Chemistry Reference
In-Depth Information
Events
*
*
*
*
Coincidences:
True
Scattered
Random
= Annihilation even
= Gamma ray
= Assigned LOR
FIgure 1.6
Different types of coincidence events.
Scattered coincidence occurs when one or both photons from a single event are scattered and both are detected; however, one
of the photons must have undergone at least one compton scattering event prior to detection. This type of event adds a background
to the true coincidence event and causes overestimation of the isotope concentration as well as decreasing image contrast.
In compton scattering, a photon interacts with an electron in the absorber material, resulting in an increase in the kinetic energy
of the electron as well as a change in direction in the photon. The energy of the photon after interaction is defined as:
E
′
=
E
+
E
mc
(
cosθ
)
(1.1)
1
−
1
2
0
where
E
is the energy of the incident photon,
E′
is the energy of the scattered photon,
m
0
c
2
is the rest mass of the electron, and q is
the scattering angle [10]. From Equation 1.1, it can be seen that fairly large deflections can occur with just a small loss of energy;
for example, for 511 keV photons, a compton scattering event results in a deflection of over 25 degrees but results in just a 10%
loss in the photon energy. random coincidence is the simultaneous detection of emission from more than one decay event. It occurs
when two photons not arising from the same annihilation event are incident on the detectors within the coincidence time-window
of the system. This contributes to statistical noise in data as well as overestimation of isotope concentration [8].
Multiple coincidences occur when more than two photons are detected by different detectors within the coincidence resolving
time. This type of event either causes event mis-positioning or rejection because it is not possible to determine the line of
response to which the event should be assigned. coincidence events are grouped together to produce projection images called
sonograms. Acquisition of PET images is not a simple process because data corrections are required for scattered, random coin-
cidences as well as for the effects of attenuation, because the data acquired from the PET camera are given as projections. The
measured projections are different from the projections assumed in image reconstruction [9]. reconstruction of images from
projections is computationally burdensome. Data reconstruction and correction are usually carried out by analytical or iterative
methods. Analytical methods are simple, fast, and usually have predictable linear behaviour. However, such methods are not
very flexible and have problems associated with noise resolution and image properties and do not allow for quantitative imaging.
on the other hand, iterative methods allow for quantitative imaging but require long calculation times as well as amplification
of the background noise; it thus requires counts to be low to reduce the projection noise.
1.2.2
Advantages and limitations
PET is a highly sensitive and popular technique in preclinical and clinical imaging. It is a very important diagnostic technique
because disease processes such as cancer often begin with functional changes at the cellular level. There are many radioactive
tracers with various half-lives applicable for different preclinical and clinical applications. The half-lives are often very short
and therefore must be injected immediately after production. Due to the mechanism of decay being the same for all different
radioactive tracers, it is only possible to trace one molecular species in a given imaging experiment or clinical scan where only
true coincidence events are used.
Tracers can be designed to be target-specific to tumours and allow study of metabolic activities such as bone metabolism and
bone metastasis that are common in a lot of cancers. Thus this technique can be used to monitor disease processes and patients'
responses to therapy. However, one of the major limitations of PET is its poor spatial resolution. It is also limited by pixel sampling
rate, quantity of the radioactive source, and blurring in the phosphor screens of the detector rings. However, the use of electronic
collimation over physical collimation helps to improve sensitivity and uniformity of the emitting source response function.
