Schematic for imaging herpes simplex virus 1 thymidine kinase reporter gene (HSV1-tk) expression with reporter probes ¹²⁴I- or ¹³¹I-labeled FIAU and ¹⁸F-labeled FHBG. The HSV1-tk gene complex is transfected into target cells by a vector. Inside the transfected cell, the HSV1-tk gene is transcribed to HSV1-tk mRNA and then translated on the ribosomes to a protein (enzyme), HSV1-TK. After administration of a radiolabeled probe and its transport into the cell, the probe is phosphorylated by HSV1-TK (gene product). The phosphorylated radiolabeled probe does not readily cross the cell membrane and is trapped within the cell. Thus, the magnitude of probe accumulation in the cell (i.e., the level of radioactivity) reflects the level of HSV1-TK enzyme activity and the level of HSV1-tk gene expression.