Corrections for Physical Factors - Correction Techniques in Emission Tomography - page 68

retaining its mass number A. One such example is the decay of the often-used
PET nuclide

18 F:

18

9 F ! 1 8 O + e + + e :

(5.1)

In PET, this decay takes place inside the body of a patient, and the emit-
ted positron loses its kinetic energy by scattering. Once slow enough (after a
maximum range in the order of a few mm, depending on the initial kinetic
energy of the positron and the nature of the the traversed material), it an-
nihilates with one of the ubiquitous electrons, resulting in most cases in two
gamma photons of the specic energy of E = 511 keV:

e + + e ! 2: (5.2)

.
As gamma radiation penetrates soft tissue quite well, it is possible that both
photons can be detected almost simultaneously outside the body by gamma-
sensitive radiation detectors (coincidence event), giving spatial information
about the location of the decay.
The gamma decay does not change the identity of the nucleus; rather,
gamma-decaying nuclei are in an energetically excited state that is usually
a consequence of a prior alpha or beta decay. The excess energy can then
be delivered either in total or partially by emission of gamma radiation of
characteristic energies. An example is the gamma decay of the metastable
SPECT nuclide

These photons are emitted in opposite directions under an angle of 180 °

99m Tc which decays by emitting 143 keV gamma radiation:

99m

43 Tc ! 9 43 Tc + :

(5.3)

If delivered inside the human body, this radiation can be detected outside
by detectors. As no coincident events as in PET are measured in SPECT,
collimator techniques have to be applied in order to gain information about
the decay location.
The detected radiation events during a certain time interval then allow
the reconstruction of image data comprising information about the activity
distribution within the human body. Both PET and SPECT potentially have
the ability to absolutely quantify the dispersed radioactivity in vivo. How-
ever, the acquired raw data has to be processed either before, during, or after
the reconstruction process to get satisfying images in terms of quality and
quantity. These corrections can be divided into two groups:

• Corrections for factors related to physical processes during radioactive

decay and during the steps until the radiation leaves the body;

• Corrections for factors related to the detection of the radiation.

The first factors will be the topic of the present chapter; the second factors
will be discussed in the next chapter.