Study of PET scanner designs using clinical metrics to optimize the scanner axial FOV and crystal thickness

Abstract

The aim of this study is to understand the trade-off between crystal thickness and scanner axial FOV (AFOV) for clinical PET imaging. Clinical scanner design has evolved towards 20–25 mm thick crystals and 16–22 cm long scanner AFOV, as well as time-of-flight (TOF) imaging. While Monte Carlo studies demonstrate that longer AFOV and thicker crystals will lead to higher scanner sensitivity, cost has prohibited the building of commercial scanners with > 22 cm AFOV. In this study, we performed a series of system simulations to optimize the use of a given amount of crystal material by evaluating the impact on system sensitivity and NEC, as well image quality in terms of lesion detectability. We evaluated two crystal types (LSO and LaBr3) and fixed the total crystal volume used for each type (8.2 liters of LSO and 17.1 liters of LaBr3) while varying the crystal thickness and scanner AFOV. In addition, all imaging times were normalized so that the total scan time needed to scan a 100 cm long object with multiple bed positions was kept constant. Our results show that the highest NEC/cm in a 35 cm diameter×70 cm long line source cylinder is achieved for an LSO scanner with 10 mm long crystals and AFOV of 36 cm while for LaBr3 scanners, the highest NEC/cm is obtained with 20 mm long crystals and an AFOV of 38 cm. Lesion phantom simulations show best lesion detection performance is achieved in scanners with long AFOV (≥ 36 cm) and using thin crystals (≤ 10 mm of LSO and ≤ 20 mm of LaBr3). This is due to a combination of improved NEC, as well as improved lesion contrast estimation due to better spatial resolution in thinner crystals. Alternatively, for lesion detection performance similar to that achieved in standard clinical scanner designs, the long AFOV scanners can be used to reduce the total scan time without increasing the amount of crystal used in the scanner. In addition, for LaBr3 based scanners, the reduced lesion contrast relative to LSO based scanners requires improved timing resolution and longer scan times in order to achieve lesion detectability similar to that achieved in an LSO scanner with similar NEC/cm.

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

“…We simulated cylindrical lesion and uniform phantoms (35 cm diameter by 70 cm long) similar to those used in our previous study (Surti et al, 2013b). The 35 cm diameter phantom has been shown to be a reference for heavy patients (BMI ~ 31) (Surti et al, 2007).…”

“…In a previous study (Surti et al, 2013b) we investigated the potential to achieve higher performance in a whole-body PET scanner by using a fixed volume of crystal (similar to that used in current commercial PET scanners); making the LSO crystals shorter while increasing the scanner AFOV, effectively trading the crystal stopping efficiency for the scanner geometric sensitivity. Our simulation studies showed that the best imaging performance, as determined by the area under the localization receiver operating characteristic curve (ALROC), is achieved when using 10 and 5 mm thick crystals and increasing the scanner AFOV to 36 and 72 cm respectively.…”

“…As opposed to previous studies (Surti et al, 2013a, Surti et al, 2013b, Thoen et al, 2013), here we focus on imaging at low activity levels which may be of interest in pediatric imaging or for performing serial PET scans for therapy monitoring. Hence we explore imaging at activity levels as low as 1/20th of that typically injected for routine 18 F-FDG scans currently (15 mCi).…”

“…Hence we explore imaging at activity levels as low as 1/20th of that typically injected for routine 18 F-FDG scans currently (15 mCi). The scanner AFOV is fixed at 72 cm based on the results of our past work (Surti et al, 2013b), and we primarily use the ALROC metric as the primary figure of merit. The cost of scintillator and photo-detector does not explicitly enter into our evaluations.…”

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.…”