An Assessment of the Impact of Incorporating Time-of-Flight Information into Clinical PET/CT Imaging

Abstract

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...

Improvement in PET/CT Image Quality with a Combination of Point-Spread Function and Time-of-Flight in Relation to Reconstruction Parameters

“…The TOF information provides a difference in arrival times between a pair of coincident photons and narrows the probability that the annihilation event will be located along the line of response. This additional information enables the reconstruction algorithm to converge to the expected image with fewer iterations and a better noise level (18).…”

“…Recent studies have reported faster convergence of lesion contrast and improvements in lesion detection performance with TOF (18)(19)(20)(21)(22). However, the effects of PSF and TOF have not been sufficiently demonstrated in different reconstruction parameters or investigated for their effects on noise levels.…”

Prognostic Value of SUVmax of <sup>18</sup>F-FDG PET/CT in Early Stage Breast Cancer with No LN Metastasis

“…Use of FDG-PET/CT for detection of the primary tumor is currently not recommended, mainly because of the supposedly low sensitivity for small carcinomas. However, the introduction of TOF technology used in this research has further increased the image quality of PET/CT and dramatically improved cancer detectability [25]. In a recent study, 67% of T1b and 98% of T1c tumors could be visualized with PET/CT.…”

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

“…Including time-of-flight (TOF) information in PET reconstruction has been shown to improve clinical PET images [1]- [3]. A variety of reconstruction approaches exist for threedimensional (3D) TOF-PET data.…”

“…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.…”