Optimization of the Energy Window for PETbox4, a Preclinical PET Tomograph With a Small Inner Diameter

Area of Science:

• Nuclear Medicine
• Medical Imaging Physics
• Pre-clinical Imaging

Background:

• Small animal positron emission tomography (PET) systems often use close geometry configurations, necessitating distinct data acquisition protocols compared to large-diameter systems.
• Geometrical factors significantly influence tomograph characteristics, impacting the optimization of data acquisition strategies for pre-clinical PET scanners.

Purpose of the Study:

• To optimize the energy window for data acquisition on the PETbox4 pre-clinical PET scanner with a 50 mm detector separation.
• To evaluate the impact of energy window optimization on noise equivalent count rate (NECR), scatter fraction (SF), and image quality metrics.

Main Methods:

• Utilized the Geant4 Application for Tomographic Emission (GATE) simulation toolkit with a voxelized mouse phantom to estimate event fractions.
• Developed custom code to classify gamma interactions, identifying trues, phantom scatters, randoms, and three types of detector backscatter events.
• Optimized the energy window based on NECR and SF, and validated results using the National Electrical Manufacturers Association (NEMA) NU-4 image quality phantom for uniformity, spillover ratio (SOR), and recovery coefficient (RC).

Main Results:

• Identified three types of detector backscatter events in addition to standard event categories.
• Optimized energy window settings (lower-level discriminators from 150 keV to 450 keV) based on NECR and SF.
• Demonstrated that a narrower energy window (350-650 keV) makes detector backscatter rejection unnecessary, significantly reducing SOR for water (45%) and air (31%) chambers without compromising uniformity.

Conclusions:

• The optimized energy window significantly improves image quality by reducing scatter and backscatter events in small animal PET imaging.
• The findings suggest that detector backscatter rejection can be avoided with appropriate energy window settings in PETbox4.
• The optimization methodology is transferable and beneficial for other small inner diameter geometry scanners in pre-clinical research.