+ - P R E S E N T E D B Y K A M A R U L A M I N A B D U L L A H Dip. MED. IMG., BSc. MED. IMG. (UiTM) 1
I N T R O D U C T I O N : An x-ray generator is a device that Supplies electrical power to x-ray tube. It provides high voltage between anode & cathode filament voltage. Controls exposure timing Turns exposure on and off High voltage switched on and off 2
Electricity & Current: Electricity is the flow of current. Current is the flow of charge. It is measured in Amperes. It is defined as the flow of 1C charge per unit time. i.e., I=Q/t where, I=current Q=charge t=time Types of Current: 1.Direct Current 2.Alternating Current 3
Alternating Current An electric current in which amplitude and polarity of the current vary with time. Its a sinusoidal wave which varies amplitude and periodic reversal of polarity. 4
Direct Current Electric current which flows in one direction only through a circuit or equipment. The associated direct voltages, in contrast to alternating voltages, are of unchanging polarity. A A 5
High Tension Circuits An x-ray equipment is just not directly connected to the source where electricity is generated. There are many other components required to complete the circuit. Components of H.T. Circuit: 1.Transformer Step up transformer Step down transformer Autotransformer 2.Rectifier 6
T R A N S F O R M E R Definition: An electrical device that transforms the voltage of alternating current electricity from one parameter to another. Principle : It is based on the principle of electromagnetic induction. 7
Types Of Transformer Used In X-ray Circuit: In x-ray circuit, three type of transformer are used: Step up transformer Step down transformer Auto transformer 8
Autotransformer The autotransformer is designed to supply voltage of varying magnitude to several different circuits of the x-ray machine including both the filament circuit and high voltage circuits. The autotransformer has only one winding and one core. The single winding has a number of connection or electric taps. It works on the principle of self induction. 9
Diagram Of Autotransformer: c A d 300V 2:2 TURN RATIO e f 300V 2:2 TURN RATIO 450V 2:3 TURN RATIO B g 10
Step Up Transformer Or High Tension Transformer: Definition: Transformers that convert a low voltage into a higher voltage are called step-up transformers. Step-up transformer primary from autotransformer secondary to rectifier circuit ma monitored at center grounded point of secondary Supplies high voltage for x-ray tube 11
Grounded metal box filled with oil oil acts as electrical insulator Function: Increases alternating voltage Also contains rectifier circuit changes alternating current into direct current Autotransformer ma Rectifier Circuit HIGH VOLTAGE TRANSFORMER 12
Step Down Transformer Also called filament transformer. A step down transformer has less turns of wire on the secondary coil, which makes a smaller induced voltage in the secondary coil. Placement : It is connected directly to the filament of x-ray tube. Purpose: the filament of the x-ray tube needs to be heated up in order to emit the electrons (thermal electrons). The Tube Current is controlled through a separate circuit called the filament circuit which is connected to filament transformer. 13
RECTIFICATION Process by which alternating current is changed to pulsating direct current A device which converts AC into pulsating DC is called rectifier. A rectifier can be valve type or vacuum type or it can be solid state/semiconductor/metallic type rectifier. X-ray tube is a sort of valve type rectifier or diode rectifier as it has two electrodes and allows the current to flow in one direction only 14
UNRECTIFIED A.C WAVE NO RECTIFICATION HALF-WAVE FULL-WAVE 15
Thermionic Diode Valves: An evacuated tube with two electrodes in it is called a thermionic diode valve. Hence it is like an X-ray tube having: A glass envelope enclosing a vacuum. Two electrodes within the glass envelope, one of which is a heated filament. The filament of a valve is heated by a step down transformer and emits electrons which are drawn across to the anode when a potential difference is applied across both the electrodes and the valve passes current. 16
Functioning of a Diode Valve: If the valve is connected in a complete circuit such that cathode is ve with respect to anode electrons are drawn towards the anode and valve passes current. If the cathode is positive with respect to anode, no electrons will be drawn across the valve and it blocks the current thus the supply of current to X-ray tube is unidirectional only. But these diode valves which were used earlier are replaced with solid state rectifiers. Hence, its function is to pass current in one direction only and to block any reverse flow. 17
HIGH TENSION GENERATORS The high tension transformer together with other components such as rectifiers is called a High Tension Generator. 18
Contd. Various types of HT generators varying from simple to complex ones are as below: 1. The self rectified HT Circuits (Single pulse generators) 2. Half wave rectified circuits (Single Pulse). 3. Single phase full wave rectified circuits (Two Pulse). 4. Three phase full wave rectified circuits (Six Pulse). 5. Three phase 12 pulse generators. 6. High frequency generators. 7. Battery powered generators. 8. Capacitor discharged generators. 9. Falling load generators. 10. Anatomical programmed generator 19
1. Self Rectified H T Generator It is an x-ray generator in which : X-Ray tube acts as rectifier Current only flows from cathode to anode Rarely seen cathode is source of free electrons Voltage applied to tube Secondary of High Voltage Transformer ma waveform 20
Applications: Portable / Low power mobile units and dental units. Advantages: Small in size. Simple design. Light in weight. Less cost. Simple to operate. 21
2. HALF WAVE RECTIFIED GENERATORS X-ray tube connected to secondary of high voltage transformer through diode rectifiers Alternating voltage applied to secondary of high voltage transformer + - Voltage applied to tube 22
Half wave Rectifier Circuit: + R1 - - R2 First Half Cycle: Diodes closed Voltage applied to tube Tube current (ma) results - R1 + - X R2 Second Half Cycle: Diodes open No voltage applied to tube No tube current (ma) 23
60 pulses per second Limitations: only positive half cycle of high tension transformer used inefficient negative half cycle wasted Secondary of High Voltage Transformer Output of High Tension Transformer Applied to x-ray tube Applied to X-ray Tube Blocked (not used) 24
3. Single Phase Full Wave Rectified H T Generator: In this circuit both half cycles of AC are used to produce X-Rays by employing a bridge of four rectifiers Secondary of High Voltage Transformer Voltage applied to tube 25
Actually what happens? + First Half Cycle - Voltage applied to tube (also ma waveform) Second Half Cycle R1 X R4 R1 X R4 - R2 X R3 + R2 X R3 26
Advantages: less difference in Average and peak current. Peak current is 1 ½ times of the average current. both the halves of AC cycle and voltage during both halves is alike. both + & - half cycle of high tension transformer used Short exposure time: for conventional exposure switching minimum duration for single pulse is 1/100 sec, as we are getting 100 half waves in 50 cycles in 1 sec. 27
Higher output than self or half wave rectified circuits. Less strain on HT cables and less insulation cost. Tube Output of High Tension Transformer Applied to X-ray Tube 28
Limitations: Costly. More complex. Heavier, not easy to transport. Larger in size. Ripple factor is 100% as it is pulsating X-Ray beam with voltage variation between zero to peak and again to zero. 29
4. THREE PHASE, SIX PULSE, SIX RECTIFIER GENERATOR This design employs a wound 1* transformer with a wound 2* transformer. The out put of the 2* winding is rectified with six solid state rectifier. Winding A and B works as a system with R2, R4, R1, R5 B A C R1 R4 Winding B and C works as a system with R1, R6, R3, R4 R2 R3 R6 R5 Winding A and C work as a system with R2, R6, R3, R5. 30
When rectified there will be six positive maximum voltage per cycle thus the term six pulse. Input 3 Phase Voltage Rectified To X-Ray Tube 31
5. Three Phase, Six Pulse, Twelve Rectifier The circuit is twelve pulse but this circuit has fixed potential to ground. This allows a 150 KVp generator to have a transformer that provide a voltage of -75KV to +75KV and hence, further reduces ripple. 32
Application: Useful in procedures requiring shortest or repetitive exposures e.g. angiography or serial radiography. Advantages: As load is distributed equally overall three phases of the supply when the x-ray exposure is made it can draw larger amounts of power. Hence three phase HT generators can supply higher ma e.g. 1000-1200mA for the x-ray tube than single phase generators. Disadvantages: More expensive. More complex in circuitry Larger and occupy more space. 33
6. High Frequency Generator The newest development in high-voltage generator design uses high frequency circuits. An inverter circuit creates the high frequency AC waveform. This AC current supplies the high voltage transformer and creates a waveform of fixed high voltage and corresponding low current. 34
Block Diagram Of High Frequency Generator 35
Advantages: It s small size can be placed within the tube housing, produce a nearly constant voltage waveform, resulting into improved quality at lower pt. dose. Portable x-ray imaging system were the first to use this technology, but now a days, all stationary imaging systems use H.F. voltage generation. 36
contd. Real advantage is their much smaller, less costly and more efficient than three phase circuit and less ripple (<1%). Because of closed looped voltage regulations, autotransformers for kvp selection and input line voltage compensation are not necessary unlike other generator design. More accuracy in voltage and current. 37
Typical home & small business power Industrial power inexpensive transformer windings expensive 1 primary coil transformer windings 1 secondary coil 3 primary coils 100% ripple 8 ms minimum exp. Time 1/120th second lower output intensity puts less heat in tube for same technique one for each phase 6 secondary coils 4-13% ripple 2 secondary coils induced per primary) slightly less patient exposure <=1 ms minimum exp. time higher output intensity puts more heat in tube 38
Voltage Ripple It is the variation between the peak voltage and the minimum voltage in the circuit. Variation of kilovoltage from maximum Usually expressed as percentage of maximum kv. 39
Ripple Example: 80 kvp 72 kvp % voltage ripple= (V V ) max min X 100 V max Ripple = 80-72 = 8 kvp %= 8 / 80 =.1 = 10% 40
Ripple Typical Values single phase always 100 % (kv ranges from zero to maximum) three phase 4-13% Single Phase Output Three Phase Output high frequency very low; approx 1% Constant Potential or High Frequency Output 41
7. Battery Powered Generators: Some portable x-ray machines employ a series of batteries to generate the high voltage and filament currents and are useful to operate in areas where the electric supply is inadequate for the conventional generators. Each cell in the battery pack supplies potential difference of 1.5 volts, so thousands of cells are required to provide high voltages used in diagnostic radiology. 42
The batteries must be recharged periodically for numerous x-ray exposures. ma Selector X ray Tube S A KV Selector 43
8. Capacitor Discharge Generators Capacitor stores charge (or voltage) Capacitor stores high voltage Special tube used Contains grid Grid close to filament Low voltage on grid controls flow of electrons to anode Exposure start & stop controlled by voltage on grid + 44
9. Anatomically Programmed Generator 1. Radiographers have been traditionally selecting & settings of kv, ma & sec. 2. But this problem is overcome by using modern circuitry in anatomically programmed generator. 3. In this generator there is a no. Of push button setting. 4. Each pushbutton is designated to a particular anatomical part or region. 5. When the chosen push- button is pressed, the circuitry associated with it automatically sets the appropriate factor s of kv, ma & selects the appropriate focal spot on the x-ray tube which is to be used. 45
Advantages: 1. Technologists perform speedier exam. 2. Technologists operating unfamiliar equipment may work with greater certainty no need of use of technique chart. 3. In fluoroscopic room, if anatomical programming is used for spot filming, there is obvious benefit when exam are changed 4. e.g. a pt. being fluoroscoped to investigate a possible gastric ulcers succeeded by one undergoing an oral cholecystogram. Disadvantages: 1. Technologist may find the range of selection not wide enough to embrace all the examinations. 2. Technologist must manipulate the exposure factor like in case of pt. physical build and certain known pathologies (osteoporosis, pleural effusion). 3. Well trained and experienced radiographers must be needed for evaluating the good quality of radiograph. 46
10. Falling Load Generators: 1. These are usually specially designed three phased or high frequency generators. 2. They take full advantage of the current loading capacity of the X-ray tube by beginning the exposure with a high milliamperage and then allows it to fall during the exposure. 3. This can be achieved with a constant potential circuit. It requires that both ma and KV be regulated independently. 4. These must be used with automatic exposure controls or rely on mas timer instead of independent ma and time controls. 47
Advantage : Shorter times in heavy load situations and simpler operations. Disadvantage : Can shorten x-ray tube life considerably as they use higher ma settings, thus causing the filament to wear out more quickly. Function with ma unknown to the operator so it is impossible for the operator to set the correct time to achieve the desired mas, therefore, required to be used with automatic exposure controls or rely on mas timer instead of independent ma and timer controls. ma TIME IN AN EXPOSURE 48
Safety Rules for Radiological Technologist The radiological technologists operate two main categories of X-Ray equipment: 1.Permanent Installation 2.Movable Equipment Electrical hazards are most communally to be greater with movable equipment. 49
There are certain rules to be observed by radiological technologist: 1. All movable x-ray equipment should be checked regularly and often by electrician. 2. The R. Technologist using the equipment should at once any damage or defect. 3. R. Technologist should not put plugs into or out of sockets which are live. 4. Cables and plugs should be treated as kindly as if they are patient! 5. Do not stretch the cable. 6. Do not run the equipment on the cable. 50
1. The essential function of high tension generators in x-ray equipments is to provide such power as is needed by the x-ray tube. 2. No doubt, x-ray tube is the heart of the x-ray equipment, like human body, heart has no importance without blood, similarly, x-ray generators provide power which acts as blood in the x-ray tube. 3. The development in high tension generator has always enhanced the quality of production in x-ray beam by reducing the ripple factor. 51
References: Radiographic Equipments By Chesney. Christensen s Physics For Diagnostic Radiology. www.xray2000.uk www.radiologyinfo.com 52