Abstract
While the first time-of-flight (TOF)-positron emission tomography (PET) systems were already built in the early 1980s, limited clinical studies were acquired on these scanners. PET was still a research tool, and the available TOF-PET systems were experimental. Due to a combination of low stopping power and limited spatial resolution (caused by limited light output of the scintillators), these systems could not compete with bismuth germanate (BGO)-based PET scanners. Developments on TOF system were limited for about a decade but started again around 2000. The combination of fast photomultipliers, scintillators with high density, modern electronics, and faster computing power for image reconstruction have made it possible to introduce this principle in clinical TOF-PET systems. This paper reviews recent developments in system design, image reconstruction, corrections, and the potential in new applications for TOF-PET. After explaining the basic principles of time-of-flight, the difficulties in detector technology and electronics to obtain a good and stable timing resolution are shortly explained. The available clinical systems and prototypes under development are described in detail. The development of this type of PET scanner also requires modified image reconstruction with accurate modeling and correction methods. The additional dimension introduced by the time difference motivates a shift from sinogram- to listmode-based reconstruction. This reconstruction is however rather slow and therefore rebinning techniques specific for TOF data have been proposed. The main motivation for TOF-PET remains the large potential for image quality improvement and more accurate quantification for a given number of counts. The gain is related to the ratio of object size and spatial extent of the TOF kernel and is therefore particularly relevant for heavy patients, where image quality degrades significantly due to increased attenuation (low counts) and high scatter fractions. The original calculations for the gain were based on analytical methods. Recent publications for iterative reconstruction have shown that it is difficult to quantify TOF gain into one factor. The gain depends on the measured distribution, the location within the object, and the count rate. In a clinical situation, the gain can be used to either increase the standardized uptake value (SUV) or reduce the image acquisition time or administered dose. The localized nature of the TOF kernel makes it possible to utilize local tomography reconstruction or to separate emission from transmission data. The introduction of TOF also improves the joint estimation of transmission and emission images from emission data only. TOF is also interesting for new applications of PET-like isotopes with low branching ratio for positron fraction. The local nature also reduces the need for fine angular sampling, which makes TOF interesting for limited angle situations like breast PET and online dose imaging in proton or hadron therapy. The aim of this review is to introduce the reader in an educational way into the topic of TOF-PET and to give an overview of the benefits and new opportunities in using this additional information.
Highlights
Positron emission tomography (PET) is based on the principle of opposed 511-keV photons originating from the annihilation of emitted positron with a nearby electron
It was shown that there was a significant TOF gain. This gain is difficult to quantify into one single factor, because TOF improves imaging performance of a PET scanner in several ways comparing to non-TOF-PET:
The evaluation on patient data was performed by matching PET and TOF-PET reconstructions, using iterations with equal noise levels in the liver
Summary
Positron emission tomography (PET) is based on the principle of opposed 511-keV photons originating from the annihilation of emitted positron with a nearby electron. The principle of TOF has been proposed in the early days of PET technology (Fig. 1) It has resulted in different developments of prototypes during the 1980s [1,2,3,4,5,6,7]. Due to the limited stopping power of the used scintillators, these systems had limited spatial resolution and sensitivity They were not competitive with the high-density scintillator bismuth germanate (BGO)-based systems, which were developed in the same period. The major technology used were block detectors based on pixelated BGO with light sharing to large photomultiplier tubes (PMTs) These had better spatial resolution than the systems based on CsF or BaF2, which used one-to-one coupling on PMTs (except the SuperPET systems from TerPogossian in late 1980s based on BaF2). During the 1990s, the developments on TOF technology were limited and BGO was the dominant scintillator for PET systems
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