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K-edge imaging is commonly used for viewing contrast pharmaceuticals in avariety of multi-energy x-ray imaging techniques, ranging from dual-energy andspectral computed tomography to fluoroscopy. When looking for the K-edge signalof a specific contrast, by taking measurements either side of the K-edge, it isfound that the K-edge is not always observable for low concentrations. We havealso observed that the ability to see the K-edge is unit dependent - a K-edgethat is not observable in computed tomography (CT) reconstructed linearattenuation units can often be made visible by converting to Hounsfield units.  This paper presents an investigation of this K-edge hiding phenomenon. Weconclude that if a multi-energy x-ray measurement of any K-edge materialcontains a signal of any other material, then there will be a positiveconcentration of that K-edge material below which its K-edge cannot be observedwithout extracting the K-edge signal through means of basis decomposition.Mathematical descriptions of this limiting minimum concentration is provided inthree cases - multi-energy radiographic projection images; and reconstructedmulti-energy CT images for both linear attenuation and Hounsfield units. Twoimportant properties of this limiting concentration are provided: the minimumconcentration for Hounsfield units is always strictly less than the minimumconcentration for linear attenuation; and the minimum concentration forradiographic projections will typically be much larger than the minimumconcentration for reconstructed linear attenuation. Finally experimentalobservation of this phenomenon is presented using data collected by the MARSspectral CT scanner equipped with a CdTe Medipix-3RX camera operating incharge-summing mode, where it is also shown that the K-edge can be recoveredthrough basis decomposition.

The Hidden K-edge Signal in K-edge Imaging