Which of the following grid types can't be used when tube angulation is required?

A crossed grid, also known as an "X-pattern" grid, is made up of two sets of lead strips that are oriented perpendicular to each other, which results in very high scatter cleanup efficiency. However, this grid design is highly susceptible to positioning errors. When tube angulation is introduced, it can drastically affect the way that the x-rays interact with the grid. Due to the orientation of the lead strips, angling the tube can lead to significant grid cutoff, reducing the quality of the image.

In contrast, focused grids, linear grids, and parallel grids can accommodate tube angulation within certain limits. Focused grids are designed to match the divergence of the x-ray beam, allowing for some angulation while maintaining image quality. Linear grids can be used in various positions, and parallel grids, while less efficient at reducing scatter than focused grids, still allow for some angling without excessive loss of image quality. Therefore, the unique design of a crossed grid makes it incompatible with situations requiring tube angulation, which is why it is identified as the correct choice in this context.