Diagnostic Imaging flashcards
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| Bremsstrahlung Radiation 70-90% of primary X-ray beam. Energy loss via interaction e- with nucleus |
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| e- brakes or changes direction results in energy loss. If e- collides directly with nucleus |
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| releases 100% of its radiation. Otherwise may just chagne direction and release partial E. Range of energies formed forms the X-ray spectrum. Effects of increasing kV E- will have higher kinetic energy at collision. This means increased amount of Br | of collisions, increased in number of Bremsstrahlung radiation energy and increased intensity (density). No effect on energy!
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| increased in number of Bremsstrahlung radiation energy and increased intensity (density). No effect on energy! Characteristic Radiation Interactions between incoming e- and the inner shell electrons of the target. The e- is ejected |
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| cascades e- to lower levels resulting in energy release (different depending of binding energy in the level). 10-25% of primary x-ray beam |
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| usually just ignore. What is the heel effect? The x-ray beam has uneven beam intensity (higher intensity at cathode and more absorption near the anode). Can utilise for body parts of variable thickness (place thicker end at cathode and thinner end at |
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| small enables increased image detail) |
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| Focal spot size (small area means decreased heat capacity with better image detail) |
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| Heat Capactiy of Target (stationary anode withstands less heat) and Circuits/Power supply Compare the Actual to Effective Focal Spot Actual focal spot is the area of the target bombarded by electrons and is determined by the area of the electron cloud | to air)
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| the filament size and the focusing cup. The Effective focal spot is the actual focal spot viewed through the tube window and is therefore ALWAYS SMALLER than the actual focal spot. A large focal spot will increase the possible exposures and heat dissipa |
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| can be completely absorbed (White) or transmitter as a scatter photon (won't reflect true density). 4 things that affect Attenuation Tissue thickness (increase = increased attenuation and therefore need to increase kVp); Density of tissue |
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| Beam Energy (penetration) |
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| Atomic Number of the Tissue (bone has higher composite atomic # to air) How does tissue density affect Attenuation? More density = more attenuation. Density of bone is 2 |
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| Density of soft tissue is 1 | = Z. PE = Z^3
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| density of gas is negligible. Therefore you have twice the attenuation in bone as you do in soft tissue |
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| What are the two mechanisms of attenuation? 1. Photoelectric Effect (absorption) and Compton Scatter (Scatter) What is the photoelectric effect? All gamma ray's energy is transferred to the electron (of atom in tissue) |
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| all energy is absorbed and transferred to electron. That photon does not reach the film and therefore results in decreased radiographic density on the film (because the tissue atom doesn't produce radiation). What is the significance of the photoelect |
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| or miss the film completely (risk of radiation safety). What is the significance of Compton Scatter? Causes increased scatter production |
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| decreased film quality and increased worked dose. What 2 things influence compton scatter? Primarily Tissue density but also beam energy (kV) How does tissue density affect Compton scatter? Increased tissue density increases compton scatter produced (i |
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| ~31% PE and 63% Compton How does scatter affect film qualty? Decreases it and increases worker dose. Begins to significantly affect quality at ~10cm. Methods of scatter control? Reduce scatter production and reduce scatter from reaching the film |
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| What are ways to reduce scatter production? Which is the most effective? 1.) Decrease the kV |
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| which decreases the Compton scatter. Note that tissue thickness and density will limit how much you can decrease kV |
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| limiting the effectiveness. 2.) Decrease the volume of tissue irradiated (most effective) = collimate! Note that grids do NOT affect scatter production. How do you reduce the scatter reaching the film? 1.) Air gap technique - increase the Object film d |
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| which reduces energy of scatter and increases PE. 4.) Grids (most effective) to absorb scatter before it reaches film - increase exposure factors to compensate. What is the structure of a grid? Flat plate with a series of lead foils trips and radioluce |
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| will hit lead foil and be absorbed. What are grid lines? Shadow of the lead strips in the grid |
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| are a series of fine white lines superimposed over the image |
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| usually uniform in size and distribution. They should not be obvious What is the grid ratio? Measures the scatter absorbing capacity of a grid. Ranges from 5:1 to 16:1. A higher grid ratio means greater capacity to absorb scatter and the primary X-ra |
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| and increases need for accuracy in grid placement |
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| What is the grid factor? The amount of exposure to increase when using the grid. It will depend on the grid ratio and type of grid. Start without a grid |
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| then add it and take a series of exposures until it matches the original exposure. Parallel Grids Lead strips are parallel to each other - Focused Grids Most common - lines gridlines up with angle of X-ray beam. Must use at specified focal distance t |
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| most efficient with potter-bucky. Potter-Bucky grids Focused grid under the table |
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| moves at exposure to blur grid lines . Most efficient. Grid Cut-off Grid cuts off more of the primary beam than it should |
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| causing pale "underdeveloped" area and uneven lines. (abnormal grid lines of reduced radiographic density - wide and uneven). Distribution will depend on the method of cutt-off and the grid type Grid Cut-off: Combined (Not Centered & Incorrect FFD) Mo |
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| wide and uneven |
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| and they decrease as you move across the film. This is more severe with a higher grid ratio |
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| an increased distance of lateral positioning and is inversily proportional to FFD Grid Use and Selection Horses; use highest grid ratio you can |
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| crosshatched. Smallies: generally 1:5 ratio.Usually use <8:1 with less than 90kV |
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| and portable/stationary machines. Higher capacity machines can use Potter-Bucky. Beam Filtration Filter material between x-ray tube and patient |
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| reduces patient dose by preventing low energy X-rays fromb eing absorbed by the patient. 3mm Aluminum will reduce patient dose by 80%. It does increase the average beam energy. Grid Cut-off: Parallel Grid Results in cut off at edges of the film. Sign |
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| small FFD and large field sizes. Grid Cut-off: Inverted Focused Grid Results in severe peripheral cut0off with a dark band of exposure at the center of the film. Higher ratio results in narrower band being exposed. Grid Cut-off: Not Centered or Perpe |
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| can be identified by abnormally wide grid lines. Incorrect Film Focal Distance Can occur either above or below focal range |
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| and the cut off will be seen at both film edges. Function of Film Screen Absorbs the X-rays. Phorsphors in screen fluoresce and emit visible |
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| intensified light |
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| which exposes the film. Brightness of the light is proportional to number of x-rays absorbed. Some detail lost with screen use. How does screen speed affect film exposure FAST: means a lower exposure is required |
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| but results in poor detail. MEDIUM: medium exposure is needed. SLOW: higher exposures required. What are the 4 properties of screens? 1. |
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| Absorption Efficiency |
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| 2. |
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| Conversion efficiency of X-ray to light |
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| 3. |
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| Screen speed |
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| 4. |
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| Speed class Absorption Efficiency The amount of X-ray energy able to be absorbed by a screen. It's a function of the phosphor type andd screen properties. Conversion Efficacy Measures the efficacy a screen converts absorbed X-rays into light. This i |
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| not the screen properties. Phosphors vary in Absorption Efficiency |
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| Conversion Efficiency and Fluorescent Colour. Screen Speed This is influenced by the screen properties (crystal size |
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| phosphor layer thickness and phosphor type). Large crystals give a larger light flash |
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| resulting in a larger area exposed with fewer X-rays needed and have a greater absorption efficiency but poor DPI |
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| poor detail (grainy image). Increased thickness results in a larger volume of light produced |
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| so a greater absorption efficacy |
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| but also causes increased spread of light (poor detail). Usually combine thick crystals with thick layers. Speed Class A number given to a screen-film combination to indicate relative speed (higher number = faster speed). Compare phosphor types Calc |
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| 5% CE |
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| speed class 25; Calcium Tungstate Regular = Low AE |
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| 5% CE |
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| speed calss 100 Rare Earth Fine = Higher AE |
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| 20% CE |
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| 100 Speed class and then Rare Earth Regular = Higher AE |
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| 20% CE |
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| 400 speed class Compare double to single mounted screens Screen Artefacts Appear as focal white artefacts/marks on radiographs (due to lack of light from screens reaching the films). Dust hair etc (clean screen) or damaged/old phosphors no longer flo |
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| with silver halid crystals within the emulsion. |
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