What is the FFD for an AP ankle positioning?

For an anteroposterior (AP) ankle positioning in radiography, the standard focal film distance (FFD), also known as source-to-image distance (SID), is typically set at 40 inches. This distance is critical because it helps to minimize the potential for distortion and magnification of the image, providing a clearer and more accurate representation of the ankle anatomy. A distance of 40 inches allows for optimal image quality while maintaining an acceptable level of radiation exposure.

Using a smaller or larger FFD can adversely affect the image quality, leading to either a loss of detail or an increased exposure to radiation without notable benefits. Hence, the correct value of 40 inches is widely used in practice for AP ankle positioning, making it the accepted standard.