  A significant fraction of non-baryonic or baryonic dark matter in galactic
halos may consist of MASsive Compact Objects (MASCOs) with mass
M=10^{1-4}M_{sun}. Possible candidates for such compact objects include
primordial black holes or remnants of primordial (Population III) stars. We
propose a method for directly detecting MASCOs in extragalactic halos, using
the VLBI techniques with extremely high resolution. If a galactic halo
comprising a large number of MASCOs produces multiple images of a background
radio-loud QSO by gravitational lensing, then a high-resolution radio map of
each macro-lensed image should reveal microlensing effects by MASCOs. To assess
their observational feasibility, we simulate microlensing of the radio-loud,
four-image lensed QSO, B1422+231, assuming angular resolution of ~0.01 mas.
MASCOs are represented by point masses. For comparison, we also simulate
microlensing of B1422+231 by singular isothermal spheres. We find that the
surface brightness of the macro-lensed images shows distinct spatial patterns
on the scale of the Einstein radius of the perturbers. In the case of
point-mass perturbers, many tiny dark spots also appear in the macro-lensed
images associated with a decrease in the surface brightness toward the fringe
of the original QSO image, whereas no such spots are available in the SIS
models. Based on the size, position and magnified or demagnified patterns of
images, we shall be able to determine the mass and density profile of a MASCO
as well as its spatial distribution and abundance in a galactic halo.
