[ 18 F]-FDG and [ 68 Ga]Ga-FAPI-04 PET/CT in Radioiodine-refractory Differentiated Thyroid Cancer: A Comparative Pictorial Review along with Insights into Potential for Theranostics and Nonspecific Uptake Patterns on [ 68 Ga]Ga-FAPI-04 PET-CT