Filament Thoriated tungsten rhenium or molybdenum Thermionic emission

Filament: Thoriated tungsten, rhenium, or molybdenum. Thermionic emission at 2, 200 o. C forms space charge Vaporization leads to arcing and tube failure, grounded metal jacket elevates arcing. Duel focus is common. Ave. tube life is 10, 000 to 20, 000 exposures. Appendix D 1: Components of the x-ray tube Focusing cup: see appendix D 2 Anode: Stationary and rotating at 3400 rpms (low speed) and 10, 000 rpms (high speed rotors). (See composition of anode illustration appendix D 2) Target, focus, focal point, focal spot, and focal track are terms used to describe area of electron bombardment on anode. 7 -15 mm L 1 -2 mm W. 1 -. 2 mm T Tube rating charts and anode cooling curve Actual and effective focus: See Line focus principle appendix D 2 Off focus radiation: Photons produced on tube parts other than the focal spot. Lead shielding protects patients from off focus and leakage radiation. Heat Units (HU) = k. Vp x m. As x S x generator constant Heat unit constants for: 1 = 1 (Correction factors) 3 = 1. 35 HF = 1. 4

Appendix D 2: Components of the x-ray tube Focusing cup: Nickel housing around filament repels electrons into narrow beam. Space charge effect: 1, 200 m. A is upper limit due to the repulsion of electrons in the space charge. Focusing cup with space charge Grid biased (pulsed or controlled) tubes: A 2000 volt negative potential on the focusing cup attracts the space charge for precision pulse control Saturation current: When k. Vp is sufficently high all electrons are drawn from the space charge during the exposure. Space charge compensator ups m. A Line focus principle: The effective focus is controlled by the actual focus and the angle of the anode. The actual focus is controlled by the size of the filament and the charge on the focusing cup. When size of focal spot is referred to, it is the effective that’s cited. Anode angles must be less than 45 degrees for the line focus principle to work. The result is improved thermal loading and better spatial resolution. Angles range from 7 to 17 degrees, 12 being average. To cover a 17” field at a 40” SID a 12 degree angle or greater is required. Anything less results in anode cutoff. Pin hole camera photograph of a focal spot. “Double banana” shape Composition of anode Anode heel effect: Due to the angle of the anode amd the absorbtion of photons in the anode, distribution of photons is greater on cathode side of tube. Effect is most pronounced on a 17” field size. AP thoracic spine is classic example. Envelope: Pyrex glass vacuum tube. Photon energy exits window. Focal spot size ranges between. 1 and 3 mm Fractional focal spots: < 1 mm Blooming: Increase in focal spot size due to increases in m. A. Rotors: 3400 rpms (low speed) and 10, 000 -12, 000 rpms (high speed rotors).