Introduction ¹⁸F- Fluoro-deoxy-glucose Positron Emission Tomography (FDG-PET) scanning is gaining popularity in the assessment of several inflammatory disorders including Crohn’s Disease (CD).

Methods 11 patients with established CD,underwent ¹⁸F- FDG-PET scanning that was compared with recent endoscopic data, provided treatment had been unchanged between the two tests. Patients were fasted for 6 h and received 185 MBq of iv FDG followed by 800 ml of oral 2.5% mannitol. A low dose CT scan of the abdomen was performed, followed by PET, at 60 min post FDG injection. PET data was acquired over a maximum of 3 bed positions (10 min/ bed position).

Analysis involved dividing the gut into 6 segments on CT (terminal ileum, ascending, transverse, descending and sigmoid colon and rectum). Segmental maximum standardised uptake value (SUV_MAX) and SUV intestine-to-liver ratio (SUV_ITL= SUV_MAX/Liver SUV_MEAN) were calculated. A segment is defined as abnormal (PET +ve) when its SUV_MAX > Liver SUV_MEAN as per previous literature.^1–3 SUV_MAX and SUV_ITL of endoscopically abnormal versus endoscopically normal PET +ve segments were compared using the Mann-Whitney test.

Results 11 patients (52 gut segments) had PET within a median of 1 month of endoscopy. 21/52 segments were active on endoscopy. Of these 20/21 were also PET +ve. However, 17/31 of endoscopically negative segments were alsoPET+ve suggesting a sensitivity of 95% and a specificity of 45% in our cohort.

Raising the SUV_MAX threshold for defining a PET +ve segment from the existing (>SUV_LIVER) to > 3.5x SUV_LIVER reduced sensitivity from 95% to 86%, but improved specificity from 45% to 82% compared to the gold-standard of endoscopy.

Conclusion FDG-PET appears to be up to 95% sensitive in identifying segments with endoscopically active CD.

Several ‘false positive’ segments are also observed conferring a low specificity.

A threshold of segmental SUV_MAX signal > 3.5 x SUV_LIVER greatly improves sensitivity with a minimar reduction in specificity.

Segments which demonstrate FDG signal but are negative on endoscopy may reflect disease undetected by endoscopy, or may be false positives. A comparison with histological activity is required to clarify this.

References

1. Louis E, JNM 2007

2. Meisner, IBD 2007

3. Das CJ, Eur J Nucl Med Mol Imaging 2007

Disclosure of Interest None Declared.