Disclosed are systems, devices and methodologies related to calibration of an ion based imaging apparatus such as a proton computed tomography scanner. In some implementations, energy degrader plates having known water-equivalent thickness (WET) values can be introduced to an ion beam to introduce different energy degradation settings. Energy detector responses to individual ions subject to such energy degradation settings can be obtained. Such responses can be normalized and correlated to water-equivalent path lengths (WEPL) of the ions based on the known WET values. Such calibration utilizing degrader plates can be performed relatively quickly and can yield accurate WEPL values that facilitate estimation of, for example, a CT image based on relative stopping power of an object.
