We report micro-Raman imaging of in-plane SiGe alloy nanowires grown by molecular beam epitaxy on Si(001) substrates. The spatial resolution of the Raman images allows us to study individual nanowires. We observe differences in Raman scattering intensity when the light polarization is parallel or perpendicular to the nanowire axis. These variations are shown to be originated from anisotropic absorption in thin nanowires whereas the intrinsic Raman efficiency is less affected by the nanostructuration. Quantitative analysis of the Raman spectra yields the composition and strain variations in the sample, in particular within each nanowire in which we resolve vertical gradients from the base to the top surfaces. These Raman results provide unique insights into the growth processes.