Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Rotor~~ ~+-statorFIGURE 5-11. The anodeof a rotating anode x-raytube comprises a tungstendisk mounted on a bearingsupportedrotor assembly(front view, top left; sideview, top right). The rotorconsists of a copper andiron laminated core andforms part of an inductionmotor (the other componentis the stator, whichexists outside of the insert-seeFig. 5-12). Amolybdenum stem (molybdenumis a poor heat conductor)connects the rotorto the anode to reduceheat transfer to the rotorbearings (bottom).The focal track area of the rotating anode is equal to the product of the tracklength (21tr) and the track width (~r), where r is the radial distance from the trackto its center. A rotating anode with a 5-cm focal track radius and a 1-mm trackwidth provides a focal track with an annular area 314 times greater than that of afixed anode with a focal spot area of 1 mm X 1 mm. The allowable instantaneousheat loading depends on the anode rotation speed and the focal spot area. Fasterrotation speeds distribute the heat load over a greater portion of the focal track area.Power input ------(time 1)Magnetic 1field linesRotorassemblyPowerinput(time 2)"---X-ray tubeinsert wallMagneticfield lines~FIGURE 5-12. A cross-sectional end viewof the stator/rotor assembly shows electromagneticcoils energized at two differenttimes. Power applied sequentially topairs of electromagnets produces a rotatingmagnetic field with an angular frequencyof 3,000 (single-phase operation)to 10,000 (three-phase operation) rpm.The magnetic field "drags" the iron componentsof the rotor to full rotationalspeed in about 1 second, during whichtime x-ray exposure is prohibited.