Final answer:
An SID somewhat longer or shorter than the 100cm focal distance of a 16:1 grid can be used; typically, an SID within the range of 100cm to 120cm is acceptable without significant loss of primary radiation, due to the grid's ability to clean up scatter radiation effectively.
Step-by-step explanation:
When considering the selection of a Source-to-Image Distance (SID) when using a 16:1 grid focused at 100cm for a cross-table lateral view of the hip, it is important to understand that in radiography, grids are used to absorb scattered radiation and improve image quality. The purpose of a grid is to allow primary radiation to pass through to the image receptor while absorbing scattered radiation that does not contribute to image formation. The focus of a grid is set based on the distance from the x-ray tube at which the grid's lead strips are aligned with the primary beam of radiation. In this case, the focus is at 100cm, which typically matches the SID for optimal imaging.
However, grids have a range of SID within which they are effective, commonly referred to as the 'grid focus range' or 'focal range.' A 16:1 grid has a high ratio of height to the space between the lead strips, making it very efficient in scatter cleanup but also more sensitive to positioning and distance changes. While there is no specific value mentioned for an optimal SID beyond the 100cm focal distance, it is generally recommended to stay within a reasonable range of that focal distance to prevent significant loss of primary radiation (also known as grid cutoff).
In practice, SIDs somewhat shorter or longer than the focal distance can be used with attention to avoiding excessive distances that would increase the divergence of the x-ray beam and potential for grid cutoff. It is still considered that an SID of 100cm to 120cm would typically be acceptable for most 16:1 focused grids without significant primary radiation loss.