Overview
This lecture covers the basics of radiographic grids, including their purpose, construction, grid ratio calculations, and how to adjust exposure settings when using or not using a grid.
Grid Basics and Purpose
- Grids reduce scatter radiation and improve image contrast by blocking angled secondary (scatter) radiation before it reaches the detector.
- Grids are mainly used for body parts 10 cm or larger.
- Grids consist of lead strips (absorb scatter) and interspace material (allows primary radiation through).
Grid Ratio and Materials
- The grid ratio is defined as the height of the lead strips divided by the distance between them (h/d).
- Higher grid ratios clean up scatter more effectively and produce better image contrast.
- Interspace material cannot be lead; it must allow primary radiation to pass.
- Common grid ratios: 6:1, 8:1, 10:1, 12:1; 16:1 is rare.
Grid Ratio Calculation Examples
- Grid ratio formula: Grid Ratio = Height of strips (h) / Distance between strips (d).
- Example: 1.6 mm height / 0.1 mm distance = 16:1 grid ratio.
- Ignore distractors such as thickness or width of lead strips; only use height and distance between strips.
Grid (Bucky) Factor
- Each grid ratio has a corresponding Bucky factor used to adjust exposure:
- 5:1 = 2; 6:1 = 3; 8:1 = 4; 10:1 = 5; 12:1 = 5; 16:1 = 6.
- The Bucky factor is used to multiply or divide mAs when switching between grid and non-grid exposures.
Exposure Calculations with Grids
- To convert from non-grid to grid: Multiply the original mAs by the grid's Bucky factor.
- To convert from grid to non-grid: Divide the mAs by the grid's Bucky factor.
- If given mA and time, multiply to get mAs before applying the Bucky factor.
- When adding a grid, required mAs always increases compared to non-grid exposure.
Key Terms & Definitions
- Scatter Radiation — Unwanted radiation deflected from its original path, reducing image quality.
- Primary Radiation — The useful radiation that travels in a straight line from the source to the detector.
- Grid Ratio — The ratio of the height of lead strips to the distance between them in a grid.
- Interspace Material — The material between lead strips that lets primary radiation through (cannot be lead).
- Bucky Factor — A number representing the increase in mAs needed when using a grid.
Action Items / Next Steps
- Complete the assigned homework with practice grid ratio and Bucky factor calculations.
- Memorize common Bucky factors for each grid ratio.