What type of grid error occurs when the x-ray beam is not aligned with the grid lines?

When the x-ray beam is not aligned with the grid lines, it results in a type of grid error known as off-angulation. In this situation, the angulation of the x-ray tube relative to the grid can cause an ineffective interaction between the x-rays and the grid, leading to an inadequate image contrast due to unwanted scatter radiation reaching the detector.

Off-angulation specifically refers to the improper angle at which the x-ray beam is directed in relation to the specific orientation of the grid lines. If the beam is angled differently than intended, it can exacerbate the grid's ability to absorb scatter, leading to a noticeable reduction in image quality.

Other options involve different circumstances: off-center refers to misalignment between the central ray of the beam and the center of the grid; off-level indicates that the grid itself may be tilted or the x-ray tube is positioned improperly in relation to the grid plane; and upside-down describes a scenario where the grid is placed incorrectly. Each of these situations affects image quality in specific ways, but they do not pertain to the alignment of the beam with the grid lines that defines off-angulation.