Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-06-15
2001-12-04
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363090
Reexamination Certificate
active
06326624
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a device for determining the presumed position of an event inducing a signal, in photodetectors, this position being, for example, located in relation to the photodetector assembly.
The invention applies in particular to determination of the position of an event from signals supplied by photomultipliers equipping a gamma camera, the position being located in relation to the photomultipliers themselves. Gamma camera means a camera sensitive to gamma (&ggr;) radiation. Such cameras are used notably for the purposes of medical imaging.
2. Discussion of the Background
At the present time, the majority of gamma cameras used in nuclear medicine are cameras operating according to the principle of Anger type cameras. This subject can be referred to U.S. Pat. No. 3,011,057.
Gamma cameras make it possible in particular to visualize the distribution, in an organ, of molecules labelled with a radioactive isotope previously injected into the patient.
The structure and operation of a known gamma camera are described and summarized below with reference to the accompanying
FIGS. 1
,
2
A and
2
B.
FIG. 1
shows a detection head
10
of a gamma camera disposed opposite an organ
12
containing molecules labelled with a radioactive isotope.
The detection head
10
has a collimator
20
, a scintillator crystal
22
, a light guide
24
and a plurality of photomultiplier tubes
26
juxtaposed so as to cover one face of the light guide
24
. The scintillator is, for example, a crystal of NaI (Tl).
The function of the collimator
20
is to select from among all the gamma radiations
30
emitted by the organ
12
those which reach the detection head substantially at normal incidence. The selective nature of the collimator makes it possible to increase the resolution and clarity of the image produced. However, the increase in resolution is made to the detriment of the sensitivity. By way of example, for around 10,000 &ggr; photons emitted by the organ
12
, one single photon is actually detected.
The &ggr; photons which have passed through the collimator reach the scintillator crystal
22
where nearly every &ggr; photon is converted into a plurality of light photons. In the remainder of the text, event designates each interaction of a gamma photon with the crystal, causing a scintillation.
The photomultipliers
26
are designed to emit an electrical pulse proportional to the number of light photons received from the scintillator for each event.
So that a scintillation event can be localized more precisely, the photomultipliers
26
are not placed directly side by side with the scintillator crystal
22
but are separated from the latter by the light guide
24
.
The photomultipliers emit a signal whose amplitude is proportional to the total quantity of light produced in the scintillator by gamma radiation, that is to say proportional to its energy. However, the individual signal from each photomultiplier also depends on the distance which separates it from the point of interaction
30
of the gamma radiation with the material of the scintillator. This is because each photomultiplier delivers a current pulse proportional to the light flux it has received. In the example of
FIG. 1
, small graphs A, B, C show that photomultipliers
26
a
,
26
b
and
26
c
situated at different distances from a point of interaction
30
deliver signals with different amplitudes.
The position of the point of interaction
30
of a gamma photon is calculated in the gamma camera from signals coming from the photomultiplier assembly by performing a barycentric weighting of the contributions of each photomultiplier.
The principle of barycentric weighting as implemented in Anger type cameras emerges more clearly on referring to the accompanying
FIGS. 2A and 2B
.
FIG. 2A
shows the electrical wiring of a detection head
10
of a gamma camera, which connects this camera to an image forming unit. The detection head has a plurality of photomultipliers
26
.
As shown in
FIG. 2B
, each photomultiplier
26
of the detection head is associated with four resistors denoted RX
−
, RX
+
, RY
−
and RY
+
. The values of these resistors are specific to each photomultiplier and depend on the position of the photomultiplier in the detection head
10
.
The resistors RX
−
, RX
+
, RY
−
and RY
+
of each photomultiplier are connected to the output
50
of the said photomultiplier, represented in
FIG. 2B
by a current generator symbol. They are moreover respectively connected to common collector lines denoted LX
−
, LX
+
, LY
−
and LY
+
, in FIG.
2
A.
The lines LX
−
, LX
+
, LY
−
and LY
+
are in turn connected respectively to analog integrators
52
X
−
,
52
X
+
,
52
Y
−
and
52
Y
+
, and, by means of the latter, to analog-to-digital converters
54
X
−
,
54
X
+
,
54
Y
−
and
54
Y
+
. The output of the converters
54
X
−
,
54
X
+
,
54
Y
−
and
54
Y
+
is taken to a digital operator
56
. The lines LX
−
, LX
+
, LY
−
and LY
+
are furthermore connected to a common path, referred to as the energy path. This path also has an integrator
57
and an analog-to-digital converter
58
and its output is also taken to the operator
56
.
By virtue of the device of
FIG. 2
, the position of the interaction is calculated according to the following equations (U.S. Pat. No. 4,672,542):
X
=
X
+
-
X
-
X
+
+
X
-
and
Y
=
Y
+
-
Y
-
Y
+
+
Y
-
in which X and Y indicate the coordinates, along two orthogonal directions, of the position of the interaction on the crystal and in which X
+
, X
−
, Y
+
, Y
−
indicate respectively the weighted signals delivered by the integrators
52
X
+
,
52
X
−
,
52
Y
+
,
52
Y
−
.
The values of X and Y, as well as the total energy E of the gamma ray which has interacted with the crystal, are produced by the digital operator
56
. These values are next used for constructing an image as described, for example, in the document FR-2 669 439.
The calculation of the position of the interaction is marred by an uncertainty related to the statistical Poisson fluctuations of the number of light photons and the number of photoelectrons produced for each event, that is to say for each gamma photon detected. The higher the number of photons or photoelectrons, the smaller is the standard deviation of the fluctuation. Because of this phenomenon, the light should be collected as carefully as possible. The intrinsic spatial resolution of the camera is characterised by the mid-height width of the distribution of the positions calculated for one and the same collimated point source placed on the scintillator crystal.
For gamma rays with an energy of 140 keV, the resolution is generally of the order of 3 to 4 mm.
The energy of a detected gamma photon is calculated by summing the contributions of all the photomultipliers which have received light. This is also marred by a statistical fluctuation. The resolution energy-wise of the camera is characterised by the ratio of the mid-height width of the distribution of the calculated energies to the mean value of the distribution, for one and the same source.
The resolution energy-wise is generally of the order of 9 to 11% for gamma rays with an energy of 140 keV.
Finally, an Anger type gamma camera has the advantage of making it possible to calculate in real time the barycentre of the signals from photomultipliers with very simple means.
This is because the system described previously has a limited number of components. Moreover, the resistors used to inject the signal from the photomultipliers into the collector lines are very inexpensive.
Such a camera has however also a major drawback, which is a reduced counting rate. Counting rate means the number of events, that is to say interactions between a &ggr; photon and the scintillator, which the camera is capable of processing per
Chapuis Alain
Janin Claude
Mestais Corinne
Tararine Michel
Commissariat A l'Energie Atomique
Epps Georgia
Hanig Richard
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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