A portable animal holder configured for use in one or more in vivo imaging systems, said animal holder comprising: two adjustable, substantially planar and parallel frames, comprising a first frame and a second frame, wherein the first frame comprises a first imaging window and the second frame comprises a second imaging window, the first imaging window being secured to the first frame and the second imaging window being secured to the second frame, wherein said imaging windows are substantially parallel, and wherein each of the first imaging window and the second imaging window allow transmission of electromagnetic radiation therethrough; and an adjustment mechanism adapted (i) to secure said animal within said animal holder, thereby preventing substantial movement of said animal during imaging of said animal in said one or more in vivo imaging systems, wherein, upon positioning of the animal between the first frame and the second frame, a first portion of the animal is flush to the first imaging window, a second portion of the animal is flush to the second imaging window, and the animal is arranged such that electromagnetic radiation can pass through the first imaging window, the animal, and the second imaging window during in vivo imaging; and (ii) to deterministically and repeatably set a separation distance between the first imaging window and the second imaging window, wherein the adjustment mechanism is configured, upon adjustment, to maintain parallelism between the first imaging window and the second imaging window, such that the first frame and the second frame are connected to each other via the adjustment mechanism, thereby maintaining a separation between the first frame and the second frame. two adjustable, substantially planar and parallel frames, comprising a first frame and a second frame, wherein the first frame comprises a first imaging window and the second frame comprises a second imaging window, the first imaging window being secured to the first frame and the second imaging window being secured to the second frame, wherein said imaging windows are substantially parallel, and wherein each of the first imaging window and the second imaging window allow transmission of electromagnetic radiation therethrough; and an adjustment mechanism adapted (i) to secure said animal within said animal holder, thereby preventing substantial movement of said animal during imaging of said animal in said one or more in vivo imaging systems, wherein, upon positioning of the animal between the first frame and the second frame, a first portion of the animal is flush to the first imaging window, a second portion of the animal is flush to the second imaging window, and the animal is arranged such that electromagnetic radiation can pass through the first imaging window, the animal, and the second imaging window during in vivo imaging; and (ii) to deterministically and repeatably set a separation distance between the first imaging window and the second imaging window, wherein the adjustment mechanism is configured, upon adjustment, to maintain parallelism between the first imaging window and the second imaging window, such that the first frame and the second frame are connected to each other via the adjustment mechanism, thereby maintaining a separation between the first frame and the second frame. (i) to secure said animal within said animal holder, thereby preventing substantial movement of said animal during imaging of said animal in said one or more in vivo imaging systems, wherein, upon positioning of the animal between the first frame and the second frame, a first portion of the animal is flush to the first imaging window, a second portion of the animal is flush to the second imaging window, and the animal is arranged such that electromagnetic radiation can pass through the first imaging window, the animal, and the second imaging window during in vivo imaging; and (ii) to deterministically and repeatably set a separation distance between the first imaging window and the second imaging window, wherein the adjustment mechanism is configured, upon adjustment, to maintain parallelism between the first imaging window and the second imaging window, such that the first frame and the second frame are connected to each other via the adjustment mechanism, thereby maintaining a separation between the first frame and the second frame.