An alloy filter (16) in the form of a plate made from a homogeneous alloy comprising 85-95 wt.% cadmium and 5-15 wt.% copper is interposed between a collimator (12) and sodium iodide crystal (18) of a gamma camera. The alloy filter improves image resolution by allowing the passage only of emissions (A,C) which impinge substantially perpendicularly on the collimator, while filtering out others (B,D) which impinge obliquely.