Why are grids used

Courses in radiation sciences and radiation physics, both required classes for student radiographers, highlight the concepts of grid use for medical imaging. There are a lot of UVA patients who currently have chest tubes in place for different medical conditions or post-surgical care for heart and lung procedures. These patients need chest X-rays routinely every morning until their lungs are healthy enough to warrant removing the chest tube.

For many years, UVA mandated the use of grids on all portable chest X-rays. However, this practice created a problem; normal grid lines run longitudinally, but portable equipment required technologists to turn the cassette crosswise, resulting in unsightly grid lines that made diagnosis difficult.

To solve the problem, Hooper purchased short-axis grids that allow for crosswise placement and give technologists the ability to angle the tube without resulting in unsightly grid lines on the finished radiograph. At UVA, the radiology department provided its technologists with gridded cassettes used with computed radiography CR imaging, which are lighter in weight and easier to manipulate.

Focused grids have lead strips that are oriented parallel at the center along the x-ray central axis and progressively slanted to the periphery to match the beam divergence from the focal spot.

The anti-scatter grid is typically manufactured with lead strips oriented along one dimension separated by a low attenuating interspace material such as carbon fiber or aluminum. For specialized applications, there are cross-hatched grids lead strips in both directions, perpendicular to each other for specialized applications such as dedicated chest imaging, and in mammography where a "cellular" grid design made of copper with air interspaces is used clinically by one manufacturer.

By selectively allowing primary x-rays to be transmitted and scattered x-rays to be absorbed in the grid, image contrast is significantly enhanced; however, the grid attenuates some of the desired primary x-rays that are incident directly on the lead strips and allows transmission of some scattered radiation photons that have a small scattering angle, or scatter in a direction parallel to the lead strips, or are multiply scattered with an exit angle from the patient that can be transmitted through the grid.

Grids are chiefly characterized by the grid ratio, grid frequency, and focal distance. The grid ratio is a measure of the height of the lead strip to the interspace distance, and is a good measure of the selectivity of primary to scatter transmission. In general, a grid with a higher grid ratio will reject scatter better than a lower grid ratio, due to the limited angle that is allowed by the grid structure.

However, a higher ratio grid typically has a higher dose penalty for its use for screen-film imaging this is known as the "Bucky Factor" which represents the increased dose to the patient when using a grid compared to not using a grid when the film optical density is matched. With digital imaging, there is also a dose penalty when using a grid is used, and the benchmark is the signal to noise ratio as opposed to film optical density.

Low frequency grids are used with systems having a moving grid assembly known as a Bucky device that oscillates during the exposure to blur the grid lines. Medium and high frequency grids are typically used with stationary grid holders e.

A key aspect of the grid is its ability to help determine and define proportion. In print, proportions most commonly echo the size of the media; the shape and orientation of the paper are often reflected in the size and shape of images included within a layout, for example. This feels comfortable because the reader subliminally understands the context of the layout as a result of the physical shape and size of the delivery mechanism, such as a piece of paper.

On the web, this idea of reflection isn't quite so important, but grids can be used in the same way to anchor content back to the screen. Screens can be more fluid, and as a designer it's not possible to know with the same confidence what size and shape of screen will be used to view content. Regardless of this, proportion and scale are important tools in a layout, so using a grid to determine and enforce rules helps define that all-important set of signposts that enable the reader to access and understand content.

The whole concept of a definitive grid 'system' is a relatively recent invention in the world of design. Grids have existed intuitively since the earliest days of drawing and writing, but it's only recently that layout has been considered in a scholarly fashion, and as such it has never existed in isolation from other best-practice layout rules. One such example of crossover is where the Golden Ratio meets the grid. The Golden Ratio also known as the golden mean determines the most pleasing set of proportions for an element, and is simplified to the 'rule of thirds'.

When used in combination with a grid, these simple rules for size, position and proportion can help ensure a layout feels coherent, but also aesthetically appealing.

This is important because, once again, it can help make the content more accessible. Remember that a grid is the invisible glue behind the content — in most cases it should be transparent to the viewer. Once you know the benefits of having a grid system in place, it makes sense that web designers have adopted grids.

To make things more practical, a few common sizes have become the standard.