Fluorescence tomography is an imaging technique used mainly in in vivo diagnostics. It is based on the use of fluorescent dyes that serve as biomarkers. The technique is now mostly used in research or prenatal studies.
What is fluorescence tomography?
Fluorescence tomography detects and quantifies the three-dimensional distribution of fluorescent biomarkers in biological tissues. Figure shows injection of biomarker. Fluorescence tomography detects and quantifies the three-dimensional distribution of fluorescent biomarkers in biological tissues. The so-called fluorophores, i.e. the fluorescent substances, first absorb electromagnetic radiation in the near-infrared range. They then re-emit radiation in a slightly lower energy state. This behavior of biomolecules is called fluorescence. The absorption and emission takes place in the wavelength range between 700 – 900 nm of the electromagnetic spectrum. Polymethines are usually used as fluorophores. These are dyes that have conjugating electron pairs in the molecule and are thus able to accept photons to excite the electrons. This energy is then released again with the emission of light and the formation of heat. As the fluorescent dye glows, its distribution in the body can be visualized. Fluorophores, like contrast agents, are used in other imaging procedures. They can be applied intravenously or orally, depending on the area of application. Fluorescence tomography is also suitable for use in molecular imaging.
Function, effect, and goals
The application of fluorescence tomography usually takes place in the near-infrared range, because the short-wave infrared light can easily cross the body tissue. Only water and hemoglobin are capable of absorbing radiation in this wavelength range. In a typical tissue, hemoglobin is responsible for approximately 34 to 64 percent of the absorption. Therefore, it is the determining factor for this procedure. There is a spectral window in the range of 700 to 900 nanometers. The radiation of the fluorescent dyes also lies in this wavelength range. Therefore, the short-wave infrared light can penetrate biological tissue well. Residual absorption and scattering of the radiation are limiting factors of the method, so its application is restricted to small tissue volumes. Fluorophores used today are mainly fluorescent dyes from the polymethine group. However, since these dyes are slowly destroyed upon exposure, their application is considerably limited. As an alternative, quantum dots made of semiconductor materials can be used. These are nanobodies, but they may contain selenium, arsenic and cadmium, so their use in humans must be ruled out in principle. Proteins, oligonucleides or peptides act as ligands for conjugation with the fluorescent dyes. In exceptional cases, non-conjugated fluorescent dyes are also used. For example, the fluorescent dye “indocyanine green” has been used in humans as a contrast agent in angiography since 1959. Conjugated fluorescent biomarkers are not currently approved in humans. Therefore, for application research for fluorescence tomography, only animal experiments are performed today. In these experiments, the fluorescence biomarker is applied intravenously and then the dye distribution and its accumulation in the tissue under investigation are examined in a time-resolved manner. The body surface of the animal is scanned with an NIR laser. During this process, a camera records the radiation emitted by the fluorescent biomarker and assembles the images into a 3D movie. This allows the path of the biomarker to be tracked. At the same time, the volume of the labeled tissue can also be recorded, making it possible to estimate whether it may be tumor tissue. Today, fluorescence tomography is used in a variety of ways in preclinical studies. However, intensive work is also being done on possible applications in human diagnostics. In this context, research for its application in cancer diagnostics, especially for breast cancer, plays a prominent role. For example, fluorescence mammography is believed to have the potential to be a cost-effective and rapid screening method for breast cancer. As early as 2000, Schering AG presented a modified indocyanine green as a contrast agent for this procedure.However, an approval is not yet available. An application for the control of lymph flow is also being discussed. Another potential field of application would be the use of the procedure for risk assessment in cancer patients. Fluorescence tomography also has great potential for the early detection of rheumatoid arthritis.
Risks, side effects, and hazards
Fluorescence tomography has several advantages over some other imaging techniques. It is a highly sensitive technique in which even minute amounts of the fluorophore are sufficient for imaging. Thus, its sensitivity is comparable to nuclear medicine PET (positron emission tomography) and SPECT (single photon emission computed tomography). In this respect, it is even superior to MRI (magnetic resonance imaging). Furthermore, fluorescence tomography is a very inexpensive procedure. This applies to the investment in equipment and the operation of the equipment as well as to the performance of the examination. In addition, there is no radiation exposure. A disadvantage, however, is that the spatial resolution decreases drastically with increasing body depth due to the high scattering losses. Therefore, only small tissue surfaces can be examined. In humans, the internal organs cannot currently be imaged well. However, there are attempts to limit the scattering effects by developing runtime-selective methods. In this process, the strongly scattered photons are separated from the only slightly scattered photons. This process is not yet fully developed. There is also a need for further research in the development of a suitable fluorescence biomarker. The current fluorescence biomarkers are not approved for use in humans. The currently used dyes are degraded by exposure to light, which is a considerable disadvantage for their use. Possible alternatives are so-called quantum dots made of semiconductor materials. However, due to their content of toxic substances, such as cadmium or arsenic, they are not suitable for the use of in vivo diagnostics in humans.
