Modern Diagnostic X-Ray Sources

Technology, Manufacturing, Reliability

Rolf Behling

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000376166
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Biomedical Science
• Index: Technology & Engineering

Now fully updated, the second edition of Modern Diagnostic X-Ray Sources: Technology, Manufacturing, Reliability gives an up-to-date summary of X-ray source technology and design for applications in modern diagnostic medical imaging. It lays a sound groundwork for education and advanced training in the physics of X-ray production, X-ray interactions with matter, and imaging modalities and assesses their prospects. The book begins with a comprehensive and easy-to-read historical overview of X-ray tube and generator development, including key achievements leading up to the current technological and economic state of the field.

The book covers the physics of X-ray generation, including the process of constructing X-ray source devices. The stand-alone chapters can be read in order or in selections. They take you inside diagnostic X-ray tubes, illustrating their design, functions, metrics for validation, and interfaces. The detailed descriptions enable objective comparison and benchmarking.

This detailed presentation of X-ray tube creation and functions enables you to understand how to optimize tube efficiency, particularly with consideration for economics and environmental care. It also simplifies faultfinding. Along with covering the past and current state of the field, the book assesses the future regarding developing new X-ray sources that can enhance performance and yield greater benefits to the scientific community and to the public.

After heading international R&D, marketing and advanced development for X-ray sources with Philips, and working in the X-ray industry for more than four decades, Rolf Behling retired in 2020 and is now the owner of the consulting firm XtraininX, Germany. He holds numerous patents and is continuously publishing, consulting and training.

Chapter 1

Historical introduction and survey

Prof. Dr. Wilhelm Conrad Roentgen, posing with one of his early tubes in January 1897 for a statue in his honor, to be erected in Berlin, Germany. (Courtesy of German Roentgen Museum, Remscheid-Lennep, Germany.)

In January 1897, summarizing an intense year of ups and downs, Professor Dr. Wilhelm Conrad Roentgen confessed to his dear friend Ludwig Zehnder: “Meanwhile, I have provisionally sworn that I do not want to deal with the behavior of the [X-ray] tubes, as these dingus are … capricious and erratic …” (see Zehnder, 1935, p. 66, translated). After the following, readers may dare to disagree.

1.1 The discovery—November 1895

The history of X-rays and the evolution of the technology of its sources and its medical and industrial application has fascinated now for more than 125 years in English literature (see Behling, 2018a,b; Behling, 2020b; Assmus, 1995; Hofman, 2010a,b; Kemerink, 2012; Kemerink et al., 2013, 2016, 2019; Luis & Nascimento, 2014; Mould, 2007), in German (Clebsch, 1872; Glasser, 1959; Kuetterer, 2005; Stamer, 1995), and in many others. Although alternative sources exist and in spite of their deficiencies (see Behling & Grüner, 2018; Behling, 2016), vacuum electronics in the form of sealed-off X-ray tubes have been and will also remain the affordable sources of medical diagnostic X-rays in the future. The following will track the various branches of the development of the technology, including adjacencies, and briefly honor a few of those many scientists, developers, craftsmen, business leaders, and artisans who have pushed for innovation.

Modern scientists and developers might misconceive the undertaking of the professor as a typical end-of-the-week Friday afternoon gambling session. However, Roentgen’s well-known custom of rigorously planning his experiments tells a different story. Roentgen discovered the invisible light of X-radiation on Friday, November 8, 1895. He had set out to confirm Philip Lenard’s results on cathode rays using partially evacuated glass tubes, similar to the replica depicted in Figure 1.1, and a large Ruhmkorff inductor.

Figure 1.1 A “tube”; replica of a very early version of a lengthy tube-like X-ray tube. Electrons were freed in a gas discharge and upon ion bombardment of the aluminum cathode (left) when this electrode was charged close to the highest negative potential. The origin of X-rays was the interior of the glass wall to the right end, the landing spot of “cathode rays.” The anticathode (top right) pulled scattered electrons off. To work correctly, the glass wall had to show a greenish fluorescence upon the impact of the electrons. During the first years of investigation, Roentgen consumed dozens of tubes, only very few survived, cited from Kuetterer (2005). (Picture courtesy of Philips.)

Figure 1.2 portrays Roentgen’s original laboratory setting. The darkness of a rainy late fall evening in Würzburg, Germany, may have helped sharpen his eyesight.¹ As his adopted daughter J. B. Donges-Roentgen later reported (see Glasser, 1959, p. 3), he was experimenting alone at late hours in his laboratory, after all other personnel had left. His color-blind eyes were extraordinarily sensitive. He was surprised by a peculiar glow of special salt crystals scattered on the table and a sheet of cardboard painted with barium tetracyanoplatinate at one side (see Dyson, 1990; Kuetterer, 2005; Pavlinskii, 2016). Roentgen aimed at investigating cathode rays and ultraviolet light. When the gas pressure in the tube was comparatively low, the spark inductor generated relatively high spikes of alternating high voltage, and the envelope of the glass tube fluoresced, it struck Roentgen that the glow of the scintillator resisted measures to douse it. Seemingly opaque objects downstream of the tube in the pathway to the screen were not completely extinguishing the glow, as Roentgen had expected. Flipping the cardboard screen did not significantly weaken the glow either.

1 Around November 8, 1895, Würzburg probably experienced a series of rainy days and overcast skies with ­sunset at 4:45 P.M. Reports for Bremen und Berlin, Germany, at Chroniknet.de (2020).

Figure 1.2 Ruhmkorff inductor (left) connected to Crookes tube. Picture of the original setting in the University of Würzburg, Germany. (Courtesy of German Roentgen Museum, Remscheid-Lennep, Germany.)

Using a Hittorf–Crookes tube and risking destruction, Roentgen varied voltage and vacuum to extremes, the key to his success. Roentgen successfully tried other ways to generate cathode rays and with it X-rays like Tesla’s incandecent lamps (predecessor to Coolidge-Lilienfeld tubes) and even electrode-less samples. He punctured many of them (the metal target or the glass), which must have been a painful experience. Only very few artifacts, now at the German Museum in Munich, seem to have survived until present time. Roentgen optimized detection and, within a period of weeks, identified key characteristics of the newly discovered agent. He stayed for more than 2 weeks overnight in the laboratory and later wrote (see Glasser, 1959, p. 87): “I hadn’t told anyone about my work; I informed my wife that I was doing something about which, if they found out, people would say, Roentgen must have gone crazy.”

Shadows of objects became visible when the residual gas pressure was in the optimal range, and the voltage from the inductor reached a level of some dozen kilovolts. The spark gap emitted an audible crackle. The shadows on the paper screen pointed to the glass tube as the source of the unknown agent rather than electric leads or other items. X-rays emerged from the location on the glass wall that showed the brightest fluorescence, the area where cathode rays, later identified by Thomson as streams of electrons, hit. Metals, such as aluminum and platinum, were better targets. As Julius Pluecker and others had experienced before, magnetic deflection of cathode rays changed the paths of light emission and now also the origin of X-rays. Roentgen employed magnetic focal spot deflection, which has become a state-of-the-art means to cancel out artifacts in modern computed tomography (CT) systems. Given constant high voltage limited by the spark gap, the tube current turned out proportional to the intensity of the luminescence of the scintillator screen. The shadows became better defined when the tube was further away from and the screen closer to the object. Roentgen analyzed the system magnification M. The transmitting agent propagated rectilinearly. Its intensity fell with the inverse square of the increasing distance to the source. Initially, Roentgen failed to identify the characteristics of light such as reflection or interference. He concluded an upper limit of the refractive index of aluminum of 1.05. Later, the deviation of the absolute value from unity indeed turned out comparatively small and even negative. Although not sure about the basic physics, he laid the firm ground for early diagnostic and therapeutic applications of X-rays.

The working principle until the development of thermionic cathodes in 1913 by Coolidge, when gas eventually became obsolete, was always similar. Upon ionization, the number of electrons moving through a gas cloud in ion tubes increased exponentially along the journey, characterized by the first Townsend coefficient. Electrons escaped toward the target, where they interacted with atomic nuclei to generate X-ray photons. Ions were drawn to the cathode and released more electrons upon impact, a process characterized by the second Townsend coefficient. While processes in the Geissler tubes from the 1850s were described well enough by the first Townsend coefficient, the second became the dominating parameter for X-ray production. A sufficiently low gas pressure was required to allow electrons to gain enough speed before the next collision with a gas molecule and high-energy impact on the target. A characteristic “dark space” had to extend throughout most of the tube. Ions impacting on the cathode cup at the left in Figure 1.1 released up to a dozen electrons, depending on the condition of the surface, like dielectric layers, see the close-up picture in Figure 1.12d, adsorbed gas molecules and other coverage. The electrostatic field generated by the cathode cup accelerated the electrons between the cathode and the target. Concave cathodes focused them into the typically elliptic focal spot on the target, which was glass in the tube type of Figure 1.1 and, for example, a platinum slab in many Hittorf tubes. As the metal evaporated and chemically reacted with residual gas, and as ions were implanted in the cathode, the gas pressure inside the sealed vessel decreased over time of operation. A sealed “soft” tube turned “hard,” as the high voltage required to maintain the discharge had to increase and with it the average photon energy and the maximum X-ray energy at the Duane–Hunt limit. Increasing object transparency yielded reduced contrast.

The efficiency of gas ionization sensibly depends on the high voltage amplitude, its frequency (influenced by the interrupter and other circuitry), cathode size and material, gas composition, pressure, charge condition of the chamber walls, the X-ray target, and other parameters. Loeb (1939), Chapter XI, A.1, states: “Below these pressures, say 0.01 mm (remark: Torr, ca 1 Pa), the Crookes dark space nearly fills the tube. The cathode glow fades. The positive column is gone, and the negative glow at the anode end of the tube plus the brilliant fluorescence on the walls is all the luminosity observed. The faint bluish cathode streamers can be seen if looked for with care.² This is the X-ray emissive stage studied by Roentgen when he observed the first evide...