INTRODUCTION AND OVERVIEW

CONTENTS

  X-radiation is created by taking energy from electrons and converting it into photons with appropriate energies. This energy conversion takes place within the x-ray tube. The quantity (exposure) and quality (spectrum) of the x-radiation produced can be controlled by adjusting the electrical quantities (KV, MA) and exposure time, S, applied to the tube. In this chapter we first become familiar with the design and construction of x-ray tubes, then look at the x-ray production process, and conclude by reviewing the quantitative aspects of x-ray production.

THE X-RAY TUBE

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   Function

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    An x-ray tube is an energy converter. It receives electrical energy and converts it into two other forms: x-radiation and heat. The heat is an undesirable byproduct. X-ray tubes are designed and constructed to maximize x-ray production and to dissipate heat as rapidly as possible.

   The x-ray tube is a relatively simple electrical device typically containing two principle elements: a cathode and an anode. As the electrical current flows through the tube from cathode to anode, the electrons undergo an energy loss, which results in the generation of x-radiation. A cross-sectional view of a typical x-ray tube is shown in below.

Cross-Section of a Typical X-Ray Tube

   Anode

CONTENTS

  The anode is the component in which the x-radiation is produced. It is a relatively large piece of metal that connects to the positive side of the electrical circuit.

   The anode has two primary functions: (1) to convert electronic energy into x-radiation, and (2) to dissipate the heat created in the process. The material for the anode is selected to enhance these functions.

   The ideal situation would be if most of the electrons created x-ray photons rather than heat. The fraction of the total electronic energy that is converted into x-radiation (efficiency) depends on two factors: the atomic number (Z) of the anode material and the energy of the electrons. Most x-ray tubes use tungsten, which has an atomic number of 74, as the anode material. In addition to a high atomic number, tungsten has several other characteristics that make it suited for this purpose. Tungsten is almost unique in its ability to maintain its strength at high temperatures, and it has a high melting point and a relatively low rate of evaporation. For many years, pure tungsten was used as the anode material. In recent years an alloy of tungsten and rhenium has been used as the target material but only for the surface of some anodes. The anode body under the tungsten-rhenium surface on many tubes is manufactured from a material that is relatively light and has good heat storage capability. Two such materials are molybdenum and graphite. The use of molybdenum as an anode base material should not be confused with its use as an anode surface material. Most x-ray tubes used for mammography have molybdenum-surface anodes. This material has an intermediate atomic number (Z = 42), which produces characteristic x-ray photons with energies well suited to this particular application. Some mammography tubes also have a second anode made of rhodium, which has an atomic number of 45. This produces a higher energy and more penetrating radiation, which can be used to image dense breast.

   The use of a rhenium-tungsten alloy improves the long-term radiation output of tubes. With x-ray tubes with pure tungsten anodes, radiation output is reduced with usage because of thermal damage to the surface.

     Design

CONTENTS

   Most anodes are shaped as beveled disks and attached to the shaft of an electric motor that rotates them at relatively high speeds during the x-ray production process. The purpose of anode rotation is to dissipate heat and is considered in detail in another chapter..

 Focal Spot

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   Not all of the anode is involved in x-ray production. The radiation is produced in a very small area on the surface of the anode known as the focal spot. The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode. In most x-ray tubes, the focal spot is approximately rectangular. The dimensions of focal spots usually range from 0.1 mm to 2 mm. X-ray tubes are designed to have specific focal spot sizes; small focal spots produce less blurring and better visibility of detail, and large focal spots have a greater heat-dissipating capacity.

   Focal spot size is one factor that must be considered when selecting an x-ray tube for a specific application. Tubes with small focal spots are used when high image visibility of detail is essential and the amount of radiation needed is relatively low because of small and thin body regions as in mammography.
Most x-ray tubes have two focal spot sizes (small and large), which can be selected by the operator according to the imaging procedure. 

    Cathode

CONTENTS

   The basic function of the cathode is to expel the electrons from the electrical circuit and focus them into a well-defined beam aimed at the anode. The typical cathode consists of a small coil of wire (a filament) recessed within a cup-shaped region, as shown below.

 
Energy Exchange within an X-Ray Tube

   Electrons that flow through electrical circuits cannot generally escape from the conductor material and move into free space. They can, however, if they are given sufficient energy. In a process known as thermionic emission, thermal energy (or heat) is used to expel the electrons from the cathode. The filament of the cathode is heated in the same way as a light bulb filament by passing a current through it. This heating current is not the same as the current flowing through the x-ray tube (the MA) that produces the x-radiation. During tube operation, the cathode is heated to a glowing temperature, and the heat energy expels some of the electrons from the cathode. 

   Envelope

CONTENTS

    The anode and cathode are contained in an airtight enclosure, or envelope. The envelope and its contents are often referred to as the tube insert, which is the part of the tube that has a limited lifetime and can be replaced within the housing. The majority of x-ray tubes have glass envelopes, although tubes for some applications have metal and ceramic envelopes.

   The primary functions of the envelope are to provide support and electrical insulation for the anode and cathode assemblies and to maintain a vacuum in the tube. The presence of gases in the x-ray tube would allow electricity to flow through the tube freely, rather than only in the electron beam. This would interfere with x-ray production and possibly damage the circuit.

   Housing

CONTENTS

    The x-ray tube housing provides several functions in addition to enclosing and supporting the other components. It functions as a shield and absorbs radiation, except for the radiation that passes through the window as the useful x-ray beam. Its relatively large exterior surface dissipates most of the heat created within the tube. The space between the housing and insert is filled with oil, which provides electrical insulation and transfers heat from the insert to the housing surface.

THE X-RAY CIRCUIT

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ELECTRON ENERGY

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    The energy that will be converted into x-radiation (and heat) is carried to the x-ray tube by a current of flowing electrons as shown above. As the electrons pass through the x-ray tube, they undergo two energy conversions, as illustrated previously: The electrical potential energy is converted into kinetic (motion) energy that is, in turn, converted into x-radiation and heat.
 

   Potential

CONTENTS

     When the electrons arrive at the x-ray tube, they carry electrical potential energy. The amount of energy carried by each electron is determined by the voltage or KV, between the anode and cathode. For each kV of voltage, each electron has 1 keV of energy. By adjusting the KV, the x-ray machine operator actually assigns a specific amount of energy to each electron.

   Kinetic

CONTENTS

    After the electrons are emitted from the cathode, they come under the influence of an electrical force pulling them toward the anode. This force accelerates them, causing an increase in velocity and kinetic energy. This increase in kinetic energy continues as the electrons travel from the cathode to the anode. As the electron moves from cathode to anode, however, its electrical potential energy decreases as it is converted into kinetic energy all along the way. Just as the electron arrives at the surface of the anode its potential energy is lost, and all its energy is kinetic. At this point the electron is traveling with a relatively high velocity determined by its actual energy content. A 100-keV electron reaches the anode surface traveling at more than one half the velocity of light. When the electrons strike the surface of the anode, they are slowed very quickly and lose their kinetic energy; the kinetic energy is converted into either x-radiation or heat.

   The electrons interact with individual atoms of the anode material, as shown below. Two types of interactions produce radiation. An interaction with electron shells produces characteristic x-ray photons; interactions with the atomic nucleus produce Bremsstrahlung x-ray photons.

 

Electron-Atom Interactions That Produce X-Ray Photons

   Binding Energy

CONTENTS

The electrons within an atom each have a specific amount of binding energy that depends on the size (atomic number, Z) of the atom and the shell in which the electron is located.  As described in a previous chapter the binding energy is the energy that would be required to remove the electron from the atom.  It is actually an energy deficit rather than an amount of available energy.
The binding energy of electrons within an atom plays a major role in the production of characteristic x-radiation as described later.

BREMSSTRAHLUNG

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   Production Process

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     The interaction that produces the most photons is the Bremsstrahlung process. Bremsstrahlung is a German word for "braking radiation" and is a good description of the process. Electrons that penetrate the anode material and pass close to a nucleus are deflected and slowed down by the attractive force from the nucleus. The energy lost by the electron during this encounter appears in the form of an x-ray photon. All electrons do not produce photons of the same energy.

    Spectrum

CONTENTS

    Only a few photons that have energies close to that of the electrons are produced; most have lower energies. Although the reason for this is complex, a simplified model of the Bremsstrahlung interaction is shown below. First, assume that there is a space, or field, surrounding the nucleus in which electrons experience the "braking" force. This field can be divided into zones, as illustrated. This gives the nuclear field the appearance of a target with the actual nucleus located in the center. An electron striking anywhere within the target experiences some braking action and produces an x-ray photon. Those electrons striking nearest the center are subjected to the greatest force and, therefore, lose the most energy and produce the highest energy photons. The electrons hitting in the outer zones experience weaker interactions and produce lower energy photons. Although the zones have essentially the same width, they have different areas. The area of a given zone depends on its distance from the nucleus. Since the number of electrons hitting a given zone depends on the total area within the zone, it is obvious that the outer zones capture more electrons and create more photons. From this model, an x-ray energy spectrum, such as the one shown below, could be predicted.

A Model for Bremsstrahlung Production and the Associated Photon Energy Spectrum

   The basic Bremsstrahlung spectrum has a maximum photon energy that corresponds to the energy of the incident electrons. This is 70 keV for the example shown. Below this point, the number of photons produced increases as photon energy decreases. The spectrum of x-rays emerging from the tube generally looks quite different from the one shown here because of selective absorption within the filter.

   A significant number of the lower-energy photons are absorbed or filtered out as they attempt to pass through the anode surface, x-ray tube window, or added filter material. X-ray beam filtration is discussed more extensively in a later chapter. The amount of filtration is generally dependent on the composition and thickness of material through which the x-ray beam passes and is generally what determines the shape of the low-energy end of the spectrum curve.

 Effect of KV

CONTENTS

   The high-energy end of the spectrum is determined by the KV (kilovoltage) applied to the x-ray tube. This is because the KV establishes the energy of the electrons as they reach the anode, and no x-ray photon can be created with an energy greater than that of the electrons. The maximum photon energy, therefore, in keV is numerically equal to the maximum applied potential in kV (kilovolts). In some x-ray equipment, the voltage applied to the tube might vary during the exposure because of the cycle nature of the AC (alternating current) electrical system. . The maximum photon energy is determined by the maximum, or peak, voltage during the voltage cycle. This value is generally referred to as the kilovolt peak (KV_P) and is one of the adjustable factors of x-ray equipment.

   In addition to establishing the maximum x-ray photon energy, the KV_P has a major role in determining the quantity of radiation produced for a given number of electrons, such as 1 mAs, striking the anode. Since the general efficiency of x-ray production by the Bremsstrahlung process is increased by increasing the energy of the bombarding electrons, and the electronic energy is determined by the KV_P, it follows that the KV_P affects x-ray production efficiency.

   Changing the KV_P will generally alter the Bremsstrahlung spectrum, as shown below. The total area under the spectrum curve represents the number of photons or quantity of radiation produced. If no filtration is present where the spectrum is essentially a triangle, the amount of radiation produced is approximately proportional to the KV squared. With the presence of filtration, however, increasing the KV also increases the relative penetration of the photons, and a smaller percentage is filtered out. This results in an even greater increase in radiation output with KV_P.

Comparison of Photon Energy Spectra Produced as Different KV_P Values

CHARACTERISTIC RADIATION

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Production

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    The type of interaction that produces characteristic radiation, also illustrated above (in the "Kinetic" paragraph), involves a collision between the high-speed electrons and the orbital electrons in the atom. The interaction can occur only if the incoming electron has a kinetic energy greater than the binding energy of the electron within the atom. When this condition exists, and the collision occurs, the electron is dislodged from the atom. When the orbital electron is removed, it leaves a vacancy that is filled by an electron from a higher energy level. As the filling electron moves down to fill the vacancy, it gives up energy emitted in the form of an x-ray photon. This is known as characteristic radiation because the energy of the photon is characteristic of the chemical element that serves as the anode material. In the example shown, the electron dislodges a tungsten K-shell electron, which has a binding energy of 69.5 keV. The vacancy is filled by an electron from the L shell, which has a binding energy of 10.2 keV. The characteristic x-ray photon, therefore, has an energy equal to the energy difference between these two levels, or 59.3 keV.

   Actually, a given anode material gives rise to several characteristic x-ray energies. This is because electrons at different energy levels (K, L, etc.) can be dislodged by the bombarding electrons, and the vacancies can be filled from different energy levels. The electronic energy levels in tungsten are shown below, along with some of the energy changes that give rise to characteristic photons. Although filling L-shell vacancies generates photons, their energies are too low for use in diagnostic imaging. Each characteristic energy is given a designation, which indicates the shell in which the vacancy occurred, with a subscript, which shows the origin of the filling electron. A subscript alpha (a) denotes filling with an L-shell electron, and beta ((3) indicates filling from either the M or N shell.

   Tungsten Spectrum

CONTENTS

   The spectrum of the significant characteristic radiation from tungsten is shown below. Characteristic radiation produces a line spectrum with several discrete energies, whereas Bremsstrahlung produces a continuous spectrum of photon energies over a specific range. The number of photons created at each characteristic energy is different because the probability for filling a K-shell vacancy is different from shell to shell.

Electron Energy Levels in Tungsten and the Associated Characteristic X-Ray Spectrum

   Molybdenum Spectrum

CONTENTS

  Molybdenum anode tubes used for mammography produce two rather intense characteristic x-ray energies: K-alpha radiation, at 17.9 keV, and K-beta, at 19.5 keV. as shown below.

The optimum spectrum to produce the best balance between contrast sensitivity and radiation dose for an average size breast is one with most of the radiation with photon energies below about 20 keV.  However, there is considerable Bremsstrahlung above this energy.  In the typical mammography equipment a molybdenum filter is used to remove that undesirable part of the spectrum.  This is an application of a filter that works on the "K edge" principle.  It absorbs radiation that is above the K-edge energy that corresponds to the binding energy of the electrons in the K shell of the molybdenum atom. 

   Rhodium Spectrum

CONTENTS

Rhodium has an atomic number (Z) of 45 compared to a Z of 42 for molybdenum. Therefore the characteristic x-radiation produced with a rhodium anode will have energies that are slightly higher than produced with molybdenum and are more penetrating.  This is of value for imaging dense breast.
Anodes that have dual surface areas, molybdenum and rhodium, make it possible for the operator to select a spectrum that is more optimized for different breast sizes and densities.

 KV Effect on Spectrum

CONTENTS

   The KV value also strongly influences the production of characteristic radiation. No characteristic radiation will be produced if the KV is less (numerically) than the binding energy of the K-shell electrons. When the KV is increased above this threshold level, the quantity of characteristic radiation is generally proportional to the difference between the operating KV and the threshold KV.

   The x-ray beam that emerges from a tube has a spectrum of photon energies determined by several factors. A typical spectrum is shown below and is made up of photons from both Bremsstrahlung and characteristic interactions.

Typical Photon Energy Spectrum from a Machine Operating at KV = 80

   The relative composition of an x-ray spectrum with respect to Bremsstrahlung and characteristic radiation depends on the anode material, KV, and filtration. In a tungsten anode tube, no characteristic radiation is produced when the KV is less than 69.5. At some higher KV values generally used in diagnostic examinations, the characteristic radiation might contribute as much as 25% of the total radiation. In molybdenum target tubes operated under certain conditions of KV and filtration, the characteristic radiation can be a major part of the total output.

 EFFICIENCY

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   Concept

CONTENTS

    Only a small fraction of the energy delivered to the anode by the electrons is converted into x-radiation; most is absorbed by the anode and converted into heat. The efficiency of x-ray production is defined as the total x-ray energy expressed as a fraction of the total electrical energy imparted to the anode. The two factors that determine production efficiency are the voltage applied to the tube, KV, and the atomic number of the anode, Z. An approximate relationship is

  KV Effect

CONTENTS

   The relationship between x-ray production efficiency and KV  has a specific effect on the practical use of x-ray equipment. As we will see in a later chapter, x-ray tubes have a definite limit on the amount of electrical energy they can dissipate because of the heat produced. This, in principle, places a limit on the amount of x-radiation that can be produced by an x-ray tube. By increasing KV, however, the quantity of radiation produced per unit of heat is significantly increased.

   Anode Material

CONTENTS

   The relationship of x-ray production efficiency to anode material is only of academic interest because most tubes use tungsten. The exception is molybdenum and rhodium used in mammography. The x-ray production efficiency of these tubes is significantly less than that of tungsten anode tubes because of their lower atomic numbers.

EFFICACY (OUTPUT)

CONTENTS

   Definition and Concept

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    The x-ray efficacy of the x-ray tube is defined as the amount of exposure, in milliroentgens, delivered to a point in the center of the useful x-ray beam at a distance of 1 m from the focal spot for 1 mAs of electrons passing through the tube.

   The efficacy value expresses the ability of a tube to convert electronic energy into x-ray exposure. Knowledge of the efficacy value for a given tube permits the determination of both patient and image receptor exposures by methods discussed in later chapters. Like x-ray energy output, the efficacy of a tube depends on a number of factors including KV, voltage waveform, anode material, filtration, tube age, and anode surface damage. The illustration below gives typical efficacy values for tungsten anode tubes with normal filtration.

 KV Control

CONTENTS

   KV is very useful in controlling the radiation output of an x-ray tube. The figure below shows a nonlinear relationship. It is normally assumed that the radiation output is proportional to the square of the KV. Doubling KV quadruples the exposure from the tube.

Typical X-Ray Tube Efficacy (Exposure Output) for Different KV_P Values

   Waveform

CONTENTS

   Waveform describes the manner in which the KV changes with time during the x-ray production process because of the cyclic nature of the electrical supply.; several different KV waveforms are used. A general principle is that the waveform with the least KV variation during the exposure is the most effective x-ray producer. Most new x-ray equipment now use  generators that produce relatively constant KV throughout the exposure.  Other waveforms are described in more detail in another chapter. 

   SUMMARY and MIND MAP

CONTENTS

The mindmap below provides a summary of the major concepts associated with x-ray production.