ABSTRACT
A system operates on an input signal to produce an output signal. Systems can be used to model all parts of radiologic imaging. Systems can be classified in terms of their properties. They can be classified as continuous or discrete, having memory or memoryless, linear or nonlinear, time invariant or nontime invariant, and statistical or deterministic. The matrix model can be used to represent both time invariant and nontime invariant systems. The complexity of the various system models is of importance in understanding why certain models can be used for systems of a particular type. The eigenfunctions of a system are signals which pass through a system unchanged except possibly for a change in scale. The eigenfunctions are also called the characteristic functions of a system since they can be used to characterize the system. A particularly simple description of a system can be obtained using the eigenvalues.
