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

TC Rust and DJ Kadrmas University of Utah 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

TC Rust and DJ Kadrmas University of Utah 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 SlatVariable Focal Length Fan-Slat

TC Rust and DJ Kadrmas University of Utah 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

TC Rust and DJ Kadrmas University of Utah 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 x 10 6 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)

TC Rust and DJ Kadrmas University of Utah Performance Comparisons for 3-head System Increased trues sensitivity & NEC in the VOI (center 1/3 rd of FOV) Whole body NEC equal to nominal parallel slat design Reduced scatter fraction in comparison to fully 3-D imaging

TC Rust and DJ Kadrmas University of Utah Methods: 18 F 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

TC Rust and DJ Kadrmas University of Utah Post-filter: 7.7mm Gauss Parallel Var. Fan No post-filter: Parallel Var. Fan

TC Rust and DJ Kadrmas University of Utah Short-axis Views of Phantom Myocardium Parallel Var. Fan Improved visualization of uniform activity in heart wall

TC Rust and DJ Kadrmas University of Utah Whole Body Phantom Experiment w/Hot Lesions: 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 18 F-FDG clinical activity distributions Parallel Slat MIPVariable Fan-Slat MIP

TC Rust and DJ Kadrmas University of Utah Transaxial Slices of Individual Lesions Parallel Var. Fan

TC Rust and DJ Kadrmas University of Utah 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

TC Rust and DJ Kadrmas University of Utah 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

TC Rust and DJ Kadrmas University of Utah Acknowledgements American Cancer Society Research Project Grant RPG CCE Nuclear Fields Australia