2. The Imaging Techniques
In this chapter we will teach you several important frets about plain films and introduce you to the other diagnostic imaging techniques in clinical use today. You will learn more about these procedures in later chapters. Remember to think three-dimensionally when looking at x-rays and other diagnostic images.
The three radiographs of a finger in Figure 2.1 illustrate at once how important it is for you to learn to think in three dimensions about x-ray shadows, reconstructing form from two views at right angles. Note that the soft tissues in A are seen as a faint uniform gray outline encompassing the bones. In B and C, however, the skin with its wrinkles and folds and the crevice between the cuticle and nail all seem to become visible. They appear so because they have been coated with a creamy substance containing a metallic salt.
Actually, the skin itself is no more visible than it was before, but the radiopaque cream collecting on its patterned, irregular surface forms a visible coating that marks the position of the skin. A and B were made in the frontal projection; C is made from the side and is called a lateral view.
Although A probably looks very flat to you and B and C give an illusion of depth, you will have realized that you can look at a medical x-ray film and think about it three-dimensionally even through you do not see it that way. The radiograph is a composite shad-owgram and represents tile added densities of many layers of tissue. You must think in layers when looking at any radiograph.
The most striking contrasts in radiodensity exist in the region of the chest, where air-filled lungs (radiolucent) on both sides of the muscular fluid-tilled heart relatively opaque) occupy the inside of a bony cage (a fretwork of crossed radiopaque strips).
In Figure 2.2A imagine the structures through which the x-ray beam has passed from back to front: the skin of the back; subcutaneous fat; lots of muscle encasing the flat blades of the scapulae, the vertebral column, and the posterior shell of the rib cage; then the lungs with the heart and other mediastinal structures between them; the sternum in and the anterior shell of the ribs; the pectoral muscles and subcutaneous fat; breast tissue and, finally, skin.
Note the crescents of density that are added in Figure 2.2B, where the x-rays have had to traverse the female breasts in addition to all the other tissue layers Below the shadow of the breasts and above that of the diaphragm the film is blacker where more rays have reached it.
One of the problems that will worry you as you begin looking at chest films is how to put them up on the light boxes against which they are viewed: since they are transparent, you can look through them from either side. Always place them so that you seem to be facing the patient. Naturally this is only possible with PA and AP (anteroposterior) views.
X-ray films are usually marked by the technologist to indicate which was the patient's right side-or, in the case of films of the extremities, whether it was the right of left leg. for example. In chest films you can usually be somewhat independent of marker because the left ventricle and the arch of the aorta cast more prominent shadows oil the left side of the patient's spine. Always view a chest film, then, so that the patient is facing you with the patient's left on your right, and remember that "left" in regard to a finding on the film invariably means the patient's left. When you read "the right breast is missing" you are going to check, automaticallv, the breast shadow to your left.
Most of the chest films you see will have been made with the beam passing in a sagittal direction "posteroanteriorly," the x-ray tube behind and the film in front of the patient. This is the standart PA chest film, and films of all kinds are called PA views if the beam passes through the patient from back to front. It is customary to make a PA chest film of any patient whois able to stand and be positioned.
Less satisfactory but often valuable AP films of the chest are made with a portable x-ray the patient is too sick to leave bed. propped up against pillows and the behind the patient, the exposure being x-ray tube over the bed. Thus the ray the patient "anteroposteriorly." You will be seeing such, portable AP films of your very especially those in intensive care units. Although they do not compare in quality with the PA films made with better technical facilities in the x-ray department, they do offer important information about the progress of the patient's disease. Sometimes patients who cannot stand are not too sick to be taken in their beds to the x-ray department and filmed AP with the equipment available there, a better film being obtained in this way than is possible with the portable machine.
After looking at many normal PA chest films, you will have formed a mental image of what a normal chest film looks like. A normal AP chest film, however, looks different for the following reasons. The divergence of the rays enlarges the shadow of the heart, which is far anterior in the chest, and the position of the patient, leaning back, makes the posterior ribs look more horizontal. These differences are even more marked at the shorter tube-film distances used in portable radiography at the. bedside. Remember that, in addition, the diaphragm will be higher and the lung volumes less than in a standing patient.
Consider a patient who presented to the emergency ward with a persistent cough, one episode of blood-streaked sputum, weight loss, and a daily fever. The routine PA chest film in Figure 2.7 is not strikingly abnormal at first glance, but there was a strong clinical suspicion of pulmonary tuberculosis, so a special projection called a lordoric view was made. Because the patient stands leaning backward in exaggerated lordosis, the horizontal beam of AP x-rays foreshortens the chest by penetrating it at such an oblique angle that the anterior and posterior segments of the same ribs are superimposed. The result of this maneuver is, of course, to project the clavicles upward so that by looking between the ribs you can much more effectively visualize the lung tissue of the apex. Note that this case is a good exercise in the use of bilateral symmetry in examining films made with a sagittal beam. Now you are able to see that there is a fluffy white shadow in the upper part of the left lung, best seen in the second interspace. Note that there is nothing like it in the same interspace on the other side. Analysis of the patient's sputum confirmed the diagnosis of active tuberculosis.
Figure 2.9 Position in which Figure 2.8 was made. (After Seurat.)
The chest film in Figure 2.10 offers you an opportunity to test your pi-ogress in three-dimensional thinking. There is an obvious metallic radiodensitv. The shape of the metal object suggests it might be a bullet. This, in fact, is a film made of a soldier wounded in the Sicilian campaign during World War II. He was transformed to a hospital, where the surgeons observed what you observe. They requested, as you are about to do, a lateral view to determine the location of the bullet. It might, of course, be in any of the structures whose roentgen shadows superimpose in this view on the origin of the fifth rib.
The importance of localizing a bullet is illustrated by the cross-sectional drawing shown in Figure 2.11 If the bullet is lodged in the spinal cord or the trachea, or in one of the major vascular structures at this level, there may be less hope of saving the patient. In fact, the bullet was located harmlessly in the anterior mediastinum, had not injured any vital structure, and was removed without incident. (For the lateral view see Figure 2.12.)
You can never know precisely where a foreign body is located from a single radiograph. A film made at right angles to the first is essential, and minute metallic foreign bodies in the soft tissues of the extremities are localized very accurately by a refinement of this procedure. Fractured bones can appear to be in good position, end to end, in one film, while a second film made at right angles shows that the fragments are separated and do not align. Often the making of such supplementary lateral films is a routine matter. At other times you will have to ask that they be made on your patients. Always ask for a right lateral chest film if you think the lesion is on the right, so that the structure to be studied is as close as possible to the film. In most medical centers in this country a chest film series includes a PA chest film and a lift lateral. Can you decide why?
At about this point you will begin to say to yourself "How am I going to know which views are important for me to understand and learn to use?" if in the course of your training in medicine, you can familiarize yourself with the chest structures and their shadows as seen in the standard PA and lateral views, you will have built yourself a "cry useful and satisfying tool, and you should have no trouble in doing so. Do not feet confused or defeated if occasionally you see chest films that look like nothing you have ever seen before. Some of these will in fact be films of grossly abnormal chests. Others, however, will turn out to be films made by special or rarely used x-ray projections with which you arc not yet familiar. You should rely comfortably on your acquaintance with the standard views, but not be incurious about or resistant to the possibilities of other modes of examination.
There are all sorts of ingenious obliquities of projection and many fascinating special procedures in the armamentarium of the radiologist that you will want to know about. Two of them, the posteroanterior obliques of the chest, arc sometimes used in studying the heart or hila of the lung. Detailed study of the ribs is obtained by obliques made anteoposteriorlv. Other procedures, designed for visualizing a particular structure in a particular way, also offer anatomic information not otherwise available. Sometimes the radiologist decides which views or procedures to obtain. At other times you will ask specifically for certain projections, or, better yet, discuss with the radiologist the advantages of their being used in the study of von patient's particular problem.
Figure 2.13A PA film of upper bony thorax.
The two films on this page spread were made of the same patient. Figure 2.1 3A is an ordinary PA radiograph; Figure 2.13B is a special-procedure film called a tomogram (or a conventional tomogram). Tomography will help you to visualize better the shadows that must be added together to make up the usual x-ray film. It is important, therefore, to understand how such radiographs are made.
Imagine that a frozen cadaver is sawed into coronal slices about 1 inch thick and that you then make a radiograph of each slice. Each film will have on it only the shadows cast by the densities of the structures in that slice. There will be no confusing superimposition of the shadows of structures from other slices to trouble you. How much simpler it would be, for example, to be able to study the manubrium and medial halves of the clavicles if they were not superimposed upon the shadow of the thoracic spine as they are in the standard PA chest film. On the next few pages von will find some radiographed cadaver slices to study. View are arranged in order from front to back, the first slice having been omitted. (It included the anterior chest wall, rib cartilages, and sternum) You will find it helpful to refer back to these slices as you learn the x-ray appearance of various organs and structures. Now notice how well you can see Figure 2.13B the shadows cast by tile clavicles Where they join the manubrium.
Conventional tomographic studies effectively slice the living patient so that von can study the shadows cast by certain structures free of superimposed shadows. The term "tomogram" is a general one and there are different types of tomographic studies, the techniques of which depend upon the result desired, that is, the shadows intended for study and those you wish to distort.
Conventional tomograms arc less likely to be requested today than in the past. For many procedures conventional tomography has been replaced by computed tomography, which you will learn about later,
On first acquaintance tomograms may look blurred and confusing to von. Whenever you are puzzled by one you see in this book, try coming back to the cadaver slices in Figures 2.14-2.17 to get your bearings, remembering that only the structures in one plane will be in focus in the tomogram. Remember too that the thickness of these particular cadaver slices may not match perfectly the chosen plane of the study you happen to be looking at, since lie pivot point determining the plane of a tomographic study is calculated arbitrarily for a certain distance in centimeters from the surface of the body.
Figures 2.14 to 2.17. Radiographs of a series of coronal slices of a cadaver, arranged from front to back Identity the following:
Junction of manubrium and clavicles
Note the change in shape of the liver from section to section. Note too that these radiographs are unlike tomograms because there are no blurred images of structures in other slices.
Conventional tomograms are made by moving both the x-ray tube and the film a round the patient during the exposure, as in Figure 2.20. They are about a pivot point calculated to fall in the plane of the object to be studied. In this way the shadows of all the structures not in the plane selected the study are intentionallv blurred because they move relative to the film. Thus in the diagram (Figure 2.21) the object to be studied, b, will be "in focus'' in the film, while the shadow of an object at a will be magnified, blurred, and distorted to lie between a' and a" on the film. Only the structures in the plane of the pivot point will be recognizable (as in Figure 2 .22C); the shadows representing organs in front of or behind iv are distorted in such a way that shape and form are no longer recognizable and the blurred images are easy for your eye to ignore. Conventional tomograpy is used as an adjunctive study whenever detail is needed of a structure superimposed on and obscured by other structures in the line of the x-ray beam.
Figure 2.20 Conventional tomographic x-ray unit showing range of tube motion.
Figure 2.21 How conventional tomography obtains a coronal slice.
Figure 2.22 Demonstration of
the effect obtained with conventional tomography.
Fluoroscapy is a common radiological technique that allows real-time visualization of the patient. You may already be familiar with the use of fluoroscopy during contrast examinations of the gastrointestinal tract to follow the course of barium through the esophagus, stomach, and bowel. Fluoroscopy is also used to guide the radiologist performing selective arterial and venous catheter placement for angiographic procedures. In addition, most interventional radiological procedures require fluoroseopic guidance.
During fluoroscopy a continuous beam of x-rays passes through the patient to cast an image on a fluorescing screen, which is amplified by an electronic image intensifier and viewed on a high-resolution television screen. Figure 2.23 shows an angiographic suite in which a coronary arteriogram is being performed. The x-ray tube is located underneath the patient and the large cylindrical image intensifier above; the beam of x-rays is passing through the patient from below. The angiographer observes a coronary artery contrast injection on the television monitors. Note that on the fluoroseopic image, black and white are reversed so that bone and contrast agents appear (lark and radiolucent structures such as the lungs appear light.
Figure 2.24A shows a simpler fluoroscopic room, such as might be used for gastrointestinal examinations. Again the image intensifier is located above the patient; the x-ray tube is concealed below within the x-ray table Films taken by the radiologist during the fluoroscopic portion of the procedure are called "spot films." They detail small areas of special interest observed during the fluoroscopic segment (Figure 2.24B). On completion of fluoroscopy the x-ray technologist obtains a set of larger conventional x-rays (Figure 2.24C), which are called "overhead" films because they are taken with a second x-ray tube that is rolled along a track suspended above the patient.
Figure 2.25 Right pulmonary arteriogram
Angiography includes a variety of procedures in which the vascular system is imaged by x-ray during intravascular injection of iodinated contrast agents. Images of arterial structures are called arteriogranis, and images of venous structures are called venograms. The arterial system is usually opacified by contrast injection into a percutaneously placed small-caliber, flexible, arterial catheter, usually introduced through a femoral artery. Under fluoroscopic guidance the catheter is manipulated through the arterial system until its tip is in position within the artery tinder examination. A wide variety of catheter shapes and soles are available, as well as sophisticated directional equipment that permits selective catheterization of virtually every major artery in the body. Once the catheter tip is positioned, contrast material is infused by a power injector at a controlled rate and volume, while x-ray filming is tamed with a rapid film changer, digital radiography system, or movie camera (cineradiography).
Venography of major veins such as the superior vena cava, inferior vena cava, and renal veins is performed by similar techniques, using a femoral vein approach. Some venous structures, such as the pulmonary veins and portal vein, do not anatomically lend themselves to direct catheterization techniques and are usually imaged by contrast injection into the supplying arteries (pulmonary artery, superior mesenteric artery) and filming through the venous phases of the arterial contrast injection. Venous studies of the extremities (leg venogram, arm venogram) do not require actual catheter techniques. These studies require only a simple injection of contrast material into the peripheral veins of the foot or hand.
Figure 2.26 Left renal venogram.
Computed tomography (CT) gives you a whole new way of looking at the body because it provides the equivalent of cross-sectional slice radiographs of the living body. These are what the lay public calls CAT (computerized axial tomography) scans, anti they are a vital source of radiological information in medicine.
You can begin by understanding the difference between plain radiographs, conventional (or plain) tomography, and CT. Remember that ordinary plain x-ray films are superimposition shadowgrams: the images of all superimposed structures appear on the film. Conventional tomography gives you sharply focused radiographic images of' one plane of the patient, upon which arc superimposed (unfortunately) the blurred images of structures in slices on both sides of the plane chosen for study.
A CT scan, on the other hand, gives you focused radiographic information about one cross-sectional slice of the patient only, without any confusing superimposed images. Thus a CT scan gives you a range of density values for a particular chosen slice of the patient, which should be studied with regional cross-sectional anatomy in mind. You will be able to learn the relationships between structures in the body much more accurately with the help of the added dimension CT provides.
An awareness of the relative x-ray densities of different tissues and organs and their interfaces with Lit planes in the body helps von as you look at CT' scans. In computed tomography a pencil-thin collimated beam of x-rays passes through the body in the axial plane chosen for study as the x-ray tube moves in a continuous arc around the patient. Carefully aligned and placed directly opposite the x-ray tube are special electronic detectors, a hundred times more sensitive than ordinary x-ray film. These detectors convert the exiting beam on the other edge of the body slice into amplified electrical pulses, the intensity of' which depends upon the amount of the remaining beam of x-ray's that has not been absorbed by the intervening tissues. Thus if the beam has passed mainly through dense areas of the body (such as bone), fewer x-rays will emerge than when the beam traverses mainly low density tissue (such as lung). The x-ray tube and detectors are housed in the gantry the doughnut-shaped structure through which the patient passes during scanning. The gantry can actually be tilted to take slices at an angle to the long axis of the patient.
Figure 2.27 Simplified diagram of computed tomography.
If you conceive of a body slice as a mosaic of unit volumes, or voxels (see Figure 227), forming a geometric grid, you can see that a single denser unit volume (perhaps calcium containing, like the small black square in Figure 2.27) will absorb more of the beam than other, less dense neighboring voxels.
As fast as it is received by the detectors, this information is conveyed to a computer, which the calculates the x-ray absorption for each voxel in the mosaic. The pictorial arrangement of absorption values makes up the final CT image. The absorption value is expressed in Hounsfield units (after one of the inventors of CT). Water was arbitrarily assigned the value of zero, while denser values range upward to bone (which can be + 500 or more). Less dense structures range downward through fat to air (which call be -500 or less).
The attenuation number so obtained for each voxel in the mosaic matrix slice is converted into a dot on a television monitor screen, the brightness of which depends on the density of that unit volume and thus reflects its anatomic structure. Denser tissues (such as bone) appear white; less dense tissues appear darker; and air appear darker; and air appear black. The "picture" produced is equivalent to rradiograph o fthat cross-sectional slice of the living patient.
It is conventional to view the CT scan so produced as through you were looking up at it from the patient's feet (Figures 2.28A and 2.2813), and it is important to remember that therefore the structures seen on your right are those on the left side of the patient's body, just as they are when you view an ordinary chest film. Permanent images are produced by transferring the images onto x-ray film with a laser camera. For each G.E slice obtained, the imaging settings (window and level) can be altered on the CT scanner controls to better show individual tissues (bone versus lung versus heart and great vessels). You will find that most CT scans are filmed at more than one setting. For example, a chest CT (Figure 2.29) is generally filmed with "lung windows" to optimally show the lung parenchyma and "soft-tissue windows" to best show the heart, blood vessels, and other structures in the mediastinum and chest wall. A head CT in a trauma patient is filmed with soft-tissue windows to show any brain injuries and also bone windows to show any fractures.
The CT scans in your patient's film envelope are documented on 14><17 inch x-ray films (called "hard copies"), each such film having from 6 to 12 to 20 scan slices in sequence, so that you can look from one slice to the next, above or below, for additional information about the form of a structure or organ. The actual CT images are usually also stored in computer form (magnetic tape, laser disk), so that additional hard copies can be obtained, if necessary, at a later date.
The usual CT series of scans for examining the chest and abdomen consists of contiguous 5- or 10-milli-meter-thick, slices, but slices slices, but slices as thin as 3.0 millimeters or even 1.0 millimeter can be obtained when finer detail is needed for diagnosis. In most radiology departments CT protocols are written and followed that detail the most optimal CT technique for examining various body regions or for evaluating various clinical conditions. 11w protocols describe not only the slice thickness and extent of study (location of first and last slices), but gantry angle (0 degrees for a true cross section versus tilted to better show structures in other angles of section), whether any oral, intravenous, or other contrast material is required, and whether any computer rearrangements (special reformations) of the axial slices are required. The protocols also describe what kind of "windows" (bone, soft-tissue, lit rig, liver, brain, etc.) should be hard copied for each slice. The x-ray dose per slice of a CT scan varies from I to 4 rads (but only to the slice being imaged) and is comparable to the exposure for conventional x-ray studies of the area.
High-density materials sue Ii as ban m or metal (a hip prosthesis or metal surgical clips) may produce artifacts like bright stars with sharply geometric radiating white lines that may degrade the image obtained and interfere with the information available it. Motion also degrades the image, but this effect is minimized with newer, high-speed scanners.
Conventional CT scanners require only I to 2 seconds to complete a slice, but patients who cannot hold their breath may have motion artifacts on their scans. This may be a problem with unconscious, very ill, or dyspneic patients and small children requiring CT studies. But the newer, high-speed scanners can virtually eliminate respiratory motion. A conventional CT scan may take 10 to 20 minutes for completion of slices; with a high-speed scanner (helical or spiral scanner) an entire chest or abdomen can be scanned in 90 seconds, or the time equivalent of one breath hold.
Mild sedation and reassurance by you as well as by the radiologist may help calm an anxious patient. The gantry is huge and may be frightening (Figure 2.30). It behooves you to inspect the CT rooms in your x-ray department so that you can explain to your patient beforehand that, unless intravenous (IV) contrast material is required, the procedure is as painless as having a photograph taken, in spite of the look of the machine. As compared to magnetic-resonance scanning (discussed later in this chapter), in which the patient's entire body is placed within the bore of a superconducting magnet, only a portion of the body is surrounded by the more doughnut-shaped gantry of a CT scanner.
Body CT scans in the axial plane can be produced with the patients supine or prone or lying on their side. Other planes of imaging, especially of the head and extremities, are possible, but you will need to learn to think of most CT body scans as transaxial and supine, since patients are most comfortable and relaxed lying on their back.
It is important for you to realize that CT should be considered most of the rime as a sophisticated study for special problems, usually arranged following con -sultation with the radiologist. Other, less expensive, procedures like plain Elms and ultrasound should be used when the information obtained is comparable.
The important exceptions to this principle are in traumatized patients and central nervous system emergencies. In head trauma the superior capacity of CT to recognize intracranial hemorrhage often makes ordinary skull films a dangerous waste of time. Patients with abdominal trauma too should be taken straight to the CT suite for serial scans from the diaphragm through tile pelvis, supplying a wealth of emergency information about hemorrhage and organ injury that can save lives.
Depending on the clinical condition tinder investigation, contrast media may be used during CT scanning to enhance the difference in density of various structures. Pie gastrointestinal (GI) tract can be illuminated by giving the patient diluted water-soluble oral contrast material, which will help to distinguish the stomach and bowel from other soft-tissue structures and masses. Intravenous administration of water-soluble contrast material will produce a temporary increase in the density of vascular structures and highly vascularized organs. This effect is referred to as enhancement and is extremely useful. For example, a great vessel and the tumor mass encasing And constricting it will appear as one homogeneously dense mass unless the vessel is enhanced with contrast material, when its narrowing.
Three-dimensional CT images may be produced by computer stacking of a series of contiguous CT slices. Figure 2.31 shows the 3DCT of a patient with a facial fracture. A 3DCT image can provide the surgeon with an image that most realistically displays the position and orientation of displaced fracture fragments. Although the fracture line and fracture fragments were clearly apparent on the individual axial slices, it is much easier to perceive the "big picture" by looking at the 3D image than by mentally stacking all the individual axial CT slices. No additional scanning is required to produce a 3DCT image. The CT scanner computer or a free-standing computer is directed to make a 3D model from the series of axial CT scans. After the model is generated, it can be rotated in real time to be viewed from every side and even sliced to reveal the interior three-dimensional anatomy.
Conventional CT scanning is performed by obtaining a series of individual axial scans during suspended respiration. The x-ray tube and detector assembly rotate about the patient while scanning each slice. Between scans, motion of the tube and detector ceases, and the patient is allowed to breathe during a 5-10 second delay, during which the scanner table moves the patient to the next scanning position. The recent introduction of improved mechanics in helical or spiral high-speed CT gantries allows for uninterrupted scanning in such a way that the patient moves through the scanner at a constant rate during continuous rotation of the x-ray tube-detector assembly. Scanning is so fast that an entire CT examination of the head, chest, or abdomen may be performed within 90 seconds, often with only a single breath bold required by the patient. Consequently, motion artifact is virtually eliminated with high-speed scanning. While scanning, the x-ray tube traverses a helical or spiral path around the patient.
As you no doubt have already realized, high-speed scanning is helpful not only when motion is a prob1em, for example during imaging of pediatric and sick adult patients, but also when scanning is done to evaluate structures that move within the patient, such as blood vessels (arterial pulsation) and lungs (respiratory motion). High-speed CT is capable of producing detailed three-dimensional displays of blood vessels (Figure 2.32). Referred to as CT angiograpry, this technique may be used to evaluate aortic aneurysms and aortic dissections, renal artery stenoses, and a variety of other vascular conditions.
Ultrasound, or ultrasonoqraphy, also gives you an image of a slice of the body, by directing a narrow beam of high-frequency sound waves into the body and recording the manner in which sound is reflected from organs and structures. The ultrasonographer uses a hand-held transducer (Figure 2.33) containing piezoelectric crystals, which change electrical energy into 'high-frequency sound waves. The sound beam is directed into the region of interest and then reflected back toward the transducer at interfaces between tissues of different acoustic impedance (which is determined by the physical density of the tissue and the velocity of the sound). As the acoustic impedance mismatch between two tissues increases at any given interface, the reflected sound, or echo ,becomes stronger. When the reflected sounds return to the transducer, they are converted to electrical signals, which are then computer analyzed to produce the ultrasound images.
These images are viewed in "real time" and can be used to display motion of the heart and blood vessels.
At ultrasound solid organs (Figure 2.34) appear as echogenic structures because they consist of tissues with multiple acoustic interfaces, whereas cysts and fluid collections (Figure 2.35) appear echo-free (echolucent or anechoic) because they lack internal acoustic reflectors. Air and bone cannot be adequately visualized with ultrasound because the acoustic impedance mismatch between these structures and the adjacent soft tissues is very great; most of the sound energy is reflected, so that little is left to visualize the structures beyond the interface.
Ultrasound does not produce an image that is as sharp and clear as UT, but it has five singular advantages. First, ultrasound is a safe procedure that does not employ ionizing radiation and that produces no biological injury. Consequently, it has found wide applications in the imaging of obstetrical, gynecological, pediatric, and testicular conditions. Second, ultrasound can be employed in the transaxial plane or sagittally or at any obliquity required to show the anatomic region being investigated. Third, it is far less expensive than either CT or magnetic-resonance imaging. Fourth, ultrasound can be performed portably at the bedside of very sick patients. Fifth, real-time ultrasound can provide moving images of the heart, fetus, and other structures.
Figure 2.33 Patient undergoing an abdominal ultrasound examination.
Like ultrasound, magnetic-resonance imaging (MM) or magnetic-resonance scanning (MR scanning) does not use ionizing radiation as ordinary x-rays and CT do. This technique for imaging places the patient within the bore of a powerful magnet and passes radio waves through the body in a particular sequence of very short pulses (Figure 2.36). Each pulse causes a responding pulse of radio waves to be emitted from the patient's tissues. The location from which the signals have originated is recorded by a detector and sent to a computer, which then produces a two-dimensional picture representing a predetermined section or slice of the patient.
The specific principles involved in magnetic-resonance imaging are quite complex and beyond the scope of this book. But a basic understanding of this imaging technique will give you a better understanding of its clinical applications and the patient's experience. MIII uses very powerful magnets, ranging in field strength from 0.3 to 1.5 Tesla. By comparison, 1 Tesla is equivalent to 10,000 gauss, and the earth's magnetic field is only 0.5 gauss. Consequently, patients with cardiac pacemakers and certain metallic implants cannot be examined with MR scanning.
Current diagnostic magnetic-resonance scanning is based on imaging hydrogen atoms in fat and water molecules. In a magnetic field, the hydrogen atoms, which are small magnets themselves, align themselves with the magnetic field, in much the same way that a compass aligns itself with the earth's magnetic field. During scanning, pulsed radiowaves of a particular radiofrequency (RE) are directed at the patient, causing these small atomic magnets to be knocked out of ulignment. The hydrogen atoms will eventually reestablish the previous equilibrium with the surrounding magnet, and when they do so, they will emit the absorbed radiofrequency waves. The distribution of the emitted radiofrequency waves is analyzed by computer to produce the image. The time required by the hydrogen atoms to regain the equilibrium in state is referred to as the relaxation time. Two relaxation times are recognized with MR scanning: the T1, or longitudinal relaxation time; and the T2, or transverse relaxation time.
A wide variety of MR techniques are available to optimally visualize different tissues and disease processes. The most commonly used are spin-echo sequences. Two other MR terms you will hear about are the repetition time (TR) and the echo time (TE). The repetition time is the time between successive RF pulses; the echo time is the time between the RE pulse that excites the hydrogen atoms and the arrival of the return signal at the detector. Longer TR and TE values will produce images that are more dependent upon the T2 values of the tissues; shorter TR and TE values will produce images that are more T1 dependent. By changing the TR and TE values one can alter the relative siqnal intensities of different tissues to better visualize the organ or clinical condition under investigation.
Various body tissues emit characteristic MR signals, which determine whether they will appear white, gray, or black on the final scans. Tissues that emit strong MR signals appear white in MR scans, whereas those emitting little or no signal appear black. Note that the x-ray terms radiolucent and radiodense do not apply to MRI; instead, structures that appear white on MR scans are said to have high signal strength, whereas dark gray or black objects are said to have a low signal, or no signal at all. Compact bone will generally appear black. Fat will appear bright on a TI-weighted image, but decrease in intensity slightly on a T2-weighted image. Most tumors and inflammatory masses appear bright on 12-weighted images. With most MR techniques, rapidly moving blood appears black because the blood moves out of the anatomic section being imaged before the emission of the RF signal from the excited protons.
A great advantage of MR over CT scanning is that direct multiplanar scanning is possible. MRI can produce primary images in almost any imaging plane, including the axial, coronal, sagittal, or any specially chosen oblique plane. In addition, greater differentiaction of soft-tissue structures is possible with CT than with CT. A disadvantage of MRI is the longer acquisition time (time to collect data for imaging) of several minutes, which results in greater motion artifact. This is a problem with MR scans of the thorax and abdomen because of respiratory motion, but not with MR scans of the head and extremities. As compared with a 10- to 20-minute CT scan, an MR examination may take 30 to 45 minutes to acquire the data to produce the scan in ages. Three - dimensional reformations can be produced of both CT and MR. images; but since MR. shows blood vessels without contrast media, it permits contrast-free 3D vascular imaging (Figure 2.40).
You should be aware that many patients feel claustrophobic within the bore of an MR scanner, some so severely that the examination may have to be discontinued before completion. These svmptoms are often alleviated by a reassuring discussion before the examination, as well as by sedation to relax anxious patients. We strongly recommend that addition observing MR scanning, you observe as many radiological procedures as possible so that you are in a position to describe them accurately and to prepare your patients for what to expect.
Finally, nuclear imaging, another branch of radiology, offers important physiological information that you must be familiar with. This branch of radiology is based on the visualization of particular living organs and tissues through the injection of a radioactive isotope (radionuclide) that takes tip residence there briefly. It does so because the selected chemical substance to which the isotope has been attached (radionuclide-labeled substance) is normally involved in the physiologic metabolism of that organ or will remain there long enough to be imaged. An image is obtained because the radioactive isotope emits gamma rays for a brief period of time. The emitted rays are recorded by a gamma camera (Figure 2.41) or, less commonly, by a rectilinear seamier during the period of gamma emission. A few hours or day's later, the isotope will stop emitting detectable rays as it returns to a stable state. Its return to stability is measured in terms of its half-lift: the period until it is seen to be emitting half as much radiation as it did initially isotopes chosen for tagging are those that will remain in the organ to be studied long enough to produce a usable image but with relatively short half- lives, so as to minimize radiation to the patient's tissues.
Technetium-99m (Tc99m) has proved to be the most useful radioactive tracer; it is relatively inexpensive, has a short but useful half-life, and is readily available from portable generators. Tc99m is linked to various physiological substances that will seek different organs. Technetium-Tc99m-pertechnetate is trapped by the thyroid gland and can be used for thyroid imaging. Two other examples of other useful Tc99m compounds are Tc99m- macroaggregated albumin (which is trapped in pulmonary capillaries) for lung scanning (Figure 2.42) and Tc-99m -methylene diphosphonate for bone scanning (Figure 2.43). Other radionuclides are also used for diagnostic imaging. You may be already familiar, for example, with the use of thallium-201 scanning in the evaluation of myocardial blood flow.
A commonly requested radioisotope examination is the bone scan. The image obtained shows areas of more or less intensity of radiation related to portions of the bone having increased turnover. Tb us "hot spots" showing markedly increased activity of bone will be seen as dense black areas on a gamma camera or rectilinear scan of the whole skeleton (see bone scan, Figure 2.44). Unfortunately, these are very nonspecific and do not tell us the cause of the increased bone turnover. If they are located in symmetrical joint areas, for example, they may be being caused by acute arthritis, and if they are located eccentrically like those in Figure 2.44, they may be assumed to indicate the location of bone metastases from the patient's known or suspected cancer. A new technique you may hear about is SPECT imaging, which uses a gamma camera that rotates around the patient to produce tomographic like nuclear images.
As we proceed through this book, other important procedures using radioactive isotopes will be described. Remember flit now that nuclear medicine gives you less precise anatomic information but much more important physiological information, which will help you to understand and remember metabolic processes, both normal and abnormal.
Realize that in the usual isotope scan, the image obtained is produced by gamma radiation from the entire thickness of the organ, not from a single slice of it as in CT, MR, and ultrasonography. Realize also that just as fluoraseopy in plain radiography consists of continuous or intermittent observation of tissues penetrated by x-rays and produces dynamic radiographic information, so too ally of the other imaging methods we have been discussing can be used dynamically. The motion of the fetal heart is routinelv monitored by real-time ultrasound as evidence. that a quiet fetus is, in fact, alive. Dynamic studies using rapidly sequenced CT scans during the intravenous injection of contrast material produce time-lapse information bout the vascularity of a liver mass. Similary, sequential isotope scans are in use to document flow patterns such as blood flow through the heart chambers in a patient suspected of having a congenital heart anomaly.
As a student, you certainly can learn to recognize some of the basic changes imaged on plain films of the chest, abdomen, and bones. You cannot expect to learn to recognize all of the innumerable more subtle plain radiographic changes the radiologist identifies. Neither will you be able to interpret the findings the radiologist recognizes in the many supplementary techniques such as CT, MR, and ultrasound. A four year residency in radiology is scarcely enough time in which to learn to do that.
It is important, though,
1kw you to learn while on are in medical school how to use the help the
radiologist can give you in p1anning which procedures ought to be included
in your diagnostic workup plan, and the order in which they should be