Internal dosimetry in F-18 FDG PET examinations based on long time measured organ activities using total-body PET/CT: dose it makes any difference from a short-time measurement?

Objectives: The 2-meter long total-body PET/CT scanner (uEXPLORER) has been developed recently while its total-body coverage and ultra- high sensitivity provide opportunities for in vivo time-activity curve (TAC) measurement of all investigated organs with high temporal resolution simultaneously. This study aims at quantifying the cumulated activity and patient dose with long-time measured time-activity curves (TACs) of different organs, so that the comparison between estimation of quantifying methods using short-time or long-time TACs could be performed.

Methods: Organ TACs of 10 healthy volunteers were collected by the newly developed dynamic total-body PET/CT system in 4 periods after the intravenous injection of 2-[F-18]Fluoro-2-deoxy-D-glucose (18F-FDG). The 8-hour TACs of 6 source organs were fitted by using a spline method. Comparing cumulated activity estimated from spline fitted curves, the cumulated activity estimated from multi-exponential curve was also calculated. Exponential curve was fitted with shorter series of data consisting with clinical procedure and previous dosimetry works. An 8-hours dynamic bladder wall dose model considering 2 voiding were demonstrated to illustrate the differences in bladder dose caused by the different measurement durations. Organ absorbed doses were further estimated by using MIRD method and voxel phantoms for effective dose estimations.

Results: Short-time measurement could bring significant differences in estimated cumulated activity for liver compared with long-time measured spline fitted method while the differences of cumulated activity were 18.38% on average. For myocardium, the estimated cumulated activity difference was statistically neglectable due to the individual variation in metabolism. The average residence time differences of brain, heart, kidney, liver and lungs are 8.38%, 15.13%, 25.02%, 23.94% and 16.50% between spline fitted curve and multi-exponential fitted curve (fitted using the data from 57 minutes to 75 minutes after injection). When considering effective dose,the maximum differences of residence time between long-time measured spline fitted curve and multi-exponential fitted curve (fitted with shorter series of data) was 9.93%. The bladder contributed the most to the effective dose among all the investigated organs with the value of 21.18%. The bladder wall dose calculated using a long-time dynamic model was 13.79% larger than the two-voiding dynamic model, while at least 50.17% smaller than previous study using fixed bladder content volume.

Conclusions: Multi-organ TACs in-vivo long-time measurement with high temporal resolution by using total-body PET/CT proved that the data of clinical procedure with 20 minutes PET scan at 1 hour after injection could be used for retrospective dosimetry analysis. As the bladder content contributed the most to the effective dose, a long-time dynamic model was recommended for the bladder wall dose estimation.