Limitations to iodine isolation using a dual beam non-K-edge approach.

In dual-beam selective iodine imaging, images of an object are made with each of two spectrally different x-ray beams. The mean beam energies may either straddle the 33 keV iodine K-edge or both lie above the K-edge. Both patient exposure considerations and the availability of sufficient x-ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone-to-iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x-ray spectra. Observed contrast ratios match the predicted value very well. In dual-beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum, energies for dual beam images are discussed.