Patient cohorts

The present study involved two patient cohorts: an initial exploration cohort for the analysis of different acquisition times and a clinical validation cohort for the validation of the initial results and the optimization of acquisition times. The results for these two cohorts are presented below.

Initial exploration cohortPatient characteristics

The demographic and clinical data of the patients are summarized in Table 1. We enrolled a total of 46 patients (34 men, 12 women) in the first cohort, with a mean age of 61.5 ± 8.8 years, a mean body mass index (BMI) of 24.1 ± 3.5 kg/m2, and a mean injected FDG dose of 1.88 ± 0.09 MBq/kg. The overall distribution of primary neoplasms in this cohort was as follows: lung neoplasms, 19 patients; colorectal cancer, 17 patients; liver cancer, 4 patients; biliary tract cancer, 2 patients; and stomach neoplasms, 4 patients. Lymph node metastases were suspected in 18 patients, while distant metastases were not present in any patient. The distribution of the primary tumors and lymph node lesions is shown in detail in Table 2.

Table 1 Characteristics of the patients in the exploration and validation cohortsTable 2 Pathological distribution of primary tumors and suspected metastases in both cohortsEvaluation of image quality

As shown in Table 3, the average qualitative scores for images with acquisition times of 1, 2, 3, 5, and 8 min (henceforth referred to as G1, G2, G3, G5, and G8, respectively) were 2.0 ± 0.2, 2.9 ± 0.3, 3.0 ± 0.0, 3.9 ± 0.2, and 4.2 ± 0.4, respectively. Even for the G1 images, the average score indicated visually acceptable images with no need for rescanning. In the case of two patients, however, the G1 images were given a score of 1 point. The subjective scores of the G5 and G8 images were significantly higher than those of G1, G2, and G3 (all p < 0.05). No significant difference in subjective scores was observed between the G5 and G8 images (p > 0.99). In addition, no distinct difference was identified between the G2 and G3 images (p > 0.99).

Table 3 Qualitative image analysis in the exploration and validation cohorts

The results of the objective image quality assessments are presented in Table 4 and Fig. 2a–d. In general, the background uptake and image noise decreased as the acquisition time increased, while the liver SNR gradually increased. For the liver uptake, there were no significant differences in SUVmax between G5, G8, and G15 (all p ≥ 0.16, G15 served as control). The liver SD, SNR, and COV in G15 images were not significantly different from those of G8 images (all p ≥ 0.10), while they differ significantly from those of the other short-duration images (all p < 0.05). For the blood pool uptake, the SUVmax was significantly lower in the G15 images than in the G3, G2, and G1 images (all p < 0.05), but did not differ between the G15 and G8 images (p > 0.99) or between the G15 and G5 images (p = 0.55). The average SD of the regions of interest (ROIs) in the blood pool significantly differed between the G15 images and the other images (all p < 0.05).

Table 4 Quantitative parameters of the background and lesions in the exploration cohortFig. 2

Box plots for comparison of quantitative parameters in liver (a–d) and lesion (e–f) among groups. In general, the liver uptake and image noise decreased as the acquisition time increased, while the SNR of liver gradually increased. G15 served as control; there were no significant differences between G8 and G15 for any of these parameters (a–d). For lesions, SUVmax (e) was significantly lower on G15 images than on G3, G2, and G1 images, but did not differ significantly between G15 and G8 (p > 0.99) or between G15 and G5 (p = 0.35). The TBR (f) significantly differed between short-duration images and G15 images. SUV = standardized uptake value, SD = standard deviation, SNR = signal-to-noise ratio, COV = coefficient of variation, TBR = tumor-to-background ratio (*indicate p < 0.05; ns, not significant)

Lesion detectability

Pathological examination confirmed a total of 75 lesions in the 46 patients. Of these, 7 lesions in 6 patients (2 liver lesions, 1 lung lesion, and 4 lymph node lesions) were not recognizable on G15 images. In all, 47 primary lesions and 21 suspicious lymph node metastases were detected on G15 images. G15 served as control; the lesion detection rates were 85.3% (58/68) and 97.1% (66/68) for the G1 and G2 images, respectively, and 100% (68/68) for the remaining images. On the G1 images, 10 lesions from 6 patients were not identifiable, including 1 lesion in the liver and 9 lesions in the lymph nodes. The lesion detection rate significantly differed between the G15 and G1 images (p < 0.05).

For the assessment of lesion conspicuity, a total of 56 out of 75 lesions were pathologically malignant and included for analysis (Table 4 and Fig. 2e, f). The lesion SUVmax was significantly lower on G15 images than on G3, G2, and G1 images, but did not differ significantly between G15 and G8 (9.99 ± 7.94 vs. 11.60 ± 9.82; p > 0.99) or between G15 and G5 (9.99 ± 7.94 vs. 11.68 ± 9.92; p = 0.35). In addition, the corresponding TBRs of the lesions on G15 images were lower than all the short-duration images (all p < 0.05). No significant difference in lesion SUVpeak was found between G15 and the short-duration images. Detailed SUVs and TBRs of different primary tumors and metastatic lymph nodes are summarized in Additional file 2: Table S2.

Clinical validation cohortPatient characteristics

A total of 147 eligible patients (79 men, 68 women) with a mean age of 59.4 ± 12.1 years, mean body weight of 63.8 ± 11.7 kg, mean BMI of 23.6 ± 3.6 kg/m2, and mean injected FDG dose of 1.88 ± 0.10 MBq/kg were included in this cohort (Table 1). After integrating the pathological data, we included 240 lesions in the final analysis: 163 primary tumors, 69 suspicious lymph node metastases, and 8 distant metastases. Three patients had multiple primary tumors: one patient had small intestine cancer with schwannomas; one had bladder cancer with ureteral cancer; and another patient had liver cancer with gastrointestinal stromal tumor. Distant metastases were present in 4 patients, including 3 patients with colorectal cancer and liver metastasis and 1 patient with breast cancer and bone metastasis. The distribution of all pathologically confirmed lesions in the validation cohort is shown in detail in Table 2.

Subjective image quality

As presented in Table 3, the subjective scores for the G2, G3, G5, G8, and Gs (acquisition time, 10 or 15 min) images were 3.0 ± 0.2, 3.0 ± 0.1, 3.6 ± 0.5, 4.0 ± 0.3, and 4.4 ± 0.5, respectively. The scores for the G2 and G3 images were approximately 3 points or slightly lower than 3 points. Significant differences in these scores were observed between any two groups (all p < 0.05), except for G2 and G3 (p > 0.99).

Lesion detectability

Of the 240 lesions, 36 lesions were not clearly identified on Gs images: 11 liver lesions, 5 biliary tract lesions, 2 pancreatic lesions, 2 bladder lesions, 1 gallbladder lesion, and 15 lymph node lesions. Compared to the Gs images, the G2, G3, G5, and G8 images had lesion detection rates of 90.2% (184/204), 94.1% (192/204), 99.0% (202/204), and 100% (204/204), respectively. The distribution of the 20 lesions that went undetected on G2 images was as follows: biliary tract (n = 3), liver (n = 2), pancreas (n = 1), stomach (n = 1), bladder (n = 1), small intestine (n = 1), lymph nodes (n = 10), and liver metastasis (n = 1). The distribution of lesions that were unidentifiable on G3 images was as follows: biliary tract (n = 2), bladder (n = 1), liver (n = 1), and lymph nodes (n = 8). The lesion detection rates for the G5 and G8 images were not significantly lower than the rates for the Gs images (all p > 0.99).