2010 Issue

17 W ITH THE ADVENT OF THE modern com- puterized tomography (CT) scanning systems, doctors can now go on a virtual tour of the complete body and can diagnose and treat illnesses that could not be done a few decades ago. At the heart of every x-ray system is an x-ray tube and receptor device that translates x- rays into an image or analytical information on material types. X-Ray CT Housing and Tube The technology of designing andmanufacturing the modern x-ray tube and accompanying solid state flat panel imaging device press the state of the art in many of the technical disciplines. This article will center on the technical challenges of the x-ray tube. VarianMedical Systems, X-Ray Products Division in Salt Lake City, Utah, is the largest independent supplier of x-ray tubes in the world. They set the standard and supply the x-ray industry with cutting edge, high performance x-ray tubes and flat panel imaging devices. An x-ray tube is a vacuum device (10-6 Torr) with a source of electrons (cathode) and a target (anode). Electrons are produced by thermionic emission from a hot tungsten filament in the cathode structure. They are accelerated in a high voltage field between the cathode and anode. The high energy electrons (40kev to 150kev) Technical Challenges of the Modern X-Ray Tube JOHN RICHARDSON PE Varian Medical Systems Since the discovery of x-rays by Conrad Roentgen in 1895, x-ray technology has affected the lives of many people. Anyone who has been to a hospital, clinic, passed through airport security, has worked in the identification of materials has been the beneficiary of x-ray technology. strike the target surface and the interaction of the electrons in the atomic structure of the target produce x-rays. Targets are either stationary or a rotating disc depending on the required energy density of the electron beam and impact area on the target. The high energy electrons from the cathode impacting the material in the target produce various energy levels of x-rays. High energy elec- trons that displace electrons in the various shells of the target atoms will produce characteristic x-rays particular to the materials in the anode. The braking action of the electrons that go into partial orbit around the target atoms produce a range of x-rays called Bremsstrahlung. Most of the energy from the electron beam is con- verted to heat (~99%). The remaining energy is converted to photon energy, or x-rays. In some CT applications, the exposure energy in some diagnostic techniques get as high as 1.4 Mega- Joules; 70kw for 20 seconds. Image resolution from an x-ray tube is a func- tion of the size of the incident x-ray beam from the target surface (focal spot). The ideal focal spot for diagnostic imaging is a point source. However, the x-ray flux needed for various x-ray receptor devices require a certain amount of electron beam energy that would melt target surfaces if the focal spot was too small. So, there is trade-off between focal spot size and image quality in each x-ray tube depending on the application. The “quality” of x-rays from a tube must comply with Federal Regulations. The quality is a func- tion of the kilo-voltage applied to the x-ray tube. Soft x-rays that could be absorbed in the skin are filtered out. The size of the x-ray beam from an x-ray tube is shaped (or collimated) to conform to only the area being examined or treated. Performance ratings for each X-ray tube are dependent on the heating and cooling of x-ray tube components. The electron beam kinetic energy from the cathode is immediately transformed to heat at the target surface and is limited by the melting temperature of the target material. Each x-ray tube specification continued on page 18