Grid cutoff

Grid cutoff is an unwanted absorption of x-rays via an x-ray grid, observed when a grid is employed incorrectly, most often seen with parallel grids. The term cutoff stems from the phenomenon in which the primary x-ray beam is 'cut off' by grid lines, leading to an overall decrease in optical density or a decrease in radiographic exposure (more opaque). The mechanism in which grid cutoff occurs differs based on the grid used. 

Parallel grid cut off

Most commonly observed grid cutoff, most prominent at a short source to image receptor distance (SID), due to the divergent beam geometry, a small SID will lead to undesired absorption of the primary x-ray beam via the grid lines. Taking on the appearance of an overall decrease in optical density or variable optical density across the image (one side appearing less dense than the other). 

The distance in which this occurs can be calculated using:

• distance to grid cut off = SID / grid ratio

Crossed grid cut off

A crossed grid is two parallel grids running perpendicular to reduce scatter in both directions; they are proven to clean up scatter radiation more effectively than parallel grids. However, due to beam geometry, the primary beam must be centred on the centre of the grid, at the same angle as the crossed grid. Any deviation can cause significant grid cut off, creating a decrease in primary beam reaching the detector often unilaterally, with the potential to simulate pathology.

Practical points

A sound method in identifying cut off is to compare the bony detail each side of the image, images with grid cut off, for example, an AP erect chest will have a notable difference in bony detail of the shoulder girdles. 

The problem of grid cutoff is almost exclusive to stationary grids, most images performed in table or upright stands utilise oscillating grids, at fixed points.