I think each body structure has different uptake values for an abnormal diagnosis. I've read the SUV uptake goes from 0 to 15, and the cut off value above 2.5 could be a sign for malignancy. In neck nodes, above 8 is possible indication for advanced disease. FDG-PET scans are especially good for identifying recurrent neck nodal disease, and diagnosis of distant Metasteses, and FDG-PET/CT is better than FDG-PET. FDG-PET, including with CT, is said to be superior in sensitivity, specificity, and accuracy over MRI and CT alone, but there are different percentages for diagnosis, recurrence, nodal disease, cancer type, and from what I've seen PET usually has the higher finding, but not in all cases. The drawbacks with PET is that it is more expensive, needs specislized tech training, does not usually show tumors less than 1cm, and has a high rate of false positives, if taken less than 3 months or 12 weeks.