Radiant energy – Calibration or standardization methods
Reexamination Certificate
2000-03-01
2001-12-11
Kim, Robert H. (Department: 2882)
Radiant energy
Calibration or standardization methods
C250S370010, C250S366000, C324S102000
Reexamination Certificate
active
06329651
ABSTRACT:
FIELD OF THE INVENTION
This invention involves a process for real time sorting of detection signals from a □ radiation detector allowing for distinguishing between signals corresponding to direct radiation and signals corresponding to indirect or diffused radiation.
Direct radiation means radiation which, after having been emitted from a radioactive zone, interacts directly with the detector. Indirect radiation on the other hand interacts one or several times with a medium surrounding the radioactive zone before interacting with the detector.
The invention also concerns a device and particularly a gamma-camera which uses the sorting process.
The invention also involves a process for correcting the detection uniformity for several detection elements of the detector.
The invention is applied in particular in the field of medical imaging and in medicine for the gamma-camera.
STATE OF THE PRIOR ART
In nuclear medicine and according to certain diagnostic methods, a patient is injected with radioisotopes in the form of molecules marked with a radioactive tracer such as, for example, Technetium, Iodine or Thallium. The molecules, depending on the type used, attach selectively to certain tissues or organs.
A gamma-camera is then used to detect gamma rays emitted from the patient and to make an image of the tissues or organ in question. The image contrast depends on the binding of the radioisotopes by the tissues.
The gamma-cameras used are mostly Anger type cameras. Such a camera is shown schematically in FIG.
1
.
The gamma-camera includes essentially a crystal scintillator
10
, equipped with a collimator
12
, and several photomultipliers
14
optically paired with the crystal scintillator by a transparent material
16
.
A barycentric calculation done on the signal emitted by each photomultiplier in response to an event allows for localisation of the place of the crystal scintillator where the gamma ray interacted with the material.
This localisation also allows for identification of the position of a radioactive zone
18
from which the radiation was emitted. This is possible due in particular to the collimator
12
which, as is shown in
FIG. 1
, allows for elimination of rays of which the incidence is not normal with respect to the entry side of the detector.
The gamma rays which reach the crystal scintillator can, like the ray
20
in
FIG. 1
, be rays which, after having left the radioactive zone
18
, interact directly with the detector. These rays are part of a phenomenon referred to in the remainder of the text as “direct radiation”.
Other rays however interact once or several times with the material surrounding the radioactive zone before reaching the scintillator. This is the case for ray
21
for example in the figure, which interacts once in the patient outside of the radioactive zone. After this first interaction, a diffused gamma ray of lower energy reaches the scintillator. This phenomenon, due to the Compton effect, is referred to as “diffused radiation” in the remainder of the text.
It can be seen on the figure that the diffused radiation can result in erroneous localisation of the radioactive zone and can contribute to degradation of the contrast of the medical image by addition of noise.
As mentioned above, diffused radiation is characterised by the fact that its energy is lower than that of direct radiation.
By distinguishing the detection signals, i.e. by rejecting those for which the amplitude as a function of energy is below a determined threshold or for which the amplitude as a function of energy is outside of a defined window, the contribution from diffused radiation can be eliminated.
In general, an amplitude window is set around the maximum amplitude value as a function of the energy of the signals for a given energy emission from the radioactive zone.
If the window is narrow, the contrast of the image is increased by limiting the acceptance of diffused radiation. This occurs to the detriment of the number of effective events detected however, i.e. the number of events which can be used to form an image.
Inversely, if the window is wider, the number of events is greater for a given measurement time, but the image contrast is poorer.
In the case of medical imaging applications, it is not possible to inject patients with highly radioactive doses nor is it comfortable for the examination to last a long time. The number of effective events measured per unit time and the energy resolution of the detector are thus important parameters.
The energy resolution is understood to be the ratio of the width at half-height of a distribution of the energy peak around the emission energy value to the emission energy.
A recent development of gamma-cameras, in which t he scintillator detectors are replaced by semi-conductor detectors, allowed for improved acquisition of events in terms of efficiency and energy resolution.
Semi-conductor detectors such as CdTe, CdZnTe, AsGa, PbI
2
directly convert gamma photons into charge bearers. For radiation of the same intensity, the number of charges created is of an order of magnitude greater than that obtained by indirect detection with scintillator detectors. The resolution of the semiconductor detectors is thus also improved.
FIG. 2
shows the structure of a semi-conductor detector. It includes a platform
30
equipped with integrated electronic circuits
32
on which several detection elements
34
are mounted.
These detection elements
34
are each in the form of a semi-conductor block with two parallel opposite sides on which electrodes are to be placed. An electric field applied to the electrodes allows for migration of the charge bearers, i.e. the electrons and the holes formed by interaction of the radiation with the semi-conductor. The electrodes, not shown on the figure, will also receive charges and transfer them to the integrated circuits of platform
30
for formation of a detection signal.
Not all of the charges created in the semi-conductor migrate directly to the electrodes. Flaws in the semi-conductors trap some charge bearers during their migration and reduce their life span, this effect increasing as the semi-conductor becomes thicker.
The charge created by gamma radiation is divided between a charge borne by the electrons and a charge borne by the holes. The mobility of the holes is lower than that of the electrodes and their collection efficiency is not as good. Thus the charges created do not all contribute equally to the detection signal which is finally delivered.
In the energy spectrum of the detection signal, this results in a “drag” corresponding to energy which is weaker than the energy of the photons which reach the detection element material.
The “drag” is characteristic of the trapping of the charges in the material before their collection.
The events detected, the energy of which is less than the energy of the gamma photons received due to the trapping phenomenon, are therefore not distinguishable from those which result from the diffused radiation mentioned previously, for which the energy is also less than that of the direct radiation.
For the semi-conductor detectors, the establishment of an acceptance amplitude window for the signals eliminates not just the events corresponding to diffused radiation, but also the events affected by the trapping of charges which is however direct radiation.
The difficulties in charge collection and the influences on the detection signal are described in patent FR-2 738 919. This patent proposes a process and an operating device with a signal supplied by a detector which overcomes the poor transport properties of the holes in the detection element material. The purpose of the process described is to make an electronic correction after acquisition of a biparametric spectrum of the contribution of the events for which all of the charge deposited in the detector could not be collected and of which the signals have a total amplitude which is less than the expected amplitude.
The practical application of this method in a gamma-camera for medical use involves acqu
Chapuis Alain
Mathy Franåoise
Mestais Corinne
Verger Loick
Commissariat A l'Energie Atomique
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
Kim Robert H.
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