Phantom-based image quality assessment of clinical 18F-FDG protocols in digital PET/CT and comparison to conventional PMT-based PET/CT

Abstract

Background: We assessed and compared image quality obtained with clinical ¹⁸F-FDG whole-body oncologic PET protocols used in three different, state-of-the-art digital PET/CT and two conventional PMT-based PET/CT devices. Our goal was to evaluate an improved trade-off between administered activity (patient dose exposure/signal-to-noise ratio) and acquisition time (patient comfort) while preserving diagnostic information achievable with the recently introduced digital detector technology compared to previous analogue PET technology.

Methods: We performed list-mode (LM) PET acquisitions using a NEMA/IEC NU2 phantom, with activity concentrations of 5 kBq/mL and 25 kBq/mL for the background (9.5 L) and sphere inserts, respectively. For each device, reconstructions were obtained varying the image statistics (10, 30, 60, 90, 120, 180, and 300 s from LM data) and the number of iterations (range 1 to 10) in addition to the employed local clinical protocol setup. We measured for each reconstructed dataset: the quantitative cross-calibration, the image noise on the uniform background assessed by the coefficient of variation (COV), and the recovery coefficients (RCs) evaluated in the hot spheres. Additionally, we compared the characteristic time-activity-product (TAP) that is the product of scan time per bed position × mass-activity administered (in min·MBq/kg) across datasets.

Results: Good system cross-calibration was obtained for all tested datasets with < 6% deviation from the expected value was observed. For all clinical protocol settings, image noise was compatible with clinical interpretation (COV < 15%). Digital PET showed an improved background signal-to-noise ratio as compared to conventional PMT-based PET. RCs were comparable between digital and PMT-based PET datasets. Compared to PMT-based PET, digital systems provided comparable image quality with lower TAP (from ~ 40% less and up to 70% less).

Conclusions: This study compared the achievable clinical image quality in three state-of-the-art digital PET/CT devices (from different vendors) as well as in two conventional PMT-based PET. Reported results show that a comparable image quality is achievable with a TAP reduction of ~ 40% in digital PET. This could lead to a significant reduction of the administered mass-activity and/or scan time with direct benefits in terms of dose exposure and patient comfort.

Keywords: Digital PET/CT; Dose reduction; Image quality; Protocol optimization.

Fig. 1

BG_cal as a function of the number of iterations used in the iterative reconstruction setups without Gaussian smoothing (a) and comparison of Gaussian vs. non-Gaussian setups (c) for devices that used the Gaussian smoothing in the clinic. Similarly, we reported BG_cal as a function of the time per bed position (b and d). Digital devices were labeled with full lines; dashed lines represent obtained results with analogue devices

Fig. 2

COV as a function of the number of iterations used in the iterative reconstruction setups without Gaussian smoothing (a) and comparison of Gaussian vs. non-Gaussian setups for devices that used the Gaussian smoothing in the clinic (c). Similarly, we reported the COV as a function of the time per bed position (b and d). Digital devices were labeled with full lines and dashed lines represent the obtained results with analogue devices

Fig. 3

COV as a function of the time-activity-product (TAP) for all tested clinical setups. Iterative reconstruction setups without Gaussian smoothing (a) and comparison of Gaussian vs. non-Gaussian setups for devices that used the Gaussian smoothing in the clinic (b). Digital devices were labeled with full lines and dashed lines represent the obtained results with analogue devices

Fig. 4

RC_A50 and RC_max values as a function of the sphere diameter for acquisitions performed with a fixed scan duration of 180 s per bed position. Iterative reconstruction setups without Gaussian smoothing (a and b) and comparison of Gaussian vs. non-Gaussian setups for devices that used the Gaussian smoothing in the clinic (c and d). Digital devices were labeled with full lines and dashed lines represent the obtained results with analogue devices. Upper and lower RC boundaries specified by the EANM/EARL accreditation protocols are labeled with black dashed lines. EARL RC values (black dashed lines) refers to January 2017 version as reported in the EARL website [17]

Fig. 5

Transaxial views of the image quality of phantom images obtained with clinical setups