Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1999-06-01
2004-01-20
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C250S363020
Reexamination Certificate
active
06680750
ABSTRACT:
TECHNICAL DOMAIN
This invention relates to a device and process for collection, encoding and processing of signals output from photo-detectors.
The invention is particularly applicable to collection and encoding of signals output by photo-multipliers used in gamma-cameras. A gamma-camera is a camera 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 (1) in the list of references at the end of this description 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
24
, a light guide
22
and several photo-multiplier tubes
26
placed adjacent to each other so as to cover one surface of the light guide
22
opposite the scintillator crystal
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
24
, 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 localized, photo-multipliers
26
are not directly fixed to the scintillator crystal
24
but are separated from it by the light guide
22
.
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 all 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 a digital 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 equipped 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 lines denoted LX
−
, LX
+
, LY
−
and LY
+
respectively in FIG.
2
A.
Lines 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
. Lines 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 sent towards operator
56
.
The device in
FIG. 2
is used to calculate the position of the interaction according to the following equations:
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 X
+
, X
−
, Y
+
, Y
−
represent the weighted signals output by integrators
52
+
,
52
−
,
52
+
,
52
−
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.
Documents (2), (3) and (4), listed in the references at the end of this description, contain more information about this subject.
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 intrinsic 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 lines 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.
The limitation in the count rate is particularly due to the fact that the camera is incapable of processing two events that take place approximately simultaneously at separate points in the scintillator crystal.
Simultaneous but geometrical
Janin Claude
Mestais Corinne
Tournier Edmond
Commissariat a l'Energie Atomique
Garber Wendy R.
Nguyen Luong
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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