An essential component of any gamma camera is the collimator. Essentially a hunk of lead with a bunch of tiny little holes in it (modern collimators are made from ridged sheets of lead bonded together to form hexagonal holes like this one).
The purpose of the collimator is to restrict the direction from which gamma rays are detected by the gamma camera. They operate like blinders and make it so that the gamma camera can only see gamma rays coming from a particular direction (usually straight ahead perpendicular to the face of the camera). Collimators are necessary because by itself, the gamma camera has no idea what direction a detected gamma ray came from. By limiting the direction gamma rays are detected from, a useful image can be created by the computer.
This image I took by placing the collimator up against the front of my camera lens and a wide open aperature (f3.2). You can see just how limited the field of view becomes with the collimator.