Time-of-flight (TOF) measurement capability promises to improve PET image quality. We characterized the physical and clinical PET performance of the first Biograph mCT TOF PET/CT scanner (Siemens Medical Solutions USA, Inc.) in comparison with its predecessor, the Biograph TruePoint TrueV. In particular, we defined the improvements with TOF. The physical performance was evaluated according to the National Electrical Manufacturers Association (NEMA) NU 2-2007 standard with additional measurements to specifically address the TOF capability. Patient data were analyzed to obtain the clinical performance of the scanner. As expected for the same size crystal detectors, a similar spatial resolution was measured on the mCT as on the TruePoint TrueV. The mCT demonstrated modestly higher sensitivity (increase by 19.7 ± 2.8%) and peak noise equivalent count rate (NECR) (increase by 15.5 ± 5.7%) with similar scatter fractions. The energy, time and spatial resolutions for a varying single count rate of up to 55 Mcps resulted in 11.5 ± 0.2% (FWHM), 527.5 ± 4.9 ps (FWHM) and 4.1 ± 0.0 mm (FWHM), respectively. With the addition of TOF, the mCT also produced substantially higher image contrast recovery and signal-to-noise ratios in a clinically-relevant phantom geometry. The benefits of TOF were clearly demonstrated in representative patient images.

With the increased interest in new PET tracers, gene-targeted therapy, immunoPET, and theranostics, other radioisotopes will be increasingly used in clinical PET scanners, in addition to 18F. Some of the most interesting radioisotopes with prospective use in the new fields are not pure short-range β+ emitters but can be associated with gamma emissions in coincidence with the annihilation radiation (prompt gamma), gamma-gamma cascades, intense Bremsstrahlung radiation, high-energy positrons that may escape out of the patient skin, and high-energy gamma rays that result in some e+/e− pair production. The high level of sophistication in data correction and excellent quantitative accuracy that has been reached for 18F in recent years can be questioned by these effects. In this work, we review the physics and the scientific literature and evaluate the effect of these additional phenomena on the PET data for each of a series of radioisotopes: 11C, 13N, 15O, 18F, 64Cu, 68Ga, 76Br, 82Rb, 86Y, 89Zr, 90Y, and 124I. In particular, we discuss the present complications arising from the prompt gammas, and we review the scientific literature on prompt gamma correction. For some of the radioisotopes considered in this work, prompt gamma correction is definitely needed to assure acceptable image quality, and several approaches have been proposed in recent years. Bremsstrahlung photons and 176Lu background were also evaluated.Electronic supplementary materialThe online version of this article (doi:10.1186/s40658-016-0144-5) contains supplementary material, which is available to authorized users.

Time-of-flight (TOF) PET uses very fast detectors to improve localization of events along coincidence lines-of-response. This information is then utilized to improve the tomographic reconstruction. This work evaluates the effect of TOF upon an observer's performance for detecting and localizing focal warm lesions in noisy PET images. Methods: An advanced anthropomorphic lesion-detection phantom was scanned 12 times over 3 days on a prototype TOF PET/CT scanner (Siemens Medical Solutions). The phantom was devised to mimic whole-body oncologic 18 F-FDG PET imaging, and a number of spheric lesions (diameters 6-16 mm) were distributed throughout the phantom. The data were reconstructed with the baseline line-of-response ordered-subsets expectation-maximization algorithm, with the baseline algorithm plus point spread function model (PSF), baseline plus TOF, and with both PSF1TOF. The lesion-detection performance of each reconstruction was compared and ranked using localization receiver operating characteristics (LROC) analysis with both human and numeric observers. The phantom results were then subjectively compared to 2 illustrative patient scans reconstructed with PSF and with PSF1TOF. Results: Inclusion of TOF information provides a significant improvement in the area under the LROC curve compared to the baseline algorithm without TOF data (P 5 0.002), providing a degree of improvement similar to that obtained with the PSF model. Use of both PSF1TOF together provided a cumulative benefit in lesion-detection performance, significantly outperforming either PSF or TOF alone (P , 0.002). Example patient images reflected the same image characteristics that gave rise to improved performance in the phantom data. Conclusion: Time-of-flight PET provides a significant improvement in observer performance for detecting focal warm lesions in a noisy background. These improvements in image quality can be expected to improve performance for the clinical tasks of detecting lesions and staging disease. Further study in a large clinical population is warranted to assess the benefit of TOF for various patient sizes and count levels, and to demonstrate effective performance in the clinical environment. The PET components in the most recent generation of combined PET/CT scanners are equipped with timeof-flight (TOF) capability. The premise for TOF PET is illustrated in Figure 1, 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 f...

TOF PET is characterized by a better trade-off between contrast and noise in the image. This property is enhanced in more challenging operating conditions, allowing for example shorter examinations or low counts, successful scanning of larger patients, low uptake, visualization of smaller lesions, and incomplete data sampling. In this paper, the correlation between the time resolution of a TOF PET scanner and the improvement in signal-to-noise in the image is introduced and discussed. A set of performance advantages is presented which include better image quality, shorter scan times, lower dose, higher spatial resolution, lower sensitivity to inconsistent data, and the opportunity for new architectures with missing angles. The recent scientific literature that reports the first experimental evidence of such advantages in oncology clinical data is reviewed. Finally, the directions for possible improvement of the time resolution of the present generation of TOF PET scanners are discussed.

The introduction of fast scintillators with good stopping power for 511-keV photons has renewed interest in time-of-flight (TOF) PET. The ability to measure the difference between the arrival times of a pair of photons originating from positron annihilation improves the image signal-to-noise ratio (SNR). The level of improvement depends upon the extent and distribution of the positron activity and the time resolution of the PET scanner. While specific estimates can be made for phantom imaging, the impact of TOF PET is more difficult to quantify in clinical situations. The results presented here quantify the benefit of TOF in a challenging phantom experiment and then assess both qualitatively and quantitatively the impact of incorporating TOF information into the reconstruction of clinical studies. A clear correlation between patient body mass index and gain in SNR was observed in this study involving 100 oncology patient studies, with a gain due to TOF ranging from 1.1 to 1.8, which is consistent with the 590-ps time resolution of the TOF PET scanner. The visual comparison of TOF and non-TOF images performed by two nuclear medicine physicians confirmed the advantages of incorporating TOF into the reconstruction, advantages that include better definition of small lesions and image details, improved uniformity, and noise reduction. The potential for PET to measure the difference in arrival times of a pair of photons from the annihilation of the positron was first explored during the early 1980s (1,2), and an improvement in signal-to-noise ratio (SNR) due to time-of-flight (TOF) information was expected. The scintillators that were available at that time were fast but had lower stopping power than bismuth germanate, the scintillator conventionally used for PET imaging because of its excellent stopping power for 511-keV photons. The low sensitivity of these early TOF PET systems could not be offset by the SNR improvement due to TOF, and thus interest in this approach declined. The introduction, during the late 1990s, of cerium-doped lutetium oxyorthosilicate (3), LSO(Ce), a scintillator that is both fast and has good stopping power for PET imaging, and the more recent development of very fast scintillators such as LaBr 3 , has reawakened interest in TOF PET (4-6), and the first commercial TOF PET scanners have been recently introduced (7,8). A fast scintillator can provide good timing resolution and therefore information on the position of the positron annihilation: the relationship between the spatial uncertainty (Dx) and the timing resolution (Dt) is given by the expression Dx 5 cDt/2, where c is the speed of light. With LSO-based PET scanners, the time difference between the arrival times can be measured to be better than 600 ps, which corresponds to a spatial uncertainty of less than 9 cm. While insufficient to place the annihilation within a single voxel, such an uncertainty is better than having no localizing information and assigning equal probability to all voxels along the line of response (LOR).When data a...