Biomedical Engineering Reference
In-Depth Information
Normalized attenuation coefficient for water
1,0
Rayleigh effect
Compton effect
Photoelectric effect
Pair production (nuclear)
Pair production (atomic)
Total attenuation
0,8
0,6
0,4
0,2
0,0
0,01
0,05
0,20
0,80
2,00
6,00
11,00
16,00
26,00
60,00
E /MeV
Fig. 3
Variation of attenuation coefficient due to various effects normalized to total attenuation.
The shaded area represents the energy range used in nuclear medicine (Data obtained from [
3
])
As can be observed in Fig.
3
, the Compton effect is predominant at the energy
range of nuclear medicine (100-511 keV - shaded area), unfortunately contributing
to the degradation of image quality.
2.3
Gamma Camera
In order to detect the gamma photons, they must interact within the detector and
the deposited energy should be directly related with his characteristics. Therefore,
gamma radiation interacts with the detector by one of the mechanisms described
above, leading to the collection of charge which is, generally, proportional to the
energy of radiation.
The gamma camera is the detector that has been adopted in conventional nuclear
medicine. Its configuration, due to Hal Anger [
4
], has been improved and at
the present is close to the maximum ratio efficiency/cost. The main components
of the gamma camera are the collimator, the scintillation crystal, the optical contact,
the photomultipliers and the electronics needed for the signal processing (Fig.
4
).
The collimator is a device, made usually of lead, which is interposed between
the crystal and the object and whose function is to absorb the scattered rays that
can not be used for the image formation. It is a circular or rectangular plate
. 50
cm
/
with thousands of tiny holes that are evenly distributed, forming a
regular lattice. The direction of these holes determines the function and designation
of collimators, such as the parallel holes, convergent and divergent collimators. Due
to the impossibility of refraction of the gamma photons, the collimators are essential
for forming the image since they can establish a spatial correlation between the
detection point and the place of emission. However, the impact on sensitivity is very
