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Chap 15,16,18

Imaging1. 15&16&18. Beam restriction&PT beam emitter&grid

QuestionAnswer
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.
Created by: Zoest35