SUV Analysis of F-18 FDG PET Imaging in the Vicinity of the Bladder

Positron Emission Tomography with 18F-FDG can be used as a predictor for post-therapeutic tumor response in rectal and gynecologic cancers. An issue with assessing lesions in the vicinity of the bladder is the radioactivity that accumulates in the bladder due to constant filling. SUV analysis is used to discriminate between therapy responders and non-responders based on percentage change/threshold, but it is not yet known how much variability can be attributed to the bladder radioactivity. The purpose of this research is to understand the effects of bladder radioactivity on surrounding concentration measurements with different scanning and image reconstruction techniques.

Methods: ROI analysis was performed on 67 PET scans from DUMC. Typical values of bladder volume, radioactivity, radioactivity concentration and bladder-to-background uptake ratio were determined and incorporated into phantom studies. A bladder phantom insert was created for a 25 L torso phantom to explore effects of a bladder in the center of the FOV, on the edge of the FOV, and outside the FOV on the GE Discovery STE. The bladder insert was also used in a phantom study to assess the effects of different, realistic bladder radioactivity levels on surrounding 1-cm lesions on both the GE Discovery STE and GE Discovery 690. Background activity and in-air environments were explored, as well 2D, 3D and time-of-flight PET acquisitions with different image reconstruction techniques.

Results: The DSTE 2D PET data suffered large void artifacts that worsened as radioactivity in the bladder increased. The DSTE 3D PET data over-estimated background measurements up to 30 slices with the bladder outside the FOV. Concentration measurements within 4 cm distal to the bladder showed great variability and were generally recovered best with TOF PET or 3D PET with an increased number of iterations. Lesions greater than 4 cm distal to the bladder showed consistent recovery in both the 3D and TOF PET data.

Discussion: Radioactivity within the bladder has substantial effects on surrounding radioactivity concentration measurements. There are limits to the measurements of the radioactivity concentration values used in SUV analysis in the vicinity of the bladder.

A general theme is that more accurate results are produced with smaller amounts of radioactivity in the bladder. This validates the DUMC protocol that begins acquisition just below the pelvic area to reduce as much filling as possible and highlights the usefulness of patient voiding prior to scanning.