Generators And The X-ray Circuit Flashcards
Flow of electrical current is in the _______ direction of the flow of electrons
OPPOSITE
Electrodynamics
Two types of electric current
- alternating current (AC) -> changes direction in cycles as the electrical potential changes
- direct current (DC) -> flows only in one direction, from positive to negative
Alternating current (AC)
-changes direction in cycles as the electric potential changes
Direct Current (DC)
-flows only in one direction (from positive to negative, opposite the direction of electron flow)
General x-ray circuit
Basic x-ray circuit can be divided into:
- main x-ray circuit
- purpose: to produce x-rays
- supplies x-ray tube with properly modified power so that it can produce x-rays
- filament circuit
- purpose: to create appropriate electron cloud
- supplies filament of x-ray tube with properly modified power to produce appropriate thermionic cloud
Components of general x-ray circuit
Main x-ray circuit
-primary side/circuit
🔹main power switch, circuit breakers, autotransformer, timer circuit and primary side of the step up transformer
-secondary side/circuit
🔹secondary side of step up transformer, mA meter, rectifier bank, x-ray tube (except filaments)
-filament circuit
🔹rheostat, step down transformer, filament
Primary circuit
- main power switch
- line compensator: used because the incoming power isn’t a consistent 220 volts, usually wired to the autotransformer, automatically adjusts the power supply to precisely 220 volts
- circuit breakers: protect against short circuits and electric shock
Primary curcuit
Autotransformer (kVp selector)
-adjustable transformer controlled by kVp selector on control panel
-when tech selects kVp setting, they are controlling this transformer
🔹determines # of turns on the secondary side to be included: which controls the output voltage sometimes called kVp selector
-primary purpose is to provide a voltage that will be increased by the step up transformer to produce the kilo-voltage selected at the operating console
Primary circuit
Step up transformer
-dividing line between primary and secondary circuits
-increases voltage from the autotransformer to the kV needed for x-ray production
🔹not adjustable, increases kV by a fixed amount
-primary coil is in primary circuit, secondary coil is in secondary circuit
Primary circuit
Timer circuit (exposure timer)
- located in primary circuit because its easier to control a low voltage vs a very high one
- purpose of the device is to ‘make or break’ the high voltage across the tube
- four types: synchronous timer, electronic timer, mAs timers, AEC
Secondary circuit
-begins with other side of step up transformer
-mA meter is there to monitor x-ray tube current
-AC is required for the transformers to work properly, but the x-ray tube requires DC current
🔹in the tube, electrons must always flow cathode to anode
Rectifiers (in secondary circuit)
- purpose is to convert AC (alternating current) to DC (direct current)
- solid state rectifier is commonly used
- consists of two semiconducting crystals joined together to form a solid state diode
Why do we need rectifiers
-when the cathode is negative and the anode is positive
🔹electrons will move across and x-rays will be produced
-if the anode of the x-ray tube is negative and the cathode positive
🔹no x-rays will be produced
🔹anode is not constructed to emit electrons, so no electrons moving to the cathode
Rectifiers
-when a positive charge is placed on the p-type crystal, and negative charge on the n-type crystal, the diode will conduct electricity
-both the traps and the electrons will move across the PN junction allowing current flow
-when AC cycle reverses, the diode will not conduct, the traps and electrons do not move to the junction and no current is conducted
-solid state p-n junction conducts electricity in only one direction (junction is called a diode)
-solid state diodes are rectifiers because they conduct electric current in one direction
🔹one way valve
🔹necessary to route the electricity through the x-ray tube correctly
Unrectified voltage
- current that passes through the x-ray tube exists only during the positive half of the cycle, where cathode is negative and anode is positive
- during the negative half of the cycle, current flows from anode to cathode, but this is not possible as the anode cannot emit electrons
Half wave rectification
- during positive portion of AC waveform, the rectifier allows electric current to pass through tube
- during negative portion of AC waveform, the rectifier does not conduct, no electric current is allowed
- result is positive pulses with gaps when current is not being conducted
- resultant electric current is rectified because the electrons flow in only one direction
- called half wave rectification because only one half of the AC waveform appears in the output
Full wave rectification
- shortcoming of half wave rectification is that it wastes half of the power supply and thus requires double the exposure time
- with 4 rectifiers, can utilize the negative half cycle
- inverts the negative half so that the anode is always positive
- energy fluctuates from max voltage to zero-100% ripple
- this is a problem for x-ray production
- x-rays produced will vary from 0 to kVp selected on console
Rectification
- not very effective when the energy level drops to 0
- one solution is to use 3 AC waveforms at the same time, but slightly out of phase or synchronized
- out of step by 1/3
- called 3 phase power
- depending on the number of rectifiers and engineering, can have ripple anywhere from 4-14%
Three phase power
-major improvement over single phase power generator where 100% ripple mean voltage supply to the tube would produce x-rays between 0-100% of the kVp set
-3 phase 6 pulse:
🔹14% ripple
🔹voltage supply to x-ray tube never falls below 86%
-3 phas 12 pulse
🔹4% ripple
🔹volatge supply to the tube never falls below 96%
High frequency generators
- the next step in improving the waveform is the use of HF generators
- takes the regular 60Hz and converts it to higher frequency (500-2500Hz) and transfers it to high voltage
- the standard 60 Hz incoming power is first fully rectified, then sent through a capacitor bank where it is smoothed
- from there it passes through an inverter circuit that ‘chops’ the DC waveform and converts it to high frequency AC
- now it is low voltage high frequency AC, which will pass through a step up transformer to turn it into high voltage AC
- then it passes through rectifiers again, to become fully rectified and finally through high voltage capacitors where it is smoothed to provide the x-ray tube with a near constant potential voltage waveform
- ripple reduced to less than 1%
Advantages of HF generators
- smaller, lightweight
- less costly
- better exposure reproducibility
Efficiency
Advantages of having less ripple are:
- greater radiation quantity and quality that results from the more constant voltage supplied to the tube
- quantity of photons is higher because the efficiency of the x-ray production is higher when the tube voltage is high
- fewer low energy photons
Falling load generator
- uses the shortest exposure time possible
- method: initial tube loading is higher, and drops during exposure
- results in achieving the set mAs, in a shorter exposure time
- principle use in areas where the shorter the exposure time, the better (interventional radiology)
In the secondary circuit
-when the exposure switch is pressed
-voltage goes through step up transformer, to increase to the kilovltage selected on control panel
-kV goes through rectifiers so that it travels through the tube correctly to produce x-rays
🔹large positive charge on anode
🔹large negative charge on cathode
-to create x-rays, need one more thing: electrons
Filament circuit
-purpose: to burn off electrons
rheostat (thermostat)
-controlled by mA selector on console
-controls the filament temp and the rate at which electrons are boiled off
-so when tech is adjusting mA on console, they are adjusting the rheostat, which adjusts the amount of resistance in the filament circuit, and ultimately the amount of current applied to the filament
-the higher the mA station #, the lower the resistance
-rheostat works together with exposure timer
-rheostat controls filament temp and the electrons being emitted, and the time determines the duration of this process
-together they determine the quantity of electrons boiled off and available to produce x-rays
-step down transformer is used to reduce the voltage applied to the filament
Filament circuit summary
- when exposure button is pressed, voltage and current is sent through the filament circuit
- quantity is controlled by rheostat to reflect the mA selected
- step down transformer is used to reduce the voltage and increase current (inverse relationship)
- current heats the selected filament and begins boiling off electrons
- time selected controls how long the voltage and current is applied to the tube
Summary of circuit operation
- on the console, select kVp, mA, exposure time and focal spot size
- follow voltage through the primary and secondary circuit
- follow current through the filament circuit
Path of circuit (important to understand)
- kVp selected adjusts the autotransformer
- determines the # of turns on the secondary side necessary to produce a voltage to send to the step up transformer
- step up transformer increase this voltage by a fixed amount and produces the kV selected on the console
- this transformer represents the transition from primary (low voltage) to the secondary (high voltage) circuit - kV is created, now it must be rectified for the x-ray tube
- electrons must flow from cathode to anode
- rectifiers (one way valves) will route the electricity through the tube correctly
- after passing through the rectifiers, the electricity will create a large positive charge on the anode, and a large negative charge on the cathode (focusing cup)
- filament circuit draws electricity from the autotransformer to the rheostat
- rheostat is controlled by the mA selector on the console and is connected to the focal spots
- focal spot selector represents the filaments in the tube
- mA station selection will also determine which filament is selected
- selected mA sets the resistance in the filament circuit
- from the rheostat, electricity travels to the step down transformer
- adjusted current travels to the filament
- filament is heated up and thermionic emission occurs
- now the paths join
- electron cloud created by the filament is ready and waiting
- kV applied to the x-ray tube creates a large positive charge on the anode and a large negative charge on the cathode (focusing cup)
- positive charge on anode draws the electrons over, giving them tremendous kinetic energy in the process
- opposites attracted and in the short 1-3 cm gap between cathode and anode the electrons reach speeds of up to 1/2 the speed of light
- negative charge on cathode serves to keep electrons crowded together
- electrons travel to the anode and interact there to produce x-rays until the timer circuit terminates the process
Emission spectrum and generator
-as the efficiency of the voltage wave form increases, the efficiency of x-ray production increases also
-high frequency generators have the lowest ripple, so they will be at peak voltage for most of the duration of the exposure
🔹meaning more effieicient production of x-rays (quantity) and higher energy x-rays produced (higher average)
-single phase generators have the most ripple
🔹less efficient production of x-rays, lower average energy
