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varkey chapter 5
study guide
| Question | Answer |
|---|---|
| What are the three principle parts of the xray imaging system? | 1.X-ray tube 2.operating console 3.High voltage generator |
| What are the functions of the operating console? | allows the tech to control the xray tube current and voltage so that the useful xray beam is of proper quantity and quality |
| All the devices at the operating console use: | low voltage |
| What is the function of a line voltage compensator? | measures the voltage provided to the xray imaging system and adjust the voltage to precisely 220 volts |
| The power supplied to the xray imaging system is delivered first to the: | autotransformer |
| What are the functions of autotransformer? | -It is designed to supply voltage of varying magnitude to several different circuits including the filament circuit and high voltage circuit. -Design system voltage to approximately twice the input voltage. -This works on self induction |
| functions of autotransformer continued... | -Daughter transformer has only one winding and one core. -The single winding has number of connections or electric taps. |
| Autotransformer works under the principle of: | Self-induction |
| True/false: The autotransformer law is the same as the transformer law. | True |
| KVP determines the _____ of the xray beam. | Quality |
| Understand the function of the KVP meter | -Allows the voltage to be monitored before an exposure. -Place across the output terminals of the autotransformer. -This type of meter is known as pre-reading KVP meter |
| What is the other name used for a KVP meter? | Pre-reading volt meter |
| Where is the KVP meter located? | Placed across the output terminals of the autotransformer |
| What is the difference between tube current and filament current? | -tube current is the flow of electrons filament to anode -filament current is current that heats filament to anode |
| difference between tube current and filament current continued... | -Xray tube current normally is not continuously variable precision resistors results in fixed stations that provide x2 currents of 100,200, or 300 mA and higher. |
| The number of electrons emitted by the filament is controlled by: | temperature |
| What is thermionic emission? | -The release of electrons from a heated filament |
| Filaments usually operate at a current of: | 3 - 6 amperes, 12 volts |
| What is space charge? | Cloud of electrons |
| What is space charge effect? | -When the electrons are all together and cannot be released, not even with more heat and you cannot produce more electrons. -Makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion |
| How a falling load generator operates | -Exposure begins at maximum mA -mA drops as the anode heats up -Exposure time is minimized avoiding overheating |
| What is the advantage of a falling load generator? | -The result is minimum exposure time -avoids overheating |
| The filament transformer is a ____ transformer | step down |
| What is the output voltage of a filament transformer? | Primary:150 volts Secondary:12 volts |
| What is the output current of a filament transformer? | Primary: 0.5-1 ampere Secondary: 5-8 ampere |
| What is the function of a guard timer(backup timer)? | will terminate an exposure after a prescribed time, usually approximately 6 seconds thus it is not possible for any timing circuit to continuously irradiate a patient for an extensive amount of time |
| What are the four types of timing circuits? | -Synchronous timers -Automatic Exposure Control -mAs timers -Electronic timers |
| What is the frequency of electric current in the U.S? | 60 Hertz |
| What is the minimum exposure time possible with a synchronous timer? | 1/60 seconds (17ms) |
| What is the disadvantage of a synchronous timer? | Cannot be used for serial exposures because they must be reset after each exposure |
| Most exposure timers are ______ and are controlled by a _____. | -Electronic -Microprocessor |
| What is the function of an mAs timer? Where is it located? | -The mAs timer is usually designed to provide the highest safe tube current for the shortest exposure for any mAs selected. -Because the mAs timer must monitor the actual tube current, it is located on the secondary side of the high voltage transformer. |
| How do you calculate mAs if mA and exposure time is given? | mA x seconds = mAs |
| How do you calculate exposure time if mAs and mA is given? | mAs divided by mA = time |
| What is the function of an automatic exposure control? | The AEC is a device that measures the quantity of radiation that reaches the image receptor, it automatically terminates the exposure when the image receptor has received the required radiation. |
| What are the two types of automatic exposure controls used? | 1. ionization chamber, placed between the patient and IR 2.Photodiode, placed behind the image receptor |
| A backup timer is set at _____ x the expected exposure time | 1.5 |
| What is the function of a back up timer? | a precaution taken for the protection of the patient and the xray tube incase the AEC fails to terminate |
| The accuracy of an xray timer can be checked by using a _____ | -Spinning top test -Solid state radiation detectors |
| The high voltage generator contains 3 primary parts: | -High voltage transformer -The filament transformer -Rectifier |
| What is meant by turns ratio? | The ratio of the number of secondary windings to the number of primary windings called the turns ratio |
| The turns ratio of the high voltage transformer is 1000:1 and the supply voltage is 110:? , what is the secondary voltage supply to the xray tube | 0.11 |
| What is rectification? | The process of converting alternating current to direct current. |
| What is rectifier? | An electric device that allows current flow only in one direction |
| In an xray machine the electrons must flow from : | cathode to anode |
| Rectification is accomplished by using : | diodes |
| Originally all diodes rectifiers were : | vacuum tubes called valve tubes |
| What kind of rectifiers are used in modern xray circuits? | Solid state rectifier made of silicone |
| Read and understand how a PN junction semiconductor is used as a solid state diode | Semiconductors are classed into two types: N-type and P-type. N-type semiconductors have loosely bound electrons that are relatively free to move. P-type semiconductors have spaces called holes where there are no electrons. |
| What is half wave rectification? | remove the negative half of the AC wave form |
| What is full wave rectification? | Device a circuit that rectifies the entire AC wave form |
| what is single phase power ? | Single-phase power results in a pulsating x-ray beam. This is caused by the alternate swing in voltage from zero to maximum potential 120 times each second under full-wave rectification. |
| what is three phase power ? | With three-phase power, the voltage applied across the x-ray tube is nearly constant, never dropping to zero during exposure. |
| What are the advantages of a high frequency generator? | -Its size (smaller) -Improved image quality with lower patient radiation dose. -Nearly constant waveform all the time |
| What is a pulsating direct current? | A periodic current which changes in value but never changes direction. |
| Understand how a capacitor discharge generator operates | Some portable x-ray imaging systems still use a highvoltage generator, which operates by charging a series of SCRs from the DC voltage of a nickel–cadmium (NiCd) battery. |
| During capacitor discharge the voltage falls at a rate of | 1 kv/mAs |
| Understand how a falling load generator operates | Many x-ray imaging systems today engage a falling load technique to ensure the shortest possible exposure time. The x-ray tube anode can accommodate only a limited heat level |
| What is a voltage ripple? | Another way to characterize these voltage waveforms is by voltage ripple. Single-phase power has 100% voltage ripple: The voltage varies from zero to its maximum value. |
| voltage ripple continued | Three-phase, six-pulse power produces voltage with only approximately 14% ripple; consequently, the voltage supplied to the x-ray tube never falls to below 86% of the maximum value. |
| single phase formula | kvp x mA x seconds |
| 3 phase 6 pulse formula | kvp x mA x seconds x 1.35 |
| 3 phase 12 pulse formula | kvp x mA x seconds x 1.41 |
| high frequency formula | kvp x mA x seconds x 1.44 |
Created by:
daniel.velazco004
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