By Harrison H Barrett; William Swindell

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Many factors contribute to this variation. The more important ones are the acoustical transducer characteristics and the electronic signal processing. Many disease states affect the distribution of collagenous material. For example,in cirrhosis of the liver, there is often a massive buildup of collagencontaining scar tissue with a corresponding and characteristic increase in echographic signal strength. Also, soft tissue responds to injury and to invasion by some malignant diseases by proliferating the growth ofcollagenous fibers.

Brain and bone tumors are also detectable with nuclear tracers. Brain cells are particularly interesting because their membranes are remarkably impervious to all materials except essential nutrients. This so-called bloodbrain barrier breaks down when the cell becomes cancerous. Many tagged pharmaceuticals will then accumulate in the malignant region, rendering it visible on a gamma-ray image as in Fig. 16. A similar situation holds with bone tumors. Here the tumor has many of the characteristics of rapidly growing bone.

Even when this is not the case, when an involved model eventually yields to sophisticated mathematics and powerful computers, the result is often a disappointing paucity of physical insights. A simple model is usually the most fertile. Probably the most important simplification we can introduce in describing any physical system is to treat it as a linear system. Logic would demand that we define the term "system" before attempting to define a "linear system," but we shall be content with the statement that a system is anything we care to analyze.

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