A detailed method for measuring and characterizing intrinsic spatial resolution is discussed in Section E.1. Typically, a lead mask containing a number of narrow (~1 mm) slits is placed on the face of the gamma camera (without the collimator) and the camera is irradiated using a 99mTc (140-keV) point source. The resulting image is a series of lines corresponding to the locations of the slits (e.g., see Fig. 14-10A). The resolution is calculated as the full width at half maximum (FWHM) of a profile drawn perpendicular to the image of the lines at various locations in the field of view. The intrinsic spatial resolution of modern large field-of-view gamma cameras measured with 99mTc in this manner is in the range of 2.9- to 4.5-mm FWHM. Because the resolution is considerably worse than the width of the slits, the contribution of the slits themselves to the measured resolution is very small (10% for measured resolution 2.5 mm).