Final answer:
Lateral chest radiographs are taken with the left side against the IR to provide a clearer view of the heart and the smaller left lung, which is particularly useful for diagnosing conditions like pneumonia that can result in lung infiltrates.
Step-by-step explanation:
Lateral projections of the chest are taken with the left side against the Image Receptor (IR) primarily to get a better view of the heart and left lung. This preference is due to the fact that the left lung is smaller than the right, as it needs to accommodate the space for the heart. In diagnostic imaging, this can be particularly important when trying to identify pathologies such as pneumonia, which may present as an infiltrated area with fluid, appearing as a white shadow on an X-ray where the tissue would typically appear black.
By placing the left side closer to the IR, the image can provide a clearer view of this area and assist in the accurate diagnosis of such conditions, taking into account anatomical differences like the cardiac notch in the left lung and variations in lung size. Also, in the incidence of pneumonia on the left side, a lateral X-ray with the left side against the IR can show lung infiltrates more clearly.