|March 2009 No. 61|
1. Overview of X-ray tube development in Japan
1-1. The early stage of manufacture of X ray equipment
The development of X-ray equipment was early in Japan. At the end of 1895, X-ray was discovered in Germany. In the following year, Shimadzu Corp. already succeeded to generate X-ray.
X-ray tubes were imported from Germany. We feared that the outbreak of World War I in 1914 would stop the import. "Tokyo Denki K.K, later Toshiba K.K." quickly developed a gas tube "Giba tube, type E", the first domestic product in 1915. Since then, Shimadzu has made X-ray equipment, while Tokyo Denki has made X-ray tubes. But, Tokyo Denki failed to make enough number Giba tubes. So, it acquired the exclusive sales right of the U.S. GE-made "Coolidge tube" in 1916, selling both Giba and Coolidge tubes. In 1920, Tokyo Denki became licensed to make the Coolidge tube in Japan. (In 1923, however, the Tokyo Denki factory was totally destroyed by the Great Kanto Earthquake, resulting in temporary suspension of production of X-ray tube.)
At the end of the Taisho era (1912-1926), Toshiba investigated foreign products and succeeded in the trial production of the Coolidge tube of complete X-ray shielding type. This was called a Japanix tube and was produced commercially in 1930. Tokyo Denki in the Kanto area and Shimadzu in the Kansai area had an agreement. X-ray tubes were to be supplied by Tokyo Denki, while X-ray equipment was to be supplied by Shimadzu. In 1930, however, Toshiba also started to make X-ray equipment, and the above-mentioned agreement lapsed.
1-2. Into the Showa era (1926-1989)
At the beginning of the Showa era, we saw active development and improvement of X-ray equipment, especially of X-ray tubes. In 1929, Philips in Holland succeeded in making Rotalix (X-ray tube of a rotating anode type). This invention triggered the technical innovation of X-ray tubes globally. The era of the rotating anode type tube began. In Japan, Shimadzu and Tokyo Denki advanced the research and development of X-ray tubes.
In 1929, Toshiba started to import the U.S. Victor product. In 1930, it developed dental X-ray equipment, partially incorporating the imported components. Toshiba's own X-ray equipment was rated at 75 kV and 100 mA.
In 1967, "The History of Shimadzu Corp." was published, which described the development of the rotating anode type tube as follows. In 1950, the company received a grant from the Ministry of Education for scientific research and started development. In 1951, it produced a prototype. In April 1951, it displayed the product at the exhibition of Japan Radiology Congress. After subsequent improvement, in 1954, the company named the product "CIRCLEX" and started a full-scale sale.
On the other hand, in 1938, Toshiba made and marketed an air-cooling rotating anode X-ray tube "Mazda SP-RA", which was named "Rotanode", because Rotalix was the trademark of Philips. Although improvement efforts were stepped up actively after that, every activity was stopped by the outbreak of the Pacific War. Moreover, the factory buildings were burned down by an air raid, and all the research and production was suspended. It was in 1949 that Toshiba completed a rotating anode X-ray tube oil immersion type "XDO-R-70" and started production of a tube of full-scale oil immersion type.
1-3. Change of postwar Japanese society and development of X-ray tube
Before and during the war, the morbidity of tuberculosis was so high that it was called a national disease in Japan. Soon after the end of the war, the morbidity was decreased through mass chest screening, where we used the capacitor discharge type X-ray equipment. For this equipment, we developed a new, small triode X-ray tube (DR-66, 67), which was effective to exterminate tuberculosis.
After that, the morbidity of stomach cancer increased, and it was called the second national disease. We needed mass stomach screening at the early stage of disease. For this purpose, we developed a medium sized Rotanode for examination of digestive organs, and it was widely used. But, we had several technical problems. The tube for mass stomach screening was used for a long time under heavy loading conditions. The same problem of heavy load occurred later when we developed cinematographic X-ray examination for cardiovascular diagnosis. The age of the heavy-load tube came with the increasing demand and popularity.
In the oversea market, such a heavy-load tube was already popular. The specification was standardized for a small focus, large electric current and large heat capacity. The diameter of rotating anode target should be 125 mm and the heat capacity should be 500 kHU. If the target of 125 mm in diameter rotates at the speed of 10,000 rpm, then the circumferential speed will reach of 250 km/h. The target will be heated due to friction, resulting in trouble in the rotating mechanism such as the bearings. The rotation noise will be loud and rotation will sometimes even stop. The most important challenge was how to develop a new tube to cope with the high-tech cardiovascular radiology.
1-4. Demand for large heat capacity and countermeasure for new requirements
In 1978, we developed a fixed anode type X-ray tube CT-111. This was used for head CT and whole-body CT examination. It was the beginning of the age of CT. In the same year, the total number of Rotanode tubes produced reached 100,000. According to the rate of cardiovascular X-ray equipment becoming popular, the demand for X-ray tube of larger heat capacity increased. We developed a tube of 400 kHU and then 500 kHU with a target of 125 mm in diameter, and opened the market of large-capacity tubes. The 125 mm Rotanode was also useful for the magnified stereoscopic radiography of cerebral blood vessels, before the 3D CT image became popular.
In order to meet the increasing demand and to achieve technical innovation and modernization of the production system, we moved the X-ray tube factory in 1987. We moved from Horikawa-cho Works located in Kawasaki City to Nasu Electronic Tube Works that was newly built in Tochigi Pref.
In 1988, we developed a 1.8 MHU Rotanode for CT to meet the demand in the CT age. This tube was also used for cardiovascular X-ray equipment. We saw an increasing trend of X-ray tube of large size and large heat capacity. Generally speaking, the new trend would motivate technical innovation for the next generation. This principle applied also to X-ray tubes. The year of a big leap in innovation came. In 1994, we completed a 4 MHU X-ray tube of "liquid metal lubrication dynamic slide bearing type." We finally shifted from the mechanical bearing system to the new bearing-less system.
The above describes the prewar days partly and subsequent postwar days. According to the high-tech development of medical imaging, the X-ray tube also made technical progress for higher output and smaller focus. Finally, the mechanical bearing system was replaced by a new bearing-less system. Medical imaging by means of radiology is expected to make further advances. The X-ray tube is the foundation of such a radiology system, and its technical innovation will continue. I hope that the subsequent chapters of the history will be written by our successors.
(* The star-marked writers belonged to Toshiba Corporation, Tube and Valve Division of those days).
III. X-ray tube development in Shimadzu
1. The history of X-ray tube development before and during the war
It began in 1932 when Shimadzu placed an order for a prototype X-ray tube with "Nihon Sekiei Kogyo K.K." whose capital was 50% shared by Shimadzu. This move was in preparation for the expiry of the GE patent for X-ray tubes, which was to occur two years later in 1934. After expiry of this patent, Shimadzu acquired Nihon Sekiei Kogyo K.K. and formed Shimadzu X-ray Manufacturing Division, starting the manufacture of universal use X-ray tubes from then until now. During the war, the scarcity of raw materials made it difficult to continue manufacture. So, engineers of several X-ray manufacturing companies had regular meetings to exchange information or to study technical documents.
2. From the immediate postwar days to the Showa 40s (1965-1974)
Society was in confusion. The Allied-Forces General Headquarters issued a recommendation that vacuum tube technology should be promoted as a peaceful industry, and that several companies should make joint research for that purpose. The engineers of the related companies exchanged technical information. The X-ray tube council (under the jurisdiction of the Ministry of Education) was established in 1947, being chaired by Professor Kanji Honda of the University of Tokyo. This council included many researchers from universities, academic laboratories, and manufacturers for active discussion and investigation, contributing much to the improvement of manufacturing technology for X-ray tubes at several companies.
The demand increased for an X-ray tube of higher output. In order to meet the demand for large heat capacity, the company received a grant of the Ministry of Education for scientific research in 1950. It started development of a rotating anode type X-ray tube and completed trial production in 1951. It displayed the product at the exhibition of Japan Radiology Congress. How to transport the exhibit tube from Kyoto to Tokyo, the venue of the exhibition became a problem, because of its fragility. An engineer decided to ride on the night train. He put the packed tube on his knees and spent the whole night without sleeping. On his way he heard a short noise coming from the package. He unpacked at the exhibition site and felt relief to see that the tube was not broken.
When a rotating anode tube is manufactured, the most difficult issue is the anode rotating mechanism and the bearing lubrication. Naturally we cannot use lubrication oil. We looked for suitable substances, which is solid at room temperature and liquid at high temperature. Its vapor pressure must be low. Finally, we selected "lead."
We improved the bearing and rotation mechanism not only at the early stage of development but also thereafter. We consulted with specialists in Mechanical Engineering in universities and the company. All their advice failed to meet the requirements. The bearing mechanism under extreme conditions (vacuum, high temperature, abnormal lubrication) was beyond their comprehension. The trial and error of on-site engineers was the only solution. After that, a Shimadzu rotating anode tube was named CIRCLEX, and was mass-produced from 1954.
Shimadzu developed a triode X-ray tube in 1958 when Toshiba also developed the same kind of tube. This tube is used for a capacitor-discharge type X-ray equipment, which is unique in Japan. Negative voltage is applied to the cathode of this tube to switch the X-ray on/off. The triode tube is used widely to control the X-ray output in various applications. Initially, a triode tube used a fixed anode, and gradually it used a rotating anode.
Electrons may be reflected on the surface of anode and they may hit the anode again, emitting the so-called off-focus X-ray of low energy. Shimadzu developed an H-type CIRCLEX for the first time in the world in 1960, which shielded the off-focus X-ray. It also developed S-type CIRCLEX having an anode having the largest diameter at that time (diameter of 110 mm, 2 electrodes/3 electrodes). The maximum working voltage was 125 kVp and 150 kVp.
In 1965, Shimadzu received a grant from the Ministry of International Trade and Industry, developing a rotating anode tube having a small focus and large heat capacity. It completed its prototype in 1966. The rotating speed of anode was increased three times. The target was made of lamination tungsten and molybdenum. Zirconium was deposited on the back of the target. The specification was intended for higher performance without increasing the weight of target. In 1966, Shimadzu also developed a TG (Tiny Grid-controlled) type CIRCLEX, the smallest tube in the world at that time for mobile X ray equipment (capacitor-discharge type X-ray equipment).
In addition to diagnostic X-ray tubes, it developed various kinds of special tubes. In 1949, it received a grant from the Ministry of Education for scientific research. In 1950, it completed an X-ray tube SCR-45-2 for contact irradiation. The X-ray is emitted out from the tip of tube in the direction of the tube axis, being used especially for intracavitary radiotherapy.
For industrial use, the company developed an X-ray tube for X-ray TV fluoroscopic equipment to test non-destructively the helical welded part of steel pipe for oil transportation. This tube has a small focus, which is suitable for magnified fluoroscopy. The tube must be capable of releasing a large current at a high voltage. The tube was called 150-0.4/2.5, and was completed in 1963. Depending on the purpose of use, X-rays can be emitted from the tube tip perpendicularly to the tube axis, with the anode grounded. A variation of this type was also developed. In addition, it developed a beryllium-window tube and a full circumferential radiation type, which is suitable for the fluoroscopic examination of the rotary blade of a helicopter.
3. Reliability of X-ray tube and image performance
Another topic is IEC/JIS standards about X-ray equipment. The standards specify the maximum working voltage of an X-ray tube, and the maximum voltage of an X-ray high voltage generator. These two maximum voltages seem to mean the same thing. But, this is where opinion divides between the tube specialists and the equipment specialists. A conclusion still remains to be reached. The tube specialists insist that the specified voltage is the absolute upper limit. On the other hand, the equipment specialists retort that any electric equipment has transient phenomenon, and that the maximum voltage should have a certain allowable range including usual errors. In other words, it is difficult to increase the withstand voltage of tube. It is also difficult to completely eliminate the error of equipment of output. Therefore, the specialists of both fields cannot reach consensus, because Toshiba started from tube technology, and Shimadzu started from equipment technology. This was the general impression among the engineers involved in this controversy.
(Back to the subject) For example, it is possible to apply the maximum voltage of 150 kV to an X-ray tube of maximum working voltage of 150 kVp. Originally, the unit of kVp meant the peak of the rectified sine wave voltage. In the case of an X-ray tube, however, it means the peak that includes the surge voltage and that can be applied to an X-ray tube.
The exhaustion and aging stages in the manufacturing process of an X-ray tube directly affect the improvement of the withstanding voltage of tube. The process was improved by around 1990 to the extent that an X-ray tube of nominal 150 kVp withstands 185 kVp.
Furthermore, the exhaustion stage was fully automated through computer control. Manual work is needed only to connect a tube for exhaustion and to perform final tip off. The automation helped to ensure the uniform quality of product.
On the other hand, we attempted to improve the image quality as follows. The distribution of electrons in the X-ray tube focus (within the electronic beam) affects the sharpness of the X-ray image. This effect was studied from the viewpoint of MTF in order to improve the image quality. For example, the distribution of electrons in the triode X-ray tube focus becomes non-uniform. As a result, a sufficient tube voltage cannot be obtained. To solve this problem, we place two cathodes (filament and convergent electrode, two each) and form the focus at slightly different points on the anode surface. Each cathode does not use the grid voltage. A bias voltage is applied to the convergent electrode in order to control the current. In the actual operation, the two focuses are caused to work at the same time. This method increases the tube current, making the electron distribution uniform, and improving the X-ray image quality. This method can be applied to make a triode X-ray tube that ensures better image quality.
The magnified contrast radiography of abdominal artery requires a 0.1 mm focus and the exposure of 0.1 sec, 30 mA. During an experiment to make a 0.1 mm focus, we have found the tube current affects the electron distribution and focus size itself. After several trials, we produced a 0.1 mm focus tube for the selective magnified continuous radiography of hepatic artery, for the first time in Japan. This tube is used effectively for the magnified contrast radiography of cerebral blood vessels.
The X-ray diagnosis of digestive organs requires snapshot at a good timing. So, it is necessary to rapidly switch from fluoroscopy to radiography. Shimadzu's over-tube type fluoroscopic/radiographic table requires a 0.8 mm focus and the exposure of 300 mA, 0.1 sec. To meet this requirement, an X-ray tube must rotate three times faster. It took 4 sec for the anode to switch from stationary state to a rotation three times the speed. We rotated the anode even during fluoroscopy at three times the speed, and shortened the switching time from fluoroscopy to radiography. To enable this operation, we needed to modify the bearing and rotation mechanism of the tube, and improve the rotation life to a great extent.
4. The recent trend
The Eimac (later, Varian) product was once extremely popular as a rotating anode X-ray tube of large heat capacity for X-ray CT. Japanese companies bought and used the products. Shimadzu developed the same metal housing as Varian. Later, we found that the glass housing is more cost-effective than the metal housing, and developed a 2 MHU tube having the glass housing.
In recent years, an X-ray tube unit including the housing, not an X-ray tube alone, is required to function as a component of the X-ray system. Shimadzu made an X-ray tube unit containing an X-ray tube having the anode diameter of 100 mm. This is a world record in terms of small size and lightweight. This model is being produced in the greatest quantity. It is incorporated into an X-ray system, contributing to the improvement of functionality and performance.