This work determines the relative importance of noise from blank (B), transmission (T) and emission (E) scans in PET using a GE Advance scanner on a 20 cm cylinder, a brain phantom, and a torso-like ellipse (18/35 cm) with examples of human scans (brain O-15 water and F-18 FDG, heart FDG). Phantom E scans were acquired in both 2D and 3D modes as decay series with C-11 or F-18 over 3-6 decades of Noise Equivalent Counts (NEC). B and T scans were made using two pin sources (approximate to 500 MBq total) over 64-32768 sec. In humans only a Limited subset was available. In homogeneous phantoms normalized variance (var) was estimated from pixel distributions in single images. In other objects, including the human studies, calculations were performed on differences of paired images. In all cases a fit was made to a simple noise model. The cylinder data show expected relations of T to B noise proving the adequacy of B scan times less than or equal to 20 min for most purposes. For the brain phantom, a contour plot is provided for var(E,T). In a typical 3D O-15 water study with 0.5M counts per central slice, a 10 min T-scan adds less than 10% to the total noise level. An example shows how to split a total scan time between E and T scans, in order to minimize the variance.
