Radiographic grid
History:
The concept of using a grid to improve radiographic image quality was first proposed by Gustav Bucky in 1913. At the time, Bucky was working as a radiologist at the University of Würzburg in Germany, and he recognized that scattered radiation was a major cause of image degradation in X-ray imaging. He proposed the use of a grid consisting of thin lead strips placed perpendicular to the plane of the image receptor to absorb scattered radiation and improve image contrast. The first radiographic grid was developed in the early 1920s by Hollis Potter, a radiologist at the Hospital for Special Surgery in New York City. Potter's grid consisted of lead strips that were approximately 0.8 mm thick and spaced 3.2 mm apart. The grid was positioned between the X-ray tube and the image receptor and was effective at reducing scatter radiation.
Design:
Radiographic grids are typically made of thin lead strips or other high atomic number materials, such as tungsten, that are placed perpendicular to the plane of the image receptor. The spacing between the lead strips can vary depending on the application but is typically between 0.2 mm and 2 mm. The thickness of the lead strips also varies but is typically between 0.05 mm and 0.5 mm.
The design of a radiographic grid depends on several factors, including the X-ray energy, the image receptor size, and the distance between the X-ray tube and the image receptor. Grids can be designed to be stationary, where the grid remains in a fixed position between the X-ray tube and the image receptor, or they can be designed to move during the exposure to minimize the appearance of grid lines on the image.
Use:
Radiographic grids are used to improve image quality in a variety of medical imaging applications, including chest X-rays, mammography, and CT imaging. They are particularly useful in situations where there is a large amount of tissue or bone that can scatter radiation and reduce image contrast.
To use a radiographic grid is positioned between the X-ray tube and the image receptor. The X-ray source is then activated, and the image is captured on the image receptor. The grid absorbs scattered radiation, which improves image contrast and reduces the appearance of artefacts on the image.
Types:
There are several different types of radiographic grids that are available, each with its own design and application. The three main types are:
Parallel grids: Parallel grids consist of lead strips that are
placed parallel to one another, with the strips spaced at regular intervals.
They are effective at reducing scatter radiation but can produce grid lines on
the image.
Crossed grids: Crossed grids consist of two parallel grids that are
placed perpendicular to one another. They are effective at reducing scatter
radiation and do not produce grid lines on the image.
Criss-cross grids: Criss-cross grids consist of two sets of parallel
lead strips that are angled at 45 degrees to one another. They are effective at
reducing scatter radiation and do not produce grid lines on the image.
Grid
ratio
Grid ratio refers to the relationship between the
height of the lead strips in a radiographic grid and the spacing between them.
A higher grid ratio means that the lead strips are taller and the spaces
between them are narrower. The grid ratio is calculated by dividing the height
of the lead strips by the distance between them.
Grid ratio is an important parameter in
radiographic grid design because it affects the amount of scattered radiation
that is absorbed and the contrast improvement that can be achieved. A higher
grid ratio is generally more effective at reducing scatter radiation and improving
image contrast, but it also requires more radiation exposure to the patient.
The grid ratio can range from 5:1 to 16:1, with
higher ratios generally being used for larger body parts and higher energy
X-rays. For example, a 5:1 grid might be used for a small body part, such as a
hand or foot, while a 16:1 grid might be used for a large body part, such as
the chest or abdomen, with higher energy X-rays.
Here's an example to illustrate how grid ratio
affects image quality:
Imagine a radiographic grid with a 5:1 ratio and a
second grid with a 16:1 ratio. Both grids have the same number of lead strips
per inch, but the taller lead strips and narrower spacing of the 16:1 grid make
it more effective at reducing scatter radiation.
If an X-ray is taken without a grid, scattered
radiation will reach the image receptor and degrade image quality. When a 5:1
grid is used, some scattered radiation will still reach the image receptor, but
the grid will absorb a significant amount, resulting in improved image
contrast. When a 16:1 grid is used, even less scattered radiation will reach
the image receptor, resulting in even better image contrast than with the 5:1
grid.
However, the use of a higher grid ratio also means
that more radiation exposure is required to produce a quality image. The choice
of grid ratio depends on the specific imaging situation, including the body
part being imaged, the X-ray energy, and the image receptor size.