EMISSION TOMOGRAPHY by PET or SPECT allows after certain corrections, to calculate the isotope concentration in the tissue. However, if one wants to image the underlying physiological parameter that the tracer is designed to trace, then these concentration images must be transformed pixel-by-pixel, using appropriate tracer kinetic models. These transformed images are called Parametric Images. They are scaled in the relative units or absolute units pertaining to the parameter imaged, conventionally mi/100 g/min for blood flow, ml/100 g for distribution volume and nM/litre for receptor density to give some specific examples. The text gives a brief review of parametric imaging. The basic tool is the convolution integral. It was introduced for tracer kinetic analysis by Seymour S. Kety in 1951,' and soon after applied by his group for processing autoradiographic images of brain slices to calculate cerebral blood flow in ml/100 g/min' - . The first sections explain the convolution integral and the basic equation of Kety for readers having elementary knowledge of calculus.