kV imaging utilizes a different method of filtration to attenuate the beam. The filter may be something like aluminum, ex 200 kVp x-ray beam filtered by 1 mm thick aluminum filter. This distribution includes the glass envelope of the x-ray tube, the surrounding oil, and the exit window of the tube housing as well. This so called inherent filtration is equivalent to approximately 1 mm Al in most x-ray tubes. Can also be a combination filter such as a Thoraeus filter TCA (tin copper aluminum)
For a megavoltage x-ray beam however, the beam is hardened by the inherent filtration of the transmission target as well as by transmission through the flattening filter.
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