PET image reconstruction using physical and mathematical modelling for time of flight PET-MR scanners in the STIR library

Wadhwa, Palak; Thielemans, Kris; Efthimiou, Nikos; Wangerin, Kristen A.; Keat, Nicholas; Emond, Elise; Deller, Timothy; Bertolli, Ottavia; Deidda, Daniel; Delso, Gaspar; Tohme, Michel; Jansen, Floris; Gunn, Roger N.; Hallett, William; Tsoumpas, Charalampos

doi:10.1016/j.ymeth.2020.01.005

Cited by 19 publications

(10 citation statements)

References 25 publications

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“…Another area where wall-free phantoms could play an important role is in the development of new image reconstruction techniques. Novel techniques are being developed all the time [23,24], and it is essential that the imaged object has a welldefined shape and known activity concentration. Often, this requirement is met by using traditional phantoms, but the potential to use more complex geometries such as those seen in clinical practice (tumours [2,25] and adrenal glands [9]) will potentially enable better algorithms to be developed by being more life-like or even more challenging.…”

Section: Discussionmentioning

confidence: 99%

3D printing 18F radioactive phantoms for PET imaging

et al. 2021

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Purpose Phantoms are routinely used in molecular imaging to assess scanner performance. However, traditional phantoms with fillable shapes do not replicate human anatomy. 3D-printed phantoms have overcome this by creating phantoms which replicate human anatomy which can be filled with radioactive material. The problem with these is that small objects suffer to a greater extent than larger objects from the effects of inactive walls, and therefore, phantoms without these are desirable. The purpose of this study was to explore the feasibility of creating resin-based 3D-printed phantoms using 18F. Methods Radioactive resin was created using an emulsion of printer resin and 18F-FDG. A series of test objects were printed including twenty identical cylinders, ten spheres with increasing diameters (2 to 20 mm), and a double helix. Radioactive concentration uniformity, printing accuracy and the amount of leaching were assessed. Results Creating radioactive resin was simple and effective. The radioactive concentration was uniform among identical objects; the CoV of the signal was 0.7% using a gamma counter. The printed cylinders and spheres were found to be within 4% of the model dimensions. A double helix was successfully printed as a test for the printer and appeared as expected on the PET scanner. The amount of radioactivity leached into the water was measurable (0.72%) but not visible above background on the imaging. Conclusions Creating an 18F radioactive resin emulsion is a simple and effective way to create accurate and complex phantoms without inactive walls. This technique could be used to print clinically realistic phantoms. However, they are single use and cannot be made hollow without an exit hole. Also, there is a small amount of leaching of the radioactivity to take into consideration.

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Section: Discussionmentioning

confidence: 99%

3D printing 18F radioactive phantoms for PET imaging

et al. 2021

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“…Indeed, detectordependent factors, such as crystal size and crystal penetration during detection, and inherent limitations such as the noncollinearity of the annihilation photons are also present and can explain why a reduced positron range with an increasing FIGURE 6 | Spatial distribution of the simulated annihilation endpoints in the z/y plane parallel to the magnetic field for a 68 Ga point source positioned at the interface between lung and soft tissue (dashed black line) for a field strength of 0 T (left) and 3 T (right). magnetic field does not translate directly into improved image quality (Herzog et al, 2010;Bertolli et al, 2016;Caribé et al, 2019;Wadhwa et al, 2020). However, in a preclinical setting with small diameter detector rings and crystal sizes, the impact of a reduced positron range on the PET image quality is expected to be much higher while the use of monolithic crystals or recordings of the depth of interaction in clinical PET systems can further enhance the PET resolution such that it becomes more sensitive to positron range effects (Hammer et al, 1994;Stockhoff et al, 2019).…”

Section: Discussionmentioning

confidence: 99%

“…However, it has to be noted that this effect was negligible for low energy positron emitters, such as 18 F. Moreover, the expected improvement of the PET image resolution, resulting from a reduced positron range by the presence of a 3T magnetic field, will only partially be observed in the resolution properties of the GE Signa PET/MR system due to the limited resolution of the PET detectors. Indeed, detector-dependent factors, such as crystal size and crystal penetration during detection, and inherent limitations such as the non-collinearity of the annihilation photons are also present and can explain why a reduced positron range with an increasing magnetic field does not translate directly into improved image quality ( Herzog et al, 2010 ; Bertolli et al, 2016 ; Caribé et al, 2019 ; Wadhwa et al, 2020 ). However, in a preclinical setting with small diameter detector rings and crystal sizes, the impact of a reduced positron range on the PET image quality is expected to be much higher while the use of monolithic crystals or recordings of the depth of interaction in clinical PET systems can further enhance the PET resolution such that it becomes more sensitive to positron range effects ( Hammer et al, 1994 ; Stockhoff et al, 2019 ).…”

Section: Discussionmentioning

confidence: 99%

Monte Carlo Simulations of the GE Signa PET/MR for Different Radioisotopes

et al. 2020

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NEMA characterization of PET systems is generally based on 18 F because it is the most relevant radioisotope for the clinical use of PET. 18 F has a half-life of 109.7 min and decays into stable 18 O via β+ emission with a probability of over 96% and a maximum positron energy of 0.633 MeV. Other commercially available PET radioisotopes, such as 82 Rb and 68 Ga have more complex decay schemes with a variety of prompt gammas, which can directly fall into the energy window and induce false coincidence detections by the PET scanner. Methods: Aim of this work was threefold: (A) Develop a GATE model of the GE Signa PET/MR to perform realistic and relevant Monte Carlo simulations (B) Validate this model with published sensitivity and Noise Equivalent Count Rate (NECR) data for 18 F and 68 Ga (C) Use the validated GATE-model to predict the system performance for other PET isotopes including 11 C, 15 O, 13 N, 82 Rb, and 68 Ga and to evaluate the effect of a 3T magnetic field on the positron range. Results: Simulated sensitivity and NECR tests performed with the GATE-model for different radioisotopes were in line with literature values. Simulated sensitivities for 18 F and 68 Ga were 21.2 and 19.0 /kBq, respectively, for the center position and 21.1 and 19.0 cps/kBq, respectively, for the 10 cm off-center position compared to the corresponding measured values of 21.8 and 20.0 cps/kBq for the center position and 21.1 and 19.6 cps/kBq for the 10 cm off-center position. In terms of NECR, the simulated peak NECR was 216.8 kcps at 17.40 kBq/ml for 18 F and 207.1 kcps at 20.10 kBq/ml for 68 Ga compared to the measured peak NECR of 216.8 kcps at 18.60 kBq/ml and 205.6 kcps at 20.40 kBq/ml for 18 F and 68 Ga, respectively. For 11 C, 13 N, and 15 O, results confirmed a peak NECR similar to 18 F with the effective activity concentration scaled by the inverse of the positron fraction. For 82 Rb, and 68 Ga, the peak NECR was lower than for 18 F while the corresponding activity concentrations were higher. For the higher energy positron emitters, the positron range was confirmed to be

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“…In this paper, we used TOF Listmode Maximum Likelihood-Expectation Maximisation (LM-MLEM) as it is the most robust option and is guaranteed to converge to a solution [47]- [49]. The validation of the TOF reconstruction with Gaussian [49]- [51] and non-Gaussian [52] kernels, has been presented in detail previously. In this study, the application of the TOF kernel is done in a similar manner to that of the simple Gaussian.…”

Section: Image Reconstruction Toolkitmentioning

confidence: 99%

TOF-PET image reconstruction with multiple timing kernels applied on Cherenkov radiation in BGO

Efthimiou¹,

Kratochwil²,

Gundacker³

et al. 2020

Preprint

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Today Time-Of-Flight in PET scanners assumes a unique, well-defined timing resolution for all coincidence events. However, recent BGO-Cherenkov detectors, combining prompt Cherenkov emission and the typical BGO scintillation signal, are capable of sorting events into multiple mixture timing kernels and therefore increase the available information for the image reconstruction. The number of Cherenkov photons detected per event impacts directly the signal rise time, which can therefore be used to improve the timing resolution. In this work, we present a simulation toolkit that outputs data with multiple timing resolutions and image reconstruction that incorporates this information. A full cylindrical BGO-Cherenkov PET model was compared, in terms of contrast recovery and contrast-to-noise ratio, against non-TOF and an LYSO model with time resolution of 213 ps. Two reconstruction approaches for the mixture kernels were tested; mixture Gaussian and decomposed simple Gaussian models. The decomposed model used the exact mixture component applied in the simulation. Images reconstructed using mixture kernels provided similar mean value and less noise as compared to the decomposed simple Gaussian kernels. Although, the later converged faster. Related to the standard LYSO model, the BGO-Cherenkov provided similar contrast, although, in most cases, with more iterations. However, due to the higher sensitivity, the contrast-to-noise ratio was 26.4% better for the BGO model.

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PET image reconstruction using physical and mathematical modelling for time of flight PET-MR scanners in the STIR library

Cited by 19 publications

References 25 publications

3D printing 18F radioactive phantoms for PET imaging

3D printing 18F radioactive phantoms for PET imaging

Monte Carlo Simulations of the GE Signa PET/MR for Different Radioisotopes

TOF-PET image reconstruction with multiple timing kernels applied on Cherenkov radiation in BGO

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