The PET components in the most recent generation of combined PET/CT scanners are equipped with time-of-flight (TOF) capability. The premise for TOF PET is illustrated in , which also shows the point spread function (PSF) for 2 source positions. When a PET radioisotope decays, it emits a positron that annihilates with a nearby electron, giving rise to a pair of 511-keV photons emitted simultaneously in (nearly) opposite directions. If both these photons interact with and are detected by the PET tomograph, they give rise to a prompt coincidence event—providing the primary imaging signal measured by the scanner. When the annihilation event occurs at the midpoint of the line-of-response (LOR) between the detector elements, both photons reach the detector at the same instant in time. However, when the annihilation event occurs away from the midpoint of the LOR, one photon travels a shorter distance than the other and reaches the corresponding detector earlier.
FIGURE 1 Recent advances in PET technology include tomographs with time-of-flight capability and point spread function modeling. TOF PET utilizes very fast detectors and electronics to measure the time difference between detection of each photon of annihilation (more ...)
In TOF PET, the time difference ΔT between detection of each annihilation photon is measured using very fast detectors and electronics. This information is used to estimate the distance along the LOR where the annihilation occurred: d = cΔT/2, where distance d = 0 is the midpoint of the LOR and c is the speed of light (2.998 × 108
m/s). Though TOF PET was first explored in the early 1980s (1
), the extremely fast timing resolution (sub-nanosecond) required to yield benefits in image quality precluded development of clinical TOF tomographs until many years later. Recent advancements in fast, high-sensitivity scintillators and detector electronics have made clinical TOF PET tomographs a reality (4
At the ultimate limit, TOF PET could potentially localize annihilation events to within a single image voxel—effectively measuring the activity distribution directly and eliminating the need for tomographic reconstruction. However, this would require a timing-resolution of approximately 10 ps to isolate events to within a 3-mm voxel. Current TOF systems have timing resolutions closer to 600 ps, yielding TOF depth resolutions on the order of 9 cm. Time-of-flight PET with these timing resolutions does not directly
lead to an improvement in the spatial resolution of the reconstructed image. It does, however, reduce noise propagation by localizing events along segments of each LOR rather than spreading statistical noise across the full length of each LOR. This reduction in noise can be interpreted as an effective gain in signal-to-noise that is dependent on the object size (8
), and this improvement may support better spatial resolution in the final image.
Recent work with TOF PET has demonstrated improvement in image characteristics such as spatial resolution, contrast, and voxelwise noise (5
). Such measures directly relate to quantitative imaging tasks such as computing standardized uptake values (SUVs), but they are less predictive of an observer's ability to detect lesions and distinguish them from a noisy background. Surti et al. assessed a detectability measure in simulations (6
), and later in cylindric phantoms (11
), demonstrating improvement due to TOF. This work assesses the impact of TOF upon observer performance for detecting focal lesions using localization receiver operating characteristics (LROC) analysis (12
). An advanced whole-body phantom with distributed lesions was used to mimic whole-body oncologic PET with 18
F-FDG, relating to both cancer detection and staging. This custom phantom was previously used to compare the lesion-detection performance of several PET tomographs (14
) and reconstruction algorithms (15
). The phantom was imaged repeatedly on a prototype TOF PET/CT (Siemens Medical Solutions) scanner with cerium-doped lutetium oxyorthosilicate (LSO) crystals, producing numerous lesion-present and lesion-absent images with known truth. Lesion-detection performance for iterative reconstructions with and without TOF was assessed using both human and model (numeric) observers. The degree of improvement with TOF was compared to that attained by modeling the spatially-variant point spread function (PSF) during reconstruction. The results are discussed, and conclusions drawn regarding the impact of TOF PET for clinical applications. Such applications are illustrated with 2 cancer patient studies acquired on a PET/CT scanner with TOF capability.