Whole Body PET imaging using variable acquisition times

Whole Body PET scans are typically performed as a series of image sets acquired at discrete axial positions to cover most or all of the body. The acquisition time at each axial position is typically kept fixed for all positions although the imaging time is typically adjusted according to the patient´s weight. Because of the varying amount of attenuation for different sections of the body, it is expected that the image S/N will vary accordingly. The aim of this work is to investigate the possibility of varying the acquisition time at different sections of the body such that the image S/N is kept relatively constant for all slices. To estimate the acquisition times for the different sections of the body we propose to use the attenuation correction (AC) generated from the CT scan that is acquired prior to the PET scan. Simulations of activity and attenuation distributions based on whole body CT scans were performed. Different diameter cylinders, a NEMA Image Quality phantom and an Anthropomorphic phantom filled with 18F-FDG were also acquired in a Siemens Biograph 64 Truepoint PET scanner. The image noise was estimated with generating multiple noise replicates by adding poisson noise to the emission sinograms for the simulated images, and by using the bootstrap method for the phantom measurements. By comparing the square of image noise (SD/Mean) for all the image slices, the acquisition time for each section could be adjusted to yield uniform image noise for all slices. The image noise was also compared to the average AC factors through the center of each body slice. A simple polynomial function was found for both the simulations and the phantom measurement images to accurately describe the image noise as a function of AC factors. These results indicate that the noise properties of whole body images can be more uniform axially by adjusting the acquisition time according to the amount of attenuation. Instead of using a fixed scan time per bed position, the acquisition- time can be reduced in areas of lower attenuation and increased in more absorbing sections of the body. Since there is a strong correlation of the image noise and the AC factors, the relative acquisition times can be quickly calculated using a simple functional relationship.