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What devices are used for the generation of x-rays
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Xray final
Harley final
| Term | Definition/statement |
|---|---|
| What devices are used for the generation of x-rays | Electrical device used for the generation of x-rays |
| How does it work? | This is accomplished by acceleration of electrons & then suddenly decelerating them |
| The energy of the x-rays is dependent on | the kinetic energy of the electrons |
| The energy of the x-rays is | controlled by kVp |
| X-ray Tube Components | Glass Envelope Cathode – negative side Anode – positive side Protective Housing |
| Glass Envelope | Made of Pyrex glass – withstands high heat Tube maintains a vacuum |
| Tube window | A segment of glass that is thinner than the rest of the glass |
| Where do the xrays exit? | Tube window – this is where the x-rays exit the tube through Contribute to inherent (how it’s built) filtration |
| Cathode | negative |
| What are the two parts of a cathode | Filament Focusing cup |
| Focusing Cup | Metallic shroud containing 2 filaments of different size |
| What does a Focusing Cup contain | Contains a negative charge |
| What is a Focusing Cup designed to do | repel electrons condense electron beam to small area on focal track – the Anode |
| Filament | Small coil of thoriated tungsten |
| How many Filaments do Modern tubes have? | 2 |
| Where does the small filament fo? | goes with the small focal spot, etc. |
| Where is the focal spot? | focal spot is the area on the Anode that is bombarded with electrons |
| What happens when the machine is turned-on | a small current flows thru them to heat them up |
| How do you adjust the Tube current? | Tube current is adjusted by controlling the filament current |
| Anode | Positively charged electrode |
| Two types of Anode | Stationary – doesn’t move – seen in dental x-ray Rotating – moves |
| Advantages of Rotating Anode | Provides greater target area and heat dissipation Can handle greater exposure loads – higher kVp & mA Heating capacity is further enhanced with high speeds – 3400 RPM |
| Rotating anode Can handle greater exposure loads | higher kVp & mA |
| Heating capacity is further enhanced with high speeds | 3400 RPM |
| Line Focus Principle | Used to reduce the effective focal spot |
| The effective focal spot is controlled by | the size of the actual focal spot and the Anode target angle |
| What is the effective focal spot? | is the area projected onto the patient & film – it is the one stated as the focal spot size |
| What is the advantage of the Line Focus Principle? | The advantage of this principle is that it provides the detail of a small focal spot while allowing for a large amount of heat dissipation |
| What is the disadvantage of the Line Focus Principle? | The disadvantage is the “Anode heel effect” The radiation intensity is less on the Anode side of the beam Photons on the Anode side have less energy – not as much penetrating power |
| What is the Anode heel effect? | The “Anode heel effect” is used to advantage when anatomical parts of unequal thickness are radiographed throughout their respective lengths – thicker on Cathode side |
| Protective Housing | Prevents excessive leakage radiation and prevents electric shock to the patient and operator |
| How does Protective Housing work? | Uses specially designed high-voltage receptacles for the high-voltage cables that connect to the Cathode & Anode Some contain oil for cooling while others have cooling fans |
| X-ray Tube Rating Charts | Graphs that indicate the maximum exposure values that may be made without damaging the tube – kVp, mA, time |
| How to use an X-ray Tube Rating Charts ? | For a given mA – any combination of kVp & time that lies below the mA curve is safe |
| Anode Cooling Chart | Shows the thermal capacity & its heat dissipation characteristics generated by the production of x-rays stored Anode Shows length of time required for complete cooling following level of heat input measured in Heat Units HU |
| Calculating Heat Units (we don’t do this) | Single phase units = kVp x mA x s 3 phase 6 pulse units = 1.35 x kVp x mA x s 3 phase 12 pulse units = 1.41 x kVp x mA x s High frequency units = 1.44 x kVp x mA x s |
| Cardinal Principles of Radiation Control | Time Distance Shielding |
| Time | minimize exposure time |
| Distance | maximize the distance from the source of radiation greater distance the lower the exposure doubling the distance from the source of radiation decreases exposure rate to ¼ the original exposure rate half distance increases the exposure by a factor of 4 |
| Shielding | shield yourself from the source of radiation – lead aprons, lead gloves, etc. – placing an “absorber” between you and the source of radiation |
| The atom | Made up of protons, neutrons, & electrons we are only concerned with electrons when talking about x-radiation |
| What happens when electrons can gain or lose energy? | when electrons lose energy they release what is called a photon x– radiation is made up of photons |
| What is a photon? | a photon is a discreet packet of energy and is considered to be a subatomic particle – x-ray photons are not visible |
| Two types of x-radiation produced when high speed electrons interact with matter | Bremsstrahlung radiation Characteristic radiation |
| Bremsstrahlung radiation | this type of x-radiation is produced when an electron changes direction due to the attractive force of the nucleus of an atom Most medical X-rays are produced this way Not a one-to-one interaction |
| Characteristic radiation | this type of x-radiation is produces when a high speed electron collides with an orbital electron. Not a one-to-one interaction |
| How do we get electrons to travel from one place to another at high speeds? | We apply a lot of voltage (potential difference) around 150,000Vp otherwise expressed as 150kVp |
| What does kVp determine? | kVp determines the quality of the x-ray photons |
| How much energy do they have (kinetic energy) | the energy of the photons does not change significantly – the opposite is also true |
| How well can they penetrate the human body? | A lower kVp will make the x-ray beam less penetrating. This will result in a greater difference in attenuation between the different parts of the subject, leading to higher contrast. A higher kVp will make the x-ray beam more penetrating. |
| If you were to increase the kVp, what would happen to the number of electrons passing from the Cathode to the Anode? | There would be more of them and faster. |
| The X-ray Emission Spectrum | Details how changes in kVp or mA effect the number of photons produced and how much energy the photons possess |
| As kVp increases (all other conditions remain constant) | the number of x-ray photons produced increases as well – so too does the energy that the photons possess – the opposite is also true |
| As mA (mAs) increases (all other conditions remain constant) | the number of x-ray photons produced increases as well – but, the energy of the photons does not change significantly – the opposite is also true |
| 15% rule of x-ray | a 15% increase in kVp is equivalent to doubling the mAs – ex – if you change the kVp from 72 kVp to 82 kVp (15% increase) you get the same effect as doubling the mAs (mA x seconds) |
| mAs formula | mAs (mA x seconds) |
| Filtration is | Inherent, Added, Overall effect of filtration |
| Inherent | by virtue of how it is made |
| Added | placing additional filters in the x-ray beam – sometimes called hardening the x-ray beam – it increases the average energy (quality) of the beam |
| Added filtration | more effectively absorbs low-energy x-ray photons before they can interact with the patient or film/image receptor |
| Filtration reduces | x-ray beam intensity (number of photons – quantity) but it increases the average energy of the photons |
| Overall effect of filtration | quality goes up, quantity goes down |
| Changes in X-ray Beam Quality & Quantity An increase in Current(mAs) | Results in Increase quantity |
| Changes in X-ray Beam Quality & Quantity An increase in Voltage(kVp) | Results in Increase both |
| Changes in X-ray Beam Quality & Quantity An increase in Added filtration | Results in Decrease quantity, increase quality |
| Changes in X-ray Beam Quality & Quantity An increase in Increase voltage ripple | Results in Decrease both |
| X-ray photons interact with matter in 5 different ways | Coherent Scattering, Photodisintegration, Pair Production. Compton Scattering |
| Coherent Scattering | photon changes direction – low energy photons that are outside the range for diagnostic imaging and most don’t reach the film/image receptor |
| Photodisintegration | the collision of a photon with the nucleus of an atom – the photon is completely absorbed in the process – a nuclear fragment is emitted |
| Pair Production | a high-energy photon is completely transformed into an electron & positron (basically a positive electron) – a process by which energy is transformed into matter |
| Compton Scattering | caused by the Compton Effect a photon interacts with an electron & transfers part of its energy to the electron the electron is either ejected from the nucleus or simply moves to an excited state the photon changes direction & has less energy these photon |
| Photoelectric Effect | process by which a photon transfers all of its energy to an electron in the material it interacts with discovered by Einstein |
| Compton Scatter | scatter radiation from the patient is the primary cause of occupational radiation exposure – safety issue |
| 3 things can happen to x-ray photons when they interact with the body | Absorbed (photoelectric effect), Transmission, Scatter (Compton effect) |
| Absorbed (photoelectric effect) | photons are absorbed by the tissues – don’t contribute to image quality – remember – when 1 photon interacts with the film/image receptor that equals 1 black dot |
| Scatter (Compton effect) | contributes to “fog” – radiograph isn’t clear |
| Transmission | penetrates through the body to interact with the radiographic film/image receptor – these photons contribute to the radiographic image |
| ATTENUATION | is the reduction of the intensity of an x-ray beam as it traverses matter. |
| What causes the attenuation reduction? | The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber (what it is made of). |
| ALARA | keep radiation exposures As Low As Reasonably Achievable |
| Rad (rad) = Radiation Absorbed Dose | biologic effects are related to rads – describes the amount of radiation absorbed by a patient – quantifies the amount of ionizing radiation energy transferred to any target material (human tissue in our case) – this is the one we are most concerned with! |
| Rem (rem) = Rad Equivalent Man | measures occupational exposure and effective dose – expresses the amount of radiation received by radiation workers – SI unit of measure is the sievert (Sv) |
Created by:
saleclair
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