[en] It is particularly important for a single photon tomographic system (SPECT) that the measurement conditions be-well-defined. Even then, traps exist. It has been observed, in comparing two ''identical'' systems that, for the same resolution, sensitivity, uniformity etc, differences of almost an order in the apparent contrast were observed. This effect was a result of a slightly different energy window setting, and caused by the effect of scatter increasing the tail effect. The definition of a set of standard phantoms and measurement protocols should be a high priority for one of the international standards agencies. Without such a programme, it is possible that some centres will use SPECT systems without them being properly set-up, and, as a result, obtain potential very high clinical false positive rates