Traditional research on disease mechanisms in animal models mainly has relied either on the detection of morphological changes of the diseased tissues obtained through anatomical imaging, or on the excision and pathological study of the tissues of interest. These methods often require long time periods for measurable changes to occur and large numbers of test animals because multiple animals are often sacrificed at each experimental point for histological testing.
During the last few years, exciting new molecular imaging agents have emerged from research laboratories that allow highly specific fluorescent-, luminescent-, and radioisotope-based imaging of disease processes at the molecular level within living animals. These in vivo molecular imaging agents provide the potential for rapid detection of specific molecular and metabolic changes within target tissues in animals (or humans) long before morphologic changes can be detected. In addition, these molecular changes can be monitored in vivo without sacrificing the animal, resulting in lower cost, time savings, and improved data by using the same live animal for continued studies.
One major advantage of recent molecular imaging agents over anatomical imaging is their applicability to “dark-field” imaging methods that allow high levels of target signal over the surrounding background signal. Dark-field contrast, however, does not typically provide the appropriate contextual anatomic information for useful localization of the molecular imaging signals within the animal. Limited anatomic context of dark-field agents has been provided using digital imaging overlay techniques in which the dark-field contrast is super-imposed on a reflection image of an experimental animal.
Although the overlay methods are beneficial, repeated imaging of the same animal in different imaging sessions often results in misinterpretation of the signal localization, as animal repositioning is difficult. The white-light reference image may be suitable for localization of large tumor masses, but lacks the anatomical context required for repeatedly localizing smaller signals of interest and/or mapping the molecular signals to bones or other anatomical structures within the animal. Hoping to realize the full potential of the dark-field molecular imaging agents, researchers are beginning to apply multimodal instrumentation that combines dark-field contrast with radiographic anatomical imaging in one system.
To illustrate this move towards multi-modal molecular imaging, Kodak Molecular Imaging Systems (New Haven, CT) has introduced a line of in vivo small-animal imaging systems, including a model that allows the capture of X-ray images. These X-ray images provide the detailed penetrating anatomical guideposts that greatly enhance the localization of the in vivo optical or radioisotopic molecular images. The Kodak Image Station In-vivo F allows for very high-resolution, multi-wavelength fluorescence, luminescence, and radioisotopic imaging in small animals, while the Kodak Image Station In-vivo FX includes all of the capabilities of the In-vivo F and the high-resolution X-ray Imaging Module using a Radiographic (X-ray) Imaging Screen.
In these Kodak systems, phosphor screens coupled to speed-enhancing interference optics efficiently convert the ionizing radiation into light. The light is emitted by the screens and captured by the charge-coupled device (CCD) camera to form the image. Two different screen assemblies are available: one optimized for the high-energy radioisotopes such as 111In, 99Tc, and 18F, and the other for the low-energy, high-resolution requirements of X-ray imaging.
For ease of use, multimodal systems should use the same operation and hardware for optical imaging of an animal (or multiple animals) as they do for X-ray imaging, with the animal immobilized and positioned in the chamber directly above the imaging chamber window. For fluorescence, excitation light from a high-intensity lamp is directed through the selected excitation filter to the animal. Fluorescence from the imaging agent inside the animal is then emitted and separated from the excitation light as it passes through a wide-angle emission filter. The fluorescence enters the 10x lens zoom and is focused onto a 4-million-pixel, cooled CCD. The digitized readout is efficiently interfaced to a personal computer (Windows or Mac). Multiple optical images of different molecular entities with different fluorescent tags can be captured in the same animal by simply selecting different filters and capturing additional images. In a time-lapse mode, multiple images can be captured in the same session to track the bio-distribution of the imaging agent.
Once the desired optical images are captured, the radiographic (X-ray) screen can be moved into the imaging field by simply sliding the screen under the animal chamber, for instance. The phosphor screen comes into close contact with the thin plastic sheet that supports the animal in the animal chamber, placing the screen essentially at the same focal plane setting used with the optical images. In the Kodak system, the image capture setting in software is switched to X-ray and the microfocus X-ray generator emits a maximum energy of 35 Kvp for the desired imaging time (typically
As the images of each modality are captured without movement of the animal and with no change in optical focus or zoom, the images can easily be merged or overlaid in software for precise co-registration.
In vivo imaging systems greatly enhance the localization of molecular signals in live animals. These systems are now used by top academic, biotechnology, and pharmaceutical research institutes worldwide. The flexibility of these systems allows the combination and co-registration of multiple wavelengths and multiple modalities of imaging including optical, radioisotopic, and radiographic imaging. Several studies are now in progress that will further detail the utility of combining, co-registering, and performing the appropriate analysis of the multiple imaging modalities.