  In the past, magnetic images acquired using scanning Superconducting Quantum
Interference Device (SQUID) microscopy have been interpreted using simple
models for the sensor point spread function. However, more complicated modeling
is needed when the characteristic dimensions of the field sensitive areas in
these sensors become comparable to the London penetration depth. In this paper
we calculate the response of SQUIDs with deep sub-micron pickup loops to
different sources of magnetic fields by solving coupled London's and Maxwell's
equations using the full sensor geometry. Tests of these calculations using
various field sources are in reasonable agreement with experiments. These
calculations allow us to more accurately interpret sub-micron spatial
resolution data obtained using scanning SQUID microscopy.
