I. History of the Grid
With the widespread use of X-rays in medical imaging, the impact of scattered radiation on image quality became increasingly evident, leading to the invention of the anti-scatter grid. Early grids had a simple structure with thick lead strips and limited specifications in terms of grid ratio and line density. While they could reduce scatter radiation, they also absorbed a significant amount of primary radiation.
As medical imaging technology evolved, manufacturing processes improved. The lead strips became thinner, and a greater variety of grid ratios and line densities became available. Materials were also optimized, and different filler substances were explored to meet various imaging requirements. With the advent of digital radiography (DR), grids were designed to be removable, allowing radiographers to decide whether to use them based on the body part being imaged—achieving a better balance between image quality and radiation dose.
II. What is a Grid?
A grid, fully named an “X-Ray Antiscatter Grid” is comprised of alternating lines of lead and another, much more X-ray transparent material, that is put directly in front of the detector or film. The purpose of them is to remove x-rays that have scattered and are not moving in a direct line from the X-ray tube to the detector.
Ⅲ. Key Concepts
- Focal Distance (f): The perpendicular distance from the convergence point of the extended lead strip lines to the grid plate; also known as the grid’s focal distance (or radius).
- Grid Density (N): The number of lead strips per centimeter, usually ranging from 40 to 65 lines/cm.
- Grid Ratio (R): The ratio of lead strip height to the distance between strips. A higher ratio improves scatter rejection, typically ranging from 8:1 to 14:1.
Structure:
A grid typically consists of a 3–5 mm thin plate, made with 0.05–0.1 mm wide lead strips aligned in parallel or angled orientation, and held between layers of aluminum or cardboard. The spaces between the strips are filled with materials that are transparent to X-rays, such as wood, paper, or aluminum, for positioning and bonding.
Ⅳ. Usage Considerations
Positioning:
The grid should be placed between the patient and the image receptor (cassette or DR detector). The focused side should face the incoming X-ray beam, with the focal point of the X-ray tube aligned with the convergence line of the lead strips. The grid must not be placed backward. The center of the X-ray beam should be aligned with the center of the grid, with lateral deviation no more than 3 cm. If the X-ray tube is tilted, the direction should be parallel to the orientation of the lead strips.
Other Notes:
- When changing focal distance during imaging, the deviation should not exceed 25% of the specified grid focal distance.
- For moving (reciprocating) grids, the movement duration should be at least one-fifth of the exposure time.
- Exposure settings should be increased to compensate for the absorption of primary radiation by the grid.
- Grids are generally not required for imaging thin body parts like the limbs.
- Different tube voltages (kVp) require matching grid ratios.
- Special imaging setups like upright chest stands and mammography have dedicated grid requirements.
Ⅴ. To Use or Not to Use?
Whether to use a grid in DR imaging depends on several factors. When imaging thicker body parts (over 15 cm) or using high tube voltages (above 60 kVp), a grid can effectively reduce scatter radiation and improve image contrast. However, since grids also absorb some primary radiation, exposure conditions must be appropriately adjusted.
For thinner areas like the extremities, where scatter radiation is minimal, a grid is usually unnecessary.
In conclusion, the decision to use a grid should be made based on the specific body part, imaging conditions (e.g., thickness and kVp), and clinical requirements to achieve optimal image quality while minimizing unnecessary radiation dose.
