  Magnetic Resonance Imaging (MRI) is universally acknowledged as an excellent
tool to extract detailed spatial information with minimally invasive
measurements. Efforts toward ultra-low-field (ULF) MRI are made to simplify the
scanners and to reduce artefacts and incompatibilities. Optical Atomic
Magnetometers (OAMs) are among the sensitive magnetic detectors eligible for
ULF operation, however they are not compatible with the strong field gradients
used in MRI. We show that a magnetic-dressing technique restores the OAMs
operability despite the gradient, and we demonstrate sub-millimetric resolution
MRI with a compact experimental setup based on an in situ detection. The
proof-of-concept experiment produces unidimensional imaging of remotely
magnetized samples with a dual sensor, but the approach is suited to be adapted
for 3-D imaging of samples magnetized in loco. An extension to multi-sensor
architectures is also possible.
