Kidneys

Evaluation of renal lesions with PET imaging is limited because significant portions of the kidney are normally hypermetabolic due to the normal renal excretion of FDG.  

The lack of intravenous contrast administration further limits evaluation of renal lesions on PET/CT scans. 

Many times what appears to be an intensely avid lesion is simply pooling of FDG within a calyx or calyceal diverticulum.

Assessment of potential malignancy with PET/CT imaging relies on the CT characteristics of the lesion as much as it does its metabolic activity (e.g. the presence of fat in a lesion seen with angiomyolipomas).

Further complicating matters is the fact that many renal cell carcinomas are only mildly FDG-avid.

Very often, a potential lesion suspected on PET/CT will require a contrast exam or ultrasound for further characterization.

A more thorough discussion of renal lesions and representative cases (benign & malignant) can be found here.