  A comprehensive model study of Bose-Einstein correlation radii in heavy ion
collisions is presented. The starting point is a longitudinally and
transversally expanding fireball, represented at freeze-out by an azimuthally
symmetric emission function. The freeze-out temperature is allowed to feature
transverse and temporal gradients. Their effects on the correlation radii are
studied. In particular, we evaluate numerically their dependence on the
transverse mass of the particle pairs and check a recent suggestion, based on
analytical approximations, that for certain reasonable source parameters all
three correlation radii satisfy simultaneously a 1/\sqrt{M_\perp} scaling.
