  We present a new method to systematically include corrections to the eikonal
approximation in the background field formalism. Specifically, we calculate the
subleading, power-suppressed corrections due to the finite width of the target
or the finite energy of the projectile. Such power-suppressed corrections
involve Wilson lines decorated by gradients of the background field - thus
related to the density - of the target. The method is of generic applicability.
As a first example, we study single inclusive gluon production in pA
collisions, and various related spin asymmetries, beyond the eikonal accuracy.
