  Previous dilepton measurements in heavy-ion collisions have mainly focused on
invariant mass spectra to clarify in-medium changes of vector meson properties.
However, a dimuon is characterized by two scales -- invariant mass $M$ and
transverse momentum $p_T$. Like transverse momentum spectra of hadrons, $p_T$
spectra of dileptons arise from an interplay between emission temperature and
collective transverse flow, whereas the invariant mass is insensitive to flow.
Having two control parameters of which only one is sensitive to flow allows at
given $M$ to characterize the emission region in terms of average temperature
and flow. Thus, one is able to study what phases of the fireball evolution
radiate into a given mass window. We demonstrate this technique using the
dimuon transverse momentum spectra measured by the NA60 collaboration and
present strong arguments that a thermalized evolution phase with $T > 170$ MeV
leaves an imprint in the spectra.
