Biomedical Engineering Reference
In-Depth Information
distance of the gamma camera to the subject. Hence the gamma camera is always
placed as near as possible to the patient. Moreover, (
7
) also shows that the lower
the quotient between the width of the hole, h, and the septa height,
`
, the greater
the capacity for discriminating thin structures. However, the decrease of the hole
area necessarily implies a reduction of the sensitivity, requiring the adoption of
compromise solutions.
The scintillator crystal is essential as a means of transforming the energy of the
gamma photons into visible light which in turn is converted into an electrical signal
by photomultiplier tubes. The gamma photons interact within the crystal either
by photoelectric or by Compton effect. Secondary interactions can occur, causing
scintillations (emission of visible light) that are then amplified and converted into
electrical signal in the photomultipliers tubes.
The most common material used to manufacture the crystal scintillator of
the gamma camera is sodium iodide activated with thallium (NaI(Tl)), whose
characteristics make it a good choice. Despite presenting a poor energy resolution,
a relatively long response time and hygroscopic properties, it still remains the most
frequent crystal used in the construction of the gamma cameras. This choice is
mainly due to the fact that sodium iodide presents a good photoelectric fraction,
2
good light yield and an advantageous cost. For the higher photon energies used in
PET (511 keV), other crystals have to be used, with higher attenuation coefficient,
such as Luthetium-oxyorthosilicate (LSO), that is also dense and luminous. An ideal
crystal should have a small attenuation length, high light yield and photoelectric
fraction and small decay constant [
7
]. The attenuation length is defined as the
length necessary to absorb 63% of the incident photons. It is common to impose
a minimum thickness of the crystal of two attenuation lengths, therefore the shorter
the attenuation length the thinner the crystal can be. Moreover, the intrinsic spatial
resolution depends on the thickness of the crystal, which is an additional reason for
using detecting materials with a short attenuation length. The photoelectric fraction
is directly related to the possibility of the energy be deposited in a unique location,
which benefits the spatial resolution, in contrast to interactions by Compton effect
whose deposition is multiple and spatially spread. Finally, the decay constant is
directly associated with the time resolution of the detector.
Photomultiplier tubes (PMTs) (Fig.
6
) are extremely sensitive detectors particu-
larly suited for near infrared, visible and ultraviolet light. PMTs combine a high gain
and low noise which makes them a good choice for detecting the scintillation light
originated at the crystal. The process for converting the scintillation light into an
electrical signal follows a simple process of avalanche amplification [
8
]. Light from
the crystal strikes the PMT's photocathode causing the ejection of electrons that
are accelerated, by an electric field, onto the first dynode. The electrons that reach
the first dynode cause the release of more electrons multiplying the charge. From
the first to the second dynode, the process repeats: more electrons are accelerated
2
The photoelectric fration,
"
,isgivenby-
"
D
F
=.
F
C
C
/
- which is the relation between the
photoelectric scattering cross section,
F
and the Compton scattering cross section
C
[
6
].
