METHODS: Single-photon-emission computed tomography studies were carried out for plastic bottles of various sizes as target objects containing Technetium-99m placed in a cylindrical water phantom. The phantoms were created to have varying ratios of target activity concentration to background activity concentration (T/B) ranging from 4.00 to 1.25. From the single-photon-emission computed tomography slices, the volume of the target object was estimated using an iterative thresholding method. A unique threshold that estimated the volume of the target object was determined and declared as ideal threshold for each level of background activity. Simple linear regression analysis was carried out to explore the relationship between the actual volume and estimated volume.
RESULTS: The analysis shows a statistically significant linear relationship (P < .01) between actual volume and estimated volume. For all ratios of target activity concentration to background activity concentration studied, the relative errors in estimated volumes of target objects are within ±7%. Thresholds that estimated volume of target objects vary depending on the level of background activity.
CONCLUSIONS: It is clearly demonstrated that background activity highly influenced the volume estimation. Therefore, background activity of the target object needs to be considered when selecting the ideal thresholds to obtain accurate volume estimation of target objects.
METHOD: A total of 2247 PET/CT patients with normal glucose level underwent 18F-FDG-whole body imaging procedures. The 18F-FDG dose of 3.7MBq per kg of patient weight administered via intravenous infusion. For CT parameters, kilovoltage of 140keV and current of 40 mAs were used for all studies. All the acquired images collected retrospectively and the effective dose was calculated for each patient using algorithm adapted from ICRP Publication 106, modified for patient weight and patient blood volume. The estimated effective doses were evaluated for patients' body weight and BMI.
RESULTS: The mean of total effective dose and standard deviation is approximately 15.08(4.52) mSv using ICRP algorithm. 56% of total patient has normal BMI and their average total effective dose is 13.6mSv. Underweight patients' effective dose can be as low as 9.6mSv even using diagnostic CT protocols.
CONCLUSION: The effective dose of PET/CT procedure in present study is one of the lowest although using diagnostic parameters for CT acquisition compared to published data worldwide. This is due to the improved sensitivity of PET and complex reconstruction technique that maintains the image quality. A significant association between body weight, BMI and effective dose is reported in present study. Therefore, it is suggested that attention must be given for underweight and ideal BMI patients while prescribing FDG activity and CT imaging parameters in order to minimize the effective dose. The effective dose reported in present study can be considered as an upper limit for effective dose in PET/CT patients with normal BMI. This upper limit can be treated as a standard limit when optimizing imaging parameters, developing algorithm for image reconstruction and prescribing activity for patients. This practice could fulfill ALARA principle that could reduce cancer risk.