The accuracy of the analogue to digital conversion (ADC) process is a critical element in the production of high quality SPECT images. Sub-optimal ADC performance can lead to the production of non-uniformities in the projection data which are amplified by the reconstruction process to produce circular artefacts in the transaxial sections. A simple method for the objective, quantitative assessment of ADC differential linearity has been developed. The analytical approach uses the fact that a differential non-linearity in the ADC will produce a non-uniformity in the digitized image which has a well defined periodicity. This can be most clearly demonstrated in the frequency space domain by determining the Fourier transform of a thick profile which is taken through the centre of a flood field image. The accuracy of the method permits deteriorations in the performance of ADCs to be detected well before significant reduction in SPECT image quality are produced.