Patent Description:
In a nuclear detection equipment such as a gamma (γ) camera, a positron emission computed tomography (abbr. PET, Positron Emission Tomography) system, a radiation detector or a crystal performance detection device, the spatial resolution of a nuclear detector is an important indicator reflecting the performance of the nuclear detection equipment. For example, in the PET system, the spatial resolution reflects the PET system's ability to identify fine tissues, which is not only one of the two most important indicators in the PET system, but also one of the important indicators for evaluating the quality of PET images. As an imaging system, the PET system's fundamental evaluation standard lies in the quality of the reconstructed images, while high-quality reconstructed images require good spatial resolution, which has been the focus of optimization in the development of PET systems for more than a decade. Especially in an animal PET system, due to the size of the animals, a higher spatial resolution for the system imaging is required than in a clinical PET system.

In the state of art, nuclear detectors whose crystal bars are cut into a size less than <NUM> are generally referred to as high spatial resolution nuclear detectors. At present, high spatial resolution nuclear detectors are typically embodied in form of the following designs:.

Firstly, by coupling a position-sensitive photomultiplier tube (PSPMT) with a scintillation crystal array, a high spatial resolution may be achieved. The position-sensitive photomultiplier tube with high gain (<NUM><NUM>) and low noise may facilitate an extremely high spatial resolution. There is a team which has implemented small animal PET systems with extremely high spatial resolution requirement by means of the coupling method (see <NPL>), and the system has achieved a favorable performance.

Secondly, by directly coupling an avalanche photodiode array (abbr. APD array) with a scintillation crystal array of the same dimension, a high spatial resolution may also be achieved. The position-sensitive APD has a small size, requires relatively low voltage during normal operation, and allows flexibility in setting up a PET detector, which may reduce the difficulty in system integration engineering. There is a team which has implemented small animal PET systems with a relatively high spatial resolution by means of the coupling method (see <NPL>).

Thirdly, a PET detector may be built by <NUM>:<NUM> directly coupling a silicon photomultiplier tube array (abbr. SiPM array) with a scintillation crystal array of the same size. A silicon photomultiplier tube has a gain of <NUM><NUM> comparable to the PMT, with low noise, small size, compact arrangement, and good time performance. When using an SiPM array to build a PET detector, the signal-to-noise ratio of the tip detector output signal is high, and the detector is flexible, which can also reduce the engineering difficulty of system integration. As a semiconductor device, SiPM array has the advantage of low price in mass production, which is especially suitable for the instrument or equipment including a large number of detectors, such as PET. There is a team which has implemented the design and production of PET detectors by <NUM>:<NUM> direct coupling of the SiPM array and the scintillation crystal array, and have integrated the system, realizing the PET system spatial resolution of approx. <NUM> (see <NPL>).

However, there are still several shortcomings in the above-mentioned designs of nuclear detector. For example, for the first type of nuclear detector based on PSPMT coupled to scintillation crystal array, its photomultiplier tube is very expensive. For the PET system with thousands of channels, there are so many detectors, leading to extremely high costs of devices. Further, the photomultiplier tube is bulk in its shape, which does not meet the requirement of flexibility to build up the system. Moreover, the operation of photomultiplier tube usually requires a high voltage up to about <NUM> volts, which will increase the engineering difficulty in PET system integration. For the second type of nuclear detector that comprises the APD array coupled to the scintillation crystal, the signal-to-noise ratio of the electric pulse signals generated by the tip detector will be lowered due to the APD's natural defects of low gain and loud noise, affecting the electronic readout effect and thus deteriorating the performance of the PET detector. For the third type of PET detector comprised of a silicon photomultiplier tube array and a scintillation crystal array of the same size coupled to each other by means of <NUM>: <NUM> direct coupling, although relatively favorable energy resolution and time resolution may be realized, the spatial resolution thereof is limited by the size of the silicon photomultiplier tube in view of the coupling and it is difficult to further improve the spatial resolution of the PET detector by cutting out smaller crystal bars in the crystal array.

In summary, in the state of art the nuclear detectors based on the PSPMT coupled to scintillation crystal array is not only expensive, but also have low system integration flexibility and high engineering difficulty. Although it can be used in the PET system with high spatial resolution, the research and development costs and production costs are relatively high. The nuclear detector based on the APD array coupled to the scintillation crystal has poor signal-to-noise ratio of the signals due to the APD's low gain, which will deteriorate the performance of the PET detector. The PET detector based on the SiPM array coupled to the scintillation crystals by means of <NUM>: <NUM> direct coupling shares the advantages of the above two types. However, a high-spatial-resolution nuclear detector with dissected crystal bars of less than <NUM> is hardly realized, due to the limitation of the size of the single SiPM of the SiPM array.

Further radiation detection devices are described in:.

In view of the technical problems, therefore, it is necessary to propose a nuclear detector with low cost, high system integration flexibility, and high spatial resolution, in order to overcome the above defects.

In the disclosure the purpose is to provide a nuclear detector, thereby solving the problems of high cost, low system integration flexibility or low spatial resolution of the nuclear detector in the state of art.

To solve the above problems, provided is a nuclear detector according to claim <NUM>, comprising a scintillation crystal array including a plurality of scintillation crystal bars of the same size arranged closely in sequence, a light guide, and a photodetector array including a plurality of silicon photomultipliers (SiPMs) arranged in sequence, wherein each of the SiPMs has a cross-sectional area greater than a cross-sectional area of each of the scintillation crystal bars, wherein the light guide includes a top surface coupled to the scintillation crystal array, an opposed bottom surface coupled to a top surface of the photodetector array and a side surface. The light guide has a thickness in a range of <NUM> to <NUM>. The light guide further includes a slit adjacent to an edge of the light guide. The slit is configured to extend from the top surface of the light guide toward the bottom surface of the light guide and the slit has a depth in a range of <NUM> to <NUM> times the thickness of the light guide.

The light guide has a shape of cuboid, and the slit has an extending direction perpendicular to the top surface and the bottom surface of the light guide. The top surface of the photodetector array has an area less than an area of the bottom surface of the light guide such that the photodetector array is recessed at the edges thereof with respect to the light guide and the scintillation crystal array.

In an embodiment of the disclosure, the slit is spaced from the side surface of the light guide in a distance of <NUM> and <NUM> times a width of the scintillation crystal bar.

In an alternative embodiment not covered by the claimed invention, the light guide is in the shape of a truncated cone with an area of the top surface of the light guide greater than an area of the bottom surface of the light guide, and the slit has an extending direction parallel to the side surface of the truncated cone.

In an embodiment of the disclosure, the slit includes a first slit spaced from the side surface of the light guide in a distance equal to a width of the scintillation crystal bar, and a second slit spaced from the side surface of the light guide in a distance of two times the width of the scintillation crystal bar, and the first slit has a depth greater than a depth of the second slit.

In an embodiment of the disclosure, there is a number of slits and the number of the slits is in a range of <NUM> to <NUM>, and the slits are sequentially arranged from the side surface of the light guide toward the center of the light guide, with the depth of the slits gradually decreased from the side surface of the light guide toward the center of the light guide.

In an embodiment of the disclosure, the scintillation crystal bars have a width in a range of <NUM> to <NUM>.

In an embodiment of the disclosure, the scintillation crystal bars have a side surface coated with an opaque material.

In an embodiment of the disclosure, the opaque material is barium sulfate powders or a specular reflection film.

In an embodiment of the disclosure, the slit and the side surface of the light guide are coated with an opaque material.

In an embodiment of the disclosure, the opaque material is black paint.

In an embodiment of the disclosure, the light guide comprises <NUM> to <NUM> layers, and the respective layers of light guide have a collective thickness in a range of <NUM> to <NUM>.

In the nuclear detector provided in the disclosure the scintillation crystal bars in the scintillation crystal array are significantly smaller than the photodetectors, that is, no <NUM>: <NUM> direct coupling between the scintillation crystal bars and the photodetectors can be realized, arranged therebetween is a light guide with slits, enabling a nuclear detector with a high spatial resolution. Because of the relatively thin thickness of the light guide, there is little scintillation photon loss of the scintillation crystals, thus leading to little loss in the signal-to-noise ratio of the scintillation photon. Therefore, the high spatial resolution of the nuclear detector can be realized without compromising the performance of the nuclear detector, such that the energy resolution and the coincidence time resolution of the nuclear detector fulfills the needs of PET detector, which is easy to be produced or manufactured cost-effectively.

In the following, the invention will be described further with reference to embodiments. It should be understood that the following embodiments are for illustrative instead of limitative purpose only.

<FIG> is a schematic front view of a nuclear detector according to a preferred embodiment of the disclosure. <FIG> is a schematic perspective view of a light guide of the nuclear detector according to <FIG>. Referring to <FIG>, in the disclosure the nuclear detector is provided and comprises a scintillation crystal array <NUM>, a light guide <NUM>, and a photodetector array <NUM>. The light guide <NUM> is disposed between, and respectively coupled to, the scintillation crystal array <NUM> and the photodetector array <NUM>. Specifically, the scintillation crystal array <NUM> includes m × n closely arranged scintillation crystal bars <NUM> of the same size, in which m and n are natural numbers not less than <NUM>. The individual scintillation crystal bar <NUM> is a six-side polished cuboid. The side surfaces of the individual scintillation crystal bar <NUM> are coated with an opaque, diffuse reflection material, such as BaSO<NUM> powders or a specular reflection film. The bottom surfaces of the individual scintillation crystal bars <NUM> collectively form the bottom surface of the scintillation crystal array <NUM>. As shown in <FIG>, the light guide <NUM> comprises a light guide body <NUM>, a first slit <NUM>, and a second slit <NUM>. The light guide body <NUM> has a shape of cuboid. The top surface of the light guide <NUM> is coupled to the bottom surface of the scintillation crystal array <NUM>. The area of the top surface of the light guide <NUM> is equal to the area of the bottom surface of the scintillation crystal array <NUM>. Provided in the top surface of the light guide <NUM> are four first slits <NUM> parallel to four sides of the top surface of the light guide <NUM>, respectively, and four second slits <NUM> also parallel to the four sides of the top surface of the light guide <NUM>, respectively. The first slits <NUM> and the second slits <NUM> extend from the top surface of the light guide <NUM> in a thickness direction of the light guide <NUM> toward inside of the light guide <NUM>, and the four second slits <NUM> are arranged more closely to the center of the light guide <NUM> than the four first slits <NUM>. In the embodiment shown in <FIG>, the first slits <NUM> are spaced from the edges of the light guide <NUM> in a distance equal to the width of the single scintillation crystal bar <NUM>, and the first slits <NUM> are spaced from the second slits <NUM> in a distance equal to the width of the single scintillation crystal bar <NUM> too. The depth of the first slits <NUM> is greater than the depth of the second slits <NUM>. The top surface of the photodetector array <NUM> is coupled to the bottom surface of the light guide <NUM>. The photodetector array <NUM> includes x × y photodetectors <NUM> of the same size arranged in sequence, in which both x and y are natural numbers. The cross-sectional area of the single photodetector is greater than the cross-sectional area of the single scintillation crystal bar <NUM>, and the top surface area of the photodetector array <NUM> is less than the bottom surface area of the light guide <NUM>.

More specifically, in the embodiment shown <FIG>, the scintillation crystal array <NUM> is formed of <NUM> × <NUM> individual scintillation crystal bars <NUM>. The scintillation crystal bars <NUM> are made from Lutetium Yttrium Orthosilicate (abbr. LYSO) scintillation crystal. The single scintillation crystal bar has a size of <NUM> × <NUM> × <NUM>, while scintillation crystal array <NUM> has an overall size of <NUM> × <NUM> × <NUM>. BaSO<NUM> powders are applied in between the respective scintillation crystal bars <NUM>. The light guide <NUM> has a thickness of <NUM>. The second slits <NUM> have a width of <NUM>, and a depth of <NUM>. The first slits <NUM> and the second slits <NUM> are filled with opaque materials, such as opaque, black paint. It is worth noting that the side surfaces of the light guide <NUM> are also coated with opaque materials, allowing to a better light guiding effect. The photodetector array <NUM> comprises <NUM> × <NUM> silicon photomultiplier tubes <NUM>. The single silicon photomultiplier tube <NUM> has a size of <NUM> × <NUM> × <NUM>. A gap is formed in between the adjacent silicon photomultiplier tubes <NUM>, with a gap width of <NUM>.

<FIG> is a schematic diagram of the multiplex circuit of the nuclear detector according to <FIG>. As can be seen from <FIG>, the multiplex circuit of the nuclear detector in the disclosure may be configured as an equalized charge distribution circuit. The circuit includes <NUM> channels and in each channel the scintillation pulse signal of the silicon photomultiplier tube <NUM> is firstly going through equalized charge distribution by two resistors <NUM> of which a resistance is of <NUM> ohms, thus generating <NUM>-way weighted signals <NUM>. The equalized charge distribution circuit allows that x × y ways of scintillation pulse signals of the silicon photomultiplier tube <NUM> can be reduced to x + y ways of signals. Finally, a position spectrum may be generated by means of Anger algorithm. It is worth noting that the Anger algorithm is conventionally used in the state of art and will not be elaborated here.

<FIG> is a schematic diagram of crystal position spectrums of the nuclear detector according to an embodiment in the disclosure. As can be seen from <FIG>, the scintillation pulse signal generated by the silicon photomultiplier tube <NUM> is processed by the multiplex circuit, and then is further processed by multi-threshold (MVT) digitalization. Unlike the traditional ADC method in which voltage is sampled at fixed time, the MVT digital method comprises pre-setting multiple voltage thresholds in the system, recording the time when the scintillation pulse signals reach each voltage threshold, and further using the prior knowledge of the scintillation pulse model to obtain time, energy, baseline drift and decay time information of the scintillation pulse signals by fitting, such that the position information can be obtained by means of the energy information. <FIG> is a position spectrum image of the nuclear detector of <FIG>, obtained by the MVT digital method. <FIG> clearly illustrates the position spectrums of the <NUM> × <NUM> crystals of the nuclear detector.

<FIG> shows the energy spectrums of the <NUM> × <NUM> scintillation crystal bars, obtained by using the position look-up table algorithm in the SiPM-based nuclear detector according to <FIG>. As can be seen from <FIG>, the energy resolution of each scintillation crystal bar can be obtained by means of Gaussian fitting, with the energy resolution of the single scintillation crystal bar in the range of <NUM>% to <NUM>%. <FIG> is a schematic diagram of the energy spectrum of the central scintillation crystal bar according to the nuclear detector of <FIG>. <FIG> is a schematic diagram of the energy spectrum of the peripheral scintillation crystal bar of the nuclear detector according to <FIG>. By comparing <FIG> with <FIG>, the energy resolution of the scintillation crystal bar at the edge of the nuclear detector is lower than that of the scintillation crystal bar at the center of the nuclear detector. <FIG> is a schematic diagram of an average energy spectrum of the scintillation crystal bars according to the nuclear detector of <FIG>. As can be seen from <FIG>, an average energy resolution of the <NUM> × <NUM> scintillation crystal bars is <NUM>%.

<FIG> is a schematic diagram of the coincidence time resolution of the nuclear detector according to <FIG>. As can be seen from <FIG>, the statistics of <NUM> pairs of adjacent lines of response (abbr. LOR, Line of Response) sampled in the SiPM-based nuclear detectors opposite to each other are consistent with the time distribution spectrum. All events are filtered through the energy window of <NUM>-650keV. The coincidence time resolution obtained by means of Gaussian fitting is <NUM>.

<FIG> is a schematic front view of a nuclear detector according to another embodiment of the disclosure. In the embodiment shown in <FIG>, the scintillation crystal array <NUM> and the photodetector array <NUM> of the nuclear detector are the same as those in the embodiment shown in <FIG>, respectively, which thus will not be elaborated. The differences lie in that in the embodiment shown in <FIG>, provided in the top surface of the light guide <NUM> are only four first slits <NUM>, which are cut parallel to the four sides of the top surface of the light guide <NUM>, respectively. The slits <NUM> extend from the top surface of the light guide <NUM> toward inside of the light guide <NUM> in the thickness direction of the light guide <NUM>. The four first slits <NUM> are spaced from the respective edges of the light guide <NUM> in a distance of <NUM>-<NUM> times the width of the scintillation crystal bar. The first slits <NUM> have a depth in a range of <NUM>-<NUM> times the thickness of the light guide <NUM>. For example, in the embodiment shown in <FIG>, the first slits <NUM> are spaced from the respective edges of the corresponding light guide <NUM> in a distance of <NUM> times the width of the scintillation crystal bar. The first slits <NUM> have a depth of <NUM> and the light guide <NUM> has a thickness of <NUM>.

<FIG> is a schematic front view of a nuclear detector according to an embodiment not covered by the claimed invention. In the embodiment shown in <FIG>, the scintillation crystal array <NUM> and the photodetector array <NUM> of the nuclear detector are the same as those in the embodiment shown in <FIG>, respectively, which thus will not be elaborated. The differences lie in that in the embodiment shown in <FIG>, the light guide <NUM> has a shape of truncated cone, including a top surface, an opposed bottom surface, and four side surfaces. The top surface area of the light guide <NUM> is greater than the bottom surface area, which is equal to the top surface area of the photodetector array <NUM>. Provided in the top surface of the light guide <NUM> are four first slits <NUM> and four second slits <NUM>. The planes where the first and second slits <NUM> and <NUM> are located are parallel to the four side surfaces of the light guide <NUM>, respectively. The four first slits <NUM> are spaced from the respective side surfaces of the light guide <NUM> in a distance equal to the width of the single scintillation crystal bar, while the four second slits <NUM> are spaced from the respective side surfaces of the light guide <NUM> in a distance of two times the width of the single scintillation crystal bar. The first slits <NUM> have a depth greater than that of the second slits <NUM>. For example, in the embodiment shown in <FIG>, the first slits <NUM> have a width of <NUM> and a depth of <NUM>, and the second slits <NUM> have a width of <NUM> and a depth of <NUM>.

According to an embodiment of the disclosure, the material used for the light guide may be a transparent element such as ordinary inorganic glass, organic glass, or scintillation crystal.

According to an embodiment of the disclosure, the light guide may comprise <NUM> to <NUM> layers, and the respective layers of light guide have a collective thickness in a range of <NUM> to <NUM>.

According to an embodiment of the disclosure, the light guide may have a shape of a truncated circular cone, a cylinder, or a cone-like polyhedron. The width or diameter of the light guide may be in between the width of the scintillation crystal array and the width of the photodetector array.

According to an embodiment of the disclosure, the opaque material filled in the first slits or the second slits of the light guide may also include a specular reflective film (aka. ESR, Enhanced Specular Reflector).

According to another embodiment of the disclosure, the number of slits of the light guide may be greater than two, but no more than <NUM>.

According to an embodiment of the disclosure, the scintillation crystal bar may be an inorganic scintillation crystal, including bismuth germanate, lutetium oxyorthosilicate, lanthanum bromide, lutetium yttrium orthosilicate, lutetium oxyorthosilicate, barium fluoride, sodium iodide, cesium iodide or the like.

According to another embodiment of the disclosure, the width of the single scintillation crystal bar in the scintillation crystal array is in a range of <NUM> to <NUM>.

According to an embodiment of the disclosure, the photodetectors in the photodetector array may be further selected from any of avalanche photodiodes (APD), multi-pixel photon counters (MPPC), and Geiger avalanche photodiodes (G-APD).

Claim 1:
A nuclear detector, comprising a scintillation crystal array (<NUM>) including a plurality of scintillation crystal bars (<NUM>) of the same size arranged closely in sequence, a light guide (<NUM>), and a photodetector array (<NUM>) including a plurality of silicon photomultipliers (SiPMs) (<NUM>) arranged in sequence, wherein each of the SiPMs (<NUM>) has a cross-sectional area greater than a cross-sectional area of each of the scintillation crystal bars (<NUM>), wherein the light guide (<NUM>) includes a top surface coupled to the scintillation crystal array (<NUM>), an opposed bottom surface coupled to a top surface of the photodetector array (<NUM>) and a side surface;
the light guide (<NUM>) has a thickness in a range of <NUM> to <NUM>;
the light guide (<NUM>) further includes a slit (<NUM>, <NUM>, <NUM>) adjacent to an edge of the light guide (<NUM>), and the slit (<NUM>, <NUM>, <NUM>) is configured to extend from the top surface of the light guide (<NUM>) toward the bottom surface of the light guide (<NUM>) and the slit (<NUM>, <NUM>, <NUM>) has a depth in a range of <NUM> to <NUM> times the thickness of the light guide (<NUM>);
the light guide (<NUM>) has a shape of cuboid;
the slit (<NUM>, <NUM>, <NUM>) has an extending direction perpendicular to the top surface and the bottom surface of the light guide (<NUM>); characterized in that
the top surface of the photodetector (<NUM>) array has an area less than an area of the bottom surface of the light guide (<NUM>) such that the photodetector array (<NUM>) is recessed at the edges thereof with respect to the light guide (<NUM>) and the scintillation crystal array (<NUM>).