To prevent shape distortion, which two elements must be parallel?

To prevent shape distortion in radiographic imaging, it is essential for the object being imaged and the image receptor to remain parallel to each other. When the object is positioned at an angle to the image receptor or is not kept parallel, the resultant image can exhibit elongation or foreshortening, which alters the true representation of the object. This parallel alignment ensures that the x-rays pass through the object and strike the image receptor uniformly, thus maintaining the accurate size and shape of the structure being imaged.

In contrast, while keeping the x-ray tube and the image receptor correctly aligned is important for optimal image quality and consistency in exposure, it does not directly influence the prevention of shape distortion. The positioning of the x-ray tube mainly affects the angle and intensity of the radiation reaching the object and image receptor, rather than the geometric fidelity of the image.