16 BIOPHYSICALLY ACTIVE X-RAYS 



The absorption of bone, composed chiefly of Ca3(P0 4 ) 2 , is proportional 

 to 



3 X 20 4 + 2 X 15 4 + 8 X 8 4 = 614,000 



so that the relative absorption 



Bone 



= 152 



Water 



i.e., bone is 152 times more opaque than water. 



If a lead bullet is embedded in the bone, then its absorption is propor- 

 tional to the fourth power of the atomic number of lead, i.e., 82 4 . Lead 

 is therefore about 74 times more opaque than bone. 



Barium sulphate, BaSC>4, which is most commonly used as an ali- 

 mentary contrast agent, has an absorption proportional to 



56 4 + 16 4 + 4 X 8 4 



It is very opaque because of the high atomic number of Ba. 



Two or more adjacent substances transmitting different proportions 

 of the incident energy show intensity contrasts which are very large 

 for long wavelengths and small for shorter ones. Thus when soft x-rays 

 are used, for radiographic work, they will produce a greater contrast 

 between adjacent substances than x-rays of shorter wavelengths. 



This contrast effectiveness is illustrated in the use of gases such as 

 sterile oxygen to give radiographic relief effects. In the well-known 

 pneumoperitoneum method, sterile oxygen is injected into the peri- 

 nephritic fatty tissues, and, in consequence of the small absorption of 

 oxygen, the more absorptive kidney is thrown into relief. The same 

 method has been used to inflate the Fallopian tubes. Carbon dioxide 

 has also been employed for this purpose. 



Quantitative Statement of the Absorption Laws 



The simple absorption law discussed on the preceding pages is a 

 logarithmic depreciation law of the form 



where I is the incident intensity, / is the intensity of the x-radiation 

 transmitted by a unit thickness, e is the base of the Napierian logarithms 

 (e = 2.72), and m is a constant for a given medium of thickness d and 

 is called the coefficient of absorption. Its numerical value, in any 

 given medium, depends upon: (a) the nature of the medium, (6) the 

 wavelength of the incident radiation. For logarithms to the base 10 



