  Understanding the 3D structure of coronal mass ejections (CMEs) is crucial
for understanding the nature and origin of solar eruptions. To derive
information on the 3D structure of CMEs from white-light (total and polarized
brightness) images, the polarization ratio technique is widely used. The
soon-to-be-launched METIS coronagraph on board Solar Orbiter will use this
technique to produce new polarimetric images. We determine the accuracy at
which the position of the centre of mass, direction and speed of propagation,
and the column density of the CME can be determined along the line of sight. We
perform a 3D MHD simulation of a flux rope ejection where a CME is produced.
From the simulation we (i) synthesize the corresponding METIS white-light
(total and polarized brightness) images and (ii) apply the polarization ratio
technique to these synthesized images and compare the results with the known
density distribution from the MHD simulation. We find that the polarization
ratio technique reproduces with high accuracy the position of the centre of
mass along the line of sight. However, some errors are inherently associated
with this determination. The polarization ratio technique also allows
information to be derived on the real 3D direction of propagation of the CME.
In addition, we find that the column density derived from white-light images is
accurate and we propose an improved technique where the combined use of the
polarization ratio technique and white-light images minimizes the error in the
estimation of column densities. Our method allows us to thoroughly test the
performance of the polarization ratio technique and allows a determination of
the errors associated with it, which means that it can be used to quantify the
results from the analysis of the forthcoming METIS observations in white light
(total and polarized brightness).
