Impact of detector design on imaging performance of a long axial field-of-view, whole-body PET scanner

Abstract

Current generation of commercial time-of-flight (TOF) PET scanners utilize 20–25 mm thick LSO or LYSO crystals and have an axial FOV (AFOV) in the range of 16–22 mm. Longer AFOV scanners would provide increased intrinsic sensitivity and require fewer bed positions for whole-body imaging. Recent simulation work has investigated the sensitivity gains that can be achieved with these long AFOV scanners, and has motivated new areas of investigation such as imaging with very low dose of injected activity as well as providing whole-body dynamic imaging capability in one bed position. In this simulation work we model a 72 cm long scanner and prioritize the detector design choices in terms of timing resolution, crystal size (spatial resolution), crystal thickness (detector sensitivity), and depth-of-interaction (DOI) measurement capability. The generated list data are reconstructed with a list-mode OSEM algorithm using a Gaussian TOF kernel that depends on the timing resolution and blob basis functions for regularization. We use lesion phantoms and clinically relevant metrics for lesion detectability and contrast measurement. The scan time was fixed at 10 minutes for imaging a 100 cm long object assuming a 50% overlap between adjacent bed positions. Results show that a 72 cm long scanner can provide a factor of ten reduction in injected activity compared to an identical 18 cm long scanner to get equivalent lesion detectability. While improved timing resolution leads to further gains, using 3 mm (as opposed to 4 mm) wide crystals does not show any significant benefits for lesion detectability. A detector providing 2-level DOI information with equal crystal thickness also does not show significant gains. Finally, a 15 mm thick crystal leads to lower lesion detectability than a 20 mm thick crystal when keeping all other detector parameters (crystal width, timing resolution, and DOI capability) the same. However, improved timing performance with 15 mm thick crystals can provide similar or better performance than that achieved by a detector using 20 mm thick crystals.

Parallax error in long-axial field-of-view PET scanners—a simulation study

“…We can understand the previous results in terms of the findings in this current study of spatial resolution. Our simulation model of a two-layer DOI detector (with modeling of inter-crystal scatter included) shows some improvement in the resolution, in both the response at FWHM and FWTM for scanner designs based on both scintillation materials (see figures 4–7), however, this improvement may not be enough to significantly affect the image quality metrics studied in Surti and Karp 2015. Note, in that study the 1-cm diameter lesions were placed in the center of the axial FOV at radial offsets of 7 cm and 13 cm, and does include attenuation and scatter.…”

“…In a recent paper by Surti and Karp 2015, a simulation study was presented that used image-based metrics to investigate the impact of detector performance on a long-axial FOV scanner having an axial FOV of 72 cm; this study included analysis of TOF resolution and a 2-layer DOI detector on image quality. It was hypothesized that detector configurations capable of measuring both TOF and DOI, as demonstrated in studies such as (Schaart et al 2009, Wiener et al 2013, Schmall et al 2014, Yeom et al 2014,), would improve imaging performance particularly for the simulated 72 cm long scanner design.…”

Total-Body PET: Maximizing Sensitivity to Create New Opportunities for Clinical Research and Patient Care

“…However, the slow and relatively low-light-yield scintillator (bismuth germanate) necessitated the use of coarse axial septa to minimize scatter and counting rate problems, greatly reducing the possible sensitivity gain. There also has been a range of computer simulation studies designed to examine the effects of longer axial FOVs and the impact on scatter and random coincidences, as well as to determine the most effective way to distribute a fixed volume of scintillator material given this is a key determinant in the overall cost of a PET system (1,7,10,(13)(14)(15)(16)(17). The availability now of improved scintillation materials based on lutetium compounds, which have a more favorable combination of speed, light output, and stopping power, has made it feasible to develop fully 3-dimensional systems that can handle the higher counting rates and scatter fraction, and additionally provide time-of-flight information to further improve performance.…”

Quantitative image reconstruction for total-body PET imaging using the 2-meter long EXPLORER scanner

“…Image quality and lesion detectability were also studied for performance evaluation between different scanner designs (Badawi et al 2013, Surti et al 2013, Surti et al 2015). Some demonstration scanners with an extended AFOV (50~70 cm) have been built and provide significant sensitivity, but achieved very limited improvement of performance due to the limitations in data acquisition electronics (Watanabe et al 2004, Conti et al 2006).…”