  The transverse momentum dependence of Hanbury-Brown/Twiss (HBT)
interferometry radii for 2-body correlation functions provides experimental
access to the collective dynamics in heavy-ion collisions. We present an
analytical approximation scheme for these HBT radii which combines the recently
derived model-independent expressions with an approximate determination of the
saddle point of the emission function. The method is illustrated for a
longitudinally boost-invariant hydrodynamical model of a heavy ion collision
with freeze-out on a sharp hypersurface. The analytical approximation converges
rapidly to the width of the numerically computed correlation function and
reproduces correctly its exact transverse momentum dependence. However, higher
order corrections within our approximation scheme are essential, and the
previously published lowest order results with simple $m_{\perp}$ scaling
behaviour are quantitatively and qualitatively unreliable.
