Impact of Time-of-Flight on PET Tumor Detection

Abstract

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...

Impact of Image Reconstruction Settings on Texture Features in 18F-FDG PET

“…The delineated lesion is also shown in green in the figure. Consistent with the previous findings (22)(23)(24)(25)(26), OSEM 1 TOF 1 PSF produced visually the best image among the chosen 4 reconstruction algorithms with their default settings. In addition, quantitative assessment supported the fact that OSEM 1 TOF 1 PSF produced the best image with an SNR of 9.1 in the liver, whereas the SNR of OSEM, OSEM 1 PSF, and OSEM 1 TOF were 5.2, 5.5, and 8.5, respectively.…”

“…Thus, the impact of the partial-volume effect can be neglected. Previous studies (22)(23)(24)(25)(26) demonstrated that OSEM 1 PSF 1 TOF produced better image quality than other methods in terms of SNR, contrast, and lesion detectability. Therefore, in this study, the tumor VOIs were delineated on the image reconstructed by OSEM 1 PSF 1 TOF with default settings and then applied to the other methods.…”

“…However, only filtered backprojection and ordered-subset expectation maximization (OSEM) were investigated. The techniques of time-of-flight (TOF) and point-spread function (PSF) modeling have been shown to clearly improve image quality in terms of signal-to-noise ratio (SNR), lesion contrast, and coefficient of variation (COV) on background and are now standard in most scanners (22)(23)(24)(25)(26). TOF improves SNR and thus reduces heterogeneity due to noise.…”

LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time

“…The use of time-of-flight (TOF) information in positron emission tomography (PET) has recently been demonstrated to enable significant improvement in image noise properties and, therefore, lesion detection, especially in heavier patients (Kadrmas et al 2009, Lois et al 2010, Moses 2007, Surti et al 2007. This warrants further research into TOF-capable PET scintillation detectors, in particular with the aim to obtain better timing resolution.…”

Comparison of list-mode and DIRECT approaches for time-of-flight PET reconstruction

“…2 in [19], one can estimate the maximum angular interval (α max ) that provides adequate sampling consistent with the spatial resolution of the data as (1) In this calculation, FWHM LOR is defined by the detector spatial resolution rather than the pixel size as was done in [19]. The subsequent minimum number of transverse angular samples (nϕ min ) and tilt angles (nθ min ) can be calculated from (2) (3) where θ max is the full axial angular range of the scanner. For a whole-body TOF-PET scanner with 6-mm resolution and 430-ps timing resolution, only 17 transverse angular samples and 2 oblique tilt angles are required; for a scanner with 700-ps timing resolution, 28 transverse views and 3 oblique tilt angles are needed.…”