Impact of detector parameters and image resolution modeling on dedicated brain PET imaging

High performance brain PET scanners suitable for accurate and precise measurement of uptake in small regions of the brain require high spatial resolution (1.5-2 mm) together with high system sensitivity. While small cross section crystals are needed to achieve high spatial resolution with pixelated detectors, effective system sensitivity requires long crystals and good time-of-flight (TOF) resolution. In addition, depending on crystal thickness, some level of depth-of-interaction (DOI) capability may also be needed to reduce parallax error. While various methods for DOI measurement can be utilized within the detector, the impact on reconstructed spatial resolution as a function of crystal thickness needs to be carefully evaluated. The focus of this work is to use Monte Carlo simulations to evaluate the impact of crystal size, thickness, and DOI resolution on the reconstructed spatial resolution in a high-resolution brain scanner design. Two crystal cross sections were chosen, 1.6 mm × 1.6 mm and 2 mm × 2 mm, and three different LSO crystal thickness (10 mm, 15 mm, and 20 mm) were evaluated. We evaluated these system designs without DOI capability and with DOI resolution (4-12 mm) for varying crystal thickness. In addition, we evaluated the use of image-space resolution modeling (IRM) techniques, which can mitigate losses in spatial resolution and positioning accuracy due to parallax error and Compton scatter, and to complement the advantages of DOI capability. A brain size elliptical phantom with small spheres was simulated to study the overall benefit in uptake measurements for small objects using the contrast recovery coefficient (CRC) metric. Our results show that a system with 2 mm × 2 mm crystals can provide comparable image quality in terms of CRC measurement than one with 1.6 mm × 1.6 mm crystals. However, crystal thickness has a bigger impact on spatial resolution and CRC with performance degrading with best performance achieved with 10 mm thick crystals. DOI measurement is necessary with longer crystals in order to maintain good imaging performance with high sensitivity. For the 15 mm thick crystals 9 mm DOI resolution is needed to compensate for the loss in spatial resolution compared to 10 mm thick crystals without DOI, but still with lower CRC. A similar conclusion is reached for 20 mm crystals with 4 mm DOI resolution. Alternatively, IRM methods can provide improved and similar performance for all crystal thicknesses studied here, indicating a pathway for achieving our brain scanner design goals without additional cost, complexity, and performance tradeoffs. This work provides a systematic investigation for designing future brain PET systems.