Time-of-Flight PET: Principles, Recent Technical Advancements, and Impact on Image Quality

摘要

Time-of-flight (TOF) PET as developed in the 1980s for brain and cardiac imaging with shortlived radio-isotopes had limited intrinsic sensitivity and spatial resolution relative to conventional PET. As result, despite reduction in image noise characteristics due to TOF information TOF PET never fully overcame its disadvantges. Since then, conventional non-TOF PET has become the primary imaging technique with further gains in spatial resolution and intrinsic sensitivity, as well as the development of fully-3D imaging mode. However, scan times for conventional PET are still relatively long (upto 30 minutes), especially when imaging large patients with high attenuation or patients with low injected activity. In recent years, development of fast, dense, and high light output scintillators has revived interest in the development of TOF PET with few or no significant compromises in the conventional PET performance. These developments promise to achieve high quality PET images with reduced noise characteristics due to the use of TOF information. A description of the new scintillators and their intrinsic performance will be presented, along with the detector arrangements that yield high performance, TOF detector modules. The image reconstruction and correction techniques necessary to achieve a quantitative TOF PET image will also be described.Simulation and measurement results depicting the image quality gains achieved in the latest generation TOF PET scanners will be shown. Our results show that TOF information leads to a faster convergence to maximum contrast recovery for hot lesions using an iterative reconstruction algorithm. This is also generally achieved at a lower noise value compared to non-TOF reconstructions. Lesion detectability also improves as timing resolution improves with greater gains in larger phantoms. For light patients this can translate into a reduced imaging time (to achieve image quality similar to that from a non-TOF scanner), while for heavier patients this can potentially lead to a substantial improvement in image quality (for similar imaging times as for a non-TOF scanner). Finally, future developments that can enhance the performance of TOF PET scanners will be described and selective results shown. At the detector end, new scintillators promise to further improve system timing resolution from currently about 600ps to 300ps or lower. Together with an optimized detector this promises to further push the system timing capabilities. For image generation, fast and efficient algorithms are being developed which can significantly reduce the image reconstruction time, making TOF PET a practical day-to-day imaging modality in a nuclear medicine clinic.