Imaging performance evaluation in depth-of-interaction PET with a new method of sinogram generation:A Monte Carlo simulation study

New PET scanners utilize depth-of-interaction (DOI) information to improve image resolution, particularly at the edge of field-of-view while maintaining high detector sensitivity. However, the inter-crystal scatter (ICS) effect cannot be neglected in DOI scanners due to the use of smaller crystals. ICS is the phenomenon wherein there are multiple scintillations for irradiation of a gamma photon due to Compton scatter in detecting crystals. In the case of ICS, only one scintillation position is approximated for detectors with Anger-type logic calculation. This causes an error in position detection and ICS worsens the image contrast, particularly for smaller hotspots. In this study, we propose to model an ICS probability by using a Monte Carlo simulator. The probability is given as a statistical relationship between the gamma photon first interaction crystal pair and the detected crystal pair. It is then used to improve the system matrix of a statistical image reconstruction algorithm, such as ML-EM in order to correct for the position error caused by ICS. We apply the proposed method to simulated data of the jPET-D4, which is a four-layer DOI PET being developed at the National Institute of Radiological Sciences. Our computer simulations show that image contrast is recovered successfully by the proposed method.

The NeuroEXPLORER (NX) system has recently been proposed for a high-resolution and high-sensitivity PET scanner dedicated for brain research. A long axial field-of-view (FOV) of 50cm was used for maximizing the sensitivity in brain region. Depth-of-interaction (DOI) modelling of NX system is necessary for generating high-resolution images and compensate for the large angle effects caused by long axial FOV.To minimize the modifications on the existing reconstruction algorithm and to minimize computational cost, a listmode-based DOI compensation method (DOI mashing) was developed. For detected event with non-zero DOI value, a new virtual detector pair was calculated along the incidence angle. The virtual detector pair has DOI value 0 and therefore has minimal DOI effect. Monte Carlo simulations were used to calculate the point-spread-functions (PSF) with the DOI effect with and without mashing. Analytical simulations were carried out with the acquired PSFs to simulate reconstructions with and without DOI corrections.The full-width-half-maximum (FWHM) of the PSF in the axial direction was 2.8mm with 0 incidence angle and was 8mm with the maximum incidence angle. With DOI mashing, the maximum FWHM reduced to 4mm. The residual blurring was found to be caused by inter-crystal scatters. Similar trend was found in the transaxial PSFs, where a large FWHM was observed in off-center locations. Image reconstruction without DOI mashing showed that without DOI mashing, image resolution degrades in off-center locations. With DOI mashing, image resolution was preserved in off-center locations.In conclusion, we proposed a DOI mashing approach that can compensate the DOI effects on the listmode data with minimal modifications of existing reconstruction methods.

High spatial resolution Positron Emission Tomography (PET) imaging provides functional data about a patient's body and provides us with a range of new possibilities, including medicine development, new diagnostic methods, and others. The use of Depth of Interaction (DOI) detectors is one effective technique for the improvement of whole-body PET imaging, as is evident in the fact that most small animal PET images are taken using a DOI system. In this study, we developed a prototype whole-body DOI- equipped PET system and evaluated its performance. This scanner's DOI detectors consist of dual-layer (front and rear) 9 x 10 array of GSO/GSO crystals, a light guide and two rectangular double-anode photo multiplier tubes (PMTs). The DOI layers are discriminated according to decay time differences, which are controlled by Ce concentrations. The DOI detectors are arranged in a circular detector ring, with a diameter of 664 mm. In order to avoid parallax errors, coincidental events, including rear layer detection, were addressed to the nearest neighbor LOR front-layer coincidence pair. To evaluate the spatial resolution of this PET system at various radial positions, 22Na point sources at 1, 10, 20, and 25 cm offset from the center were reconstructed, both with and without DOI information. Results showed that spatial resolution degradation at 25 cm improved from 195 % to 154 % as a result of using DOI data. During a visual evaluation using a Derenzo phantom, image reconstruction using DOI data clarified the image and corrected hot rod shapes. The results presented here show that DOI detectors were useful in creating an effective whole-body PET system.

Organ-dedicated PET scanners are becoming more prevalent because of their advantages in higher sensitivity, improved image quality, and lower cost. Detectors utilized in these scanners have finer pixel size with depth of interaction (DOI) capability. This work presents a LYSO(Ce) detector module with DOI capability which has the potential to be scaled up to a high-resolution small animal or organ-dedicated PET system. For DOI capability, a submodule with one LYSO block detector utilizing PETsys TOFPET2 application-specific integrated circuit (ASIC) was previously developed in our lab. We scaled up the submodule and optimized the configuration to allow for a compact housing of the dual-readout boards in one side of the blocks by designing a high-speed dual-readout cable to maintain the original pin-to-pin relationship between the Samtec connectors. The module size is 53.8 × 57.8 mm2. Each module has 2 × 2 LYSO blocks, each LYSO block consists of 4 × 4 LYSO units, and each LYSO unit contains a 6 × 6 array of 1 × 1 × 20 mm3 LYSO crystals. The four lateral surfaces of LYSO crystal were mechanically ground to W14, and the two end surfaces were polished. Two ends of the LYSO crystal are optically connected to SiPM for DOI measurement. Eight LYSO blocks performance including energy, timing, and DOI resolution is characterized with a single LYSO slab. The in-panel and orthogonal-panel spatial resolution of the two modules with 107.4 mm distance between each other are measured at 9 positions within the field of view (FOV) with a 22Na source. Results show that the average energy, timing, and DOI resolution of all LYSO blocks are 16.13% ± 1.01% at 511 keV, 658.03 ± 15.18 ps, and 2.62 ± 0.06 mm, respectively. The energy and timing resolution of two modules are 16.35% and 0.86 ns, respectively. The in-panel and orthogonal-panel spatial resolution of the two modules at the FOV center are 1.9 and 4.4 mm respectively.

Wavelength discrimination (WLD) detector optimization for time-of-flight positron emission tomography with depth of interaction information

Continuous crystals can potentially obtain better intrinsic detector spatial resolution compared to pixelated crystals, additionally providing depth of interaction (DoI) information from the light distribution. To achieve high performance sophisticated interaction position estimation algorithms are required. There are a number of algorithms in the literature applied to different crystal dimensions and different photodetectors. However, the different crystal properties and photodetector array geometries have an impact on the algorithm performance. In this work we analysed, through Monte Carlo simulations, different combinations of realistic crystals and photodetector parameters to better understand their influence on the interaction position estimation accuracy, with special emphasis on the DoI. We used an interaction position estimation based on an analytical model for the present work. Different photodetector granulation schemes were investigated. The impact of the number of crystal faces readout by photodetectors was studied by simulating scenarios with one and two photodetectors. In addition, crystals with different levels of reflection and aspect ratios (AR) were analysed. Results showed that the impact of photodetector granularity is mainly shown near the edges and specially in the corners of the crystal. The resulting intrinsic spatial resolution near the centre with a 12 × 12 × 10 mm3 LYSO crystal was 0.7–0.9 mm, while the average spatial resolution calculated on the entire crystal was 0.77 ± 0.18 mm for all the simulated geometries with one and two photodetectors. Having front and back photodetectors reduced the DoI bias (Euclidean distance between estimated DoI and real DoI) and improved the transversal resolution near the corners. In scenarios with one photodetector, small AR resulted in DoI inaccuracies for absorbed events at the entrance of the crystal. These inaccuracies were slightly reduced either by increasing the AR or reducing the amount of reflected light, and highly mitigated using two photodetectors. Using one photodetector, we obtained a piecewise DoI error model with a DoI resolution of 0.4–0.9 mm for a 1.2 AR crystal, and we observed that including a second photodetector or reducing the amount of reflections reduced the DoI bias but did not significantly improve the DoI resolution. Translating the piecewise DoI error model obtained in this study to image reconstruction we obtained a spatial resolution variability of 0.39 mm using 85% of the FoV, compared to 2.59 mm and 1.87 mm without DoI correction or with a dual layer system, respectively.

We present the first experimental evaluation of a depth-of-interaction (DOI) positron emission tomography (PET) detector using a digital silicon photomultiplier (dSiPM). To measure DOI information from a mono-layer array of scintillation crystals with a single-ended readout, our group has previously proposed and developed a new method based on light spread using triangular reflectors. Since this method relies on measurement of the light distribution, dSiPM, which has a fully digital interface, has several merits for our DOI measurement. The DOI PET detector comprised of a dSiPM sensor (DPC-3200-22-44) coupled with a 14 × 14 array of 2 mm × 2 mm × 20 mm unpolished LGSO crystals. All crystals were covered with triangular reflectors. To obtain a good performance of the DOI PET detector, several parameters of detector were selected as a preliminary experiment. Detector performance was evaluated with the selected parameters and the optimal experimental setup, and a DOI measurement was conducted by irradiating the crystal block at five DOI positions spaced at intervals of 4 mm. Maximum-likelihood estimation was employed for DOI positioning and the optimal DOI estimation scheme was also investigated in this study. As a result, the DOI PET detector showed clear crystal identification. The energy resolution (full-width at half-maximum (FWHM)) averaged over all depths was 10.21% ± 0.15% at 511 keV, and time resolution averaged over all depths was 1198.61 ± 39.70 ps FWHM. The average DOI positioning accuracy for all depths was 74.22% ± 6.77%, which equates to DOI resolution of 4.67 mm. Energy and DOI resolutions were uniform over all crystal positions except for the back parts of the array. Furthermore, additional simulation studies were conducted to verify the results of our DOI measurement method that is combined with dSiPM technology. In conclusion, our continuous DOI PET detector coupled with dSiPM is a promising PET/MRI detector with DOI-encoding capability.

Depth of interaction (DOI) errors in commonly employed crystal detectors cause severe degradation of off-axis resolution in current positron emission tomography (PET) scanners used for small animal or single organ imaging. We propose a PET camera design that uses low cost, easily fabricated, lead-walled straw (LWS) detectors. Photons converted in Pb generate photoelectrons that escape into the straw interior producing an avalanche whose position can be readily measured with high precision. Straws can be of considerable length (up to 50 cm), but small in diameter (2 mm), and can be packed to form scanners with high sensitivity and large fields of view (FOV). The rigorous three-dimensional position encoding eliminates DOI errors encountered in crystal detectors, and provides submillimeter resolution and sensitivity response that is uniform over the full extent of the FOV. We present preliminary results of the operation of a prototype 200-straw module. We then propose a PET scanner design that consists of two rotating trapezoidal detector panels, each panel containing fourteen 200-straw modules (a total of 5600 straws). The FOV has a diameter of 5 cm and extends axially to 15 cm. This unique geometry affords a long field of view, with near constant axial sensitivity of 4.7%, and uniform resolution of 0.9 times 0.9 times 1.0 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> maintained over the entire extent of the FOV, as determined in Monte Carlo simulations.

In this paper, a Compton camera capable of estimating depth of interaction (DOI) in the detector was designed and optimized using Geant4. The proposed DOI Compton camera consists of two pixelated CsI(Tl) scintillators coupled with digital silicon photomultipliers [Digital Photon Counter (DPC)-3200-22]. The parameters affecting the performance of a Compton camera were evaluated for optimization in terms of image sensitivity and image resolution with a measure of figure of merit. The considered parameters were: 1) type of the reflector surface; 2) thickness of the scintillator; and 3) the distance between the scatter and the absorber detectors. The simulation results showed that a ground white surface is better than a polished white one. The optimal thickness and the scatter-absorber distance were found to be 15 mm and 4 cm, respectively. With the optimized parameters, performance of the DOI Compton camera was estimated for a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source at 1 m using Geant4. The image resolutions were 6.06° full-width at half-maximum with the list-mode maximum likelihood expectation maximization algorithm. It took 20 s to take Compton images for the gamma ray sources of 0.1 μSv/h intensity with the DOI Compton camera. It was shown that there was a clear improvement by employing DOI estimation algorithm in combination with the use of thick scintillators for a Compton camera.

This study focuses on Monte-Carlo (MC) based techniques to calculate the system matrix (SM) for iterative image reconstruction of small animal PET data. Our goal is to determine in advance if some simplifications can be done to accelerate the MC simulation without jeopardizing the accuracy of the system model. For most PET scanners, the calculation of the SM would imply extremely long simulation times, even when considering symmetries. Our small animal scanner prototype consists of a continuous LSO crystal (42 times 42 times 10 mm3 ) attached to a flat panel position sensitive photomultiplier tube which was simulated using GATE. To compute the SM elements, we investigated three different radioactivity distribution models: (a) homogeneous distribution within the voxel, (b) one unique emission point at the center of the voxel and (c) eight point sources distributed within the voxel. Each of these models was simulated for five representative voxel positions inside the field of view (FOV), two of these voxels in the inner region of the FOV and the other three voxels were in the peripheral region of the FOV. The storage and discretization of the SM elements was performed by means of: (i) tube-of-response (TOR) histograms and (ii) sinograms. By comparing the SM's elements generated for each distribution model at each voxel's position, the accuracy of the models was studied. The results show that in the inner region of the FOV, model C distribution yields the best trade-off between simulation time and SM accuracy. Whereas, for the peripheral region of the FOV, model B yields the best compromise. These results are independent of the discretization process considered. Since the scanner is capable to yield depth-of-interaction (DOI) information, the study was performed taking DOI into account

The performance characteristics of the SPECT sub-system S102 of the ALBIRA II PET/SPECT/CT are analyzed for the 80mm field of view (FOV) to evaluate the potential in-vivo imaging in rats, based on measurements of the system response for the commonly used Technetium-99m (99mTc) in small animal imaging. The ALBIRA II tri-modal µPET/SPECT/CT pre-clinical system (Bruker BioSpin, Ettlingen, Germany) was used. The SPECT modality is made up of two opposite gamma cameras (Version S102) with Sodium doped Cesium Iodide (CsI(Na)) single continuous crystal detectors coupled to position-sensitive photomultipliers (PSPMTs). Imaging was performed with the NEMA NU-4 image quality phantom (Data Spectrum Corporation, Durham, USA). Measurements were performed with a starting activity concentration of 4.76MBq/mL 99mTc. An energy window of 20% at 140keV was selected in this study. The system offers a 20mm, 40mm, 60mm and an 80mm field of view (FOV) and in this study the 80mm FOV was used for all the acquisitions. The data were reconstructed with an ordered subset expectation maximization (OSEM) algorithm. Sensitivity, spatial resolution, count rate linearity, convergence of the algorithm and the recovery coefficients (RC) were analyzed. All analyses were performed with PMOD and MATLAB software. The sensitivities measured at the center of the 80mm FOV with the point source were 23.1 ± 0.3 cps/MBq (single pinhole SPH) and 105.6 ± 5.5 cps/MBq (multi pinhole MPH). The values for the axial, tangential and radial full width at half maximum (FWHM) were 2.51, 2.54, and 2.55mm with SPH and 2.35, 2.44 and 2.32mm with MPH, respectively. The corresponding RC values for the 5mm, 4mm, 3mm and 2mm rods were 0.60 ± 0.28, 0.61 ± 0.24, 0.29 ± 0.11 and 0.20 ± 0.06 with SPH and 0.56 ± 0.20, 0.50 ± 0.18, 0.38 ± 0.09 and 0.23 ± 0.06 with MPH. To obtain quantitative imaging data, the image reconstructions should be performed with 12 iterations. The ALBIRA II preclinical SPECT sub-system S102 has a favorable sensitivity and spatial resolution for the 80mm FOV setting for both the SPH and MPH configurations and is a valuable tool for small animal imaging.

Positron emission tomography (PET) and optical imaging are two major techniques for molecular imaging. The PET scanner which enables simultaneous detection of optical imaging will be a powerful tool in molecular imaging. Here, we propose a new detector that detects both signals: annihilation photons for PET and photons for optical imaging. It is based on a four layered depth-of-interaction (DOI) PET detector. Since the detector must be set at a close distance to an object for optical imaging, the DOI information becomes important to obtain uniform spatial resolution in PET imaging over the field-of-view. The proposed PET/optical imaging detector consists of a scintillation crystal block and a position sensitive photo multiplier tube (PS-PMT). While the bottom of the crystal block is optically coupled to the PS-PMT, four side surfaces of the crystal block are covered with reflectors. A dichroic mirror is placed on the top of the crystal block. The dichroic mirror allows fluorescence photons (longer wavelength than 600 nm) to pass through while it reflects scintillation photons (wavelength around 450 nm) generated within the crystal block. Owing to the dichroic mirror, both scintillation photons originating inside the crystal block and fluorescence photons coming from outside the block can be detected by the same PS-PMT. To evaluate of the DOI-PET/ optical detector, we first measured the influence of the dichroic mirror. The results show that the dichroic mirror maintains DOI-PET detector performance without significant loss of fluorescence photons for optical imaging.

Depth-of-interaction enhanced Compton camera using pixelated LYSO(Ce) scintillator arrays and dual-ended SiPMs.

This paper is about 3D image reconstruction from a limited set of computed tomography (CT) projections. We focus on configurations with very limited angle of view and on applications in which the image to be reconstructed is composed of one or several localized objects laying in a known background. We propose an original method based on the detection of the localized object voxels and on a sparse modeling of the image. Reconstruction is done by computing the maximum a posteriori estimator of the image parameters. To implement image reconstruction, we adopt a multigrid strategy in which coarse-to-fine resoluted images are successively reconstructed. This strategy provides detection of localized object voxels as well as accurate initial solutions at each resolution level. Each optimization stage is carried out by using an iterative deterministic descent algorithm. We propose a convergent single site update algorithm which consists of successive constrained optimizations with respect to one voxel at a time. We show the performance of the multigrid method on simulated data corresponding to a set of limited angle cone-beam projections of a synthetic image. The results are accurate, while both memory storage and numerical time of computation are dramatically reduced compared with the monogrid method.

Resolution-recovery-embedded image reconstruction for a high-resolution animal SPECT system