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Figure 5: MRI and PET over MRI superimposed brain images of the patient who had been infused with autologous cytolytic T cells expressing IL-13 zetakine and HSV1-tk genes. Images were acquired approximately two hours after 18F-FHBG injection. The patient had a surgically resected tumor (1) in the left corner and a new nonresected tumor in the center (2), near corpus callosum of his brain. The infused cells had localized at the site of tumor 1 and also trafficked to tumor 2. 18F-FHBG activity is higher than the brain background at both sites. Background 18F-FHBG activity is low within the central nervous system due to its inability to cross the blood brain barrier. Background activity is relatively higher in all other tissues. Activity can also be observed in the meninges. The tumor 1/meninges and tumor 2/meninges 18F-FHBG activity ratio in this patient were 1.75 and 1.57, respectively. Whereas the average resected tumor site/meninges and intact tumor site to meninges 18F-FHBG activity ratio in control patients were 0.86 and 0.44, respectively.