Detectability of low-contrast objects (vessels) in subtraction imaging depends on the noise properties of the image. There is a structure imposed upon the quantum noise that is due to the attenuation of the subtracted background object; this structure is referred to as a "noise print." It is shown to be a function of the manner of presubtraction processing (linear or logarithmic) and the local thickness of the attenuator in the x-ray beam path. The influence of the noise print on the visibility of vessels in subtraction images is demonstrated.