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Chap 15,16,18
Imaging1. 15&16&18. Beam restriction&PT beam emitter&grid
| Question | Answer |
|---|---|
| The principle factors that affect the amount of scatter produced are: | Kilovoltage and the irradiated material |
| As kVp increases, what happens to the total number of photons that undergo interaction? | decreases. (more pass through the patient to interact with the IR instead) |
| As kVp increases, what happens to patient dose? | Decreases. |
| The higher the atomic number of a material, the _____ the amount of PE absorption and the ____ scatter. | greater the amount of PE, less scatter. |
| Why does the atomic number of materials have an impact on the amount of scatter? | Because higher atomic numbers have more electrons which means photons have a greater chance at striking an electron, creating an absorption interaction. ex) Bone absorbs more and scatters less than soft tissue. |
| Which shutter controls the image detail/recorded edge? | bottom |
| The ____ shutter reduces the penumbra because of what? | bottom, greater distance from the focal spot. |
| What is penumbra? | geometric unsharpness around the periphery of the image. |
| What do the upper shutters of the collimator help with? | Reducing the amount of off-focus radiation by absorbing some radiation before is exits the tube. |
| What is a PBL? | Positive beam limitation device. Auto collimation. |
| The thicker the body part, the _____ the attenuation will be. | greater |
| ______ atomic number materials attenuate a greater percentage of the beam than _____ atomic number materials. | high, low. |
| Why do higher atomic numbers attenuate more? | They have more electrons with which photons can interact. |
| Along with atomic number, what else affects attenuation? | Density |
| What is subject contrast? | degree of differential absorption resulting from the differing absorption characteristics of the tissues in the body. |
| A large patient with more OID will have an impact on what? | Subject detail/recorded detail. |
| Misrepresentaion of the size or shape of the structure of interest is called? | Distortion |
| Magnification is a form of | distortion |
| Moving a patients arm may cause foreshortening/distortion, what would cause elongation? | Central ray movement. |
| When an x-ray beam passes through the body, 1 of 3 things will happen. what are the 3 options? | pass through unaffected, be absorbed by body, interact and change direction. |
| Higher atomic number materials will have more or less scatter? | less |
| What is the basic layout of a grid? | a series of radiopaque strips that alternate with radiolucent interspace strips. |
| What are the radiopaque strips for and what kind of material are they usually? | needed to absorb the scatter, usually lead |
| what are the radiolucent strips for and what kind of material are they usually? | allows radiation to pass easily through, usually aluminum. |
| What is grid ratio? | ratio of the height of the lead strips to the distance between the strips |
| what is grid frequency? | number of grid lines per inch or centimeter |
| What are linear grids? | lead strips running in only one direction |
| why are linear grids most commonly used? | they can be used when doing procedures that require tube angulation, you can angle the CR along the direction that lines are running. |
| What would result in the primary beam being directed into the lead strips? | Angulation ACROSS the long axis (towards the patients left or right side)… grid cutoff. beam angled into the lead. |
| What are parallel grids? | made with the lead and interspace strips running parallel to one another. |
| what are focused grids? | central grid strips are parallel and as the strips move away from the central axis they become more and more inclined |
| When are stationary grids primarily used? | portables or for upright or horizontal beam views. |
| Since the lead strips of the grid run along the long axis of the table, to blur the lead lines what directions will the grid be moving? | Must move at a right angle to the direction of the lines, so back and forth across the table NOT top to bottom. |
| What are the two movement mechanisms used today for grids? | reciprocating and oscillating |
| Reciprocating grid moves how? oscillating grid moves how? | Recip= back and forth. oscillating= electromagnet pulls and releases- circular motion within the frame. |
| The more efficient a grid is, the ____ exposure will be received by the IR. | less |
| What affect does a grid have on patient dose? why? | To compensate for less IR exposure, mAs must be increased. The increase in mAs increases patient dose. |
| ******** Grid conversion factors for 85 kVp | Nongrid/screen = 1 5:1 grid ratio = 2 6:1 grid ratio = 3 8:1 grid ratio = 4 10:1 or 12:1 = 5 15:1 or 16: 1 = 6 |
| modern grids absorb up to how much scatter? | 80-90% |
| ** What is the grid conversion formula? | |
| * a 10 in air gap has the same degree of clean up of scatter as a ___ grid for a 10 cm body part. | 15:1 grid |
| * What are the two types of linear grids and which is more common? | Focused and parallel, parallel are more common |
| Which grid is designed to match the divergence of the beam? | focused. The lead strips become more inclined further from the central axis |
| What is remnant radiation? | Radiation that comes out of the patient |
| commonly used grids have a frequency of what? | 85-103 lines/inch |
| Grids with a higher grid frequency have ____ lead strips. | Thinner |
| More lead with ____ ratio and ___ frequency. | high ratio and low frequency. |
| Grid ratio vs grid frequency? | ratio is height and distance between lead strips, frequency is number of grid lines per inch or cm. |
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Zoest35
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