"
"
The principle of radiation detection is based on the interaction of these radiations with the
matter. When a gamma photon enters in interaction with a detector material, it loses its energy
mainly in the form of ionizations or excitations. The excited atoms return to their ground state
through the emission of secondary low energy gamma photons. The incident gamma photon
can be partially or totally absorbed (photoelectric effect). In the first case, the energy loss is
accompanied by a deviation of the photon (Compton scattering). The photon loses "memory"
of its initial place of issue. So the photoelectric effect is the right phenomenon which must be
considered when we interest to the gamma-ray emission site.
In the gamma camera, the detection medium is historically a NaI scintillation crystal typically
doped with thallium. This crystal is able to emit light especially through a fluorescence process
after the excitation of its molecules by a charged particle (electron). The density of NaI is 3.67
g/cm3 and its atomic number 50. Its time of scintillation (fluorescence) is 230 nm and the
maximum light emission is at 4150 Angstroms wave length. Its refractive index is 1.85, and it
is relatively transparent to its own light; about 30% of emitted light is transmitted to the
detection chain [1]. The energy resolution can reach 7-8% at 1 MeV and the constant time of
their pulse is equal to ~10-7 sec. The detection efficiency of NaI is quite large, of the order of 40
photons/keV. Indeed, gamma-ray energy of 100 keV transferring all its energy in the crystal
results in the creation of approximately 4000 fluorescence light photons. These photons are
collected by the photocathode of a photomultiplier tube (Figure 1).
For the detection of the secondary light photons generated in the crystal by the interaction with
the incident gamma radiations, a photomultiplier tube (PMT) located behind the scintillator
is used (Figure 1). At the level of the PMT photocathode, each light photon is converted to
electrons. These electrons are then accelerated and multiplied by ten dynodes polarized by a
gradually increasing voltage, and finally collected by an anode placed at the other side of the
PMT where they give birth to an electrical impulse. This pulse has an amplitude proportional
to the energy of the detected gamma-ray.
The output signal is amplified by the PMT. Its amplitude is measured, digitized and stored.
Numerical analysis enables to obtain a spectrum (number of photons detected as a function of
their energy) characteristic of the detected gamma-rays. Detection time (acquisition) should
be sufficient to obtain good counting statistics. The theoretical gamma-rays spectrum reaching
the crystal is a line spectrum; the spectrum is continuous (Figure 2). The spectrum includes
the total energy peak corresponding to gamma directly emitted by the radioactive source
without any interaction before reaching the crystal and a background of lower energies due
to the partial absorption of gamma by Compton scattering. Compton scattering in the path of
the photon is changed making it impossible to locate its transmitter site. It is therefore necessary
to take into account only the events corresponding to the photoelectric interactions at the level
of the crystal with the total emission energy. This is achieved by the intermediate of a "window"
for selecting the double-threshold energy (pulse height analyzer).
Figure 2. Gamma-rays spectrum at the level of the crystal detector (ideal (top) and real (bottom) cases).
The width of the peak of total absorption depends essentially of the random statistical
fluctuations of the gain of the PMT. The width at half maximum ΔE relative to an average
energy E0 defines the energy resolution ΔE/E0. The energy resolution of PMT is about 10% at
140 keV (emission peak of technetium-99m). The pulses selected by the pulse analyzer
(maximum intensity) are directed to a time scaling circuit having a time integrator which then
delivers a count rate in counts per second (cps). This count rate can be correlated to the real
activity of the source after a number of corrections taking into account in particular the
geometric efficiency and the detection performance of the detection chain. For very high source
activity, the detector response is no longer linear so that a number of events are not taken into
account. The lapse of time in which these events are lost (not counted by the detector) is called
the dead time. In practice, it is usual, to work under conditions such that the detection dead
time correction is not necessary (medium activity source).
The Anger gamma scintillation camera (Figure 3) uses the information provided by the
amplitude of the electrical pulse not only to measure the energy of the detected radiation, but
also to locate in the space the emission site of this radiation.
The camera developed by Anger in 1953 has a crystal of sodium iodide (NaI) thallium
activated. It can take single crystal of large dimensions, up to 60x50 cm2 with a thickness
ranging from 1/4 inch to 1 inch [1]. These crystals are fragile and are highly sensitive to shocks
and moisture. The surface of the crystal is covered with a large number of PMTs (between 50
and 100). When scintillation occurs, the sum of the output signals of all the MPTs provides the
energy lost in the volume of the scintillator (Z coordinate). The large number of PMTs ensures
the collection of maximum light. Moreover, the amplitude of the output signal of PMT varies
with the distance between the centre of the photocathode and the place where the scintilaltion
is produced is in the crystal. The amplitude distribution of the output pulses of the PMT then
provides the location information (X and Y coordinates) by means of a computer listing. For
each photon interacting with the detector is thus obtained location coordinates (X and Y) and
a value of the energy given or lost in the crystal (Z coordinate). An amplitude analysis allows
selecting only the photon energy characteristic of the radionuclide used (eg. 140 keV for 99mTc)
having lost all their energy in the crystal (photoelectric peak).
The scintillation Gamma-camera was used originally for planer projection imaging is mainly
composed by the following components:
ليست هناك تعليقات:
إرسال تعليق