Takayoshi Ishimori MD, Presenter: Nothing to Disclose

Samuel P Pak MD, Abstract Co-Author: Nothing to Disclose

Richard Leo Wahl MD, Abstract Co-Author: Nothing to Disclose

We have frequently observed relatively intense rectal tracer uptake in asymptomatic patients on FDG-PET/CT. The purpose of the present study was to investigate the physiological FDG uptake in the normal rectum at PET/CT to provide normative information to assist in making an accurate diagnosis of rectal pathology.

PET/CT scans of 20 consecutive clinical PET/CT patients (pts) (10 M, 10 F, 41-81 y.o. mean 57 y.o.) who had no history of abdominal or pelvic malignancy nor rectal symptoms were retrospectively evaluated. One hour after FDG injection, whole-body CT followed by PET was performed with a PET/CT scanner (Discovery LS, GEMS). Images of rectum were divided into 3 anatomical segments (Rp: proximal, Rm: middle and Rd: distal). In total, 60 segments were evaluated. FDG uptake in each segment was visually evaluated by grading according to intensity (0:none, 1:mild, 2:moderate, 3:intense), and the maximum Standardized Uptake Value (SUVmax) of each segment was also measured. All pts had no rectal lesion by clinical follow up for at least 6 months, and this was confirmed by screening colonoscopy in one pt.

Co-registered PET / CT images made it possible to evaluate FDG uptake in the rectum precisely regardless of the intensity. However, FDG uptake of Rd portion could not be separated from that of anal sphincter muscles. By qualitative grading, all pts had increased (grade 2 or 3) FDG uptake in Rd segment, while 13 pts (65%) had grade 2-3 FDG uptake in Rp and 11 pts (55%) had in Rm segment. No increased FDG uptake was seen in Rp and Rm segment in 3 pts (15%). The average of SUVmax in Rd was significantly higher than that in Rp or Rm (4.31 ± 1.61 in Rd vs. 2.92 ± 1.55 in Rp (p=0.004), 3.10 ± 1.67 in Rm (p=0.009)).

Intense FDG uptake in the rectum is common on PET/CT imaging. Physiological uptake of FDG was higher in the distal than in proximal rectum with SUV>2.5 common in the normal distal rectum. Intense FDG uptake in the rectum could mask rectal tumors but should not be confused with rectal pathology.

R.L.W.: Honorarium from and research contract with GE Medical Systems,Honorarium from CTI Molecular Imaging

Ishimori, T, Pak, S, Wahl, R, An Analysis of Physiological FDG Uptake in the Rectum at PET/CT.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4408585.html