CN113040800B - PET detector, PET imaging system and gamma ray positioning method - Google Patents
PET detector, PET imaging system and gamma ray positioning method Download PDFInfo
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Abstract
The invention provides a PET detector, a PET imaging system and a gamma ray positioning method, wherein the PET detector comprises a detector unit and reading electronics; the detector unit comprises at least one crystal group and a light detector array corresponding to the crystal group; each crystal group comprises at least two layers of crystal arrays, and the scintillation crystals in the crystal arrays of different layers have different light attenuation time; one side face of the crystal group for incidence of the gamma ray is parallel to the cross section of each scintillation crystal; the photodetector array is coupled to a layer of crystal arrays on the bottom surface of the corresponding crystal group. This PET detector reads the multilayer crystal array that light decay time is different through the photodetector array side, not only can realize the position and the depth measurement of gamma ray and scintillation crystal effect simultaneously, has reduced the cost of PET detector moreover, can make the PET imaging system of major axis to the field of vision when improving sensitivity, keeps high spatial resolution.
Description
Technical Field
The invention relates to the technical field of medical instruments, in particular to a PET detector, a PET imaging system and a gamma ray positioning method.
Background
PET (Positron Emission Tomography) is a living body function metabolism imaging technology, has the advantages of high sensitivity and high quantitative accuracy, and has wide application in early diagnosis and early evaluation of treatment of cardiovascular diseases, cranial nerve diseases and tumors; meanwhile, PET is also an important tool for brain science research, new therapeutic method research and new drug development. PET imaging systems achieve imaging by coincidence detecting gamma (gamma) rays produced by positron annihilation of a particular drug emission. Specifically, a positron emitted by a specific drug is combined with a negative electron to generate annihilation radiation, and two gamma rays with certain energy and opposite flight directions are generated; these two gamma rays have a temporal simultaneity of generation and fly out almost in opposite directions, which makes it possible to use two oppositely placed detectors outside the body to detect them with coincidence techniques, which are now commonly measured by closed torroidal detectors.
The PET detector is the core component of a PET imaging system, which is used to detect the location where gamma rays occur, and is typically comprised of a scintillation crystal, a photodetector array, and readout electronics. The scintillation crystal is a functional material capable of effectively absorbing gamma rays and emitting visible light, the light detector array converts photons generated by interaction of the gamma rays and the scintillation crystal into current signals, and the read electronics processes the current signals and outputs detected gamma ray information.
The scintillation crystal in the PET detector generally consists of a plurality of long crystals with small cross sections, and for a PET imaging system with an ultra-long axial field of view, although the sensitivity is greatly improved, the influence of the uncertain depth effect of the PET detector on the axial spatial resolution of the PET imaging system is aggravated.
Disclosure of Invention
The invention aims to provide a PET detector, a PET imaging system and a gamma ray positioning method, which can improve the sensitivity and maintain high spatial resolution.
In a first aspect, embodiments of the invention provide a PET detector comprising a detector unit and readout electronics;
the detector unit comprises at least one crystal group and a light detector array arranged corresponding to the crystal group; each crystal group comprises at least two layers of crystal arrays, and the scintillation crystals in the crystal arrays of different layers have different light attenuation time; one side face of the crystal group for incidence of gamma rays is parallel to the cross section of each scintillation crystal; the photodetector array is mutually coupled with a layer of crystal array on the bottom surface of the corresponding crystal group;
the readout electronics is connected to the photodetector array in the detector unit; the photodetector array is used for converting photons generated by interaction of gamma rays and the crystal group into electric signals; the readout electronics are configured to process the electrical signals output by the photodetector array into position-dependent energy signals for subsequent determination of the position and depth of interaction of the gamma rays with the crystal set based on the energy signals.
Further, the detector unit comprises two crystal groups, which are arranged in a stack.
Further, the material of the scintillation crystal comprises yttrium lutetium silicate LYSO, gadolinium silicate GSO, yttrium silicate YSO, lutetium fine silicate LFS, bismuth germanate BGO, lutetium silicate LSO, barium fluoride BaF2Cesium iodide CsI, sodium iodide NaI, lead tungstate, lanthanum bromide LaBr3At leastTwo kinds.
Further, a difference between light attenuation times of scintillation crystals in the crystal arrays of different layers is greater than or equal to 10 ns.
Further, the photodetector array comprises any one of: the device comprises a position sensitive photomultiplier array, a microchannel plate, an avalanche photodiode array, a silicon photomultiplier array, a multi-pixel photon counter and a photomultiplier array.
Furthermore, the scintillation crystals in each layer of the crystal array are separated by a reflecting film, and the scintillation crystals in the different layers of the crystal arrays are coupled by a coupling agent, wherein the coupling agent comprises air or optical glue.
Furthermore, the PET detector is applied to a PET imaging system for animal body imaging, the size of the cross section of a single scintillation crystal is 1-2 mm, and the length of the single scintillation crystal is 10-20 mm; or the PET detector is applied to a PET imaging system of human body imaging, the size of the cross section of the single scintillation crystal is 3-6 mm, and the length of the single scintillation crystal is 20-30 mm.
Further, the detector unit is a plurality of, and each detector unit is arranged in a ring shape.
In a second aspect, embodiments of the present invention further provide a PET imaging system, including the PET detector of the first aspect, and further including a data processing device connected to readout electronics in the PET detector; the data processing device is configured to acquire an energy signal of the readout electronics output and determine a location and depth of interaction of a gamma ray with the crystal group based on the energy signal.
In a third aspect, an embodiment of the present invention further provides a gamma ray localization method, which is applied to the data processing device in the PET imaging system according to the second aspect; the method comprises the following steps:
acquiring an energy signal output by the readout electronics; the energy signal comprises first energy data and second energy data, the first energy data comprise energy values at different positions in the X direction after being superposed on the X direction, the second energy data comprise energy values at different positions in the Z direction after being superposed on the Z direction, the X direction is the arrangement direction of each scintillation crystal in each layer of the crystal array, and the Z direction is the length direction of the scintillation crystal;
determining the X-direction action position of the gamma ray according to the first energy data by adopting a gravity center method, an energy weighting method or a position weighting iterative algorithm;
acquiring an output pulse waveform in the Z direction and search data in the Y direction according to the second energy data; determining a target crystal array which acts with gamma rays by screening the pulse shape of the output pulse waveform; determining the Y-direction action position of the gamma ray according to the Y-direction search data, the target crystal array and a pre-established Y coordinate lookup table; the Y-direction search data comprises a standard deviation of the second energy data or a ratio of a maximum value in the second energy data to a total energy value corresponding to the second energy data;
and determining the action depth of the gamma ray according to the second energy data by adopting a gravity center method, an energy weighting method or a maximum likelihood method.
Further, the step of determining the target crystal array interacting with the gamma ray by pulse shape discrimination of the output pulse waveform includes:
and carrying out pulse shape discrimination on the output pulse waveform by a delay integration method or an over-threshold pulse width method, and determining a target crystal array which acts with gamma rays.
Further, the step of determining the depth of action of the gamma ray according to the second energy data by using a gravity center method, an energy weighting method or a maximum likelihood method comprises the following steps:
calculating the probability of each event falling to a different Z position according to the second energy data; wherein one of said events is a gamma ray interacting with said set of crystals;
and determining the action depth of the gamma ray according to the Z coordinate with the maximum probability and a pre-established depth lookup table.
Further, the method further comprises:
acquiring third energy data which are output by the readout electronics and correspond to each preset position when gamma rays are irradiated to a plurality of preset positions on a YZ plane in the PET detector; the third energy data comprise energy values which are superposed to different positions in the Z direction after the Z direction;
calculating Y-direction standard data corresponding to each preset position, wherein the Y-direction standard data comprise standard deviation of the third energy data or a ratio of a maximum value in the third energy data to a total energy value corresponding to the third energy data; establishing the Y coordinate lookup table according to Y-direction standard data corresponding to each preset position, a crystal array where each preset position is located and an actual Y coordinate of each preset position;
calculating the probability of each event falling to different Z positions according to the third energy data, and determining a Z coordinate with the maximum probability corresponding to the preset position; establishing the depth lookup table according to the Z coordinate with the maximum probability corresponding to each preset position and the action depth corresponding to each preset position; and the action depth corresponding to each preset position is the actual Z coordinate of the preset position.
In the PET detector, the PET imaging system and the gamma ray positioning method provided by the embodiment of the invention, the PET detector comprises a detector unit and readout electronics; the detector unit comprises at least one crystal group and a light detector array arranged corresponding to the crystal group; each crystal group comprises at least two layers of crystal arrays, and the scintillation crystals in the crystal arrays of different layers have different light attenuation time; one side face of the crystal group for incidence of the gamma ray is parallel to the cross section of each scintillation crystal; the optical detector array is mutually coupled with a layer of crystal array on the bottom surface of the corresponding crystal group; the readout electronics is connected to the photodetector array in the detector unit; the photodetector array is used for converting photons generated by interaction of the gamma rays and the crystal group into electric signals; readout electronics are used to process the electrical signals output by the photodetector array into position-dependent energy signals for subsequent determination of the location and depth of interaction of the gamma rays with the crystal set based on the energy signals. This PET detector reads the multilayer crystal array that light decay time is different through the photodetector array side, not only can realize the position and the depth measurement of gamma ray and scintillation crystal effect simultaneously, has reduced the cost of PET detector moreover, can make the PET imaging system of major axis to the field of vision when improving sensitivity, keeps high spatial resolution.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
FIG. 1 is a schematic diagram of a PET detector according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of a detector unit according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of another detector unit according to an embodiment of the present invention;
FIG. 4 is a schematic structural diagram of a PET imaging system according to an embodiment of the present invention;
FIG. 5 is a schematic flowchart of a gamma ray positioning method according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of a row-column addition method for a silicon photomultiplier array according to an embodiment of the present invention;
FIG. 7 is a schematic diagram illustrating a delay integration method according to an embodiment of the present invention;
FIG. 8 is a schematic diagram illustrating an over-threshold pulse width method according to an embodiment of the present invention;
FIG. 9 is a schematic view of an irradiation spot of a PET detector in a calibration stage according to an embodiment of the present invention;
FIG. 10 is a schematic view of an irradiation spot of another PET detector in a calibration stage according to an embodiment of the present invention.
Icon: 100-a detector unit; 101-a crystal array; 102-a photodetector array; 200-readout electronics; 300-a data processing device.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the following embodiments, and it should be understood that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
If within a specified time window (typically 0 ns-15 ns) two gamma rays at 180 degrees to each other (± 0.25 degrees) are detected by the PET detector, i.e. a coincidence event, the annihilation point is at the Line between the two crystal blocks at which the gamma rays were detected, forming a LOR (Line Of Reaction, Line Of response). To improve the spatial resolution of the PET detector, it is necessary to accurately measure the starting and ending positions of the lines of response, i.e., the depth of interaction of the gamma rays with the PET detector.
Based on the above, the PET detector, the PET imaging system and the gamma ray positioning method provided by the embodiment of the invention have the depth measurement capability, so that the sensitivity of the PET imaging system with the long axial view field can be improved, and the high spatial resolution can be maintained.
For the understanding of the present embodiment, a detailed description of a PET detector disclosed in the embodiments of the present invention will be given first.
Referring to the schematic structural diagram of a PET detector shown in fig. 1, the PET detector includes a detector unit 100 and readout electronics 200; referring to the schematic structural diagram of one detector unit shown in fig. 2 and the schematic structural diagram of another detector unit shown in fig. 3, the detector unit 100 includes at least one crystal group and a photodetector array 102 disposed corresponding to the crystal group; each crystal group comprises at least two layers of crystal arrays 101, and the scintillation crystals in the crystal arrays 101 in different layers have different light attenuation time; one side of the crystal group for gamma ray incidence (e.g., the left side of fig. 2 and 3) is parallel to the cross-section of each scintillation crystal, i.e., gamma rays are incident perpendicular to the cross-section of the scintillation crystal; the light detector array 102 is mutually coupled with a layer of crystal array 101 on the bottom surface of the corresponding crystal group; the readout electronics 200 are connected to the photo detector array 102 in the detector unit 100.
The photodetector array 102 is used for converting photons generated by interaction of gamma rays and the crystal group into electric signals; the readout electronics 200 are used to process the electrical signals output by the photodetector array 102 into position-dependent energy signals for subsequent determination of the location and depth of interaction of the gamma rays with the crystal set based on the energy signals.
The readout electronics 200 may be directly connected to the photodetector array 102, or may be indirectly connected to the photodetector array through a substrate, for example, and the connection manner between the two is not limited in this embodiment.
The number of the crystal groups may be one or more, and the number of the photodetector arrays 102 is the same as the number of the crystal groups, for example, the detector unit 100 shown in fig. 2 includes one crystal group and one photodetector array 102, while the detector unit 100 shown in fig. 3 includes two crystal groups and two photodetector arrays 102, and the two crystal groups are stacked (the detector unit 100 includes the crystal group, the photodetector array 102, the crystal group, and the photodetector array 102 from top to bottom). The detector unit shown in fig. 2 reads out two layers of crystal arrays 101 through the side surfaces of 1 light detector array 102, and the manufacturing cost of the PET detector is low; the detector unit shown in fig. 3 laterally reads two layers of crystal arrays 101 through 2 photo-detector arrays 102, and the size in the Y direction is smaller, so that higher position resolution and time resolution can be achieved. The number of layers of the crystal array 101 and the type of the scintillation crystal in each crystal group are both greater than or equal to 2.
Taking the example where each crystal group includes two layers of crystal arrays 101, the above-described PET detector can implement a single photodetector array 102 side readout double layer crystal array 101: firstly, discrimination of double-layer crystal arrays 101 with different decay time characteristics is realized through pulse shape discrimination, namely, a target crystal array which acts with gamma rays is determined; secondly, according to the distribution of photons generated by the interaction of the gamma rays and the crystal groups to the photodetector array 102, the action position measurement and the action depth measurement in each layer of the crystal array 101 are realized, namely, the action position measurement of the gamma rays in the stacking direction (such as the Y direction in the figures 2 and 3) of the crystal array 101 and the action depth measurement in the length direction (such as the Z direction in the figures 2 and 3) of the scintillation crystal are realized.
According to the PET detector, the multilayer crystal array 101 with different light attenuation times is read out from the side surface of the light detector array 102, so that the position and depth measurement of the action of the gamma ray and the scintillation crystal can be realized simultaneously, the cost of the PET detector is reduced, the PET imaging system with a long axial view field can keep high spatial resolution while improving the sensitivity, and meanwhile, the insensitive area of the PET detector is reduced (the area where the light detector array 102 is located belongs to the insensitive area). In summary, the PET detector can achieve high spatial resolution and high sensitivity in a full field of view at low cost. In addition, the PET detector is convenient to assemble.
The discrimination effect of the pulse shape discrimination method can be verified by testing scintillation crystal materials with two different light attenuation times. Optionally, the material of the scintillation crystal comprises yttrium lutetium silicate LYSO, gadolinium silicate GSO, yttrium silicate YSO, lutetium fine silicate LFS, bismuth germanate BGO, lutetium silicate LSO, barium fluoride BaF2Cesium iodide CsI, sodium iodide NaI, lead tungstate and lanthanum bromide LaBr3At least two of them.
Table 1 below shows performance metrics for four scintillation crystal materials, where FTRL refers to Fast LYSO.
TABLE 1
| Performance index | FTRL | LYSO | GSO | YSO |
| Attenuation coefficient/(1/cm) | 0.86 | 0.86 | 0.70 | 0.39 |
| Decay time/(ns) | 31 | 40 | 60 | 70 |
| Photon yield/(photons/MeV) | 30000 | 30000 | 10000 | 27000 |
| Refractive index | 1.81 | 1.81 | 1.85 | 1.8 |
| Luminous wavelength/(nm) | 420 | 420 | 430 | 420 |
| Deliquescence property | Is free of | Is free of | Is free of | Is free of |
In some possible embodiments, the materials of the scintillation crystal can be arbitrarily chosen from two of table 1. Because the adopted scintillation crystals have fast decay time, the PET detector can achieve higher time resolution. The intersection part of the response line of the long axial view PET imaging system and a measured object such as a human body is longer, so that the positron annihilation position range is enlarged, the time resolution of the PET detector is improved, the positron annihilation positioning precision can be further improved, the signal-to-noise ratio of a reconstructed image is improved, and the imaging quality of the long axial view PET imaging system is remarkably improved.
Preferably, the difference between the light attenuation times of the scintillation crystals in the crystal arrays 101 of different layers is greater than or equal to 10 ns. This facilitates identification of the crystal arrays 101 of different layers, i.e. determination of the target crystal array that is to be subjected to gamma radiation.
Optionally, the photodetector array 102 includes any one of the following: PSPMT (Position Sensitive Photo-Multiplier Tubes) arrays, MCP (MicroChannel Plates), APD (Avalanche photodiode) arrays, SiPM (Silicon photomultiplier) arrays, MPPC (Multi-Pixel Photon Counter), and PMT (Photo-Multiplier Tubes) arrays. In this embodiment, the coupling manner between the photodetector array 102 and the corresponding crystal array 101 is not limited, and taking the photodetector array 102 as an SiPM array as an example, the SiPM array and the corresponding crystal array 101 may be coupled by, but not limited to, air or silicon oil.
OptionallyIn each layer of the crystal array 101, the scintillation crystals can be separated by a reflective film, i.e., the scintillation crystals can be separated by a reflective film in the X direction, which can be, but is not limited to, ESR reflective film, Teflon reflective film, or BaSO reflective film4A reflective film. The scintillation crystals in the crystal arrays 101 of different layers may be coupled with each other by using a coupling agent, that is, the scintillation crystals are coupled with each other by using a coupling agent along the Y direction, and the coupling agent may include air (that is, the two are directly attached) or optical glue.
Alternatively, the detector unit 100 may be wrapped with Teflon film or ESR reflective film on all sides and top. The surface roughness of the crystal group in the detector cell 100 may be designed to be one of three types of polishing modes of only bottom and side polishing, only 4 side polishing, and full surface polishing.
Alternatively, when the PET detector is applied to a PET imaging system for imaging animal bodies (such as mice and other small animals), the cross section of each single scintillation crystal can be 1-2 mm in size, and the length of each single scintillation crystal can be 10-20 mm. When the PET detector is applied to a PET imaging system for human body imaging, the size of the cross section of a single scintillation crystal can be 3-6 mm, and the length of the single scintillation crystal can be 20-30 mm. For example, when the PET detector is applied to a PET imaging system for human body imaging, the size of a single scintillation crystal can be 3X 6X 18mm3The scintillation crystals in the crystal group can be arranged in a 2 × 4 array, and 3 × 3mm can be adopted24 x 6 photo detector array 102.
Alternatively, as shown in fig. 1, the detector units 100 are plural, and the detector units 100 are arranged in a ring.
In summary, the PET detector provided by the embodiment of the invention can simultaneously achieve high position resolution, high depth resolution and high time resolution, thereby meeting the requirements of a long axial view PET imaging system on full view high spatial resolution, high efficiency and high definition.
An embodiment of the present invention further provides a PET imaging system, referring to a schematic structural diagram of a PET imaging system shown in fig. 4, the PET imaging system includes the above-mentioned PET detector, and further includes a data processing device 300 connected to the readout electronics 200 in the PET detector; the data processing device 300 is configured to acquire an energy signal output by the readout electronics 200 and determine, based on the energy signal, a location and depth at which the gamma ray interacts with the crystal group.
The implementation principle and the technical effect of the PET imaging system provided by the present embodiment are the same as those of the foregoing PET detector embodiment, and for the sake of brief description, reference may be made to the corresponding content in the foregoing PET detector embodiment for the part where the embodiment of the PET imaging system is not mentioned.
The embodiment of the invention also provides a gamma ray positioning method, which is applied to the data processing equipment in the PET imaging system and is used for determining the position and the depth of the interaction between the gamma ray and the crystal group in the PET detector. Referring to fig. 5, a schematic flow chart of a gamma ray positioning method is shown, which mainly includes the following steps S502 to S508:
step S502, acquiring an energy signal output by the readout electronics; the energy signal comprises first energy data and second energy data, the first energy data comprise energy values at different positions in the X direction after being superposed on the X direction, the second energy data comprise energy values at different positions in the Z direction after being superposed on the Z direction, the X direction is the arrangement direction of each scintillation crystal in each layer of crystal array, and the Z direction is the length direction of the scintillation crystal.
In this embodiment, as shown in fig. 2 and 3, a three-dimensional coordinate system is established with the cross section of the scintillation crystal as an XY plane and the length direction of the scintillation crystal as a Z direction, where the X direction is a row direction of the photodetector array and the Z direction is a column direction of the photodetector array.
Taking the SiPM array with 4 × 6 photodetector array in the PET detector as an example, referring to the row-column addition schematic diagram of a silicon photomultiplier array shown in FIG. 6, the SiPM array with 4 × 6 can obtain X by row addition1、X2、X3And X4Four energy values, the four energy values constituting first energy data; z can be obtained by column addition1、Z2、Z3、Z4、Z5And Z6Six energy values, which constitute the second energy data.
And step S504, determining the X-direction action position of the gamma ray according to the first energy data by adopting a gravity center method, an energy weighting method or a position weighting iterative algorithm.
Taking the photo detector array as the SiPM array as an example, the X-direction action position of the gamma ray (i.e. the X-direction position where the gamma ray interacts with the crystal group) can be measured according to the center of gravity of the X-direction distribution of the photons generated when the gamma ray interacts with the scintillation crystal in the SiPM array. The present embodiment provides three methods of measuring the X-direction action position as follows:
the gravity center method comprises the following steps:
wherein X is the coordinate of the acting position of the gamma ray in the X direction, and XiIs the X coordinate of the SiPM array of row i, ExiEnergy value, N, output by row addition for the ith row SiPM arrayxIs the number of rows of the SiPM array, e.g., for the SiPM array shown in FIG. 6, NxIs 4.
Energy weighting method:
wherein, K is a preset index parameter, and the value range of K can be [1, 3 ].
Position weighting iterative algorithm:
where j is the number of iterations, j is 1, 2, … … Nx(ii) a k is a preset iteration parameter, and the value range of k can be [1, 5 ]]。
Location-weighted overlapIn the generation algorithm, the initial value X of X0Can be calculated by a gravity center method, and X is obtained after j iterationjThe corresponding weights for each SiPM array row change.
Step S506, acquiring output pulse waveform in the Z direction and search data in the Y direction according to the second energy data; determining a target crystal array which acts with gamma rays by screening the pulse shape of the output pulse waveform; determining the Y-direction action position of the gamma ray according to the Y-direction search data, the target crystal array and a pre-established Y coordinate lookup table; the Y-direction search data comprises a standard deviation of the second energy data or a ratio of a maximum value in the second energy data to a total energy value corresponding to the second energy data.
The Y-direction action position (i.e., the Y-direction position where the gamma ray interacts with the crystal group) can be determined according to the difference in the output pulse shape of the PET detector and the width of the Z-direction photon distribution, and generally, the closer the Y-direction action position is to the photodetector array, the narrower the width of the Z-direction photon distribution.
Still taking the photo detector array as the SiPM array as an example, the specific implementation process may be as follows:
firstly, waveform sampling is carried out on the second energy data, then a complete waveform (namely an output pulse waveform in the Z direction) is obtained through an interpolation method, pulse shape discrimination is carried out on the output pulse waveform through a delay integration method or an over-threshold pulse width method, and a target crystal array which acts with gamma rays is determined.
Second, the width of the Z-direction photon distribution is evaluated and the standard deviation S or ratio is calculated using the energy Ezi of each column of the SiPM array
(A larger standard deviation S indicates a closer Y-direction effect to the SiPM array; ratio
Larger, indicating a Y-direction action site closer to the SiPM array):
wherein E isziEnergy value, N, output by column addition for ith column SiPM arrayzIs the number of columns of the SiPM array, e.g., for the SiPM array shown in FIG. 6, NZIs 6.
It should be noted that, the first step and the second step are not executed in sequence, and in other embodiments, the second step may be executed first, and then the first step may be executed.
Third, by standard deviation S or ratio
And the target crystal array is used for searching and obtaining the Y-direction action position of the gamma ray in a pre-established Y coordinate lookup table.
For the convenience of understanding, the embodiment of the invention also provides a schematic diagram of a delay integration method and an over-threshold pulse width method. Referring to fig. 7, a schematic diagram of a delay integration method is shown, in which two curves respectively represent output pulse waveforms corresponding to two scintillation crystals, and R ═ I is calculatedt2/It1Or R ═ It2/(It1+It2) To distinguish different kinds of scintillation crystals (R calculated by two curves is different) so as to determine the target crystal array which reacts with gamma rays, wherein It1、It2To integrate the total area of the output pulse waveform from the (0, t1) and (t1, t2) intervals, respectively, t1 and t2 are two time points different by a preset time length from the start time of the waveform. Referring to the schematic diagram of the over-threshold pulse width method shown in fig. 8, two curves respectively show the output pulse waveforms corresponding to two scintillation crystals, and different types of scintillation crystals are distinguished by measuring the over-threshold pulse widths Δ t1 and Δ t2 (the over-threshold pulse widths corresponding to the two curves are over-thresholdDifferent pulse widths of values) to determine a target crystal array which acts with gamma rays, wherein the pulse width of the over-threshold is a time interval of an abscissa obtained after a fixed trigger threshold is set for an output pulse waveform output by the SiPM array; the threshold is preset and can be 50% -80% of the maximum energy value.
And step S508, determining the action depth of the gamma ray according to the second energy data by adopting a gravity center method, an energy weighting method or a maximum likelihood method.
When the gravity center method or the energy weighting method is used to determine the depth of interaction of the gamma ray (i.e., the depth of interaction between the gamma ray and the crystal group), reference may be made to the corresponding contents in step S504, which is not described herein again.
When the maximum likelihood method is adopted to determine the depth of action of the gamma ray, the specific process can be as follows: first, from the second energy data, the probability of each event falling to a different Z position is calculated:
wherein σ (z) is EziMean square error of distribution,. mu.z, is EziMean of the distribution, an event being a gamma ray interacting with a crystal group; then according to the Z coordinate with the maximum probability (i.e. the
Wherein
To represent
The Z coordinate at which the maximum value is taken) and a pre-established depth lookup table, the depth of action of the gamma ray is determined.
According to the gamma ray positioning method provided by the embodiment of the invention, the identification of the crystal array is carried out according to the output pulse waveform in the Z direction, the action position of the gamma ray in the Y direction in each layer of crystal is measured according to the distribution width of photons in the Z direction of the optical detector array, and in addition, the action position of the gamma ray in the X direction and the action depth of the gamma ray in the Z direction can be determined according to the distribution gravity center of the photons on the optical detector array, so that the three-dimensional positioning of the gamma ray is realized.
The embodiment of the invention also provides a calibration method of the PET detector, which is used for establishing a Y coordinate lookup table and a depth lookup table. The specific process is as follows:
(1) the method comprises the steps of obtaining third energy data which are output by electronics and correspond to each preset position when gamma rays are irradiated to a plurality of preset positions on a YZ plane in a PET detector; the third energy data comprises energy values which are superposed to different positions in the Z direction after the Z direction;
(2) calculating Y-direction standard data corresponding to each preset position, wherein the Y-direction standard data comprise standard deviation of third energy data or a ratio of a maximum value in the third energy data to a total energy value corresponding to the third energy data; establishing a Y coordinate lookup table according to Y-direction standard data corresponding to each preset position, a crystal array where each preset position is located and an actual Y coordinate of each preset position;
(3) calculating the probability of each event falling to different Z positions according to the third energy data, and determining a Z coordinate with the maximum probability corresponding to the preset position; establishing a depth lookup table according to the Z coordinate with the maximum probability corresponding to each preset position and the action depth corresponding to each preset position; and the action depth corresponding to each preset position is the actual Z coordinate of the preset position.
In particular, one may be used22The Na point source and the PET detector comprising a single detector unit irradiate a plurality of fixed (Y, Z) positions of the crystal group in the PET detector through electronic collimation, so that third energy data corresponding to the plurality of fixed (Y, Z) positions are obtained, and the establishment of a Y coordinate lookup table and a depth lookup table is further realized. The irradiation points may be as shown in fig. 9 and 10, for example, by electronically collimating the side irradiation at several positions spaced apart in the Y direction, such as 1 mm.
In addition, when the irradiation is carried out in the calibration stage, the uniform irradiation of the crystal group in the PET detector can also be realized through the coincidence of the PET detector containing the single detector unit and a large-volume reference PET detector.
When the gamma ray three-dimensional positioning is carried out, the positioning precision can be improved by the Y coordinate lookup table and the depth lookup table obtained by applying the method.
In all examples shown and described herein, any particular value should be construed as merely exemplary, and not as a limitation, and thus other examples of example embodiments may have different values.
It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures.
In addition, in the description of the embodiments of the present invention, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (4)
1. A gamma ray positioning method is characterized by being applied to a data processing device in a PET imaging system, wherein the PET imaging system comprises a PET detector and the data processing device; the PET detector comprises a detector unit and readout electronics, to which the data processing device is connected; the detector unit comprises at least one crystal group and a light detector array arranged corresponding to the crystal group; each crystal group comprises at least two layers of crystal arrays, and the scintillation crystals in the crystal arrays of different layers have different light attenuation time; one side face of the crystal group for incidence of gamma rays is parallel to the cross section of each scintillation crystal; the photodetector array is mutually coupled with a layer of crystal array on the bottom surface of the corresponding crystal group; the readout electronics is connected to the photodetector array in the detector unit; the photodetector array is used for converting photons generated by interaction of gamma rays and the crystal group into electric signals; the readout electronics for processing the electrical signals output by the photodetector array into position-dependent energy signals;
the method comprises the following steps:
acquiring an energy signal output by the readout electronics; the energy signal comprises first energy data and second energy data, the first energy data comprise energy values at different positions in the X direction after being superposed on the X direction, the second energy data comprise energy values at different positions in the Z direction after being superposed on the Z direction, the X direction is the arrangement direction of each scintillation crystal in each layer of the crystal array, and the Z direction is the length direction of the scintillation crystal;
determining the X-direction action position of the gamma ray according to the first energy data by adopting a gravity center method, an energy weighting method or a position weighting iterative algorithm;
acquiring an output pulse waveform in the Z direction and search data in the Y direction according to the second energy data; determining a target crystal array which acts with gamma rays by screening the pulse shape of the output pulse waveform; determining the Y-direction action position of the gamma ray according to the Y-direction search data, the target crystal array and a pre-established Y coordinate lookup table; the Y-direction search data comprises a standard deviation of the second energy data or a ratio of a maximum value in the second energy data to a total energy value corresponding to the second energy data;
and determining the action depth of the gamma ray according to the second energy data by adopting a gravity center method, an energy weighting method or a maximum likelihood method.
2. The gamma ray localization method of claim 1, wherein the step of determining the target crystal array interacting with the gamma ray by pulse shape discrimination of the output pulse waveform comprises:
and carrying out pulse shape discrimination on the output pulse waveform by a delay integration method or an over-threshold pulse width method, and determining a target crystal array which acts with gamma rays.
3. The method of claim 1, wherein the step of determining the depth of action of the gamma ray according to the second energy data by using a centroid method, an energy weighting method or a maximum likelihood method comprises:
calculating the probability of each event falling to a different Z position according to the second energy data; wherein one of said events is a gamma ray interacting with said set of crystals;
and determining the action depth of the gamma ray according to the Z coordinate with the maximum probability and a pre-established depth lookup table.
4. The gamma ray localization method of claim 3, further comprising:
acquiring third energy data which are output by the readout electronics and correspond to each preset position when gamma rays are irradiated to a plurality of preset positions on a YZ plane in the PET detector; the third energy data comprise energy values which are superposed to different positions in the Z direction after the Z direction;
calculating Y-direction standard data corresponding to each preset position, wherein the Y-direction standard data comprise standard deviation of the third energy data or a ratio of a maximum value in the third energy data to a total energy value corresponding to the third energy data; establishing the Y coordinate lookup table according to Y-direction standard data corresponding to each preset position, a crystal array where each preset position is located and an actual Y coordinate of each preset position;
calculating the probability of each event falling to different Z positions according to the third energy data, and determining a Z coordinate with the maximum probability corresponding to the preset position; establishing the depth lookup table according to the Z coordinate with the maximum probability corresponding to each preset position and the action depth corresponding to each preset position; and the action depth corresponding to each preset position is the actual Z coordinate of the preset position.
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