Temporal relation and frequency of PET-MRI Gallbladder ¹⁸F FDG activity: An observational study

Objectives To investigate PET-MRI gallbladder (GB) ¹⁸F FDG activity in non-oncologic patients (pts) with normal liver (L) enzymes and bilirubin levels.

Methods Retrospective observational study. 53 consecutive patients with no malignancy had 164 initial presentation and follow-up clinical PET scans that included 82 TOF PET-CT and 82 same day PET-MRI. Patient demographics are in Table-1.

GB and liver FDG activities as SUVmax and SUVmean measured on PET images of PET-CT and PET-MRI studies utilizing fused-images via manual ROIs. Early (mean ∆t: 60 min) SUV_GB, SUV_liver and delayed (mean ∆t: 144 min) SUV_GB, SUV_liver PET FDG activity values and respective Uptake Ratios (UR) as (GB/L) -SUV _max and (GB/L) -SUV _mean were calculated and analyzed. SUV is unitless, and obtained using a 1.2cm diameter VOI placed in the posterior right hepatic lobe and over the center of a GB (0.5-1.0cm) and all SUVs are calculated based on pts body weight. Statistical significance of the measurementsexpressed as p values of < 0.05.

Results Table-2:

No pts had GB or GB-fossa abnormality on CT and/or MRI images. All pts had normal serum Liver enzymes and bilirubin levels; Serum direct and total bilirubin levels, Aspartate Aminotransferase, Alanine Aminotransferase, Alkaline phosphatase values were all normal, less than reference values: < 0.3 mg/dl, 1.2 mg/dl, 40 U/L, 41 U/L, 130 U/L, respectively. An inverse relation between GB volume and GB FDG intensity were present. There were 4 patients with cholecystectomy.

Conclusions PET images of patients with normal serum liver enzymes and bilirubin levels, obtained 121-181 min after ¹⁸F FDG injection, GB FDGactivity is significantly higher than the 60 min images. Delayed uptake-phase is a major factor in prominent GB visualization on PET-MRI images. High GB volume attenuates elevated GB FDG activity; conversely, very high GB FDG activity is associated with a very small GB volume. If not recognized, an increased gallbladder FDG activity in a non-distended GB might be mistaken as a focal liver lesion, liver metastasis. Recognition of non-pathologic increased FDG gallbladder activity may be important to eliminate misdiagnosing it as a malignancy, liver metastasis or gallbladder pathology, especially when it occurs in a non-distended gallbladder.