2010 Issue

18 has a defined focal spot size and related maximum energy exposure limits that can be applied to the target. Also specified is the maximum average power in kilo-watts the x-ray tube unit can dissipate continuously. Some electrons rebound from the initial impact on the target surface and heat adjacent components. This energy can be as high as 30%of the initial electron beam energy. Components are repeatedly heated and cooled within seconds and stress the material properties. The resulting creep, fatigue and annealing of these components cause engineering challenges in the x-ray tubes. Artifacts are created in diagnostic images when electrical breakdown or arcing occurs in the x-ray tube. Each tube needs to be electrically stable at the maximum rated voltage (40kv to 150kv for medical diagnostic x-ray tubes). High voltage is needed to create an electrical field for the ac- celeration of electrons between the cathode and anode. To sustain this electrical field, components need to be specified such that the potential is stable with no partial discharge, arcing or corona during exposures. Materials and surfaces need to be “clean” of particles or void of field emission sites. Materials must be free of trapped gases that could ionize in the high voltage field and cause electrical breakdown. Dielectric materials must be chosen to stand off the rated high voltage required in the x-ray tube. Thesematerials need to bemechanically stable, resist thermal cycling and be free of gas. Most dielectric materials used in x-ray tubes are made of glass and ceramics. The targets used in x-ray tubes need to be designed from conductive, high melting point, mechanically stable materials. Targets repeatedly heat and cool withinminutes from room temperature to 800°C. They must absorb up to 1.4Mega-Joules per exposure and up to 10 Meg-Joules of bulk heat while spinning at 10,000 RPM. Modern x-ray targets aremade of a Molybdenum disc with a 1mm track made of Tungsten. Minor amounts of other materials are added for track stability and target stiffness. The bearings in x-ray tubes use metallic lubricants such as lead and silver. The thickness of these lubricants is measured in angstroms. Traditional petroleum lubricants cannot be used because they would vaporize from the heat and ionize in the high voltage field causing electrical instability in the x-ray tube. X-ray tubes are contained in an enclosure called the housing. The hous- ing is a structural support for the x-ray tube. It must sustain up to 30G’s of stress in CT systems. The housing contains dielectric fluid to sustain high voltage stability and aid in heat transfer from the x-ray tube. The housing also has a stator to boost the x-ray tube target disk to 10,000RPM within seconds. The housing also provides x-ray absorptionmaterial to attenuate stray x-rays from the tube unit. Other technical challenges in designing x-ray tubes include complex chemical surface preparations and cleaning procedures, vacuum pro- cessing equipment, high voltage power supply equipment, radiation compliance test equipment, x-ray beamquality test equipment, structural material design synthesis for 30g operation in CT applications and various braze, diffusion bonding and glass forming technologies. All of these technical challenges are addressed at Varian Medical Systems in the design of x-ray tubes. There are state of the art software tools that aid in the design and verification analysis of tube components. Mechani- cal stress, precision bearing design, electron optics, thermodynamics, radiation attenuation all have associated software simulation programs that help resolve the technical issues in x-ray tube development. These tools help us at Varian engineering create new x-ray technology which in turn will improve the health care, safety and security of our society. Mr.Richardson,analumnusoftheUniversityofUtah,isaProfessionalEngineer,andhasservedvarious membersocietiesoftheCouncilinvariouscapacitiesoveranumberofyears.Heisassociatedwithas anengineeringmanageratVarianMedicalSystemsfor42yearsdesigningx-raytubes.Heholdsnine patents on x-ray tubes. VISUALIZATION — continued from page 17