Overview
This lecture explains the purpose, design, and operation of radiographic grids in X-ray imaging, with emphasis on improving image quality and using the grid conversion formula.
Purpose of Radiographic Grids
- Grids are used to prevent scatter radiation from reaching the image receptor and degrading image quality.
- Scatter radiation arises from interactions like Compton and coherent scattering within the patient.
- Absorbed and transmitted X-ray photons help form the useful radiographic image; scatter adds unwanted noise.
Grid Design and Function
- Grids consist of thin, vertical lead strips separated by radiolucent (X-ray transparent) material.
- The grid allows useful (aligned) photons to pass through but absorbs scattered (misaligned) photons.
- Grids are positioned between the patient and the image receptor.
Grid Ratio and Efficiency
- Grid ratio = lead strip height ÷ distance between strips.
- Higher grid ratios mean greater scatter removal and better image contrast.
- Common grid ratios include 5:1, 6:1, 8:1, 10:1, 12:1, and 16:1.
- The appropriate grid ratio depends on body part thickness, patient size, and kVp (X-ray energy).
Grid Use Considerations
- Body parts less than 10 cm thick usually do not require a grid.
- High-ratio grids are recommended for larger body parts (≥10 cm) and larger patients, especially at higher kVp.
- Using a grid increases patient dose because some useful photons are also absorbed.
Grid Conversion Formula & Examples
- When changing grid ratio, the mAs (milliampere-seconds) must be adjusted to maintain exposure.
- Grid Conversion Formula:
mAs₂ = mAs₁ × (Grid Conversion Factor₂ ÷ Grid Conversion Factor₁)
- Example conversion factors: No grid = 1, 5:1 = 2, 6:1 = 3, 8:1 = 4, 10:1/12:1 = 5, 16:1 = 6.
- Example calculation: Changing from no grid (factor 1) to a 10:1 grid (factor 5) with original mAs of 2 → new mAs = 2 × 5/1 = 10.
Key Terms & Definitions
- Scatter Radiation — X-ray photons deflected from their original path, causing image noise.
- Radiographic Grid — Device made of lead strips to absorb scatter before it reaches the image receptor.
- Grid Ratio — The ratio of lead strip height to distance between strips (determines grid efficiency).
- mAs (milliampere-seconds) — Product of tube current and exposure time; controls total X-ray quantity.
- Grid Conversion Factor — Number used to adjust mAs when changing grid ratios.
Action Items / Next Steps
- Practice using the grid conversion formula with different scenarios.
- Review when to use various grid ratios based on anatomy and patient size.
- Complete any assigned textbook readings on radiographic grids.