CN105832358B - An imaging method based on system calibration of rotating dual-plate PET system - Google Patents

Abstract

The imaging method of the invention discloses a kind of rotation double flat plate PET system based on system calibration, multiple groups monochromatic light subdata is collected by rotating double flat PET system, and by after data prediction, obtain the forward projection data group needed for rebuilding, it recycles iterative reconstruction algorithm OSEM to be rebuild, obtains multiple groups and rebuild data；By the calculating to the offset vector between the registration point coordinate between two neighboring angle reconstruction result, the set of offset vectors of one group of image area is obtained, and then obtains the geometrical offset vector of system；Successively data for projection is rebuild using angle as unit dividing subset using the image area set of offset vectors and projection data set of calculating.This invention ensures that the accuracy of reconstructed results, improves the resolution ratio of plate PET reconstruction image；The calculation amount for reducing system response matrix improves the efficiency of reconstruction；Using the offset vector of image area, the calculating of indirect complete paired systems geometrical offset vector.

Description

An imaging method based on system calibration of rotating dual-plate PET system

technical field

The invention belongs to the technical field of nuclear medicine imaging, and in particular relates to an imaging method of a rotating double-plate PET system based on system calibration.

Background technique

PET (PositronEmission Tomography, PET) is the full name of Positron Emission Tomography, which is an advanced imaging technology in the field of nuclear medicine. Using the PET system detector to obtain the metabolic distribution in vivo is a functional imaging technology, especially in the early detection of tumors, which can not be ignored. Most medical PET detectors adopt a static annular structure. Although sampling data from a complete angle can be obtained, the system has high complexity and high cost. The system has a certain degree of closure, and it is not convenient for doctors or experimenters to provide technical guidance to sampling objects. . In order to reduce the cost and increase the flexibility of the detector, in recent years, researchers have proposed flat-panel PET detectors, such as Chien-Min Kao et al. in "A High-Sensitivity Small-Animal PET Scanner: Development and Initial PerformanceMeasurements, IEEE Transactions on Nuclear Science , vol.56, no.5, pp.2678-88, 2009" article proposed a static double-head flat-panel PET detector, which has a simple structure and can obtain satisfactory imaging results, but in the direction perpendicular to the detection plate The image resolution is relatively poor, and the reason is that the static flat PET cannot obtain projection data of a full angle; in order to solve the problem of the missing angle of the flat panel system, some researchers have proposed a flat panel PET rotation system, which collects single-photon data from multiple angles. , in order to achieve the purpose of improving the system resolution, for example, June et al. in "Efficient methodologies for system matrix modelling in iterative imagereconstruction for rotating high-resolution PET, Physics in Medicine and Biology, vol.55, no.7, 1833-61, 2010" used four flat-panel PET detectors with two detector heads. Two pairs, due to the small detection plate and large plate spacing, the system has a large missing angle, which solves this problem by rotating operation; in addition, Chunhui Zhang et al. system, Physics in Medicine and Biology, vol.60, no.15, pp.5873-90, 2015" uses two large-area flat-panel PET detectors as the main part of the system, in order to overcome the resolution perpendicular to the detection plate direction The problem is that it rotates the system 90 degrees to obtain full-angle sampling data; but the above two systems are both based on simulation, and the rotation process of the mechanical system does not involve positional deviation. Processing does not apply to real systems.

The calibration of the rotating system in the prior art and the reconstruction results of multiple angle data due to system geometric errors cannot be completely coincident.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide an imaging method of a rotating double-plate PET system based on system calibration, which aims to solve the problem that the calibration of the rotating system in the prior art and the reconstruction results of multiple angle data due to system geometric errors cannot be completely coincident question.

The present invention is implemented in the following way: an imaging method based on a system calibration-based rotating double-plate PET system, wherein the imaging method based on the system-calibration rotating double-plate PET system collects multiple sets of single-photon data by rotating the double-plate PET system, After data preprocessing, the forward projection data set required for reconstruction is obtained; then the iterative reconstruction algorithm OSEM is used for reconstruction to obtain multiple sets of reconstruction data; The data is rotated by a certain angle in the inverse system rotation direction; with the help of a specially designed registration point phantom, a set of image domains is obtained by calculating the offset vector between the registration point coordinates between the reconstruction results of two adjacent angles. Then, the geometric offset vector of the system is obtained; using the calculated image domain offset vector set and the projection data set, the subsets are divided in units of angles, and the projection data are reconstructed in turn. After the reconstruction of each subset is completed , after performing rotation and offset correction on the reconstruction result, it is added to the reconstruction process as the initial value of the next subset data reconstruction, and the correction of the system is completed in the reconstruction process.

Further, the imaging method of the system calibration-based rotating dual-plate PET system specifically includes the following steps:

In step 1, the data acquisition and preprocessing of the rotating double-plate PET system is adopted. The rotating double-plate PET system is composed of a double-plate PET detector, a data conformity processing system and a control system, wherein the PET detector is used for the ultra-high-speed scintillation pulse of gamma photons. Capture and analysis, the data coincidence processing system is used to perform coincidence processing on the collected data, and process the single photon events collected by each detector according to the energy and time information provided by each photon to obtain the required reconstruction. The projection data is the sinogram data; the control system completes the setting of the rotation step size, rotation angle and acquisition time of the system;

Step 2, single-angle data reconstruction

The data is reconstructed using the iterative reconstruction algorithm OSEM, and the formula is as follows:

in, is the image intensity of the jth voxel at the kth iteration under different angles of deg. For the convenience of description, the definition of deg is as follows:

The symbol S _l is the lth (l=1,2,...,L) subset, L is the total number of subsets divided, M is the overall prime number, g _i is the forward projection data; p _ij is the system response matrix The element of P, representing the probability that voxel j is detected by the corresponding line i;

Step 3: Obtain the system geometric offset vector, o represents the center of the field of view, o ⁰ is the center of the field of view at 0°, and the system geometric offset vector is defined as After a 90° rotation, the center of the field of view moves to o ¹ and defines Define the offset vector between pixels as vector and The relationship between is described as:

in,

The matrix R is a rotation parameter related to the rotation angle α. In this example, α=90°. It can be known from the above two formulas

compute vector The relationship between the two is:

Step 4: Multi-angle data reconstruction;

In step 5, the three-dimensional reconstruction result is displayed, and the reconstruction result obtained in step four is displayed in three dimensions.

Further, the data collection and preprocessing specifically include:

First, use the rotating double-plate PET system to collect data from no less than 2 angles on the object placed on the stage, and stay at each angle for the same time to obtain the reception of γ photons of a single detector at each angle. ; The double-plate PET system is rotated at equal intervals by the control system, and the detector collects a set of single-photon data and stores the real-time data every time the detector rotates by an angle, and the rotating double-plate PET system rotates 180° around the rotation axis;

Secondly, taking the angle as the unit, according to the time, energy, and position information provided by the collected data, the event coincidence of a single angle is carried out, and random noise and scattered noise are removed in turn, and the forward projection data set is obtained, that is, the sinogram data g ⁰ , g ¹ .

Further, the vector The calculation process is as follows:

The first step is to use the OSEM algorithm to reconstruct the forward projection data g ⁰ , g ¹ to obtain two single-angle reconstruction results

The second step, by rotating the image coordinates Get the rotated result

In the third step, the center coordinates of the 13 registration points at two angles are calculated respectively.

Further, the third step specifically includes:

Step one, from Select the slice perpendicular to the rotation axis related to the registration point;

Step 2, extract the boundary of each slice and calculate the center coordinates of the registration point Where i=1,...,num, num is the number of points, num=13;

Step 3, calculate the average offset of the center coordinates in Step 2:

Step 4, then the vector

Further, the multi-angle data reconstruction specifically includes:

The first step is to initialize f=f ^k =1, k=0;

The second step, initialization Use the OSEM algorithm to reconstruct the forward projection data g ⁰ to obtain the reconstruction result

The third step, will The coordinates of using the formula Perform a rotation and translation transformation to get the result in, represents the coordinates of pixel j, i.e.

The fourth step is to use As the initial value of the initial value OSEM algorithm, reconstruct the forward projection data g ¹ , and obtain the reconstruction result

The fifth step, will The coordinates of using the formula Perform a rotation and translation transformation to get the result where R' is the transpose matrix of matrix R;

The sixth step, k=k+1;

In the seventh step, jump to the second step, and end the iteration until the appropriate reconstruction result is obtained.

The imaging method of the rotating double-plate PET system based on system calibration provided by the invention can realize accurate positioning and high-resolution reconstruction of radionuclides in the living body, and can be used in the imaging field of rotating-plate PET system; and a system calibration-based imaging method is proposed. The imaging method of the rotating double-plate PET system has strong practicability. While retaining the flexible structure of the plate system, it realizes accurate localization and high-resolution reconstruction of the metabolic status of radionuclides in complex organisms.

Compared with the prior art, the present invention has the following advantages:

First, the present invention uses the designed registration point phantom to calculate the geometric offset vector of the rotating double-plate PET system, which is simple and convenient to complete the system correction while ensuring the accuracy of the reconstruction results. The resolution of the orientation is increased by about 3 times, which greatly improves the quality of the reconstructed image of the flat-panel PET.

Second, in the multi-angle reconstruction scheme adopted in the present invention, the system response matrix required for reconstruction is the same as that of the static system, and there is no need to rearrange the response lines. Efficiency of reconstruction.

Third, the present invention does not need to correct the system directly, and uses the offset vector of the image domain to indirectly complete the calculation of the geometric offset vector of the system.

Description of drawings

FIG. 1 is a flowchart of an imaging method of a rotating double-platen PET system based on system calibration provided by an embodiment of the present invention.

FIG. 2 is a description diagram of a system geometric deviation provided by an embodiment of the present invention.

FIG. 3 is a schematic diagram of a registration point phantom used in an embodiment of the present invention.

FIG. 4 is a result comparison diagram of an embodiment of the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

The application principle of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1 , the imaging method of the rotating dual-plate PET system based on system calibration according to the embodiment of the present invention includes the following steps:

S101: Collect multiple sets of single-photon data through a rotating biplane PET system, and after data preprocessing, obtain forward projection data sets required for reconstruction, and then use the iterative reconstruction algorithm OSEM for reconstruction to obtain multiple sets of reconstruction data; based on In this reconstruction data set, in addition to the initial angle reconstruction results, the reconstruction data of other angles are rotated by a certain angle in the inverse system rotation direction;

S102: With the help of a specially designed registration point phantom, by calculating the offset vector between the registration point coordinates between the two adjacent angle reconstruction results, a set of offset vector sets in the image domain are obtained, and then the the geometric offset vector of the system;

S103: Use the calculated image domain offset vector set and the projection data set to divide the subsets in units of angles, and reconstruct the projection data in turn. After the reconstruction of each subset is completed, perform rotation and offset correction on the reconstruction results, as the following The initial value of the reconstruction of a subset of data is added to the reconstruction process, so that the correction of the system can be completed at the same time during the reconstruction process.

The application principle of the present invention will be further described below with reference to the accompanying drawings.

Step 1, data collection and preprocessing

First, use the rotating double-plate PET system to collect data from no less than 2 angles (2 angles are used in this example) on the object placed on the stage. The receiving condition of the gamma photon of the detector; the data acquisition method of the present invention is: rotating the double-plate PET system at equal intervals through the control system, collecting a group of single-photon data for each rotation angle of the detector and storing the data in real time, the system Rotate 180° around the axis of rotation.

Secondly, taking the angle as the unit, according to the time, energy, and position information provided by the collected data, the event coincidence of a single angle is carried out, and random noise and scattered noise are removed in turn, and the forward projection data set is obtained, that is, the sinogram data g ⁰ , g ¹ .

Further, the rotating double-plate PET system is composed of a double-plate PET detector, a data compliance processing system and a control system. Among them, the PET detector is used for capturing and analyzing ultra-high-speed scintillation pulses of gamma photons, and the data conformity processing system is used for conformity processing of the acquired data to obtain the forward projection data required for reconstruction, namely sinogram data. The setting of rotation step length, rotation angle and acquisition time length can realize the control operation of the system.

Step 2, single-angle data reconstruction

The data is reconstructed using the iterative reconstruction algorithm OSEM, and the formula is as follows:

in, is the image intensity of the jth voxel at the kth iteration under different angles of deg. For the convenience of description, the definition of deg is as follows:

This definition still applies to other variables of the present invention. The symbol S _l is the _lth (l=1, 2, . p _ij is an element of the system response matrix P, representing the probability that voxel j is detected by the corresponding line i. The calculation of the system response matrix P is described in detail in the article "Performance evaluation of a90°-rotating dual-head small animal PET system, Physics in Medicine and Biology, vol.60, no.15, pp.5873-90, 2015" , and no longer describe its computation.

Step 3, the acquisition of the system geometric offset vector

Due to the inconsistency between the center of rotation of the system and the center of the field of view, the multi-angle reconstructed images cannot be overlapped. This problem is illustrated with the help of Figure 2. As shown in Figure 2(a), o represents the center of the field of view, o ⁰ is the center of the field of view at 0°, and the system geometric offset vector is defined as After a 90° rotation, the center of the field of view moves to o ¹ and defines Therefore, the offset between the pixels of the double-angle (0° and 90°) reconstructed image is caused, as shown in Fig. 2(b), the offset vector between the pixels is defined as vector and The relationship between can be described as:

in,

The matrix R is a rotation parameter related to the rotation angle α. In this example, α=90°. It can be known from the above two formulas

In practical work, it is difficult to directly calculate the system geometric offset vector But it is very convenient to calculate the vector The relationship between the two is:

vector The calculation process is as follows:

3a) Use the OSEM algorithm to reconstruct the forward projection data g ⁰ , g ¹ to obtain two single-angle reconstruction results

3b) By rotating the image coordinates Get the rotated result

3c) Calculate the center coordinates of the 13 registration points (as shown in Figure 3) under two angles respectively;

3c1) from Select the slice perpendicular to the rotation axis related to the registration point;

3c2) Extract the boundaries of each slice and calculate the center coordinates of the registration points where i=1,...,num, num is the number of points, in this example num=13;

3c3) Calculate the average offset of the center coordinates in step 3c2):

3c4) then the vector

Step 4. Multi-angle data reconstruction

4a) Initialize f=f ^k =1, k=0;

4b) Initialization Use the OSEM algorithm to reconstruct the forward projection data g ⁰ to obtain the reconstruction result

4c) will The coordinates of using the formula Perform a rotation and translation transformation to get the result in, represents the coordinates of pixel j, i.e.

4d) Utilize As the initial value of the initial value OSEM algorithm, reconstruct the forward projection data g ¹ , and obtain the reconstruction result

4e) will The coordinates of using the formula Perform a rotation and translation transformation to get the result where R' is the transpose matrix of matrix R;

4f) Let k=k+1;

4g) Jump to step 4b), until the appropriate reconstruction result is obtained, the iteration ends.

In step 5, the three-dimensional reconstruction result is displayed, and the reconstruction result obtained in step 4 is displayed in three dimensions.

Below in conjunction with accompanying drawing 3, the registration point phantom used in the present invention is further described:

Figure 3 is the registration point phantom used in the present invention, which consists of 3 parts, each part is 80×30×10mm ³ in size, wherein the first part consists of 5 holes, and the other two parts consist of 4 holes. The positions of the holes are respectively on the two diagonal lines and the central axis part. During the experiment, the three parts are superimposed and used as a whole.

The reconstruction result of the present invention will be further described below with reference to FIG. 4 .

FIG. 4 is a schematic diagram of a reconstruction result according to an embodiment of the present invention. Among them, the system rotation angle is 90°, and the number of collected projection data is 2 groups.

FIG. 4 is a reconstruction result obtained after performing systematic correction with the steps of the present invention, wherein the upper figure is a single-angle reconstruction result, and the lower figure is a double-angle reconstruction result.

Comparing the reconstruction effect of the present invention with the reconstruction structure in Fig. 4, it can be seen that the single-angle reconstruction results in the reconstruction results are stretched in one direction, while the multi-angle reconstruction results correct the image stretch. The invention overcomes the shortcomings of the prior art that the projection data is limited and the feature points need to be clearly identified, and can conveniently and quickly complete the correction of the poor resolution of the flat-panel PET reconstruction result.

The above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included in the protection of the present invention. within the range.

Claims (6)

1. an imaging method based on the rotating double-plate PET system of system calibration, is characterized in that, the described imaging method based on the rotating double-plate PET system of system calibration collects multiple groups of single photon data by rotating double-plate PET system, and After data preprocessing, the forward projection data set required for reconstruction is obtained; then the iterative reconstruction algorithm OSEM is used for reconstruction to obtain multiple sets of reconstruction data; Rotate a certain angle in the inverse system rotation direction; obtain a set of offset vectors in the image domain by calculating the offset vector between the coordinates of the registration points between the reconstruction results of two adjacent angles, and then obtain the geometry of the system Offset vector: Using the calculated image domain offset vector set and the projection data set, the subsets are divided in units of angles, and the projection data are reconstructed in turn. After the reconstruction of each subset is completed, the reconstruction results are rotated and offset corrected. , which is added to the reconstruction process as the initial value of the next subset data reconstruction, and the correction of the system is completed in the reconstruction process. 2. The imaging method of the rotating double-plate PET system based on system calibration as claimed in claim 1, wherein the imaging method of the rotating double-plate PET system based on system calibration specifically comprises the following steps: In step 1, the data acquisition and preprocessing of the rotating double-plate PET system is adopted. The rotating double-plate PET system is composed of a double-plate PET detector, a data conformity processing system and a control system, wherein the PET detector is used for the ultra-high-speed scintillation pulse of gamma photons. Capture and analysis, the data conformity processing system is used to conform the collected data to obtain the forward projection data required for reconstruction, that is, the sinogram data, and the control system completes the setting of the rotation step size, rotation angle, and collection duration of the system; Step 2, single-angle data reconstruction The data is reconstructed using the iterative reconstruction algorithm OSEM, and the formula is as follows: in, is the image intensity of the jth voxel at the kth iteration under different angles of deg. For the convenience of description, the definition of deg is as follows: Symbol S _l is the lth subset, l=1,2,...,L, L is the total number of subsets divided, M is the overall prime number, g _i is the forward projection data; p _ij is the element of the system response matrix P , represents the probability that voxel j is detected by the corresponding line i; Step 3: Obtain the system geometric offset vector, o represents the center of the field of view, o ⁰ is the center of the field of view at 0°, and the system geometric offset vector is defined as After a 90° rotation, the center of the field of view moves to o ¹ and defines Define the offset vector between pixels as vector and The relationship between is described as: in, The matrix R is a rotation parameter related to the rotation angle α. In this example, α=90°. It can be known from the above two formulas compute vector The relationship between the two is: Step 4: Multi-angle data reconstruction; In step 5, the three-dimensional reconstruction result is displayed, and the reconstruction result obtained in step four is displayed in three dimensions. 3. The imaging method of a system-calibrated rotating dual-plate PET system according to claim 2, wherein the data acquisition and preprocessing specifically include: First, use the rotating double-plate PET system to collect data from no less than 2 angles on the object placed on the stage, and stay at each angle for the same time to obtain the acceptance of γ photons of a single detector at each angle. ; The double-plate PET system is rotated at equal intervals by the control system, and the detector collects a set of single-photon data for each angle of rotation and stores the data in real time, and the system rotates 180° around the rotation axis; Secondly, taking the angle as the unit, according to the time, energy, and position information provided by the collected data, the event coincidence of a single angle is carried out, and random noise and scattered noise are removed in turn, and the forward projection data set is obtained, that is, the sinogram data g ⁰ , g ¹ . 4. The imaging method of a system calibration-based rotating double-plate PET system according to claim 2, wherein the vector The calculation process is as follows: The first step is to use the OSEM algorithm to reconstruct the forward projection data g ⁰ , g ¹ to obtain two single-angle reconstruction results The second step, by rotating the image coordinates Get the rotated result In the third step, the center coordinates of the 13 registration points at two angles are calculated respectively. 5. The imaging method of a system-calibrated rotating dual-plate PET system according to claim 4, wherein the third step specifically comprises: Step one, from Select the slice perpendicular to the rotation axis related to the registration point; Step 2, extract the boundary of each slice and calculate the center coordinates of the registration point Where i=1,...,num, num is the number of points, num=13; Step 3, calculate the average offset of the center coordinates in Step 2: Step 4, then the vector 6. The imaging method of a system-calibrated rotating dual-plate PET system according to claim 2, wherein the multi-angle data reconstruction specifically comprises: The first step is to initialize f=f ^k =1, k=0; The second step, initialization Use the OSEM algorithm to reconstruct the forward projection data g ⁰ to obtain the reconstruction result The third step, will The coordinates of using the formula Perform a rotation and translation transformation to get the result in, represents the coordinates of pixel j, i.e. The fourth step is to use As the initial value of the initial value OSEM algorithm, reconstruct the forward projection data g ¹ , and obtain the reconstruction result The fifth step, will The coordinates of using the formula Perform a rotation and translation transformation to get the result where R' is the transpose matrix of matrix R; The sixth step, k=k+1; In the seventh step, jump to the second step, and end the iteration until the appropriate reconstruction result is obtained.