94 ELECTROMAGNETIC RADIATIONS AND MATTER 



The electrons lost may have been valence, or bonding, electrons — active 

 in holding the molecule together. In covalent bonding two paired electrons 

 form the bond between carbon atoms, as in a sugar molecule for example. 

 Ionization weakens the bond and perhaps breaks it; in any case the unpaired 

 electron left is chemically very reactive and will make a new bond at any 

 time or place. Cross-bonding of molecules, the synthesis of new molecules, 

 polymerization of old ones, etc., all can occur. It is not hard to envisage 

 how such reactions could adversely affect the tightly geared steady-state of 

 normal living tissue. 



It is convenient to reserve further discussion of the effects of ionizing radia- 

 tions until the principles of radioactivity have been outlined. The radioac- 

 tive emanations, alpha, beta, and the nucleons, are ionizing radiations, as 

 are gamma and X, and the effects of all are conveniently discussed together. 



Diagnosis by X Rays 



The absorption of electromagnetic radiation increases with increasing 

 density of the absorbent. Differentiation of diseased tissue from normal is 

 based on this fact. The higher the speed of the electrons which impinge 

 on the target metal, the harder the X rays so produced. Machines avail- 

 able today produce X rays from electrons which have been accelerated by 

 thousands to millions of volts. In general, the greater the voltage, the greater 

 the energy of the X-ray photons, and the greater their penetrating power. 



For example, at 40,000 v (i.e., 40 kilovolt potential (kvp), in radiation 

 terminology) almost any tissue will stop some of the X radiation and cast 

 a shadow on the fluorescent screen or photographic plate behind it. At 80 to 

 100 kvp, commonly used in medical diagnosis, the radiograph displays 

 shadows which differentiate fat and other soft tissues from air space and 

 from bone. 



Whenever it is possible to insert molecules containing heavy metal atoms 

 into a region of interest, differentiation of tissues in the region is enhanced 

 (Figure 4-10). Thus barium sulfate solution is commonly administered as 

 an enema so that the lower part of the intestines may be examined (by X 

 radiation). Iodine in a variety of compounds is also widely used to increase 

 differentiation. For instance, in iodophthalien it is preferentially taken up 

 by the liver and stored in the gall bladder; thus gallstones, if present, are 

 easily seen. Similarly, the kidneys, uterus, blood vessels, and even the heart 

 can be made visible to X-radiography (see Figure 4-10 (b), for example). 

 Location of broken bones, of swallowed pins, of stomach ulcers and of 

 tumors is routine. 



The use of X rays for diagnosis introduces the serious question of the ex- 

 tent of the damage done by the rays absorbed. A complete fluoroscopic 

 gastrointestinal examination with barium sulfate can be done by a competent 

 physician with the dose to the region irradiated not exceeding 20 rads (the 



