  The spatial mode is an essential component of an electromagnetic field
description, yet it is challenging to characterize it for optical fields with
low average photon number, such as in a squeezed vacuum. We present a method
for reconstruction of the spatial modes of such fields based on the homodyne
measurements of their quadrature noise variance performed with a set of
structured masks. We show theoretically that under certain conditions we can
recover individual spatial mode distributions by using the weighted sum of the
basis masks, where weights are determined using measured variance values and
phases. We apply this approach to analyze the spatial structure of a squeezed
vacuum field with various amount of excess thermal noise generated in Rb vapor.
