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 field-of-view 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 noise equivalent counts (NEC), as well as 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 L of LSO and 17.1 L 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(-1) 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(-1) is obtained with 20 mm long crystals and an AFOV of 38 cm. Lesion phantom simulations show that the 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(-1).

Figure 1

Reconstructed images of the central transverse slices for the simulated lesion (Left) and uniform (Right) phantoms. Images are shown for data reconstructed with very high count statistics (or scan time). The distribution of 16, 1-cm diameter lesions at radial distances of 7 and 13 cm can be visualized in the Left image.

Figure 2

Relative sensitivity for a point source as a function of crystal thickness for (a) LSO and (b) LaBr₃ scanners with varying AFOV.

Figure 3

Change in relative sensitivity per crystal thickness for a point source as a function of crystal thickness for (a) LSO and (b) LaBr₃ scanners with varying AFOV.

Fig. 4

(a) Total NEC as a function of crystal thickness for scanner designs using a fixed amount of crystal (8.2 liters of LSO and 17.1 liters of LaBr₃). As the crystal thickness increases, the corresponding scanner AFOV is lower. This result corresponds to a fixed scan time for a single bed position study. (b) Simulated scanner AFOV for a given crystal thickness when using a fixed amount of crystal (8.2 liters of LSO and 17.1 liters of LaBr₃) for different scanner designs.

Figure 5

(a) Relative scan time per bed position for scanner designs using different combinations of crystal thickness and scanner AFOV while keeping the total crystal volume used in the scanner constant. Scan times are shown relative to an 18 cm long LSO scanner with 20 mm thick crystals (b) NEC/cm for performing a 100 cm long scan in a fixed total scan time of 10 minutes. All results are shown for a 35 cm diameter line source phantom.

Figure 6

Average transverse spatial resolution (FWHM) measured in varying scanner designs. Results are shown separately for source radial positions between (a) 0–5 cm and (b) 5–20 cm. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 7

Measured lesion contrast values as a function of iteration number for varying scanner designs with a timing resolution of 300ps for both LSO and LaBr₃. Note, that the simulated lesion uptake was 3:1 with respect to the background. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 8

ALROC values as calculated using the generalized scan statistic technique for (a) LSO and (b) LaBr₃ scanners as a function of timing resolution. The total scan time was fixed at 10 minutes for imaging a 100 cm long object. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 9

ALROC values as calculated using the generalized scan statistic technique for (a) LSO and (b) LaBr₃ scanners as a function of total scan time. The scan time was fixed at 600ps for LSO and 300ps for LaBr₃ scanner designs. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 9

ALROC values as calculated using the generalized scan statistic technique for (a) LSO and (b) LaBr₃ scanners as a function of total scan time. The scan time was fixed at 600ps for LSO and 300ps for LaBr₃ scanner designs. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 10

ALROC values as calculated using the generalized scan statistic technique for selected LSO and LaBr₃ scanners for varying total scan time and timing resolution while imaging a 100 cm long object. The scanner AFOV (in cm) and crystal thickness (in mm) are listed for each scanner design.

Figure 11

Reconstructed central slice of the lesion phantom from six different scanner designs. We also show the corresponding ALROC, contrast and NEC/cm values for each image.