Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-05-25
2002-02-12
Sugarman, Scott J. (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370060, C250S370130, C250S370080
Reexamination Certificate
active
06346708
ABSTRACT:
TECHNICAL FIELD
This invention relates to a device and a process for discrimination of pulses from semiconductor radiation detectors, and particularly from gamma radiation detectors.
Electrical pulses output by gamma radiation detectors depend on the number of detection parameters, the influence of each of the parameters being variable depending on detection conditions.
In particular, the invention is designed to identify the influence of a number of parameters and, as a function of this influence, to discriminate signals.
Discrimination may consist particularly of selecting pulses and therefore detected “events” that correspond to determined detection conditions, and to reject other pulses.
The invention is used in applications in the gamma spectrometry and nuclear medicine fields.
STATE OF PRIOR ART
An illustration of the state of the art is given below with the reference to examples taken from nuclear medicine.
In nuclear medicine, according to some diagnosis methods, a patient is injected with radio elements in the form of molecules marked by a radioactive tracer, for example such as technetium, iodine or thallium. These molecules will become fixed selectively on some tissues or organs, depending on the type of the molecules.
A gamma camera is then used to detect the gamma radiation produced by the patient, and to constitute an image of the tissues or the organ concerned. The image contrast depends on the fixation of the radioelements by the tissues.
Most gamma cameras used are of the Anger type. One such camera is shown diagrammatically in FIG.
1
.
The essential part of the gamma camera is a scintillator crystal
10
equipped with a collimator
12
and a plurality of photomultipliers
14
optically coupled with the scintillator crystal by a transparent material
16
.
A central of gravity calculation carried out on the signal output from each photomultiplier in response to an event can be used to identify the location in the scintillator crystal at which the gamma radiation interacts with the material.
This localization is also a means of identifying the position of a radioactive area
18
from which the radiation was emitted. This is possible particularly due to the collimator
12
which, as shown in
FIG. 1
, is a means of eliminating radiation with incidence not approximately normal to the detector input face.
Gamma radiation that reaches the scintillator crystal, like the ray
20
in
FIG. 1
, may consist of rays that interact with the detector directly after leaving the radioactive area
18
. These rays form part of a phenomenon denoted “direct radiation” in the rest of this text.
On the other hand, other rays interact with the material surrounding the radioactive area one or more times before reaching the scintillator. For example, this is the case of ray
21
in the figure which interacts a first time inside the patient, but outside the radioactive area. After this first interaction, a diffused gamma ray with lower energy reaches the scintillator. This phenomenon is due to the Compton effect and is denoted “diffuse radiation” in the rest of this text.
The figure shows that diffuse radiation can cause incorrect localization of the radioactive area and contribute to degrading the contrast of the medical image by the addition of noise.
As mentioned above, diffuse radiation is characterized by the fact that its energy is lower than the energy of direct radiation.
The contribution of diffuse radiation can be eliminated by making a discrimination between detection signals, in other words rejecting signals for which the amplitude as a function of the energy is less than a given threshold and for which the amplitude as a function of the energy is outside a determined window.
In general, a window is fixed in amplitude around the maximum amplitude value as a function of the energy of the signals for a given emission energy from the radioactive area.
If the window is narrow, the image contrast can be increased by limiting acceptance of diffuse radiation. However, this is done at the detriment of the number of detected effective events, in other words the number of events that can be used for the formation of an image.
Conversely, if the window is too wide, the number of events is greater for a given measurement time, but the image contrast is degraded.
Within the framework of medical imagery applications, it is not possible to inject excessive radioactive doses to patients, and it is not comfortable to prolong the examination duration beyond a certain time. Thus, the number of effective events measured per unit time and the energy resolution of the detector are important parameters.
The energy resolution is taken to be the ratio of the width of the distribution of an energy peak around the value of the emission energy at mid-height, to the emission energy.
A recent development in gamma cameras in which scintillator detectors are replaced by semiconducting detectors, has improved the acquisition of events in terms of efficiency and energy resolution.
Semiconductor detectors, for example such as CdTe, CdZnTe, AsGa, PbI
2
, directly convert gamma photons into charge carriers. For radiation with the same intensity, the number of charges created is an order of magnitude greater than the number created obtained in indirect detection with scintillator detectors. Thus, the resolution of semiconductor detectors is also improved.
FIG. 2
shows the structure of an individual semiconductor detector
30
. Usually, several such detectors are combined together to form a detection head.
The detector in
FIG. 2
is in the form of a semiconductor block with two opposite parallel faces on which electrodes
32
a
,
32
b
are provided. An electrical field applied to the electrodes causes the migration of charge carriers, in other words electrons and holes formed by the interaction of the radiation with the semiconductor. Electrodes are also provided to collect charges and transfer them to electronic circuits
34
for the formation of a detection signal. This signal is in the form of pulses corresponding to interactions.
These interactions are characterized by the energy that they transfer to the detector and the depth at which they take place in the detector. This depth may be understood as being the distance to be traveled by charges to reach one of the electrodes.
All charges created in the semiconductor do not migrate to the electrodes directly. Defects in semiconductors trap some charge carriers during their migration and reduce their life, particularly when the semiconductor is thick.
The charge created by a gamma radiation is distributed into a charge carried by the electrons and a charge carried by the holes. The mobility of the holes is less than the mobility of the electrons and their collection efficiency is not as good. Thus, not all created charges contribute equally to the finally output detection signal.
In the energy spectrum of the detection signal this results in a “drag”, representing a lower energy than the energy of the photons reaching the detector material.
“Drag” is a characteristic of charges being trapped in the material before their collection.
Detected events, the energy of which is less than the energy of actually received gamma photons due to the trapping phenomenon, are then combined with photons resulting from diffuse radiation mentioned above for which the energy is also less than the energy of the direct radiation.
Since the detector electrodes are usually formed on their surface, the trapping phenomenon is directly related to the depth at which the radiation interacts in the detector.
FIG. 3
is a graph that shows the variation of the amplitude (ordinate) as a function of time (abscissa) for different detection signals that may be emitted by a gamma radiation detector according to
FIG. 2
, under different detection conditions.
The signal reference
41
has the largest amplitude and corresponds to detection of direct radiation that interacted at a shallow depth in the detector.
This signal is useful, for example, for the formation of a medical image.
Signal refe
Arques Marc
Montemont Guillaume
Burns Doane , Swecker, Mathis LLP
Commissariat a l'Energie Atomique
Hasan Mohammed
Sugarman Scott J.
LandOfFree
Device and process for discrimination of pulses output from... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Device and process for discrimination of pulses output from..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Device and process for discrimination of pulses output from... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2934555