Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-06-30
2002-04-02
Porta, David P. (Department: 2882)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C378S147000, C378S154000
Reexamination Certificate
active
06365900
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a detection head and a collimator for a gamma camera and more particularly for a “pixels” gamma camera.
A pixels gamma camera means a camera sensitive to gamma radiation, in which the detection head comprises a number of adjacent individual elementary detectors.
The invention has applications in medical imagery, as for example such as scintigraphy and Single Photo-Emission Computed Tomography (SPECT).
DISCUSSION OF THE BACKGROUND
Gamma cameras conventionally used in medical imagery are of the Anger type. Document (1), listed in the references at the end of this description, contains further information about this subject.
Gamma cameras are used particularly to display the distribution of molecules marked by a radioactive isotope previously injected into the patient, throughout the body or in an organ.
FIG. 1
more precisely shows a detection head
10
of an Anger type gamma camera placed facing an organ
12
.
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 in order 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 collimator is in the form of a lead disk through which a number of ducts
21
carrying gamma radiation can pass, approximately identical and parallel to each other. The disk is placed in contact with the scintillator
24
such that the ducts
21
are perpendicular to the surface of this crystal. A divergent or convergent collimator may be used for some applications in which the object size has to be magnified or reduced to produce the image size.
The function of the collimator
20
is to select the part of the gamma radiation
30
emitted by organ
12
that reaches the detection head at approximately normal incidence.
The selective nature of the collimator is such that the resolution and sharpness of the image produced can be increased. However, the resolution is increased at the detriment of sensitivity.
The opening and the length of the ducts
21
are determined as a function of the inspection energy and the compromise between the spatial resolution and the derived sensitivity. As the ducts become longer and narrower, the spatial resolution of the detection head improves but its sensitivity reduces. Furthermore, the spacing between ducts is chosen to be higher when the energy of the received radiation is greater.
Known collimator ducts have a hexagonal cross-section (or round for high energies).
This form is dictated partly by detection uniformity requirements, but also by collimator manufacturing constraints.
It is considered that the circular shape for the collimator duct cross-section gives the most uniform and homogeneous detection possible.
However, when ducts with a circular cross-section are placed adjacent to each other, it is observed that the thicknesses of the material walls separating the ducts are not uniform. The non-uniform nature of the wall thicknesses and especially the existence of intermediate regions between the ducts in which the thickness of the absorbing material (lead) varies, is a major disadvantage.
Doses of radioactive product injected into the patient necessarily have to be limited. Thus, the intensity of the emitted radiation is relatively low. Under these conditions, the extent and thickness of intermediate walls separating the collimator ducts must be reduced in order to limit excessive losses of the “useful” radiation.
In order to limit the thickness of the walls between ducts and differences in this thickness, collimators are made with ducts with a hexagonal cross-section. This shape also has the advantage that it facilitates manufacturing of the collimators.
Finally, it can be noted that the hexagonal shape is used to the extent that it is relatively close to the circular shape, and enables approximately uniform detection.
In the case of ducts with a hexagonal cross-section, the thickness of the walls that delimit the ducts is usually chosen within a range from 0.2 to 2 mm. The characteristic size of the duct opening, in other words the distance between flats in the hexagonal cross-section, is of the order of 1.5 to 4.5 mm. Finally, the depth of the ducts is usually chosen between 30 and 50 mm.
Known collimators are usually made using a technique for the assembly of lead sheets shaped to make the ducts. According to another known technique, the collimators may also be obtained by casting in a pin mold.
With reference to
FIG. 1
, it can be seen that gamma photons that have passed through the collimator reach the scintillator crystal
24
in which practically every gamma photon is converted into several light photons
31
. Throughout the rest of this text, each detected interaction between a gamma photon and the detector material, for example with the scintillator crystal, will be denoted as an “event”.
Photo-multipliers
26
are designed to emit an electric pulse proportional to the number of light photons received on scintillator
24
, at each event.
In order to be able to locate the scintillation event more precisely, the photo-multipliers
26
are not placed immediately adjacent to scintillator crystal
24
, but are separated from it by the light guide
22
.
The 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 interaction point
30
of the gamma radiation with the scintillator material. Each photo-multiplier outputs a current pulse proportional to the light flux that it received. In the example shown in
FIG. 1
, small graphs A, B, C show that: the 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
and the energy of a gamma photon is calculated in the gamma camera starting from signals originating from all photo-multipliers by making a center of gravity weighting of the contributions of each photo-multiplier.
However, Anger type gamma cameras have a disadvantage due to the fact that the number of light photons created during each event in the scintillator crystal satisfies Poisson statistics. The number of photo-electrons torn from the photo-cathode of the photo-multipliers also satisfies Poisson statistics. Thus, the position and energy calculations are affected by an inaccuracy related to Poisson fluctuations in the number of light photons and the number of photoelectrons produced for each event.
The standard deviations of the fluctuations is lower when the number of photons or photoelectrons is high. The inherent spatial resolution of the gamma camera is characterized by the width at mid-height of the distribution of the calculated positions, for a single isolated collimated point source placed on the crystal. The resolution is of the order of 3 to 4 mm at 140 keV. Furthermore, the energy of the gamma photon is calculated by taking the sum of the contributions of all photo-multipliers that received light. This is also affected by a statistical fluctuation. The energy resolution is characterized by the ratio of the mid-height width of the distribution of calculated energies, to the average value of the distribution, for the same source. It is of the order of 9 to 11% at 140 keV.
Detection heads for gamma cameras are also known in which the scintillator crystal and photo-multipliers are replaced by solid detectors arranged in the form of a matrix of individual detectors. In this case, the spatial resolution of the gamma camera depends on the size of the individual detectors.
The attached
FIG. 2
very diagrammatically shows a detection head with solid detectors, for information. The detection head
40
comprises several individual elementary detectors
42
with semi
Allemand Robert
Campagnolo Raymond
Mestais Corinne
Commissariat a l'Energie Atomique
Hobden Pamela R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Porta David P.
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