The stellar initial mass function function (IMF) is one of the most fundamental, and hotly debated, observational topics in astrophysics, with a non negligable impact on our understand of galaxy structure. Recently, evidence for the non-universality of the stellar IMF of the most massive early-type galaxies (ETGs) has begun to mount. This evidence comes from three independent techniques: modelling of the stellar kinematics, gravitational lensing, and the analysis of gravity-sensitive spectroscopic features in galaxy spectra. In this talk I will present a new complementary technique to probe the IMF. I use the kinematics of the cold molecular gas reservoirs in early-type galaxies to constrain their mass profiles. By combining these profiles with observations of the galaxies stellar luminosity profile (and stellar population parameters from IFU spectroscopy) we are able to derive mass-to-light ratios, and constrain the IMF in a radially resolved manner. By applying this technique in a sample of 7 gas-rich ETGs I confirm that a varying IMF appears to be required, and show the extent of radial IMF variations. I will also discuss how the variation of the IMF scales with galaxy parameters, both globally and locally inside individual galaxies.