High performance under Challenging Conditions
Siemens developed the STRATON® X-ray tube for high-performance CT scanning in extreme operating conditions.
Benefits of the STRATON design
The STRATON tube differs significantly from a standard tube. Unlike conventional tubes, the anode in the STRATON tube is cooled directly, creating a powerful tube which is much smaller than conventional designs (Fig. 2). This means that two X-ray tubes can be installed in dual source platforms for scans at high temporal resolution, which is essential especially for cardiac imaging. As the anode is cooled directly, long runoff scans or perfusion studies are not interrupted by cooling processes.
Technology and operating conditions
X-ray tubes are subject to high voltages of up to 140 kV and high currents. The materials used in the tube design must be able to withstand a high vacuum, extreme temperatures and permanent
temperature changes. New technology had to be developed for the STRATON tube design, including the deflection coils which shape the electron beam. The new design can shape different focal spot sizes with just one emitter. In each X-ray tube 1% of electrical energy applied results in an X-ray beam. The rest of the electrical energy is converted into heat. When the electron beam hits the anode, the energy density reaches values up to half the energy density on the surface of the sun. A power of 100 kW is directly applied to the focal spot by the generator (Fig. 1). While the power is applied, the temperature in the focal spot reaches 2,500 °C, and the temperature on the focal track hits 2,000 °C. The back of the anode has – due to the direct cooling – a temperature of only 200 °C, resulting in a temperature gradient of more than 1,800 °C/cm. Emitter temperatures reach up to about 2,500 °C, and must also be kept very stable at high temperatures. The STRATON tube components must also endure other extreme conditions like significant centrifugal forces. The tube rotation frequency is 160 Hz. With the additional scanner rotations, the STRATON tube has to grapple with 40 times the gravity (40 g), which is about 10 times higher than the centrifugal force a passenger is exposed to in a roller coaster. Even though these extreme conditions put the tube under enormous stress, continuous technical improvements have made the Straton tube one of the most reliable and long-lasting tubes in the industry.
Wear and tear affecting an X-ray tube
All X-ray tubes are consumables and the wear caused during operation will sooner or later lead to failure. One example of deterioration is emitter erosion. Over time, the cathode material becomes thinner until it melts. Physical stress caused by the temperature changes can cause micro leaks. The constant bombardment of the anode with electrons leads to a rough surface which eventually results in insufficient dose.
The need to increase tube life
In an ideal world, a technical system should survive for its designed service life and fail after this period as illustrated in Fig. 3 (blue line). As the variables affecting wear are hard to predict, the service life of an X-ray tube cannot be ensured and a tube failure may occur at any time. By examining a large number of tubes, the failure rate and the average time until a tube will fail can be measured. The failure distribution over time can be expressed in a linear relationship Fig. 3 (red line). Siemens constantly monitors the quality, investigates the reasons for tube failure and continuously improves clinical capabilities and the quality of its X-ray tubes. With tube optimization, physical conditions will still affect the service life of the tube, however the tube failure will decrease as illustrated in Fig. 3 (green line). The area between the red line and the green line represents the average increased tube life.
Siemens service solution
Siemens offers a unique service for remotely determining the remaining life span of a tube. Thanks to the Guardian program including TubeGuard, a replacement tube is ordered before a failure occurs. To cover the financial risk, Siemens provides a warranty for its tubes and offers a service contract during the life cycle.