Improved Hybrid PET Imaging Using Variable Focal Length FanSlat Collimators

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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

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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

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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
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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

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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)

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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

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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 -gt OSEM 2/12
• 3D Fully-3D OSEM (3D-TAB) 2/12

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No post-filter
Parallel
Var. Fan

• Post-filter 7.7mm Gauss

Parallel
Var. Fan
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Short-axis Views of Phantom Myocardium
Parallel
Var. Fan

• Improved visualization of uniform activity in
  heart wall

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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
• 31 201 targetsoft tissue background
• 18F-FDG clinical activity distributions

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Transaxial Slices of Individual Lesions
Parallel
Var. Fan
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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

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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

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Acknowledgements

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