Showing papers by "Tianyu Ma published in 2016"

Event-by-Event Continuous Respiratory Motion Correction for Dynamic PET Imaging.

Abstract: Existing respiratory motion-correction methods are applied only to static PET imaging. We have previously developed an event-by-event respiratory motion-correction method with correlations between internal organ motion and external respiratory signals (INTEX). This method is uniquely appropriate for dynamic imaging because it corrects motion for each time point. In this study, we applied INTEX to human dynamic PET studies with various tracers and investigated the impact on kinetic parameter estimation. Methods: The use of 3 tracers—a myocardial perfusion tracer, 82Rb (n = 7); a pancreatic β-cell tracer, 18F-FP(+)DTBZ (n = 4); and a tumor hypoxia tracer, 18F-fluoromisonidazole (18F-FMISO) (n = 1)—was investigated in a study of 12 human subjects. Both rest and stress studies were performed for 82Rb. The Anzai belt system was used to record respiratory motion. Three-dimensional internal organ motion in high temporal resolution was calculated by INTEX to guide event-by-event respiratory motion correction of target organs in each dynamic frame. Time–activity curves of regions of interest drawn based on end-expiration PET images were obtained. For 82Rb studies, K1 was obtained with a 1-tissue model using a left-ventricle input function. Rest–stress myocardial blood flow (MBF) and coronary flow reserve (CFR) were determined. For 18F-FP(+)DTBZ studies, the total volume of distribution was estimated with arterial input functions using the multilinear analysis 1 method. For the 18F-FMISO study, the net uptake rate Ki was obtained with a 2-tissue irreversible model using a left-ventricle input function. All parameters were compared with the values derived without motion correction. Results: With INTEX, K1 and MBF increased by 10% ± 12% and 15% ± 19%, respectively, for 82Rb stress studies. CFR increased by 19% ± 21%. For studies with motion amplitudes greater than 8 mm (n = 3), K1, MBF, and CFR increased by 20% ± 12%, 30% ± 20%, and 34% ± 23%, respectively. For 82Rb rest studies, INTEX had minimal effect on parameter estimation. The total volume of distribution of 18F-FP(+)DTBZ and Ki of 18F-FMISO increased by 17% ± 6% and 20%, respectively. Conclusion: Respiratory motion can have a substantial impact on dynamic PET in the thorax and abdomen. The INTEX method using continuous external motion data substantially changed parameters in kinetic modeling. More accurate estimation is expected with INTEX.

Choice of crystal surface finishing for a dual-ended readout depth-of-interaction (DOI) detector.

Abstract: The objective of this study was to choose the crystal surface finishing for a dual-ended readout (DER) DOI detector. Through Monte Carlo simulations and experimental studies, we evaluated 4 crystal surface finishing options as combinations of crystal surface polishing (diffuse or specular) and reflector (diffuse or specular) options on a DER detector. We also tested one linear and one logarithm DOI calculation algorithm. The figures of merit used were DOI resolution, DOI positioning error, and energy resolution. Both the simulation and experimental results show that (1) choosing a diffuse type in either surface polishing or reflector would improve DOI resolution but degrade energy resolution; (2) crystal surface finishing with a diffuse polishing combined with a specular reflector appears a favorable candidate with a good balance of DOI and energy resolution; and (3) the linear and logarithm DOI calculation algorithms show overall comparable DOI error, and the linear algorithm was better for photon interactions near the ends of the crystal while the logarithm algorithm was better near the center. These results provide useful guidance in DER DOI detector design in choosing the crystal surface finishing and DOI calculation methods.

Crystal Identification in Dual-Layer-Offset DOI-PET Detectors Using Stratified Peak Tracking Based on SVD and Mean-Shift Algorithm

Abstract: An Anger-logic based pixelated PET detector block requires a crystal position map (CPM) to assign the position of each detected event to a most probable crystal index. Accurate assignments are crucial to PET imaging performance. In this paper, we present a novel automatic approach to generate the CPMs for dual-layer offset (DLO) PET detectors using a stratified peak tracking method. In which, the top and bottom layers are distinguished by their intensity difference and the peaks of the top and bottom layers are tracked based on a singular value decomposition (SVD) and mean-shift algorithm in succession. The CPM is created by classifying each pixel to its nearest peak and assigning the pixel with the crystal index of that peak. A Matlab-based graphical user interface program was developed including the automatic algorithm and a manual interaction procedure. The algorithm was tested for three DLO PET detector blocks. Results show that the proposed method exhibits good performance as well as robustness for all the three blocks. Compared to the existing methods, our approach can directly distinguish the layer and crystal indices using the information of intensity and offset grid pattern.

3D positioning and readout channel number compression methods for monolithic PET detector

Abstract: Monolithic PET detector design is promising for low cost, high packing fraction, intrinsic depth-of-interaction (DOI) capability as well as potentially better energy and timing resolution compared to the conventional segmented crystal-based detector design. In this work, we developed a monolithic detector with 3D positioning capability and readout channel number compression. The detector contains a monolithic LYSO crystal (size: 25 mm × 25 mm × 14 mm) coupled to a 6 × 6 SiPM array (SensL FC30035). Two 3D positioning algorithms including statistical-based positioning (SBP) and artificial neural network (ANN) were developed to estimate gamma ray interaction positions inside the crystal. To simplify detector electronics design, two readout channel number compression strategies respectively based on Anger and principal component analysis (PCA) were proposed. With Anger, the readout channel number was compressed from 36 to 12 while with PCA the readout channel number was respectively compressed to 12 (PCA12), 6 (PCA6) and 4 (PCA4) corresponding to using different number of principal component vectors. The results demonstrate that 3D positioning capability is achievable with both SBP and ANN and the positioning resolution in x, y and z direction are respectively 2.58 mm, 2.56 mm and 4.79 mm with SBP, and 2.02 mm, 2.03 mm and 3.96 mm with ANN. Both strategies can effectively reduce the readout channel number without inducing degradations in detector positioning performance. With Anger and PCA12, almost no degradation is observed in positioning performance compared to no compression. With PCA6 or PCA4, deteriorations in positioning performance in z direction are observed, especially with PCA4, although there is no significant degradation in × or y direction. We conclude that the developed monolithic detector is promising for its 3D positioning capability as well as engineering application value with compressed readout channel number in PET system designs.

PET/CT alignment calibration with a non-radioactive phantom and the intrinsic 176 Lu radiation of PET detector

Abstract: Positron emission tomography/computed tomography (PET/CT) is an important tool for clinical studies and pre-clinical researches which provides both functional and anatomical images. To achieve high quality co-registered PET/CT images, alignment calibration of PET and CT scanner is a critical procedure. The existing methods reported use positron source phantoms imaged both by PET and CT scanner and then derive the transformation matrix from the reconstructed images of the two modalities. In this paper, a novel PET/CT alignment calibration method with a non-radioactive phantom and the intrinsic 176 Lu radiation of the PET detector was developed. Firstly, a multi-tungsten-alloy-sphere phantom without positron source was designed and imaged by CT and the PET scanner using intrinsic 176 Lu radiation included in LYSO. Secondly, the centroids of the spheres were derived and matched by an automatic program. Lastly, the rotation matrix and the translation vector were calculated by least-square fitting of the centroid data. The proposed method was employed in an animal PET/CT system (InliView-3000) developed in our lab. Experimental results showed that the proposed method achieves high accuracy and is feasible to replace the conventional positron source based methods.

Prompt gamma imaging with a multi-knife-edge slit collimator for large FOV monitoring of scanned proton pencil beams

Abstract: Proton therapy is superior to conventional radiation therapy due to dose deposition sharply increasing at Bragg peak. However, an uncertainty in Bragg peak position as large as 15 mm could occur due to a variety of uncertainties in proton therapy treatment delivery. One solution is to monitor the treatment during delivery by prompt gamma imaging. Pencil beam scanning mode is one of the widely adopted modes in proton therapy treatment In this mode, two sets of magnets scan the proton beams in a 2D pattern, and therefore the emitted gamma photons are focused to a small region, and the position of this region are rapidly changing with time. This is different with conventional nuclear medicine imaging, when the gamma source are distributed over the entire patient body, and rapidly changing with time. To meet the requirement of 40 cm × 40 cm × 30 cm FOV for proton pencil beam scanning mode, we propose a prompt gamma imaging system design with a multi-knife-edge slit collimator based on the optimization of a single-knife-edge slit collimator design. Monte Carlo simulation was performed to evaluate the positioning accuracy of Bragg peak and the spatial resolution of the imaging system. The prompt gamma distribution was generated via Monte Carlo simulation of proton interaction with water in GATE. The generated prompt gamma distribution was subsequently used to evaluate the system performance. For the multi-knife-edge slits collimator-based imaging system, preliminary evaluation results show that it can provide a large FOV size of 40×40×30 cm3, ∼0.17% detection efficiency at FOV center, <2mm special resolution and <3 mm positioning accuracy along Bragg peak direction. Based on the preliminary results, we conclude that the proposed multi-knife-edge slits collimator-based prompt gamma imaging system design is very promising and could potentially facilitate precise proton therapy.

Collimator-less 4π gamma imaging with 3-D position-sensitive detector

Abstract: Fast and accurate lost radioactive source positioning is of critical importance for the prevention of radiation damage in the accident. Traditional instruments with low detection efficiency cost lots pf time and energy, and may lead to harms to the operator's health. A portable detector to directly determine the incident angle of gamma rays is promising for the homeland security applications. In this work, we proposed a collimator-less 4π gamma imaging with 3-D position-sensitive detector. The distribution of photon counts is dependent on the orientation angle of the point gamma source, and the angle is reconstructed with Maximum Likelihood Expectation Maximization (MLEM) algorithm. Angular resolution of this imaging system was calculated by Cramer-Rao lower bound (CRLB). We simplified the 3-D position-sensitive detector to 2-D angular discrimination design. This design is capable of positioning the incident angle of 511 keV gamma rays. Excellent angular discrimination performance can be achieved except some particular angular positions at 0°, 90°, 180° and 270°. And the angular discrimination performance can be improved with more counts. Besides, we performed a preliminary experiment with 4 BGO crystal blocks and for all the tested angular positions ranging from 15° to 75° with an increment of 15°, the maximum error was less than 6.5°. Besides, we are now developing the positioning technology in a monolithic crystal which can be applied to the gamma imaging method. From the analytical calculation and experimental results, we conclude that the proposed 4π gamma imaging method with 3-D position-sensitive detector is effective and capable of searching for radioactive sources with excellent accuracy.

Axial Movement Parameter Study on Preclinical Slit-Slat SPECT

Abstract: One challenge to discrete detector slit-slat SPECT is that the system’s resolution and sensitivity vary significantly in the axial direction. The objective of this work was to reduce the variation by introducing axial movement and to determine the corresponding parameters, i.e. step size (D) and total length (L). We calculated the system response matrix of the slit-slat SPECT system and evaluated the system’s axial-direction resolution and sensitivity as functions of axial positions in the field of view. A series of scan protocols with combined rotational and axial movements, that is, helical scans, were applied to simulated phantom studies to select the preferred L and D values. Introducing axial movement effectively improved the system’s axial-direction sensitivity uniformity and reconstructed image resolution in both axial direction and in transverse plane, and it also reduced the artifacts in the transverse plane. The preferred choice of L is the cycle of the periodic pattern formed by the slat collimator’s and detector crystals. As about the value of D, a smaller D value is preferred for better uniformity as well as resolution improvement but practical constraints would apply.

Regularized MLEM reconstruction with a strong anatomical prior using newton iterative algorithm

Abstract: MAP algorithm has outperformed in medical image reconstruction with noise suppression and edge preservation. However MAP-OSL algorithm could hardly apply a strong prior by using a large regularization parameter. In this study we proposed a MAP-Newton reconstruction framework to enable a strong prior to be applied in MAP optimization. EM iterative framework was used in MAP-Newton. In the step of maximizing expectation of the complete data log-likelihood function, MAP-Newton solves the non-linear equation accurately with Newton iterative algorithm, which is potentially better than approximate linearization method used in MAP-OSL. MAP-Newton reconstruction algorithm was implemented with both Bowsher's prior and joint total variation (JTV) prior based on anatomical image. 18F PET and CT data of an image-quality phantom were acquired on the small animal PET/SPECT/ CT Iniview 3000 system for performance evaluation. The priors with three different strength (regularization parameter was from 0.01 to 1.0) were applied in the list mode reconstruction studies with three noise levels (100 M, 12 M and 2 M LORs separately). The normalized standard derivation (NSTD) of region of interest was calculated. The results with Bowsher's prior for all noise level cases showed no significant difference between MAP-Newton and MAP-OSL when the regularization parameter was smaller than 1.0. When the parameter was set to be 1.0, the proposed MAP-Newton can greatly reduce NSTD with reasonable image quality although no reasonable images could be obtained with MAP-OSL. The results with JTV prior showed the same trend, except that increased regularization parameter did not reduce NSTD. In conclusion, with a strong prior, MAP-Newton can result in better image quality than MAP-OSL in terms of noise suppression, while no significant difference when applying a light prior. It indicated that the MAP-Newton reconstruction with a strong prior could be applied, when we have enough confidence on the prior.

SPECT (single-photon emission computed tomography) equipment for small animals

Abstract: The invention discloses SPECT (single-photon emission computed tomography) equipment for small animals. The SPECT equipment comprises a collimator and an imaging detector, wherein at least one composite hole allowing a ray to penetrate is formed in the collimator, each composite hole includes a needle hole and a parallel hole which are sequentially communicated along the transmitting direction of the ray, the passing area of each needle hole increases or decreases gradually or decreases gradually and then increases gradually in the transmitting direction of the ray, and the passing area of each parallel hole is constantly unchanged. The SPECT equipment for the small animals has the advantages that the needle holes and parallel holes which are sequentially communicated are formed in the collimator, projection overlap is limited by the parallel holes and the amplification characteristic of the needle holes, and accordingly high-resolution SPECT reconstructed images can be obtained under limited detection area.

Multi-gamma ray photon simultaneous medicine emission time-coincidence nuclear medicine imaging system and method

Abstract: A multi-gamma ray photon simultaneous medicine emission-based time-coincidence nuclear medicine imaging system and method. The system comprises: multiple detector probes arranged in a non-parallel mode, a time coincidence module (3), and a computer platform (4). Each detector probe comprises a collimator (1) and a gamma ray photon detector (2) with a time measuring function. Multiple gamma ray photons radiated by detection radionuclides within a short time form multiple gamma ray photon coincidence events. According to calculation in the method, a point having the smallest sum of distances of projection lines determined by the multiple gamma ray photon coincidence events is a disintegration position of radionuclides, and the distribution of radionuclides in a living body can be obtained by accumulating a certain number of multi-gamma ray photon coincidence events. The imaging system and method simplify a reconstruction algorithm, improve the signal to noise ratio of reconstructed images, lower the requirement for the total count of gamma ray photons, and reduce the irradiation risk to patients.

Development of a MR-compatible DOI-TOF detector module for PET imaging systems

Abstract: PET systems are widely used in clinical and preclinical molecular imaging applications. Various PET systems, including whole-body human and small animal PET, and dedicated systems such as breast and brain PET have been developed. A Standardized PET detector module with good performance will make it much easier and more efficient for different kind of PET scanner instrumentation. In this paper, we developed a compact MR-compatible PET detector with DOI and TOF capability. The detector module has staggered 15×15 +16 ×16 LYSO crystal array with single crystal size of 2×2×7mm3. The detector is coupled to SiPM array (MicroFJ-30035-TSV, SensL). The output signals of SiPMs are multiplexed by 64-channel ASIC chips and then digitized by 80MHz 12-bit ADC chips and TDC implemented inside FPGA to generate energy and timing information. The performance of the detector was preliminarily evaluated. A flood image was acquired and all crystals were clearly identified with an average energy resolution of 12.7% for 15×15 array and 14.2% for 16×16 array. About 300 ps coincidence timing resolution was achieved for a pair of single crystal. More accurate energy resolution and coincidence timing resolution for the detector module will be evaluated in the future. And the MR compatibility evaluation of this module is underway. Large quantities of detector blocks will be made and tested to verify the performance stability and reproducibility. Furthermore, we are developing standardized power and signal interface for better usability.

PET-MR joint reconstruction with information theoretic priors

Abstract: We proposed a novel idea to introduce information theoretic priors between PET and MR images to PET/MR joint reconstruction framework. To evaluate the performance of the proposed method, information theoretic priors: mutual information (MI) and joint entropy (JE) were compared with structural similarity priors: parallel level set (PLS) and joint total variation (JTV). With MI and JE, PET/MR joint reconstruction substantially improved resolution and accuracy of both PET and MR images with both simulation and real human data compared with no prior, PLS and JTV priors.