CN105832358B - A kind of imaging method of the rotation double flat plate PET system based on system calibration - Google Patents
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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
Technical Field
The invention belongs to the technical field of nuclear medicine imaging, and particularly relates to an imaging method of a rotary double-flat-plate PET system based on system calibration.
Background
PET (positron emission Tomography) is known as positron emission computed Tomography and is an advanced imaging technology in the field of nuclear medicine. The detector of the PET system can obtain the in-vivo metabolic distribution, is a functional imaging technology, and particularly occupies a non-negligible position in the aspect of early detection of tumors. Most of medical PET detectors adopt a static ring structure, and although sampling data of a complete angle can be acquired, the system is high in complexity and cost, has certain closure, and is inconvenient for doctors or experimenters to guide the technology of a sampling object. To reduce cost and increase detector flexibility, in recent years researchers have proposed flat panel PET detectors, such as the "A High-Sensitivity Small-Animal PET Scanner: Defelop" by Chien-Min Kao et alThe article "IEEE Transactions on nuclear Science, vol.56, No.5, pp.2678-88,2009" proposes a static double-headed flat-panel PET detector, which has a simple structure and can obtain satisfactory imaging results, but the image resolution in the direction perpendicular to the detection panel is poor, because the static flat-panel PET cannot obtain projection data of a complete angle; to solve the problem of angle missing of the flat panel system, researchers have proposed a flat panel PET rotation system for improving the resolution of the system by acquiring single photon data at multiple angles, for example, JuneThe inventor adopts four flat-plate PET detectors in 'effective methods for system matrix modification in iterative imaging PET, Physics in medicinal analysis biology, vol.55, No.7,1833 and 61, 2010', two pairs of the detectors are arranged, because the detection plates are smaller and the plate spacing is larger, the system has a large deletion angle, and the problem is solved by rotating operation; in addition, Chunhui Zhang et al in "Performance evaluation of a 90-rotating dual-head small PET system, Physics in Medicine and Biology, vol.60, No.15, pp.5873-90,2015" adopt two large-area flat PET probes as the main part of the system, in order to overcome the problem of poor resolution perpendicular to the probe plate, the system is rotated by 90 degrees, and full-angle sampling data can be obtained; however, in the case that both of the above two systems are based on simulation, the problem of position deviation is generated in the rotation process without involving the mechanical system, and the data processing process is not suitable for the real system.
The calibration of the rotation system and the reconstruction of multiple angle data due to system geometric errors in the prior art cannot be completely coincident.
Disclosure of Invention
The invention aims to provide an imaging method of a rotating double-flat-plate PET system based on system calibration, and aims to solve the problems that in the prior art, the calibration of a rotating system and the reconstruction results of a plurality of angle data caused by system geometric errors cannot be completely overlapped.
The invention is realized in such a way, the imaging method of the rotating double-flat-plate PET system based on the system calibration acquires a plurality of groups of single photon data through the rotating double-flat-plate PET system, and acquires a forward projection data group required by reconstruction after data preprocessing; then, reconstructing by using an iterative reconstruction algorithm OSEM to obtain a plurality of groups of reconstruction data; based on the reconstruction data set, besides the initial angle reconstruction result, the reconstruction data of other angles rotate a certain angle in the direction opposite to the rotation direction of the system; by means of a specially designed registration point phantom, calculating an offset vector between coordinates of registration points between two adjacent angle reconstruction results to obtain an offset vector set of a group of image domains, and further obtaining a geometric offset vector of the system; dividing subsets by taking an angle as a unit by utilizing the calculated image domain offset vector set and the projection data group, reconstructing projection data in sequence, after reconstruction of each subset is finished, rotating and carrying out offset correction on a reconstruction result, adding the reconstruction result as an initial value of reconstruction of next subset data into a reconstruction process, and finishing correction on a system in the reconstruction process.
Further, the imaging method of the rotating dual-plate PET system based on the system calibration specifically comprises the following steps:
the method comprises the steps that firstly, a rotating double-flat-plate PET system is adopted for data acquisition and preprocessing, the rotating double-flat-plate PET system is composed of a double-flat-plate PET detector, a data coincidence processing system and a control system, wherein the PET detector is used for capturing and analyzing gamma photon ultra-high-speed scintillation pulses, the data coincidence processing system is used for performing coincidence processing on acquired data, single photon events acquired by each detector are processed according to energy and time information provided by each photon, and projection data, namely sinogram data, required by reconstruction is acquired; the control system completes the setting of the rotation step length, the rotation angle and the acquisition duration of the system;
step two, reconstructing single-angle data
And reconstructing the data by adopting an iterative reconstruction algorithm OSEM, wherein the formula is as follows:
wherein,for the image intensity of the jth voxel at different angles deg at the kth iteration, for convenience of description, deg is defined as follows:
symbol SlIs the L (1, 2, 1, L) subset, L is the total subset number of partitions, M is the total prime number, giIs forward projection data; p is a radical ofijIs an element of the system response matrix P representing the probability that a voxel j is detected by the corresponding line i;
step three, acquiring a geometric offset vector of the system, wherein o represents the center of the visual field, and o0For the center of the field of view at 0 deg., the geometric offset vector of the system is defined asAfter 90 DEG rotation, the center of the field of view is shifted to o1And defineDefining an offset vector between pixels asVector quantityAndthe relationship between is described as:
wherein,
the matrix R is a rotation parameter related to the rotation angle α, in this case α ═ 90 °, as can be seen from the above two equations
Computing vectorsThe relationship between the two is:
step four, multi-angle data reconstruction;
and fifthly, displaying the three-dimensional reconstruction result, and displaying the reconstruction result obtained in the fourth step in a three-dimensional manner.
Further, the data acquisition and preprocessing specifically includes:
firstly, a rotating double-flat-plate PET system is utilized to acquire data of at least 2 angles of an object placed on an object stage, the retention time of each angle is the same, and the receiving condition of gamma photons of a single detector under each angle is obtained; the double-flat-plate PET system is rotated at equal intervals through the control system, a group of single photon data is collected and stored in real time when the detector rotates for each angle, and the double-flat-plate PET system is rotated for 180 degrees around the rotating shaft;
secondly, taking an angle as a unit, according to the time, energy and position information provided by the acquired data, carrying out event coincidence of a single angle, sequentially eliminating random noise and scattering noise, and acquiring a forward projection data set, namely sinogram data g0,g1。
Further, the vectorThe calculation process of (2) is as follows:
first, reconstructing forward projection data g by using an OSEM algorithm0,g1Obtaining two single-angle reconstruction results
Second, by rotating the image coordinatesObtaining the rotated result
And thirdly, respectively calculating the center coordinates of the 13 registration points under two angles.
Further, the third step specifically includes:
step one, fromSelecting a slice which is related to the registration point and is vertical to the rotating shaft;
step two, extracting the boundary of each slice and calculating the center coordinate of the registration pointWherein, the number of points is num, and num is 13;
step three, calculating the average offset of the center coordinates in the step two:
step four, then vector
Further, the multi-angle data reconstruction specifically includes:
first, initialize f ═ fk＝1,k＝0；
Second, initializationReconstruction of forward projection data g using the OSEM algorithm0Obtaining a reconstruction result
A third step of mixingUsing formula ofPerforming rotation translation transformation to obtain the resultWherein,representing the coordinates of pixel j, i.e.
The fourth step, utilizeReconstructing forward projection data g as initial values of an initial value OSEM algorithm1Obtaining a reconstruction result
The fifth step is toUsing formula ofPerforming rotation translation transformation to obtain the resultWherein R' is the transpose of matrix R;
sixthly, k is k + 1;
and seventhly, jumping to the second step until a proper reconstruction result is obtained, and ending the iteration.
The imaging method of the rotating double-flat-plate PET system based on system calibration can realize accurate positioning and high-resolution reconstruction of radionuclide in organisms, and can be used in the field of imaging of the rotating flat-plate PET system; the imaging method of the rotating double-plate PET system based on system calibration has strong practicability, and realizes accurate positioning and high-resolution reconstruction of the metabolism condition of the radionuclide by a complex organism while keeping the flexible structure of the plate system.
Compared with the prior art, the invention has the following advantages:
firstly, the invention utilizes the designed registration point phantom to calculate the geometric offset vector of the rotating double-plate PET system, thereby simply and conveniently finishing the correction of the system, simultaneously ensuring the accuracy of the reconstruction result, improving the resolution ratio vertical to the direction of the detector by about 3 times and greatly improving the quality of the plate PET reconstruction image.
Secondly, according to the multi-angle reconstruction scheme adopted by the invention, the system response matrix required by reconstruction is the same as that of a static system, the rearrangement of response lines is not needed, the calculation amount of the rotating double-flat-plate PET system matrix is reduced by at least 4 times, and the reconstruction efficiency is improved.
Thirdly, the system is not required to be directly corrected, and the calculation of the geometric offset vector of the system is indirectly finished by using the offset vector of the image domain.
Drawings
Fig. 1 is a flowchart of an imaging method of a rotating dual-plate PET system based on system calibration according to an embodiment of the present invention.
Fig. 2 is a diagram illustrating geometric deviations of a system according to 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 graph comparing the results of examples of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The following detailed description of the principles of the invention is provided in connection with 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: acquiring a plurality of groups of single photon data through a rotating biplane PET system, preprocessing the data, acquiring a forward projection data group required by reconstruction, and reconstructing by using an iterative reconstruction algorithm OSEM to acquire a plurality of groups of reconstruction data; based on the reconstruction data set, besides the initial angle reconstruction result, the reconstruction data of other angles rotate a certain angle in the direction opposite to the rotation direction of the system;
s102: by means of a specially designed registration point phantom, calculating an offset vector between coordinates of registration points between two adjacent angle reconstruction results to obtain an offset vector set of a group of image domains, and further obtaining a geometric offset vector of the system;
s103: and dividing subsets by taking an angle as a unit by utilizing the calculated image domain offset vector set and the projection data group, reconstructing the projection data in sequence, and after the reconstruction of each subset is finished, performing rotation and offset correction on the reconstruction result, wherein the reconstruction result is used as an initial value for the reconstruction of the next subset data and is added into the reconstruction process, so that the system correction is completed simultaneously in the reconstruction process.
The application of the principles of the present invention will now be described in further detail with reference to the accompanying drawings.
Step 1, data acquisition and preprocessing
Firstly, a rotating double-flat-plate PET system is utilized to acquire data of not less than 2 angles (2 angles are adopted in the embodiment) of an object placed on an object stage, each angle stays for the same time, and the receiving condition of gamma photons of a single detector under each angle is acquired; the data acquisition method comprises the following steps: the double-flat-plate PET system is rotated at equal intervals through the control system, a group of single photon data is collected when the detector rotates for one angle, real-time data storage is carried out, and the system rotates for 180 degrees around the rotating shaft.
Secondly, by anglePerforming event coincidence of a single angle according to time, energy and position information provided by the acquired data, sequentially eliminating random noise and scattering noise, and acquiring a forward projection data set, namely sinogram data g0,g1。
Furthermore, the rotating double-flat-plate PET system is composed of a double-flat-plate PET detector, a data coincidence processing system and a control system. The PET detector is used for capturing and analyzing gamma photon ultra-high-speed scintillation pulses, the data coincidence processing system is used for performing coincidence processing on acquired data to acquire forward projection data required for reconstruction, namely sinogram data, and the control system is mainly used for setting the rotation step length, the rotation angle and the acquisition duration of the system to realize control operation on the system.
Step 2, single angle data reconstruction
And reconstructing the data by adopting an iterative reconstruction algorithm OSEM, wherein the formula is as follows:
wherein,for the image intensity of the jth voxel at different angles deg at the kth iteration, for convenience of description, deg is defined as follows:
this definition is applicable to the other variables of the present invention. Symbol SlIs the L (1, 2, 1, L) subset, L is the total subset number of partitions, M is the total prime number, giIs forward projection data. p is a radical ofijIs an element of the system response matrix P representing the probability that a voxel j is detected by the corresponding line i. Calculation of the System response matrix P in the article "Performanceevaluation of a 90-rotating dual-head small animal PET system, Physics in Medicine and biology, vol.60, No.15, pp.5873-90,2015 "are described in detail and their calculations are not described.
Step 3, obtaining the geometric offset vector of the system
Due to the inconsistency between the system rotation center and the view center, the multi-angle reconstructed images cannot be overlapped, and this problem is illustrated with reference to fig. 2. As shown in FIG. 2(a), o denotes the center of the field of view, o0For the center of the field of view at 0 deg., the geometric offset vector of the system is defined asAfter 90 DEG rotation, the center of the field of view is shifted to o1And defineThus resulting in an offset between the pixels of the dual angle (0 ° and 90 °) reconstructed image, as shown in fig. 2(b), defining an offset vector between the pixels asVector quantityAndthe relationship between can be described as:
wherein,
the matrix R is a rotation parameter related to the rotation angle α, in this case α ═ 90 °, as can be seen from the above two equations
In practical work, it is difficult to directly calculate the geometric offset vector of the systemBut vectors can be conveniently calculatedThe relationship between the two is:
vector quantityThe calculation process of (a) is as follows:
3a) reconstruction of forward projection data g using the OSEM algorithm0,g1Obtaining two single-angle reconstruction results
3b) By rotating the image coordinatesObtaining the rotated result
3c) Respectively calculating the center coordinates of the 13 registration points (shown in FIG. 3) at two angles;
3c1) fromIn which a vertical associated with a registration point is selectedSlices straight to the axis of rotation;
3c2) extracting the boundary of each slice and calculating the center coordinates of the registration pointsWhere i is 1., num, num is the number of points, in this case num is 13;
3c3) calculating the average offset of the center coordinates in step 3c 2):
3c4) then vector
Step 4, multi-angle data reconstruction
4a) Initializing f ═ fk＝1,k＝0；
4b) InitializationReconstruction of forward projection data g using the OSEM algorithm0Obtaining a reconstruction result
4c) Will be provided withUsing formula ofPerforming rotation translation transformation to obtain the resultWherein,representing the coordinates of pixel j, i.e.
4d) By usingReconstructing forward projection data g as initial values of an initial value OSEM algorithm1Obtaining a reconstruction result
4e) Will be provided withUsing formula ofPerforming rotation translation transformation to obtain the resultWherein R' is the transpose of matrix R;
4f) let k be k + 1;
4g) and 4b), jumping to the step 4b), and ending the iteration until a proper reconstruction result is obtained.
And 5, displaying a three-dimensional reconstruction result, and displaying the reconstruction result obtained in the step 4 in a three-dimensional manner.
The registration point phantom used in the present invention is further described below with reference to fig. 3:
FIG. 3 is a schematic view of a registration phantom used in the present invention, which is composed of 3 parts, each part having a size of 80X 30X 10mm3The first part is composed of 5 holes, the other two parts are composed of 4 holes, the positions of the holes are respectively arranged on two diagonal lines and a central axis part, 3 parts are superposed during the experiment,as a whole.
The reconstruction results of the present invention are further described below with reference to fig. 4.
Fig. 4 is a schematic diagram of a reconstruction result of an embodiment of the present invention. The rotation angle of the system is 90 degrees, and the number of the collected projection data is 2 groups.
Fig. 4 is a reconstruction result obtained after systematic correction by the steps of the present invention, wherein the upper graph is a single angle reconstruction result and the lower graph is a dual angle reconstruction result.
Comparing the reconstruction effect of the invention with the reconstruction structure shown in figure 4, it can be seen that the single-angle reconstruction result in the reconstruction result has a stretching condition in one direction, and the multi-angle reconstruction result corrects the stretching of the image, which illustrates that the invention overcomes the defects of the prior art that the projection data is limited and the feature points are obviously easy to identify, and conveniently and quickly completes the correction of the poor resolution of the flat PET reconstruction result.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (6)
1. The imaging method of the rotating double-flat-plate PET system based on the system calibration is characterized in that a plurality of groups of single photon data are acquired by the rotating double-flat-plate PET system, and a forward projection data group required by reconstruction is acquired after data preprocessing; then, reconstructing by using an iterative reconstruction algorithm OSEM to obtain a plurality of groups of reconstruction data; based on the reconstruction data set, besides the initial angle reconstruction result, the reconstruction data of other angles rotate a certain angle in the direction opposite to the rotation direction of the system; obtaining a set of offset vectors of a group of image domains by calculating the offset vectors between the coordinates of the registration points between two adjacent angle reconstruction results, and further obtaining the geometric offset vector of the system; dividing subsets by taking an angle as a unit by utilizing the calculated image domain offset vector set and the projection data group, reconstructing projection data in sequence, after reconstruction of each subset is finished, rotating and carrying out offset correction on a reconstruction result, adding the reconstruction result as an initial value of reconstruction of next subset data into a reconstruction process, and finishing correction on a system in the reconstruction process.
2. The imaging method of a system calibration based rotating dual-plate PET system as claimed in claim 1, wherein the imaging method of a system calibration based rotating dual-plate PET system specifically comprises the steps of:
the method comprises the following steps that firstly, a rotating double-flat-plate PET system is adopted for data acquisition and preprocessing, the rotating double-flat-plate PET system is composed of a double-flat-plate PET detector, a data coincidence processing system and a control system, wherein the PET detector is used for capturing and analyzing gamma photon ultra-high-speed scintillation pulses, the data coincidence processing system is used for performing coincidence processing on acquired data to acquire forward projection data required for reconstruction, namely sinogram data, and the control system is used for setting the rotating step length, the rotating angle and the acquisition duration of the system;
step two, reconstructing single-angle data
And reconstructing the data by adopting an iterative reconstruction algorithm OSEM, wherein the formula is as follows:
wherein,for the image intensity of the jth voxel at different angles deg at the kth iteration, for convenience of description, deg is defined as follows:
symbol SlL is 1,2, …, L is the total number of subsets divided, M is the total prime number, giIs forward projection data; p is a radical ofijIs an element of the system response matrix P representing the probability that a voxel j is detected by the corresponding line i;
step three, acquiring a geometric offset vector of the system, wherein o represents the center of the visual field, and o0For the center of the field of view at 0 deg., the geometric offset vector of the system is defined asAfter 90 DEG rotation, the center of the field of view is moved to o1And defineDefining an offset vector between pixels asVector quantityAndthe relationship between is described as:
wherein,
the matrix R is a rotation parameter related to the rotation angle α, in this case α ═ 90 °, as can be seen from the above two equations
Computing vectorsThe relationship between the two is:
step four, multi-angle data reconstruction;
and fifthly, displaying the three-dimensional reconstruction result, and displaying the reconstruction result obtained in the fourth step in a three-dimensional manner.
3. The method for imaging a rotating dual-plate PET system based on system calibration as claimed in claim 2, wherein the data acquisition and pre-processing specifically comprises:
firstly, a rotating double-flat-plate PET system is utilized to acquire data of at least 2 angles of an object placed on an object stage, the retention time of each angle is the same, and the receiving condition of gamma photons of a single detector under each angle is obtained; the double-flat-plate PET system is rotated at equal intervals through a control system, a group of single photon data is collected and stored in real time when the detector rotates for one angle, and the system rotates for 180 degrees around a rotating shaft;
secondly, taking an angle as a unit, according to the time, energy and position information provided by the acquired data, carrying out event coincidence of a single angle, sequentially eliminating random noise and scattering noise, and acquiring a forward projection data set, namely sinogram data g0,g1。
4. The method for imaging a rotating dual-plate PET system based on system calibration of claim 2, wherein the vector isThe calculation process of (2) is as follows:
first, reconstructing forward projection data g by using an OSEM algorithm0,g1To obtain twoSingle angle reconstruction result
Second, by rotating the image coordinatesObtaining the rotated result
And thirdly, respectively calculating the center coordinates of the 13 registration points under two angles.
5. The imaging method of a rotating dual-plate PET system based on system calibration according to claim 4, wherein the third step specifically comprises:
step one, fromSelecting a slice which is related to the registration point and is vertical to the rotating shaft;
step two, extracting the boundary of each slice and calculating the center coordinate of the registration pointWherein, the number of points is num, and num is 13;
step three, calculating the average offset of the center coordinates in the step two:
step four, then vector
6. The method for imaging a rotating dual-plate PET system based on system calibration as claimed in claim 2, wherein the multi-angle data reconstruction specifically comprises:
first, initialize f ═ fk＝1,k＝0；
Second, initializationReconstruction of forward projection data g using the OSEM algorithm0Obtaining a reconstruction result
A third step of mixingUsing formula ofPerforming rotation translation transformation to obtain the resultWherein,representing the coordinates of pixel j, i.e.
The fourth step, utilizeReconstructing forward projection data g as initial values of an initial value OSEM algorithm1Obtaining a reconstruction result
The fifth step is toUsing formula ofPerforming rotation translation transformation to obtain the resultWherein R' is the transpose of matrix R;
sixthly, k is k + 1;
and seventhly, jumping to the second step until a proper reconstruction result is obtained, and ending the iteration.
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