PET instrumentation has undergone several incremental improvements in recent years. This chapter will discuss recent technology advances for PET/CT and PET/MR, which include improved sensitivity, new detectors, digital PET, improved electronics advanced reconstructions, implementation of time-of-flight (TOF), and motion compensation methods. We also review how these high-quality CT component and hybrid PET/MR systems enable some additional potential applications for advanced hybrid cardiac imaging.
Historically, PET scanners operated in 2D mode, with interplane septa between slices and with each slice reconstructed independently. This technique had the advantage of reducing the scatter, randoms, and singles rate of the PET scanner. This improvement in image quality came at the expense of system sensitivity. In 3D PET, there are no interplane septa that physically separate detector rings along the axial dimension, which improves count sensitivity by a factor of 4–6, as compared to traditional 2D PET scanners with interplane septa. However, the disadvantage of 3D acquisitions is an increased scatter fraction, with the number of scattered events detected as a percentage of all coincidence events in the 30%-40% range.1 As the technology for reducing system dead time and scatter correction improved, most instrumentation providers embraced the 3D imaging mode, with many modern PET/CT systems being built without a 2D option.
Importantly for first-pass cardiac imaging, with short-lived tracers such as 82Rb or 15O water, the number of single events is very high, and consequently so is the number of multiple random coincidence events (random events), which are primarily produced by the simultaneous occurrence of a true coincidence event and a single event or two single events in the detectors. This may lead to event pileups and saturation during initial phases of the first-pass cardiac imaging with PET. Clinical PET studies optimize the injected doses of 82Rb for the new 3D PET scanners, thereby avoiding the saturation issues.2 Manufacturers also introduced specially designed high-count rate detectors to withstand the high-count rates during initial phases of dynamic 82Rb imaging. However, challenges still remain with analog PET systems for the high-count rate with 82Rb.3 Despite these potential technical difficulties with 3D PET kinetic imaging, the advantages of lower injected doses and a lower overall radiation burden for the patient make 3D PET protocols preferred.
When performing 3D PET imaging with short-lived tracers such as 82Rb, attention must be paid to the approximately 13% of nuclear decays associated with a 776 keV prompt-gamma emission. These prompt gammas occur when the 82Rb nucleus decays into an unstable final state, which then decays to its ground state releasing the 776 keV photon. These prompt-gamma photons can downscatter into the 511 keV PET energy window. Prompt-gamma corrections have been now incorporated in the 3D reconstruction process together with corrections for standard scattered events. This was not necessary for 2D PET imaging, but is very important for ...