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Enhancing Hybrid PET Imaging with Variable Focal Length Fan-Slat Collimators

This study presents a novel design for a variable focal length fan-slat collimator aimed at optimizing hybrid PET imaging. The collimator enhances the count rate by capturing more true coincidence events while effectively blocking singles, randoms, and scatter events. Through Monte Carlo simulations, we demonstrate improved sensitivity in the center of the field of view (FOV) and reduced noise levels, especially in cardiac imaging. The findings indicate that this design significantly benefits visualization of lesions and enhances image quality for various applications.

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Enhancing Hybrid PET Imaging with Variable Focal Length Fan-Slat Collimators

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  1. Improved Hybrid PET Imaging Using Variable Focal Length Fan-Slat Collimators Thomas C. Rust and Dan J. Kadrmas, Ph.D. Medical Imaging Research Laboratory Department of Radiology University of Utah

  2. Objectives • To design a collimator which optimally utilizes the available count rate of cameras to process as many true coincidence events as possible, while blocking as many singles/randoms and scatters as possible • Presently, to design and evaluate a variable focal length fan-slat collimator for hybrid PET

  3. Variable Focal Length Fan-Slat Design • High sensitivity to center of FOV • Creates a Pseudo-3D geometry • Improved shielding of singles arising from outside the FOV • Good shielding of scattered events • Variable Fan-beam focused on a central volume-of-interest (VOI) • Focal point for outermost septa at midpoint between opposing heads • Focal point for innermost septa at opposing head or beyond Parallel Slat Variable Focal Length Fan-Slat

  4. Simulated Axial Sensitivity Profiles • Converging designs block singles near edge and outside FOV, and increase peak trues sensitivity in the center of the FOV • 3X Variable Fan-Slat has a broader trues peak than normal Fan-Slat

  5. Methods: Monte Carlo Simulations • GEANT Monte Carlo (CERN) • Simulated physics of attenuation & scatter in object and entire detector, with full record of all scintillating events • Penetration, partial energy deposition, energy resolution, etc. • Three-head gamma camera system • 60 - 120 x 106 annihilation pairs/simulation for low noise • Cylindrical phantom 120cm long  30cm diameter filled with water and positron emitting source • Results analyzed in terms of singles- & trues-sensitivity, scatter fraction, and noise equivalent count (NEC)

  6. Performance Comparisons for 3-head System • Increased trues sensitivity & NEC in the VOI (center 1/3rd of FOV) • Whole body NEC equal to nominal parallel slat design • Reduced scatter fraction in comparison to fully 3-D imaging

  7. Methods: 18F Cardiac Phantom Experiment • Anthropomorphic cardiac torso phantom (Data Spectrum, Inc) • Cardiac viability study activity distribution • Marconi IRIX 3-head hybrid PET gamma camera • 30 min scan at same count rate for both collimators • 105 min delay between Parallel and Variable Fan-Slat • Reconstruction methods • 2D: SSRB -> OSEM 2/12 • 3D: Fully-3D OSEM (3D-TAB) 2/12

  8. Post-filter: 7.7mm Gauss No post-filter: Parallel Var. Fan Parallel Var. Fan

  9. Short-axis Views of Phantom Myocardium Parallel Var. Fan • Improved visualization of uniform activity in heart wall

  10. Whole Body Phantom Experiment w/Hot Lesions: Parallel Slat MIP Variable Fan-Slat MIP • Hoffman Brain, RSD Thorax (modified), and elliptical cylinder pelvis. • 8 lesions placed in mediastinum and lungs • 4 – 22 mm inner diameter • 3:1 – 20:1 target:soft tissue background • 18F-FDG clinical activity distributions

  11. Transaxial Slices of Individual Lesions Parallel Var. Fan

  12. Summary & Conclusions • A Variable Focal Length Fan-Slat collimator was designed with shorter focus for outer septa and longer for inner septa • Variable Fan-Slat has high, broadly-peaked trues sensitivity profile, and reduces singles at the edge and outside FOV • While whole-body NEC of Variable Fan-Slat design is equal to Nominal Parallel, VOI NEC is double (triple conv. parallel) • Cardiac phantom experiments demonstrated 3D Variable Fan-Slat images had reduced noise compared to 2D Parallel Slat

  13. Summary & Conclusions • Variable Fan-Slat collimators improved visibility of uniform activity in short-axis views of cardiac phantom • In whole-body phantom with hot lesions, Variable Fan-Slat improved lesion visualization and contrast • Variable focal length fan-slat collimators have shown great potential for improving hybrid PET image quality, especially for VOI applications

  14. Acknowledgements • American Cancer Society Research Project Grant RPG-00-200-01-CCE • Nuclear Fields Australia

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