MEDIUM FREQUENCY GENERATORS

ADVANTAGES: Supply a constant, nearly ripple-free voltage to x- ray tube. More efficient and small size- convenient for portable units and can be integrated with x-ray tube housing.

TRANSFORMER RATING The maximum safe output of its secondary winding. Expressed in kilowatts. If rating is exceeded, transformer may over heat and burn out its insulation and windings. For 3 phase gen- kW= kV x mA 1000 For single phase- kW= kVx mAx 0.7 (As the voltage varies from 0 to some peak value in single phase generator, so to figure out the avg power an avg voltage , the R.M.S(root mean square ) voltage is used i.e peak voltage in kV/√2=.707 peak) Since the transfermer is drivig the X ray tube,the current is assumed to be fairly constant. Used for comparing generators and when the generator is under load.

EXPOSURE SWITCHING A switch is the device that turns the high voltage applied to the x ray tube on and off.when use in primary circuit(low kV high mA) called primary switching and in secondary circuit(low mA high kV ) called secondary switching. most commonly used primary switching for general purpose and also it is cheaper. PRIMARY SWITCHING There are 3 types of primary switches: electromechanical contractors thyratrons and solid state silicon-controlled rectifiers. the last 2 are phased out. Now the silicon controlled rectifiers/thyristers are found in most modern generatos.A control rectifier is a rectifier can be turned on/off by a small voltage pulse. It can produce exposures as short as 1/2 ms ,but it can’t produce these exposures at a high repetitive rate SECONDARY SWITCHING It takes place on the high voltage side of transformer or at the X ray tube itshelf. They are of two types. 1.Triode vacuum tubes 2.Grid controlled X ray tubes It is used in special purpose generators such as those needed in angiography . It has a sharp crisp exposures with rapid on-off rates with many repeated exposure rate. Triode tubes mainly used in angiography and grid controlled X ray tubes for cinefluorography

SCR/THYRISTER Similar to series of diodes in that there are NP and PN junctions. If the cathode is made –ve and anode+ve current can flow through the 2 NP junctions(though they are forward bi- ased) and no current throug the one PN junction(reversed bi-ased) As the electron in a diode will flow freely from N type to P type material. If a small +ve(1V) is applied to the gate(PN jun) the reversed bias at that jun will be overcome and electron will flow through the thrister. A small +ve pulse (1v) to gate overcome the reversed bias and a larger current to flow through the thyrister. Electrons will not flow from anode to cathode because there are two PN junctions,that prevent electron flow. The response of the gate is almost instantaneous.

GRID-CONTROLLED X RAY TUBES In addition to two electrode of Xray tube here a third electrode is used to control the flow of electrons from fillament to target. The third electrode is a focusing cup that sorrounds the fillament.,which helps to focus the electrons on the target. In grid controlled tube the focusing cup can be electrically –ve relative to the fillament. The voltage across the fillament grid produces an electric field along the path of the electron beam that pushes the electron closer together. If the voltage is made large enough the tube current may be completely pinched off.a condition in which no electrons go from the fillament to the target. The voltage applied between the focusing cup and fillament may therefore act like a switch to turn the tube on and off. As the cup and fillament are closely placed small voltage is necessory to cut-off the tube current.

EXPOSURE TIMERS Control the length of an x-ray exposure. Four basic types : 1. Mechanical timers- obsolete now 2. Electronic timers 3. Automatic exposure control -most commonly used 4. Pulse counting timers Mechanical and electronic timers are subject to human error.The exposure time is selected on the basis thickness and density of tissue under exposure.If the estimation is incorrect there is improper radiographic exposure.

ELECTRONIC TIMERS The length of X ray exposure is determined by the time required to charge a capacitor through a selected resistance. The exposure button starts the exposure and also starts charging the capaitor. The exposure is terminated when the charge in capacitor reaches a predetermined value. By varying the resistance we can increase/decrease the exposure time. PULSE COUNTING TIMERS Technique used here to measure a periodic event(the most accurate clock, the atomic clock count the very stable frequency of atomic ossilation) Timing is accomplished by counting pulses of a regular periodic voltage.

AEC (PHOTO TIMERS) Essential element is a device that can detect radiation and, in response produce a small electric current Three devices- 1. Photomultiplier detectors – most common 2. Ionization chambers, 3. Solid-state detectors. Depending on location of phototimer - Entrance type- located in front of cassette Exit type- behind cassette

PHOTOMULTIPLIER PHOTOTIMERS This is the most common type of AEC. The detector is made up of lucite coated with phosphor which emit light when irradiated with X rays and the intensity of light emited is proportional to the intensity of X ray. The lucite paddle itself is radiolucent and transmits this light to an output region called a light gate. The light gate direct the light to the photocathode of a photomultiplier tube,where the light is converted to an electric current i.e amplified to produce an electrical signal. The electric current generated by the photomultiplier tube may be used to charge a capacitor. When the charge in the capacitor reaches a predetermined value it causes termination of exposure

FUNCTIONING OF PHOTOMULTIPLIER Its a vacuum tube with a photocathode,dynodes and anode. When light strikes the photoemissive layer on photocathode it emits photoelectrons in no proportional to intensity of light. The Dynodes(intermediate electrodes) are coted with material that emits secondary electrons when struck by another electron. The electrons are accelerated by the graded positive potential from one to next dynodes,with each dynodes giving more secondary electrons and finally large no electrons reaches the anode. Such an arrangement amplify the tiny current emitted by photocathode by a factor of 1 million.

- IONISATION CHAMBERS + - + Almost always entrance type Two thin parallel aluminum plates/lead foil are used as electrodes Small voltage applied between plates before exposure Radiation causes ionization of air between two plates Ionized particles move to opposite charged electrodes Neutralize charge on plates-Then as the voltage has been reduced to a previously determined value, activates electronic circuit –terminates exposure Photon + - + -

SOLID-STATE AUTOTIMER That operate on the basis of radiation-producing ionization in or near a PN structure.

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