The term venous translucence (or translumination) has been used in phlebology since 1996 by surgeon Pedro Fernandes Neto during ambulatory clinical exams in Brazil. His results were published in the annals of the national and international congresses of angiology. Venous translucence is the process of reflective image visualization of veins by light, which reaches up to the superficial venous system. It is a non-invasive method. Since it is a simple, low-cost technique it can be repeated as needed, which is useful in disease-process monitoring. It is a new diagnostic procedure, still undergoing investigation; more analysis is necessary to hone its technical aspects. Venous translucence is based on optical physics. It is caused by the refraction, absorption and reflection of light (whose principle is the dispersion and absorption of light). The color which is not absorbed is reflected, and is the one that is seen. Therefore, venous translumination is based on the incidence of luminousity on the vein, where part of the light is absorbed and another reflected (supplying a silhouette of the vein in question).
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Technical aspects
Steps must be taken to avoid artifacts or luminous pollution during the venous translumination. Reflection and refraction of light are important to avoid distorted images which can appear during a scanning. Many are due to inappropriate sources which would provoke light loss similar to luminous pollution.
During the venous translumination, the amount of reflected light depends on the index of refraction which can be altered by the venous thrombosis. Certain types of lamps process more dispersion than reflection, and therefore favor the formation of artifacts.
Another aspect is the inability to be obtain useful images with a fiber-optic source due to the high irradiation frequency. It is also known that the reflection in an optic fiber source is initially processed internally. Certain instruments are not specific to obtain reflected images. The illumination of the skin by distant infrared light, without the impression of the source on the skin, does not reveal a diagnostic image. The translumination transductor must be in direct contact with the skin. The process should be similar to that used in Doppler ultrasonography.
Tissue
Tissue transluminated by white light has a refraction index in agreement with its texture. By the impression of the camera on the skin, red and yellow colors are observed. The red color is soon dispersed in the skin and the yellow surrounding it begins to alter its tonality with the change in source direction. A shade with the change of source direction arises. It is known that the refraction index changes in agreement with the spectrum of transmitted light; when the white light is projected and finds an obstacle, it becomes separated into the three elementary colors (red, blue and green). Light during translumination may have greater dispersion or reflection, depending on how the source is placed on the examined area. This handling may improve or degrade the image, depending on the examiner's experience.
Light interaction with skin color
Skin is opaque to light. In physics light reception, heat or other type of radiant energy on the part of molecules is called absorption. When tissues are illuminated during translumination, some light is absorbed and some reflected. It is known that an object which absorbs all radiation is seen as black; the pigments that give color to the skin and the other tissues, absorb certain wavelengths of white light and transmit radiant energy. This is an aspect of color as captured by our vision. The mechanism by which certain substances absorb more light than others apparently depends on their molecular structure. Light, when reaching more pigmented areas, disperses and reflects more easily; the refraction index is smaller, and it is difficult for white light to penetrate below the skin. Studies of the distribution of elastin and collagen fibers in patients with dermal lesions need to be analyzed for changes. A study of tissue consistency using translumination and dermatoscopy could supply important data complementing the diagnosis of some collagen diseases and study tissue aging. Skin color affects the effectiveness of translumination; individuals with light skin have better venous visualization during the translumination than those with darker skin.
Blood viscosity and flow
The erythrocytes absorb more light because they are oxygenated. Considering that aspect, we can deduce that venous blood has a light absorption different from arterial blood (in which sanguine viscosity is greater, due to the higher concentration of CO2. In that sense, venous blood has a greater ability to reflect light. When an arterial-venous fistulae is transluminated, there are few reflected images because flow velocity is higher and sanguine viscosity lower than in the venous segment. It is impossible to visualize arteries by translumination, because they do not provide a reflection due to their accelerated flow. Another aspect is that iron, which composes hemosiderin, emits light of several wavelengths when stimulated.
Histogram
In translumination, the spectrum of white light is divided into different wavelengths (colors). A histogram represents the graphic visualization of these colors and the luminescence of the obtained images. In a histogram, the intensity of the luminescence is accompanied by a gray baseline that decreases as the source approaches, where the red scale is more intense and is represented by a line in ascension. The scales of blue and green colors represent the refraction indexes of the light emitted by the transluminator in contact with the studied area. In the histogram when the selection of a scale (for example, the green scale) is disabled, the luminescence intensity of captured images may be mapped. Bollinger et al. reported their experience: denominated fluorescence videomiscroscopy, based on the video capture of images and study of their luminescence through light emission stimulated by 20% sodium-fluorocein (0.3 ml/l of blood). The principles of spectrographic analysis of this test are similar to those used to evaluate the luminescence of captured images by venous translumination, and the histogram also evaluates the scales of red, blue and green (RGB). All organic components are composed of chemical elements that emit light according to their wavelength. This is why the histogram analysis of transluminated images could define an organic element according to the quality and amount of their components.