Posterior Nipple Line

Posterior nipple line (PNL)

Drawn perpendicularly and posteriorly from nipple toward pectoralis muscle; Especially important when pectoralis muscle not seen on CC view (up to 70% of images); PNL lengths on CC and MLO should be equal (differ by <1cm) to reliably localize a mammographic finding

Localizing a lesion to a quadrant

CC:  lesion above PNL = Outer breast; below PNL = Inner breast

MLO: lesion above PNL = Upper breast; below PNL = Lower breast