Chapter 5: Radiological anatomy
General aspects
Most clinicians view internal anatomy with the aid of radiographic images and procedures. Proper interpretation of these images presupposes a detailed knowledge of anatomy. Radiography has proved particularly valuable in the detection of the early stages of deep-seated disease, when the possibility of cure is greatest. During these early stages there is little departure from the normal, hence knowledge of the earliest detectable variations, that is, of "the borderlands of the normal and early pathological..." (Kohler), is of great medical importance. Radiographic diagnosis is the most important method of non-destructive testing of the living body.
There are a wide array of procedures that fall under the perview of radiology. The earliest of these utilized the interaction of ionizing radiation with the body in order to create an image. Plain radiographs involve generating x-rays that are directed at the part of the body that is to be examined. The recognition that these rays can be harmful has resulted in procedures that restrict the area that is exposed (columation) and also that shield sensitive areas against the rays. Various methods of detection of x-rays have been developed. The earliest of these relied on the direct interaction between photographic film and the x-rays. Subsequently, there was development of methods for enhancement of images (using radiosensitive amplifying screens adjacent to x-ray film). Additionally, the interaction between x-rays and fluorescent screen permitted the viewing of moving images (fluoroscopy). More recently, methods for electronic detection and storage of information have afforded improved safety as well as the ability to store, manipulate and transmit information generated by these tests.
Radio-opacity
The fundamental principle of all radiographic tests that employ x-rays is that different body tissues have a different capacity to block or absorb x-rays. The following tissue densities produce the usual radiographic image, and they are arranged in order of increasing radio-opacity (i.e., whiteness on conventional radiographic film or computerized tomograms, blackness on fluoroscopic screens):
1. Air, as found, for example, in the trachea and lungs, the stomach and intestine, and the paranasal sinuses.
2. Fat.
3. Soft tissues, e.g., heart, kidney, muscles (these are all approximately the density of water).
4. Calcific (due to the presence of calcium and phosphorus), for example, in the skeleton.
5. Enamel of the teeth.
6. Dense foreign bodies, for example, metallic fillings in the teeth. Also radio-opaque contrast media, such as a barium meal in the stomach or intravascular contrast.
When the density of a structure is too similar to that of adjacent structures, it is possible to use contrast media to enhance or outline its contours. Contrast media are classified as radiolucent (e.g., air) and radio-opaque (e.g., barium or iodinated contrast media).
Plain film radiographs (often, but inappropriately called x-rays) display the shadow of the body part on the film. The farther the body part is from the film, the more magnified it will appear, but also its borders will be less distinct (fig. 5-1). Additionally, since the image is a 2-dimentional representation of a 3-dimentional object, the antero-posterior location of structures must be inferred. Images taken from a two perspectives may permit the skilled radiologist to interpret the antero-posterior location of objects by their displacement relative to one another in images taken from orthogonal (right angles) directions (fig. 5-2). Plane film radiographs are still the most common method of viewing osseous structures or the chest.
Positioning
The views used in plain radiographic images are named for the part of the body that is nearest the film, for example, anterior, right lateral, left anterior oblique. Alternatively, the terms anteroposterior and postero-anterior are used when the x-rays have passed through the object from front to back (tube in front of object, film behind) or from back to front (tube behind object, film in front), respectively. Radiographic postioning is highly standardized in order to facilitate interpretation. Views are selected to highlight the particular areas or structures being examined.
Skeletal radiology
The skeleton, owing to its high radio-opacity, is generally the most striking feature of a radiogram. It is important to appreciate, however, that many of the organs and soft tissues of the body can be investigated radiographically.
General features of a long bone
Radiographically, the compact substance of the bone is seen peripherally as a homogeneous band of calcific density. A nutrient canal may be visible as a radiolucent line traversing the compacta obliquely. In some areas the compacta is thinned to form a cortex. The cancellous, or spongy, substance is seen particularly toward the ends of the shaft as a network of lime density presenting interstices of soft-tissue density. Islands of compacta are visible occasionally in the spongiosa. The bone marrow and the periosteum present a soft-tissue densityand are not distinguishable as such.
In many young bones the uncalcified portion of an epiphysial disc or plate can be seen radiographically as an irregular, radiolucent band termed an epiphysial line. When an epiphysial line is no longer seen, it is said to be closed, and the epiphysis and diaphysis are said to be united or fused. The radiographic appearance of fusion, however, precedes the disappearance of the visible epiphysial disc as seen on the dried bone.
The term metaphysis is used radiologically for the calcified cartilage of an epiphysial disc and the newly formed bone beneath it.
General features of a joint
The articular cartilage presents a soft-tissue density and is not distinguishable as such. The so-called radiological joint space, that is, the interval between the radio-opaque epiphysial regions of two bones, is occupied almost entirely by the two layers of articular cartilage, one on each of the adjacent ends of the two bones. On a radiogram the" space" is usually 2 to 5 mm in width in the adult. The joint cavity is rarely visible. The "radiological joint line," that is, the junction between the radio-opaque end of a bone and the radiolucent articular cartilage, is actually the junction between a zone of calcified cartilage over the end of the bone and the uncalcified articular cartilage.
Skeletal maturation
The development of bones and skeletal maturation are discussed in Chapter 2. Tables showing the times of appearance of the postnatal ossific centers in the limbs are provided inside the back cover. The time of appearance that is given is the age at which 50 per cent of normal children show a certain center radiographically, while the remaining 50 per cent do not yet show it. It is important to appreciate that a considerable range of variation occurs on each side of the median figure.
Use of contrast
Because of limitations in resolving different structures of similar densities on plain radiographs, contrast agents (dye) has been used for a long time. Barium and/or air can be used to highlight the lumen of the grastrointestinal tract (c.f. figs. 27-3 and 27-9). This may show irregularities, abnormal growths or constrictions of these organs. Iodinated contrast media can be administered intravascularly to view arteries (arteriograms, c.f. fig. 43-19) or veins (venograms, c.f. fig. 13-2) and intrathecally to outline the spinal cord and nerve roots. The images may be viewed as plain radiographs, or may be view fluoroscopically (where movement of the dye can be followed) or in computer enhanced images that may remove adjacent structures from the image (digital subtraction angiograms, c.f. fig. 43-19). Advantages in digital subtraction angiograms include improved clarity of the image and the ability to use smaller amounts of contrast. Allergies to iodine or renal problems (the dye is excreted by the kidney) may limit the ability to use these procedures.
Fluoroscopy
Flouroscopy is less common than it once was. In this imaging procedure, the image is created by x-rays striking a fluorescent screen coated with a thin layer of fluorescent material (phosphor). When x-rays activate the screen, light is emitted. This image may be intensified electronically. Moreover, the fluoroscopic image can be photographed or electronically displayed. The great advantages of fluoroscopy are the ability to observe the motion of parts of the subject and the ability to change the position of the subject during the examination. The principal disadvantages are lower image resolution and higher radiation exosure.
Cross-sectional imaging
Pain radiographs suffer from limitations of difficulty in resolving the precise relationship between structures along the course of the x-rays as well as the difficulty in distinguishing objects that have similar densities. This has given rise to a revolution in imaging that continues to the present.
Computerized tomography
Computerized Axial Tomography (C.A.T. or CT) was the first of these revolutionary cross-sectional imaging procedures. In the 1950s, Sir Godfrey Newbold Hounsfield began working on the mathematical problem of using density calculations derived from x-ray beams directed through the body in multiple directions along a 360 degree arc (tomography) to construct a cross sectional image of the body. The original CT scanners had a source of x-rays on one side of the body and a detector on the other. After density measurements were made for this beam, the tube was rotated and another beam was passed through the patient and measured. This was continued around the entire patient (therefore, the “doughnut” shape of the CT scanner). The first functional CT scanner was constructed in 1972 and there was rapid development of this technology thereafter. Dr. Hounsfield received the Nobel Prize for this work in 1979 (along with Allan Cormack who independently worked on this problem). CT scanners have become significantly faster in creating the image by using multiple radiographic sources and detectors, but the principle of modern CT scanners are identical. A series of cross-sectional images (or “slices”), usually taken at 3mm to 1cm intervals, are collected through the body part. It is an important skill to be able to mentally visualize how the stack of slices fit into a 3-dimensional whole. Modern CT scanners, in addition to creating the common 2-dimensional images can mathematically reconstruct these images into a 3-dimentional images of the organ or body part of interest.
Limitations for the technology include the necessity for the patient to be quite still (although the speed of the test is improving) and also that the images can only be created in the plane of the gantry (the doughnut-shaped machine). It also uses ionizing radiation (x-rays) so it is not without hazard. Its ability to resolve difference between tissues of similar radiographic density has limits. However, it is of particular utility in trauma situations where it may detect fractures that are invisible to conventional radiographs and where clotted blood (for example due to traumatic hemorrhages) has a high radiographic density that can be easily seen.
Since CT scanning utilizes x-rays, the same principles apply for the use of contrast to enhance the visibility of structures. Additionally, it is possible to detect the leakage of contrast from blood vessels in regions that normally preclude the passage of intravascular contrast into the tissues (such as the brain). Such an abnormality is termed a “contrast enhancing lesion” and this can tell something about the biology of the lesion.
Magnetic resonance imaging
Magnetic resonance images are another method of creation of cross-sectional images. However, this does not involve the use of ionizing radiation. In the 1970s, Paul Lauterbur described the possibility for using magnetic fields for creating images of the human body, while much of the mathamatics of the process was worked out by Peter Mansfield. These investigators received the Nobel Prize in 2003 for their work and the applications of magnetic resonance in medicine have exploded since the first clinical scanners in the early 1980s.
The fundamental principle behind MRI is that polarized molecules in the body (the most ubiquitous being water) will align themselves when placed in a strong magnetic field. When subject to a pulse of radio waves, many of these aligned molecules will absorb energy and will be displaced from their aligned position. As these molecules return to alignment they will give up their energy, which can be detected. Additionally, the molecules will wobble their way back to alignment giving up successive small bursts of energy (termed “echos”). The ability of water molecules to move within the magnet vary with the chemical environment of the molecules. Therefore, each tissue will have its own characteristics as will each type of pathology. Variables in the creation of the image include the duration and frequency of the radio pulse as well as the timing of collection of the returning signal. The returning signal is collected by detectors arrayed around the body part and a series of images can be created by computer modeling in any desired plane.
Limitations to the technology include the fact that some objects are interfered with (such as pacemakers) or can be displaced (such as certain types of metal clips) by the magnetic field. Additionally, since it takes quite a long time to create an image, any movement can degrade the quality.
Magnetic resonance can be used to perform some unique functions. Images specifically of blood vessels can be created (magnetic resonance angiograms). Additionally, it is possible to measure the concentration of certain chemical in tissues (magnetic resonance spectroscopy). Furthermore, it is possible to determine differences in blood oxygen levels in tissues from one moment to the next. This is particularly useful in the brain, where neuronal function in an area results in an increase in blood flow that outstrips demand (resulting in increased oxygenation of hemoglobin). This “functional MRI” can be used to detect the areas of activity of the brain during certain tasks or activities.
It is clear that we have yet to explore the full capabilities of magnetic resonance, which only entered the clinical arena in the 1980s.
Nuclear medicine
Nuclear medicine scans bridge the border of anatomy and physiology. They typically involve the delivery of a radioactive contrast medium to the patient and collecting the products of radioactive decay. These may be detected in a single plane, much like a conventional radiographic image or in a manner similar to the creation of a CT scan, where a computer can depict the locations of concentration of the radioactive tracer in cross-sectional planes or even in three-dimensional reconstructions. The latter procedures typically utilize SPECT (single photon emission computerized tomography) technology.
The specificity of the scan depends on the particular substance to which the radioactive tracer is bound (the ligand). In some cases, the radioactive tracer itself provides this specificity. So, for example, radioactive iodine will be taken up by the thyroid gland, which can then be imaged, because the thyroid accumulated iodine. A radioactive tracer that is taken up by osteoblasts will show bone, with the greatest concentration at sites of active bone formation (such as around fractures or metastastatic tumors that are stimulating bone repair). If a radioactive tracer is bound to a compound that remains within the intravascular space, blood flow can be measured, while if radioactive tracers are incorporated within the patient's white blood cells, it may be able to determine the location of an infectious process.
More recently, very short half-life, positron-emitting compounds, have been applied to the study of function (positron emission tomography - P.E.T). These compounds decay with the release of a positron that travels for only a short distance in the tissues before being annihilated, with the release of a gamma ray that can be detected. The location of concentration of the positron emitter can then be calculated and a map created. Some positron emitters (such as a particular isotope of oxygen) can be used directly to create a map of oxygen delivery in the body, the positron emitters can be incorporated in many other metabolites or pharmacologic agents to show their distribution in the body. The most common uses of the procedure are for mapping brain function during cognitive tasks and for detecting malignant tumors (which usually have relatively high metabolic activity).
Additional reading
Hamilton, W. J., Simon, G., and Hamilton, S. G. I., Surface and Radiological Anatomy, 5th ed., Heffer, Cambridge, 1971.
Kohler, A., and Zimmer, E. A., Borderlands of the Normal and Early Pathologic in Skeletal Roentgenology, 3rd ed., Grone and Stratton, New York, 1968. Based on the 11th edition in German.
Tillier, H., Normal Radiological Anatomy, trans. by R. O'Rahilly, Thomas, Springfield, Illinois, 1968.
Questions
5-1 Look up the discovery of x-rays. When were they discovered and by whom?
5-2 Where would the film be placed for a left anterior oblique view of the thorax?
5-3 What are (a) the epiphysial line and (b) the joint space in terms of radiology?
Figure legends
Figure 5-1 Image magnification. X-ray images are shadows, and the geometry of their formation is similar to that relating to shadows formed by ordinary light. Thus the image becomes more enlarged the nearer the object is placed to the target, or the source of radiation. In the second diagram, the image of the object is smaller than in the first diagram because the object is farther from the target. In the third diagram, the target is more than 2 meters from the film, and the magnification is negligible.
Figure 5-2 Dissociation of planes in oblique views (after Tillier). When the incident radiation is in the direction of arrow 1, the images of parts A, B, C, and D of the object are projected in the order c, a, d, b on the film. With incident radiation in the direction of arrow 2, however, the order is a, c, b, d. Thus when the tube is displaced to the right, the upper plane (AB) becomes displaced to the left in relation to the lower plane (CD).