Development of organ-specific SPECT system using variable pinhole collimator

다국어 초록 (Multilingual Abstract)

Single-photon emission computed tomography (SPECT) is a nuclear medical imaging method enabling the user to view functional images of patients. The collimator, which is an essential component of the SPECT, limits the direction of the incident gamma rays, such that the distribution of the gamma photons from the body can be observed. Collimators are composed of shielding materials with high atomic number and density; thus, it is difficult to fabricate them in complex shapes. Among the existing collimators, the pinhole collimator consists of a small aperture perforated in a shielding material, and the modification of the pinhole parameters, such as the hole diameter and acceptance angle, during the scan is also challenging. A variable pinhole (VP) collimator comprises several thin tungsten layers with various hole sizes. Thus, the pinhole parameters can be varied for the region of interest (ROI) by forming the desired pinhole shape using a combination of holes. In this study, we implemented the concept of a VP collimator and applied it in a SPECT system to enhance its performance. We manufactured the prototype VP collimator consisting of seven layers with a diameter of 240 mm and a thickness of 0.5 mm and with 16 holes of different sizes in each layer. Two-point sources (Co-57, 122 keV) were used for the performance analysis by changing the system parameters. The spatial resolution and sensitivity of the SPECT system were affected by the variation of the magnification, with the peak-to-valley ratio increasing by 3.3 times, counts increasing by 8.81 times, and full-width half-maximum (FWHM) value decreasing by 4.69 times, with the increase in the magnification of the SPECT system. The spatial resolution and sensitivity of the SPECT system were also affected by the pinhole diameters, indicating that pinhole diameters suitable for high-resolution and high-sensitivity systems can be used, depending on the purpose. The improved VP collimator was composed of five layers of with a diameter of 170 mm and a thickness of 1 mm and with six holes of different sizes in each layer. Two line source phantoms (Tc-99m, 140 keV) with an internal diameter of 1 mm were used for the performance evaluation of the system. The phantoms were positioned 20 mm apart inside the ROI with a diameter of 50 mm at a position of 29 mm from the object center. By applying the VP collimator SPECT system, the resolution and the sensitivity performance were improved, achieving an FWHM value of 2.7 times and counts of 2.8 times those of the conventional SPECT system. Lastly, we evaluated the performance by upgrading the detector and by resetting the ROI and driving parameters to improve the resolution performance of the system. Two line source phantoms with 9 mm intervals and a Derenzo-like phantom were positioned inside the ROI with a diameter of 30 mm at a position of 30 mm from the object center. By applying the VP collimator SPECT system, the resolution performance was improved, achieving an FWHM value 3.4 times higher than that of the conventional SPECT system with the same number of counts from the line source. In the phantom experiment, only the overall shape of the phantom can be distinguished, using the conventional SPECT, whereas the second smallest rod with 2 mm diameter can be distinguished, using VP collimator SPECT. In future research, we aim to improve the system efficiency by conducting a simultaneous experimental driving test of the dual-head VP collimator SPECT system.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Motivation 1
• 1.2 Aim of the Study 2
• 2. Background Theories 4
• 2.1 Single-Photon Emission Computed Tomography 4
• 2.2 Detector 6
• 2.3 Collimator 7
• 3. Variable Pinhole Collimator SPECT System 10
• 3.1 VP collimator 10
• 3.2 Geometric Parameters of VP collimator SPECT 12
• 3.3 Simulation Study 15
• 4. Single-Head Prototype of VP Collimator SPECT 19
• 4.1 Development of Single-Head VP Collimator SPECT 19
• 4.2 Magnification Variation Experiment 25
• 4.3 VP Collimator Variation Experiment 27
• 5. Dual-Head Organ-Specific VP Collimator SPECT 30
• 5.1 Development of Dual-Head VP Collimator SPECT 30
• 5.2 Tomographic Line Source Experiment 37
• 5.3 Tomographic Experiment of Improved System 43
• 6. Conclusion 52
• Appendix 55
• References 64
• Curriculum Vitae 68