Positioning is based on body part being imaged, condition of patient, and suspected disease. Body positioning is important because many of the internal structures superimpose over each other, therefore, certain positions are required to get a clear view of the area of concern.
Ex. Chest X-ray: The heart lies more anterior and a little to the left, this is why the PA projection is preformed; so the heart will be closer to the IR. A perpendicular CR is used at 72 SID. If any of these factors are altered we have the possibility of size distortion.
X-rated 