Corrections for Physical Factors - Correction Techniques in Emission Tomography - page 71

FIGURE 5.2: Radioactive decay. The activity of a radioactive sample as a
function of time gives an exponential relationship determined by the nuclide-
specic decay constant .
emission tomography image; however, it obviously affects the quantitation of
image-derived values.
5.3 Randoms correction
The correction for random coincidence events is a PET-specific problem.
Random events contribute to both bias in measured activity distribution and
a loss of image contrast by adding additional background to the data. The
impact of randoms on the acquired images is dependent on several factors such
as the length of the coincidence timing window, the administered activity, the
energy window limits, and the scanning mode (2D vs. 3D PET).
There are basically two standard strategies that enable estimating of the
number of random coincidences between two detector elements. The first one
is a statistical calculation of random events based on measured single events;
the second one relies on a measurement of random coincidence events with
the help of a delayed coincidence window.