We propose a fast and accurate method of finding the thickness and dielectric permittivity of weak dielectric objects using a focused continuous-wave radar security scanning system, to characterize objects that may be explosive threats. Building on the previously developed ray-based Virtual Source model for sensing scattering from slabs, this work presents an inversion algorithm using two observable CW millimeter wave probe parameters by considering expected small variations of dielectric slab thickness over its surface. The determination works reasonably well with a simulated dielectric covered conductive ground plane as a representation of body worn explosives.