As SPECT imaging has become more quantitative with the advent of the attenuation correction, SPECT parametric imaging can become a reality if the data acquisition and the numeric reduction procedures can be simplified. A method that is clinically practical for acquiring quantitative parametric images of unidirectional metabolic rate constants and apparent blood space is proposed. Its application to PET imaging with FDG was investigated. This procedure requires a short postinjection waiting period, three sequential imaging scans and one blood sample (1 ml) during each scan, obviating the requirements of continuous blood sampling, "assumed" rate constants (autoradiographic method) and difficult nonlinear regression computations. The effect of the early-phase blood input function is computed directly from the image. The clinical procedure is completed 1 hr after FDG injection. The computation time for generating 21 metabolic rate image slices and blood space slices is negligible (30 sec after image reconstruction). Preliminary human studies on brain, heart, liver and tumor were performed. The method was tested on seven normal subjects. The results showed that the rapidly changing early-phase blood input can be derived from the raw image and that the metabolic rate images of this method agreed with the results from the graphical analysis method, using continuous sampling, and with published data from three-compartment models. This study is clinically more practical and computationally simpler as a method to acquire parametric images of the metabolic rate constant, Ki, and the apparent blood space Vd for unidirectional tracers. Applying this simple quantitative parametric imaging method to routine clinical studies may improve the accuracy of routine clinical evaluations.