Scattered Radiation and Contrast

Learning Objectives

References and Resources

Mind Map Summary

Learning Objectives

• Explain how scatter affects image contrast.
• Explain the general relationship between x-ray beam area and image contrast.
• Explain and illustrate the concept of grid ratio.  What are typical values?
• Distinguish between parallel and focused grids.
• Define the following grid quantities:

a.                  primary penetration

b.                  Bucky factor (B)

c.                   contrast improvement factor (K)

• Briefly explain the following three types of grid cutoff and describe their effect on images.

a.                  incorrect distance between grid and tube.

b.                  off-center grid

c.                   tilted grid

• Discuss the relationship of contrast improvement to grid ratio.
• Discuss the relationship of patient exposure to grid ratio.
• Describe the advantages and disadvantages of a low-ratio grid.
• Describe the advantages and disadvantages of a high-ratio grid.
• Discuss the use of grids in mammography.
• Describe the differences among the following beam-limiting devices and the advantages and disadvantages of each.

a.                  fixed cones

b.                  light localizing collimators

c.                   positive beam limitation (automatic collimators)

• Describe how grid lines can be eliminated from an image.
• Describe an air-gap technique.  What are the advantages and disadvantages? Identify a clinical procedure where it is routinely used.

References and Resources

1.  Chapter 13. The Physical Principles of Medical Imaging, 2^ndEd. Perry Sprawls.  Medical Physics Publishing.

Resources on the Web

Mind Map