Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-07-12
2001-12-25
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S366000
Reexamination Certificate
active
06333503
ABSTRACT:
DESCRIPTION
Technical domain
This invention relates to a device for determining the position of an event inducing a signal in photodetectors, for example this position being identified with respect to the set of photodetectors. This type of position can be identified by the center of gravity of the event in a coordinate system relative to the photodetectors.
The invention is particularly applicable to determining the position of an event starting from signals output by photo-multipliers used in a gamma-camera, the position being identified with respect to the photo-multipliers themselves. A gamma-camera is a camera that is sensitive to gamma (&ggr;) radiation. This type of camera is used particularly for medical imagery purposes.
STATE OF PRIOR ART
At the present time, most gamma-cameras used in nuclear medicine operate using the principle of Anger type cameras. Document U.S. Pat. No. 3,011,057 provides further information about this subject.
Gamma-cameras have the specific feature that they display the distribution of molecules marked by a radioactive isotope previously injected into the patient, within an organ.
The structure and operation of a known gamma-camera are described and summarized below with reference to the attached
FIGS. 1
,
2
A and
2
B.
FIG. 1
shows a detection head
10
of a gamma-camera placed facing an organ
12
containing molecules marked by a radioactive isotope.
The detection head
10
comprises a collimator
20
, a scintillator crystal
22
, a light guide
24
and several photo-multiplier tubes
26
placed adjacent to each other so as to cover one surface of the light guide
24
. For example, the scintillator may be an NaI (Tl) crystal.
The function of the collimator
20
is to select the radiation which reaches the detection head at an approximately normal incidence, among all the gamma radiation
30
emitted by organ
12
. The selective nature of the collimator can increase the resolution and the sharpness of the image produced. However, the resolution is increased at the expense of sensitivity. For example, only one photon among about 10 000 &ggr; photons emitted by organ
12
, is actually detected.
The &ggr; photons that passed through the collimator arrive at the scintillator crystal
22
, where almost all &ggr; photons are converted into several light photons. In the rest of this text, each interaction of a gamma photon with the crystal causing a scintillation is called an event.
Photo-multipliers
26
are designed to emit an electric pulse proportional to the number of light photons received from the scintillator for each event.
In order for a scintillation event to be more precisely positioned, photo-multipliers
26
are not directly fixed to the scintillator crystal
22
but are separated from it by the light guide
24
.
Photo-multipliers emit a signal, the amplitude of which is proportional to the total quantity of light produced in the scintillator by gamma radiation, in other words proportional to its energy. However, the individual signal from each photo-multiplier also depends on the distance that separates it from the point
30
at which the gamma radiation interacts with the scintillator material. Each photo-multiplier outputs a current pulse proportional to the light flux that it received. In the example in
FIG. 1
, small graphs A, B and C show that photo-multipliers
26
a
,
26
b
and
26
c
located at different distances from an interaction point
30
output signals with different amplitudes.
The position of the interaction point
30
of a gamma photon is calculated in the gamma-camera starting from signals originating from the set of photo-multipliers by taking a center of gravity weighting of the contributions of each photo-multiplier.
The principle of center of gravity weighting as used in Anger type cameras can be explained more clearly with reference to attached
FIGS. 2A and 2B
.
FIG. 2A
shows the electric wiring of a gamma-camera detection head
10
, that connects this camera to an image generation unit. The detection head comprises several photo-multipliers
26
.
As shown in
FIG. 2B
, each photo-multiplier
26
in the detection head is associated with four resistances denoted RX
−
, RX
+
, RY
−
and RY
+
. The values of these resistances are specific to each photo-multiplier and depend on the position of the photo-multiplier in the detection head
10
.
Resistances RX
−
, RX
+
, RY
−
and RY
+
in each photo-multiplier are connected to the output
50
of the said photo-multiplier, represented in
FIG. 2B
by a current generator symbol. They are also connected to common collecting rows denoted LX
−
, LX
+
, LY
−
and LY
+
respectively in FIG.
2
A.
Rows LX
−
, LX
+
, LY
−
and LY
+
are in turn connected to analog integrators
52
X
−
,
52
X
+
,
52
Y
−
and
52
Y
+
respectively, and through these integrators to analog/digital converters
54
X
−
,
54
X
+
,
54
Y
−
and
54
Y
+
respectively. The output from converters
54
X
−
,
54
X
+
,
54
Y
−
and
54
Y
+
is directed towards a digital operator
56
. Rows LX
−
, LX
+
, LY
−
and LY
+
are also connected to a common channel, called the energy channel. This channel also comprises an integrator
57
and an analog/digital converter
58
, and its output is also directed towards operator
56
.
The device in
FIG. 2
is used to calculate the position of the interaction 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 are the coordinates along two orthogonal directions of the position of the interaction on the crystal, and in which X
+
, X
−
, Y
+
, Y
−
represent the weighted signals output by integrators
52
X
+
,
52
X
−
,
52
Y
+
,
52
Y
−
respectively.
The values of X and Y, and the total energy E of the gamma ray that interacted with the crystal, are established by the digital operator
56
. These values are then used to generate an image, for example as described in document FR-2 669 439.
The calculation of the interaction position is affected by an uncertainty related to Poisson statistical fluctuations in the number of light photons and the number of photoelectrons produced for each event, in other words for each detected gamma photon. The standard deviation of the fluctuation reduces when the number of photons or photoelectrons increases. Due to this phenomenon, light should be collected as carefully as possible. The intrinsic spatial resolution of the camera is characterized by the width at the mid-height of the distribution of positions calculated for the same collimated point source placed on the scintillator crystal.
The resolution for gamma rays with an energy of 140 keV is usually of the order of 3 to 4 mm.
The energy of a detected gamma photon is calculated by taking the sum of the contributions of all photo-multipliers that received light. It is also affected by a statistical fluctuation. The energy resolution of the camera is characterized by the ratio of the width at the mid-height of the distribution of calculated energies, to the average value of the distribution, for the same source.
The energy resolution is usually 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 that it enables real time calculation of the center of gravity of photo-multiplier signals with very simple means.
The system described above has a limited number of components. Furthermore, the resistances used to inject the photo-multiplier signal in collecting rows are not very expensive.
However, this type of camera also has a major disadvantage, which is a low count rate. The count rate is the number of events, in other words the number of interactions between a &ggr; photon and the scintillator, that the camera is capable of processing per unit time.
One of the limitations in the count rate is particularly due to
Chapuis Alain
Janin Claude
Noca Alain
Burns Doane , Swecker, Mathis LLP
Commissariat A l'Energie Atomique
Gagliardi Albert
Hannaher Constantine
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