Small animal scanner

The Small Animal Scanner is an application of TCT technology for the imaging of small animals, such as mice and rats. The many advantages of TCT over fluorescence imaging make this an important tool in the study of disease and drug interactions.

Figure 1 is a photograph of the animal imaging chamber of OptoSonics' prototype small animal scanner. An array of four fiber-optic bundles delivers pulsed laser light to the animal being imaged. The wavelength of the light can be adjusted over a range of 680-950 nm. A small stepper motor (not shown) rotates the mouse about the vertical axis during TCT data collection.

Figure 1. OptoSonics Small Animal Scanner

One important advantage of TCT is the ability to image the target using different wavelengths of light. This greatly improves the sensitivity of the device to the targets of interest. As an example of the application of multi-spectral imaging, OptoSonics has studied the use of indocyanine green (ICG) as a contrast-enhancing agent. ICG is approved for use in humans and travels freely throughout the vascular system. It has been widely used in the measurement of cardiac output. Figure 2 shows the absorption spectrum of ICG with a strong peak at 750 nm that makes ICG an ideal probe for multi-spectral imaging.

Figure 2. Absorption spectrum of ICG

Data is taken at 2 wavelengths: λ₁ where absorption is greatest, and λ₂ where absorption is virtually nil. By subtracting the two images, all of the background tissue structure, which presumably has about the same absorption at both wavelengths, is removed. As a result, the sensitivity to ICG is greatly enhanced. Figure 3 shows two subtraction images at the wavelengths of 783 and 836 nm. Multi-spectral imaging improves the sensitivity of ICG detection to a remarkable 5 nanomolar (i.e., 10^-9) level.

Figure 3. ICG concentrations shown in nanomoles

Another exciting example of the potential of multi-spectral imaging lies in the inherent optical absorption properties of hemoglobin in blood. Figure 4 shows the absorption curves of oxygenated hemoglobin (Hb-O2), deoxygenated hemoglobin (Hb-R), fat (lipids), and water.

Figure 4. Absorption spectra of Hb-O₂, Hb-R, lipids, and H₂O

Any of these four constituents can be accentuated in a TCT image by choosing the appropriate wavelength of stimulating light. For example, a wavelength of 950 nm or greater could be used to interrogate the water distribution, while a wavelength of 750 nm could be used to accentuate deoxygenated hemoglobin in the blood. TCT images acquired at several wavelengths can be analyzed to determine the relative proportions of these constituents within any tissue. The oxygenation state of the tissue, an important indicator of underlying disease, can be determined in this manner, and a 3-D map of this distribution can be made.

Water-enhanced cross-sectional image of a mouse brain at 1064 nm	Vascular-enhanced mouse brain image at 750 nm

OptoSonics is exploring techniques to attach highly absorbing optical molecules, such as ICG, to antibodies and peptides that are targeted to specific receptors associated with disease. The high sensitivity of multi-spectral TCT allows us to detect and image very small amounts of these probes.