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In diagnostic CT imaging, multiple important applications depend on the knowledge of the x‐ray spectrum, including Monte Carlo dose calculations and dual‐energy material decomposition analysis. Due to the high photon flux involved, it is difficult to directly measure spectra from the x‐ray tube of a CT scanner. One potential method for indirect measurement involves estimating the spectrum from transmission measurements. The expectation maximization (EM) method is an accurate and robust method to solve this problem. In this article, this method was evaluated in a commercial CT scanner.

Two step‐wedges (polycarbonate and aluminum) were used to produce different attenuation levels. Transmission measurements were performed on the scanner and the measured data from the scanner were exported to an external computer to calculate the spectra. The EM method was applied to solve …