Scattered Radiation and Contrast

 

Outline and Learning Guide

 

Introduction and Overview
Scattered Radiation
Direction
Scatter Factor

Contrast Reduction
Grids
Grid Function
Grid Ratio

Grid Penetration
Grid Focusing
Grid Cut-off and Positioning
Buckey Factor

Moving Grids

Grid Selection

Collimation

Air Gap

 

 

 

 

 

 

 

 

 

 

 

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.

 

 

 

 

 

 

 

 

 

 

Text Reference

Chapter 13
The Physical Principles of Medical Imaging, 2nd Edition
Medical Physics Publishing, http://www.medicalphysics.org
 

 

 

 

 

 

 

 

 

 

 

Mind Map