 Introdução
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Capítulo
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1. Basic Concepts
As you probably already know, x-rays are produced by bombarding a tungsten target with an electron beam. They are a form of radiant energy similar in several respects to visible light. For example, they radiate from the source in all directions unless stopped by an absorber. As with light rays, a very small part of the beam of x-rays will be absorbed by air, whereas all of the beam will be absorbed by a sheet of thick metal. The fundamental difference between x-rays and light rays is their range of wavelengths of all x-rays being sorter than those of ultraviolet light. The useful science of diagnostic radiology is based on this difference, since many substances that are ppaque to light are penetrated by x-rays. Is was this attractive property that caught the attention of Professor Wilhelm Conrad Roentgen of the University of Würzburg on a cold November night in 1895, when he first observed certain physical phenomena he could not explain.
Roentgen had been experimenting with an apparatus that, unknown to him, caused the emission of x-rays as a by-product. Accustomed to the darkened laboratory, he observed that whenever the apparatus was working, a chemical-coated piece of cardboard lying on the table glowed with a pale green light. We know now that fluorescence, or the emission of visible light, can be produced in a variety of ways by complex nuclear energy exchanges. But is 1895 Roentgen recognized at first only the fact that he had unintentionally produced a histherto unknown form of radiant energy that was invisible, could cause fluorescence, and passed through objects opaque to light. When placed his hand between the source of the beam and the glowing cardboard, he could see the bones inside his fingers within the shadow of his hand. He found that the new rays, which he named x-rays, penetrated wood. Using Photographic paper instead of a fluorescing material, he mad an "x-ray picture" of a hand through the door of his laboratory.
Six years later the first Nobel Prize in physics was awarded to Roentgen for his discovery, and by then this remarkably systematic investigator has explored most of the basic physical and medical applications of the new ray. Figure 1.2. Staged version of the discovery of the roentgen ray.
Figure 1.2 Staged version of the discovery of the roentgen ray.
The idea of being able to see through opaque objects caught the public fancy all over the world, and a great deal of nonsense was written on the subject a many languages within the first decade after its discovery. There is a fascinating file off cartoons and articles in the Library of Congress documenting this fever.
Are you quite sure you can imagine exactly what Roentgen saw when he first observed that he could "see through his hand" with the help of the new ray? In order to grasp this clearly, you must first understand the important difference between what Roentgen saw fluoroscopically (Figure 1.3) and that you see today in an x-ray film of the hand, such as the one in Figure 1.4. You should call this a radiograph, no an x-ray.
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Figure 1.3 What Roentgen saw when he placed his hand between the tube fluorescing cardboard on the laboratory table. Areas no in the shadow of his hand fluoresced vigorously and appear white. Fewer x-rays reached the areas under the bones, so they were not so luminous and appear dark. |
When light hits photographic film, a photochemical process takes place in which metallic silver is precipitated in fine particles within the gelatin emulsion, rendering the film black when it is developed chemically. Places on the film that are not exposed to light remain clear. When a "positive" paper print is made of this "negative" film, the values are reserved: the black, silver-bearing areas prevent light from reaching the photosensitive paper, while clear areas in the film permit the paper to be blackened (Figure 1.4).
The x-ray film you will see in medical school is equivalent to the negative film you may have worded with in your own photographic darkroom. X-rays, like light rays, precipitate silver in a photographic film, but they do so much less rapidly than light rays. A patient cannot be expected to hold still long enough for films to be made by using x-rays alone, and too much exposure to radiation is both dangerous and technically undesirable. Therefore, an ingenious reinforcing technique has been worked out using a special film container, or cassette.
The cassette (Figure 1.5) contains two fluorescent intensifying screens, which are activated by the x-rays and in turn emit light rays that reinforce the photochemical effects of the x-rays themselves on the film. In this way the silver-precipitating effect of the x-rays combined with that of the light rays they generate work together to blacken the film. The use of x-ray cassettes with specialized intensifying screens and films permits diagnostic x-ray imaging with less radiation of the patient and faster exposure times. When an object interposed between the x-ray beam source and the cassette has absorbed the rays, no light activation of the fluorescent screen will take place; neither x-rays nor light rays will reach the film, and no silver will be precipitated.
Figure 1.4 Radiograph of hand …welll, actually not a hand. Guess what it might be.
Figure 1.5A Cross section of a cassette, or modern film holder. The x-ray film in use today consists of an acetate sheet coated on both sides with photographic emulsion, and the cassette is constructed so that plastic fluorescent screens are applied in contact with each side of the film. In this way right rays reinforce the photochemical effect on the film of the x-rays themselves. This effectively speeds up the exposure, shortening the time, and reduces the blurring effect of the patient's motion.
Figure 1.5B Technologist holding a film cassette in her right hand while inserting another cassette into a film processor with her left hand.
Figure 1.6 Modern radiograph of a hand, a: Blackened area where only air is interposed between beam and film. b: Soft tissues absorb part of the beam before it reaches the film. c: Calcium salts in bone absorb even more x-rays, leaving the film only lightly exposed and relatively little silver precipitated in the emulsion. d: The dense metal of the ring absorbs all rays; no silver is precipitated. (Note: This, like most x-ray illustrations in textbooks and periodicals published in this country, is a doubly reversed print, so that what you see here is what you will see whenever you hold a film of the hand against the light.)
In Figure 1 .6 a man's left hand has been placed over the cassette and exposed to a beam of x-rays. Notice that the film not covered by any part of the hand has been intensely blackened because very little of the beam was absorbed by the air, which was the only absorber interposed between x-ray tube and film. The fleshy parts of the hand (or soft tissues, as they are called by the radiologist) absorbed a good deal of the beam, so that the film appears gray. Very few x-rays reached that part of the film directly under the bones, because hones contain large amounts of calcium, which absorb more x-rays than the soft tissues. Every metal absorbs x-rays to an extent depending on its atomic number and thickness. No x-rays at all were able to pass through the gold ring, and the him underneath it was not altered photographically and appears completely white.
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Figure 1.7 Positive print of a radiograph of the hand, made by singly reversing the values. This is also approximately as the hand would lookon the fluoroscopic screen. |
What Roentgen saw, on the contrary, was the reverse of all the light-dark values you have been looking at in the film of the hand. X-rays reached the coated cardboard in abundance all around his hand so that the background fluoresced vigorously, while the shadow of his hand emitted less light and appeared gray-green. The cardboard underneath the bones of his fingers appeared darkest of all, since it received almost no activating rays. (Compare Figure 1 .3.)
Figure 1.8
Fluoroscopic light is very faint unless it is amplified electronically. Today all fluoroscopic rooms are equipped with such "image-intensification" machines, functioning in a lighted room. You may not see much fluoroscopy in your lifetime, but you will see many thousands of x-ray films. For this reason we suggest that you make a practice of thinking in terms of the white and black values that relate to the usual x-ray film as you saw them in the hand in Figure 1.6. Think of dense objects as white and of those more easily penetrated as gray or black. All the illustrations in this book are printed like Figure 1.6, and you will find that most journals and books reproduce x-ray illustrations in this way.
While it is essential to understand which are the more dense (or radiopaque) substances and which the more transparent (or radiolucent) ones, your concern, even as you first begin looking at radiographs, should not be only with density. You will often make quite reasonable and useful deductions from the form and shape of radiographic shadows. If you figured out that Figure 1.1 was, and could only be, a radiograph of a duck-billed platypus, you have experienced the sort of educated guessing used all the time in radiology. You guess imaginatively and then subject your guess to a rigorous logical analysis based on radiological and medical data. By putting together expected density and expected form, you will soon find that you can predict the appearance of the radiograph of an object or structure.
Begin, then, by applying imagination and judgment to a variety of nonmedical objects. Try to predict the type of shadow that would appear on the film if you x-rayed an egg.
Figure 1.8 is a radiograph of a woman's purse. Although the cloth from which the purse was made offered almost no obstruction to the x-rays, anything made of metal inside it, including the frame of the purse, absorbed the rays and left a white profile on the film. You will be able to identify from their outlines alone a paper clip, a bobby pin, a safety pin, a pair of rimless glasses, coins, a lipstick case, two locker keys (overlapped), a nail file, and a metal pencil. You can almost construe the woman: a poverty-stricken, myopic individual who is taking two lab courses and wears makeup. You might, of course, be mistaken about the state of her finances; folding money would be quite radiolucent.
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Figure 1.9 Radiograph of a portrait in oils painted over an earlier portrait. Only the woman with the pale eyes is visible as one looks at the painting. |
Pure metals are relatively radiopaque and so are their salts. Consequently, so also are the mixtures of and brilliantly colored metallic salts responsible for the whole field of oil painting. The radiography of paintings and other works of art is a fascinating and technically useful branch of the science. Frauds, inept reconstructions, and masterpieces painted over by amateurs may sometimes be detected by x-ray studies.
In Figure 1.9 two painters have used the same canvas-or, dissatisfied with the portrait of the man whose eyes appear as the lower pair, the same painter may have done the portrait of the woman with the light eves and severely dressed hair, covering the earlier portrait. Only the woman is visible as one looks at the painting.
Variations in the precise metallic composition of artists' colors used at different times in history may help in the identification and dating of such works of art. The pigments in use since about 1800 have been made of he salts of metals with much lower atomic numbers than those of the older pigments, and for that reason they x-ray quite differently.
Figure 1.10
Thus a modern forgery of an old master, no matter how adroit a copy, will yield a radiograph entirely different from a radiograph of the original. But a copy made by a pupil of the master or another artist of the same school, painting at about the same time in history with the same hand-ground, earth-mineral colors, can be expected to x-ray in about the same way.
The characteristic use of brush strokes, which, even better than the signature, often stamps the work of a great artist, may also help to identify a concealed painting covered by a lesser artist. You would be able to imagine the radiograph of a contemporary canvas with the vigorous, heavy brush strokes of Van Gogh showing through, the example.
Remember too that any radiograph of a painting represents the summation nor only of the various paint densities but also of the x-ray shadows of the canvas itself and the supporting structures. The wooden frame on which the canvas is stretched will cast some shadow, and if there are any nails in the wood they will appear in the radiograph also. Figure 1.10 shows a radiograph of a painting supported on wooden strips. The curious white areas are wormholes that have been filled with white lead. The x-rays have been completely absorbed, you notice, by the white-lead casts of the wormholes, and under them no x-rays have reached the film to blacken it. The white areas on the film are actually, therefore, shadow-profiles of these white-lead casts. Remember this! It has an important parallel in barium work in medical x-ray studies of the gastrointestinal tract.
The industrial uses of x-rays arc many and important. Flaws, cracks, and fissures in heavy steel can be shown by x-raying big equipment or building materials. Especially powerful machines are needed for this sort of work, ones that will produce a more penetrating beam of x-rays of very short wavelength, often called hard x-rays. X-rays of long wavelength, or soft x-rays, are used to study thin or delicate objects. Very soft x-rays are used to study tissue sections of bone I or 2 microns in thickness (microradiography), while very hard x-rays are used to penetrate deep into the body and destroy malignant tumor cells (radiation therapy). Between these two extremes fall the wavelengths that are used in medical x-ray diagnosis.
The electromagnetic spectrum is a scaled arrangement of all types of radiant energy according to wavelength. Within the range used in diagnostic radiology, x-ray technologists are trained to select and use the particular wavelength suited to the density and thickness of the part they are filming. They do this by varying the kilovoltage of the machine: the higher the kilovoltage, the harder or more penetrating the beam of rays produced. They can also vary the amount of radiation in the beam by altering the milliamperage used, and, finally, they can control the time of exposure. Thus, for instance, for a thin object like the hand they use a soft beam for a short time, and for a dense object like the pelvis, a hard beam and a long exposure.
| Radiodensity as a Function of Thickness |
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Figure 1.11 Radiodensity as a function of the thickness of the object. Here the object to be filmed is of homogeneous composition and has a stepwise range of thickness. Gray shading indicates the degree of absorption of the x-rays. |
| Radiodensity as a Function of Composition, with Thickness Kept Constant |
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Having reasoned through all this, you must consider in greater detail the relative radiodensities of various substances and tissues. To do this most easily, let us eliminate thickness completely for the moment. Consider an imaginary row of l-centimeter cubes of lead, air, butter, bone, liver, blood, muscle, subcutaneous fat, and barium sulfate. Can you arrange them in order of their radiodensity, decreasing from left to right? If they were all pure elemental chemicals, you certainly could arrange them in order by looking up their atomic numbers. Only one of them is quite as simple as that, and a judicious guess will surely place first to your left as most dense the cube of lead, with an atomic number of 82. Are you hesitating between bone and barium sulfate? Barium has an atomic number of 56, and calcium in the bone cube has an atomic number of 20. Bone, however, is lot even pure calcium salt. It has a functioning physiological structure with holes and spaces to accommodate body fluids and marrow. It is composed of an organic matrix into which the complex bone mineral is precipitated. All such organic substances reduce the radiodensitv of the cube of bone, and it will consequentlv have even less radiodensity than a similar cube of packed bone dust. The cube of barium sulfate must be placed next to the lead cube, therefore, and after it, the cube of bone.
Figure 1.12 Radiodensity as a function of composition, with thickness kept constant.
As to the most radiolucent of all, you can have no troubIe with that: surely you have put the cube of air far to the right, at the opposite end of the scale from the lead. The film under the air cube will be black, since the sparse scattering of air molecules offers almost no obstacle to the rays. The square of film tinder the lead, unaltered because no rays penetrated the cube to reach it, will be clear white, whereas that tinder the bone will show a tinge of gray.
Butter and subcutaneous fat have very similar x-ray densities. They are extremely radiolucent and must be placed next to air in the scale we are considering. Neither butter nor fatty tissue is homogeneous, since the first is never quite free of water and the second contains both circulating fluids and a supporting network of fibrous connective tissue. Their squares on the radiograph would be almost the same very dark gray. Between the three very dense cubes and the three very lucent ones there remain to be arranged the three cubes of blood, muscle, and liver. These will all x-ray an almost identical medium gray, and you should remember that all moist solid or fluid-filled organs and tissue masses will have about the same radiodensity, greater than fat or air but considerably less than bone or metal. Thus the muscular heart with its blood-filled chambers could be expected to x-ray as you see it on the chest film, a homogeneous mass much denser than the air-containing lung on both sides of it, but showing no differentiation between muscular ventricle wall and blood within the ventricle. Remember that in this discussion of relative radiodensities, we have kept thickness and form contain, as well as such technical factors as kilovoltage and time of exposure. We have planned this deliberately so that you can more easily construct a working concept of the relative densities of different tissues. In practice, the radiologist adjusts tile technical factors to accentuate these differences. Upon this useful spectrum of differing radiodensities of human tissues is based tile whole field of medical radiography. Once these basic facts are learned, radiologv becomes an exercise in logical deduction and all absorbing habit of mind. More important for you, it is also a delightful extra dimension in learning, a sort of custom-tailored illustrative tool related to nearly everything you will study in medical school. If you wish, you can use it to help you learn from the first day you begin to study anatomy, through your courses in physical diagnosis, pathology, medicine, and surgery, as a means of comprehending and remembering medical facts.
| How Roentgen Shadows Instruct You about Form |
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Figure 1.13
Consider now the contribution of form. Figure 1. 13 is a radiograph of three roses, which we can use as an example of the basic logic of the roentgen shadows of complex objects. Flowers require only a very soft beam, of course, because they arc both thin and delicate. A glance will tell you that one rose is full-blown and the other two more recently opened. You can deduce a great deal of information from the form, outline, shape, and structure of roentgen shadows. This is so true that in time you will learn to recognize with confidence the identity of certain shadows in medical radiographs because of their shape or form.
Now study the density of various parts of a single petal and compare the radiodensity, or whiteness, of the petals with that of the leaves. The leaves look less dense than the flowers and stems. Notice too that the veins within each leaf are denser than the rest of it. Veins of leaves have, of course, a structure independent of the cells composing the flatter part of the leaf. The stems arc thicker and they also convey fluid. In both medical and nonmedical radiographs you can, in general, anticipate added density wherever there is fluid.
| Radiographs as Summation Shadowgrams |
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Another reason for the denser appearance of the petals compared with the leaves in Figure 1.13 is that they do not lie flat against the film but are curved and folded and overlap one another. This gives you a clue to a very important facet of radiological interpretation. A sheet of any uniform composition, if it lies flat and parallel to the film, will have a uniform x-ray density and cast a homogeneous shadow. If it is curved, however, those parts which lie perpendicular to the plane of the film will radiograph as though they were much more dense.
This is perfectly simple. X-rays pass through a complex object and render upon the film not a picture at all but a "composite shadowgram," representing the sum of the densities interposed between beam source and film. Thus a sheet of rose petal that lies perpendicular to the film, or in the plane of the ray, is equivalent to many thicknesses of petal laid one upon another and, quite logically, is much more dense than a single sheet lying flat. Find the leaf that is turned on edge.
Curved sheets, considered geometrically, arrange themselves into groups of planes, and should be so considered in imagination when you are interpreting an x-ray film. Of course, in nature, and consequently in medicine, the curved plane is common and the symmetrical plane rare. In the radiograph of any curved plane structure, therefore, learn to think in terms of those parts of it that are relatively parallel to the film and those that are roughly perpendicular to it.
Observe, finally, that the shadow of the stem of the rose in Figure 1.13 has a form you will find characteristic of any tubular structure of uniform composition. The margins are relatively dense because they represent long, curved planes radiographed tangentially, and the center area between them appears as a darker, more radiolucent streak. Rose stems are not truly hollow as one looks at them with the naked eye, but the central core, like that of tubular bones, is filled with a structure having less radiodensity than the margins. Hence the rose stem looks hollow and tubular on the film, just as a hollow tube containing air would look.
By this time you have several important principles clearly in mind, although you have learned them largely from examples. First, you know that x -rays are radiant energy of very short wavelength, beyond light in the electromagnetic spectrum, and that they penetrate, differently according to their wavelengths, substances opaque to light.
Second, you know that a beam of x-rays penetrates a complex object like the hand in accordance with the relative radiodensities of the materials which compose the object. You know that the beam produces on the film a composite shadow gram representing the sum of those radiodensities, layer for layer and part for part. You know that radiodensity is a function of atomic number and of thickness.
Third, you have realized that the parts of an object may become recognizable as to form, and their structure may be deduced, according to whether they are constructed most like solid or hollow spheres, cubes, or cylinders, or like plane sheets lying flat or curved upward away from the film.
Because we believe that the working of problems and puzzles will greatly increase your enjoyment of this book, we have included some in almost every chapter. They are geared to the chapter in question both in subject matter and in difficulty. In general, they are presented with a few details about the patient, and you should imagine yourself the intern or practicing physician in charge of that patient. Often, especially in the early chapters, you are asked not for a diagnosis but rather for an impression of variation from the normal of a particular structure. You will see that solving these puzzles will help you gauge as you go along just how roentgen shadows can be reasoned out and used as mnemonic device in learning medicine. We think it will also persuade you that you know more and can reason better than you had realized.
Figure 1.14 (Unknown 1.1). Sometimes the radiologist figures in criminology as an adjunctive source of information. The lucky throw you see in the innocent-looking pair of dice in the photograph was actually not luck at all but planned economy. Below are two radiographs, one of a pair of loaded dice and one of a pair of unloaded dice for which they could be switched. It is simple enough to decide which are the loaded dice, but can you figure out precisely what has been done to them?
Figure 1.16 Not an unknown. This is a radiograph of the egg referred to earlier. You no doubt anticipated its shape, but did you predict the air pocket at the blunted end? Note that the density of the shell increases peripherally, as with any hollow sphere.
Sumário
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Introdução
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Topo
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Capítulo
2
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