Evaluation of a brain PET/MR system & creation and dosimetric analysis of an imaging pre-clinical database through GATE
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Κωστού, Θεοδώρα
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Abstract
The purpose of the current study is the use of Gate MC Simulations for Imaging System and for pre- clinical Studies modeling.
First, integrated PET / MR technology is optimized for human brain imaging in visualizing neurotransmitter pathways and their disruptions in order to improve overall diagnostic accuracy. In this study, we develop and compare two different portable prototype MRI-compatible PET systems dedicated to brain imaging, considered for construction in the framework of the EU funded MINDView project. One of the objectives of the MINDView project is the construction of PET, MR compatible, inserts achieving a combination of high and uniform spatial resolution throughout the field of view (FOV) with high sensitivity by exploiting depth of interaction (DOI) information on small ring diameters.
The first model, with 140 mm inner radius and 150 mm axial FOV, is comprised of a three-layer pixellated LYSO:Ce trapezoidal form crystal. The second design, based on a 165 mm inner radius and 150 mm axial FOV, is comprised of a rectangular monolithic LYSO:Ce crystal. The expected performance of the two proposed designs was evaluated using GATE MC Simulations, including the definition of the most suitable geometrical and readout configuration for overall PET performance optimization. The spatial resolution and sensitivity of the two scanners are investigated and compared based on a combination of the industry accepted NEMA standards.
Spatial resolution results indicate comparable performance for the two system designs at the center of the FOV (< 1mm) although the pixelated crystal based system has an improved spatial resolution away from the centre of the FOV (2.5 mm vs. 2 mm) but a smaller sensitivity.
The diameter, the kind and the shape of the crystals play a crucial role in the quality of spatial resolution and sensitivity. Smaller diameters lead to better spatial resolution and monolithic cystals to better sensitivity. So we have to optimize the trade-off between these two measurements.
Secondly, nuclear medicine (NM) and radiation therapy (RT) although well established, are still rapidly evolving, by exploiting animal models, aiming to define precise diagnostic, dosimetric and therapeutic protocols. The second purpose of the study is the design of a database using pre-clinical experimental data derived from literature and original simulated data, for research imaging purposes and assessment of the dosimetric characteristics of commonly used radionuclides. In addition, specific dosimetric calculations were performed in two different whole-body mouse models, with varying organs’ size, so as to determine their impact on absorbed doses and S-values. MC techniques with validated codes can serve as gold standard for such applications using computational phantoms, as the ground truth is known.
The GATE MC simulation toolkit was used. For the imaging studies we have simulated in total seven 4D digital MOBY mouse phantoms using SPECT tracers; a) five normal bio-distributions of 99mTc-MDP, 99mTc-MIBI, 99mTc-HMPAO, 99mTc-NBRH1 and 125I-HPP and b) three additional bio-distributions of tumor imaging with 99mTc-DTPA and a 99mTc-RGD peptide. Furthermore, two mouse phantoms, 28 and 34 gr respectively, were modeled based on realistic preclinical exams, to calculate the absorbed doses and S-values of five commonly used radionuclides in nuclear imaging studies namely 18F, 68Ga, 177Lu, 111In and 99mTc. The simulation procedure was validated using a 30 gr mouse phantom, by comparing the extracted S-values with already published results, using the spectrum of an 131I source. The small animal SPECT prototype system, which was used for the original acquisitions was modeled and validated in GATE and the QSPECT open-source software was used for data reconstruction with Maximum Likelihood Expectation Maximization (MLEM) algorithm.
Quantitative analysis was performed within several organs for each case and line profiles of the simulated and the clinical data are presented. The calculation of the S-values using a 131I source for several source and target organs showed very good agreement with already published S-values, with differences ranging between 19.76 and 29.20% respectively. The dose per organ in mGy and the S-values in Gy / Bq * sec of all simulated cases is given in the database and the comparison of the dosimetric calculations on the two different phantoms provides the impact of the mouse model size.
The proposed database provides a tool for evaluating preclinical protocols, optimizing absorbed doses for tumor imaging and validating several processing algorithms (reconstruction, filtering, denoising, etc.). S-values are generated to improve the accuracy of the absorbed dose calculations and demonstrate that the mouse model size is a crucial parameter.
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MC simulations, Voxelized phantoms, Dosimetry, Imaging systems