Biomedical Engineering Reference
In-Depth Information
Fig. 4
Components of a
gamma camera
Collimator
Scintilator crystal
NaI(Tl)
Optical contact
PMT
1
PMT
2
PMT
3
PMT
n
Photomultipliers
tubes
Electronic
circuit
Fig. 5
Source point seen by
a gamma camera. The
geometric relationships
between the distance of
spatial resolution (d), the
characteristics of the septa
and the distance to the source
(f) can be derived directly
from the figure
f
q
s
h
l
Collimator
c
Crystal
d
important since only a fraction less than 1% of the photons cross the collimator and
reaches the crystal [
5
]. This collimator is not necessary in PET (because electronic
collimation is used instead) and is the main reason why PET is a technique with
much higher sensitivity. Another interesting aspect associated with the collimators
is its direct influence on the spatial resolution of the system. These geometric
relationships are well known and can easily be established from Fig.
5
.
Figure
5
shows a point source viewed by a gamma camera located at a distance
f from the collimator. The width of the septa
1
is
s
and the height is
`
. The width of
the holes is represented by h and c is the distance between the collimator and the
crystal. Applying the rules of similar triangles to Fig.
5
,weget:
h
`
.c C ` C f/:
d
D
(7)
The capacity of discriminating two distinct points (spatial resolution) is inversely
related to the resolution distance, d, which in turn is directly proportional to the
distance, f, between the collimator and the source. Since, in general, the distance, f,
to the source is larger than the height of the septa,
`
, and larger than the distance,
c, from the collimator to the crystal, the resolution depends significantly on the
1
Septa are the walls that confine a hole of the collimator.
