Prediction of Absorbed Dose to Normal Organs with Endocrine Tumors for I-131 by use of 99mTC Single Photon Emission Computed Tomography/Computed Tomography and Geant4 Application for Tomographic Emission Simulation.

INTRODUCTION: This study aimed to predict the dose absorbed by normal organs with neuroendocrine tumors for 131I using single photon emission computed tomography/computed tomography (SPECT/CT) images and Geant4 application for tomographic emission (GATE) simulation. MATERIALS AND METHODS: Four to 5 whole-body planar scan series, along with one SPECT/CT image, were taken from four patients following 99mTc-hynic-Tyr3-octreotide radiotracer injection. After image quantification, the residence time of each organ was calculated using the image analysis and the activity time curves. The energy deposit and dose conversion (S-value) were extracted from the GATE simulation for the target organs of each patient. Using the residence times and S-values, the mean absorbed dose for the target organs of each patient was calculated and compared with the data obtained from the standard method. RESULTS: Very close agreement was obtained between the S-value of the self-organ irradiation. The mean percentage difference between the two methods (i.e. GATE and Medical Internal Radiation Dose [MIRD]) was 1.8%, while a weak agreement was observed for cross-organ irradiation. The percentage difference between the total absorbed doses by the organs was 2%. The percentage difference between the absorbed doses obtained for tumors and three considered normal organs estimated by the GATE method was slightly higher than the MIRD method (about 11% on average for tumors). CONCLUSION: Regardless of the small difference between the obtained results for the organs and absorbed doses of the tumors in the present study, patient-specific dosimetry by the GATE methods is useful and essential for therapeutic radionuclides such as 131I due to high cross-dose effects, especially for young adult patients, to ensure the radiation safety and increase the effectiveness of the treatment.

Attenuation correction in single-photon emission computed tomography for NURBS-based cardiac-torso phantom using dual-energy acquisition.

Single photon emission tomography is widely used to detect photons emitted from the patient. Some of these emitted photons suffer from scattering and absorption because of the attenuation occurred through their path in patient's body. Therefore, the attenuation is the most important problem in single-photon emission computed tomography (SPECT) imaging. Some of the radioisotopes emit gamma rays in different energy levels, and consequently, they have different counts and attenuation coefficients. Calculation of the parameters used in the attenuation equation N out=αNin = e- µ l Nin by mathematical methods is useful for the attenuation correction. Nurbs-based cardiac-torso (NCAT) phantom with an adequate attenuation coefficient and activity distribution is used in this study. Simulations were done using SimSET in 20-70 and 20-167 keV. A total of 128 projections were acquired over 360°. The corrected and reference images were compared using a universal image quality index (UIQI). The simulation repeated using NCAT phantom by SimSET. In the first group, no attenuation correction was used, but the Zubal coefficients were used for attenuation correction in the second image group. After the image reconstruction, a comparison between image groups was done using optimized UIQI to determine the quality of used reconstruction methods. Similarities of images were investigated by considering the average sinogram for every block size. The results showed that the proposed method improved the image quality. This study showed that simulation studies are useful tools in the investigation of nuclear medicine researches. We produced a nonattenuated model using Monte Carlo simulation method and compared it with an attenuated model. The proposed reconstruction method improved image resolution and contrast. Regional and general similarities of images could be determined, respectively, from acquired UIQI of small and large block sizes. Resulted curves from both small and large block sizes showed a good similarity between reconstructed and ideal images.

GATE MODELING OF LATERAL SCATTERING OF PROTON PENCIL BEAMS.

To validate the GATE Monte Carlo simulation code and to investigate the lateral scattering of proton pencil beams in the major body tissue elements in the therapeutic energy range. In this study, GATE Monte Carlo simulation code was used to compute absorbed dose and fluence of protons in a water cubic phantom for the clinical energy range. To apply the suitable physics model for simulation, different physics lists were investigated. The present research also investigated the optimal value of the water ionization potential as a simulation parameter. Thereafter, the lateral beam profile of proton pencil beams were simulated at different energies and depths in body tissue elements. The range results obtained using the QGSP_BIC_EMY physics showed the best compatibility with the NIST database data. Moreover, it was found that the 76 eV is the optimal value for the water ionization potential. In the next step, it was shown that the beam halo can be described by adding a supplementary Gaussian function to the standard single-Gaussian model, which currently is used by treatment planning systems (TPS).

Monte Carlo simulation and performance assessment of GE Discovery 690 VCT positron emission tomography/computed tomography scanner.

The aim of this study is to simulate GE Discovery 690 VCT positron emission tomography/computed tomography (PET/CT) scanner using Geant4 Application for Tomographic Emission (GATE) simulation package (version 8). Then, we assess the performance of scanner by comparing measured and simulated parameter results. Detection system and geometry of PET scanner that consists of 13,824 LYSO crystals designed in 256 blocks and 24 ring detectors were modeled. In order to achieve a precise model, we verified scanner model. Validation was based on a comparison between simulation data and experimental results obtained with this scanner in the same situation. Parameters used for validation were sensitivity, spatial resolution, and contrast. Image quality assessment was done based on comparing the contrast recovery coefficient (CRC) of simulated and measured images. The findings demonstrate that the mean difference between simulated and measured sensitivity is <7%. The simulated spatial resolution agreed to within <5.5% of the measured values. Contrast results had a slight divergence within the range below 4%. The image quality validation study demonstrated an acceptable agreement in CRC for 8:1 and 2:1 source-to-background activity ratio. Validated performance parameters showed good agreement between experimental data and simulated results and demonstrated that GATE is a valid simulation tool for simulating this scanner model. The simulated model of this scanner can be used for future studies regarding optimization of image reconstruction algorithms and emission acquisition protocols.

Assessment of the scatter correction procedures in single photon emission computed tomography imaging using simulation and clinical study.

BACKGROUND: Compton-scattered photons transfer incorrect spatial information. These photons are detected in used photo-peak energy window. In this study, three scatter correction procedures including dual-energy window (DEW), three energy window (TEW), and new approach were evaluated, and then the best procedure based on simulation and clinical conditions introduced. MATERIALS AND METHODS: In this study, simulation projections and three-dimensional nonuniform rational B-spline-based Cardiac-Torso phantoms were produced by GEANT4 application for emission tomography simulation code. For clinical study, 2-day stress/rest myocardial perfusion imaging (MPI) protocol was performed with 99m Tc-sestamibi for 46 patients. Image quality parameters including contrast, signal-to-noise ratio (SNR), and relative noise of the background (RNB) were evaluated. RESULTS: The simulation results showed that contrast values for DEW, TEW, and new approach were (0.45 ± 0.07, 0.5 ± 0.08, and 0.63 ± 0.09), SNR values (4.74 ± 0.94, 5.58 ± 1.08, and 6.56 ± 1.24), and RNB values (0.33 ± 0.06, 0.33 ± 0.07, and 0.33 ± 0.05), respectively. In clinical study, the contrast values for DEW, TEW, and new approach were 0.53 ± 0.03, 0.57 ± 0.07, and 0.62 ± 0.04 in rest MPI and were 0.52 ± 0.04, 0.57 ± 0.06, and 0.6 ± 0.05 in stress MPI, respectively. Moreover, for the rest images, the SNR values were 7.65 ± 1.9, 9.08 ± 2.2, and 10.2 ± 1.75 and for stress images were 7.76 ± 1.99, 9.12 ± 2.25, and 10.17 ± 2.04, respectively. Finally, RNB values for rest and stress images were 0.12 ± 0.03, 0.13 ± 0.03, and 0.13 ± 0.03, respectively. CONCLUSION: The simulation and the clinical studies showed that the new approach could be better performance than DEW, TEW methods, according to values of the contrast, and the SNR for scatter correction.