<P> The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses . Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation . The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time . </P> <P> The tube current and exposure time affect the dose and therefore the contrast of the image . </P> <P> Since the introduction of scattered electron traps, directly cooled liquid metal anode bearings, rotating frame tubes and other modern technology the term has become misleading when used for the objective comparison of the performance of medical rotating anode X-ray tubes . Accordingly, the IEC 60613 standard has been revised in 2010 . Anode heat storage capacity and implicitly "Heat Units" were abandoned and replaced by terms of practical relevance, e.g. NOMINAL CT ANODE INPUT POWER . Historically, heat units were referring to the amount of heat stored in a solely radiation cooled anode inside an electron reflecting glass envelope . In radiography, in particular in radiology, a heat unit was a usual unit derived from the Joule and measuring the quantity of heat in a device, in particular the thermal energy that an X-ray tube must be able to support and dissipate during and after an exam . </P> <P> Historically, this unit comes from the need to express a quantity of energy whatever the waveform of the electrical source power (direct or pulsating). The heat produced in these devices is (in Joule): </P>

Distance between anode and cathode in x-ray tube