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RAD 121 - PP Unit 2
PowerPoint question
| Question | Answer |
|---|---|
| What type of energy do incident electrons convert to heat and x-rays? | KINETIC energy |
| What is the ratio of heat energy to x-ray energy? | about 99% heat and 1% x-rays |
| What is classical scatter? what are other names for it? | low energy interactions within the anatomic part (less than 10keV); energy is redirected, erroneously activating pixels on the IR; Thompson, Coherent, unmodified, Rayleigh, elastic |
| What is photodisintegration? | An atomic nuclear event; the interaction destabilizes the nucleus, causing to eject material to restabilize. Occurs at photon energies over 10MeV |
| What is another word for positive contrast media? What are some examples? | RADIOPAQUE - Barium, Iodine |
| Describe FOG. | created when there is scatter being absorbed by the IR, reduces contrast, degrades image. |
| What is the general keV for Thompson scatter? | 10-20 keV |
| Does Rayleigh scatter ionize the atoms of the target? | NO |
| How does the wavelength of the incident photon relate to the wavelength of the emitted radiation during coherent scatter? | SAME - all energy is transmitted, but in different direction |
| What effect does classical scatter have on image contrast? | LOWER contrast - more photons, less contrast |
| Does coherent scatter contribute to patient dose? | YES |
| What is the general keV for Compton Scatter? | 20keV - 125keV |
| Does Compton Scatter contribute to patient dose? | |
| What happens during modified scattering? | Incident x-ray photon interacts with an outer orbital electron, removing it from its shell. This removes up to 1/3 of the original energy, which is continues in a different direction (creating scatter). |
| How does the wavelength of the incident photon relate to the wavelength of the emitted radiation during Compton scatter? | The incident wavelength is stronger than the scattered photon - its wavelength is longer and frequency is decreased (due to lower energy) |
| Compton Scatter - what does this interaction do? | 1. creates ionization (unstable atom); 2. The RECOIL (ejected) electron can go on to have more interactions.; 3. Incident photon is also deflected in another direction. |
| Why is Compton scatter important to radiographers? What can be done? | A) Major source of occupational exposure. Shielding (for us, patients and family) and dosimeters are used to monitor exposures. B) creates fog/noise that degrade image. Use grids to reduce scatter |
| Where does Compton scatter occur? What does it depend on to occur? | A) In soft tissue and bone - basically, everywhere B) It is dependent on the energy of the incident photon (not the atomic #) |
| What effect does higher kVp have on Compton scatter? | Increases Compton scatter, |
| How much energy is usually transmitted from the incident photon during a modified scatter incident? | up to 1/3 of the energy is transmitted to the ejected electron |
| What is back scatter, front scatter and side scatter? | Back scatter is Compton that scatters back towards x-ray tube (least energetic type of scatter). Front scatter is Compton that scatters towards IR (most energetic scatter) and Side scatter is Compton that scatters out towards the sides |
| As x-ray photon energy increases, the probability of that photon penetrating a given tissue without interaction _____. | INCREASES |
| With increase in photon energy, likelihood of Compton interactions relative to photoelectric interactions also _____. | INCREASES |
| Increased kVp or keV results in _____ scatter, _____ transmission, and _____ absorption. | INCREASED, INCREASED, DECREASED |
| What type of interaction is dominant with higher kVp? | COMPTON |
| Why is Compton so dangerous to the radiographer? | Because it retains about 2/3 of its energy and may leave the patient to expose the radiographer. MUST use shields to prevent |
| What is a photoelectric effect? | Photon interacts w/ inner-shell electron of tissue atom. The interaction causes electron to absorb all incident photon energy and eject the inner shell electron from atom. This electron is called PHOTOELECTRON. The vacancy created results in characteristi |
| What is a photoelectron? | the electron ejected from an atom during a photoelectric event. It carries all the energy of the incident photon. |
| At what energies do photoelectric interactions occur? | Occurs throughout diagnostic range - incident x-ray photon must be equal to or greater than orbital shell binding energy. |
| In which shells do photoelectric events occur? | K shell or L shell |
| Is the characteristic cascade in the x-ray tube the same as the characteristic cascade in the patient? Why or Why not? | NO. Characteristic cascade in the x-ray tube occurs at 69.5keV due to TUNGSTEN, so energy is stronger. Characteristic cascade in tissue occurs at lower energies due to lower atomic numbers. |
| How does contrast medium such as Barium affect atomic number of tissue? | INCREASES atomic number |
| What does the probability of photoelectric interaction depend on? | Energy of incident photon and atomic # of tissue. The incident photon must have high enough energy to remove the inner shell electrons. |
| What is pair production? | Occurs with HIGH energy photons of 1.02 MeV or higher. The incident photon has high enough energy to interact with the NUCLEUS of the atom, absorbing and emitting a POSITRON (unnatural) and ELECTRON which share the original energy. |
| What imaging modality(ies) uses pair production? | Nuclear medicine (PET scans) or Radiation Therapy |
| What energy do the positron and electron carry? | .51 MeV each |
| What happens after the positron and electron leave the atom? | The positron travels until it unites with a free electron, destroying both during an annihilation event, transferring the energy into 2 x-ray photons traveling in opposite directions. The free electron travels until it unites with another atom. |
| What is photodisintegration? | occurs with SUPER HIGH energy photons (more than 10MeV) strike the nucleus, making it unstable, The nucleus ejects a nuclear particle (proton, neutron, alpha) |
| Which x-ray interaction is the most hazardous to the radiographer and patient? | COMPTON scattering |
| During pair production, the resulting electrons have how much energy? | .51 MeV each |
| What is (are) the atomic result of Compton scatter? | recoil electron and scattered x-ray photon |
| Materials or tissues that attenuate small amounts of x-ray photons are called _____. | radiolucent |
| What factors affect the emission spectrum? | mA, kVp, tube filtration, generator type, target material (anode) |
| There are two portions to the emission spectrum. What are they? | Continuous portion - the interactions that occur under the entire curve of energy used. DISCRETE portion represents the line that indicates the CHARACTERISTIC interactions (occur at the binding energy of the K shell (with Tungsten it's 69.5) |
| In an emission spectrum graph, what does the x-axis represent? | X-ray energy - KeV |
| In an emission spectrum graph, what does the y-axis represent? | Quantity of x-ray photons present in beam |
| Why is the DISCRETE portion of the emission spectrum a line? | because the photon energies are limited to just a few EXACT values (69.5/70 KeV for tungsten) |
| What does the continuous portion represent? | Bremsstrahlung interactions - can be low or high energy depending on how close the interaction occurs to the nucleus of the atom |
| What do Brem photon energies depend on? | The strength of the filament electron attraction to the nucleus (how/where it interacts with the atom) Can range from just above 0 to max kVp selected. |
| How much energy does the peak of curve represent? | 1/3 of total energy of photons (1/3 of kVp energy) |
| What are BEAM Properties? | properties that characterize a given x-ray based on how it was produced and how it behaves in its interaction with matter |
| Which BEAM PROPERTIES are important to the radiographer? | beam QUANTITY and beam QUALITY |
| What is Beam Quantity? | the total number of x-ray photons in a beam |
| What affects BEAM QUANTITY? | mAs, kVp, distance and filtration |
| What beam property should ALWAYS be linked to patient dose in the radiographer's mind? | BEAM QUANTITY |
| What is mAs relationship to quantity? | DIRECTLY PROPORTIONAL - because mAs controls the number of electrons thermionically produced at the filament (the number of available electrons for possible interactions) |
| How is beam quantity affected by kVp? | Beam quantity varies as the source of the ratio of the change in kVp. If kVp is doubled, the intensity increases by a factor of 4. 15% increase in kVp is equivalent to doubling the mAs. |
| Why does kVp affect beam quantity? | kVp gives kinetic energy to filament electrons |
| Why isn't kVp the most desirable method of changing quantity? | because it is not proportional and kVp influences too many other factors - penetrability, scatter |
| How does distance affect beam quantity? | varies inversely as the square of the distance (inverse square law) I1/I2 = D2 squared/D1 squared |
| What is BEAM QUALITY? | the penetrating (energy) power of the beam |
| What affects BEAM QUALITY? | kVp and filtration (mainly controlled by kVp) |
| What is beam penetration? | x-ray photons that are transmitted through the anatomical part to the IR |
| How does kVp affect beam quality? | increasing kVp increases the beam's ability to penetrate matter. |
| What is a HARD beam? | a high-quality beam; the low energy photons have been removed, increasing the average energy of the beam |
| What is a SOFT beam? | a low-quality beam - low energy photons have not been removed (via filtration, etc) |
| How does FILTRATION affect beam quality? | filtration removes lower energy photons, increasing the average energy (quality) of the beam |
| What is the relationship of filtration to quality? | DIRECT - the more filtration, the more energetic the beam |
| What is HVL? | Half Value Layer - measurement of beam quality - definition - the thickness of absorbing material (Aluminum or its equivalent) necessary to reduce the energy of the beam to 1/2 original intensity |
| How do you determine HVL? | measure the intensity of the beam with radiation detector, then place aluminum filters of known thickness until intensity reading is reduced to 1/2 original value |
| What is the normal HVL of a diagnostic beam? | 3-5 mm of Aluminum |
| What is differential absorption? | Anatomic tissues absorb and transmit x-rays differently based on their composition (atomic number and tissue density) |
| What is attenuation? | the primary x-ray beam loses some of its photons as it interacts with anatomical tissues. Includes absorption and scatter |
| What is absorption? | the x-rays lose all energy and remains in the tissue |
| What is the remnant beam? | transmitted and scattered radiation. also known as exit radiation |
| What is scatter? | When x-ray photons interact with tissue atoms and are redirected from original trajectory |
| What is secondary radiation? | Radiation emitted during interactions between photons and tissue atoms. (secondary to original x-ray beam); also leakage radiation |
| Is secondary radiation the same as scatter? | NO, they are not created the same way - but it has the same effect on both the patient and the IR |
Created by:
Larobbins
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