To ensure that the greatest amount of tissue has been included,
a generous amounta of pectoralis muscle should be visualised. It is
desirablea for the inferior extent of the muscle to be visible down
to the posterior nipple line (PNL). The latter criterion can be
achieved in more than 80% of women. The PNL is drawn at an
angle approximately perpendicular to the muscle, extending
posteriorly from the nipple to the pectoralis muscle or the edge
of the film, whichever comes first. The pectoralis muscle shoulda
also be sufficiently wide