Optimal clinical protocols for total-body 18F-FDG PET/CT examination under different activity administration plans

Abstract

Background: Highly sensitive digital total-body PET/CT scanners (uEXPLORER) have great potential for clinical applications and fundamental research. Given their increasing sensitivity, low-dose scanning or snapshot imaging is now possible in clinics. However, a standardized total-body ¹⁸F-FDG PET/CT protocol is still lacking. Establishing a standard clinical protocol for total-body 18F-FDG PET/CT examination under different activity administration plans can help provide a theoretical reference for nuclear radiologists.

Methods: The NEMA image quality (IQ) phantom was used to evaluate the biases of various total-body ¹⁸F-FDG PET/CT protocols related to the administered activity, scan duration, and iterations. Several objective metrics, including contrast recovery (CR), background variability (BV), and contrast-to-noise ratio (CNR), were measured from different protocols. In line with the European Association of Nuclear Medicine Research Ltd. (EARL) guidelines, optimized protocols were suggested and evaluated for total-body ¹⁸F-FDG PET/CT imaging for three different injected activities.

Results: Our NEMA IQ phantom evaluation resulted in total-body PET/CT images with excellent contrast and low noise, suggesting great potential for reducing administered activity or shortening the scan duration. Different to the iteration number, prolonging the scan duration was the first choice for achieving higher image quality regardless of the activity administered. In light of image quality, tolerance of oncological patients, and the risk of ionizing radiation damage, the 3-min acquisition and 2-iteration (CNR = 7.54), 10-min acquisition and 3-iteration (CNR = 7.01), and 10-min acquisition and 2-iteration (CNR = 5.49) protocols were recommended for full-dose (3.70 MBq/kg), half-dose (1.95 MBq/kg), and quarter-dose (0.98 MBq/kg) activity injection schemes, respectively. Those protocols were applied in clinical practices, and no significant differences were observed for the SUV_max of large/small lesions or the SUV_mean of different healthy organs/tissues.

Conclusion: These findings support that digital total-body PET/CT scanners can generate PET images with a high CNR and low-noise background, even with a short acquisition time and low administered activity. The proposed protocols for different administered activities were determined to be valid for clinical examination and can maximize the value of this imaging type.

Keywords: 18F-FDG; Fast scanning; Low-dose; Protocol optimization; Total-body PET/CT.

Fig. 1

The schematic figure of transverse image slices of the NEMA IQ phantom achieved at the different acquisition time, iteration number, and injected activity. The PET raw-data were measured from the NEMA IQ phantom with a background activity of ~ 5.2 MBq/L (hot sphere-to-background ratio = 4:1) scanned for 30 min. By truncating the list-mode PET raw-data, a series of PET images were reconstructed into 40 s, 1 min, 2 min, 3 min, 5 min, 10 min and 30 min (ordered from left to right column), where the iteration number applied in OSEM-TOF-PSF reconstruction process was 2, 3, 4, 5, and 15 from top to bottom line, respectively. The activity per body weight was estimated from the instantaneous background activity concentration taking into account of the nature decay of ¹⁸F nuclide for a waiting time of 60 min after injection. The activity per body weight ranged from 0.24 to 7.61 MBq/L

Fig. 2

Background variability (BV) map, which is derived from NEMA IQ phantom measurement (hot sphere: background, 4:1) at four estimated doses, as a function of both factors—acquisition time and iterations. The dark area represents that BV value is more than 15%

Fig. 3

Contrast-to-noise ratio (CNR) map as a function of both factors—acquisition time and iterations at different estimated injected dose. The dark area represents the corresponding BV value that is more than 15%. The CNR values that excessed 5.0 were considered as acceptable for diagnosis

Fig. 4

Relationship between the objective indicator (CNR) and the subjective assessment (5-point Likert scale). A The reference example images of grades 1–5 in the 5-point Likert scale (L1: poor, L5: excellent). B Scatter plot for the CNR value vs. 5-point Likert scale upon the total body PET/CT scanner and the conventional short AFOV PET/CT scanner (derived from Siemens Biograph mCT scanner in this study), respectively. The 5-point Likert scale was independently rated by two nuclear radiologists in blind

Fig. 5

Iteration-dependent SUV_mean at muscle, lung, liver, and blood pool, and SUV_max at suspected major lesions and metastatic small lesions upon full-dose group for an acquisition time from 40 to 600 s, where the iterative numbers set at 2, 3, 4, 5, and 15

Fig. 6

Comparison of SUV_mean at lung, muscle, blood pool and liver, and SUV_max at suspected major lesions and small metastatic lesions among six different suggested protocols that involves full-dose, half-dose, and quarter-dose