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Development of Japanese Radiological Equipment in the Post-World War II
Period (26

The history of the development of X-ray tubes

Sumio Makino (Advisor, JIRA) Sumio Makino (Advisor, JIRA)

I. The pioneering spirit of Kikuta Koizumi in developing soft X-ray tubes

1. The history of X-ray tubes

Dr. Roentgen discovered the X-ray in November 1895. In those days, he and many other physicists did much of the experimental work of vacuum discharge, using the Geissler tube or the Crookes tube. Their results are the foundation of electric discharge physics even today. It is well known that their experimental work led to the discovery of X-rays.

The history of X-ray tube is described in detail in the book "The development of X-ray tube in Japan" written by Kikuta Koizumi, one of senior members of JIRA. This book is retained by JIRA Library as one of the precious collection of historical books.

2. About Kikuta Koizumi

The detailed history of Koizumi and his company Softex is described in the voluminous document "This Eternal Light, vol. 1 (113 pages) and vol. 2 (186 pages)" which were written by Mr. Yoshio Homma, his brother. This document is not published, and I would like to summarize it below in remembrance of his pioneering spirit.
Koizumi was born on Nov. 24, 1903 and died in 1988. His relation with JIRA was long and close. He served as Vice Chairman in 1980, contributing much as senior management for the progress of JIRA.
He was born in Sadogun, Niigata Pref. as a son of Shinto priest, graduating from the elementary school and entering a company "Taka Roentgen Rays" at the age of 12. He was recruited as an apprentice by this company that was run by a friend of the principal of his elementary school. He described his experience by saying that he was proud of his work related to X-ray, which was novel and strange in those days.
The company was officially called "Takaoka Rikagaku K.K.", and headquartered in Takaoka City, Toyama Pref. The Physics Dept. manufactured "the X-ray tube called Taka tube" in Kanazawa City, while the Chemistry Dept. manufactured potassium cyanide. The manufacturing process of the X-ray tube ranged from vacuum exhaustion to forming a glass bulb. It was supervised by Mr. Sadao Yamamuro a then leading expert on the technology. He came from Tokyo Electric (later Toshiba), which was a pioneering company in manufacturing vacuum tubes. After several years of trial, a glass X-ray tube was completed around 1920.
Through social changes of that era, "Taka Roentgen Rays" disappeared in 1921. Then, the main activity of manufacture and research of X-ray tube moved to the Kansai area. For the effort made by Koizumi in those days, readers are encouraged to refer to the above-mentioned book "The development of X-ray tube in Japan". Now moving on to the Showa era (1926-1989).


3. Establishment of Softex

(1) In the Showa era, Japanese industries were affected by the preparation for war and the subsequent war, and they failed to achieve something worth recording. For this reason, the story covers only the postwar period starting in 1945 when the war ended.
Koizumi established "Koizumi X-ray Kosha" at 4-37, Kitazawa, Setagaya-ku, Tokyo on Feb. 1, 1946. His house narrowly escaped the Tokyo air raids. At a vacant lot to the west of his house, he built a small workshop measuring about 6 m by 8 m, where five persons worked including Koizumi and his wife. Koizumi was helped by Mr. Ichimura, an expert of vacuum exhaustion and Mr. Itagawa, an expert of metalworking for anodes and cathodes. The news of the establishment of an X-ray company spread throughout Japan with the rumor that this company can repair defective X-ray tubes. Many repair orders came from X-ray technicians and related companies that are located in Kanto, Tohoku, Nagoya, Kinki, and even Shikoku, Kyushu and Hokkaido far from Tokyo.
President Koizumi said to the employees: "In the future, we will surely make our own new tubes. When we repair Japanese tubes and foreign tubes brought here, we must thoroughly study their performance and features. We have much to learn from these tubes. We must absorb the knowledge as much as possible to improve our technology." He believed that the simple work of repair contains much technological information, which will be useful for future business development. He wanted to convey his belief to the employees and encouraged them to absorb such information for future use.
Thus, the work was started vigorously. "The postwar society was in confusion. We had no materials, money, and machines. The manufacture of tubes requires the supply of gas and electricity, both of which were time-rationed. Gas was supplied for certain time zones that were suited to ordinary daily life, not a factory. The time zone was one hour around 5 o'clock in the morning, 11:30 and 5 o'clock in the evening respectively. So, the glasswork was possible only for these time zones, and the work schedule was quite irregular. Electricity was supplied at night, but it was often suspended during the daytime almost daily." Text is partly omitted. This is an extract from the above-mentioned "This Eternal Light".
-- Makino's note -- We had these inconveniences in daily life and the industrial activity for several years after the end of war. This is inconceivable now. The enthusiasm to continue to work even under such circumstances is the industrial infrastructure of present Japan. I worked for Toshiba, where we could not make an X-ray table because we could not obtain the material (plywood) of the tabletop. In 1949, when we had steep inflation, Mr. J. Dodge came to Japan as an economic advisor to GHQ. He enforced the so-called Dodge line policy, suppressing the depression and inflation, but increasing the bankruptcy and unemployment. In 1950, a war broke out in the Korean Peninsula, bringing about the so-called "special procurement boom," which triggered the restart of Japanese industries.


(2) The encounter with soft X-rays: After 1950, they welded the mica foil onto the glass surface and made a vacuum tube container. Finally they made an X-ray tube with a mica window. They were impressed with the beautiful soft X-ray image taken with that tube.
Many questions about the technology and theory were raised at once, and the challenging work for solutions started. They wanted to apply their tube technology to the manufacture of the X-ray tube. Big companies, such as Nihon Denki (NEC) and Kobe Kogyo (TEN), started the sale of medical X-ray tubes on a large scale. So, they thought that the direct competition in this was not advantageous. They had a slight hope in their repair of X-ray tubes used for X-ray analysis, in which neither Toshiba nor Shimadzu was involved in those days. They repaired the analytical X-ray tubes of different makers. Not only that, they attempted to make their own high-performance tube, model KM-3, which had a mica window.
-- Thus, Softex was established -- : Softex started to develop soft X-ray equipment. It was dubious whether it grew without any problems. External experts advised that the soft X-ray is very dangerous, and that much care and study is needed to handle it. However, Koizumi thought that the slow decision-making was an obstacle to future growth. He was determined to continue development of soft X-ray technology, and to make step-by-step efforts with patience. Thus, they started the subsequent business activity.
It was January 1953. Koizumi consulted with Dr. Takahashi, Advisor, to look for objects to be X-rayed. They walked around parks and tea gardens in Setagaya-ku to collect some insects, such as bagworms, mantises, and larvae of swallowtails. They X-rayed the insects in 5 to 8 seconds and the fish in 10 to 15 seconds, and obtained excellent images to their satisfaction. They toasted the New Year and the successful start of the new business.
Subsequently, Softex developed as a unique maker of special soft X-ray generator for small animals and plants, which was used in the field of research and education. Like every other business, their management experienced rises and falls. The tenth anniversary of foundation came in 1962. Their product was named "Genkaisen" and occupied about 80% of the market share. In commemoration of the anniversary, Koizumi left the following message.

Fig. 1 Cassette used in the early period of Showa Era (1945-1964)

The word "X-ray" reminds everyone of the radiology department of a hospital. It reminds the persons related to a science and engineering course of X ray analysis and a non-destructive test. For a long period of about 60 years since the discovery of X-ray, the research and development has been remarkable.

Immediately after the end of the war, I was hopeless and at a loss about what to do. Although I had been involved in X-ray technology, I was not sure whether it would be useful or not in the future. But, I was determined to leave it to the next generation, building a small workshop in the back yard of my house, and starting to make vacuum tubes by using the city gas, which was supplied only at mealtimes because of shortage production in the postwar period. It was the end of 1946.

Then, I managed to procure glass material. But, I could not find any materials for X-ray tube electrodes. So, I started to repair defective tubes, which were sent to us by hospitals. The glass materials and pump oil were poor. But, we attempted to repair the tubes to the satisfaction of customers, and two or three years passed like a dream.

Our company as a repairer of X-ray tubes became known to the people related to science and engineering. A customer sent us a Lindemann glass tube for analytical use. The repair was difficult. In the meantime, several companies introduced the technology of attaching a mica sheet at the window of a Geiger-Mueller counter tube. We applied the technology to an X-ray tube, and succeeded in making an X-ray tube having a mica window. We made an experiment to find what kind of X-ray would be generated by that tube. We obtained soft X-ray that is very useful at the far lower X-ray tube voltage compared with X-ray for medical use. We made an experimental X-ray apparatus for educational use and named it "SOFTEX". Since then, ten years have passed.

We would like to report our development and express our thanks to our customers who extended their support and guidance to us.

Nov. 3, 1962.

Kikuta Koizumi


4. Conclusion

In any industry, we find anecdotes and episodes. The soft X-ray tube developed by Koizumi is worth mentioning in the long history of Japanese X-ray tubes, which dates back to the Meiji era (1868-1912). Softex was particularly famous in the postwar radiation industry in Japan. The above description is only an example of Japanese industrialists in the postwar period, to which we would like to pay our respects. The present Softex Company is not related to radiation now and is run by Kazuhiko Koizumi, grandson of the founder. The company is headquartered in Ebina City, Kanagawa Pref., being specialized in IT-related products.

Kikuta Koizumi, the founder of Japan Softex K.K., was awarded the Yellow Ribbon Medal in 1971, and The Order of the Sacred Treasure, Gold and Silver Rays in 1977. He is a pioneer in soft X-ray equipment and soft X-ray tubes. The products contributed much to research and education in Biology, Physics and other special fields.


II. X-ray tube development in Toshiba

Sumio Makino, Motoshige Yoshida*, Toshio Yamamura*

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.


2. Summary

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

Sadao Tanabe, Hiroshi Sagane, Takashi Kagawa, Motohisa Tsuda (who belonged to Shimadzu formerly)

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.