Quiz No. 1
A cathode is an electrode connected to the negative terminal of the voltage supply. It consists of a wire filament which contains electrons.
An anode is an electrode connected to the positive terminal of the voltage supply. It is usually composed of Tungsten because of tungsten's high melting and boiling points, and also its high atomic number (increases x-ray production efficiency).
Thermionic emission is the process through which electrons boil off the wire filament in the cathode. A current which flows through the filament increases the filament temperature. At a certain temperature, electrons start jumping out of the wire and form a space charge around the filament.
Exposure factors are exposure time, tube current and tube voltage. These are the variables we can manipulate as radiographers to obtain best images for lowest patient dose.
A line compensation circuit ensures that the 240V from the mains supply is constant. It must be supplied consistently at 240V for maximum x-ray production efficiency.
Rectification is the process of converting an AC signal to a DC signal. It involves the use of diodes which are composed of a semiconductor material which only allow the flow of electrons in a single direction. The arrow on the diode symbol indicates the flow of conventional current, which is the opposite direction of flow of electrons. DC voltages do not vary in polarity, meaning the anode is always positive and electrons are always accelerated towards the anode. A full-wave rectifier circuit consists of 4 diodes which effectively convert an AC voltage into a pulsating DC voltage.
How does an X-ray tube work?
The cathode is connected to the negative terminal of the voltage supply. When a current passes through the filament, the filament temperature increases and through thermionic emission the electrons boil off the cathode into the X-ray tube. The electrons are accelerated towards the negative anode and the kinetic energy that they gain is numerically equal to the tube voltage. The rapid deaccleration of the electrons as they smash into the anode target causes the production of x-rays. The number of electrons passing from the cathode to anode per second is the filament current.
What are the four main processes involved in standard radiographic imaging?
What is three phase power?
Three phase power is to reduce voltage ripple. It uses three wires, each arranged at 1/3 of a circuit after one another, to increase the voltage pulses to either 6 or 12 times the frequency of the original pulsating DC voltage. Three phase voltage power also has a neutral or ground wire to prevent electric shock. By minimising voltage ripple, we are maximising the efficiency of x-ray production.
What is voltage ripple?
Voltage ripple is the degree to which the AC voltage differs from a DC voltage. A smaller voltage ripple means better x-ray production.
What is a space charge?
A space charge is produced when electrons are boiled off the filament wire due to an increased temperature and accumulate around the wire as a cloud of charge. This only occurs when they are not accelerated towards the anode target.
What is line focussing?
Line focussing uses a focusing cup. At each cathode, there are up to 2 filament wires each with a focusing cup. The function of the focusing cup is to provide an electric field which compresses the electron stream into the smaller possible focal spot on the anode target. By doing so, we increase the spatial resolution of the image. However, because of the small focal spot, the heat energy produced but the accelerated electrons when they hit the anode is only in a very small area, damaging the anode. Although this can be somewhat rectified by having a rotating anode, by arranging the anode at a slight angle to the electron stream, we increase the focal spot area with the apparent focal spot from below being smaller. This means that the heat energy can be more safely absorbed.
What are the roles of the following components of an X-ray tube?
An anode is an electrode connected to the positive terminal of the voltage supply. It is usually composed of Tungsten because of tungsten's high melting and boiling points, and also its high atomic number (increases x-ray production efficiency).
Thermionic emission is the process through which electrons boil off the wire filament in the cathode. A current which flows through the filament increases the filament temperature. At a certain temperature, electrons start jumping out of the wire and form a space charge around the filament.
Exposure factors are exposure time, tube current and tube voltage. These are the variables we can manipulate as radiographers to obtain best images for lowest patient dose.
A line compensation circuit ensures that the 240V from the mains supply is constant. It must be supplied consistently at 240V for maximum x-ray production efficiency.
Rectification is the process of converting an AC signal to a DC signal. It involves the use of diodes which are composed of a semiconductor material which only allow the flow of electrons in a single direction. The arrow on the diode symbol indicates the flow of conventional current, which is the opposite direction of flow of electrons. DC voltages do not vary in polarity, meaning the anode is always positive and electrons are always accelerated towards the anode. A full-wave rectifier circuit consists of 4 diodes which effectively convert an AC voltage into a pulsating DC voltage.
How does an X-ray tube work?
The cathode is connected to the negative terminal of the voltage supply. When a current passes through the filament, the filament temperature increases and through thermionic emission the electrons boil off the cathode into the X-ray tube. The electrons are accelerated towards the negative anode and the kinetic energy that they gain is numerically equal to the tube voltage. The rapid deaccleration of the electrons as they smash into the anode target causes the production of x-rays. The number of electrons passing from the cathode to anode per second is the filament current.
What are the four main processes involved in standard radiographic imaging?
- Generation of a nearly uniform beam of x-rays.
- Differential attenuation of the x-rays by different tissues.
- Detection of the radiation passing through the body.
- Image analysis and interpretation.
What are the three main components of an X-ray imaging system?
- X-ray tube
- Operating console
- High voltage generator
*What are we actually changing when we change mA value?
The mA value is the tube current which is completely dependent on filament temperature, which is completely dependent on filament current. When we apply a filament current, the filament temperature increases, causing electrons to boil off in thermionic emission. The number of electrons that boil off and are accelerated towards the anode is the tube current. Thus, the filament current is what is changed when we alter mAs.
In more detail, the autotransformer provides a low voltage for the high voltage generator in terms of kVp. However, there is a second connection in the autotransformer that provides voltage for the filament circuit. Precision resistors provide the voltage corresponding to the selected mA value and the voltage is then applied to the step-down filament transformer. The lower output voltage corresponds to a higher output current, which heads the filament.
In more detail, the autotransformer provides a low voltage for the high voltage generator in terms of kVp. However, there is a second connection in the autotransformer that provides voltage for the filament circuit. Precision resistors provide the voltage corresponding to the selected mA value and the voltage is then applied to the step-down filament transformer. The lower output voltage corresponds to a higher output current, which heads the filament.
*What are we actually changing when we change the kVp value?
The kVp value is the tube voltage. 240V AC main voltage is supplied to an autotransformer, which in turn supplies the high voltage generator circuit, which then enters the rectifying circuit. The voltage output of the autotransformer is approximately 100V to 300V AC. This means when we change the kVp, we are actually changing the small output voltage of the of the autotransformer.
What does a high voltage generator consist of?
- High voltage step-up transformer (has a turns ratio of 500:1)
- Filament transformer
- Rectifier circuit
What is three phase power?
Three phase power is to reduce voltage ripple. It uses three wires, each arranged at 1/3 of a circuit after one another, to increase the voltage pulses to either 6 or 12 times the frequency of the original pulsating DC voltage. Three phase voltage power also has a neutral or ground wire to prevent electric shock. By minimising voltage ripple, we are maximising the efficiency of x-ray production.
What is voltage ripple?
Voltage ripple is the degree to which the AC voltage differs from a DC voltage. A smaller voltage ripple means better x-ray production.
What is a space charge?
A space charge is produced when electrons are boiled off the filament wire due to an increased temperature and accumulate around the wire as a cloud of charge. This only occurs when they are not accelerated towards the anode target.
What is line focussing?
Line focussing uses a focusing cup. At each cathode, there are up to 2 filament wires each with a focusing cup. The function of the focusing cup is to provide an electric field which compresses the electron stream into the smaller possible focal spot on the anode target. By doing so, we increase the spatial resolution of the image. However, because of the small focal spot, the heat energy produced but the accelerated electrons when they hit the anode is only in a very small area, damaging the anode. Although this can be somewhat rectified by having a rotating anode, by arranging the anode at a slight angle to the electron stream, we increase the focal spot area with the apparent focal spot from below being smaller. This means that the heat energy can be more safely absorbed.
What are the roles of the following components of an X-ray tube?
- Tube housing: oil insulates and cools down the x-ray tube, glass housing encloses the x-ray tube, lead sheath reduces leakage radiation
- Aluminium filters: remove low energy (soft) x-rays from the x-ray beam as these only contribute to patient dose
- Focusing cup: compresses the electron stream into a smaller focal spot to increase spatial resolution (using an electric field)
- Collimator: with help of a light beam, the collimators can adjust the field size to reduce patient dose
- Anode: electrode connected to the negative terminal of the voltage supply, can be stationary or more commonly rotating to disperse heat energy more safely without damaging anode, made of tungsten, connected to rotor by anode stem, rotor is inside an induction motor, stator of the motor outside of the x-ray tube consists of coils of wire, an AC current through the coils of wire creates a magnetic field which induces an electric current in the rotor, interplay of these fields causes the anode to turn
- Space charge compensation circuit: this means that when the kVp value is changed, the filament current changes automatically to maintain a constant tube current
What is the relationship between filament current and tube current?
As filament current increases, tube current increases rapidly, provided that an adequate filament temperature has been reached to stimulate thermionic emission. The reason the graph increases so rapidly is because increasing filament current increases filament temperature which means electrons are boiling off. As soon as the temperature is reached, electrons move straight away from cathode to anode.
What is the relationship between tube voltage and tube current?
Well, as you increase tube voltage, you increase the attraction between electrons and the positive anode target. This means that virtually all electrons that are boiled of the filament wire in thermionic emission are accelerated towards the anode. Increasing tube voltage instigates a rapid increase in tube current until the tube current plateaus, meaning virtually all electrons are being accelerated towards the target. The only way we can escape the temperature limited region is if the filament current is increased, thus increasing filament temperature, thus increasing the number of electrons being boiled off that can then move towards the anode target.
What are the advantages of operating an X-ray tube with a DC voltage, compared to an AC voltage?
- DC voltage is constant in polarity, meaning the anode can always be positive
- More efficient
- The x-ray beam is more uniform and is being produced at all times, rather than half the time
When we use an AC circuit, this means the anode is positive half the time, and negative the rest of the time. The reason this is bad is because electrons are only accelerated towards the anode half the time and therefore x-rays are only being produced half the time.
What is capacitor smoothing?
Capacitor smoothing is the process of placing a capacitor after the rectifier circuit. On the upward swing of the pulsating DC voltage the capacitor charges, while it discharges on the downward swing. This reduces voltage ripple.
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