CN108542409B - Method for measuring noise equivalent counting rate of double-panel PET system - Google Patents
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Abstract
The invention discloses a method for measuring the noise equivalent counting rate of a double-flat-plate PET system. The method comprises the following steps: 1) defining a coordinate space, and determining the axial direction of the double-flat-plate PET system; 2) placing the selected die body in the double-flat-plate PET system for data acquisition; organizing the collected coincidence data into a histogram form, and recording the collection duration; wherein, the axial direction of the die body is consistent with the axial direction of the system; 3) recombining each axial gradient data in the histogram into a corresponding single-layer histogram perpendicular to the axial direction; 4) calculating the accidental coincidence and scattering coincidence count of each layer according to the accidental coincidence and scattering coincidence count of each layer under each projection angle; 5) calculating the real coincidence counting rate of each layer of the histogram according to the total coincidence counting rate, the accidental coincidence counting rate and the scattering coincidence counting rate of each layer; 6) and obtaining the noise equivalent counting rate of the PET system according to the real coincidence counting rate of each layer. The invention is suitable for a double-flat-plate PET system.
Description
Technical Field
The invention relates to a method for measuring and calculating the noise equivalent counting rate of a double-panel PET system, belonging to the technical field of PET imaging.
Background
Positron Emission Tomography (PET) is an advanced nuclear medicine imaging technology, can detect physiological and biochemical information of biological tissues such as blood flow, metabolism, receptor molecule combination and the like at a molecular level, and is an important component of modern high-quality medical imaging technology.
At present, most of PET is in an annular or regular polygon structure, and has the advantages that more complete three-dimensional data can be acquired, but the imaging space is relatively closed, and the imaging mode is relatively fixed and limited. In recent years, more and more PET imaging systems with different structures appear, and especially the compact dual-plate PET has become a hot research and development point in recent years. The double-flat-plate PET is composed of a pair of parallel flat-plate detectors, the distance between the double plates can be adjusted according to the size of an imaging target, the detection condition is flexible, and high sensitivity can be achieved. The application scene is wide, and the system comprises clinical local imaging of mammary glands, thyroid glands, limbs and the like, animal research imaging and the like, dynamic imaging and image guidance under high sensitivity performance, an insert of a radiotherapy and nuclear magnetic resonance MRI mode and the like.
PET images by annihilation of electrons and electrons emit a pair of gamma photons of opposite directions and energies of 511 KeV. Ideally, two gamma photons generated by annihilation of the positive and negative electrons exit the tissue along a straight line and are coincidently detected by the detector units, and a straight line (coincidence response line, LOR for short) connecting the two detector units that detected the photons passes through the location where the annihilation event occurred. However, scattering of gamma rays produced by positron annihilation can lead to false coincidence event localization, and accidental coincidence between photons produced by different annihilation events can also lead to false coincidence events, which not only increase Noise and reduce the signal-to-Noise ratio of the system, but also reduce the contrast of the image, causing image quality degradation, and measuring and calculating the Noise Equivalent Count Rate (NECR) of the system is a method for measuring the sensitivity of the PET system to Noise, such as accidental coincidence and scatter coincidence, and for data containing a certain proportion of scatter and accidental coincidence Counts, NECR represents the true event Rate with the same signal-to-Noise ratio under non-scatter and accidental coincidence conditions.
The prior art computing principle and Measuring method of Noise Equivalent counting rate are mainly based on the article entitled "Measuring PET scanner Sensitivity: Relating Count-Rates to Image Signal-to-Noise Ratios Using Noise Equivalent Counts" published on IEEE Trans Nucl Sci NS 37(2) page 783-788 in S.C., Casey, M.E. and Hoffman, E.J. 1990. These methods are used for the description of intrinsic background count rate scanners (see Watson c.c. et al, 2004, J Nucl Med,45(5), 822-page 826, entitled "Modification from the NEMA- (2001) -NU2procedure for system using scanning with natural radio activity videos of C.C.Watson, M.E.Casey"). Generally, the phantom used for the measurements is a solid right circular cylinder and a hole is drilled at a certain radial offset parallel to the central axis of the cylinder. The line source inserted into the test phantom was a transparent plastic hose filled with a known amount of radioactive material and passed through an off-center hole in the phantom. During measurement, the prepared die body is placed in the center of the visual field for data acquisition.
In prior art solutions, according to experimental measurements, the acquired data are first organized into a sinusoidal histogram (sinogram), and the position of the line source response center is determined from the pixel position of the maximum at each projection angle in the sinusoidal histogram, then each projection is moved accordingly so that the pixel containing the maximum is aligned with the central pixel of the sinusoidal histogram, a total projection is generated after alignment so that one pixel in the total projection is equal to the sum of the pixels with the same radial offset in each angular projection, and finally the total event rate and the noise equivalent event rate of the system are calculated from the total projection data.
After studying the prior art solution, the inventor finds that the prior art solution is only applicable to a ring or polygon PET system, and the following problems exist when the prior art solution is applied to a dual-flat PET imaging system: (1) in a dual-panel PET system, data acquired at all projection angles are not complete data, and not all radial offset data exist at each projection angle, and projection lines at some projection angles do not pass through a line source, so that the pixel position of the maximum value at the projection angles is not the central position of the line source response; (2) because the pixel position of the maximum value in the double-flat-plate system under each projection angle is not the central position of the ray source response under the angle, the projection data can not be moved according to the central position; (3) since data movement cannot be performed, a total projection with the center aligned cannot be generated, and the total event rate of the system and the noise equivalent event rate cannot be calculated according to the total projection.
In summary, there is no method for measuring and calculating the noise equivalent count rate for the dual-panel PET in the prior art.
Disclosure of Invention
Aiming at the technical problems in the prior art, the invention aims to provide a method for accurately measuring and calculating the noise equivalent counting rate of a dual-panel PET system so as to make up the blank in the prior art.
A method for measuring and calculating the noise equivalent count rate of a dual-panel PET system comprises the following steps:
step 1: defining a coordinate space of a double-flat-plate system, and determining the axial direction of the system;
step 2: according to the system space coordinate definition, a prepared right cylinder die body is arranged at the center of a system visual field, the axial direction of the cylinder die body is consistent with the axial direction defined by the system, and a line source in the die body is positioned at a designated position in the visual field by rotating the die body; wherein, the line source in the die body is filled with known amount of radioactive substances and passes through the hole which is deviated from the center in the die body;
and step 3: completing data acquisition of the die body, organizing the acquired coincidence data into a histogram form, and recording acquisition duration;
and 4, step 4: according to the coordinate definition of a system, recombining each axial inclined layer data in the histogram into a corresponding single-layer histogram vertical to the axial direction by adopting a single-layer recombination method, and keeping the total coincidence counting value of the histogram unchanged;
and 5: setting pixel values, which are outside the motif and have a distance with the motif edge larger than a set distance threshold value, in the histogram to 0 aiming at each layer of recombined data, so as to remove edge noise;
step 6: calculating the distance between the LOR line corresponding to each pixel in each layer of recombined data and the line source in the phantom;
and 7: determining the pixels which are measured and counted as accidental coincidence and scattering coincidence under each projection angle of each layer according to the distance between the LOR line corresponding to each pixel in the data and the line source in the phantom, calculating the accidental coincidence and scattering coincidence counts in the pixels which are possible to simultaneously have real coincidence, accidental coincidence and scattering coincidence according to the count values of the pixels, and calculating the accidental coincidence and scattering coincidence counts under each projection angle of each layer;
and 8: accumulating the accidental coincidence and scattering coincidence counts under all projection angles in each layer, and calculating the accidental coincidence and scattering coincidence counts of each layer;
and step 9: calculating the total coincidence counting rate of each layer of the system according to the data obtained in the step 5;
step 10: calculating the real coincidence counting rate of each layer of the system according to the total coincidence counting rate, the accidental coincidence and the scattering coincidence counting rate of each layer;
step 11: and calculating the noise equivalent count rate of each layer of the system and the total noise equivalent count rate of the system.
Compared with the prior art, the invention has the positive effects
The invention does not need to align the center of the projection data under each angle and is not limited by whether the pixel of the maximum value under each projection angle is the position of the response center of the line source. Specifically, different conditions of data under each angle are analyzed and processed, different solutions are provided, the noise equivalent counting rate of the system can be calculated more accurately and reasonably, and the method is suitable for a double-flat-plate PET imaging system.
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FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a schematic diagram of a system coordinate space;
(a) the Z axis represents the axial direction of the system, the Z axis is parallel to the long axis of the flat plate, and the XY plane is the cross section direction of the system;
(b) the Z axis represents the axial direction of the system, the Z axis is parallel to the short axis of the flat plate, and the XY plane is the cross section direction of the system;
FIG. 3 is a schematic view of a mold body placement method.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The noise equivalent count rate is a description of the susceptibility of the PET imaging system to scatter and incidental coincidence times. The mold bodies adopted for measuring the noise equivalent counting rate are generally uniformly filled solid right-circular cylindrical mold bodies with special sizes, and the radial offset distance in the mold bodies is doffsetThe position of the test die body is drilled with a hole parallel to the central axis of the cylinder, a radioactive ray source is inserted into the hole of the die body during the experiment, the ray source inserted into the test die body is a transparent plastic hose, and the plastic hose is filled with known radioactive substances.
The size of the right cylindrical mold body and the location of the holes in the mold body are generally relevant to the use of the PET system, for example: PET systems for whole-body imaging usually employ a mold body having a diameter of 20cm, a length of 70cm and a small hole offset from the center by 4.5 cm; PET systems for animal studies such as rats often employ a mold body having a diameter of 50mm, a length of 150mm, and a center 17.5mm offset from the orifice.
Imaging with this phantom allows for better discrimination between scattering and non-scattering events. In a PET system, if a pair Of detector cells is hit by two gamma photons at the same time, indicating that the pair Of detector cells detects an annihilation coincidence event, the Line connecting the pair Of detector cells is called a Line Of Response (LOR). The annihilation event occurs at a location that is likely to be at any point on the LOR line to which all coincidence events detected by the pair of detector cells belong. Theoretically only the detected counts on LOR lines that pass through the line source are likely to be true coincidence counts, while the counts on LOR lines that do not pass through the line source are scatter and occasional coincidence counts. In practice, because the circular hole has a certain size, the line source is not an ideal line source, and simultaneously is influenced by the spatial resolution capability of the system, each real event cannot be accurately positioned, so that the real coincidence event has a certain extension around the central position of the line source, and within a certain range from the central position of the line source, a real coincidence count, an accidental coincidence count and a scattering coincidence count exist simultaneously, and beyond the range, the real coincidence count, the accidental coincidence count and the scattering coincidence count cannot exist. The size of the range of true coincidence events extension is related to the structural and spatial resolution capabilities of the PET system. Due to the special structure of the phantom, the scattering coincidence events and the accidental coincidence events are uniformly distributed in the whole visual field range, so that the scattering coincidence and the accidental coincidence counts at the positions near a line source can be obtained by using the coincidence events collected at a far distance from the line source through a fitting or linear interpolation method. The total coincidence count minus the scatter and incidental coincidence counts yields a true coincidence count. PET systems generally provide a method of occasional coincidence acquisition, such as acquisition by direct measurement methods. Thus, scatter coincidences, accidental coincidences, and true coincidences in the acquisition count can be distinguished. Accordingly, the system noise equivalent count rate can be calculated.
According to the expression, the key point of calculating the noise equivalent counting rate is to obtain the scattering and accidental coincidence counting of the area where the line source is positioned.
In the dual-panel PET system, not all the data acquired at all the projection angles are complete data, that is, not all the radial offset data are possessed at each projection angle, and the projection lines at some projection angles do not pass through the line source, which results in that the pixel position of the maximum value at these projection angles is not the central position of the line source response, and the position of the line source at each angle cannot be aligned in a data translation manner, and a total projection is obtained.
The embodiment of the present invention will be further described in detail with reference to the accompanying drawings, and as shown in fig. 1, a schematic flow chart of a method for measuring and calculating a noise equivalent count rate of a dual-panel PET system according to the embodiment of the present invention is provided, where the method includes:
step 1: and defining a coordinate space of the dual-flat-plate system and determining the axial direction of the system.
Due to the flexibility of the structure of the dual-plate PET system, the axial direction of the dual-plate PET system has two different definition modes, as shown in (a) and (b) of FIG. 2, the Z axis in (a) of FIG. 2 represents the axial direction of the system, the Z axis is parallel to the long axis of the flat plate, and the XY plane is the cross-sectional direction of the system; the Z-axis in fig. 2(b) represents the axial direction of the system, the Z-axis is parallel to the short axis of the flat plate, and the XY-plane is the transverse direction of the system. In the experiment, either one or two of the above-mentioned steps can be selected and carried out separately.
Step 2: placing the prepared right cylinder mold body in the center of the system view field, making the axial direction of the cylinder mold body consistent with the axial direction defined by the system, rotating the mold body to make the line source in the mold body located at the designated position in the view field, and recording the position coordinate (x) of the central axis of the cylinder mold body in the system cross sectioncenter,ycenter) Position coordinate (x) of the line source in the cylindrical mold in the transverse sectionline,yline)。
For example, one method is to place a cylindrical phantom in the center of the system field of view and align the axial direction of the cylindrical phantom with the axial direction of the system, and rotate the phantom manually or mechanically so that the line source in the phantom is positioned perpendicular to the plane defined by the center axis of the cylinder and closest to the flat panel detector in the dual flat PET, as shown in fig. 3.
And step 3: completing system data acquisition, organizing the acquired data into a histogram form, and recording acquisition duration Tacq。
The data acquired by the PET are recorded sequentially one by one to form a stream data format, i.e., list mode data, in which information such as the location, energy, time, etc. of each annihilation coincidence event is recorded. The position information is information of detector units or LOR lines which detect coincident photons, data can be converted into a histogram format according to the position information, specifically, encoding can be carried out according to the angle and the radial distance of the LOR lines which coincide with an event, coincidence encoding can also be carried out according to encoding information of the detector units connected with the LOR lines, data are counted, and records are organized in a histogram mode. The data are subjected to statistics by coding according to the angle and the radial distance of the LOR line conforming to the event, and the data organized and recorded in a histogram form are sinusoidal histograms (sinograms).
Specifically, the coincidence coding is performed according to the coding information of the detector units connected to the LOR line, the data is counted, and the records are organized in a histogram form, for example, the specific organization method of the data is as follows: by xDRepresenting the encoding of the detector units in a dual-panel PET system in the x-direction, with zDRepresenting the encoding of the detector unit in the z-direction. For any LOR line in the system, use (x)D1,zD1) The number of detector units on one of the plates to which the LOR line is connected is denoted by (x)D2,zD2) Denotes the number of detector units on the other plate to which the LOR line is connected, so (x)D1,zD1,xD2,zD2) The spatial position and orientation of this LOR line is uniquely indicated. Establishing an array according to the number of two detector units connected with the LOR line as a coding variable, accumulating the number of all collected events of the detector unit corresponding to the LOR line, wherein the accumulated value is corresponding to an element (x) in the arrayD1,zD1,xD2,zD2) And (4) finishing the organization of the data histogram.
And 4, step 4: and according to the coordinate definition of the system, recombining each inclined layer in the histogram into the corresponding single-layer histogram by adopting a single-layer recombination method, and keeping the total count value of the histogram unchanged.
The double-flat PET consists of a pair of parallel flat detectors, the two parallel flat detectors are oppositely arranged in parallel, and when the serial numbers of the two detector units connected with the LOR line in the direction of the z axis of the system are the same, the formed histogram data is parallel to the cross section and is straight data; when the numbers of two detector units connected by the LOR line in the z-axis direction of the system are different, the formed histogram data are crossed with the cross section and have a certain included angle, and the histogram data are data of an inclined layer.
The data processing efficiency can be improved by recombining the inclined layer data into the straight layer, and a single-layer recombination method, a multi-layer recombination method and a Fourier recombination method can be generally adopted. The statistics of the data is not changed only by a single-layer recombination method, and because of the symmetry of a right cylindrical model adopted by the system, each inclined layer in the histogram is recombined into a corresponding single-layer histogram by adopting the single-layer recombination method, so that the total count value of the histogram can be kept unchanged, and the influence on the distribution of scattering coincidence data, accidental coincidence data, real coincidence data and the like in each layer can be ignored. Therefore, for the convenience of data statistics, the inclined layer histogram data is recombined into the corresponding single layer histogram data by adopting a single layer recombination method.
In particular, will be located at zD1And zD2The coincidence between the two sets of detectors is equivalent to the median plane z ═ of the two sets of detectors (z ═D1+zD2) Per 2, data format conversion after reassembly to (x)D1,xD2Z), wherein z is:
z＝(zD1+zD2)/2 (1)
and 5: aiming at each layer of data after recombination, the distance between all the layers of data and the die body edge is greater than a certain distance (marked as d)edge) All set to 0.
The specific implementation manner of step 5 may be: recording the radius of the positive cylinder mould body in the step 2 as rphantomIf the die body in the step 2 is a right cylinder, and the axial direction of the cylinder die body is consistent with the axial direction (z-axis direction) defined by the system, the distance between the cylinder die body and the edge of the die body is greater than dedgeThe pixel positions of the pixels in each layer of data are the same, and the distance between the pixels and the central position of the cylinder in the layer is larger than (d)edge+rphantom). Based on the above relationship, the center position (x) of the cylinder at any one of the transverse layers is calculatedcenter,ycenter) Distances to LOR lines corresponding to each pixel in the same transverse-layer histogram, all distances marked greater than (d)edge+rphantom) The value of the marked pixel position in each layer of the histogram is set to zero, and the process is finishedAll distances from the edge of the mould body being greater than a certain distance (denoted d)edge) Is set to 0.
Step 6: calculating the distance between the LOR line corresponding to each pixel in each layer of recombined data and the source of the phantom line, and recording the distance between the LOR line corresponding to each pixel in each layer and the source of the phantom line as di,jWhere i represents the number of the data layer in which the LOR line is located, and j represents the number in each layer.
The specific implementation manner of step 6 may be: according to the step 2, the die body is in the shape of a right cylinder, the axial direction of the cylindrical die body is consistent with the axial direction (z-axis direction) defined by the system, and the line source is positioned in a hole parallel to the central axis of the cylinder, so that the line source is parallel to the axial direction of the system. The distance between the LOR line and the line source can be converted into the distance between the projection of the LOR line in the transverse layer and the projection point of the line source in the transverse layer, and the calculation of the distance can be completed according to the mathematical method of the distance between the points and the line.
And 7: and calculating the count of accidental coincidence and scattering coincidence under each projection angle of each layer according to the characteristics of the data under each different projection angle in each layer of data.
The method comprises the following steps of acquiring scattering coincidence and accidental coincidence counts at positions near a line source by adopting a coincidence event acquired at a position far away from the line source according to the characteristic that the scattering coincidence event and the accidental coincidence event are uniformly distributed in the whole visual field range through a fitting or linear interpolation method. The size of the interpolated range is usually determined according to the size of the selected phantom, and the size of the range exceeds 2-3 times of the spatial resolution of the system. The distance threshold is defined as M, and in the following embodiments, the value of M is set as 7 mm.
Specifically, the distance between the line source and the LOR line at each projection angle in each layer of histogram is denoted as di，j，kWhere i represents the layer number, j represents the angle number, k represents the data number at that angle of the layer, and d is defined in terms of coordinates when the LOR line is to the left or below the line sourcei,j,kNegative values, otherwise zero or positive values. In addition to this, the present invention is,if classification is carried out according to the distance between the LOR line and the line source under each projection angle in each layer of histogram, processing is carried out according to different conditions:
● first case: the distances between all pixels and the line source at the angle satisfy | di,j,kIf | is greater than 7mm, then all counts detected at that angle are coincidental and scatter coincidences counts. Specifically, the counts of all pixels at the jth angle of the ith layer are accumulated to generate a value C of the accidental coincidence count plus the scatter count at the jth angle of the ith layerr+s,i,jWhere the subscript r represents the chance coincidence and s represents the scatter coincidence.
● second case: in all pixels under this angle, d is presenti,j,k> 7mm, also present di,j,k< -7mm, with | d also presenti,j,kAnd | is less than or equal to 7mm, then:
● third case: in all pixels under this angle, d is present only and simultaneouslyi,j,kLess than-7 mm and less than or equal to-7 mmi,j,kPixels ≦ 0, then:
● fourth case: in all pixels under this angle, d is present only and simultaneouslyi,j,kD is more than 7mm and not more than 0i,j,k7mm or less pixels, then:
● fifth case: the distance between the pixel and the line source at the angle has di,j,k-7mm and di,j,kThe absolute values of the distances from the line source to the remaining pixels at this angle are less than 7mm for a 7mm pixel:
● sixth case: the distance between the pixel and the line source at the angle has di,j,k-7mm, but no di,j,k7mm pixels and the absolute values of the distances from the line source to the remaining pixels at that angle are less than 7mm, then:
● seventh case: the distance between the pixel and the line source at the angle has di,j,k7mm, but no di,j,k-7mm of pixels and the absolute values of the distances from the line source for the remaining pixels at that angle are all less than 7mm, then:
● eighth case: the distance between the pixel and the line source at the angle is di,j,kPixels of < 0mm, also present di,j,kPixels > 0mm, and all di,j,kAll satisfy | di,j,kIf | is less than 7mm, then
● ninth case: the number of the pixels under the angle is more than or equal to N, and the distances between all the pixels under the angle and the line source all meet the condition that d is more than 7mmi,j,kLess than or equal to 0mm, then:
● tenth case: the number of pixels under the angle is more than or equal to N, and d is more than or equal to 0mmi,j,kIf < 7mm, then:
● eleventh case: the number of pixels under the angle is less than or equal to (N' -1), and d is less than or equal to 0mmi,j,kLess than 7mm, or all satisfy-7 mm < di,j,kLess than or equal to 0mm, and the number of (N' -1) is too small, so that the accidental coincidence and the accidental scattering counting are judged according to the number, the generated error is overlarge, and the counting of the pixels in the angle is abandoned. In particular, can beThe count values of these pixels are labeled 0 and thus do not participate in the count statistics calculation. Generally, the value of N 'is more than or equal to 5 and more than or equal to 2, and the value of N' can be judged according to a system error and can also be accurately estimated by a Monte Carlo simulation method.
And 8: calculating the accidental coincidence and scattering coincidence count C of each layer according to the calculated accidental coincidence and scattering coincidence counts at each angle of each layerr+s,i。
Specifically, the scatter coincidence count and the accidental coincidence count at all angles calculated in each layer are added to obtain an accidental coincidence and scatter coincidence count C of the layerr+s,i。
And step 9: the total coincidence count rate for each layer of the system is calculated.
Specifically, the counts of all projected pixels of the ith layer are summed to obtain a total count C of the layerTOT,i。
Record the duration of data acquisition as TacqTotal count rate R for each layer iTOT,iSee formula (2):
step 10: the true coincidence count rate for each layer of the system is calculated.
In particular, the real count rate R for each layer it,iSee formula (3):
step 11: and calculating the equivalent count rate of each layer and the total noise of the system.
Aiming at a system capable of directly measuring the accidental coincidence count, the measured accidental coincidence data is organized into a histogram format as the original measurement data, all pixels which are more than a certain distance away from the edge of the motif are set to be 0, and the sum of the accidental coincidence data measured by each layer is the total accidental coincidence count C of the layerr,iThe calculation of the occasional coincidence count rate is shown in equation (4):
for systems that can directly measure the occasional coincidence count, the noise equivalent count rate R for each layer iNEC,iSee formula (5):
noise equivalent count rate R for each layer i except for imaging systems that can directly coincide with count rate by chanceNEC,iSee formula (6):
system noise equivalent count rate RNECThe calculation for the sum of the noise equivalent count rates for all layers i is given in equation (7):
the embodiment of the invention provides a method for measuring and calculating the noise equivalent counting rate of a system aiming at double-flat-plate PET for the first time. In the existing technical scheme, the position of a pixel of a maximum value under each projection angle is generally considered as the position of a line source response center, then data are aligned to generate a total projection, and finally the total counting rate and the noise equivalent counting rate of the system are calculated according to the total projection data. However, the method provided by the embodiment of the present invention does not need to center the projection data at each angle, and is not limited by whether the pixel with the maximum value at each projection angle is the position of the response center of the line source. Specifically, different conditions of data under each angle are analyzed and processed, different solutions are provided, the noise equivalent counting rate of the system can be calculated more accurately and reasonably, and the method is suitable for a double-flat-plate PET imaging system.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (6)
1. A method for measuring the noise equivalent count rate of a double-plate PET system comprises the following steps:
1) defining a coordinate space of the double-flat-plate PET system, and determining the axial direction of the double-flat-plate PET system;
2) placing the selected die body in the double-flat-plate PET system for data acquisition; organizing the collected coincidence data into a histogram form, and recording the collection duration; wherein, the axial direction of the die body is consistent with the axial direction of the double-flat PET system;
3) recombining each axial gradient data in the histogram into a corresponding single-layer histogram perpendicular to the axial direction;
4) counting the total coincidence counting rate of each layer of the histogram; determining the pixels which are measured and counted as accidental coincidence and scattering coincidence under each projection angle of each layer according to the distance between the LOR line corresponding to each pixel in each layer of data of the histogram and the line source in the phantom; then calculating the count of accidental coincidence and scattering coincidence under each projection angle of each layer according to the count values of the pixels;
5) accumulating the accidental coincidence and scattering coincidence counts under all projection angles in each layer, and calculating the accidental coincidence and scattering coincidence counts of each layer;
6) calculating the real coincidence counting rate of each layer of the histogram according to the total coincidence counting rate, the accidental coincidence counting rate and the scattering coincidence counting rate of each layer of the histogram;
7) calculating the noise equivalent counting rate of the double-flat-plate PET system according to the real coincidence counting rate of each layer of the histogram;
the method for calculating the accidental coincidence and scattering coincidence counts under each projection angle of each layer comprises the following steps: let the distance between the LOR line and the line source at each projection angle in each layer of histogram be recorded as di,j,kWhere i represents the layer number, j represents the angle number, k represents the data number at the jth angle of the ith layer, and d is the LOR line left or below the line sourcei,j,kIs a negative value; setting a distance threshold value M;
a) if the distances between all the pixels and the line source at the jth angle of the ith layer satisfy | di,j,k|>M, accumulating the counts of all the pixels at the jth angle of the ith layer to obtain a count value C of the accidental coincidence and scattering coincidence at the jth angle of the ith layerr+s,i,j；
b) If the distance between the pixel at the jth angle of the ith layer and the line source exists di,j,k>M, also present is di,j,k<-M, with | d also presenti,j,kIf the pixel with the angle less than or equal to M is in the ith layer, obtaining the counting value C of the position point which is at the jth angle of the ith layer and has the distances of-M and M from the line sourceL,i,jAnd CR,i,j(ii) a Then, linear interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
c) If the distance between the pixel and the line source at the jth angle of the ith layer is only and simultaneously di,j,k<-M and-M ≦ di,j,kIf the pixel is less than or equal to 0, obtaining the counting value C of the position point with the distance of-M from the line sourceL,i,j(ii) a Then, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
d) If the distance between the pixel and the line source at the jth angle of the ith layer is only and simultaneously di,j,k>M and 0. ltoreq. di,j,kPixels less than or equal to M; a count value C of the position point at a distance M from the line source is obtainedR,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
e) If the distance between the pixel at the jth angle of the ith layer and the line source exists di,j,kIn the presence of di,j,k-M, the absolute values of the remaining distances being less than M; get the ith layerCounting value C of position points which are at the j-th angle and have the distances of M and M from the line sourceL,i,jAnd CR,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
f) If the distance between the pixel at the jth angle of the ith layer and the line source exists di,j,kis-M and is absent di,j,kM, and the absolute values of the other distances are less than M; a count value C of the position point at a distance-M from the line source is acquiredL,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
g) If the distance between the pixel at the jth angle of the ith layer and the line source exists di,j,kM and no di,j,k-M, the absolute values of the remaining distances being less than M; a count value C of the position point at a distance M from the line source is obtainedR,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
h) If the distance between the pixel at the jth angle of the ith layer and the line source exists di,j,k<0, also in the presence of di,j,k>0, and all absolute values of the distances are less than M; a count value C of position points at distances M and-M from the line source is acquiredL,i,jAnd CR,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
i) If the number of the pixels at the jth angle of the ith layer is more than or equal to N, and the distances between all the pixels and the line source meet-M<di,j,kLess than or equal to 0; a count value C of the position point at a distance-M from the line source is acquiredL,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
j) If the number of the pixels at the jth angle of the ith layer is more than or equal to N, and the distances between all the pixels and the line source satisfy d which is more than or equal to 0i,j,k<M; a count value C of the position point at a distance M from the line source is obtainedR,i,jThen, the method of nearest neighbor interpolation is adopted to determine | di,j,k|<M pixel count value to obtain count value C of coincidence and scattering coincidence under the jth angle of ith layerr+s,i,j；
k) If the ith layer has (N '-1) pixels under the jth angle, and the distances between the (N' -1) pixels and the line source all satisfy d is more than or equal to 0i,j,k<M or-M<di,j,kLess than or equal to 0, N' is less than a set threshold value; the accidental coincidence and scattering coincidence at the jth angle of the ith layer is counted as value Cr+s,i,j＝0。
2. The method as claimed in claim 1, wherein in the step 3), each axial tilt layer data in the histogram is recombined into a corresponding single layer histogram perpendicular to the axial direction by using a single layer recombination method.
3. The method of claim 1, wherein the distance threshold M is greater than or equal to twice the spatial resolution of the center of the field of view of a dual panel PET system; n' is more than or equal to 5 and more than or equal to 2; n is more than or equal to 2.
4. The method according to claim 1, characterized in that for each layer of data after reorganization, the pixel values outside the phantom in the histogram, which are more than a set distance threshold from the edge of the phantom, are all set to 0, i.e. edge noise is removed; then step 4) is performed.
5. The method of claim 1, wherein the noise equivalent count rate comprises a noise equivalent count rate for each layer and a total system noise equivalent count rate.
6. The method of claim 5, wherein the noise equivalent count rate per layer
Wherein R ist,iIs the real count rate of the i-th layer, RTOT,iIs the total count rate of the ith layer.
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