Among the analyze of the physical action of radiations on biological systems, there is the subject of the biological effects. We shall discuss the following examples of biological effects: mutation, ultraviolet-light inactivation and reactivation.
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Heat is familiar to all of us. We can feel heat entering our bodies from the summer Sun or from hot coffee or tea after a winter stroll. We can also feel heat leaving our bodies as we feel the chill of night or the cooling effect of sweat after exercise.
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The light which is not absorbed is re-emitted in all directions, producing what is called scattering of the light; we mentioned the two extreme cases of total reflection and total transmission. Most cases are intermediate. If electron oscillations are equally possible in all directions, the incident unpolarized light beam will emerge essentially as it entered the suspension—except for the phase shift already mentioned. If, however, the particles or molecules in suspension are not isotropic, i.e., the electrons can oscillate more readily in one direction than in another, an incident unpolarized light beam will be split, because waves oscillating in one direction will have their phases shifted more than those oscillating in another direction. The net result is that so-called anisotropic molecules produce two plane polarized emergent beams. Since these emerge in somewhat different directions, the phenomenon is called double refraction or birefringence.
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A host of medical imaging techniques employ nuclear radiation. What makes nuclear radiation so useful? First, γ radiation can easily penetrate tissue; hence, it is a useful probe to monitor conditions inside the body. Second, nuclear radiation depends on the nuclide and not on the chemical compound it is in, so that a radioactive nuclide can be put into a compound designed for specific purposes. The compound is said to be tagged. A tagged compound used for medical purposes is called a radiopharmaceutical. Radiation detectors external to the body can determine the location and concentration of a radiopharmaceutical to yield medically useful information. For example, certain drugs are concentrated in inflamed regions of the body, and this information can aid diagnosis and treatment as seen in Figure 32.4. Another application utilizes a radiopharmaceutical which the body sends to bone cells, particularly those that are most active, to detect cancerous tumors or healing points. Images can then be produced of such bone scans. Radioisotopes are also used to determine the functioning of body organs, such as blood flow, heart muscle activity, and iodine uptake in the thyroid gland.
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