Evaluation of method to minimize the effect of X-ray contrast in PET-CT attenuation correction

In the Discovery ST PET-CT scanner, the attenuation map used for correction of the 511 keV emission attenuation is derived from the CT scan using a continuous conversion scale which has a range of slopes dependent on the CT kV and the CT number. High CT numbers are assumed to be bone, and contrast material which generates CT numbers in the range of bone results in over-estimated 511 keV attenuation values. For normal concentrations of contrast media, the elevation in the emission counts of contrast-enhanced organs is not significant, but artifacts can be produced when there are unusually high concentrations of contrast material, especially when the contrast has moved between the CT and the emission scan. In these cases it is possible to reduce the artifactually high emission counts using a tissue-contrast conversion scale derived from CT number and attenuation measurements made on various concentrations of contrast in phantoms and measurements of contrast enhancement in patients. If this conversion scale is applied to an image containing bone, the emission image is under-corrected for the bone attenuation. The effect on reconstructed activity of using either the bone scale or the contrast scale is evaluated in a range of organs. The use of the contrast scale results in a reduction of activity, which is not significant except in the vicinity of dense bone.