Figure 4.

Mechanisms of optical imaging modalities and examples from oncolytic virotherapy and gene therapy studies

(A) Fluorescence: Incident excitation wavelength (Ex. λ) excites the fluorophore within the intracellular GFP and causes an emission wavelength (Em. λ) to be released for detection. Imaging panel: GFP distribution in neonatal mice after intravenous (i.v.) administration of AAV-8-CMV-GFP. Left to right: whole body, eyeball, liver, kidney, muscle, and brain. (B) Bioluminescence: Intracellularly expressed firefly luciferase (Fluc), Renilla or Gaussia luciferase (Rluc/Gluc), or NanoLuc bind with substrates D-luciferin, coelenterazine (CTZ), or furimazine, respectively, which enters the cell from exogenous sources. Emission wavelengths (Em. λ) are released upon processing of the substrate by luciferase. Imaging panel: Firefly luciferase imaging in mice after i.v. administration of an AAV-8-NFκB-Luc-2A-GFP biosensor or the constitutively expressed control AAV-8-SFFV-Luc-2A-GFP. (C) Cerenkov luminescence: NIS expressed on the cell surface concentrates ¹³¹I radiotracer and concentrates it into the cell. Upon radioactive decay, ¹³¹I releases light (Em. λ), which can be captured by a charge-coupled camera device. Imaging panel: Medulloblastoma xenografts in mice infected with measles virus expressing NIS (MV-NIS). Top panels are tumor bioluminescence and lower panels are ¹³¹I imaging from radiotracer concentrated into the tumor by NIS. (D) Photoacoustic imaging (PAI): Laser light absorbed by tissue is translated to heat and converted into ultrasound waves (photoacoustic effect), which are used for imaging. Tyr produces melanin, which readily absorbs the laser. Imaging panel: PAI and ultrasound imaging (US) of MCF-7 tumors with or without Tyr expression in mice. Figure made with BioRender (https://biorender.com/) and Photoshop CC 2019. With the exception of GFP, protein shapes are artistic interpretations. Permissions for imaging panels: (A and B) From Karda et al.,¹²¹ modified via cropping of the original figure and reproduced under CCA 4.0 International License (https://creativecommons.org/licenses/by/4.0/). (C) From Hutzen et al.¹²² and reproduced under CC Attribution License (https://creativecommons.org/licenses/by/2.0). (D) From Qin et al.¹⁰³ and reproduced under CC Attribution 3.0 Unported License (https://creativecommons.org/licenses/by/3.0/).