Module 6 - Week 8 Flashcards
How does increasing kVp affect scatter radiation?
Increasing kVp increases photon energy and penetrability, reducing photoelectric absorption while relatively increasing Compton scatter. Higher kV results in more transmitted photons.
How does tissue size affect scatter radiation?
As tissue thickness or density increases, photon interactions and scatter also increase, even at constant kV.
How does collimation affect scatter radiation?
Opening the collimators increases field size, allowing more x-rays to pass and interact with tissue, generating more scatter. Closely collimating to include only necessary structures reduces scatter.
What are the components of grid construction?
Grids have parallel lead strips separated by radiolucent materials (aluminum or plastic fiber), enclosed in an aluminum casing.
What is the function of a grid?
Grids prevent scatter from reaching the receptor and degrading image quality. They are typically used for body parts 10cm or thicker.
Define grid ratio.
Grid ratio is the ratio of the height of the lead strips to the distance between them. A higher grid ratio increases the likelihood of absorbing scattered radiation but also absorbs some of the useful beam.
Define grid frequency.
Grid frequency refers to the number of lead strips per inch or cm. A higher frequency improves scatter reduction but also absorbs more of the useful beam.
Describe a parallel grid and its appropriate use.
Parallel grids consist of vertically oriented lead strips that absorb off-focus photons. However, they also absorb some useful diverging photons, especially at the periphery, leading to grid cut off.
Describe a focused grid and its appropriate use.
A focused grid aligns lead strips with beam divergence and requires a specific SID. Correct SID and central ray alignment are crucial for optimal performance.
Describe a crossed grid and its appropriate use.
Crossed grids consist of two stacked parallel grids, effectively reducing scatter but causing grid cut-off at the periphery. They require higher technique settings, leading to increased patient exposure, which outweighs their benefits, so they are rarely used.
Describe an oscillating grid and its appropriate use.
Oscillating grids, found in bucky mechanisms, use motorized movement to reduce scatter and blur grid lines. They can be parallel or focused in design.
How does using a grid affect image contrast?
Using a grid significantly enhances image contrast.
How does applying a grid affect technical factors (mAs)?
Applying a grid requires an increase in exposure (typically mAs) to compensate for the absorption of useful photons.
What is inherent grid error?
Inherent grid error, which is only grid cutoff not caused by operator error, occurs at the edges of parallel grids when collimation is fully opened because the parallel lead strips absorb part of the useful beam due to the beam’s divergence.
What is off level grid error?
Off level error is due to a tilted or off-level grid and results in a majority of the useful beam being absorbed.
What is off center grid error?
Off center grid error happens when the x-ray beam is not accurately aligned with a focused grid, causing grid cut-off because of beam divergence.
What is off focus grid error?
Off focus error occurs when the incorrect SID is applied to a focused grid.
What is backwards grid error?
Backwards grid error results in near complete image cut off due to the opposing angles of the divergent beam and the focused grid lines.
How do modern computing algorithms affect the use of grids?
Modern computing algorithms can enhance image contrast, reducing the need for traditional grids in some situations. These software solutions require little to no increase in exposure and can simulate up to a 6:1 grid ratio.
