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Purpose PET image quality is influenced by the patient size according to the current guideline. The purpose was to propose an optimized dose regimen to yield a constant image quality independent of patient size to meet the clinical needs.Methods A first patient cohort of 78 consecutives for oncological patients (59.7±13.7 years) who underwent a total-body PET/CT scan were retrospectively enrolled to develop the regimen. The patients were equally distributed in four BMI groups according to WHO criteria. The liver SNR (Signal noise ratio, SNR L ) was obtained through manually drawing ROIs and normalized (SNR norm ) by the injected activity and acquisition time. And fits of SNR norm against different patient-dependent parameters were performed to determine the best correlating parameter and fit method. A qualitative assessment on image quality was performed using a 5-point Likert scale to determine the acceptable threshold of SNR L . And thus, an optimized regimen was proposed and validated by a second patient cohort with prospectively enrolled 38 oncological patients. ResultsThe linear fit showed SNR norm was the strongest correlation (R 2 = 0.69) with the BMI than other patient-dependent parameters. The qualitative assessment revealed a SNR L of 14.0 as a threshold to achieve a sufficient image quality. The optimized dose regimen was determined as a quadratic relation with BMI: Injected activity = 39.2 MBq/(-0.03*BMI+1.49) 2 . In the validation study, the SNR L no longer decreased with the increase of BMI. There was no significant difference of the image quality, the SNR L , between different BMI groups (p > 0.05). In addition, the injected activity was reduced by 75.6±2.9 %, 72.1±4.0 %, 67.1±4.4 % and 64.8±3.5 % compared to the first cohort for the four BMI groups, respectively. ConclusionThe study recommended a quadratic relation between the 18 F-FDG injected activity and the patient's BMI and propose a regimen for total-body PET imaging. In the regimen, the image quality can maintain in a constant level independent of patient size and meet the clinical requirement even with a reduced injected activity.