1. Method for the non-intrusive measurement of the volume density of a specific phase in a part, comprising the following steps: producing a volume image of said part, said image being formed by a three-dimensional grid of voxels, the values of which indicate the disposition of said specific phase in said part, associating a binary coefficient with each voxel of said volume image, thus constructing an initial three-dimensional matrix representation of binary coefficients, said binary coefficients representing a presence or absence of said specific phase in zones of said part corresponding to the voxels, convoluting said initial matrix representation with a convolution matrix kernel corresponding to a predetermined reference volume, said convolution being performed by effecting a composition of three monodimensional convolutions in three independent directions, thus forming a resultant matrix representation, each resultant coefficient of which represents a volume ratio of said specific phase in said reference volume, wherein said part is a part of an aircraft engine.