  Spin waves - the elementary excitations of magnetic materials - are prime
candidate signal carriers for low dissipation information processing. Being
able to image coherent spin-wave transport is crucial for developing
interference-based spin-wave devices. We introduce a platform for probing
coherent spin waves based on magnetic resonance imaging with electron spins in
diamond. Focusing on a thin-film magnetic insulator, we quantify spin-wave
amplitudes, visualize the dispersion, and demonstrate time-domain measurements
of spin-wave packets. We use our platform to study spin-wave interference,
revealing uni-directional, autofocused spin-wave patterns with
frequency-controlled numerical apertures. A theoretical analysis explains the
patterns in terms of chiral spin-wave excitation and stray-field coupling to
the sensor spins. These results pave the way for probing spin waves in
atomically thin magnets, even when embedded between opaque materials.
