Biocompatible polymerized 2-(hydroxyethyl) methacrylate (pHEMA) is a very popular material for medical applications. There are still numerous questions to be answered concerning the polymerisation processes of HEMA upon compression [1]. For this reason, the application of non-destructive contactless time-domain Brillouin scattering (TDBS) pump-probe technique [2] to the evaluation of HEMA properties looks very promising. We report here the extension of the TDBS technique to the 3D imaging of evolution of HEMA compressed in a diamond anvil cell (DAC). First, experiments were performed using classical picosecond ultrasonic set-up with a mechanical delay line [2]. To polymerise HEMA, it was compressed to 6.5 GPa and then decompressed immediately to 0.5 GPa. After 10 minutes at this pressure it was decompressed to 0 GPa. This procedure led to the increase of the Brillouin frequency from 6.3 GHz in HEMA monomer to 11.3 GHz in pHEMA. The measurements by Raman spectrometry confirmed the polymerisation of HEMA. Then, to perform 3D imaging possible in a reasonable time, we have applied an ultrafast laser technique based on an asynchronous optical sampling (ASOPS). The maps of the Brillouin frequencies obtained using TBDS technique could significantly extend our knowledge about the earlier recognised two different states of HEMA above and below 6.5 GPa [1], which cannot be distinguished by any other technique. Moreover, it should potentially allow following in time the 3D spatial spreading of the polymerization reaction starting from the nucleation sites. In perspective, the 3D imaging of the transient processes at high pressures with nanometers depth resolution could be possible. Acknowledgments: This research is supported by the grant ANR-18-CE42-017. [1] E. Evlyukhin, et al Scientific Reports 5, 18244 (2015). [2] C. Thomsen, H.T. Graham, H.J. Maris, J. Tauc, Optics Communications 60, 55 (1986).