Dual-energy subtraction imaging techniques allow the tissue and bone structures in the patient to be imaged separately, thus removing some obscurity resulted from the overlapping of the two structures. Furthermore, they provide the potential for the tissue or bone contents to be quantified for diagnostic use. Thus, capabilities for dual-energy subtraction imaging are often incorporated with new digital radiography techniques. There are three different schemes for implementing dual-energy subtraction imaging techniques. Among them, dual-kVp and sandwich detector approaches are two most often used schemes. A third scheme is the single kVp-dual filter approach which allows a more flexible control of the spectra while avoiding kVp switching. It is suitable for digital radiography techniques using two linear detector arrays. In this paper, the signal-to-noise properties of these three schemes is computed for various combinations of kVp, filters and patient thicknesses (tissue and bone). Based on the signal-to-noise analysis, they are compared to each other for the efficiency of x ray usage, dose efficiency, and accuracy for background subtraction and thickness measurement.