By virtue of its extremely bright gaseous field ion source, the ORION (TM) helium ion microscope has demonstrated a probe size smaller than 0.3 nm, when operating the microscope in its (typical) high-resolution imaging mode, i.e., surface imaging with secondary electron signal. The authors combined SRIM-models of beam spread in the sample with models for secondary electron signal generation and escape, for a wide range of beam energies and sample materials, in order to calculate the effect of beam spread on image resolution. It was found that the effect on resolution is larger for sample materials with higher atomic numbers, and that the effect is inversely proportional to beam energy. The magnitude of the calculated effect on image resolution ranges from 0.005 to 0.08 nm, which is typically an order of magnitude smaller than the currently experimentally measured image resolution of the helium ion microscope. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3497012]