  Objective. Prompt-gamma imaging encompasses several approaches for online
monitoring of beam range or deposited dose distribution in proton therapy. We
test one of the imaging techniques - a coded mask approach - both
experimentally and via simulations. Approach. Two imaging setups have been
investigated experimentally. Each of them comprised a structured tungsten
collimator in a form of a MURA mask and a LYSO:Ce scintillation detector of
fine granularity. The setups differed in the detector dimensions and the
operation mode (1D or 2D imaging). A series of measurements with radioactive
sources have been conducted, testing the setups' performance of near-field
gamma imaging. Additionally, Monte Carlo simulations of a larger setup of the
same type were conducted, investigating its performance with a realistic gamma
source distribution occurring during proton therapy. Main results. The images
of point-like sources reconstructed from two smallscale prototypes' data using
the MLEM algorithm constitute the experimental proof of principle for the
near-field coded-mask imaging modality, both in the 1D and the 2D mode. Their
precision allowed us to calibrate out certain systematic offsets appearing due
to the misalignment of setup elements. The simulation of the full-scale setup
yielded a mean distal falloff retrieval precision of 0.72 mm in the studies for
beam energy range 89.5-107.9 MeV and with 1x10^8 protons (typical number for
single distal spots). The implemented algorithm of image reconstruction is
relatively fast - a typical procedure needs several seconds. Significance.
Coded-mask imaging appears a valid option for proton therapy monitoring. The
results of simulations let us conclude that the proposed fullscale setup is
competitive to the knife-edge-shaped and the multiparalell slit cameras
investigated by other groups.
