31 : 3/ Electronic Computers 



575 



If the bone is surrounded by soft tissue, the problem is more difficult. 

 The X-ray absorption coefficients of soft tissue and of bone have very 

 different variations with wavelength. Thus, any wedge which is a 



TRACE PATHS 



Dista 

 Center 2 



Proximal 3 



Figure I. Bones and wedge. X-ray photograph showing rat, 

 the three trace paths used, and the aluminum-zinc alloy wedge 

 used for calibration. After H. Schraer, The Pennsylvania State 

 University; original figure. 



good analog of bone is a poor analog of soft tissues. Although various 

 approximations are made to try to use the apparent equivalent wedge 

 mass of the soft tissue, these approximations introduce errors as large as 

 ± 30 per cent into the computed equivalent wedge thickness of a bone 

 surrounded by a large amount of soft tissue. 



