  Nuclear magnetic resonance (NMR) is a powerful method for determining the
structure of molecules and proteins. While conventional NMR requires averaging
over large ensembles, recent progress with single-spin quantum sensors has
created the prospect of magnetic imaging of individual molecules. As an initial
step towards this goal, isolated nuclear spins and spin pairs have been mapped.
However, large clusters of interacting spins - such as found in molecules -
result in highly complex spectra. Imaging these complex systems is an
outstanding challenge due to the required high spectral resolution and
efficient spatial reconstruction with sub-angstrom precision. Here we develop
such atomic-scale imaging using a single nitrogen-vacancy (NV) centre as a
quantum sensor, and demonstrate it on a model system of $27$ coupled $^{13}$C
nuclear spins in a diamond. We present a new multidimensional spectroscopy
method that isolates individual nuclear-nuclear spin interactions with high
spectral resolution ($< 80\,$mHz) and high accuracy ($2$ mHz). We show that
these interactions encode the composition and inter-connectivity of the
cluster, and develop methods to extract the 3D structure of the cluster with
sub-angstrom resolution. Our results demonstrate a key capability towards
magnetic imaging of individual molecules and other complex spin systems.
