Contrast-enhanced digital mammography using spectroscopic x-ray detectors may improve image quality relative to existing contrast-enhanced breast imaging approaches. We present a framework for theoretical modelling of signal and noise in contrast-enhanced spectral mammography (CESM) and apply our framework to systems that use a spectroscopic amorphous selenium (a-Se) field-Shaping multi-Well Avalanche Detector (SWAD) which uses avalanche gain to overcome the low conversion gain of a-Se. We modelled an approach that uses an a-Se SWAD with 100x100 μm2 detector elements, a converter thickness of 300 μm, an avalanche gain of ten, a 10-keV electronic noise floor and two energy bins. We modelled the influence of quantum efficiency, conversion gain, avalanche gain, characteristic emission, electronic noise, energy thresholding and image subtraction on the modulation transfer function (MTF), noise power spectrum (NPS) and iodine contrast. We investigated the choice of energy thresholds for the task of visualizing iodine signals. Our analysis demonstrates that reabsorption of characteristic photons yields energy-bin-dependent MTFs. As a result, spectral subtraction of low-energy and high-energy images enhances high spatial frequencies relative to low spatial frequencies. This effect, combined with better noise performance when using the lowest possible threshold to separate low-energy photons from electronic noise, results in better imaging performance than when reabsorption is suppressed through thresholding. Our theoretical framework enables quantifying trade offs between contrast, spatial resolution and noise for analysis of novel approaches for CESM, and provides a theoretical platform for comparison of CESM with existing approaches.