In situ measurement of iron overload in liver tissue by dual-energy methods

The errors for determining liver iron content by dual-energy computed tomography (dual-energy CT) are calculated for the ideal case where only monochromatic x-ray beams are used. Because of the strong influence of spatial resolution on the radiation dose needed to reduce the error to a given level, we have also calculated the error in dual-energy transmission measurements alone, where the spatial information along the beam path is lost. The prediction of error was tested by simulations and measurements using x-rays emitted by radioactive isotopes and synchrotron radiation. Good agreement between calculation, simulation and measurement was found. It is shown that concentrations of liver iron content (disregarding variation of tissue composition) can be studied with a skin dose of about 30 mGy using dual-energy CT and even with much lower dose using dual-energy transmission measurements. However, there are sources of error besides photon noise, especially errors caused by variation of tissue composition. For example dual-energy CT, although suggested to avoid artifacts caused by fat in the case of a fatty liver, still is affected by fat. The magnitude of these errors is discussed qualitatively, and possibilities for their reduction are suggested. For a definitive estimate of errors of iron content measurements with optimized apparatus more experimental data for well defined variations of body tissue, especially in the case of haemochromatosis, are needed.