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Wilhelm Conrad Rontgen was born in Remscheid-Lenep, Germany on March 27,1845 and died on February10,1923 in Munich, Germany. As a small child, his family moved to Apeldoorn, in the Netherlands, when Rontgen began his education in public schools. In 1865, after a brief period of study at Utrecht Technical School, he went to the Polytechnical School in Zurich, Switzerland. There he received a diploma as a mechanical engineer. Rontgen obtained his doctor's degree in 1869 at the University of Zurich. In 1885, he became a professor of physics and director of the new Physical Institute of the University of Wurzburg.
Rontgen's most famous contribution to science was the discovery on November 8, 1895 of the x-ray. "Gas tubes" (Hittorf-Crookes' tubes) were being used at that time to conduct experiments with cathode rays (Fig. 2-1). In these tubes air was pumped out of the tube and a current passed through the tube. The rarified gas was partially ionized and the positive ions were attracted to the cathode with sufficient energy to release additional electrons. A new larger current of electrons interacted with gas molecules in the tube, causing further ionization of the gas molecules and production of additional electrons. These electrons also passed toward the glass wall. If the energy of these electrons was sufficiently high, x-rays were produced when they interacted with the glass wall. Tube current or electron flow was dependent on the presence of the residual gas in the tube.
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This is the type of cathode ray tube Rontgen experimented with at the time of his discovery of x-rays.
Rontgen wrapped a Hittorf-Crookes' tube with dark paper. During tube activation, he saw a greenish illumination from the cardboard across the room that had been painted with a fluorescent material (barium platinocyanide). The fluorescent screens had been used earlier to detect cathode rays. In the next few weeks, Rontgen investigated and correctly interpreted many of the physical characteristics of this new invisible radiation. Rontgen presented his first paper to the Society of Physics and Medical Sciences at Wurzburg University on December 28,1895. At this time, he exhibited a radiograph of his wife's hand that he had produced.
In these first communications, Rontgen showed that: although the new rays were invisible they caused fluorescence in certain substances; they darkened the photographic plate; they were propagated in straight lines; their behavior differed fundamentally from that of cathode rays; they were neither reflected nor refracted by (then known) experimental methods; they were not deviated by the influence of electro-magnetic fields. He investigated the startling penetration of the rays through many materials and observed the "hardening" of x-ray beams after penetrating several absorbers. He noted that different types of scattered and of secondary radiations were produced, and also that air traversed by the rays was made electrically conductive.
The possibility of using x-ray pictures in medical and surgical diagnosis was recognized at once and exploited immediately. Radiographs were produced in Great Britain on January 13, 1896. Within the first year after the announcement of the discovery almost a thousand papers and many books on x-rays were published. In an editorial in The Journal of The American Medical Association of February, 1896, a cautious opinion was expressed that possibly the new rays might have therapeutic applicability. Unfortunately the first physiologic effects of the x-rays came as a surprise and marked the beginning of a distressing chapter of suffering endured by many of the pioneers. As early as April, 1896, reports came from several sources that x-rays would produce "changes of the skin which are very similar to the effects of sunburn." Naturally, many observers of such changes reasoned that the x-rays had healing powers and thus laid the foundation for radiation therapy (Glasser, 1934). In recognition of his discovery, Rontgen was presented the first Nobel Prize awarded in Physics in 1901.
It is interesting to note that a Professor Goodspeed in Philadelphia had made a radiograph earlier on February 22, 1890, but unfortunately did not recognize the importance of this discovery. (Departments of Goodspeedology might seem a bit strange when compared with Departments of Rontgenology.)
Veterinary Radiology has a unique history that is closely related with Radiology as it is used in man. It is encouraging to note some effort to document this history (Patterson, 1982; Gillette, et al, 1977).
T o evaluate properly the discovery of x-rays, it is necessary to review the work of the "scientific forefathers" of Wilhelm Conrad Rontgen. Even a condensed list of those milestones in discoveries relating to electricity during the last three centuries gives a splendid sequence leading to the production of high-tension currents and of highly evacuated tubes, and the study of electric discharges through those tubes, and finally to the important discovery of the roentgen rays.
The foregoing resume outlines the progress made in the construction of high-tension apparatus, and the state of knowledge of discharges of electricity through rarefield gases. In November, 1895, Wilhelm Conrad Rontgen, professor of physics at the University of Wurzburg, Germany, observed a strange phenomenon while experimenting with cathode rays. He pursued this finding feverishly during the next few weeks, in an amazing number of experiments carefully designed to investigate the cause of this effect. He discovered it to be a "new kind of rays," which he subsequently called "x-rays."
The replacement of the fluorescent screen by the recording photographic plate was one of Rontgen's early and most important steps in experimenting with the new radiation. By means of the screen and the photographic plate, Rontgen made many fundamental observations that were reported in his three classic communications in December 1895, March 1896 and May 1897.
The contributions of the scientists who lived and worked during the three centuries previous to the discovery of x-rays were matched by the brilliant achievements of many scientists in the decades since this discovery, as shown by the following milestones.
Max Theodol Felix von Laue was awarded a Nobel Prize in Physics "for his discovery of the diffraction of roentgen rays in crystals."
G. Bucky developed the grid, which later became known as the Bucky Diaphragm. This was perfected by H. E. Potter in 1919 and was known as the Potter -Bucky diaphragm.
A Vallebona reported on his technique of making roentgenograms of plane sections of solid objects, which he called straitigraphy. Similar techniques were developed by B.G. Ziedses des Plantes (planigraphy) in 1931, G. Grossman (tomography) in 1935, J. R. Andrews in 1936, and J. Kieffer (laminography) in 1938. The term laminography is now generally used.
DuPont introduces polyester base film.
Development of high speed film-screen systems using rare earth screens and orthochromatic film. Development of carbon fiber cassette fronts and table tops.
Development of CT units.
Development of digital radiography.
X-rays are ionizing radiation that result from the transfer of the kinetic energy held by electrons into electromagnetic radiation.
When electrons interact with matter, they lose their energy and x-rays are produced by two different methods:
The quality and quantity of x-rays produced by an x-ray tube can be specifically described. The milliamperage mA, or flow of electrons placed across the x-ray tube determines the number of electrons that will interact with the target in the tube and thus determines the number of resulting x-ray photons that will be produced. Remember that this
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An electron from the cathode is given high energy and attracted toward the anode. If the electron passes close to the nucleus of an atom in the target it is attracted and alters its path and speed releasing a photon of energy. These photons are called general or Bremsstrahlung radiation.
of x-ray photons begins to decrease immediately because of the attenuation as the x-ray beam passes through matter. The maximum speed of the electron flowing across the x-ray tube determines the maximum energy of the x-ray photon produced. In an x-ray tube, the resulting beam is polychromatic, that is having photons of varying energies. The kilovoltage peak, kVp, is the terms used to describe the maximum energy within the beam. Remember that many of the photons will have a lesser energy and that the mean energy of the beam will change as the beam passes through any attenuater. This brief discussion will suffice until more complete explanations are presented later.
When an electron passes near the nucleus of a tungsten atom in the target of the x-ray tube, the positive charge of the nucleus acts on the negative charge of the electron causing a marked change in direction of travel and a marked deceleration (Fig. 2-2). The kinetic energy lost through this process of radiative interaction is called general radiation or Bremsstrahlung (from the German "bremsen," to brake) . The energy of the photon produced is dependent on the energy of the incoming electron and cannot exceed this energy. Usually the electron gives up only a part of its energy so that a continuous spectrum of x-ray energies is produced (Fig. 2-3). The amount of energy in the x-ray photon emitted depends on:
A given electron may interact several times giving up a small amount of energy with each interaction. It is in this manner that so much of the energy carried by the incoming electron beam is converted into heat and not into useful x-rays. Maximum energy of the x-ray produced is dependent on the maximum kVp setting on the machine and loss of all of the electrons' energy in the
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The ordinate represents the intensity of x-rays in an arbitrary unit and the abscissa is wavelength in A or nm) Maximum photon energy (minimum wavelength) is indicated The continuous spectrum of x-rays produced by Bremsstrahlung can be seen.
creation of one photon through one interaction, (Table 2-1). The minimum wavelength in Ångstroms or nm that can result from a given kVp setting, can be determined.
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Generation of x-rays by radiative deceleration of electrons in matter is in competition with collisional interactions which result in dissipation of electronic energy as heat. Efficiency of production of x-rays by radiative interactions is proportional to:
General radiation results from radiative interactions of high speed electrons. It is also possible that one of the incoming electrons will collide with an electron tightly bound in one electron shell of an atom of target material.
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This interaction results in ejection of the electron from its shell if the energy of the impinging electron exceeds the binding energy of the ejected electron. Both initial and ejected electrons travel away from the atom in which the interaction took place. The vacancy in the electron shell will not remain. Immediately, another electron:
This shifting of electrons continues until a stable energy state is reached. Each one of the electronic shifts results in release of an x-ray photon. The energy of this photon can be determined by subtracting the binding energy of the electron if it originates from a shell from the binding energy of the new position to be assumed (Fig. 2-4).
The photon of greatest energy results from filling a K shell space by an electron from outside the atom. The most commonly occurring electronic transition is the filling of a vacancy by an electron from the next outer shell and subsequent filling of that vacancy by an electron from the next outer shell. Since the binding energies of each
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CHARACTERISTIC RADIATION. An electron with sufficient energy (excess of 70 keV) strikes an electron in the K shell of the tungsten target. The K shell electron is ejected and the impinging electron continues with a lower energy. The vacancy in the K shell is filled quickly by an electron from the L shell. The difference in binding energies is emitted as an x-ray photon.
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shell and subshell electron are known, it is possible to predict the exact energies of the photons that can result from the interaction of the high energy incoming electron with the electrons of the target atoms.
Because of this predictability, this type of radiation is called characteristic radiation. Energy of the characteristic radiation depends on the shell of the atom from which the electron was ejected and the shell from which the replacing electron was derived (Fig. 2-5). Energy levels of characteristic radiation vary with the target material. Remember that this type of radiation can only be produced if the energy of the incoming electron is sufficient to exceed the binding energy of the target electrons.
Relative numbers of characteristic and general radiations for a tungsten target are dependent on x-ray tube potential. This can be seen in Table 2-2.
Interaction of an electron beam with a target results in production of a spectrum of characteristic and general radiation. It can be seen from Table 2-2 that most of the
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exposures made in veterinary radiography are the result of Bremsstrahlung or general radiation. This is one reason to consider the advantages derived from filtration of the primary beam.
X-rays are classified on the basis of their energies into functional groups. The names are indicative of their use (Table 2-3) The equipment used to produce the various energy levels varies markedly from the type of target and tube window to the cooling mechanism required because of high tube Current
Attenuation of an x-ray beam by matter is the reduction in intensity of the x-ray beam as it passes through any matter by either absorption or deflection of photons within the beam Passage of the x-ray beam through tube wall, filters, air, patient, table top, grid, cassette, and finally the interaction of the x-ray photon with the crystals on the film can be better understood by considering the attenuation that takes place within the beam. This attenuation assumes two forms. In the first form the photon is absorbed by matter and transfer of total energy carried by the photon to the matter takes place. The second form of attenuation is a scattering of photons
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___________________________________________________
with a deflection of the photon from its original direction with or without a transfer of some of the energy from the photon to the absorber. In this discussion it is of value to assume that :
Attenuation of x-rays in diagnostic radiology is dependent on the energy of the incident photon and the linear attenuation coefficient of the material.
The linear attenuation coefficient (mue) a quantitative measurement of attenuation per centimeter of absorber and tells us how much of the x-ray beam remains after passage through a given thickness of an absorber. The linear attenuation coefficient can be used only with monochromatic radiation (radiation of one energy) and is specific for a certain energy x-ray beam and for a certain absorber such as bone or muscle (water).
This concept is important in our understanding half value layer.This is a constant that conveniently characterizes the attenuation of a beam of x-rays in a given absorber. It is equal to the amount of absorber that is required to reduce the number of incident photons to onehalf after passage through the absorber . Linear attenuation coefficients are listed for substances such as water, bone sodium iodide and most of the elements. From this information we can derive the half-value layer for many absorbers for whatever photon energy is of interest.
Attenuation of polychromatic radiation is more complex than the attenuation of monochromatic radiation. The polychromatic x-ray beam contains a whole spectrum of energies with the peak kilovoltage (kVp) used to describe the most energetic. In general the mean energy of the polychromatic x-ray beam lies between one-third and one-half of its peak energy. This is why kVp is sometimes referred to as kilovoltage "peak" instead of kilovoltage "potential." As the polychromatic x-ray beam passes through an absorber; it becomes more nearly monochromatic and acts more like a monochromatic beam
This can be understood better by considering a 100 kVp heterochromatic x-ray beam that has a mean energy of 35 kev. As the 100 kVp beam passes through a filter that reduces the intensity to one-half (a half-value layer) the mean energy of the beam is increased to 40 kev. This is because the filter selectively removes the lower energy portion of the beam to a greater extent. Therefore we need to add a slightly thicker filter to this new beam to cut the quantity of the beam in half. The mean energy of the beam increases because of selective filtration and a progressively thicker filter is needed to function as the new half-value layer so as to decrease the quantity of the beam in half (Fig 2-6).
There are three important ways in which an x-ray photon can inteact with matter These are:
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The energy spectrum of the unfiltered beam is on the left while the effect of passing the beam through successive half-value layers is seen on the right. Note that with a heterochromatic beam the number of photons (Y axis) decreases as the white area under the curve becomes shorter and the energy level of the curve shifts to the left (X axis) as the beam becomes more energetic.
that scatter may possess enough remaining energy to interact additional times. Once they are altered from their original course, they no longer can contribute accurately to the production of the x-ray image, but will contribute in an inaccurate manner through production of radiation fog which tends to blacken the film without any consideration of the overlying tissue.
Coherent scattering, or classical scattering, describes the photon inter- action with electrons of an atom causing excitation with no reduction in energy of the incident photon and minimum change in the direction of the photon. This interaction is important in tissue struck by low energy photons (< 15 keV) and the probability is increased with high Z material. No ionization occurs. Coherent scatter causes radiation fog but the frequency of the event is low (Fig. 2-7).
Photoelectron absorption is the second type of attenuation possible and is important because the energy of the photon is transferred to an inner electron of all atom. The electron is ejected from the atom with a kinetic energy that equals the energy of the incident photon minus the binding energy that is necessary to remove the atom. The ejected electron is called a photoelectron. Photoelectric absorption increases as the third power of the atomic number but decreases rapidly as the photon energy is increased. An electron from an adjacent shell usually immediately fills the electron void releasing a characteristic photon. Photoelectron absorption produces:
Photoelectric absorption requires the energy of the incoming photon exceed the binding energy of the electron and occurs when the two energies are relatively close. This will be relatively low energy photon. Hence the interaction is inversely proportional to the third power of the photon energy (1/energy3). The frequency of photoelectric absorption increases with the third power of the atomic number (Z3)-Remember that
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The photon enters from the left, interacts with the atom, and is deflected in a new direction with the same energy. This type of interaction occurs with low kev photons and is relatively unimportant in diagnostic radiology
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photoelectric absorption produces a low energy photon and consequently does not cause radiation fog.
Compton or incoherent scattering is an interaction in which a part of the energy of the incident photon is transferred to a loosely bound or free electron within the attenuating medium. The electron recoils at a specific angle and the photon scatters at another specific angle (Fig. 2-9) . The kinetic energy of the recoil or Compton electron equals the energy lost by the photon assuming that the binding energy of the electron is negligible The energy transfer is dependent on the angle of defection of the Compton photon and the energy level of the incident photon. The probability of this interaction is dependent on density and is nearly independent of the atomic number of the attenuating medium. It decreases gradually with increasing photon energy. Compton scatter produces most of the radiation fog seen in diagnostic radiology.
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Pair production and photo disintegration are two additional ways in which an x-ray photon can interact with tissue. They have no importance in diagnostic radiology because they occur at photon energies in excess of the 150 kev that is a common maximum in diagnostic radiology.
The attenuation of the x-ray beam by tissue has been shown to be dependent on:
It does not matter in what type of mixture or compound the atoms present nor whether the substance is liquid gas or solid. The attenuation is determined by the nature and number of atoms and not by their chemical composition or physical state. Tissue is made up principally of hydrogen (Z = 1) carbon (Z = 6) nitrogen (Z = 7) and oxygen (Z = 8). 'The bones are composed mainly of calcium (Z = 80) and phosphorous (Z = 15) (Fig. 2-10). It is important to remember that the probability of photoelectric absorption is proportional to the third power of the atomic number (Z) per gram of the absorbing material while the probability of Compton scattering is independent of atomic number of the absorbing material. The difference in atomic number between bone and soft tissues partially explains the difference in attenuation noted. Since the average atomic number of bone is 74 and that of soft tissue is 7 at identical densities the attenuation would be 2(3) or 8 times as great assuming that the attenuation was due to photoelectric adaptation and scatter was ignored.
The interaction between photons and absorbing material is also proportional to the density of the tissue. Density is defined as the quantity of matter per unit volume. The more tightly the atoms are packed the greater is the density. When the density is doubled the number of electrons available for interaction is doubled and twice as many photons will interact. This explains the great difference in the appearance on the
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radiograph of the air containing organs (lungs) as compared to fluid density soft tissues (heart) although the average atomic number given to air and soft tissues is the same. The great difference radiographically is due to the enormous difference in density. Density is very important in influencing the frequency of both Compton scatter and photoelectric absorption (Table 2-4).
The thicker the tissue to be penetrated by the x-ray beam, the greater the amount of attenuation. This concept is important in understanding the half-value layer.
Penetration of the tissue depends on a wavelength of a photon with attenuation due to photoelectric absorption being directly proportional to the third power of the wavelength. When the energy of the photon increases photoelectric absorption decreases and the probability of Compton scatter increases. (Table 2-5 2-6).
Secondary radiation is a term used in practice that includes both scattered or characteristic photons as well as secondary electrons all of which contribute to a compromise of film quality. However most of the problem is caused by Compton scattering and the results of attenuation
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Human tissue |
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Muscle |
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Fat |
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Bone |
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Lung |
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Contrast material |
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Barium |
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Iodine |
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Air |
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Other |
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Concrete |
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Tungsten |
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Lead |
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Produce radiation fog |
Yes |
No |
Relationship with atomic number of absorbing material |
Independent |
Increases with the third power |
Relationship with electron binding energy. |
Decreases directly with increasing |
Most likely when photon energyand binding energyare similar |
Relationship with photon energy level |
Predominately occurs at high energy |
Predominately occurs at low |
ionization occurs |
Yes (Compton electron) |
Yes(photo-electron) |
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and scatter have been referred to as scatter radiation. There are three factors that affect scatter:
While most secondary scatter is thought of as being a forward direction it should be realized that if the photon strikes the orbital electron directly the recoil electron will be directed in a forward direction with a high level of energy while the scattered photon is directed backwards toward the tube. This will leave the photon having lost the greatest amount of energy. This type of scatter radiation is referred to as backscatter and is that secondary radiation that may originate from behind the image plane. Its effect can be controlled by use of lead foil on the back of the cassette or film holder to absorb this new radiation to prevent film fogging.
Many techniques are available to reduce the production of scatter or its effect on radiograph.This includes
These are discussed in greater detail later in the text.
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'The Fundamentals of Radiography 12th Edition. East-man Kodak CO., Rochester 1980.
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