What is the recommended source-to-image distance (SID) for an anteroposterior (AP) projection of the fifth digit of the foot?

For the anteroposterior (AP) projection of the fifth digit of the foot, a source-to-image distance (SID) of 40 inches is commonly recommended. This distance helps to minimize distortion and ensures a better image quality, which is critical for accurately visualizing the bony structures of the foot, particularly in a smaller area such as the fifth digit.

Using the appropriate SID is essential because it influences the magnification and sharpness of the image. A 40-inch SID strikes an optimal balance, allowing for detailed visualization while maintaining clarity and reducing the potential for distortion. Compared to other distances such as 30, 50, or 60 inches, 40 inches aligns with standard radiographic practices, particularly for lower extremity imaging where precise detail is needed for diagnosis and treatment planning.