Purpose: The image quality of Positron Emission Tomography (PET) using ¹⁸F-fluoro-2-deoxy-D-glucose is extremely important to diagnose cancer precisely. The purpose of this study is to inspect the physical indexes that are correlated with image quality and optimization of the scan time, utilizing the physical index. Methods: This study calculated two kinds of patient noise equivalent counts (NEC_patient and NEC_density) and signal to noise ratio in liver (SNR_liver) to measure PET image quality. We estimated the correlation coefficient between the visual assessment of PET image quality and physical indexes to assess the degree of correlation. We also set the optimal scan time, depending on body mass index (BMI), using the physical index and examined the efficacy of the optimization. Results: NEC_density showed the highest correlation coefficient in PET image quality (r=0.743, p<0.001). By optimizing the scan time using NEC_density, we showed there was no correlation between BMI and NEC_density (r=0.192, p=0.047). In addition, reduced dependence of the PET image quality by BMI was showed. Conclusion: The optimization of the scan time using NEC_density reduces deterioration of the PET image quality by patient habitus.