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The effect of object size on the capability of positron emission computed tomography to measure isotope concentrations in a cross section was studied. The relationship between the apparent isotope concentration in an image and the true concentration was measured as a function of object size for three instrument resolutions and four convolution filters. The relationship between image size and object size was also measured under the same conditions. Depression of apparent isotope concentration in an image for objects equal in size to the instrument resolution (FWHM) was significant (50% for a cylinder and 25% for a bar). For objects larger than 1.0 FWHM, accurate object sizes can be estimated from the images. Thus, reasonably accurate and practical schemes of compensation for object size effects can be implemented for objects larger than 1.0 FWHM. Accuracy in quantitating isotope concentrations in smaller objects is seriously compromised by the loss of sensitivity to the object size and the large correction factors required to compensate for instrument response. The results of the measurements were found to be in good agreement with theoretical predictions for ideal systems of comparable resolution.