Corrections for Scanner-Related Factors - Correction Techniques in Emission Tomography - page 109

FIGURE 6.2: During dead time the system cannot process a following in-
coming event. Taken from [10].

the order of (1/processing time) the chance of pulse pile up (two or
more events arriving during the processing time) increases. For a non-
paralyzable system the count rate response will start to deviate from
linearity, and it will level off to a maximum count rate.

• the system processes piled-up events, increasing the effective processing

time each time a pile-up event is added. After onset of non-linear behav-
ior of the detected count rate versus the expected count rate (based on a
linear increase with radioactivity in the field of view) the count rate will
level off, but may start decreasing as well. A maximum in the expected
vs. measured count rate curve indicates the presence of a paralyzable
pulse processing system.

Although the term is known and can be found in many pulse processing
systems, it has been of major concern in 3D PET. Correction for dead time is
usually provided by reference to a lookup table that links the measured singles
rate to a dead-time correction factor. In practice a 10% dead-time correction
is taken to constitute an upper limit to the count rate that can be processed
in a study that will be quantified.
Recently, interesting developments in preclinical systems have been de-
scribed: the Quicksilver electronics platform for the Siemens Inveon scanner
family has signicantly reduced dead-time{related problems, and the degree
of multiplexing in various experimental systems has also been reflected in the
reported count rate capability of the systems (e.g., MADPET, SiPM).
For example, cardiac studies with Rb are currently limited with respect to
the injected dose by the dead-time characteristics of the PET scanner. On the
other hand, increasing the level at which 10% dead-time correction is applied
is useful only in case this leads to an improved NEC rate, which might not be
the case for a situation in which considerable scatter is present in the system.