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Improved Hybrid PET Imaging Using Variable Focal Length Fan-Slat Collimators

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. Objectives.

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Improved Hybrid PET Imaging Using 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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